33345

8/17/2019 33345

http://slidepdf.com/reader/full/33345 1/83

Dept. of Mechanical Engineering , St. Joseph ’ s College of Engin eering

1

ME 2253 ENGINEERING MATERIALS & METALLURGY

UNIT – I CONSTITUTION OF ALLOYS AND PHASE DIAGRAMS

Constitution of alloys – Solid solutions, substitutional and interstitial – phase diagrams,

Isomorphous, eutectic, peritectic, eutectoid and peritectroid reactions, Iron – Iron carbideequilibrium diagram. Classification of steel and cast Iron microstructure, properties andapplication.

Materials

Materials are very important in development of human civilization. In respect, theirnames are associated in history, e.g. stone age, Bronze age, Iron age, etc.

Properties of materials

i. All solid engineering materials are characterized for their properties.ii. Engineering use of a material is reflection of its properties under conditions of

use.iii. All important properties can be grouped into six categories: Mechanical,

Electrical, Thermal, Magnetic, Optical, and Deteriorative.iv. Each material possess a structure, relevant properties, which dependent on

processing and determines the performance.v. Since there are thousands of materials available it is almost impossible to select a

material for a specific task unless otherwise its properties are known.vi. There are several criteria on which the final decision is based on.

vii. There are less chances of material possessing optimal or idle combination of properties.

viii. A need to trade off between number of factors!

Material ScienceIt can be defined as science dealing the relationships that exist between the structures and

properties of materials, which are useful in practice of engineer’s prof ession.

Classification of MaterialsThree basic groups of solid engineering materials based on atomic bonds and structures:

Metals Ceramics

Polymers

Classification can also be done based on either properties (mechanical, electrical,optical), areas of applications (structures, machines, devices). Further we can subdivide

these groups. According to the present engineering needs:

Composites,

Semiconductors, Biomaterials

Modern Material needs

8/17/2019 33345



2

Engine efficiency increases at high temperatures; requires high temperaturestructural materials.

Use of nuclear energy requires solving problems with residue, or advance innuclear waste processing.

Hypersonic flight requires materials that are light, strong and resist high

temperatures. Optical communications require optical fibers that absorb light negligibly.

Civil construction – materials for unbreakable windows.

Structures: materials that are strong like metals and resist corrosion like plastics.

ALLOYAlloy is a substance that has metallic properties and is composed of two or more

chemical elements, of which at least one is a metal. In which the major proportion of themetal is called as base metal and the minor proportion of the metal or element is alloying

element.

Alloys can be according to classified by1.

Based on the no of components

2.

Based on the no of equilibrium or phases3. Based on the structures

Alloys may be homogeneous (uniform) or mixtures. If the alloy is homogeneous it will

consist of a single phase and if it is a mixture it will be a combination of several phases.

PhaseA phase can be defined as a homogeneous portion of a system that has uniform physical

and chemical characteristics i.e. it is a physically distinct from other phases, chemicallyhomogeneous and mechanically separable portion of a system.

Pure Metal

In nature 100% pure metal is impossible to get, hence only 99.99% of pure metalcan exist with 0.01% of impurities.

In the equilibrium conditions, all the metals exhibit a definite melting or freezing point

This condition implies extremely slow heating and cooling (i.e for any change isto occur sufficient time must be allowed)

The cooling cure for the pure metal will be a horizontal line at the melting or

freezing point

Based on the no of components

1. Binary alloy system – The system if made up of two elements2. Ternary alloys system - The system is made up of three elements

8/17/2019 33345



3

Based on the structures

Solid SolutionA solid solution is formed when two metals are completely soluble in liquid state and

also completely soluble in solid state. In other words, when homogeneous mixtures oftwo or more kinds of atoms (of metals) occur in the solid state, they are known as solid

solutions. The more abundant atomic form is referred as solvent and the less abundantatomic form is referred as solute. For example sterling silver (92.5 percent silver and the

remainder copper) is a solid solution of silver and copper. In this case silver atoms aresolvent atoms whereas copper atoms are solute atoms. Another example is brass. Brass is

a solid solution of copper (64 percent) and zinc (36 percent). In this case copper atomsare solvent atoms whereas zinc atoms are solute atoms.

Types of Solid Solutions

Solid solutions are of two types. They are(a) Substitutional solid solutions.

(b) Interstitial solid solutions.

Substitutional Solid SolutionsIf the atoms of the solvent or parent metal are replaced in the crystal lattice by atoms of

the solute metal then the solid solution is known as substitutional solid solution.

The substitutional solid solution is further subdivided into1.

Disordered substitutional solid solution

2. Ordered substitutional solid solutionIn disordered substitutional solid , where the solute atoms are randomly substituted for

the solvent atoms. Its shown in the given figure below.

In ordered substitutional solid , where the solute atoms are orderly substituted for thesolvent atoms. Its shown in the given figure below.

Alloy

Homogeneous Mixture

Solid Solution

Interstitial

Intermediate

allo hase

Intermetallic Interstitial Electron

Any combination of

solid phases

8/17/2019 33345



4

Example , copper atoms may substitute for nickel atoms without disturbing the F.C.C.structure of nickel (Fig. a). In the substitutional solid solutions, the substitution can be

either disordered or ordered. Fig. b shows disordered substitutional solid solution. Herethe solute atoms have substituted disorderly for the solvent atoms on their lattice site. Fig.

c shows an ordered substitutional solid solution. Here the solute atoms have substituted inan orderly manner for the solvent atoms on their lattice site.

Interstitial Solid Solutions

In interstitial solid solutions, the solute atom does not displace a solvent atom, but ratherit enters one of the holes or interstices between the solvent atoms. Interstitial solid

solutions normally have a limited solunility. Example is iron-carbon system which isshown in fig.

In this system the carbon (solute atom) atom

occupies an interstitial position between iron

(solvent atom) atoms. Normally, atoms which haveatomic radii less than one angstrom are likely toform interstitial solid solutions. Examples are atoms

of carbon (0.77 A°), nitrogen (0.71 A°), hydrogen(0.46 A°), Oxygen (0.60 A°) etc.

Intermetallic Compounds

Intermetallic compounds are generally formed when one metal (for example magnesium)has chemical properties which are strongly metallic and the other metal (for example

antimony, tin or bismuth) has chemical properties which are only weakly metallic.Examples of intermetallic compounds are Mg2Sn, Mg2Pb, Mg3Sb2 and Mg3 Bi2. These

intermetallic compounds have higher melting point than either of the parent metal. Thishigher melting point indicates the high strength of the chemical bond in intermetallic

compounds.

8/17/2019 33345



5

Hume Rothery rules – Controls the range of Solubility in alloy system

(a) Crystal structure factor: For complete solid solubility, the two elements should have

the same type of crystal structure i.e., both elements should have either F.C.C. or B.C.C.

or H.C.P. structure.

(b) Relative size factor: As the size (atomic radii) difference between two elements

increases, the solid solubility becomes more restricted. For extensive solid solubility thedifference in atomic radii of two elements should be less than about 15 percent. If the

relative size factor is more than 15 percent, solid solubility is limited. For example, bothsilver and lead have F.C.C. structure and the relative size factor is about 20 percent.

Therefore the solubility of lead in solid silver is about 1.5 percent and the solubility ofsilver in solid lead is about 0.1 percent. Copper and nickel are completely soluble in each

other in all proportions. They have the same type of crystal structure (F.C.C.) and differin atomic radii by about 2 percent.

(c) Chemical aff in ity factor: Solid solubility is favoured when the two metals have lesser

chemical affinity. If the chemical affinity of the two metals is greater then greater is thetendency towards compound formation. Generally, if the two metals are separated in the

periodic table widely then they possess greater chemical affinity and are very likely toform some type of compound instead of solid solution.

(d ) Relative valence factor: If the solute metal has a different valence from that of the

solvent metal, the number of valence electrons per atom called the electron ratio. It isfound that a metal of lower valence tends to dissolve more of a metal of higher valence

than vice versa. For example in aluminium-nickel alloy system, nickel (lower valance)dissolves 5 percent aluminium but aluminium (higher valence) dissolves only 0.04

percent nickel.

Phase Diagrams

Many of the engineering materials possess mixtures of phases, e.g. steel, paints, and

composites. The mixture of two or more phases may permit interaction between different phases, and results in properties usually are different from the properties of individual

phases. Different components can be combined into a single material by means ofsolutions or mixtures. A solution (liquid or solid) is phase with more than one

component; a mixture is a material with more than one phase. Solute does not change thestructural pattern of the solvent, and the composition of any solution can be varied. In

mixtures, there are different phases, each with its own atomic arrangement. It is possibleto have a mixture of two different solutions!

A pure substance, under equilibrium conditions, may exist as either of a phase namely

vapor, liquid or solid, depending upon the conditions of temperature and pressure. A phase can be defined as a homogeneous portion of a system that has uniform physical

and chemical characteristics i.e. it is a physically distinct from other phases, chemicallyhomogeneous and mechanically separable portion of a system. In other words, a phase is

a structurally homogeneous portion of matter. When two phases are present in a system,

8/17/2019 33345



6

it is not necessary that there be a difference in both physical and chemical properties; adisparity in one or the other set of properties is sufficient.

There is only one vapor phase no matter how many constituents make it up. For puresubstance there is only one liquid phase, however there may be more than one solid phase

because of differences in crystal structure. A liquid solution is also a single phase, even

as a liquid mixture (e.g. oil and water) forms two phases as there is no mixing at themolecular level. In the solid state, different chemical compositions and/or crystalstructures are possible so a solid may consist of several phases. For the same

composition, different crystal structures represent different phases. A solid solution hasatoms mixed at atomic level thus it represents a single phase. A single-phase system is

termed as homogeneous, and systems composed of two or more phases are termed asmixtures or heterogeneous. Most of the alloy systems and composites are heterogeneous.

It is important to understand the existence of phases under various practical conditionswhich may dictate the microstructure of an alloy, thus the mechanical properties and

usefulness of it. Phase diagrams provide a convenient way of representing which state ofaggregation (phase or phases) is stable for a particular set of conditions. In addition,

phase diagrams provide valuable information about melting, casting, crystallization, andother phenomena.

Usefu l terminology: -

Component – is either pure metal and/or compounds of which an alloy is composed. Thecomponents of a system may be elements, ions or compounds. They refer to the

independent chemical species that comprise the system.

System – it can either refer to a specific body of material under consideration or it may

relate to the series of possible alloys consisting of the same components but withoutregard to alloy composition.

Soli d solution – it consists of atoms of at least two different types where solute atomsoccupy either substitutional or interstitial positions in the solvent lattice and the crystalstructure of the solvent is maintained.

Solubility limit – for almost all alloy systems, at a specific temperature, a maximum ofsolute atoms can dissolve in solvent phase to form a solid solution. The limit is known as

solubility limit. In general, solubility limit changes with temperature. If solute available ismore than the solubility limit that may lead to formation of different phase, either a solid

solution or compound.

Phase equi li bri um – it refers to the set of conditions where more than one phase may

exist. It can be reflected by constancy with time in the phase characteristics of a system.In most metallurgical and materials systems, phase equilibrium involves just solid phases.However the state of equilibrium is never completely achieved because of very slow rate

of approach of equilibrium in solid systems. This leads to non-equilibrium or meta-stablestate, which may persist indefinitely and of course, has more practical significance than

equilibrium phases. An equilibrium state of solid system can be reflected in terms ofcharacteristics of the microstructure, phases present and their compositions, relative

phase amounts and their spatial arrangement or distribution.

8/17/2019 33345



7

Var iables of a system – these include two external variables namely temperature and pressure along with internal variable such as composition (C ) and number of phases ( P ).

Number of independent variables among these gives the degrees of freedom ( F ) orvariance.

Gibbs Phase Rule

In a system under a set of conditions, the relationship between number of phases (P) exist

can be related to the number of components (C) and degrees of freedom (F) by Gibbs phase rule.

P + F = C + 2

Where, P – no of phases (solid, liquid, Gaseous etc)C – No of components in the alloy

F – Degrees of freedom refers to the number of independent variables (e.g.: pressure, temperature) that can be varied individually to effect changes in a

system.

Thermodynamically derived Gibbs phase rul e:

In practical conditions for metallurgical and materials systems, pressure can be treated as

a constant (1 atm.). Thus Condensed Gibbs phase rul e is written as:

P + F = C + 1

Polymorphism and Allotropy

Polymorphism is a physical phenomenon where a material may have more than one

crystal structure. A material that shows polymorphism exists in more than one type ofspace lattice in the solid state. If the change in structure is reversible, then the

polymorphic change is known as allotropy . The prevailing crystal structure depends on both the temperature and the external pressure.

One familiar example is found in carbon: graphite is the stable polymorph at ambient

conditions, whereas diamond is formed at extremely high pressures.

The best known example for allotropy is iron. When iron crystallizes at 2800oF(1536

oC)

it is B.C.C. (d -iron), at 2554oF(1391

oC) the structure changes to F.C.C. (g -iron or

austenite), and at 1666oF(914

oC)it again becomes B.C.C. (a -iron or ferrite).

α(bcc) ↔γ(fcc) ↔δ(bcc) ↔Liquid

8/17/2019 33345



8

Unl imi ted Solid Solubili ty: Solute and solvent are mutually soluble at all concentrations,

e.g., Cu-Ni system Meets the requirements of the Hume-Rothery Rules,result is a “single phase alloy”

L imi ted or Partial Soli d Solubil ity: There is a limit to how much of the solute can dissolvein the solvent before “saturation” is reached, e.g., Pb-Sn and most other systems. Does not

meet the requirements of the Hume-Rothery Rules. Results in a “multi- phase alloy”

Equilibrium Phase Diagrams

A diagram that depicts existence of different phases of a system under equilibrium istermed as phase diagram. It is also known as equilibrium or constitutional diagram.

Equilibrium phase diagrams represent the relationships between temperature and thecompositions and the quantities of phases at equilibrium. In general practice it is

sufficient to consider only solid and liquid phases, thus pressure is assumed to beconstant (1 atm.) in most applications. These diagrams do not indicate the dynamics

when one phase transforms into another. However, it depicts information related tomicrostructure and phase structure of a particular system in a convenient and concise

manner. Important information, useful for the scientists and engineers who are involved

with materials development, selection, and application in product design, obtainable froma phase diagram can be summarized as follows:

- To show phases are present at different compositions and temperatures underslow cooling (equilibrium) conditions.

- To indicate equilibrium solid solubility of one element/compound in another.

- To indicate temperature at which an alloy starts to solidify and the range of

solidification.

8/17/2019 33345



9

- To indicate the temperature at which different phases start to melt.

- Amount of each phase in a two-phase mixture can be obtained.

A phase diagram is actually a collection of solubility limit curves. The phase fields in

equilibrium diagrams depend on the particular systems being depicted. Set of solubilitycurves that represents locus of temperatures above which all compositions are liquid arecalled liquidus, while solidus represents set of solubility curves that denotes the locus of

temperatures below which all compositions are solid.

Every phase diagram for two or more components must show a liquidus and a solidus,and an intervening freezing range, except for pure system, as melting of a phase occurs

over a range of temperature. Whether the components are metals or nonmetals, there arecertain locations on the phase diagram where the liquidus and solidus meet.

For a pure component, a contact point lies at the edge of the diagram. The liquidus andsolidus also meet at the other invariant positions on the diagram. Each invariant point

represents an invariant reaction that can occur only under a particular set of conditions between particular phases, so is the name for it.

Phase diagrams are classi f ied based on the number of components in the system.

Single component systems have unary diagrams,

Two-component systems have binary diagrams,

Three-component systems are represented by ternary diagrams, and so on.

When more than two components are present, phase diagrams become extremely

complicated and difficult to represent.

Experimental Methods:

The data for the construction of equilibrium diagrams are determined experimentally by a

variety of methods, the most common methods are:

Metallographic Methods

X-ray Diffraction Technique

Thermal Analysis

Metallographic Methods:This method is applied by heating samples of an alloy to different temperatures, waitingfor equilibrium to be established, and then quickly cooling to retain their high-

temperature structure. The samples then examined microscopically. This method isdifficult to apply to metals at high temperatures because the rapidly cooled samples do

not always retain their high-temperature structure, and considerable skill is then requiredto interpret the observed microstructure correctly.

8/17/2019 33345



10

X-ray Di ff raction Technique:

This method is applied by measuring the lattice dimensions and indicating the appearance

of a new phase either by the change in lattice dimension or by the appearance of a newcrystal structure. This method is very precise and very useful in determining the changes

in solid solubility with temperature.

Thermal Analysis:

This is by far the most widely used experimental method. It relies on the information

obtained from the cooling diagrams. In this method, alloys mixed at differentcompositions are melted and then the temperature of the mixture is measured at a certain

time interval while cooling back to room temperature.

A cooling diagram for each mixture is constructed and the initial and final phase change

temperatures are determined. Then these temperatures are used for the construction of the

phase diagrams.

Cooling Curve of a Pure Metal:

Under equilibrium conditions, all metals exhibit a definite melting or freezing point. If a

cooling curve is plotted for a pure metal. It will show a horizontal line at the melting orfreezing temperature.

Figure 2. Cooling curve for the solidification of a pure metal.

Cooling Curve of a Solid Solution:

A solid solution is a solution in the solid state and consists of two kinds of atoms

combined in one type of space lattice. A solution is composed of two parts: a solute and asolvent. The solute is the minor part of the solution or the material which is dissolved,while the solvent constitutes the major portion of the solution. When solidification of the

solution starts, the temperature may be higher or lower than the freezing point of the puresolvent. Most solid solutions solidify over a range in temperature. Figure 3 shows the

cooling curve for the solidification of a solid solution.

8/17/2019 33345



11

Figure 3. Cooling curve for a solid solution.

Figure 4. Series of cooling curves for different alloys in a completely soluble system. The

dotted lines indicate the form of the phase diagram.

8/17/2019 33345



12

Figure 5. Phase Diagram of all oy A+B

Unary diagrams: In these systems there is no composition change (C =1), thus onlyvariables are temperature and pressure. Thus in region of single phase two variables

(temperature and pressure) can be varied independently. If two phases coexist then,according to Phase rule, either temperature or pressure can be varied independently, but

not both. At triple points, three phases can coexist at a particular set of temperature and pressure. At these points, neither temperature nor the pressure can be changed without

disrupting the equilibrium i.e. one of the phases may disappear. Figure-1 depicts phasediagram for water.

Figure-1: Unary phase diagram for water.

Binary diagrams: These diagrams constitutes two components, e.g.: two metals (Cu and Ni), or a metal and a compound (Fe and Fe

3C), or two compounds (Al

2O

3and Si

2O

3), etc.

In most engineering applications, as mentioned before, condensed phase rule is

applicable. It is assumed that the same is applicable for all binary diagrams, thus the presentation of binary diagrams becomes less complicated. Thus binary diagrams areusually drawn showing variations in temperature and composition only. It is also to be

noted that all binary systems consist only one liquid phase i.e. a component is completelysoluble in the other component when both are in liquid state.

8/17/2019 33345



13

Hence, binary systems are classified according to their solid solubility.

1. Components completely soluble in the liquid state and completely soluble in thesolid state – Type – I

2.

Components completely soluble in the liquid state and insoluble in the solid state – Type – II

3. Components completely soluble in the liquid state and partly soluble in the solid

state – Type – III

4. Components completely soluble in the liquid state and formation of congruent-

melting intermediate phase – Type – IV

5. The Peritectic reaction – Type – V

6. Components partly soluble in liquid state – Monotectic reaction – Type – VI

7. Components insoluble in the liquid state and insoluble in the solid state Type – VII

8.

Transformation in solid statea. Allotropic change

b. Order – Disorder

c. The Eutectoid reaction

d. The Peritectoid reaction

Binary Phase diagrams

Components completely soluble in the liqui d state and completely soluble in the soli d

state – Type – I

If both the two components are completely soluble in each other in both solid and liquid

state, then the system is called isomorphous system. Hence it can be called as Binary Isomorphous phase diagram or system

E.g.: Cu-Ni, Ag-Au, Ge-Si, Al2O

3-Cr

2O

3.

Extent solid solubility for a system of two metallic components can be predicted based on

Hume-Ruthery conditions, summarized in the following:

- Crystal structure of each element of solid solution must be the same.

- Size of atoms of each two elements must not differ by more than 15%.

- Elements should not form compounds with each other i.e. there should be noappreciable difference in the electro-negativities of the two elements.

- Elements should have the same valence.

All the Hume-Rothery rules are not always applicable for all pairs of elements whichshow complete solid solubility.

In systems other than isomorphous systems i.e. in case of limited solid solubility, thereexist solid state miscibility gaps; number of invariant reactions can take place;

intermediate phases may exist over a range of composition (intermediate solid solutions)

8/17/2019 33345



14

or only at relatively fixed composition (compound ). These intermediate phases mayundergo polymorphic transformations, and some may melt at a fixed temperature

(congruent transformations, in which one phase changes to another of the samecomposition at definite temperature). A solid solution based on a pure component and

extending to certain finite compositions into a binary phase diagram is called a terminal

solid solution, and the line representing the solubility limit of a terminal solid solutionw.r.t a two-phase solid region is called a solvus line ( figure-4).

Binary I somorphous System

If both the two components are completely soluble in each other in both solid and liquidstate, then the system is called isomorphous system. Hence it can be called as Binary

Isomorphous phase diagram or system

E.g.: Cu-Ni, Ag-Au, Ge-Si, Al2O

3-Cr

2O

3.

A typical phase diagram for an isomorphous system made of two metallic elements A and

B is shown in figure(a). Any phase diagram can be considered as a map. A set ofcoordinates – a temperature and a composition – is associated with each point in the

diagram. If the alloy composition and temperature specified, then the phase diagramallows determination of the phase or phases that will present under equilibrium

conditions. There are only two phases in the phase diagram, the liquid and the solid phases. These single-phases regions are separated by a two-phase region where both

liquid and solid co-exist. The area in the figure above the line marked liquidus ( A’bB’ )corresponds to the region of stability of the liquid phase i.e beginning of the

solidification, and the area below the solidus line ( A’dB’ ) represents the stable region forthe solid phase i.e end of the solidification.

Between two extremes of the horizontal axis of the diagram, cooling curves for different

alloys are shown in below as a function of time and temperature. Cooling curves shownin figure(b) represent A, U’ , X , V’ and B correspondingly in figure-(a). Change in slope

of the cooling curve is caused by heat of fusion. In fact these changes in slope are nothing but points on either solidus or liquidus of a phase diagram. An experimental procedure

Figure (a)

Figure(b)

8/17/2019 33345



15

where repeated cooling/heating of an alloy at different compositions, and correspondingchanges in slope of cooling curves will be used to construct the phase diagram.

For the interpretation of the phase diagram, let’s consider the vertical line ae drawn

corresponding to composition of 50%A +50%B and assume that the system is undergoingequilibrium cooling. The point a on the line ae signifies that for that particular

temperature and composition, only liquid phase is stable. This is true up to the point b

which lies on the liquidus line, representing the starting of solidification. Completion of

solidification of the alloy is represented by the point, d . Point e corresponds to single- phase solid region up to the room temperature. Point c lies in the two-phase region made

of both liquid and solid phases. Corresponding micro-structural changes are also shownin figure-2. As shown in figure (a) above liquidus only a liquid phase exists, and below

the solidus single solid phase exists as completely solidified grains. Between these twolines, system consist both solid crystals spread in liquid phase. It is customary to use L to

represent liquid phase(s) and Greek alphabets (α, β , γ) for representing solid phases.

Another important aspect of interpreting phase diagrams along with phases present is

f inding the relative amount of phases present and their individual composit ion.

Procedure to fi nd equi li brium concentr ations of phases:

- A tie-line or isotherm (UV ) is drawn across two-phase region to intersect the

boundaries of the region.

- Perpendiculars are dropped from these intersections to the composition axis,

represented by U’ and V’ in figure-(a), from which each of each phase is read. U’

represents composition of liquid phase and V’ represents composition of solid

phase as intersection U meets liquidus line and V meets solidus line.

Procedure to fi nd equi li brium relati ve amounts of phases (lever r ule):

- A tie-line is constructed across the two phase region at the temperature of thealloy to intersect the region boundaries.

- The relative amount of a phase is computed by taking the length of tie line fromoverall composition to the phase boundary for the other phase, and dividing by the

total tie-line length. From figure-(a), relative amounts of liquid and solid phases isgiven respectively by

Liquid percent= (cV/UV) X 100

Solid percent% = (cU/UV) X 100

U – 28 percent

V – 74 percent

c – 50 percent

Hence cV = 74 – 50 = 24 and cU = 50 – 28 =22, UV = 74-28 = 46

Liquid percent = 24 / 46 = 52.18 percent

8/17/2019 33345



16

Solid percent = 22 / 46 = 47.82 percent

Example is Cu – Ni alloy system – Binary Isomorphous system

Example of isomorphous system:Cu-Ni (the complete solubilityoccurs because both Cu and Ni

have the same crystal structure,FCC, similar radii,

electronegativity and valence).

The melting point of Cu is 1085ºCand for Ni is 1453ºC. This consist

of three phases liquid, solid (α)and mixture of liquid and solid

(α). Liquidus line represents the beginning of solidification and

Solidus line represents the end ofthe solidification. Here both Cu

and Ni are completely soluble in both liquid and solid state.

Components completely soluble in the liquid state and Insoluble in the solid state –

Type – II

Eutectic phase diagram describes behavior of the alloys, two components of which arecompletely soluble in liquid state and entirely insoluble in solid state.

This diagram has two liquidus curves,

starting from the freezing points of the twometals and intersecting in a minimum point

– Consider solidification of an alloy withconcentration C. When the alloy

temperature is higher than TL , single liquid phase exists (point M on the diagram).

When the temperature reaches the value TL(point M1 on the liquidus curve)

solidification starts. The primary crystals,

forming in this case are the crystals of themetal “A”.

8/17/2019 33345



17

Further cooling of the alloy causes enrichment of the liquid phase with the metal “B” according to the liquidus curve and when the alloy temperature reaches a certain

intermediate value T (position MT ), liquid phase of composition Cy and solid phase,consisting of “A” crystals, are in equilibrium.

At the temperature equal to TE (eutectic temperature) formation of the primary crystalsstops and the remainding liquid phase , having composition CE (eutectic composition),transforms to an intimate mixture of small “A” and “B” solid crystals. This is the

eutectic phase transformation.

Relative amounts of the primary crystals and the eutectic mixture may be calculated bythe “lever rule” :

WP / WE = (CE-C) / C

Where: WP – weight of the primary crystals;

WE – weight of the eutectic mixture;

Components completely soluble in the liquid state and partly soluble in the solid

state – Type – III

Eutectic system: Many binary systems have components which have limited solid

solubility, e.g.: Cu-Ag, Pb-Sn. The regions of limited solid solubility at each end of a phase diagram are called terminal solid solutions as they appear at ends of the diagram.

Many of the binary systems with limited

solubility are of eutectic type, which consists ofspecific alloy composition known as eutecticcomposition that solidifies at a lower

temperature than all other compositions. Thislow temperature which corresponds to the

lowest temperature at which the liquid can existwhen cooled under equilibrium conditions is

known as eutectic temperature. Thecorresponding point on the phase diagram is called eutectic point . When the liquid of

eutectic composition is cooled, at or below eutectic temperature this liquid transformssimultaneously into two solid phases (two terminal solid solutions, represented by α and

β ). This transformation is known as eutectic reaction and is written symbolically as:Liquid (L ) ↔ solid solution-1 (α) + solid solution-2 ( β )

This eutectic reaction is called invariant reaction as it occurs under equilibriumconditions at a specific temperature and specific composition which can not be varied.Thus, this reaction is represented by a thermal horizontal arrest in the cooling curve of an

alloy of eutectic composition. A typical eutectic type phase diagram is shown in figure-4along with a cooling curve.

8/17/2019 33345



18

As shown in figure-4, there exist three single phase regions, namely liquid (L ), α and β

phases. There also exist three two phase regions: L+α, L+β and α+ β . These three two phase regions are separated by horizontal line corresponding to the eutectic temperature.

Below the eutectic temperature, the material is fully solid for all compositions.

Compositions that are on left-hand-side of the eutectic composition are known as hypo-

eutectic compositions while compositions on right-hand-side of the eutectic compositionare called hyper-eutectic compositions. The phase that forms during cooling but before

reaching eutectic temperature is called pro-eutectic phase.

Example for completely

soluble in the liquid state and partly soluble in the solid

state is Pb – Sn alloy. In thisthe melting point of the Pb is

327C and Sn is 232C. In thisthere are 6 phases, liquid,

solid-α, solid–β and mixtureof liquid and solid-α, liquid

and solid- β, solid-α andsolid –β. Where the Eutectic

reaction takes place atEutectic isotherm

temperature is 183C wherethe eutectic reaction takes

place at the 61.9% of Sn.

Eutectic reaction is

Liquid ↔ Solid-α + Solid-β

Type – IV

Congruent Melting intermediate phase

When one phase changes into

another phase isothermally(atconstant temperature) and without

any change in chemicalcomposition, it is said to be

congruent phase change orcongruent transformation. All pure

metals solidify congruently. Thegiven phase diagram shows the

congruent melting intermediate phase. The vertical line shows the

8/17/2019 33345



19

intermediate alloy phase, since it is a compound, it is indicated as AB2, where 2 is the noof atoms combined in the compound. E.g is Mg2Sn.This consist of more than one eutectic

reaction also.

Type V – Peritectic Reaction

An isothermal, reversible reaction between two phases, a liquid and a solid, that results,

on cooling of a binary, ternary, ... , n system in one, two, ... (n – 1) new solid phases.

Liquid + Solid-α ↔ Solid - β

In the given figure, the peritectic reaction takes place at

the point of P.

Invariant reactions: The eutectic reaction, in which a liquid transforms into two solid

phases, is just one of the possible three-phase invariant reactions that can occur in binarysystems those are not isomorphous. Schematically it can be shown as in figure-9. It

represents that a liquid phase, L , transforms into two different solids phases (α and β )upon cooling during the eutectic reaction.

In the solid state analog of a eutectic reaction, called a eutectoid reaction, one solid phase

having eutectoid composition transforms into two different solid phases.

8/17/2019 33345



20

Another set of invariant reactions that occur often in binary systems are - peritecticreaction where a solid phase reacts with a liquid phase to produce a new solid phase, and

in peritectoid reaction, two solid phases react to form a new solid phase. Peritecticreaction is commonly present as part of more-complicated binary diagrams, particularly if

the melting points of the two components are quite different. Peritectic and peritectoid

reactions do not give rise to micro-constituents as the eutectic and eutectoid reactions do.Another invariant reaction that involves liquid phase is monotectic reaction in which aliquid phase transforms into a solid phase and a liquid phase of different composition.

Over a certain range of compositions the two liquids are immiscible like oil and waterand so constitute individual phases, thus monotectic reaction can said to be associated

with miscibility gaps in the liquid state. Example system for monotectic reaction: Cu-Pbat 954

C and 36%Pb. Analog to monotectic reaction in solid state is monotectoid

reaction in which a solid phase transforms to produce two solid phases of differentcompositions. Another notable invariant reaction that is associated with liquid

immiscibility is syntectic reaction in which two liquid phases react to form a solid phase.All the invariant reactions are summarized in the table-1 showing both symbolic reaction

and schematic part of phase diagram.Table-1: Summary of invariant reactions in binary systems.

Intermediate phases: An intermediate phase may occur over a composition range(intermediate solid solution) or at a relatively fixed composition (compound) inside the

phase diagram and are separated from other two phases in a binary diagram by two phaseregions. Many phase diagrams contain intermediate phases whose occurrence cannot be

readily predicted from the nature of the pure components. Intermediate solid solutionsoften have higher electrical resistivities and hardnesses than either of the two

components. Intermediate compounds form relatively at a fixed composition when thereexists a stoichiometric relationship between the components, for example: Mg2Ni and

MgNi2 in Mg-Ni system. These are called inter-metallic compounds, and differ fromother chemical compounds in that the bonding is primarily metallic rather than ionic or

covalent, as would be found with compounds in certain metal-nonmetal or ceramicsystems. Some metal-nonmetal compounds, Fe3C, are metallic in nature, whereas in

others, MgO and Mg2Si, bonding is mainly covalent. When using the lever rules, inter-

8/17/2019 33345



21

metallic compounds are treated like any other phase, except they appear not as a wideregion but as a vertical line.

Number of phase transformations may takes place for each system. Phase transformationsin which there are no compositional alternations are said to be congruent

transformations, and during incongruent transformations at least one of the phases will

experience a change in composition. Examples for (1) congruent transformations:allotropic transformations, and melting of pure materials (2) incongruent transformations:all invariant reactions, and also melting of alloy that belongs to an isomorphous system.

Intermediate phases are sometimes classified on the basis of whether they meltcongruently or incongruently. MgNi2, for example, melts congruently whereas Mg2Ni

melts incongruently since it undergoes peritectic decomposition.

8/17/2019 33345



22

8/17/2019 33345



23

The iron – carbon system, phase transformations

A study of iron-carbon system is useful and important in many respects. This is because(1) steels constitute greatest amount of metallic materials used by man (2) solid state

transformations that occur in steels are varied and interesting. These are similar to those

occur in many other systems and helps explain the properties.Iron-carbon phase diagram shown in figure given below is not a complete diagram. Partof the diagram after 6.67 wt% C is ignored as it has little commercial significance. The

6.67%C represents the composition where an inter-metallic compound, cementite (Fe3C),with solubility limits forms. In addition, phase diagram is not true equilibrium diagram

because cementite is not an equilibrium phase. However, in ordinary steelsdecomposition of cementite into graphite never observed because nucleation of cementite

is much easier than that of graphite. Thus cementite can be treated as an equilibrium phase for practical purposes.

The Fe-Fe3C is characterized by five individual phases and four invariant reactions. Five

phases that exist in the diagram are: α – ferrite (BCC) Fe-C solid solution, γ-austenite(FCC) Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or

cementite - an inter-metallic compound and liquid Fe-C solution. Four invariant reactionsthat cause transformations in the system are namely eutectoid, eutectic, monotectic and

peritectic.As depicted by left axes, pure iron upon heating exhibits two allotropic changes. One

involves α – ferrite of BCC crystal structure transforming to FCC austenite, γ-iron, at910 C. At 1400

C, austenite changes to BCC phase known as δ-ferrite, which finally

melts at 1536 C.

Carbon present in solid iron as interstitial impurity, and forms solid solution with ferrites/ austenite as depicted by three single fields represented by α, γ and δ.

Carbon dissolves least in α – ferrite in which maximum amount of carbon soluble is 0.02%

at 723 C. This limited solubility is attributed to shape and size of interstitial position in

BCC α – ferrite. However, carbon present greatly influences the mechanical properties of

α – ferrite. α – ferrite can be used as magnetic material below 768 C.

Solubility of carbon in γ-iron reaches its maximum, 2.11%, at a temperature of 1147 C.

Higher solubility of carbon in austenite is attributed to FCC structure and corresponding

interstitial sites. Phase transformations involving austenite plays very significant role inheat treatment of different steels. Austenite itself is non-magnetic.

Carbon solubility in δ-ferrite is maximum (0.1%) at 1495 C. As this ferrite exists only

at elevated temperatures, it is of no commercial importance

8/17/2019 33345



24

8/17/2019 33345


Dept. of Mechanical Engineering, St. Joseph ’ s College of Engin eering

25

8/17/2019 33345



26

Cementite, Fe3C an inter-metallic compound forms when amount of carbon presentexceeds its solubility limit at respective temperatures. Out of these four solid phases,

cementite is hardest and brittle that is used in different forms to increase the strength ofsteels. α – ferrite, on the other hand, is softest and act as matrix of a composite material.

By combining these two phases in a solution, a material’s properties can be varied over a

large range.

For technological convenience, based on %C dissolved in it, a Fe-C solution is classified

as: commercial pure irons with less than 0.008%C; steels having %C between 0.008-2.11; while cast irons have carbon in the range of 2.11%-6.67%. Thus commercial pure

iron is composed of exclusively α – ferrite at room temperature. Most of the steels and castirons contain both α – ferrite and cementite. However, commercial cast irons are not

simple alloys of iron and carbon as they contain large quantities of other elements such assilicon, thus better consider them as ternary alloys. The presence of Si promotes the

formation of graphite instead of cementite. Thus cast irons may contain carbon in form of both graphite and cementite, while steels will have carbon only in combined from as

cementite.As shown in figure-above and mentioned earlier, Fe-C system constitutes four invariant

reactions:- peritectic reaction at 1495

C and 0.16%C, δ-ferrite + L ↔ γ-iron (austenite)

- monotectic reaction 1495 C and 0.51%C, L ↔ L + γ-iron (austenite)

- eutectic reaction at 1147 C and 4.3 %C, L ↔ γ-iron + Fe3C (cementite) [ledeburite]

- eutectoid reaction at 723 C and 0.8%C, γ-iron ↔ α – ferrite + Fe3C (cementite)

[pearlite]

Product phase of eutectic reaction is called ledeburite, while product from eutectoid

reaction is called pearlite. During cooling to room temperature, ledeburite transforms into pearlite and cementite. At room temperature, thus after equilibrium cooling, Fe-C

diagram consists of either α – ferrite, pearlite and/or cementite. Pearlite is actually not asingle phase, but a micro-constituent having alternate thin layers of α – ferrite (~88%) and

Fe3C, cementite (~12%). Steels with less than 0.8%C (mild steels up to 0.3%C, mediumcarbon steels with C between 0.3%-0.8% i.e. hypo-eutectoid Fe-C alloys) i.e. consists

pro-eutectoid α – ferrite in addition to pearlite, while steels with carbon higher than 0.8%(high-carbon steels i.e. hyper-eutectoid Fe-C alloys) consists of pearlite and pro-eutectoid

cementite. Phase transformations involving austenite i.e. processes those involveeutectoid reaction are of great importance in heat treatment of steels.

In practice, steels are almost always cooled from the austenitic region to room

temperature. During the cooling upon crossing the boundary of the single phase γ-iron,first pro-eutectoid phase (either α – ferrite or cementite) forms up to eutectoid temperature.

With further cooling below the eutectoid temperature, remaining austenite decomposes toeutectoid product called pearlite, mixture of thin layers of α – ferrite and cementite.

Though pearlite is not a phase, nevertheless, a constituent because it has a definiteappearance under the microscope and can be clearly identified in a structure composed of

several constituents.

8/17/2019 33345



27

The decomposition of austenite to form pearlite occurs by nucleation and growth. Nucleation, usually, occurs heterogeneously and rarely homogeneously at grain

boundaries. When it is not homogeneous, nucleation of pearlite occurs both at grain boundaries and in the grains of austenite. When austenite forms pearlite at a constant

temperature, the spacing between adjacent lamellae of cementite is very nearly constant.

For a given colony of pearlite, all cementite plates have a common orientation in space,and it is also true for the ferrite plates. Growth of pearlite colonies occurs not only by the

nucleation of additional lamellae but also through an advance at the ends of the lamellae.Pearlite growth also involves the nucleation of new colonies at the interfaces between

established colonies and the parent austenite. The thickness ratio of the ferrite andcementite layers in pearlite is approximately 8 to 1. However, the absolute layer thickness

depends on the temperature at which the isothermal transformation is allowed to occur.

The temperature at which austenite is transformed has a strong effect on the inter-lamellar spacing of pearlite. The lower the reaction temperature, the smaller will be inter-

lamellar spacing. For example, pearlite spacing is in order of 10-3 mm when it formed at700 C, while spacing is in order of 10-4 mm when formed at 600

C. The spacing of

the pearlite lamellae has a practical significance because the hardness of the resultingstructure depends upon it; the smaller the spacing, the harder the metal. The growth rate

of pearlite is also a strong function of temperature. At temperatures just below theeutectoid, the growth rate increases rapidly with decreasing temperature, reaching a

maximum at 600 C, and then decreases again at lower temperatures.

Additions of alloying elements to Fe-C system bring changes (alternations to positions of phase boundaries and shapes of fields) depends on that particular element and its

concentration. Almost all alloying elements causes the eutectoid concentration todecrease, and most of the alloying elements (e.g.: Ti, Mo, Si, W, Cr) causes the eutectoid

temperature to increase while some other (e.g.: Ni, Mn) reduces the eutectoidtemperature. Thus alloying additions alters the relative amount of pearlite and pro-

eutectoid phase that form.

Fe-C alloys with more than 2.11% C are called cast irons. Phase transformations in castirons involve formation of pro-eutectic phase on crossing the liquidus. During the further

cooling, liquid of eutectic composition decomposes in to mixture of austenite andcementite, known as ledeburite. On further cooling through eutectoid temperature,

austenite decomposes to pearlite. The room temperature microstructure of cast irons thusconsists of pearlite and cementite. Because of presence of cementite, which is hard, brittle

and white in color, product is called white cast iron. However, depending on cooling rateand other alloying elements, carbon in cast iron may be present as graphite or cementite.

Gray cast iron contains graphite in form of flakes. These flakes are sharp and act as stress

risers. Brittleness arising because of flake shape can be avoided by producing graphite inspherical nodules, as in malleable cast iron and SG (spheroidal graphite) cast iron.

Malleable cast iron is produced by heat treating white cast iron (Si < 1%) for prolonged periods at about 900

C and then cooling it very slowly. The cementite decomposes and

8/17/2019 33345



28

temper carbon appears approximately as spherical particles. SG iron is produced byadding inoculants to molten iron. In these Si content must be about 2.5%, and no

subsequent heat treatment is required.

Review on Fe – Fe3C system

Phases in Fe – Fe3C Phase Diagram

α-Ferr ite - solid solution of C in BCC Fe

• Stable form of iron at room temperature. • The maximum solubility of C is 0.022 wt%

• Transforms to FCC g-austenite at 912 °C

Austeni te - soli d solution of C in FCC Fe • The maximum solubility of C is 2.14 wt %. • Transforms to BCC d-ferrite at 1395 °C

• Is not stable below the eutectic temperature (727 ° C) unless cooled rapidly

δ-Ferr ite solid solution of C in BCC Fe

• The same structure as a-ferrite• Stable only at high T, above 1394 °C

• Melts at 1538 °C

Fe3C (ir on carbide or cementi te)

• This intermetallic compound is metastable, it remains as a compoundindefinitely at room T, but decomposes (very slowly, within several years) into a-

Fe and C (graphite) at 650 - 700 °C

Fe-C liquid soluti on

C is an interstitial impurity in Fe. It forms a solid solution with a, g, d phases of iron

Maximum solubility in BCC a-ferrite is limited (max. 0.022 wt% at 727 °C) - BCC hasrelatively small interstitial positions Maximum solubility in FCC austenite is 2.14 wt% at

1147 °C - FCC has larger interstitial positions Mechanical properties: Cementite is veryhard and brittle - can strengthen steels. Mechanical properties also depend on the

microstructure, that is, how ferrite and cementite are mixed. Magnetic properties: a -ferrite is magnetic below 768 °C, austenite is non-magnetic

Classification. Three types of ferrous alloys:

• Iron: less than 0.008 wt % C in a-ferrite at room T• Steels: 0.008 - 2.14 wt % C (usually < 1 wt % )

α-ferrite + Fe3C at room T (Chapter 12)• Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)

8/17/2019 33345



29

Classification Of Metal Alloys

Classification of Steels

8/17/2019 33345



30

Classification of Steels

8/17/2019 33345



31

Low Alloys: Low Carbon

• Composition: less than ~ 0.25% C

• Microstructure: ferrite and pearlite• Properties: relatively soft and weak, but possess high ductility and toughness

• Other features: moderately priced, machinable and weldable

• Applications: auto-body components, structural shapes, sheets etc.

• Plain carbon steels : residual concentration of impurities – are unresponsible to heat

treatment• H igh-strength low alloy (HSLA) steels: up to 10 wt% of alloying elements, such as

Mn, Cr, Cu, V, Ni, Mo – can be strengthened by heat-treatmentExamples of HSLA Steels:

•Weathering steels , exhibit superior atmospheric corrosion resistance•Control-roll ed steels , has a highly deformed austenite structure that transforms

to a very fine equiaxed ferrite structure on cooling•Pearli te-reduced steels , strengthened by very fine-grain ferrite and precipitation

hardening with a little or no pearlite in the microstructure•Microalloyed steels , with very small additions of Ni, V and/or Ti for refinement of

grain size and/or precipitation hardening•Acicul ar f erri te steel , very low carbon steels with sufficient hardenability to

transform on cooling to a very fine high-strength acicular ferrite structure

Low All oys: Medium Carbon Steels

• Composition: 0.25< C0 <0.6 C wt.%

• Processing: Increasing the carbon content to approximately 0.5% with anaccompanying

increase in manganese allows medium carbon steels to be used in the quenched and

tempered condition.• Microstructure: typically tempered martensite• Properties: stronger than low-carbon steels, but in expense of ductility and toughness

• Applications: couplings, forgings, gears, crankshafts other high-strength structuralcomponents. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels

and rail axles.

Low Al loys: H igh& Ul tra High - Carbon Steels

• High-carbon steels contain from 0.60 to 1.00 % C with manganese contents ranging

from0.30 to 0.90%. High-carbon steels are used for spring materials, high-strength wires,

cutting tools and etc.• Ultrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These

steelsare thermo-mechanically processed to produce microstructures that consist of ultra-fine,

equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

8/17/2019 33345



32

H igh-Al loy Steels: Stainless Steels (SS)

• The primarely alloying element is Cr (≥11 wt.%) • Highly resistance to corrosion

• Four major classes: (a) The austenitic SS has γ-Fe microstructure at room temperature!! This fcc

structure normally stable only above 910 C is stabilized in SS by addition of Ni – extremely corrosion resistant and NOT magnetic;

(b) The ferritic SS has α−Fe bbc structure. Not so corrosion resistant as austenitic SS, but less expensive magnetic steel;

(d) The martensitic SS this fine magnetic bct structure is produced by rapid

quenching and possesses high yield strength and low ductility. Applications:springs.(e) The precipitation hardening SS – producing multiple microstructure form

a single-phase one, leads to the increasing resistance for the dislocation motion• (a) and (b) are hardening and strengthening by cold work

8/17/2019 33345



33

H igh-Al loy Steels: Tools steels

• Provide the necessary hardness with simpler heat-tr eatment and

retain this hardness at high temperatu re

• The primary alloying elements are: Mo, W and Cr

CAST I RON

The cast irons are the ferrous alloys with greater that 2.14 wt % carbon, but typically

contain 3-4.5 wt % of C as well as other alloying elements, such as silicon (~3 wt.%)which controls kinetics of carbide formation

There are four general types of cast irons:

1. White iron has a characteristics white, crystalline fracture surface. Large amount ofFe3C are formed during casting, giving hard brittle material

2. Gray iron has a gray fracture surface with finely faced structure. A large Si content (2-3 wt %) promotes C flakes precipitation rather than carbide

3. Ductile iron: small addition (0.05 wt.%) of Mg to gray iron changes the flake Cmicrostructure to spheroidal that increases (by factor ~20) steel ductility

4. Malleable iron : traditional form of cast iron with reasonable ductility. First cast towhite iron and then heat-treated to produce nodular graphite precipitates.

White and Malleable cast iron

The low-sil icon cast irons (<1.0wt.%), produced under rapid cooling conditions

• Microstructure: most of cementite

• Properties: extremely hard very but brittle

• White iron is an intermediate for the production of malleable iron

In white cast iron light Fe3C regions surrounded by pearlite In malleable cast iron dark graphite rosettes in α-Fe matrix

Gray and Ducti le Cast I rons

• The gray irons contain 1-31.0 wt.% of Si• Microstructure: flake – shape graphite in ferrite matrix

• Properties : relatively weak and brittle in tension BUT very effective in damping

vibrational energy an high resistive to wear!!

Ductile (or Nodular) i ron : small addition of Mg or/and Ce to the gray iron composition

before casting• Microstructure: Nodular or spherical-like graphite structure in pearlite or ferric matrix

• Properties: Significant increase in material ductility• Applications: valves pump bodies, gears and other auto and machine components.

8/17/2019 33345



34

Micro Structures of steelCementite

Cementite is also known as iron carbide which has a chemical formula, Fe3C. It contains6.67 % Carbon by weight. It is a typical hard and brittle interstitial compound of low

tensile strength (approximately 5,000 psi) but high compressive strength. Its crystal

structure is orthorhombic

Austenite

It is also known as (γ ) gamma-iron, which is an interstitial solid solution of carbondissolved in iron with a face centered cubic crystal (F.C.C) structure. Average properties

of austenite are: High toughness.

Austenite is normally unstable at room temperature. Under certain conditions it is

possible to obtain austenite at room temperature.

Ferrite

It is also known as (α ) alpha -iron, which is an interstitial solid solution of a smallamount of carbon dissolved in iron with a Body Centered Cubic (B.C.C.) crystal

structure. It is the softest structure on the iron-iron carbide diagram. Average propertiesare: Low toughness and low hardness

Pearlite:

It is the eutectoid mixture containing 0.83 % Carbon and is formed at 1333oF on very

slow cooling. It is very fine platelike or lamellar mixture of ferrite and cementite. Thestructure of pearlite includes a white matrix (ferritic background) which includes thin plates of cementite.

Alloying Elements of Steel

Plain carbon steels contain traces of certain elements (Si, Mn, S, P), which unavoidablyentered the steel during the iron and steel making processes. However, these are not

considered alloying elements, unless their content exceeds the amount corresponding tothe production process. Steel is considered alloyed when in addition to the basic

components ( Fe & C), other alloying y p ), y g elements are added intentionally to assure

certain properties, that cannot be assured without alloying.

The most important and most frequently applied alloying elements of steel are manganese

(Mn), nickel (Ni), chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo),titanium (Ti), niobium (Nb) and boron (B).

8/17/2019 33345



35

The main aim of alloying steels is to:

Improve mechanical properties (e.g. strength, ductility, toughness)

Increase resistance to corrosion (chemical resistance) Improve certain physical properties (e.g. magnetic, electrical properties)

Improve complex properties of technological workability (e.g. formability,

weldability, machinability)The objectives can be achieved in various ways depending on the relationship of the

alloying elements to the base material (Fe) and to the fundamental alloying element ofsteels, that is carbon (C ).

8/17/2019 33345



36

Effect of Alloying elements

8/17/2019 33345



37

8/17/2019 33345



38

UNIT – II HEAT TREATMENT 11

Definition – Full annealing, stress relief, recrystallisation and spheroidizing – normalising,hardening and Tempering of steel. Isothermal transformation diagrams – cooling curves

superimposed on I.T. diagram CCR - Hardenability, Jominy end quench test –

Austempering, martempering – case hardening, carburising, nitriding, cyaniding,carbonitriding – Flame and Induction hardening.

Heat Treatment

A combination of heating and cooling operations timed and applied to a metal or alloy inthe solid state in a way that will produce desired properties.

All basic heat treatment process for steel involve the transformation or decomposition of

austenite. In the heat treatment of steel is to heat the material to some temperature in orabove the critical range in order to form the austenite

I. HEAT TREATMENT PROCESSES

A. The process of heating a metal workpiece to a high temperature to changeit's

properties, normally to make the workpiece harder or softer.

B. Basic heat treatment steps and equipment:

1. Heating to the correct temperature.

a. Equipment normally a heat-treating furnace, a blowtorch, gas welding torch, or a forge.

2. Holding or soaking at this temperature for a certain length of time.

a. Equipment can be the same furnace or forge.

3. Cooling in a way that will produce the desired results.

a. Equipment can be container of water, tempering oil or brine.

C. Heat treatment processes and uses.

1. Hardening

a. The process:

(1). Heating and cooling steel to increase its hardness and tensile strength, to reduce its

ductility.

(2). Requires a steel with a minimum of 0.20% carbon content.

b. Uses:

(1). To produce sharp-edged cutting tools.

(2). To make bearing surfaces wear better/longer.

8/17/2019 33345



39

(3). To put "spring" in a spring.

2. Tempering

a. The process

(1). Process of reducing the degree of hardness (removing brittleness) and strength and

increasing toughness.

(2). If a part is too hard, it will chip; if it is not hard enough, it will bend. Tempering

achieves the balance between hardness and strength.

b. Uses:

(1). Knife blades, screw driver tip, cold chisel tip, axes, shears.

3. Annealing

a. The process:

(1). Process of softening metal to relieve internal strain and to make the metal easier to

shape and cut.

b. Uses:

(1). Reusing old springs or files for other projects not requiring hardness.

(2). To relieve built up stresses and prevent cracking of manufactured metals.

II. HOT METAL FORMING PROCESSES

A. Techniques to give objects shape or form without adding or removing any

materials from the part.

B. Metal forming operations.

1. Bending.

a. The process:

(1). Process by which metal is uniformly strained or stretched around a straight axis

and results in a product having a linear or straight shape.

b. Uses:

(1). Decorative grillwork.

2. Forging (blacksmithing).

a. The process:

(1). Forming is achieved by hammering or applying steady pressure to a workpiece,

forcing it to take the shape of a die. May be done with the workpiece either hot or

cold.

8/17/2019 33345



40

b. Uses:

(1). Making small tools (chisels and center punches), horseshoes, ornamental ironwork.

I. CLASSIFICATION AND USES OF METALS

A. Ferrous metal types ("ferrous" = containing iron and alloys)

1.Iron.

a. Rare in the pure state; pure iron is not used commercially.

2. Wrought iron.

a. Contains:

(1). Iron, alloyed (combined) with,

(2). Less than 0.03% carbon.

b. True wrought iron is scarce and expensive.

c. True wrought iron forges well, can be easily bent hot or cold and can be welded.

d. "Wrought iron" is currently used to refer to almost any malleable low carbon steel.

3. Carbon steels, or "steel".

a. Contains:

(1). Iron, alloyed (combined) with,

(2). Carbon,

(3). Less than 1.65% manganese,

(4). Less than 0.60% copper, and

(5). Smaller amounts of silicon, sulfur and phosphorous.

b. Types:

(1). Low-carbon ("mild") steels

(a). Between 0.05% and 0.30% carbon.

(b). Tough and ductile. Easily formed, machined and welded.

(c). Most commonly used of the carbon steel types.

(2). Medium-carbon steels


(b). Strong and hard, but less ductile.

(c). Not as easily welded, due to tendency to crack after welding.

(d). Used for gears.

8/17/2019 33345



41

(3). High-carbon steels


(b). Very hard and strong, less ductile.

(c). Special electrodes and welding procedures are required, to prevent brittleness

and cracking.

(d). Used for cold chisels and hammers.

(4). Very-high-carbon steels


(b). Super hard and strong.

(c). Seldom welded; special electrodes and procedures used.

(d). Used for tools and springs.

(e). Can be used for items that must be hardened and tempered. (See paragraph IV.

below for definition of these terms).

4. Rolled steels.

a. Bar, rod and structural steels produced by rolling the steel into shape, much like an old

clothes wringer.

Cold rolled steel.

Steel formed when cold.

Results in more accurately sized, better surface finished product.

Hot rolled steel.

Metal formed into shape while the metal is red hot.

Produces a uniform quality, commonly used steel.

Bluish scale on the surface formed when water sprayed on the steel as it

passes between rollers.

5. Galvanized steel.

a. Mild steel coated with zinc to prevent rusting.

b. Care should be taken not to inhale toxic fumes when welding this

material.

II. PROPERTIES OF METALS

A. Tensile Strength.

8/17/2019 33345



42

1. Ability to resist being pulled apart in tension. Metal failures are often caused by forces

exceeding the tensile strength of the part.

B. Ductility.

1. Ability to be stretched or pulled through a die to form wire.

2. Copper is a very ductile metal.

C. Hardness.

1. Ability to resist penetration.

2. Hardness can be increased by heat treatment or work hardening.

III. IDENTIFICATION OF METALS

A. Numbering system for carbon and alloy steels.

1. Four digit (sometimes five digits) numbering system to identify carbon and alloy

steels:

a. First digit usually indicates the principle element in the steel as follows:

SERIES TYPES AND

DESIGNATION CLASSES

10XX Non-resulferized carbon steel grades (plain carbon steel)

13xx Manganese 1.75%

20xx Nickel steels

23xx Nickel 3.5%

30xx Nickel-chromium steels*

31xx Nickel 1.25% - chromium 0.65 or 0.80%

40xx Molybdenum 0.25%

41xx Chromium 0.50 – 0.95% - molybdenum 0.15 or 0.20%

43xx Nickel 1.80% - chromium 0.50 or 0.80% - molybdenum 0.25%*

50xx Chromium 0.28 or 0.40%

51xx Chromium 0.80, 0.90, 0.95, 1.00 or 1.05%

5xxxx Carbon 1.00% - chromium 0.50, 1.00 or 1.45%

60xx Chrome-vanadium steels

61xx Chromium 0.80 or 0.95% - vanadium 0.10 or 0.15% min.

70xx Heat resisting casting alloys

80xx Nickel – chrome – molybdenum steels*

8/17/2019 33345



43

86xx Nickel 0.55% - chromium 0.50 or 0.65% - molybdenum 0.20%

90xx Silicon – manganese steels

92xx Manganese 0.85% - silicon 2.00%

93xx Nickel 3.25% - chromium 1.20% - molybdenum 0.12%

*Stainless steels always have a high chromium content, often considerable amounts of

nickel, and sometimes contain molybdenum and other elements. Stainless steels are

identified by a three-digit number beginning with 2, 3, 4, or 5.

b. Second digit (in alloy steels) represents the approximate percentage of alloy

element.

c. Third and fourth digits show the carbon content in points (where a point

equals 100 times the percentage carbon).

d. Examples:

(1). 1095 steel is a carbon steel with 0.95% (95 points) carbon.

(2). 2511 steel is nickel steel with approximately 5% nickel and 0.11% carbon.

Various heat treatment process are

1. Annealing

a. Process annealing

b. Full annealing

c. Stress relief annealing

d.

Spheroidize annealing

2. Normalizing

3. Hardening

4. Tempering

a. Austempering

b. Martempering

8/17/2019 33345



44

Annealing Processes

Annealing is a heat treatment where the material is taken to a high temperature, kept therefor some time and then cooled. High temperatures allow diffusion processes to occur

fast. The time at the high temperature (soaking time) is long enough to allow the desiredtransformation to occur. Cooling is done slowly to avoid the distortion (warping) of the

metal piece, or even cracking, caused by stresses induced by differential contraction dueto thermal inhomogeneities. Benefits of annealing are:

• relieve stresses • increase softness, ductility and toughness

• produce a specific microstructure

8/17/2019 33345



45

Annealing techniques

Process anneali ng – applied to

cold worked materials to negate

effects of cold work. Commonlysandwiched between two coldwork operations. Improves

ductility. This is used to treatworked metals, such as two

pieces of metal that have beenwelded together. This makes it

possible for the metal to undergofurther work without fracturing.

The metal is heated to just belowthe A1 temperature line (see blue

arrow); it is held there longenough for the metal to change

the size and distribution of itsgrain structure and then cooled

naturally in air. This process ischeaper than Full Annealing or

Normalizing because the metal isnot heated to high temperatures

Stress reli ef – purpose of it is to remove stresses. Temperatures are low such that cold

work effects are not affected. Reduces the residual stresses in large castings and welded parts. These stresses are caused by thermal cycling or work hardening. The metal is

heated to 600 degrees Celsius (see green line), held at that temperature level for over anhour and then cooled in air.

Fu ll annealing – used for products that are to be machined later-on. Cooling is done in

furnace itself. Hardness and strength are restored by additional heat treatments aftermachining. Is the process of slowly raising the temperature above the Austenitic.

Temperature line A3 (see the red arrow). Austenitic steel is the most ductile of the steelsand has a very high relative strength. It is held at this temperature until all the material

transforms and then slowly cools in a furnace to about 50 degrees Celsius when it can bethen cooled through convection in the room

Normalizing – used to refine the grains and produce a more uniform and desirable size

distribution. It involves heating the component to attain single phase (e.g.: austenite insteels), then cooling in open air atmosphere.Is the same as the Full Anneal, the metal is

heated above the A3 temperature line (see red arrow). However, the metal is cooled rightaway through room convection, rather than through a furnace. This makes normalizing

cheaper since a furnace is not used to cool the metal in a controlled environment. Both ofthese processes make the metal more soft, which makes it more machineable, the

8/17/2019 33345



46

difference is that the Full annealed metal is uniformly soft while the Normalized metalvaries in its softness.

Tempering: This is used to remove brittleness found in quench hardened parts. It is done

to improve hardness, ductility, toughness and strength. It must be done immediately after

a metal has been quenched and then cooled to 40 degrees Celsius. It is then reheated to between 150-400 degrees Celsius. This heating is usually done in an oil bath; this ensuresthat every part of the metal will undergo the same temperature and tempering.

Quenching operation is usually fol lowed by temperi ng.

Tempering involves heating martensitic steel at a temperature below the eutectoid

transformation temperature to make it softer and more ductile. Here Martensitetransforms to ferrite embedded with carbide particles.

Martempering is used to minimize distortion and cracking. It is the interrupted cooling

procedure used for steels to minimize the stresses, distortions and cracking of steels thatmay develop during rapid quenching. It involves cooling the austenized steel to

temperature just above Ms temperature, holding it there until temperature is uniform,followed by cooling at a moderate rate to room temperature before austenite-to-bainite

transformation begins. The final structure of martempered steel is tempered Martensite.

Austempering involves austenite-to-bainite transformation. Thus, the final structure ofaustempered steel is bainite. Austempering is an isothermal heat treatment that, when

applied to ferrous materials, produces a structure that is stronger and tougher thancomparable structures produced with conventional heat treatments. Conventional heat

treaters heat the parts to "red heat" in a controlled atmosphere and then quench them in a bath of oil or water that is near room temperature. (Maybe even as high as a few hundred

degrees Fahrenheit). This produces a crystalline structure known as Martensite, a hard, brittle phase. The parts are then tempered in another furnace at 350°F (177c) to 1100°F

(593c) to decrease the "brittleness."

Austempering starts the same way. The parts areheated to red heat in a controlled atmosphere (so

they don't scale) but then are quenched in a bathof molten salt at 450°F (232c) to 750°F (399c).The quench temperature is above the Martensite

starting temperature. Therefore, a differentstructure (not Martensite) results. In

Austempered Ductile Iron and AustemperedGray Iron the structure is Ausferrite, and in steel,

it is Bainite.

8/17/2019 33345



47

Transformation rate effects and TTT diagrams, Microstructure and Property

Changes in Fe-C Alloys

Solid state transformations, which are very important in steels, are known to be

dependent on time at a particular temperature, as shown in figure-14(b). Isothermal

transformation diagram, also known as TTT diagram, measures the rate of transformationat a constant temperature i.e. it shows time relationships for the phases during isothermaltransformation. Information regarding the time to start the transformation and the time

required to complete the transformation can be obtained from set of TTT diagrams. Onesuch set of diagram for reaction of austenite to pearlite in steel is shown in figure-17 . The

diagram is not complete in the sense that the transformations of austenite that occur attemperatures below about 550

C are not shown.

As mentioned in previous section, thickness of layers in pearlite depends on thetemperature at which the transformation occurred. If the transformation took place at a

temperature that is just below the eutectoid temperature, relatively thick layers of α –

ferrite and cementite are produced in what is called coarse pearlite. This is because ofhigh diffusion rates of carbon atoms. Thus with decreasing transformation temperature,sluggish movement of carbon results in thinner layers α – ferrite and cementite i.e. fine

pearlite is produced.

At transformation temperatures below 550 C, austenite results in different product

known as bainite. Bainite also consists of α – ferrite and cementite phases i.e.

transformation is again diffusion controlled but morphologically it consists of very small

8/17/2019 33345



48

particles of cementite within or between fine ferrite plates. Bainite forms needles or plates, depending on the temperature of the transformation; the microstructural details of

bainite are so fine that their resolution is only possible using electron microscope.

It differs from pearlite in the sense that different mechanism is involved in formation ob

bainite which does not have alternating layers of α – ferrite and cementite. In addition, because of equal growth rates in all directions pearlite tends to form spherical colonies,whereas bainite grows as plates and has a characteristic acicular (needlelike) appearance.

Upper bainite, formed at the upper end of the temperature range (550 C-350

C), ischaracterized by relatively coarse, irregular shaped cementite particles in α – ferrite plates.

If the transformation is taking place at lower temperatures (350 C-250

C), the α –

ferrite plates assume a more regular needlelike shape, and the transformation product iscalled lower bainite. At the same time carbide particles become smaller in size and

appear as cross-striations making an angle of about 55 to the axis of the α – ferrite plate.

Upper bainite has large rod-like cementite regions, whereas lower bainite has much finer

cementite particles as a result of sluggish diffusion of carbon atoms at lowertemperatures.

Lower bainite is considerably harder than upper bainite. Another characteristic of bainite

is that as it has crystallographic orientation that is similar to that found in simple ferritenucleating from austenite, it is believed that bainite is nucleated by the formation of

ferrite. This is in contrast to pearlite which is believed to be nucleated by formation ofcementite.

Basically, bainite is a transformation product that is not as close to equilibrium as

pearlite. The most puzzling feature of the bainite reaction is its dual nature. In a numberof respects, it reveals properties that are typical of a nucleation and growth type of

transformation such as occurs in the formation pearlite and also a mixture of α – ferrite andcementite though of quite different morphology (no alternate layers), but at the same time

it differs from the Martensite as bainite formation is athermal and diffusion controlledthough its microstructure is characterized by acicular (needlelike) appearance.

The time-temperature dependence of the bainite transformation can also be presented

using TTT diagram. It occurs at temperatures below those at which pearlite forms i.e. itdoes not form until the transformation temperature falls below a definite temperature,

designated as Bs. Above this temperature austenite does not form bainite except underexternal stresses. Below Bs, austenite does not transform completely to bainite. The

amount of bainite formed increases as the isothermal reaction temperature is lowered. Byreaching a lower limiting temperature, Bf , it is possible to transform austenite completely

to bainite. The Bs and Bf temperatures are equivalent to the Ms and Mf temperatures forMartensite.

8/17/2019 33345



49

In simple eutectoid steels, pearlite and bainite transformations overlap, thus transition

from the pearlite to bainite is smooth and continuous i.e. knees of individual pearlite and bainite curves are merged together. However each of the transformations has a

characteristic C-curve, which can be distinguishable in presence of alloying elements.

As shown in complete TTT diagram for eutectoid steel in figure-18, above approximately550

C-600 C, austenite transforms completely to pearlite. Below this range up to

450 C, both pearlite and bainite are formed. Finally, between 450 C and 210 the

reaction product is bainite only. Thus bainite transformation is favored at a high degree of

supercooling, and the pearlite transformation at a low degree of supercooling. In middleregion, pearlitic and bainitic transformations are competitive with each other.

As explained in earlier section, martensitic transformation can dominate the proceedings

if steel is cooled rapid enough so that diffusion of carbon can be arrested. Transformationof austenite to Martensite is diffusion-less, time independent and the extent of

transformation depends on the transformation temperature. Martensite is a meta-stable phase and decomposes into ferrite and pearlite but this is extremely slow (and not

noticeable) at room temperature. Alloying additions retard the formation rate of pearliteand bainite, thus rendering the martensitic transformation more competitive. Start of the

transformation is designated by Ms, while the completion is designated by Mf in atransformation diagram. Martensite forms in steels possesses a body centered tetragonal

crystal structure with carbon atoms occupying one of the three interstitial sites available.This is the reason for characteristic structure of steel Martensite instead of general BCC.

Tetragonal distortion caused by carbon atoms increases with increasing carbon contentand so is the hardness of Martensite. Austenite is slightly denser than Martensite, and

therefore, during the phase transformation upon quenching, there is a net volumeincrease. If relatively large pieces are rapidly quenched, they may crack as a result of

internal stresses, especially when carbon content is more than about 0.5%.

Mechanically, Martensite is extremely hard, thus its applicability is limited by brittlenessassociated with it. Characteristics of steel Martensite render it unusable for structural

applications in the as-quenched form. However, structure and thus the properties can bealtered by tempering , heat treatment observed below eutectoid temperature to permit

diffusion of carbon atoms for a reasonable period of time.

During tempering, carbide particles attain spherical shape and are distributed in ferrite phase – structure called s pheroidite. Spheroidite is the softest yet toughest structure that

steel may have. At lower tempering temperature, a structure called tempered Martensiteforms with similar microstructure as that of spheroidite except that cementite particles are

much, much smaller.

The tempering heat treatment is also applicable to pearlitic and bainitic structures. Thismainly results in improved machinability. The mechanism of tempering appears to be

first the precipitation of fine particles of hexagonal ε-carbide of composition aboutFe2.4C from Martensite, decreasing its tetragonality. At higher temperatures or with

8/17/2019 33345



50

increasing tempering times, precipitation of cementite begins and is accompanied bydissolution of the unstable ε-carbide. Eventually the Martensite loses its tetragonality and

becomes BCC ferrite, the cementite coalesces into spheres. A schematic of possibletransformations involving austenite decomposition are shown in figure-19.

Tempering of some steels may result in a reduction of toughness what is known as

temper embrittlement . This may be avoided by (1) compositional control, and/or (2)tempering above 575 or below 375, followed by quenching to room temperature. Theeffect is greatest in Martensite structures, less severe in bainitic structures and least

severe in pearlite structures. It appears to be associated with the segregation of soluteatoms to the grain boundaries lowering the boundary strength. Impurities responsible for

temper brittleness are: P, Sn, Sb and As. Si reduces the risk of embrittlement by carbideformation. Mo has a stabilizing effect on carbides and is also used to minimize the risk of

temper brittleness in low alloy steels.

TTT diagrams are less of practical importance since an alloy has to be cooled rapidly andthen kept at a temperature to allow for respective transformation to take place. However,

most industrial heat treatments involve continuous cooling of a specimen to roomtemperature. Hence, Continuous Cooling Transformation (CCT) diagrams are generally

more appropriate for engineering applications as components are cooled (air cooled,furnace cooled, quenched etc.) from a processing temperature as this is more economic

than transferring to a separate furnace for an isothermal treatment. CCT diagramsmeasure the extent of transformation as a function of time for a continuously decreasing

temperature.

For continuous cooling, the time required for a reaction to begin and end is delayed, thusthe isothermal curves are shifted to longer times and lower temperatures. Both TTT and

CCT diagrams are, in a sense, phase diagrams with added parameter in form of time.Each is experimentally determined for an alloy of specified composition. These diagrams

allow prediction of the microstructure after some time period for constant temperatureand continuous cooling heat treatments, respectively. Normally, bainite will not form

during continuous cooling because all the austenite will have transformed to pearlite bythe time the bainite transformation has become possible. Thus, as shown in figure-20,

region representing austenite-pearlite transformation terminates just below the nose.

8/17/2019 33345



51

Hardenability of steels

Jomniy End Quench TestFull annealing and Spheroidizing: to produce softer steel for good machining and forming

Normalization: to produce more uniform fine structure that tougher than coarse-grainedone Quenching: to produce harder alloy by forming martensitic structure

Note: cooling rates before reaching Austenite – Martensite transformation are in therange 1 -50 C/s. Measuring cooling rates at every point (e.g. by thermocouples) and

finding rates correlations with the hardness one may develop quenching rate – hardnessdiagram

8/17/2019 33345



52

Instead, a Jominy Test is used to compare hardenabilities of steel. This is a standard testadopted by ASTM (American Society for Testing & Materials) and by SAE (Society for

Automotive Engineers). uto ot e g ee s). A steel bar 100mm long and 25mm in diameteris austenitised, placed into a fixture and sprayed with one end with a jet of water of

specified flow rate and temperature. After the test, Rockwell hardness measurements are

made along the test specimen every 1/16 inch from the quenched end for the first inch.The distance(s) between hardness tests for the remaining length of the bar is at thediscretion of the tester.

A plot of hardness vs. distance from quenched end is plotted to produce a hardenability

curve. This method of quenching results in different rate of cooling along the length ofthe test piece. The quenched end cools the most rapidly and exhibits the maximum

hardness, since 100% martensite is formed. The cooling rate at the opposite end can beconsidered to be air cooled, a very slow cooling rate, producing a structure which is less

hard. The distance from the quenched end is the Jominy distance and is related to thecooling rate. We need to know the hardness at different cooling rates, rather than the

hardness at different distances.Each steel alloy has its own unique hardenability curve.This standard quench process produces a common cooling rate gradient along the Jominy

bar, for all carbon and low alloy steels. Thermal properties are nearly identical for theseiron carbon alloys

8/17/2019 33345



53

Precipitation Hardening

Enhancing the strength and hardness of the alloy by the formation (as a result of specific

heat-treatment) of the fine uniformly dispersed particles of the second phase, whichimpede dislocations.

8/17/2019 33345



54

Two-stage heat-treatment:(1) Solution formation: relatively high T to provide single-phase solution formation +

quenching to form a non-equilibrium supersaturated solution;(2) Precipitation: relatively low T, within two-phase zone, to form second phase particles

Effect of Alloying Elements

Hardenability - Alloy steels have high hardenability.

Effect on the Phase Stability - When alloying elements are added to steel, the

binary Fe-Fe3C stability is affected and the phase diagram is altered.

Shape of the TTT Diagram - Ausforming is a thermomechanical heat treatment in

which austenite is plastically deformed below the A1 temperature, then permitted

to transform to bainite or martensite.

Tempering - Alloying elements reduce the rate of tempering compared with that

of a plain-carbon steel.

Surface Treatments

Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon

steel above the A3 temperature and then quench the steel.

Case depth - The depth below the surface of a steel at which hardening occurs by

surface hardening and carburizing processes.

Carburizing - A group of surface-hardening techniques by which carbon diffuses

into steel.

Cyaniding - Hardening the surface of steel with carbon and nitrogen obtained

from a bath of liquid cyanide solution.

Carbonitriding - Hardening the surface of steel with carbon and nitrogen obtained

from a special gas atmosphere.

Case Hardening or Surface HardeningMany materials applications require good wear resistance, and resistance to high contact

stresses at the surface, while at the same time providing good toughness at the core of thematerial. These requirements need hi h hardness and strength at the surface while keeping

the ore requirements high strength surface, while keeping core or inner section of thematerial soft and having low strength. This gradient in properties through the section of

8/17/2019 33345



55

the part or component can be achieved through

surface hardening in whicha hard martensitic structure

is produced at the surface,

while non martensiticstructures, pearlite or bainite, are at the inner

section or core of the part.Surface hardening is

usually specified to acertain depth from the

surface resulting in a shellor case of hard material. Thus, it is also commonly referred to as case hardening. It may

be achieved with or without change of the surface composition.

Purpose of Case or Surface hardening

• Increase wear resistance• Increase surface strenght for load carrying (crush resistance)

• Induce suitable residual and compressive stresses • Improve fatigue life

• Impact resistance

Carburising

Involves changing the carbon content of the surface, followed by a quenching process to

convert the surface layers to martensite. Carbon content in the surface layers increasesfrom less than 0.2% to 0.7 to 0.8%. Carburising can be done when the source of carbon is

either in the:• solid state – Pack carburising

8/17/2019 33345



56

• liquid state – Liquid carburising or salt bath carburising or cyaniding• gaseous state – Gas carburising

A case-hardening process in which carbon is dissolved in the surface layers of a low-

carbon steel part at a temperature sufficient to render the steel austenitic, followed by

quenching and tempering to form a martensitic microstructure;2CO ↔ C(in Fe) +CO2 CO + H2 ↔C(in Fe) + H2O

Methods: gas carburizing, vacuum carburizing, plasma (ion) carburizing, salt-bathcarburizing, and pack carburizing; Majority of carburized parts processed by gas

carburizing, using natural gas, propane, or butane.

Gas CarburisingCarbonsource: gaseous hydrocarbons (methane (CH4), propane (C3H3), butane

(C4H10)), vaporized hydro-carbon liquids;

Pack carburizing:

Carbon monoxide derived from a solid compound decomposes at the metal surface intonascent carbon and carbon dioxide; nascent carbon is absorbed into the metal, and the

carbon dioxide immediately reacts with carbonaceous material present in the solidcarburizing compound to produce

fresh carbon monoxide;

Carburizing compounds: contain 10-20% alkali or alkaline earth metal

carbonates bound to hardwoodcharcoal or to coke by oil, tar, or

molasses; Barium carbonate(BaCO3) is the principal energizer

comprising about 50 to 70% of thetotal carbonate content (CaCO3&

Na2CO3);Process control: carbon potential,

temperature (815-955oC), time, steelcomposition;

The steel component is heated to above the A3 temperature while in a sealed metal box

which contains the carburising medium, e.g. a carbon‐ rich Liquid carburising or salt bath carburizing material such as charcoal and an energiser such as barium carbonate.

The oxygen present in the box reacts with the carbon to produce carbon monoxide. Thiscarbon rich atmosphere in contact with the hot steel results in carbon diffusing into the

surface austenite layers.

Liquid Carburising

8/17/2019 33345



57

Parts are held in a molten salt that will introduce carbon and nitrogen, or carbon alone,into the metal. Diffusion of the carbon from the surface toward the interior produces a

case that can be hardened, usually by fast quenching from the bath;

This involves heating the component in a bath of suitable carbon rich salts. Sodium

cyanide is mainly used. The carbon from the molten salt diffuses into the component.This method produces relatively thin, hardened layers with high carbon content. Thisoccurs because carburising takes place very quickly. Problems with this method include

the dangers that the poisonous cyanide may pose, and the removal of salt from thehardened component after treatment, which can be particularly difficult with threaded

parts and blind holes.

Nitriding:

Nitrogen diffused into surface of special alloy steels (aluminum or chromium) Nitride compounds precipitate out

– Gas nitriding - heat in ammonia – Liquid nitriding - dip in molten cyanide bath

Case thicknesses between 0.001 and 0.020 in. with hardness up to HRC 70.

Involves changing the surface composition of a steel by diffusing nitrogen into it to produce hard nitride compounds. Prior to the nitriding treatment, the steel is hardened &

tempered to the properties required by the core, provided that the tempering temperatureis higher than the temperature of the nitriding process. Otherwise, the nitriding process

would temper the steel & change the properties of the core. Unlike carburising, nitridingis carried out at temperatures below the stable austenitic state. The process consists of

heating a component in an atmosphere of ammonia gas & hydrogen (temperatures ≈ 500to 530°C).

The time taken for the nitrogen to react with the elements in the surface of the steel is

often as much as 100 hours. The depth to which the nitrides are formed in the steeldepends on the temperature & the time allowed for the reaction. The depth of hardening

is unlikely to exceed about 0.7mm. After the treatment, the component is allowed to coolslowly in the ammoniahydrogen atmosphere. Since with nitriding, no quenching

treatments are involved, cracking and distortion are less likely than with other surfacehardening treatments.

Nitrogen diffused into the surface of a ferrous product to produce a hard case →high

surface hardness, improved wear resistance, increased fatigue resistance, and improvedcorrosion resistance;

Performed between 500 and 550oC, quenching is not required →minimum distortion andexcellent control;

Purposes: to obtain high surface hardness; to increase wear resistance and antigalling properties; to improve fatigue life; to improve corrosion resistance (except for stainless

steels); to obtain a surface that is resistant to the softening effect of heat at temperaturesup to the nitriding temperature;

8/17/2019 33345



58

Processes: gas nitriding, liquid nitriding, plasma (ion) nitriding;Structure: diffusion zone (original core microstructure with nitride precipitates and

nitrogen solid solution) with or without a compound zone (Fe4 N and Fe2-3Nintermetallics).

Factors affecting the microhardness profile of a nitrided steel.The hardness of thecompound zone is unaffected by alloy content, while the hardness of the diffusion zone isdetermined by nitride-forming elements (Al, Cr, Mo, Ti, V, Mn).

Applications

Aluminum-containing low- alloy steels: Nitralloys;Medium-carbon, chromium-containing low-alloy steels: 4100, 4300, 5100, 6100, 8600,

8700, and 9800 series;Low-carbon, chromium-containing low-alloy steels: 3300, 8600, and 9300 series;

Hot-working die steels containing 5% Cr: H11, H12, and H13;Air-hardenabletool steels: A2, A6, D2, D3, and S7;

High-speed tool steels: M2 and M4; Nitronic stainless steels: 30, 40, 50, and 60;

Ferritic and martensitic stainless steels: 400 series;Austenitic stainless steels: 200 and 300 series;

Gas nitriding:

Nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at asuitable temperature (495 to 525oC) in contact with a nitrogenous gas, usually ammonia;

Single-stage process: →brittle nitrogen-rich compound zone known as the white nitride

layer at the surface of the nitrided case;

Double-stage process (Floe process):reducing the white nitrided layer thickness; thesecond stage is added which either by continuing at the first-stage temperature or

increasing the temperature to 550 to 565oC →higher -temperature second stage lowers the

case hardness and increases the case depth.

FLAME HARDENING

This involves heating the surface of a steel with an oxyacetylene flame (transforming the

structure of the surface layers to austenite), and then immediately quenching the surfacewith cold water (changing the austenite to martensite). The depth of hardening depends

on the heat supplied per unit surface area per unit time. Thus, the faster the burner ismoved over the surface, the less the depth of hardening. The temperatures used in this

method are typically of the order of 850°C or more, i.e. above the A temperature

8/17/2019 33345



59

INDUCTION HARDENING

This method involves placing the steel component within a coil, through which a highfrequency current is passed. The form of the induction coil depends on the shape of the

component being hardened hardened. The alternating current induces another alternatingcurrent to flow within the surface layers of the steel component, the induced electrical

currents heating the surface layers. The temperatures so produced cause the surface layersto change to austenite. When the surface has reached the austenitizing temperature, the

surface is sprayed with cold water to transform the austenite to martensite. The depth of

heating produced by this method, and hence the depth of hardening, is related to thefrequency of the alternating current used. The higher the frequency, the less the hardeneddepth

8/17/2019 33345



60

ProcessMetals

hardened

Element

added to

surface

Procedure General

Characteristics

Typical

applications

Carburizi

ng

Low-

carbon

steel

(0.2%

C), alloy

steels

(0.08 –

0.2% C)

C Heat steel at 870 –

950 °C (1600 – 1750

°F) in an atmosphere

of carbonaceous

gases (gas

carburizing) or

carbon-containing

solids

(pack carburizing).

Then quench.

A hard, high-

carbon surface

is produced.

Hardness 55 to

65 HRC. Case

depth < 0.5 – 1.5

mm ( < 0.020 to

0.060 in.). Some

distortion of

part during heat

treatment.

Gears, cams,

shafts,

bearings,

piston pins,

sprockets,

clutch plates

Carbonitr

iding

Low-

carbon

steel

C and N Heat steel at 700 –

800 °C (1300 – 1600


of carbonaceous gas

and ammonia. Thenquench in oil.

Surface

hardness 55 to

62 HRC. Case

depth 0.07 to

0.5 mm (0.003to 0.020 in.).

Less distortion

than in

carburizing.

Bolts, nuts,

gears

Cyanidin

g

Low-

carbon

steel

(0.2%

C), alloy

steels

(0.08 –

0.2% C)

C and N Heat steel at 760 –

845 °C (1400 – 1550

°F) in a molten bath

of solutions of

cyanide (e.g., 30%

sodium cyanide) and

other salts.

Surface

hardness up to

65 HRC. Case

depth 0.025 to

0.25 mm (0.001

to 0.010 in.).

Some distortion.

Bolts, nuts,

screws,

small gears

8/17/2019 33345



61

Nitriding Steels

(1% Al,

1.5% Cr,

0.3%

Mo),

alloy

steels

(Cr, Mo),

stainless

steels,

high-

speed

tool

steels

N Heat steel at 500 –

600 °C (925 – 1100


of ammonia gas or

mixtures of molten

cyanide salts. No

further treatment.

Surface

hardness up to

1100 HV. Case

depth 0.1 to 0.6

mm (0.005 to

0.030 in.) and

0.02 to 0.07 mm

(0.001

to 0.003 in.) for

high speed steel.

Gears,

shafts,

sprockets,

valves,

cutters,

boring bars,

fuel-

injection

pump parts

Boronizi

ng

Steels B Part is heated using

boron-containing

gas or solid in

contact with part.

Extremely hard

and wear

resistant

surface. Case

depth 0.025 – 0.075 mm

(0.001 –

0.003 in.).

Tool and die

steels

Flame

hardenin

g

Medium-

carbon

steels,

cast irons

None Surface is heated

with an

oxyacetylene torch,

then quenched with

water spray or other

quenching methods.

Surface

hardness 50 to

60 HRC. Case

depth 0.7 to 6

mm (0.030 to

0.25 in.). Little

distortion.

Gear and

sprocket

teeth, axles,

crankshafts,

piston rods,

lathe beds

and centers

Induction

hardenin

Same as

above

None Metal part is placed

in copper induction

Same as above Same as

above

8/17/2019 33345



62

g coils and is heated

by high frequency

current, then

quenched.

8/17/2019 33345



63

UNIT III MECHANICAL PROPERTIES AND TESTING

Mechanism of plastic deformation, slip and twinning – Types of fracture – Testing of

materials under tension, compression and shear loads – Hardness tests (Brinell, Vickers

and Rockwell), Impact test - Izod and Charpy, Fatigue and creep tests, fracture toughnesstests.

Mechanism of Plastic Deformation

Permanent plastic deformation is due to shear process – atoms change their neighbors.

Inter- atomic forces and crystal structure plays an important role during plasticdeformation.

Cumulative movement of dislocations leads to gross plastic deformation.

Edge dislocation move by slip and climb, while screw dislocation move by slip andcross-slip.

During the movement, dislocations tend to interact. The interaction is very complex because of number of dislocations moving over many slip systems in different directions.

While some materials are elastic in nature up point of fracture, many engineeringmaterials like metals and thermo-plastic polymers can undergo substantial permanent

deformation. This characteristic property of materials makes it feasible to shape them.However, it imposes some limitations on the engineering usefulness of such materials.

Permanent deformation is due to process of shear where particles change their neighbors.During this process inter-atomic or inter-molecular forces and structure plays important

roles, although the former are much less significant than they are in elastic behavior.Permanent deformation is broadly two types – plastic deformation and viscous flow.

Plastic deformation involves the relative sliding of atomic planes in organized manner incrystalline solids, while the viscous flow involves the switching of neighbors with much

more freedom that does not exist in crystalline solids.

It is well known that dislocations can move under applied external stresses. Cumulative

movement of dislocations leads to the gross plastic deformation. At microscopic level,dislocation motion involves rupture and reformation of inter-atomic bonds. The necessity

of dislocation motion for ease of plastic deformation is well explained by the discrepancy between theoretical strength and real strength of solids, as explained in chapter-3. It has

been concluded that one-dimensional crystal defects – dislocations – plays an importantrole in plastic deformation of crystalline solids. Their importance in plastic deformation

is relevant to their characteristic nature of motion in specific directions (slip-directions)on specific planes (slip-planes), where edge dislocation move by slip and climb while

screw dislocation can be moved by slip and cross-slip.

8/17/2019 33345



64

The onset of plastic deformation involves start of motion of existing dislocations in real

crystal, while in perfect crystal it can be attributed to generation of dislocations andsubsequently their motion. During the motion, dislocations will tend to interact among

themselves. Dislocation interaction is very complex as number of dislocations moving on

number of slip planes in various directions. When they are in the same plane, they repeleach other if they have the same sign, and annihilate if they have opposite signs (leaving behind a perfect crystal). In general, when dislocations are close and their strain fields

add to a larger value, they repel, because being close increases the potential energy (ittakes energy to strain a region of the material). When unlike dislocations are on closely

spaced neighboring slip planes, complete annihilation cannot occur. In this situation, theycombine to form a row of vacancies or an interstitial atom.

An important consequence interaction of dislocations that are not on parallel planes isthat they intersect each other or inhibit each others motion. Intersection of two

dislocations results in a sharp break in the dislocation line. These breaks can be of twokinds:

(a) A jog is break in dislocation line moving it out of slip plane.

(b) A kink is break in dislocation line that remains in slip plane.

Other hindrances to dislocation motion include interstitial and substitutional atoms,foreign particles, grain boundaries, external grain surface, and change in structure due to

phase change. Important practical consequences of hindrance of dislocation motion arethat dislocations are still movable but at higher stresses (or forces), and in most instances

that leads to generation of more dislocations. Dislocations can spawn from existingdislocations, and from defects, grain boundaries and surface irregularities. Thus, the

number of dislocations increases dramatically during plastic deformation. As further

motion of dislocations requires increase of stress, material can be said to be strengthened

i.e. materials can be strengthened by controlling the motion of dislocation.

Mechanisms of plastic deformation in metals

Plastic deformation, as explained in earlier section, involves motion of dislocations.There are two prominent mechanisms of plastic deformation, namely slip and twinning .

Slip is the prominent mechanism of plastic deformation in metals. It involves sliding of blocks of crystal over one other along definite crystallographic planes, called slip planes.

In physical words it is analogous to a deck of cards when it is pushed from one end. Slipoccurs when shear stress applied exceeds a critical value. During slip each atom usuallymoves same integral number of atomic distances along the slip plane producing a step,

but the orientation of the crystal remains the same. Steps observable under microscope asstraight lines are called slip lines.

Slip occurs most readily in specific directions (slip directions) on certain crystallographic planes. This is due to limitations imposed by the fact that single crystal remains

homogeneous after deformation. Generally slip plane is the plane of greatest atomicdensity, and the slip direction is the close packed direction within the slip plane. It turns

8/17/2019 33345



65

out that the planes of the highest atomic density are the most widely spaced planes, whilethe close packed directions have the smallest translation distance. Feasible combination

of a slip plane together with a slip direction is considered as a slip system. The commonslip systems are given in table

In a single crystal, plastic deformation is accomplished by the process called slip, andsometimes by twinning. The extent of slip depends on many factors including external

load and the corresponding value of shear stress produced by it, the geometry of crystal

structure, and the orientation of active slip planes with the direction of shearing stressesgenerated. Schmid first recognized that single crystals at different orientations but ofsame material require different stresses to produce slip. The dependence of various

factors has been summarized using a parameter – critical resolved shear stress, τR, givenas

Slip in polycrystalline material involves generation, movement and (re-)arrangement ofdislocations. Because of dislocation motion on different planes in various directions, they

may interact as well. This interaction can cause dislocation immobile or mobile at higherstresses. During deformation, mechanical integrity and coherency are maintained along

the grain boundaries; that is, the grain boundaries are constrained, to some degree, in theshape it may assume by its neighboring grains. Once the yielding has occurred, continued

plastic deformation is possible only if enough slip systems are simultaneously operativeso as to accommodate grain shape changes while maintaining grain boundary integrity.

According to von Mises criterion, a minimum of five independent slip systems must beoperative for a polycrystalline solid to exhibit ductility and maintain grain boundary

integrity. This arises from the fact that an arbitrary deformation is specified by the sixcomponents of strain tensor, but because of requirement of constant volume, there are

only independent strain components. Crystals which do not possess five independent slipsystems are never ductile in polycrystalline form, although small plastic elongation may

be noticeable because of twinning or a favorable preferred orientation.

8/17/2019 33345



66

Twinning

plane (twin boundary) mirror the atoms on the other side.

Displacement magnitude in the twin region is proportional to the atom�s

distance from the twin plane

takes place along defined planes and directions depending upon the system.

o Ex: BCC twinning occurs on the (112)[111] system

The second important mechanism of plastic deformation is twinning. It results when a

portion of crystal takes up an orientation that is related to the orientation of the rest of theuntwined lattice in a definite, symmetrical way. The twinned portion of the crystal is a

mirror image of the parent crystal. The plane of symmetry is called twinning plane. Each

atom in the twinned region moves by a homogeneous shear a distance proportional to itsdistance from the twin plane. The lattice strains involved in twinning are small, usually inorder of fraction of inter-atomic distance, thus resulting in very small gross plastic

deformation. The important role of twinning in plastic deformation is that it causeschanges in plane orientation so that further slip can occur. If the surface is polished, the

twin would be still visible after etching because it possesses a different orientation fromthe untwined region. This is in contrast with slip, where slip lines can be removed by

polishing the specimen.

Properti es of Twinning

It occurs in metals with BCC or HCP crystal structure

occurs at low temperatures and high rates of shear loading (shock

loading) pccurs in conditions in which there are few present slip systems

(restricting the possibility of slip)

In which small amount of deformation when compared with slip.

8/17/2019 33345



67

Twinning also occurs in a definite direction on a specific plane for each crystal structure.However, it is not known if there exists resolved shear stress for twinning. Twinning

generally occurs when slip is restricted, because the stress necessary for twinning isusually higher than that for slip. Thus, some HCP metals with limited number of slip

systems may preferably twin. Also, BCC metals twin at low temperatures because slip isdifficult. Of course, twinning and slip may occur sequentially or even concurrently in

some cases. Twinning systems for some metals are given in table-

8/17/2019 33345



68

Fractures

Failure of a material component is the loss of ability to function normally or to performthe intended job

Three general ways failure: Excessive elastic deformation, E.g.:buckling. Controlled by

design and elastic modulus of the material. Excessive plastic deformation, Controlled byyield strength of the material. E.g.:loss of shape, creep and/ or stress-rupture at elevated

temperatures. Fracture, involves complete disruption of continuity of a component – understatic load: brittle or ductile, under fluctuating/cyclic load: fatigue, mode in which most

machine parts fail in service.

Fracture defined as the separation or fragmentation of a solid body into two or more partsunder the action of stress.

Fracture is classified based on several characteristic features:

8/17/2019 33345



69

Fracture ModesDuctile and Brittle are relative terms. Most of the fractures belong to one of the following

modes: (a) rupture, (b) cup-&-coneand (c) brittle.

Ductile fracture in tension occurs after appreciable plastic deformation. It is usually

preceded by necking. It exhibits three stages -(1) formation of cavities (2) growth ofcavities (3) final failure involving rapid crack propagation at about 45 to the tensile axis.

Fractography of ductile fracture reveals numerous spherical dimples separated by thinwalls on the fractured surface.

Stages of void nucleation, void growth, crack initiation and eventual fracture under

ductile fracture mode:

8/17/2019 33345



70

Brittle Fracture

Brittle fracture intakes place with little or no preceding plastic deformation. It occurs,often at unpredictable levels of stress, by rapid crack propagation. Crack propagates

nearly perpendicular to the direction of applied tensile stress, and hence called cleavagefracture. Most often brittle fracture occurs through grains i.e. transgranular. Three stages

of brittle fracture -(1) plastic deformation that causes dislocation pile-ups at obstacles, (2)micro-crack nucleation as a result of build-up of shear stresses, (3) eventual crack

propagation under applied stress aided by stored elastic energy.

Brittle farcture - Griffth Theory

Creep Fracture

Deformation that occurs under

constant load/stress and elevatedtemperatures which is time-dependent

8/17/2019 33345



71

is known as creep. Creep deformation (constant stress) is possible at all temperaturesabove absolute zero. However, it is extremely sensitive to temperature. Hence, creep in

usually considered important at elevated temperatures (temperatures greater than 0.4 Tm,Tm is absolute melting temperature). Creep test data is presented as a plot between time

and strain known as creep curve. The slope of the creep curve is designated as creep rate.

Creep curve is considered to be consists of three portions. After initial rapid elongation,ε0, the creep rate decreases continuously with time, and is known as primaryor transient

creep. Primary creep is followed by secondaryor steady-stateor viscous creep, which ischaracterized by constant creep rate. This stage of creep is often the longest duration of

the three modes. Finally, a third stage of creep known as, tertiary creepoccurs that ischaracterized by increase in creep rate.

First stage creep is associated with strain hardening of the sample. Constant creep rate

during secondary creep is believed to be due to balance between the competing processesof strain hardening and recovery. Creep rate during the secondary creep is called the

minimum creep rate. Third stage creep occurs in constant load tests at high stresses athigh temperatures. This stage is greatly delayed in constant stress tests. Tertiary creep is

believed to occur because of either reduction in cross-sectional area due to necking orinternal void formation. Third stage is often associated with metallurgical changes such

as coarsening of precipitate particles, recrystallization, or diffusional changesin the phases that are present. Two most important parameter that influence creep rate are:

stress and temperature. With increase in either stress or temperature (a) instantaneouselastic strain increases (b) steady state creep rate increases and (c) rupture lifetime

decreases.

Brinell Hardness Test

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness

test methods in common use today, the Brinell test is frequently used to determine thehardness of forgings and castings that have a grain structure too course for Rockwell or

Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying thetest force and ball size, nearly all metals can be tested using a Brinell test. Brinell values

are considered test force independent as long as the ball size/test force relationship is thesame.

Brinell Test Method

All Brinell tests use a carbide ball indenter. The test procedure is as follows:

The indenter is pressed into the sample by an accurately controlled test force.The force is maintained for a specific dwell time, normally 10 - 15 seconds.

After the dwell time is complete, the indenter is removed leaving a round indentin the sample.

8/17/2019 33345



72

The size of the indent is determined optically by measuring two diagonals of theround indent using either a portable microscope or one that is integrated with the load

application device.The Brinell hardness number is a function of the test force divided by the curved

surface area of the indent. The indentation is considered to be spherical with a radius

equal to half the diameter of the ball. The average of the two diagonals is used in thefollowing formula to calculate the Brinell hardness.

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, willincrease as the sample gets harder. Tables are available to make the calculation simple. Atypical Brinell hardness is specified as follows:

356HBW

Where 356 is the calculated hardness and the W indicates that a carbide ball was used.

Note- Previous standards allowed a steel ball and had an S designation. Steel balls are nolonger allowed.

Applications

Because of the wide test force range the Brinell test can be used on almost any metallicmaterial. See the application guide. The part size is only limited by the testing

instrument's capacity.

http://www.wilsoninstruments.com/applications/test_method_guide.aspx




8/17/2019 33345



73

UNIT – IV FERROUS AND NON FERROUS METALSEffect of alloying additions on steel (Mn, Si, Cr, Mo, V Ti & W) - stainless and tool

steels – HSLA - maraging steels – Gray, White malleable, spheroidal - Graphite - alloycastirons

Copper and Copper alloys – Brass, Bronze and Cupronickel – Aluminum and Al-Cu – precipitation strengthening treatment – Bearing alloys.

HSLA steel

High strength low alloy (HSLA) steel is a type of alloy steel that provides bettermechanical properties or greater resistance to corrosion than carbon steel. HSLA steels

vary from other steels in that they aren't made meet a specific chemical composition, butrather to specific mechanical properties. They have a carbon content between 0.05 – 0.25%

to retain formability and weldability.

Other alloying elements include up to 2.0% manganese and small quantities of copper,nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare

earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added forstrengthening purposes. These elements are intended to alter the microstructure of carbon

steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion ofalloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing

effect of a pearlitic volume fraction, yet maintains and increases the material's strength byrefining the grain size, which in the case of ferrite increases yield strength by 50% for

every halving of the mean grain diameter.

CAST IRON

The cast irons are the ferrous alloys with greater that 2.14 wt % carbon, but typically

contain 3-4.5 wt % of C as well as other alloying elements, such as silicon (~3 wt.%)which controls kinetics of carbide formation

There are four general types of cast irons:

1. White iron has a characteristics white, crystalline fracture surface. Large amount ofFe3C are formed during casting, giving hard brittle material

2. Gray iron has a gray fracture surface with finely faced structure. A large Si content (2-3 wt %) promotes C flakes precipitation rather than carbide

3. Ductile iron: small addition (0.05 wt.%) of Mg to gray iron changes the flake Cmicrostructure to spheroidal that increases (by factor ~20) steel ductility

4. Malleable iron : traditional form of cast iron with reasonable ductility. First cast towhite iron and then heat-treated to produce nodular graphite precipitates.

White and Malleable cast iron

The low-sil icon cast irons (<1.0wt.%), produced under rapid cooling conditions

8/17/2019 33345



74

• Microstructure: most of cementite

• Properties: extremely hard very but brittle

• White iron is an intermediate for the production of malleable iron

In white cast iron light Fe3C regions surrounded by pearlite

In malleabl e cast iron dark graphite rosettes in α-Fe matrix

Gray and Ducti le Cast I rons

• The gray irons contain 1-31.0 wt.% of Si• Microstructure: flake – shape graphite in ferrite matrix

• Properties : relatively weak and brittle in tension BUT very effective in damping

vibrational energy an high resistive to wear!!

Ductile (or Nodular) i ron : small addition of Mg or/and Ce to the gray iron composition

before casting• Microstructure: Nodular or spherical-like graphite structure in pearlite or ferric matrix

• Pro perties: Significant increase in material ductility• Applications: valves pump bodies, gears and other auto and machine components.

8/17/2019 33345



75

Effect of Alloying elements

8/17/2019 33345



76

8/17/2019 33345



77

Copper and its Alloys

Copper: soft and ductile; unlimited cold-work capacity, high electrical conductivity, highthermal thermal conductivity, resist to corrosion, cannot be welded, but difficult to

machine. Cold-working and solid solution alloying. It is the oldest one widely used.

Main types of Copper Alloys: – Brasses(Cu-Zn) : 36% of zinc (Zn) and Pb, Al, Mn are added with it. It exists in α-

brasses, β-brasses. It is soft ductile, cold worked, stronger than copper, lowthermal andelectrical conductivity than copper. Applications: cartridges, auto-radiator. Musical

instruments, coins. – Bronzes(Cu-Sn) : tin (Sn), aluminum (Al), Silicon (Si) and nickel (Ni); stronger than

brasses with high degree of corrosion resistance. It can be shaped or rolled into wires,rods and sheets.

– Heat-tr eated (precipitation hardening) Cu-alloys : beryllium coopers; relatively high

strength, excellent electrical and corrosion properties BUT expensive; applications: jetaircraft landing gear bearing, surgical and dental instruments.

- Cupronickel (Cu-Ni ) It is ductile and malleable and it is good corrosive resistant for seawater. It can be shaped to any form.

Copper advantage as primary metal and recycled metal, for brazed, long-life radiators andradiator parts for cars and trucks:

Applications: Manufacturing of power cables, telephone cables, water tanks in food andchemical industries and in heat exchangers

Aluminum and its Alloys

It is light weight, low specific gravity, high strength to weight ratio. Low density (~2.7g/cm3), high ductility (even at room temperature), high electrical and thermal

conductivity and resistance to corrosion BUT law melting point (~660°C).

Main types of Aluminum Alloys:- Wrought Alloys

- Cast Alloys- Others: e.g. Aluminum-Lithium Alloys

Applications: form food/chemical handling to aircraft structural parts

8/17/2019 33345



78

Magnesium and its Alloys

Key Properties:

Light weight

Low density (1.74 g/cm3 two thirds that of aluminium)Good high temperature mechanical properties

Good to excellent corrosion resistanceVery high strength-to density ratios (specif ic strength )

In contrast with Al alloys that have fcc structure with (12 ) slip systems and thus highductility, hcp structure of Mg with only three slip systems leads to its brittleness.

Applications: from tennis rockets to aircraft and missiles

Example: Aerospace

RZ5 (Zn 3.5 - 5,0 SE 0.8 - 1,7 Zr 0.4 - 1,0 Mg remainder), MSR (AG 2.0 - 3,0 SE 1.8 -

2,5Zr 0.4 - 1,0 Mg remainder) alloys are widely used for aircraft engine and gearboxcasings. Very large magnesium castings can be made, such as intermediate compressorcasings for turbine engines. These include the Rolls Royce Tay casing in MSR, which

weighs 130kg and the BMW Rolls Royce BR710 casing in RZ5. Other aerospaceapplications include auxiliary gearboxes (F16, Euro-fighter 2000, Tornado) in MSR or

RZ5, generator housings (A320 Airbus, Tornado and Concorde in MSR) and canopies,generally in RZ5.

Titanium and its Alloys (1)

Titanium and its alloys have proven to be technically superior and cost-effectivematerials of construction for a wide variety of aerospace, industrial, marine and

commercial applications.The properties and characteristics of titanium which are important to design engineers in

a broad spectrum of industries are:- Excellent Corrosion Resistance: Titanium is immune to corrosive attack by salt water

or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids,alkalis, natural waters and industrial chemicals.

8/17/2019 33345



79

- Superior Erosion Resistance: Titanium offers superior resistance to erosion, cavitationor impingement attack. Titanium is at least twenty times more erosion resistant than the

copper-nickel alloys.- High Heat Transfer Efficiency: Under "in service" conditions, the heat transfer

properties of titanium approximate those of admiralty brass and copper-nickel.

Titanium alloys capable of operating at temperatures from sub zero to 600°C are used inengines for discs, blades, shafts and casings from the front fan to the last stage of the high

pressure compressor, and at the rear end of the engine for lightly loaded fabrications suchas plug and nozzle assemblies.

Alloys with strength up to 1200MPa are used in a wide variety of airframe applications

from small fasteners weighing a few grams to landing gear trucks and large wing beamsweighing up to 1 ton.

Currently titanium makes up to 10% of empty weight of aircraft such as the Boeing 777.

Other Alloys

The Refractory Metals : Nb (m.p.=2468°C); Mo (°C); W (°C); Ta(3410°C)

- Also: large elastic modulus, strength, hardness in wide range of temperatures

The Superall oys – possess the superlative combination of properties

- Examples:- Applications: aircraft turbines; nuclear reactors, petrochemical equipments

The Noble Metal Al loys : Ru(44), Rh (45), Pd (46), Ag (47), Os (75), Ir (77), Pt (78),Au (79)

- expensive are notable in properties: soft, ductile, oxidation resistant

- Applications: jewelry (Ag, Au, Pt), catalyst (Pt, Pd, Ru), thermocouples (Pt, Ru),dental materials etc.

Miscell aneous Nonf err ous Al loys :

Nickel and its alloy: high corrosion resistant (Example: monel – 65Ni/28Cu/7wt%Fe – pumps valves in aggressive environment)

Lead, tin and their alloys : soft, low recrystalization temperature, corrosion resistant(Applications: solders, x-ray shields, protecting coating).

8/17/2019 33345



80

UNIT-V

Polymers are materials of complex structure. Polymer consists of many units.In polymer, molecules of ordinary sizes are combined in long chains.

Types of polymers

Plastics Fibers

Elastomers

Polymers with fillers, solvents, plasticers and pigments are known as plastic.

Types of plastics:

Thermo plastic

Thermoset plastic

Major Characteristics of plastics are:

Non crystalline structure Low softening temperature

Resistance to chemical reaction

Visco- elastic behaviour

Non-conductors

Low thermal conductivity

Engineering thermoplastics are:

Polythene

Polyvinylchloride

Polypropene

Polystyrene

Polymethyl Methacrylate

Polyesters

Polycarbonates

Polyamides

ABS

Polymers

The word ‘polymer’is originated from Greek word meros, which means ‘a part’.Polymers are primarily organic compounds, however few polymers are made of inorganic

compounds. Characteristics of ceramics are:-low temperature stability-low hardness-lowmechanical strength-high elongation under application of stress-low thermal and

electrical conductivities-high sensitivity of properties to their morphology

Polymer classificationPolymers are classified in many ways. The prime classification based on their industrial

usage is: plastics and elastomers.

Plastic polymers are again classified based on their temperature dependence of theirstructure as follows:-thermoplastsand-thermosets

8/17/2019 33345



81

Thermo Plastic

Plastics which softens up on heating and hardens up on cooling where the softening andhardening are totally reversible processes. Hence thermoplastscan be recycled. They

consist of linear molecular chains bonded together by weak secondary bonds or by inter-

winding. Cross-linking between molecular chains is absent in theromplasts. E.g.:Acrylics, PVC, Nylons, Perspex glass, etc.

ThermosetsPlastics which are ‘set’under the application of heat and/or pressure. This process is not

reversible, hence thermosetscan not be recycled.

They consist of 3-D network structures based on strong covalent bonds to form rigidsolids. linear molecular chains bonded together by weak secondary bonds or by inter-

winding.

Characterized by high modulus / rigidity / dimensional stability when compared withthermoplasts.

E.g.: Epoxies, Amino resins, some polyester resins, etc.

Elastomers

These polymers are known for their high elongations, which are reversible upon releaseof applied loads. They consist of coil-like molecular chains, which straightens up on

application of load. Characterized by low modulus / rigidity / strength, but hightoughness.E.g.: natural and synthetic rubber.

Polymer Synthesis

Processing of polymers primarily limits to synthesis followed by forming. Polymers aresynthesized by process known as polymerization. Polymerization is process in which

multi-functional monomers are attached to form linear/3-D macro molecular chains.When polymerization process involves single kind of monomers i.e. in Additional

polymerization, resultant macro-molecule’s composition is an exact multiplication ofcomposition of individual monomer.

Additional polymerization process involves three stages namely initiation, propagation

and termination. Initiation process will be started by an initiator (e.g.benzoylperoxide)which forms an reactive site where carbon atom of another monomer is attracted, upon

which reaction site transfers to different place leading to molecular chain growth. Asmolecular chain grows longer, reaction rate decreases. However the growth process is

terminated either by the combination or disproportionation process.

Polymer Synthesis

8/17/2019 33345



82

Degree of Polymeri sation

Extant of polymerization is

characterized in terms of ‘degree of polymerization’. It is defined as

number of mer units in molecular chain or ration of average molecular weight of polymer

to molecular weight of repeat unit. Average molecular weight is defined in two ways:Weight average molecular weight (based on weight fraction) and Number averagemolecular weight (based on the number fraction). Number average molecular weight is

always smaller than the weight average molecular weight.

Ceramics

The word ‘ceramic’is originated from greekword keromikos, which means ‘burnt stuff’.Ceramics are compounds of metallic and non-metallic elements. Characteristics of

ceramics are:-high temperature stability-high hardness-brittleness-high mechanicalstrength-low elongation under application of stress-low thermal and electrical

conductivities. Ceramics are classified in many ways. It is due to divergence incomposition, properties and applications. Based on their composition, ceramics are:-

Oxides-Carbides-Nitrides-Sulfides-Fluoridesetc.

Classification of CeramicsBased on their specific applications, ceramics are classified as:-Glasses-Clay products-

Refractories-Abrasives-Cements-Advanced ceramics for special applications

Based on their engineering applications, ceramics are classified into two groups as:traditional and engineering ceramics.

Traditional ceramics – most made-up of clay, silica and feldspar

Engineering ceramics – these consist of highly purified aluminium oxide (Al2O3), siliconcarbide (SiC) and silicon nitiride(Si3N4)

8/17/2019 33345



Documents

33345