ch05 casting processes

8/20/2019 Ch05 Casting Processes

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Manufacturing Processes for Engineering Materials, 5th ed.Kalpakjian • Schmid

© 2008, Pearson EducationISBN No. 0-13-227271-7

Temperature & Density for Castings



FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals. Note that freezingtakes place at a constant temperature. (b) Density as a function of time.

T e m p e r a t u r e

Time

Cooling of liquid

Cooling ofsolid

B A

LiquidLiquid

+solid

Solid

Freezing begins

Freezingends

Freezingtemperature

S p e c i f i c d e n s i t y

Time

Shrinkage of liquid

Shrinkage of solid

Solidificationshrinkage

(a) (b)





Two-Phased Alloys

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

(b)(a)





Phase Diagram for Nickel-Copper

FIGURE 5.3 Phase diagram for nickel-copper alloy system obtained by a low rate of solidification. Note that pure nickel and purecopper each have one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals; the secondcircle shows the formation of dendrites; and the bottom circle shows the solidified alloy with grain boundaries.

Solid(42% Cu-58% Ni)

Liquid(50% Cu-50% Ni)

First solid(36% Cu-64% Ni)

Liquid(58% Cu-42% Ni)

0 36 42 50 58 67 100

Solidsolution

Alloy composition

Liquid solution

Fraction liquid

0 1

S o l i d u s

L i q u i d u s L + S

Solid solution(50% Cu-50% Ni)

1981

CS CO CL

1082 1980

2280

23502395

26511455

1249

12881313

T e m p e r a t u r e ( ° F )

° C

Composition (% by weight)

Copper (Cu)

100 64 58 50 42 33 0 Nickel (Ni)





Irn-Iron Carbide Phase Diagram

FIGURE 5.4 (a) The iron-iron carbide phase diagram. (b) Detailed view of the microstructures above and below the eutectoidtemperature of 727°C (1341°F). Because of the importance of steel as an engineering material, this diagram is one of the mostimportant phase diagrams.

400

500

600

700

800

900

1000

1100

0 0.5 1.0 1.5 2.0 2.5

1000

1500

2000

727°C ° F

T e

m p e r a t u r e ( ° C )

Carbon (% by weight)

! + Fe3C

"

" + Fe3C

Fe3C

Ferrite

!!

! !

!

" + !

T

e m p e r a t u r e ( ° C )

Carbon (% by weight)

1000

1200

1400

800

600

400

1600

0 1 2 3 4 5 6 6.67

1000

1500

2500

2000

Cementite (Fe3C)

Liquid

727°C

1495°C1538°C

1394°C

912°C ° F

0.77%0.022%

4.30%2.11%

1148°C

" (ferrite)

" + cementite

! + cementite

! + liquid

# (Ferrite)

Detail view

(a) (b)

! (austenite)

"+!





Texture in Castings

FIGURE 5.5 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals,with preferred texture at the cool mold wall. Note in the middle of the figure that only favorable oriented grainsgrow away from the mold surface; (b) solid-solution alloys; and (c) structure obtained by heterogeneous nucleationof grains.

(a)

Chill zone

Equiaxed structureEquiaxed zone

(b) (c)

Columnar zone





Alloy Solidification & Temperature

FIGURE 5.6 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Notethe formation of dendrites in the semi-solid (mushy) zone.

L + ST S

T L

Liquid

Solid

Solid

Solid

Mushy zone

Dendrites

Moldwall

Liquid

Liquid


Alloying element (%)

S

L

L i q u i d u s

S o l i d

u s

Pure metal





Solidification Patterns for Gray Cast Iron

FIGURE 5.7 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxeddendritic; and (c) equiaxed nondendritic. Source: After D. Apelian.

(a)

(b)

0.05–0.10% CSteel

0.25–0.30% CSteel

Minutes after pouring

Minutes after pouring

0.55–0.60% CSteel

8 11 40 60 90 102

5 2 15 2 16 2

Sand

mold

Chill

mold

Sand

mold

Chill

mold

Sand

mold

Chill

mold





Cast Structures

FIGURE 5.9 Schematic illustration of cast structures in (a) plane front,single phase, and (b) plane front, two phase. Source: After D. Apelian.

(a) (b) (c)

SolidSolidSolid Liquid Liquid LiquidMoldwall

(a)

Solid LiquidMoldwall Liquid

(b)

FIGURE 5.8 Schematic illustration of three basictypes of cast structures: (a) columnar dendritic;(b) equiaxed dendritic; and (c) equiaxednondendritic. Source: After D. Apelian.





Mold Features

FIGURE 5.10 Schematic illustration of a typical sand mold showing various features.

Open riser

Vent

Pouring basin (cup)

Drag

Cope

Sand

Sprue

Sand

Flask

PartinglineMold

cavity WellGate

Core(sand)

Blindriser

Runner





Temperature Distribution

FIGURE 5.11 Temperature distribution at the mold wall and liquid-metal interface during solidification of metals in casting.

Roomtemperature

Distance

at mold–airinterface

at metal–moldinterface

Meltingpoint


Air Solid Liquid

!T

!T

Mold





Skin on Casting

FIGURE 5.12 Solidified skin on a steel casting; the remainingmolten metal is poured out at the times indicated in the figure.Hollow ornamental and decorative objects are made by aprocess called slush casting, which is based on this principle. Source: After H.F. Taylor, J. Wulff, and M.C. Flemings.

5 s 1 min 2 min 6 min

A

B

Chvorinov’s Rule:

Solidification time=C

VolumeSurface area

n





Shrinkage

Contraction (%) Expansion (%)

Aluminum 7.1 Bismuth 3.3Zinc 6.5 Silicon 2.9Al - 4.5% Cu 6.3 Gray iron 2.5Gold 5.5White iron 4-5.5

Copper 4.9Brass (70-30) 4.5Magnesium 4.290% Cu - 10% Al 4Carbon steels 2.5-4Al - 12% Si 3.8Lead 3.2

TABLE 5.1 Volumetric solidification contraction or expansionfor various cast metals.

300 300





Cast MaterialProperties

FIGURE 5.13 Mechanical properties for various

groups of cast alloys. Compare with varioustables of properties in Chapter 3. Source: Courtesy of Steel Founders' Society of America.

S t e e l

N o d u l a r i r o n

G r a y i r o n

M a l l e a b l e i r o n

A l u m i n u m

b a s e d

C o p p e r b a s e d

M a g n e s i u m

b a s e d

N i c k e l b a s e d

Z i n c b a s e d

U l t i m a t e t e n s i l e s t r e n g t h ( p s i x 1 0 3 )

300280

260240

220200

180160

140120100

8060

4020

0

2000

1800

2000

1800

1600

1400

1200

1000

800

600

400

200

M P a

(a)

300280

260240

220200

180160

140120100

8060

4020

0

1600

1400

1200

1000

800

600

400

200

S t e e l


G r a y i r o n


A l u m i n u m

b a s e d


M a g n e s i u m

b a s e d


Z i n c b a s e d

Y i e l d s t r e n g t h ( p s i x 1 0 3 )

(b)


G r a y i r o n


A l u m i n u m

b a s e d


M a g n e s i u m

b a s e d


Z i n c b a s e d

T i t a n i u m

m e t a l

T i t a n i u m

a l l o y s

C a s t s t e e l

0

5

10

20

25

30

15

0

50

100

200

150

G P a

M o d u l u s o f e l a s t i c i t y ( p s i x 1 0 6 )

0

10

12

8

6

4

2

Wrought

Cast

S t e e l


G r a y i r o n


A l u m i n u m

b a s e d

M a g n e s i u m

b a s e d

T i t a n i u m

m e t a l T

i t a n i u m

a l l o y

T e n s i l e s t r e n g t h / d e n s i t y r a t i o ( i n x 1 0 5 )

(d)(c)


G r a y i r o n

A l u m i n u m

b a s e d


M a g n e s i u m

b a s e d


Z i n c b a s e d

S t e e l


800

700

600

500

400

300

200

100

0

B r i n e l l h a r b n e s s ( H B )

(e)


70

60

50

40

30

20

10

0

90

80

70

60

50

40

30

20

10

0

S t e e l


G r a y i r o n I m

p a c t e n e r g y ( f t - l b ,

C h a r p y V - n o t c h )

(f)

70

60

50

40

30

20

10

0





S t e e l

G r a y

i r o n



R e d u c t i o n o f a r e a ( % )

(h)


G r a y i r o n

A l u m i n u m

b a s e d C

o p p e r

b a s e d

M a g n e s i u m

b a s e d N

i c k e l b a s e d

Z i n c b a s e d

S t e e l


70

60

50

40

30

20

10

0

E l o n g a t i o n ( % )

(g)





General Characteristics of Casting

TABLE 5.2 General characteristics of casting processes.

Evaporative PermanentSand Shell pattern Plaster Investment mold Die Centrifugal

Typical materials cast All All All Nonferrous All All Nonferrous All

(Al, Mg, (Al, Mg,Zn, Cu) Zn, Cu)

Weight (kg):minimum 0.01 0.01 0.01 0.01 0.001 0.1 < 0.01 0.01maximum No limit 100+ 100+ 50+ 100+ 300 50 5000+

Typ. surface finish (µm Ra) 5-25 1-3 5-25 1-2 0.3-2 2-6 1-2 2-10Porosity1 3-5 4-5 3-5 4-5 5 2-3 1-3 1-2Shape complexity1 1-2 2-3 1-2 1-2 1 2-3 3-4 3-4Dimensional accuracy1 3 2 3 2 1 1 1 3Section thickness (mm):

minimum: 3 2 2 1 1 2 0.5 2maximum: No limit — — — 75 50 12 100

Typ. dimensional tolerance 1.6-4 ±0.003 ±0.005−

±0.005 ±0.015 ±0.001−

±0.015(0.25 for 0.010 0.005

small)Cost1,2

Equipment 3-5 3 2-3 3-5 3-5 2 1 1Pattern/die 3-5 2-3 2-3 3-5 2-3 2 1 1Labor 1-3 3 3 1-2 1-2 3 5 5

Typical lead time2,3 Days Weeks weeks Days Weeks Weeks Weeks- Monthsmonths

Typical production rate2,3 1-20 5-50 1-20 1-10 1-1000 5-50 2-200 1-1000Minimum quantity2,3 1 100 500 10 10 1000 10,000 10-10,000

Notes:1. Relative rating, 1 best, 5 worst. For example, die casting has relatively low porosity, mid- to low shape complexity, high dimensionalaccuracy, high equipment and die costs and low labor costs. These ratings are only general; significant variations can occur dependingon the manufacturing methods used.2. Data taken from Schey, J.A., Introduction to Manufacturing Processes , 3rd ed, 2000.3. Approximate values without the use of rapid prototyping technologies.





Typical Applications & Characteristics

TABLE 5.3 Typical applications for castings and casting characteristics.

Type of Alloy Application Castability∗ Weldability∗ Machinability∗

Aluminum Pistons, clutch housings, intake mani-

folds, engine blocks, heads, cross mem-bers, valve bodies, oil pans, suspensioncomponents

G-E F* G-E

Copper Pumps, valves, gear blanks, marine pro-pellers

F-G F G-E

Gray Iron Engine blocks, gears, brake disks anddrums, machine bases

E D G

Magnesium Crankcase, transmission housings,portable computer housings, toys

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 com-ponents for chemical plants

F F F

Nodular iron Crankshafts, heavy-duty gears G D GSteel (carbonand low alloy)

Die blocks, heavy-duty gear blanks, air-craft undercarriage members, railroadwheels

F E F-G

Steel (high al-loy)

Gas turbine housings, pump and valvecomponents, rock crusher jaws

F E F

White iron

(Fe3C)

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 & Applications of Cast Iron

TABLE 5.4 Properties and typical applications of cast irons.

UltimateTensile Yield Elonga-

Cast Strength Strength tion inIron Type (MPa) (MPa) 50 mm (%) Typical ApplicationsGray Ferritic 170 140 0.4 Pipe, sanitary ware

Pearlitic 275 240 0.4 Engine blocks, machine toolsMartensitic 550 550 0 Wear surfaces

Ductile Ferritic 415 275 18 Pipe, general service(Nodular) Pearlitic 550 380 6 Crankshafts, highly stressed parts

Tempered 825 620 2 High-strength machine parts, wear

Martensite resistanceMalleable Ferritic 365 240 18 Hardware, pipe fittings, general

engineering servicePearlitic 450 310 10 CouplingsTempered 700 550 2 Gears, connecting rods

White Pearlitic 275 275 0 Wear resistance, mill rolls





Nonferrous Alloys

TABLE 5.5 Typical properties of nonferrous casting alloys.

Casting UTS Yield Strength Elongation Hardness

Alloy Condition Method

∗

(MPa) (MPa) in 50 mm (%) (HB)Aluminum357 T6 S 345 296 2.0 90380 F D 331 165 3.0 80390 F D 279 241 1.0 120

MagnesiumAZ63A T4 S, P 275 95 12 —AZ91A F D 230 150 3 —QE22A T6 S 275 205 4 —

Copper

Brass C83600 — S 255 177 30 60Bronze C86500 — S 490 193 30 98Bronze C93700 — P 240 124 20 60

ZincNo. 3 — D 283 — 10 82No. 5 — D 331 — 7 91ZA27 — P 425 365 1 115

∗ S, sand; D, die; P, permanent mold.





Microstructure for Cast Irons

FIGURE 5.14 Microstructure for cast irons. (a) ferritic gray iron with graphite flakes; (b) ferritic nodular iron, (ductile iron) with

graphite in nodular form; and (c) ferritic malleable iron. This cast iron solidified as white cast iron, with the carbon present ascementite (Fe3C), and was heat treated to graphitize the carbon.

(a) (b) (c)





Continuous-Casting

FIGURE 5.15 (a) The continuous-castingprocess for steel. Note that the platform isabout 20 m (65 ft) above ground level.

Source: American Foundrymen's Society. (b)Continuous strip casting of nonferrousmetal strip. Source: Courtesy of HazelettStrip-Casting Corp.

Electric furnace

Tundish

Argon

X-ray receiver(controls pouring rate)

X-ray transmitter

Molten metal

Solidified metal

Oil

Cooling water

Platform; 20 m (701 ft)above ground level

Air gap

Catch basin

Pinch rolls

Oxygen lance(for cutting)

Starting dummy

Tundish

Top belt (carbon steel)

High-velocitycooling water jetsBack-up rolls

Bottombelt Water gutters

Nip pulley Synchronizedpinch rolls

Tensionpulley

Edge dam blocks

Water nozzle

(a) (b)





SandCasting

FIGURE 5.16 Schematic illustration of the sequence of operations in sand casting. (a) A mechanical drawing of the part, used tocreate patterns. (b-c) Patterns mounted on plates equipped with pins for alignment. Note the presence of core prints designed tohold 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 flaskwith aligning pins, and attaching inserts to form the sprue and risers. (g) The flask is rammed with sand and the plate and insertsare removed. (h) The drag half is produced in a similar manner. (j) The core is set in place within the drag cavity. (k) The mold isclosed by placing the cope on top of the drag and securing the assembly with pins. (l) After the metal solidifies, the casting isremoved from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated(when necessary). Source: Courtesy of Steel Founders' Society of America.

Cope ready for sand

Cope after rammingwith sand and

removing pattern,sprue, and risers

Drag readyfor sand

Drag afterremoving pattern

Core halvespasted together

(e) (f) (g) (h) (i)

Sprue

Risers

Flask

Drag with coreset in place

(j)

Cope and drag assembledand ready for pouring

(k)

Cope

Drag

Closingpins

Casting as removedfrom mold; heat treated

(l)

Casting readyfor shipment

(m)

(a)

Mechanical drawing of part Core boxesCope pattern plate Drag pattern plate

(d)(b) (c)

Core prints Gate

Core prints





Shell-Molding Process

FIGURE 5.17 Schematic illustration of the shell-molding process, also called the dump-box technique.

Pattern

Coatedsand

Dump box

1. Pattern rotatedand clamped to dump box

Shell

Excesscoated sand

4. Pattern and shellremoved from dump box

Coatedsand

3. Pattern and dump boxin position for the investment

Investment

Pattern

Coated sand

2. Pattern and dumpbox rotated

Adhesive Clamps

5. Mold halves joined together

Flask

Sand ormetal beads

Shells

6. Mold placed in flaskand metal poured





Caramic Mold Manufacture

FIGURE 5.18 Sequence of operations in making a ceramic mold.

3. Burn-off2. Stripping green mold1. Pouring slurry

Flask

Greenmold

Pattern

Plate

Ceramic slurry

Pattern

Transfer bowl

Flask

Torch

Mold





Vacuum-Casting Process

FIGURE 5.19 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: After R. Blackburn.

Mold

(a) (b)

Induction furnace

Vacuum

Casting

Molten metal

Gate





Evaporative Pattern Casting

FIGURE 5.20 Schematic illustration of the expendable-pattern casting process, also known as lost-foam or evaporative-pattern casting.

1. Pattern molding

4. Compacted in sand 5. Casting

6. Shakeout

2. Cluster assembly3. Coating

Cluster

Parts





Investment Casting

FIGURE 5.21 Schematic illustration of investment casting (lost wax process). Castings by this methodcan be made with very fine detail and from a variety of metals. Source: Steel Founders' Society ofAmerica.

9. Shakeout8. Pouring7. Pattern meltout6. Completed mold

Casting

10. Pattern

Moltenmetal

Autoclaved

Molten waxor plastic

Heat Heat

1. Injection wax orplastic pattern

4. Slurry coating2. Ejectingpattern

5. Stucco coating3. Patternassembly (tree)

Waxpattern

Mold to make pattern





Rotor Microstructure

FIGURE 5.22 Microstructure of a rotor that has been investment cast (top) andconventionally cast (bottom). Source: Advanced Materials and Processes, October 1990, p. 25.ASM International.





Pressure & Hot-Chamber Die Casting

FIGURE 5.23 The pressure casting process,

utilizing graphite molds for the productionof steel railroad wheels. Source: GriffinWheel Division of Amsted IndustriesIncorporated.

Airtightchamber

Ladle Refractory tube

Molten metal

Air pressure

Railroad wheelGraphite mold

FIGURE 5.24 Schematic illustration of the hot-chamber

die-casting process.

Gooseneck

Nozzle

Die cavity

Hydraulic shotcylinder

Plunger rod

Plunger

Molten metal

Pot

Ejector die

Cover die

Furnace





Cold-Chamber Die Casting

FIGURE 5.25 Schematic illustration of the cold-chamber die-casting process. These machines are largecompared to the size of the casting, because highforces are required to keep the two halves of the dieclosed under pressure.

Shotcylinder

Metalsleeve

Coverdisc

Closingcylinder

Ejectorbox

Ejector platen(Moves)

Ejectordie half

Hydrauliccylinder

Shotsleeve

Ejector box

Ladle

Stationarydie half

Plungerrod

Stationary platenCavity

Pouring hole

Plunger

Plunger rod





Properties of Die-Casting AlloysUltimate Elonga-

Tensile Yield tion

Strength Strength in 50 mmAlloy (MPa) (MPa) (%) ApplicationsAluminum 380 320 160 2.5 Appliances, automotive

(3.5 Cu-8.5 Si) components, electrical motorframes and housings, engineblocks.

Aluminum 13 300 150 2.5 Complex shapes with thin(12 Si) walls, parts requiring strength at

elevated temperatures

Brass 858 (60 Cu) 380 200 15 Plumbing fixtures, lock hard-ware, bushings, ornamental cast-ings

Magnesium 230 160 3 Power tools, automotiveAZ91B (9 Al - 0.7 Zn) parts, sporting goods

Zinc No. 3 (4 Al) 280 — 10 Automotive parts, office equip-ment, household utensils, build-ing hardware, toys

Zinc No. 5 (4 Al - 1 Cu) 320 — 7 Appliances, automotive parts,

building hardware, businessequipment

Source: The North American Die Casting Association

TABLE 5.6 Properties and typical applications of common die-casting alloys.





Centrifugal Casting

FIGURE 5.26 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and

similarly shaped hollow parts can be cast by this process.

Free rollerDrive roller

Mold

(a) (b)

Drive shaft

Spout

Rollers

Ladle

Molten metalMold





Semicentrifugal Casting

FIGURE 5.27 (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 themachine, and the molten metal is forced into the molds by centrifugal forces.

(a) (b)

Mold

Molten metal

CastingFlasks

Holdingfixture

Drag

Revolvingtable

CastingPouring basinand gate

Cope





Squeeze-Casting

FIGURE 5.28 Sequence of operations in the squeeze-casting process. This process combines theadvantages of casting and forging.

1. Melt metal 2. Pour moltenmetal into die

3. Close die andapply pressure

4. Eject squeeze casting,charge melt stock,

repeat cycle

Die

Ejectorpin

Finishedcasting

Cavity





Turbine Blade Casting

FIGURE 5.29 Methods of casting turbine blades: (a) directional solidification; (b) method to produce asingle-crystal blade; and (c) a single-crystal blade with the constriction portion still attached. Source: (a)and (b) After B.H. Kear, (c) Courtesy of ASM International.

(c)(a) (b)

Radiantheat

Columnarcrystals

Constriction

Chill plate

Columnarcrystals

Heatbaffles

Radiantheat

Chill plate





Crystal Growing

FIGURE 5.30 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) floating-zonemethod. Crystal growing is especially important in the semiconductor industry. (c) A single-crystal silicon ingotproduced by the Czochralski process. Source: Courtesy of Intel Corp.

(c)(a) (b)

~1 rev/s 1 0 µ m / s

Liquid

Seed

2 0 µ m / s

Inductioncoil

S i n g l e

c r y s t a l

P o l y c r y s t a l l i n e

f e e d





Melt-Spinning Process

FIGURE 5.31 (a) Schematic illustration of the melt-spinning process to produce thin strips of

amorphous metal. (b) Photograph of nickel-alloy production through melt-spinning. Source: Courtesyof Siemens AG.

(b)(a)

Crucible

Induction coil

Melt

Strip

Gas

Copper disk





Austenite-Pearlite Transformation

FIGURE 5.32 (a) Austenite to pearlitetransformation of iron-carbon alloys as a functionof time and temperature. (b) Isothermal

transformation diagram obtained from (a) for atransformation temperature of 675°C (1247°F). (c)Microstructures obtained for a eutectoid iron-carbon alloy as a function of cooling rate. Source: Courtest of ASM International.

(a)

25

75

50

0

100

75

25

50

100

0

A u s t e n i t e ( % )

P e a r l i t e ( % )

Time (s)

600°C 650° 675°

1 10 102 103

(b) (c)

50

0

100

P e r c e n t o f a u s t e n i t e

t r a n s f o r m e d t o p e a r l i t e

T e m p e r a t u r e ( ° C )

A u s t e n i t e ( % )

° F

50

100

0

600

400

500

700

800

1000

1200

1400

Time (s)

50% Completion curve

Pearlite

Completion curve(~100% pearlite)

Eutectoidtemperature

Austenite

(unstable)

Begin curve(~0% pearlite)

Transformationtemperature675°C

Transformationbegins

1 10 103 104 105

Transformationends

1 10 102 103 104 105

Austenite (stable)

102

Time (s)

T e m p e r a

t u r e ( ° C )

200

100

0

200

400

600

800

1000

1200

1400

300

400

5003 5 ° C

/ s 1 4 0

° C / s

600

700

800

Eutectoid temperature

M (start)

Criticalcooling

rate

Time (s)

1 10 10210-1 103 104 105

° F

PearliteMartensite

Martensite+ pearlite

Austenite pearlite





Phase Diagram for Aluminum-Copper

FIGURE 5.33 (a) Phase diagram for the aluminum-copper alloy system. (b) Variousmicrostructures obtained during the age-hardening process.

X


X —solid solution

XA—quenched, solid solution retained

AB—age-hardened, precipitation starts

(submicroscopic)

AC—over-aging, precipitateagglomerates

100 95 90 Aluminum (Al)

0 5 10 Copper (Cu)

70

400

900

1100

Liquid

+ liquid

20

200

500

600

700

(b)(a)

1300

Composition (% by weight)


° F

A B C

Time

+ !





Outline of Heat Treating

TABLE 5.7 Outline of heattreatment processes forsurface hardening.

ElementMetals added to General Typical

Process hardened surface Procedure characteristics applicationsCarburizing Low-carbon

steel (0.2%C), alloy steels(0.08-0.2% C)

C Heat steel at 870-950◦

(1600-1750◦F) in an at-mosphere of carboaceousgases (gas carburizing) orcarbon-containing solids(pack carburizing). Thenquench.

A hard, high-carbon surfaceis produced. Hardness 55-65 HRC. Case depth <0.5-1.5 mm (<0.020 to 0.060 in.).Some distortion of part dur-ing heat treatment.

Gears, cams,shafts, bearings,piston pins,sprockets, clutchplates

Carbonitriding Low-carbonsteel

C a nd N Hea t s teel a t 7 00 -8 00◦C(1300-1600◦F) in an atmo-sphere of carbonaceous gasand ammonia. Then quenchin oil.

Surface hardness 55-62 HRC.Case depth 0.07-0.5 mm(0.003-0.020 in.). Less distor-tion than in carburizing.

Bolts, nuts,gears.

Cya ni di ng Low-car bo nsteel (0.2%C), alloy steels

(0.08-0.2% C)

C a nd N Hea t s teel a t 7 60 -8 45◦C(1400-1550◦F) in a moltenbath of solutions of cyanide

(e.g., 30% sodium cyanide)and other salts.

Surface hardness up to 65HRC. Case depth 0.025-0.25mm (0.001-0.010 in.). Some

distortion.

Bolts, nuts,screws, smallgears.

Nitriding Steels (1% Al,1.5% Cr, 0.3%Mo), alloy steels(Cr, Mo), stain-less steels, high-speed steels

N Heat steel at 500-600◦C (925-1100◦F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No fur-ther treatment.

Surface hardness up to 1100HV. Case depth 0.1-0.6 mm(0.005-0.030 in.) and 0.02-0.07 mm (0.001-0.003 in.) forhigh speed steel.

Geards, shafts,sprockets,valves, cutters,boring bars

Boronizing Steels B Part is heated using boron-containing gas or solid in con-tact with part.

Extremely hard and wear-resistance surface. Cased ep th 0 .0 25 -0 .0 75 m m(0.001-0.003 in.).

To ol a nd d iesteels.

Flame

hardening

Medium-carbon

steels, cast irons

None Surface is heated with an

oxyacetylene torch, thenquenched with water sprayor other quenching methods.

Surface hardness 50-60 HRC.

Case depth 0.7-6 mm (0.030-0.25 in.). Little distortion.

Axles,

crankshafts,piston ro ds,lathe beds, andcenters.

Inductionhardening

Same as above None Metal part is placed in cop-per induction coils and isheated by high frequency cur-rent, then quenched

Same as above Same as above





Heat Treatment Temperature Ranges

FIGURE 5.34 Temperature ranges for heat treating plain-carbon steels, as indicated

on the iron-iron carbide phase diagram.

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6


600

700

800

900

1000

1200

1400

1600

1800

Normalizing

Fullannealing

Acm

Spheroidizing

738°C

A3

A1

Composition (% C)

° F





Casting Processes Comparison

TABLE 5.8 Casting Processes, and their Advantages and Limitations.

Process Advantages Limitations

Sand Almost any metal is cast; no limit to

size, shape or weight; low tooling cost.

Some finishing required; somewhat

coarse finish; wide tolerances.Shell mold Good dimensional accuracy and sur-

face finish; high production rate.

Part size limited; expensive patterns

and equipment required.

Expendable pattern Most metals cast with no limit to size;

complex shapes

Patterns have low strength and can

be costly for low quantities.

Plaster mold Intricate shapes; good dimensional

accuracy and finish; low porosity.

Limited to nonferrous metals; limited

size and volume of production; mold

making time relatively long.

Ceramic mold Intricate shapes; close tolerance

parts; good surface finish.

Limited size.

Investment Intricate shapes; excellent surface fin-

ish and accuracy; almost any metal

cast.

Part size limited; expensive patterns,

molds, and labor.

Permanent mold Good surface finish and dimensional

accuracy; low porosity; high produc-

tion rate.

High mold cost; limited shape and in-

tricacy; not suitable for high-melting-

point metals.

Die Excellent dimensional accuracy and

surface finish; high production rate.

Die cost is high; part size limited; usu-

ally limited to nonferrous metals; long

lead time.

Centrifugal Large cylindrical parts with go od

quality; high production rate.

Equipment is expensive; part shape

limited.





Chills

FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings toeliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, asshown in (c).

(a)

(b)

(c)

Porosity Chill

Casting

Boss

Chill

Sand

Casting Chill

Sand





Hydrogen Solubility in Aluminum

FIGURE 5.36 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify.

H y d r o g e n

s o l u b i l i t y

Fusion

S o l i d

L i q u i d

Melting point





Elimination of Porosity in Castings

FIGURE 5.37 (a) Suggested design modifications to avoid defects in castings. Note that sharp corners are

avoided to reduce stress concentrations; (b, c, d) examples of designs showing the importance of maintaininguniform cross-sections in castings to avoid hot spots and shrinkage cavities.

(a)

Poor

Good

(b) (c) (d)

Shrinkagecavity

Poor Poor

Good Good





Design Modifications

FIGURE 5.38 Suggesteddesign modifications to avoiddefects in castings. Source:

Courtesy of The NorthAmerican Die CastingAssociation.

Use radii or fillets to avoid cornersand provide uniform cross-section.

Wall sections should be uniform.

Sloping bosses can be designed forstraight die parting to simplify die design.

Ribs and/or fillets improve bosses.

Side cores can be eliminatedwith this hole design.

Deep cavities should be on oneside of the casting where possible.

Poor Good

Poor Good

Poor GoodPoor Good

Poor Good

Poor Good

Core incover half

Core inejector half





Economics of Casting

FIGURE 5.39 Economic comparison of making a part by two different casting processes. Note that because of the high cost ofequipment, die casting is economical mainly for large production runs. Source: The North American Die Casting Association.

C o s t p e r p i e c e ( r e l a t i v e )

8

7

6

5

4

3

2

1

0100 101 102 103 104 105 106

Number of pieces

Die cast

Sand cast

Permanent-moldcasting

Plaster cast




Lost-Foam Casting of Engine Blocks

FIGURE 5.40 (a) An engine block for a 60-hp 3-cylinder marine engine, produced by the lost-foam castingprocess; (b) a robot pouring molten aluminum into a flask containing a polystyrene pattern. In the pressurizedlost-foam process, the flask is then pressurized to 150 psi (1000 kPa). Source: Courtesy of Mercury Marine

(a) (b)

ch05 casting processes

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