CHAPTER 5

Metal-Casting Processes and Equipment; Heat Treatment

(주조공정및장비; 열처리)

Casting Examples

Figure extra Typical gray-iron castings used in automobiles, including transmission valve body (left) and hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation.

Figure extra A cast transmission housing.

Die-Casting Examples

(a) (b)

Figure extra (a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast, high purity magnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die casting process. Source: Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc.

Chapter 5 Outline

-Solidification of the metal from its molten state, and accompanying shrinkage (volume change)

-Flow of the molten metal into the mold cavity

-Heat transfer during solidification and cooling of the metal in the mold

-Influence of the type of mold material

Cooling Curve

Figure 5.1 Cooling curve for the solidification of pure metals. Note that freezing takes place at a constant temperature; during freezing the latent heat of solidification is given off.

Solidification of Metals

Two-Phase System

Two-Phase System

Figure 5.2 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead-copper alloy. The grains represent lead in solid solution in copper, and the particles are lead as a second phase. (b) Schematic illustration of a two-phase system consisting of two sets of grains: dark, and light. The dark and the light grains have separate compositions and properties.

Nickel-Copper Alloy Phase Diagram

Figure 5.3 Phase diagram for nickel-copper alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each has one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals. The second circle shows the formation of dendrites. The bottom circle shows the solidified alloy, with grain boundaries.

Iron-Iron Carbide Phase Diagram

Figure 5.4 The iron-iron carbide phase diagram. Because of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams.

Austenite, Ferrite, and Martensite

Figure 5.5 The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of carbon (by weight) on the lattice dimensions for martensite is shown in (d). Note the interstitial position of the carbon atoms (see Fig. 3.8). Note, also, the increase in dimension c with increasing carbon content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism.

Iron-Carbon Alloy Above and Below Eutectoid Temperature

Figure 5.6 Schematic illustration of the microstructures for an iron-carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727 °C (1341 °F).

Extended Iron-Carbon Phase Diagram

Figure 5.7 Phase diagram for the iron-carbon system with graphite (instead of cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.8.

Cast Structures of Metals

Figure 5.8 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) structure obtained by using nucleating agents. Source: G. W. Form, J. F. Wallace, J. L. Walker, and A. Cibula.

Cast Structures

Preferred Texture Development

Figure 5.9 Development of a preferred texture at a cool mold wall. Note that only favorably oriented grains grow away from the surface of the mold.

Alloy Solidification

Figure 5.10Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone.

Solidification Patterns

Figure 5.11 (a) Solidification patterns for gray cast iron in a 180-mm (7- in.) square casting. Note that after 11 min. of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. Source: H. F. Bishop and W. S. Pellini.

↑ Solidificationrange

↓

Cast Structures

Figure 5.12Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxeddendritic; and (c) equiaxednondendritic. Source: D. Apelian.

Figure 5.13 Schematic illustration of cast structures in (a) plane front, single phase, and (b) plane front, two phase. Source: D. Apelian.

convection strong convection

Higher solute concentration

Fundamentals of Metal-Casting

Sand Mold Features

Figure 5.14 Schematic illustration of a sand mold, showing various features.

Fluid Flow and Heat Transfer

Riser-Gated Casting

Figure extra Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. See also Fig. 11.4 Source: American Foundrymen’s Society.

Fluidity Test

Figure extra A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater is its fluidity.

Temperature Distribution

Figure 5.15 Temperature distribution at the interface of the mold wall and the liquid metal during solidification of metals in casting.

Solidification Time

Figure 5.16 Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: H. F. Taylor, J. Wulff, and M. C. Flemings.

Solidification Contraction for Various Cast Metals

TABLE 5.1 Solidification for various cast metals

Metal or alloy

Volumetric solidification

contraction (%) Metal or alloy

Volumetric solidification

contraction (%) Aluminum 6.6 70%Cu–30%Zn 4.5 Al–4.5%Cu 6.3 90%Cu–10%Al 4 Al–12%Si 3.8 Gray iron Expansion to 2.5 Carbon steel 2.5–3 Magnesium 4.2 1% carbon steel 4 White iron 4–5.5 Copper 4.9 Zinc 6.5 Source: After R. A. Flinn.

Hot Tears

Figure extra Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heat-producing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing.

Casting Defects

Figure extra Examples of common defects in castings. These defects can be minimized or eliminated by proper design and preparation of molds and control of pouring procedures. Source: J. Datsko.

Internal and External Chills

Figure 5.17Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metals, as shown in (c).

Solubility of Hydrogen in Aluminum

Figure 5.18 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify.

Types of Melting Furnaces

Figure extra Two types of melting furnaces used in foundries: (a) crucible, and (b) cupola.

Melting Practice and Furnaces

Mechanical Properties for Various Groups of Cast Alloys

Figure 5.19 Mechanical properties for various groups of cast alloys. Note that gray iron has very little ductility and toughness, compared with most other cast alloys, some of which undergo considerable elongation and reduction of area in tension. Note also that even within the same group, the properties of cast alloys vary over a wide range, particularly for cast steels. Source: Steel Founders' Society of America.

Casting Alloys

Mechanical Properties for Various Groups of Cast Alloys (cont.)

Figure 5.19 Mechanical properties for various groups of cast alloys. Note that gray iron has very little ductility and toughness, compared with most other cast alloys, some of which undergo considerable elongation and reduction of area in tension. Note also that even within the same group, the properties of cast alloys vary over a wide range, particularly for cast steels. Source: Steel Founders' Society of America.

Typical Applications for Casting and Casting Characteristics

TABLE 5.2 Type of alloy Application Castability* Weldability* Machinability* Aluminum Pistons, clutch housings, intake

manifolds E F G–E

Copper Pumps, valves, gear blanks, marine propellers

F–G F F–G

Ductile iron Crankshafts, heavy-duty gears G D G Gray iron Engine blocks, gears, brake disks

and drums, machine bases E D G

Magnesium Crankcase, transmission housings G–E G E Malleable iron Farm and construction machinery,

heavy-duty bearings, railroad rolling stock

G D G

Nickel Gas turbine blades, pump and valve components for chemical plants

F F F

Steel (carbon and low alloy)

Die blocks, heavy-duty gear blanks, aircraft undercarriage members, rail-road wheels

F E F

Steel (high alloy) Gas turbine housings, pump and valve components, rock crusher jaws

F E F

White iron Mill liners, shot blasting nozzles, railroad brake shoes, crushers and pulverizers

G VP VP

Zinc Door handles, radiator grills, E D E

*E, excellent; G, good; F, fair; VP, very poor; D, difficult.

Properties and Typical Applications of Cast Irons

TABLE 5.3

Cast iron Type

Ultimate tensile

strength (MPa)

Yield strength (MPa)

Elongation in 50 mm

(%) Typical applications Gray Ferritic

Pearlitic Martensitic

170 275 550

140 240 550

0.4 0.4 0

Pipe, sanitary ware Engine blocks, machine tools Wearing surfaces

Ductile (Nodular)

Ferritic Pearlitic Tempered martensite

415 550 825

275 380 620

18 6 2

Pipe, general service Crankshafts, highly stressed parts High-strength machine parts,wear-resistant parts

Malleable Ferritic Pearlitic Tempered martensite

365

450 700

240

310 550

18

10 2

Hardware, pipe fittings, general engineering service Railroad equipment, couplings Railroad equipment, gears, connecting rods

White Pearlitic 275 275 0 Wear-resistant parts, mill rolls

VIDEO

Mechanical Properties of Gray Cast Irons

TABLE 5.4 ASTM class

Ultimate tensile

strength (MPa)

Compressive

strength (MPa)

Elastic

modulus (GPa)

Hardness (HB)

20 152 572 66 to 97 156 25 179 669 79 to 102 174 30 214 752 90 to 113 210 35 252 855 100 to 119 212 40 293 965 110 to 138 235 50 362 1130 130 to 157 262 60 431 1293 141 to 162 302

Properties and Typical Applications of Cast Nonferrous Alloys

TABLE 5.4

Alloys (UNS) Condition

Ultimate tensile

strength (MPa)

Yield strength (MPa)

Elongation in 50 mm

(%) Typical applications Aluminum alloys 195 (AO1950) 319 (AO3190) 356 (AO3560)

Heat treated Heat treated Heat treated

220–280 185–250 260

110–220 125–180 185

8.5–2 2–1.5 5

Sand castings Sand castings Permanent mold castings

Copper alloys Red brass (C83600) Yellow brass (C86400)

Annealed Annealed

235 275

115 95

25 25

Pipe fittings, gears Hardware, ornamental

Manganese bronze (C86100)

Annealed 480 195 30 Propeller hubs, blades

Leaded tin bronze (C92500)

Annealed 260 105 35 Gears, bearings, valves

Gun metal (C90500) Annealed 275 105 30 Pump parts, fittings Nickel silver (C97600) Annealed 275 175 15 Marine parts, valves Magnesium alloys AZ91A AZ63A AZ91C EZ33A HK31A QE22A

F T4 T6 T5 T6 T6

230 275 275 160 210 275

150 95 130 110 105 205

3 12 5 3 8 4

Die castings Sand and permanent mold castings High strength Elevated temperature Elevated temperature Highest strength

Microstructures for Cast Irons

(a) (b) (c)

Figure 5.20 Microstructure for cast irons. Magnification: 100X. (a) Ferritic gray iron with graphite flakes. (b) Ferritic Ductile iron (nodular iron), with graphite in nodular form. (c) Ferritic malleable iron; this cast iron solidified as white cast iron, with the carbon present as cementite, and was heat treated to graphitize the carbon. Source: ASM International.

Continuous Casting

Figure 5.21 The continuous-casting process for steel. Typically, the solidified metal descends at a speed of 25 mm/s (1 in./s). Note that the platform is about 20 m (65 ft) above ground level. Source: Metalcaster's Reference and Guide, American Foundrymen's Society.

Ingot Casting and Continuous Casting

Summary of Casting

Processes

TABLE 5.5Process Advantages LimitationsSand Almost any metal cast; no limit

to size, shape or weight; lowtooling cost.

Some finishing required;somewhat coarse finish; widetolerances.

Shell mold Good dimensional accuracy andsurface finish; high productionrate.

Part size limited; expensivepatterns and equipmentrequired.

Expendable pattern Most metals cast with no limitto size; complex shapes

Patterns have low strength andcan be costly for low quantities

Plaster mold Intricate shapes; gooddimensional accu- racy andfinish; low porosity.

Limited to nonferrous metals;limited size and volume ofproduction; mold making timerelatively long.

Ceramic mold Intricate shapes; closetolerance parts; good surfacefinish.

Limited size.

Investment Intricate shapes; excellentsurface finish and accuracy;almost any metal cast.

Part size limited; expensivepatterns, molds, and labor.

Permanent mold Good surface finish anddimensional accuracy; lowporosity; high production rate.

High mold cost; limited shapeand intricacy; not suitable forhigh-melting-point metals .

Die Excellent dimensional accuracyand surface finish; highproduction rate.

Die cost is high; part s izelimited; usually limited tononferrous metals; long leadtime.

Centrifugal Large cylindrical parts withgood quality; high productionrate.

Equipment is expensive; partshape limited.

Casting Processes

General Characteristics of Casting Processes

TABLE 1extra

Typical Weig ht (kg)Typicalsurface Section thic kness (mm)

Processmaterials

cast Minimum Maximum

finish

(µm, Ra) Porosity*Shape

complexity*Dimensionalaccuracy* Minimum Maximum

Sand All 0.05 No limit 5-25 4 1-2 3 3 No limitShell All 0.05 100+ 1-3 4 2-3 2 2 --Expendablemoldpattern All 0.05 No limit 5-20 4 1 2 2 No limit

Plastermold

Nonferrous(Al, Mg, Zn,

Cu) 0.05 50+ 1-2 3 1-2 2 1 --

Inves tment

All(High melting

pt.) 0.005 100+ 1-3 3 1 1 1 75Permanentmold All 0.5 300 2-3 2-3 3-4 1 2 50

Die

Nonferrous(Al, Mg, Zn,

Cu) <0.05 50 1-2 1-2 3-4 1 0.5 12Centrifugal All -- 5000+ 2-10 1-2 3-4 3 2 100

*Relative rating: 1 bes t, 5 wors t.Note : These ratings are only general; significant variations can occur, depending on the methods used.

Figure extra Outline of production steps in a typical sand-casting operation.

Steps in Sand Casting

Expendable Mold

VIDEO

Pattern Material Characteristics

Table extra

Ratinga

Characteristic Wood Aluminum Steel Plastic Cast ironMachinability E G F G GWear resis tance P G E F EStrength F G E G GWeightb E G P G PRepairability E P G F GResistance to:Corrosionc E E P E PSwellingc P E E E EaE, Excellent; G, good; F, fair; P, poor.bAs a factor in operator fatigue.cBy water.Source : D.C. Ekey and W.R. Winter, Introduction to Foundry Technology. New York. McGraw-Hill, 1958.

Patterns for Sand Casting

Figure 5.23 A typical metal match-plate pattern used in sand casting.

Figure extra Taper on patterns for ease of removal from the sand mold.

Examples of Sand Cores and Chaplets

Figure extra Examples of sand cores showing core prints and chaplets to support cores.

Squeeze Heads

Figure extra Various designs of squeeze heads for mold making: (a) conventional flat head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm. Source: © Institute of British Foundrymen. Used with permission.

Vertical Flaskless Molding

Figure extra Vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assembled molds pass along an assembly line for pouring.

VIDEO

Sequence of Operations for Sand Casting

Figure extra Schematic illustration of the sequence of operations for sand casting. Source: Steel Founders' Society of America. (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b-c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins, and attaching inserts to form the sprue and risers. (continued)

Figure extra (g) The flask is rammed with sand and the plate and inserts are removed. (g) The drag half is produced in a similar manner, with the pattern inserted. A bottom board is placed below the drag and aligned with pins. (i) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled and the casting is cleaned, inspected, and heat treated (when necessary).

Sequence of Operations for Sand Casting (cont.)

VIDEO: green sand

Surface Roughness for Various Metalworking Processes

Figure extra Surface roughness in casting and other metalworking processes.

Dump-Box Technique

Figure extra A common method of making shell molds. Called dump-boxtechnique, the limitations are the formation of voids in the shell and peelback (when sections of the shell fall off as the pattern is raised). Source: ASM International.

Composite Molds

Figure 5.24 (a) Schematic illustration of a semipermanent composite mold. Source: Steel Castings Handbook, 5th ed. Steel Founders' Society of America, 1980. (b) A composite mold used in casting an aluminum-alloy torque converter. This part was previously cast in an all-plaster mold. Source: Metals Handbook, vol. 5, 8th ed.

Expendable Pattern Casting

Figure extra Schematic illustration of the expendable pattern casting process, also known as lost foam or evaporative casting.

VIDEO

Ceramic Molds

Figure 5.25 Sequence of operations in making a ceramic mold. Source: Metals Handbook, vol. 5, 8th ed.

Figure extra A typical ceramic mold (Shaw process) for casting steel dies used in hot forging. Source: Metals Handbook, vol. 5, 8th ed.

Figure 5.26 Schematic illustration of investment casting, (lost-wax process). Castings by this method can be made with very fine detail and from a variety of metals. Source: Steel Founders' Society of America.

Investment Casting

VIDEO

VIDEO 2

Investment Casting of a Rotor

Figure extra Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a vacuum, with molten superalloy. (d) The cast rotor, produced to net or near-net shape. Source: Howmet Corporation.

Investment and Conventionally Cast Rotors

Figure 5.27 Cross-section and microstructure of two rotors: (top) investment-cast; (bottom) conventionally cast. Source: Advanced Materials and Processes, October 1990, p. 25 ASM International

Vacuum-Casting Process

Figure 5.28 Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source: From R. Blackburn, "Vacuum Casting Goes Commercial," Advanced Materials and Processes, February 1990, p. 18. ASM International.

Pressure Casting

Figure 5.29 (a) The bottom-pressure casting process utilizes graphite molds for the production of steel railroad wheels. Source: The Griffin Wheel Division of Amsted Industries Incorporated. (b) Gravity-pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser. Railroad wheels can also be manufactured by forging.

VIDEO

Permanent Mold

Hot- and Cold-Chamber Die-Casting

Figure 5.30 (a) Schematic illustration of the hot-chamber die-casting process. (b) Schematic illustration of the cold-chamber die-casting process. Source: Courtesy of Foundry Management and Technology.

(a) (b)

Cold-Chamber Die-Casting Machine

(a)

Figure extra (a) Schematic illustration of a cold-chamber die-casting machine. These machines are large compared to the size of the casting because large forces are required to keep the two halves of the dies closed.

VIDEO

Figure extra (b) 800-ton hot-chamber die-casting machine, DAM 8005 (made in Germany in 1998). This is the largest hot-chamber machine in the world and costs about $1.25 million.

(b)

Hot-Chamber Die-Casting Machine

VIDEO

Die-Casting Die Cavities

Figure 5.32 Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute.

Figure extra Examples of cast-in- place inserts in die casting. (a) Knurled bushings. (b) Grooved threaded rod.

VIDEO

Properties and Typical Applications of Common Die-Casting Alloys

TABLE 5.7

Alloy

Ultimatetensile

strength(MPa)

Yieldstrength(MPa)

Elongationin 50 mm

(%) ApplicationsAluminum 380 (3.5 Cu-8.5 Si) 320 160 2.5 Appliances, automotive components ,

electrical motor frames and hous ings 13 (12 Si) 300 150 2.5 Complex shapes with thin walls , parts

requiring strength at elevatedtemperatures

Brass 858 (60 Cu) 380 200 15 Plumbing fiztures , lock hardware,bushings, ornamental cas tings

Magnes ium AZ91 B (9 Al-0.7 Zn) 230 160 3 Power tools, automotive parts , sportinggoods

Zinc No. 3 (4 Al) 280 -- 10 Automotive parts, office equipment,household utens ils, building hardware,toys

5 (4 Al-1 Cu) 320 -- 7 Appliances, automotive parts, buildinghardware, bus ines s equipment

Source : Data from American Die Casting Institute

Centrifugal Casting Process

Figure 5.33 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process.

Semi-centrifugal Casting

Figure 5.34 (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal force.

Squeeze-Casting

Figure 5.35 Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.

VIDEO

Single Crystal Casting of Turbine Blades

(c)

Figure 5.36 Methods of casting turbine blades: (a) directional solidification; (b) method to produce a single-crystal blade; and (c) a single-crystal blade with the constriction portion still attached. Source: (a) and (b) B. H. Kear, Scientific American, October 1986; (c) Advanced Materials and Processes, October 1990, p. 29, ASM International.

Single Crystal Casting

Figure 5.37 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) the floating-zone method. Crystal growing is especially important in the semiconductor industry. Source: L. H. Van Vlack, Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

Melt Spinning

Figure 5.38 Schematic illustration of melt-spinning to produce thin strips of amorphous metal.

Other Casting Processes

* VIDEOs

1. Cold box molding, automatic cold box molding2. Hot box molding3. Hunter process4. Pit molding5. Gravity die casting6. Tilt casting

Optimization of Casting Processes 1, 2

Austenite to Pearlite

Transformation

Figure 5.39 (a) Austenite-to-pearlite transformation of iron-carbon alloy as a functionof time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675 °C (1247 °F). (continued)

Heat Treatment

Austenite to Pearlite Transformation (cont.)

Figure 5.39 (c) Microstructures obtained for a eutectoid iron-carbon alloy as a function of cooling rate. Source: ASM International.

Eutectoid Steel Microstructure

Figure extra Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700 °C (1292 °F). Magnification: 1000X. Source: Courtesy of USX Corporation.

Pearlite Microstructure

Figure extra Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide. Magnification: 2500X. Source: Courtesy of USX Corporation.

Aluminum-Copper Phase Diagram

Figure 5.40 (a) Phase diagram for the aluminum-copper alloy system. (b) Various micro-structures obtained during the age-hardening process. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

Martensite

(b)

Figure extra (a) Hardness of martensite, as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000X. Source: Courtesy of USX Corporation.

Hardness of Tempered Martensite

Figure extra Hardness of tempered martensite, as a function of tempering time, for 1080 steel quenched to 65 HRC. Hardness decreases because the carbide particles coalesce and grow in size, thereby increasing the interparticle distance of the softer ferrite.

End-Quench Hardenability

Test

Figure extra (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals, for better control of properties. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

Age Hardening

Figure extra The effect of aging time and temperature on the yield stress of 2014-T4 aluminum alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.

Properties of Oil-Quenched Steel

Figure extra Mechanical properties of oil-quenched 4340 steel, as a function of tempering temperature. Source: Courtesy of LTV Steel Company

Mechanical Properties of Annealed Steels

Figure extra Mechanical properties of annealed steels, as a function of composition and microstructure. Note (in (a)) the increase in hardness and strength and (in (b)) the decrease in ductility and toughness, with increasing amounts of pearlite and iron carbide. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

Hardness and Toughness of Annealed Steels

Figure extra (a) and (b) Hardness and (c) toughness for annealed plain-carbon steels, as a function of carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has spherelike carbide particles. Note htat the percentage of pearlite begins to decrease after 0.77% carbon. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

Outline of Heat Treatment Processes for Surface Hardening

TABLE 5.8 Process Metals hardened Element added to

surface Procedure General Characteristics Typical applications

Carburizing 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

Carbonitriding Low-carbon steel C and N Heat steel at 700–800 °C (1300–1600 °F) in an atmosphere of carbonaceous gas and ammonia. Then quench in oil.

Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm (0.003 to 0.020 in.). Less distortion than in carburizing.

Bolts, nuts, gears

Cyaniding 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

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 °F) in an atmosphere 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

Boronizing 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 hardening 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 hardening

Same as above None Metal part is placed in copper induction coils and is heated by high frequency current, then quenched.

Same as above Same as above

Induction Heating

Figure extra Types of coils used in induction heating of various surfaces of parts.

Heat Treatment ProcessesFigure 5.41 Heat-treating temperature ranges for plain-carbon steels, as indicated on the iron-iron carbide phase diagram. Source: ASM International.

Figure extra Hardness of steels in the quenched and normalized conditions, as a function of carbon content.

Metal Casting: Design, Materials, and Economics

Casting Design Modifications

Figure 5.42 Suggested design modifications to avoid defects in castings. Note that sharp corners are avoided to reduce stress concentrations.

Casting Cross-Sections(continued)

Figure 5.42 Examples of designs showing the importance of maintaining uniform cross- sections in castings to avoid hot spots and shrinkage cavities.

Avoiding Shrinkage Cavities

Figure extra Examples of design modifications to avoid shrinkage cavities in castings. Source: Steel Castings Handbook, 5th ed. Steel Founders' Society of America, 1980. Used with permission.

Chills

Figure extra The use of metal padding (chills) to increase the rate of cooling in thick regions in a casting to avoid shrinkage cavities. Source: Steel Castings Handbook, 5th ed. Steel Founders' Society of America, 1980. Used with permission.

Normal Shrinkage Allowance for Some Metals Cast in Sand Molds

TABLE 5.9 Metal Percent Gray cast iron White cast iron Malleable cast iron Aluminum alloys Magnesium alloys Yellow brass Phosphor bronze Aluminum bronze High-manganese steel

0.83–1.3 2.1

0.78–1.0 1.3 1.3

1.3–1.6 1.0–1.6

2.1 2.6

Parting Line

Figure extra Redesign of a casting by making the parting line straight to avoid defects. Source: Steel Casting Handbook, 5th ed. Steel Founders' Society of America, 1980. Used with permission.

Casting Design Modifications

Figure 5.43 Examples of casting design modifications. Source: Steel Casting Handbook, 5th ed. Steel Founders' Society of America, 1980. Used with permission.

Desirable and Undesirable Die-Casting Practices

Figure 5.44 Examples of undesirable and desirable design practices for die-cast parts. Note that section-thickness uniformity is maintained throughout the part. Source: American Die Casting Institute.

General Cost Characteristics of Casting Processes

TABLE 5.10 Cost*

Process Die Equipment Labor Production rate (Pc/hr)

Sand L L L–M <20 Shell-mold L–M M-H L–M <50 Plaster L–M M M–H <10 Investment M–H L-M H <1000 Permanent mold M M L–M <60 Die H H L–M <200 Centrifugal M H L–M <50 * L, low; M, medium; H, high.

Economics