Case Hardening Report

Prepared by: Eng. Marwa Alsayed Ali

Presented to: Prof. Dr. Mamdouh

2012

Metallurgy Department – Cairo University

1

Contents Page no.

1. Introduction……………………………... 2

2. History…………………………………... 3

3. Chemistry……………………………….. 4

4. Modern use……………………………… 5

5. Processes…………………………………6

5.1 Carburizing………………………….6

5.2 Nitriding…………………………… 12

5.3 Cyaniding ―Liquid Carbonitriding‖…………19

5.4 Carbonitriding………………………22

5.5 Ferritic nitrocarburizing…………….24

5.6 Flame and induction hardening……..27

6. Applications…………………………….. 31

7. References……………………………… 32

2

Introduction

Case hardening (or) Surface hardening:-

Case hardening (or) Surface hardening is a

process of heating the metal over its surface so

as to harden it. This process is adopted, as

many of the applications like gears, cams, and

crankshafts desire high hardness on the outer

surface and softer core, which is tough enough

to withstand the shocks. So to attain such

properties it is very difficult to employ low

carbon steels or high carbon steels as low carbon steels cannot be hardened where

as high carbon steels have poor toughness.

So for obtaining the required properties in general medium carbon steels are

used. These steels have intermediate properties of both the above-mentioned

steels. So the hardness of these materials over the surface is increased by any of

the following ways

1) So as to increase the surface of hardness of the material it is subjected to

heating so as to reduce the carbon in free form and helps in forming of cementite.

2) Nitriding is done so as to form nitrides which are very strong in nature

3) Hardening with out change of composition can also be done by some surface

hardening processes like flame hardening, electric arc hardening etc.

4) Steels can also harden by sending both nitrogen and carbon at a time along

with heating.

Surface hardening is done in many methods if the composition of the surface is

changed then the hardening is called as case hardening and if there is no change

in surface composition then the hardening technique is called surface hardening.

3

History

Early iron melting made use of bloomeries,

which produced two layers of metal: one

with a very low carbon content that is

worked into wrought iron, and the rest a high

carbon cast iron. Since the high carbon iron

is hot short, meaning it fractures and

crumbles when forged, it was not useful

without more smelting. The wrought iron,

with nearly no carbon in it, was

very malleable and ductile, but not very

hard.

Case hardening involves packing the low-carbon iron within a substance high in

carbon, then heating this pack to encourage carbon migration into the surface of

the iron. This forms a thin surface layer of higher carbon steel, with the carbon

content gradually decreasing deeper from the surface. The resulting product

combines much of the toughness of a low-carbon steel core, with the hardness

and wear resistance of the outer high-carbon steel.

The traditional method of applying the carbon to the surface of the iron involved

packing the iron in a mixture of ground bone and charcoal, or a combination

of leather, hooves, saltand urine, all inside a well-sealed box. This carburizing

package is then heated to a high temperature, but still under the melting point of

the iron, and left at that temperature for a length of time. The longer the package

is held at the high temperature, the deeper the carbon will diffuse into the surface.

Different depths of hardening is desirable for different purposes: sharp tools need

deep hardening to allow grinding and resharpening without exposing the soft

core, while machine parts like gears might need only shallow hardening for

increased wear resistance.

The resulting case hardened part may show distinct surface discoloration. The

steel darkens significantly, and shows a mottled pattern of black, blue and purple,

caused by the various compounds formed from impurities in the bone and

charcoal. This oxide surface works similarly to bluing, providing a degree of

corrosion resistance, as well as an attractive finish. Case coloring refers to this

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pattern and is commonly encountered as a decorative finish

on replica historic firearms.

With modern steelworking techniques, it is possible to make homogeneous steels

of low to high carbon content, removing much of the original motivation for case

hardening. However, the heterogeneous nature of case hardened steel may still be

desirable, as it can combine both extreme hardness and extreme toughness,

something which is not readily matched by homogeneous alloys.

Chemistry

Carbon itself is solid at case-hardening

temperatures and so is immobile. Transport to

the surface of the steel was as gaseous carbon

monoxide, generated by the breakdown of the

carburising compound and the oxygen packed

into the sealed box. This takes place with pure

carbon, but unworkably slowly. Although

oxygen is required for this process it's re-

circulated through the CO cycle and so can be

carried out inside a sealed box. The sealing is necessary to stop the CO either

leaking out, or being oxidised to CO2 by excess outside air.

Adding an easily decomposed carbonate "energiser" such as barium

carbonate breaks down to BaO + CO2 and this encourages the reaction

C (from the donor) + CO2 <—> 2 CO

increasing the overall abundance of CO and the activity of the carburising

compound. Its 'common knowledge' that case-hardening was done with bone, but

this is misleading. Although bone was used, the main carbon donor was hoof and

horn. Bone contains some carbonates, but is mainly calcium phosphate

(as hydroxylapatite). This doesn't have the beneficial effect on encouraging CO

production and it can also supply phosphorus as an impurity into the steel alloy.

5

Modern Use

Both carbon and alloy steels are suitable for case-hardening; typically mild steels

are used, with low carbon content, usually less than 0.3% (see plain-carbon

steel for more information). These mild steels are not normally hardenable due to

the low quantity of carbon, so the surface of the steel is chemically altered to

increase the hardenability. Case hardened steel is formed by diffusing carbon

(carburization), nitrogen (nitriding) and/or boron (boriding) into the outer layer of

the steel at high temperature, and then heat treating the surface layer to the

desired hardness.

The term case hardening is derived from the practicalities of the carburization

process itself, which is essentially the same as the ancient process. The steel work

piece is placed inside a case packed tight with a carbon-based case hardening

compound. This is collectively known as a carburizing pack. The pack is put

inside a hot furnace for a variable length of time. Time and temperature

determines how deep into the surface the hardening extends. However, the depth

of hardening is ultimately limited by the inability of carbon to diffuse deeply into

solid steel, and a typical depth of surface hardening with this method is up to

1.5 mm. Other techniques are also used in modern carburizing, such as heating in

a carbon-rich atmosphere. Small items may be case hardened by repeated heating

with a torch and quenching in a carbon rich medium, such as the commercial

products Kasenit /Casenite or "Cherry Red". Older formulations of these

compounds contain potentially toxic cyanide compounds, such

as ferrocyanide compounds, while the more recent types

6

Processes

The following are the processes in case hardening

1. Carburizing

2. Nitriding

3. Cyano Nitriding

4. Carbo Nitriding

The following are the processes in surface hardening

5. Flame hardening

6. Induction hardening

Carburizing

is a process of adding Carbon to the surface. This is done by exposing the part to

a Carbon rich atmosphere at an elevated temperature and allows diffusion to

transfer the Carbon atoms into steel. This diffusion will work only if the steel has

low carbon content, because diffusion works on the differential of concentration

principle. If, for example the steel had high carbon content to begin with, and is

heated in a carbon free furnace, such as air, the carbon will tend to diffuse out of

the steel resulting in Decarburization.

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To summarize, carburizing

methods include :

1. Gas carburizing

2. Vacuum carburizing

3. Plasma carburizing

4. Salt bath carburizing

5. Pack carburizing

These methods introduce carbon by the use of gas (atmospheric-gas, plasma, and

vacuum carburizing), liquids (salt bath carburizing), or solid compounds (pack

carburizing). All of these methods have limitations and advantages, but gas

carburizing is used most often for large-scale production because it can be

accurately controlled and involves a minimum of special handling.

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Vacuum carbunzing and plasma carburizing have found applications because of

the absence of oxygen in the furnace atmosphere. Salt bath and pack carburizing

arc still done occasionally, but have little commercial importance today.

Carburizing is the addition of carbon to the surface of low-carbon steels at

temperatures generally between 850 and 950°C (1560 and 1740°F), at which

austenite, with its high solubility for carbon, is the stable crystal structure.

Hardening is accomplished when the high-carbon surface layer is quenched to

form martensite so that a high-carbon martensitic case with good wear and

fatigue resistance is superimposed on a tough, low-carbon steel core.

The Carbon content in the steel determines whether it can be directly hardened. If

the Carbon content is low (less than 0.25% for example) then an alternate means

exists to increase the Carbon content of the surface. Depending on the amount of

time and temperature, the affected area can vary in carbon content. Longer

carburizing times and higher temperatures lead to greater carbon diffusion into

the part as well as increased depth of carbon diffusion.

When the iron or steel is cooled rapidly by quenching, the higher carbon content

on the outer surface becomes hard via the transformation from austenite to

martensite, while the core remains soft and tough as a ferritic and/or pearlite

microstructure.This manufacturing process can be characterized by the following

key points: It is applied to low-carbon workpieces; workpieces are in contact with

a high-carbon gas, liquid or solid, it produces a hard workpiece surface;

workpiece cores largely retain their toughness and ductility and it produces case

hardness depths of up to 0.25 inches (6.4 mm).

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Carburization of steel involves a heat treatment of the metallic surface using a

source of carbon.

Early carburization used a direct application of charcoal packed onto the metal

(initially referred to as case hardening), but modern techniques apply carbon-

bearing gases or plasmas (such as carbon dioxide or methane)

. Carburizing is the addition of carbon to the surface of low-carbon steels at

temperatures generally between 850 and 950°C (1560 and 1740°F), at which

austenite, with its high solubility for carbon, is the stable crystal structure.

Hardening is accomplished when the high-carbon surface layer is quenched to

form martensite so that a high-carbon martensitic case with good wear and

fatigue resistance is superimposed on a tough, low-carbon steel core.

In its earliest application, parts were simply placed in a suitable container and

covered with a thick layer of carbon powder (pack carburizing). In gas

carburizing, the parts are surrounded by a carbon-bearing atmosphere that can be

continuously replenished so that a high carbon potential can be maintained. In

efforts required to simplify the atmosphere, carburizing in an oxygen-free

environment at very low pressure (vacuum carburizing) has been explored and

developed into a viable and important alternative.

Furthermore, because the parts are

heated in an oxygen-free environment,

the carburizing temperature may be

increased substantially without the

risk of surface or grain-boundary

oxidation. Because vacuum

carburizing is conducted at very low

pressures, and the rate of flow of the

carburizing gas into the furnace is

very low, the carbon potential of the

gas in deep recesses and blind holes is

quickly depleted. Unless this gas is

replenished, a great nonuniformity in

case depth over the surface of the part

is likely to occur. If, in an effort to

overcome this problem, the gas pressure is increased significantly, another

problem arises, that of free-carbon formation, or sooting.

Case hardness of carburized steels is primarily a function of carbon content.

When the carbon content of the steel exceeds about 0.50% additional carbon has

no effect on hardness but does enhance hardenability. Carbon in excess of 0.50%

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may not be dissolved, which would thus require temperatures high enough to

ensure carbon-austenite solid solution.

Case depth of carburized steel is

a function of carburizing time

and the available carbon

potential at the surface. When

prolonged carburizing times are

used for deep case depths, a high

carbon potential produces a high

surface-carbon content, which

may thus result in excessive

retained austenite or free

carbides. Consequently, a high

carbon potential may be suitable

for short carburizing times but

not for prolonged carburizing.

Gas carburizing is normally carried out at a temperature within the range of 900

to 950 °C. In oxy-acetylene welding, a carburizing flame is one with little

oxygen, which produces a sooty, lower-temperature flame. Carburizing steels for

case hardening usually have base-carbon contents of about 0.2%, with the carbon

content of the carburized layer generally being controlled at between 0.8 and 1%

C. However, surface carbon is often limited to 0.9% because too high a carbon

content can result in retained austenite and brittle martensite.

Pack carburizing containers are usually made of carbon steel coated with

aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with

fire clay. There are different types of elements or materials that can be used to

perform this process, but these mainly consist of high carbon content material.

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A few typical hardening agents include carbon monoxide gas (CO), sodium

cyanide and barium chloride, or hardwood charcoal. In gas carburizing, the CO is

given off by propane or natural gas. In pack carburizing, carbon monoxide is

given off by coke or hardwood charcoal.

Plasma carburization is increasingly used in major industrial regimes to improve

the surface characteristics (such as wear and corrosion resistance, hardness and

load-bearing capacity, in addition to quality-based variables) of various metals,

notably stainless steels. The process is used as it is environmentally friendly (in

comparison to gaseous or solid carburizing). It also provides an even treatment of

components with complex geometry (the plasma can penetrate into holes and

tight gaps), making it very flexible in terms of component treatment.

Steels made to coarse grain practices can be carburized if a double quench

provides grain refinement. Many alloy steels for case hardening are now

specified on the basis of core hardenability. First, in a case-hardened steel, the

hardenability of both case and core must be considered.

The relationship between the thermal gradient and the carbon gradient during

quenching of a carburized part can make a measurable difference in the case

depth as measured by hardness. That is, an increase in base hardenability can

produce a higher proportion of martensite for a given carbon level, yielding an

increased measured case depth. Therefore, a shallower carbon profile and shorter

carburizing time could be used to attain the desired result in a properly chosen

steel.

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Nitriding

THE NITRIDING PROCESS: is a heat treating process that diffuses nitrogen

into the surface of a metal to create a case hardened surface. It is predominantly

used on steel, but also titanium, aluminum and molybdenum. In this process the

steel material is heated to a temperature of around 5500C and then exposed to

atomic nitrogen. This atomic nitrogen reacts with iron and other alloying

elements and forms nitrides, which are very hard in nature. By this process both

wear resistance and hardness of the product can be increased. Atomic nitrogen

only has the property to penetrate in the steel but where as molecular nitrogen

lacks in that property. So molecular nitrogen is never used as a nitriding medium

while hardening of steels. The atomic nitrogen required for this process is

generated by the decomposition of salt bath like NaCN.

first developed in the early 1900s, continues to play an important role in many

industrial applications. Along with the derivative nitrocarburizing process,

nitriding often is used in the manufacture of aircraft, bearings, automotive

components, textile machinery, and turbine generation systems. Though wrapped

in a bit of ―alchemical mystery,‖ it remains the simplest of the case hardening

techniques.The secret of the nitriding process is that it does not require a phase

change from ferrite to austenite, nor does it require a further change from

austenite to martensite. In other words, the steel remains in the ferrite phase (or

cementite, depending on alloy composition) during the complete procedure.

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This means that the molecular structure of the ferrite (body-centered cubic, or

bcc, lattice) does not change its configuration or grow into the face-centered

cubic (fcc) lattice characteristic of austenite, as occurs in

more conventional methods such as carburizing. Furthermore, because only free

cooling takes place, rather than rapid cooling or quenching, no subsequent

transformation from austenite to martensite occurs. Again, there is no molecular

size change and, more importantly, no dimensional change, only slight growth

due to the volumetric change of the steel surface

caused by the nitrogen diffusion. What can (and does) produce distortion are the

induced surface stresses being released by the heat of the process, causing

movement in the form of twisting and bending.

The processes are named after the medium used to donate. The three main

methods used are: gas nitriding, salt bath nitriding, and plasma nitriding.

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Gas nitriding

In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3), which is

why it is sometimes known as ammonia nitriding. When ammonia comes into

contact with the heated work piece it disassociates into nitrogen and hydrogen.

The nitrogen then diffuses onto the surface of the material creating a nitride layer.

This process has existed for nearly a century, though only in the last few decades

has there been a concentrated effort to investigate the thermodynamics and

kinetics involved. Recent developments have led to a process that can be

accurately controlled. The thickness and phase constitution of the resulting

nitriding layers can be selected and the process optimized for the particular

properties required.

The advantages of gas nitriding over the other variants are:

Precise control of chemical potential of nitrogen in the nitriding

atmosphere by controling gas flow rate of nitrogen and oxygen.

All round nitriding effect (can be a disadvantage in some cases,

compared with plasma nitriding)

Large batch sizes possible - the limiting factor being furnace size

and gas flow

With modern computer control of the atmosphere the nitriding

results can be closely controlled

Relatively low equipment cost - especially compared with plasma

The disadvantages of gas nitriding are:

Reaction kinetics heavily influenced by surface condition - an oily

surface or one contaminated with cutting fluids will deliver poor

results

Surface activation is sometimes required to treat steels with a high

chromium content - compare sputtering during plasma nitriding

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Ammonia as nitriding medium - though not especially toxic it can be

harmful when inhaled in large quantities. Also, care must be taken

when heating in the presence of oxygen to reduce the risk of

explosion.

Salt bath nitriding

In salt bath nitriding the nitrogen

donating medium is a nitrogen-

containing salt such as cyanide salt. The

salts used also donate carbon to the

workpiece surface making salt bath a

nitrocarburizing process. The

temperature used is typical of all

nitrocarburizing processes: 550–590 °C

(1022–1094 °F).

The advantages of salt nitriding are:

Quick processing time - usually in the order of 4 hours or so to

achieve

Simple operation - heat the salt and workpieces to temperature and

submerge until the duration has transpired

The disadvantages are:

The salts used are highly toxic - Disposal of salts are controlled by

stringent environmental laws in western countries and has increased

the costs involved in using salt baths. This is one of the most

significant reasons the process has fallen out of favor in recent

decades.

Only one process possible with a particular salt type - since the

nitrogen potential is set by the salt, only one type of process is

possible

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Plasma nitriding

Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-

discharge nitriding, is an industrial surface hardening treatment for metallic

materials.

In plasma nitriding, the reactivity of the nitriding media is not due to the

temperature but to the gas ionized state. In this technique intense electric fields

are used to generate ionized molecules of the gas around the surface to be

nitrided. Such highly active gas with ionized molecules is called plasma, naming

the technique. The gas used for plasma nitriding is usually pure nitrogen, since no

spontaneous decomposition is needed (as is the case of gas nitriding with

ammonia). There are hot plasmas typified by plasma jets used for metal cutting,

welding, cladding or spraying. There are also cold plasmas, usually generated

inside vacuum chambers, at low pressure regimes.

Usually steels are very beneficially treated with plasma nitriding. Plasma

nitriding advantage is related to the close control of the nitrided microstructure,

allowing nitriding with or without compound layer formation.

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Not

only

the

performance of metal parts gets enhanced but working lifespan gets boosted. So

does the strain limit, and the fatigue strength of the metals being treated. For

instance, mechanical properties of austenitic stainless steel like wear can be

significantly reduced and the hardness of tool steels can be double on the surface.

A plasma nitrided part is usually ready for use. It calls for no machining, or

polishing or any other post-nitriding operations. Thus the process is user-friendly,

saves energy since it works fastest, and causes little or no distortion.

This process was invented by Dr. Bernhardt Berghaus of Germany who later

settled in Zurich to escape Nazi persecution. After his death in late 1960s the

process was acquired by Klockner group and popularized world over.

Plasma nitriding is often coupled with physical vapor deposition (PVD) process

and labeled Duplex Treatment, with enhanced benefits. Many users prefer to

have a plasma oxidation step combined at the last phase of processing to produce

a smooth jetblack layer of oxides which is resistant to wear and corrosion.

Since nitrogen ions are made available by ionization, differently from gas or salt

bath, plasma nitriding efficiency does not depend on the temperature. Plasma

nitriding can thus be performed in a broad temperature range, from 260°C to

more than 600°C. For instance, at moderate temperatures (like 420°C), stainless

steels can be nitrided without the formation of chromium nitride precipitates and

hence maintaining their corrosion resistance properties.

In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying

gas. Other gasses like hydrogen or Argon are also used. Indeed, Argon and H2

can be used before the nitriding process during the heating of the parts to clean

the surfaces to be nitrided. This cleaning procedure effectively removes the oxide

18

layer from surfaces and may remove fine layers of solvents that could remain.

This also helps the thermal stability of the plasma plant, since the heat added by

the plasma is already present during the warm up and hence once the process

temperature is reached the actual nitriding begins with minor heating changes.

For the nitriding process H2 gas is also added to keep the surface clear of oxides.

This effect can be observed by analysing the surface of the part under nitriding .

Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100,

8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS

3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for

example) and certain cast irons.

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Cyaniding

Cyaniding, or salt-bath carbonitriding, is a

heat-treating process that produces a file-

hard, wear-resistant surface on ferrous parts.

When steel is heated above Ac1 in a suitable

bath containing alkali cyanides and

cyanates, the surface of the steel absorbs

both carbon and nitrogen from the molten

bath. When quenched in mineral oil,

paraffin-base oil, water, or brine, the steel

develops a hard surface layer, or case, that contains less carbon and more

nitrogen than the case developed in activated liquid carburizing baths.

Because of greater efficiency and lower cost, sodium cyanide is used instead of

the more expensive potassium cyanide. The active hardening agents of cyaniding

baths--carbon monoxide and nitrogen--are not produced directly from sodium

cyanide.

Molten cyanide decomposes in the presence of air at the surface of the bath to

produce sodium cyanate, which in turn decomposes in accordance with the

following chemical reactions:

2NaCN + O2 2NaNCO

4NaNCO Na2CO3 + 2NaCN + CO + 2N

2CO CO2 + C

NaCN + CO2 NaNCO + CO

The rate at which cyanate is formed and decomposes, liberating carbon and

nitrogen at the surface of the steel, determines the carbonitriding activity of the

bath. At operating temperatures, the higher the concentration of cyanate, the

faster the rate of its decomposition.

Because the rate of cyanate decomposition also increases with temperature, bath

activity is greater at higher operating temperatures. A fresh cyaniding bath must

be aged for about 12 h at a temperature above its melting point to provide a

21

sufficient concentration of cyanate for efficient carbonitriding activity. For the

aging cycle to be effective, any carbon scum formed on the surface must be

removed. To eliminate scum, the cyanide content of the bath must be reduced to

the 25 to 30% range by addition of inert salts (sodium chloride and sodium

carbonate).

At the bath aging temperature--usually about 700 °C (1290 °F)--the rate of its

decomposition is low.

Bath Composition. A sodium cyanide mixture such as grade 30 in Table 3,

containing 30% NaCN, 40% Na2CO3, and 30% NaCl, is generally used for

cyaniding on a production basis. This mixture is preferable to any of the other

compositions given in the next table.

(a) Appearance: white crystalline solid. This grade also contains 0.5% sodium cyanate (NaNCO) and 0.2% sodium hydroxide

(NaOH); sodium sulfide (Na2S) content, nil.

(b) Appearance: white granular mixture.

The inert salts sodium chloride and sodium carbonate are added to cyanide to

provide fluidity and to control the melting points of all mixtures. The 30% NaCN

mixture, as well as the mixtures containing 45, 75, and 97% NaCN, may be

added to the operating bath to maintain a desired cyanide concentration for a

specific application.

Mixture grade

designation

Composition, wt% Melting point Specific gravity

NaC

N

NaCO3 NaCl °C °F 25 °C

(75

°F)

861 °C

(1580

°F)

96-98(a) 97 2.3 Trac

e

560 1040 1.50 1.10

75(b) 75 3.5 21.5 590 1095 1.60 1.25

45(b) 45.3 37.0 17.7 570 1060 1.80 1.40

30(b) 30.0 40.0 30.0 625 1155 2.09 1.54

Mixture grade

designation

Composition, wt% Melting point Specific gravity

NaC

N

NaCO3 NaCl °C °F 25 °C

(75

°F)

861 °C

(1580

°F)

96-98(a) 97 2.3 Trac

e

560 1040 1.50 1.10

75(b) 75 3.5 21.5 590 1095 1.60 1.25

45(b) 45.3 37.0 17.7 570 1060 1.80 1.40

30(b) 30.0 40.0 30.0 625 1155 2.09 1.54

Mixture grade

designation

Composition, wt% Melting point Specific gravity

NaC

N

NaCO3 NaCl °C °F 25 °C

(75

°F)

861 °C

(1580

°F)

96-98(a) 97 2.3 Trac560 1040 1.50 1.10

21

The carbon content of the case developed in cyanide baths increases with an

increase in the cyanide concentration of the bath, thus providing considerable

versatility. A bath operating at 815 to 850 °C (1500 to 1560 °F) and containing 2

to 4% cyanide may be used to restore carbon to decarburized steels with a core

carbon content of 0.30 to 0.40% C, while a 30% cyanide bath at the same

temperature will yield a 0.13 mm (0.005 in.) case containing 0.65% C at the

surface in 45 min.

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Carbonitriding

Carbonitriding is a metallurgical surface modification technique that is used to

increase the surface hardness of a metal, thereby reducing wear. During the

process, atoms of carbon and nitrogen diffuse interstitially into the metal,

creating barriers to slip, increasing the hardness and modulus near the surface.

Carbonitriding is often applied to inexpensive, easily machined low carbon steel

to impart the surface properties of more expensive and difficult to work grades of

steel. Surface hardness of carbonitrided parts ranges from 55 to 62 HRC.

Certain pre-industrial case hardening processes include not only carbon-rich

materials such as charcoal, but nitrogen-rich materials such as urea, which

implies that traditional surface hardening techniques were a form of

carbonitriding.

Process

Carbonitriding is similar to gas carburization with the addition of ammonia to the

carburizing atmosphere, which provides a source of nitrogen. Nitrogen is

adsorbed at the surface and diffuses into the work piece along with carbon.

Carbonitriding (around 850 °C / 1550 °F) is carried out at temperatures

substantially higher than plain Nitriding (around 530 °C / 990 °F) but slightly

lower than those used for carburizing (around 950 °C / 1700 °F) and for shorter

times. Carbonitriding tends to be more economical than carburizing, and also

reduces distortion during quenching. The lower temperature allows oil

quenching, or even gas quenching with a protective atmosphere.

Characteristics of carbonitrided parts

Carbonitriding forms a hard, wear-resistant case, is typically 0.07mm to 0.5mm

thick, and generally has higher hardness than a carburized case. Case depth is

tailored to the application; a thicker case increases the wear life of the part.

Carbonitriding alters only the top layers of the work piece; and does not deposit

an additional layer, so the process does not significantly alter the dimensions of

the part.

Maximum case depth is typically restricted to 0.75mm; case depths greater than

this take too long to diffuse to be economical. Shorter processing times are

preferred to restrict the concentration of nitrogen in the case, as nitrogen addition

is more difficult to control than carbon. An excess of nitrogen in the work piece

can cause high levels of retained austenite and porosity, which are undesirable in

producing a part of high hardness.

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Advantages

Carbonitriding also has other advantages over carburizing. To begin, it has a

greater resistance to softening during tempering and increased fatigue and impact

strength. It is possible to use both carbonitriding and carburizing together to form

optimum conditions of deeper case depths and therefore performance of the part

in industry. This method is applied particularly to steels with low case

hardenability, such as the seat of the valve. The process applied is initially

carburizing to the required case depth (up to 2.5mm) at around 900-955°C, and

then carbonitriding to achieve required carbonitrided case depth. The parts are

then oil quenched, and the resulting part has a harder case than possibly achieved

for carburization, and the addition of the carbonitrided layer increases

the residual compressive stresses in the case such that the contact fatigue

resistance and strength gradient are both increased.

24

Ferritic nitrocarburizing

Ferritic nitrocarburizing is a range of case hardening processes

that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures.

The processing temperature ranges from 525 °C (977 °F) to 625 °C (1,157 °F),

but usually occurs at 565 °C (1,049 °F). At this temperature steels and other

ferrous alloys are still in a ferritic phase, which is advantageous compared to

other case hardening processes that occur in the austenitic phase. There are four

main classes of ferritic nitrocarburizing: gaseous, salt bath, ion or plasma,

and fluidized-bed.

The process is used to improve three main surface integrity aspects:

scuffing resistance

fatigue properties

corrosion resistance

It has the added advantage of inducing little shape distortion during the hardening

process. This is because of the low processing temperature, which reduces

thermal shocks and avoids phase transitions in steel.

History

The first ferritic nitrocarburizing methods were done at low temperatures, around

550 °C (1,022 °F), in a liquid salt bath. The first company to successfully

commercialize was the Imperial Chemical Industries in England. They called

their process a "Sulfinuz" treatment because it had sulfur in the salt bath. While

the process was very successful with high-speed spindles and cutting tools, there

were issues with cleaning the solution off because it was not very water soluble.

Because of the cleaning issues the Joseph Lucas Limited company began

experimenting with gaseous forms of ferritic nitrocarburizing in the late 1950s.

The company applied for a patent by 1961. It produced a similar surface finish as

the Sulfinuz process with the exception of the formation of sulfides. The

atmosphere consisted of ammonia, hydrocarbon gases, and a small amount of

other carbon-containing gases.

This spurred the development of a more environmentally friendly salt bath

process by the German company Degussa. Their process is the widely known

Tufftride process. Following this the ion nitriding process was invented in the

early 1980s. This process had faster cycle times, required less cleaning and

preparation, formed deeper cases, and allowed for better control of the process.

25

Processes

Despite the naming the process is a modified form of nitriding and not

carburizing. The shared attributes of this class of this process is the introduction

of nitrogen and carbon in the ferritic state of the material. The processes are

broken up into four main classes: gaseous, salt bath, ion or plasma, or fluidized-

bed. The trade name and patented processes may vary slightly from the general

description, but they are all a form of ferritic nitrocarburizing.

Salt bath ferritic nitrocarburizing

Salt bath ferritic nitrocarburizing is also known as liquid ferritic nitrocarburizing

or liquid nitrocarburizing and is also known by the trademarked names Tufftride

and Tenifer.

The simplest form of this process is encompassed by the trademarked Melonite

process, also known as Meli 1. It is most commonly used on steels, sintered irons,

and cast irons to lower friction and improve wear and corrosion resistance.

The process uses a salt bath of alkali cyanate. This is contained in a steel pot that

has an aeration system. The cyanate thermally reacts with the surface of the

workpiece to form alkali carbonate. The bath is then treated to convert the

carbonate back to a cyanate. The surface formed from the reaction has a

compound layer and a diffusion layer. The compound layer consists of iron,

nitrogen, and oxygen, is abrasion resistant, and stable at elevated temperatures.

The diffusion layer contains nitrides and carbides. The surface hardness ranges

from 800 to 1500 HV depending on the steel grade. This also inversely affects

the depth of the case; i.e. a high carbon steel will form a hard, but shallow case.

A similar process is the trademarked Nu-Tride process, also known incorrectly as

the Kolene process (which is actually the company's name), which includes a

preheat and an intermediate quench cycle. The intermediate quench is an

oxidizing salt bath at 400 °C (752 °F). This quench is held for 5 to 20 minutes

before final quenching to room temperature. This is done to minimize distortion

and to destroy any lingering cyanates or cyanides left on the workpiece.

Other trademarked processes are Sursulf and Tenoplus. Sursulf has a sulfur

compound in the salt bath to create surface sulfides which creates porosity in the

workpiece surface. This porosity is used to contain lubrication. Tenoplus is a

two-stage high-temperature process. The first stage occurs at 625 °C (1,157 °F),

while the second stage occurs at 580 °C (1,076 °F).

Gaseous ferritic nitrocarburizing

26

Gaseous ferritic nitrocarburizing is also known as controlled nitrocarburizing,

soft nitriding, and vacuum nitrocarburizing or by the tradenames Nitrotec,

Nitemper, Deganit, Triniding, Corr-I-Dur, Nitroc, and Nitrowear. The process

works to achieve the same result as the salt bath process, except gaseous mixtures

are used to diffuse the nitrogen and carbon into the workpiece.

The parts are first cleaned, usually with a vapor degreasing process, and then

nitrocarburized around 570 °C (1,058 °F), with a process time that ranges from

one to four hours. The actual gas mixtures are proprietary, but they usually

contain ammonia and an endothermic gas.

Plasma-assisted ferritic nitrocarburizing

Plasma-assisted ferritic nitrocarburizing is also known as ion nitriding, plasma

ion nitriding or glow-discharge nitriding. The process works to achieve the same

result as the salt bath and gaseous process, except the reactivity of the media is

not due to the temperature but to the gas ionized state. In this technique intense

electric fields are used to generate ionized molecules of the gas around the

surface to diffuse the nitrogen and carbon into the workpiece. Such highly active

gas with ionized molecules is called plasma, naming the technique. The gas used

for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition

is needed (as is the case of gaseous ferritic nitrocarburizing with ammonia). Due

to the relatively low temperature range (420 °C (788 °F) to 580 °C (1,076 °F))

generally applied during plasma-assisted ferritic nitrocarburizing and gentle

cooling in the furnace, the distortion of workpieces can be minimized. Stainless

steel workpieces can be processed at moderate temperatures (like 420 °C (788

°F)) without the formation of chromium nitride precipitates and hence

maintaining their corrosion resistance properties.

27

Flame and induction Hardening

FLAME HARDENING is a heat-treating

process in which a thin surface shell of a

steel part is heated rapidly to a temperature

above the critical point of the steel. After the

grain structure of the shell has become

austenitic (austenitized), the part is quickly

quenched, transforming the austenite to

martensite while leaving the core of the part

in its original state. In contrast, slow cooling

causes transformation, as the temperature

passes through the corresponding ranges, to

pearlite, bainite, and martensite, with the

final structure being a combination of the three. The result is relatively soft and

ductile steel. To achieve hardness, therefore, the steel must be cooled rapidly so

that it bypasses the first two transformation phases and transforms directly from

austenite to martensite.

Flame hardening employs direct impingement of a high-temperature flame or

high-velocity combustion product gases.

The part is then cooled at a rate that will produce the desired levels of hardness

and other properties. The high temperature flame is obtained by combustion of a

mixture of fuel gas with oxygen or air; flame heads are used for burning the

mixture. Depths of hardening from about 0.8 to 6.4 mm ( 1/32 to ¼ in.) or more

can be obtained, depending on the fuels used, the design of the flame head, the

duration of heating, the hardenability of the work material, and the quenching

medium and method of quenching used. The process can be used for the through

hardening of work 75 mm (3 in.) or less in cross section, depending on the

hardenability of the steel.

Hardening by flame differs from true case hardening because the hardenability

necessary to attain high levels of hardness is already contained in the steel, and

hardening is obtained by localized heating. Although flame hardening is mainly

used to develop high levels of hardness for wear resistance, the process also

improves bending and torsional strength and fatigue life. One of the major

advantages of flame hardening is the ability to satisfy stringent engineering

requirements with carbon steels.

28

Methods of Flame Hardening

The versatility of flame-hardening equipment and the wide range of heating

conditions obtainable with gas burners often permit flame hardening to be done

by a variety of methods, of which the principal ones are:

· Spot, or stationary

· Progressive

· Spinning

· Combination progressive-spinning

The selection of the appropriate method depends on the shape, size, and

composition of the workpiece; the area to be hardened; the depth of case

required; and the number of pieces to be hardened. In many instances, more than

one method will provide the desired result; the choice will then depend on

comparative costs.

The spot (stationary) method, shown in Fig. 1(a), consists of locally heating

selected areas with a suitable flame head and subsequently quenching. The

heating head may be of either single-orifice or multiple-orifice design, depending

on the extent of the area to be hardened. The heat input must be balanced to

obtain a uniform temperature over the entire selected area. After being heated, the

parts are usually immersion quenched; however, in some mechanized operations,

a spray quench may be used.

29

Fuel Gases

Several different fuel gases are used in flame hardening. In selecting a fuel gas

for a given application, the required rate of heating and the cost of the gas must

be considered, along with the initial cost of equipment and maintenance costs.

Flame hardening does not alter the composition of the base metal if done

properly. Carburizing, neutral, and oxidizing flames can be used. Oxidizing

flames have high oxygen ratios and can be detrimental because they produce

extremely hot temperatures that can cause decarburization and overheating. A

carburizing flame can prevent some decarburization but can also introduce

unwanted carbon into the surface. For best results, neutral or slightly carburizing

flames should be used.

A comparison of the heating rates of fuel gases can be made when certain

fundamental properties of usable mixtures with oxygen are known. A parameter

that correlates well with actual heating speed is combustion intensity, or specific

flame output. This is the product of the normal velocity of burning multiplied by

the net heating value of the mixture of oxygen and fuel gas.

A knowledge of these two properties often permits the selection of the most

suitable fuel gas for a specific hardening speed and depth of case. The fuels of

greatest commercial interest are ranked by combustion intensity (at

metallurgically suitable ratios of mixture with oxygen) in the following order:

Depth of Heating. Shallow hardness patterns (less than 3.2 mm, or 0.125 in.,

deep) can be attained only with oxy-gas fuels. The high-temperature flames

obtained with oxy-gas fuels provide the fast heat transfer necessary for effective

localization of the heat pattern.

Deeper hardness patterns permit the use of either oxy-gas fuels or air-gas fuels.

Oxy-gas fuels will localize the heat, but care is required in their application to

avoid overheating the surface during the development of the deeper-seated heat.

Air-gas fuels, with their slower rates of heat transfer (lower flame temperatures),

minimize or eliminate surface overheating but generally extend the heat pattern

beyond the desired hardness pattern. For this reason, air-gas flame hardening is

generally limited to steels of shallow hardenability.

In this manner, the hardness pattern is controlled by the quench rather than by

the heating. The deeper-seated heat produced by air-gas flames may preclude the

use of air-gas mixtures because excessive distortion may occur.

31

In consideration of these factors, the use of air-gas heating will depend primarily

on the shape of the part insofar as the configuration favors heat localization and a

lower rate of heat transfer.acetylene, MAPP (methylacetylene propadiene),

propane, methane.

31

Application

Parts that are subject to high pressures and sharp

impacts are still commonly case hardened.

Examples include firing pins and rifle bolt faces, or

engine camshafts. In these cases, the surfaces

requiring the hardness may be hardened selectively,

leaving the bulk of the part in its original tough

state.

Firearms were a common item case hardened in the

past, as they required precision machining best done

on low carbon alloys, yet needed the hardness and

wear resistance of a higher carbon alloy. Many modern replicas of older firearms,

particularly single action revolvers, are still made with case hardened frames, or

with case coloring, which simulates the mottled pattern left by traditional

charcoal and bone case hardening.

Another common application of case hardening is on screws, particularly self-

drilling screws. In order for the screws to be able to drill, cut and tap into other

materials like steel, the drill point and the forming threads must be harder than

the material(s) that it is drilling into. However, if the whole screw is uniformly

hard, it will become very brittle and it will break easily. This is overcome by

ensuring that only the case is hardened and the core remains relatively soft. For

screws and fasteners, case hardening is less complicated as it is achieved by

heating and quenching in the form of heat treatment

For theft prevention, lock shackles and chains are often case hardened to resist

cutting, whilst remaining less brittle inside to resist impacts. As case hardened

components are difficult to machine, they are generally shaped before hardening.

32

References

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