Temperature Density for Castings

  Temperature & Density for Castings Cooling of liquid

  Shrinkage of solid Freezing begins Freezing ends

  B A Solidification Freezing shrinkage temperature emperature T

  Cooling of

Specific density

  solid Liquid Liquid Solid

• Shrinkage of liquid solid

  Time Time (a)

  (b)

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

  

Two-Phased Alloys

FIGURE 5.2 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as lead-copper alloy. The grains represent lead in solid solution of 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. Dark and light grains have their own compositions and properties.

  

(b) (a)

  

Phase Diagram for Nickel-Copper

Alloy composition First solid

  2651 Liquid solution (36% Cu-64% Ni) 1455 Liquid

  Liquidus (50% Cu-50% Ni)

  Solidus L + S

  Solid (42% Cu-58% Ni) 1313 2395 2350 1288

  Liquid °C

  (58% Cu-42% Ni) 1249 2280 emperature (°F) T Solid

  Solid solution solution (50% Cu-50% Ni) 1082 1980

  1981 67 100 36 42 50 58 Copper (Cu) 100 64 58 50 42

  33 Nickel (Ni) Composition (% by weight) _{S O L} C C C

FIGURE 5.3 Phase diagram for nickel-copper alloy system obtained by a low rate of solidification. Note that pure nickel and pure copper each have one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals; the second

  circle shows the formation of dendrites; and the bottom circle shows the solidified alloy with grain boundaries.

Irn-Iron Carbide Phase Diagram

  1100 1600 2000 1538°C

  (Ferrite) # 1495°C

  1000 Liquid ! !

  1400 !

  2500 ! !

• 1394°C ! liquid

  900 ! Fe C +

  3 1200 1148°C

  ! (austenite) 2.11% 4.30% 2000

  1500 800 1000

• " !

  °F °F 727°C

• cementite ! 912°C

  700 "+! Detail view 1500 emperature (°C) 800

  " 727°C emperature (°C) T T

  Ferrite 600 0.77% " + cementite

  0.022% Fe C

  600

  3 1000 1000

  (ferrite) " 500 Cementite (Fe

  C)

  3 " + Fe C

  3 400

  1

  2

  3

  4 5 6 6.67 400

  0.5

  1.0

  1.5

  2.0

  2.5 Carbon (% by weight) Carbon (% by weight) (a)

  (b)

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

  important phase diagrams.

  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 grains

  grow away from the mold surface; (b) solid-solution alloys; and (c) structure obtained by heterogeneous nucleation of grains.

  (a) Chill zone Equiaxed structure Equiaxed zone

  (b) (c) Columnar zone

  

Alloy Solidification & Temperature

Liquid

  T L Liqu idu

  L

• S s

  T S S Solid oli emperature du

  S T s

  L Solid Liquid Alloying element (%)

  Pure metal Mushy zone Mold wall Solid

  Liquid Dendrites

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

Solidification Patterns for Gray Cast Iron

  8

  11

  40

  60 90 102

Minutes after pouring

(a)

0.05–0.10% C 0.25–0.30% C 0.55–0.60% C

  

Steel Steel Steel

Sand Chill Sand Chill Sand Chill

mold mold mold mold mold mold

  5

  2

  15

  2

  16

  2 Minutes after pouring

(b)

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

  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) Solid Solid Solid Liquid

  Liquid Liquid Mold wall

  (a) Solid Liquid Mold wall

FIGURE 5.8 Schematic illustration of three basic types of cast structures: (a) columnar dendritic;

  (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: After D. Apelian.

Liquid (b)

Mold Features

  Open riser Pouring basin (cup) Vent

  Flask Blind Cope

  Sprue riser Core

  Sand (sand)

  Drag Parting line

  Gate Mold Well cavity

  Sand Runner

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

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

  Room temperature Distance at mold–air interface at metal–mold interface Melting point

  T emperature Air Solid Liquid

  ! T

  ! T

  Mold Skin on Casting B A

  Chvorinov’s Rule: n

  ! " Volume 5 s 1 min 2 min 6 min

  Solidification time = C Surface area

FIGURE 5.12 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: After H.F. Taylor, J. Wulff, and M.C. Flemings.

Shrinkage

  Contraction (%) Expansion (%) Aluminum

  7.1 Bismuth

  3.3 Zinc

  6.5 Silicon

  2.9 Al - 4.5% Cu

  6.3 Gray iron

  2.5 Gold

  5.5 White iron 4-5.5 Copper

  4.9 Brass (70-30)

  4.5 Magnesium

  4.2 90% Cu - 10% Al

  4 Carbon steels 2.5-4 Al - 12% Si

  3.8 Lead

  3.2 TABLE 5.1 Volumetric solidification contraction or expansion for various cast metals. Man ufacturing Pr ocesses f or Engineering Materials, 5th ed. Kalpakjian •  Schmid

  © 2008, P earson Education

  Cour tesy of Steel F ounders' Society of America.

  Cast Material Pr oper ties

  FIGURE

  5.13 Mechanical pr oper ties for various gr oups of cast allo ys.

  ISBN No . 0-13-227271-7

Compar e

  Steel Nodular iron

  Malleable iron

  Magnesium based Steel

  Malleable iron 100 200 300 400 500 600 800 700

  Brinell harbness (HB) (e)

  Nodular iron Gray iron

  Aluminum based Copper based

  Magnesium based Steel

  70

  Nodular iron Gray iron

  60

  50

  40

  30

  20

  (g)

  Aluminum based Copper based

  15 Modulus of elasticity (psi x 10 ⁶ ) (c)

  Gray iron Malleable iron

  20 (a)

  Aluminum based Copper based

  Magnesium based Nickel based

  Ultimate tensile strength (psi x 10 ³ ) 100 120 140 160 180 200 220 240 260 300 280

  80

  60

  40

  Nodular iron Gray iron

  30

  Aluminum based Copper based Magnesium based

  Nickel based Zinc based Titanium metal

  Titanium alloys

  5

  with various tables of pr oper ties in Cha pter

  20

  25

  10

General Characteristics of Casting

  Evaporative Permanent Sand 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

  0.01 maximum No limit 100+ 100+ 50+ 100+ 300 50 5000+

Typ. surface finish (µm R ) 5-25 1-3 5-25 1-2 0.3-2 2-6 1-2 2-10

_{1 a} Porosity 3-5 4-5 3-5 4-5 ₁

  5 2-3 1-3 1-2 Shape complexity 1-2 2-3 1-2 1-2 ₁ 1 2-3 3-4 3-4 Dimensional accuracy

  3

  2

  3

  2

  1

  1

  1

  3 Section thickness (mm): minimum:

  3

  2

  2

  1

  1

  2

  0.5

  2 maximum: 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) 1,2

  Cost Equipment 3-5 3 2-3 3-5 3-5

  2

  1

  1 Pattern/die 3-5 2-3 2-3 3-5 2-3

  2

  1

  1 Labor 1-3

  3 3 1-2 1-2

  3

  5

  5 2,3

  

Typical lead time Days Weeks weeks Days Weeks Weeks Weeks- Months

months 2,3

  

Typical production rate 1-20 5-50 1-20 1-10 1-1000 5-50 2-200 1-1000

2,3

  Minimum quantity 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 dimensional

accuracy, high equipment and die costs and low labor costs. These ratings are only general; significant variations can occur depending

on 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.

TABLE 5.2 General characteristics of casting processes.

  Typical Applications & Characteristics ∗ ∗ ∗

  Type of Alloy Application Castability Weldability Machinability Aluminum Pistons, clutch housings, intake mani- G-E F* G-E folds, engine blocks, heads, cross mem- bers, valve bodies, oil pans, suspension components Copper Pumps, valves, gear blanks, marine pro- F-G F G-E pellers Gray Iron Engine blocks, gears, brake disks and E D G drums, machine bases Magnesium Crankcase, transmission housings, G-E G E portable computer housings, toys Malleable iron Farm and construction machinery, heavy- G D G duty bearings, railroad rolling stock Nickel Gas turbine blades, pump and valve com- F F F ponents for chemical plants Nodular iron Crankshafts, heavy-duty gears G D G

  Steel (carbon Die blocks, heavy-duty gear blanks, air- F E F-G and low alloy) craft undercarriage members, railroad wheels Steel (high al- Gas turbine housings, pump and valve F E F loy) components, rock crusher jaws White iron Mill liners, shot blasting nozzles, railroad G

  VP

  VP

  3 (Fe

  C) brake shoes, crushers and pulverizers Zinc Door handles, radiator grills E D E ∗ E, excellent; G, good; F, fair; VP, very poor; D, difficult.

TABLE 5.3 Typical applications for castings and casting characteristics.

Properties & Applications of Cast Iron TABLE 5.4 Properties and typical applications of cast irons

  Ultimate Tensile Yield Elonga- Cast Strength Strength tion in Iron Type (MPa) (MPa) 50 mm (%) Typical Applications Gray Ferritic 170 140

  0.4 Pipe, sanitary ware Pearlitic 275 240

  0.4 Engine blocks, machine tools Martensitic 550 550 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 resistance Malleable Ferritic 365 240

  18 Hardware, pipe fittings, general engineering service Pearlitic 450 310

  10 Couplings Tempered 700 550

  2 Gears, connecting rods White Pearlitic 275 275 Wear resistance, mill rolls Nonferrous Alloys TABLE 5.5 Typical properties of nonferrous casting alloys.

  Casting UTS Yield Strength Elongation Hardness Alloy Condition Method ∗

  60 Bronze C86500 — S 490 193

  7

  82 No. 5 — D 331 —

  10

  60 Zinc No. 3 — D 283 —

  20

  98 Bronze C93700 — P 240 124

  30

  30

  (MPa) (MPa) in 50 mm (%) (HB) Aluminum 357 T6 S 345 296

  Brass C83600 — S 255 177

  95 12 — AZ91A F D 230 150 3 — QE22A T6 S 275 205 4 — Copper

  AZ63A T4 S, P 275

  80 390 F D 279 241 1.0 120 Magnesium

  3.0

  90 380 F D 331 165

  2.0

  91 ZA27 — P 425 365 1 115 ∗ S, sand; D, die; P, permanent mold.

  

Microstructure for Cast Irons

(a)

  

(b)

(c)

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 as

  3

  cementite (Fe C), and was heat treated to graphitize the carbon.

  Continuous-Casting Electric furnace Tundish

Platform; 20 m (701 ft) above ground level

  Oil Argon Cooling water X-ray receiver

  X-ray transmitter (controls pouring rate) Molten metal Solidified metal

  Top belt (carbon steel) Air gap High-velocity Tundish Back-up rolls cooling water jets

  Catch basin

FIGURE 5.15 (a) The continuous-casting process for steel. Note that the platform is

  Tension pulley Pinch rolls about 20 m (65 ft) above ground level.

  Source: American Foundrymen's Society. (b) Nip pulley Synchronized Continuous strip casting of nonferrous pinch rolls Water nozzle

  metal strip. Source: Courtesy of Hazelett

Bottom Strip-Casting Corp

  Oxygen lance Edge dam blocks belt Water gutters Starting dummy (for cutting)

  (a) (b)

  Core prints Core prints Gate

  Sand Mechanical drawing of part Cope pattern plate Drag pattern plate Core boxes (a) (b) (c) (d)

Casting

  Sprue Risers Flask Cope after ramming with sand and Core halves removing pattern, Drag ready Drag after pasted together Cope ready for sand sprue, and risers for sand removing pattern (e) (f) (g) (h) (i)

  Cope Drag Closing pins

  Drag with core Cope and drag assembled Casting as removed Casting ready set in place and ready for pouring from mold; heat treated for shipment (j) (k) (l) (m)

FIGURE 5.16 Schematic illustration of the sequence of operations in sand casting. (a) A mechanical drawing of the part, used to create patterns. (b-c) Patterns 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. (g) The flask is rammed with sand and the plate and inserts are 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 is closed by placing the cope on top of the drag and securing the assembly with pins. (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). Source: Courtesy of Steel Founders' Society of America.

Shell-Molding Process

  Pattern Coated sand Coated sand

  Investment Pattern Dump box Coated sand

  1. Pattern rotated

  2. Pattern and dump

  3. Pattern and dump box box rotated and clamped to dump box in position for the investment Flask

  Shell Shells Excess

  Sand or coated sand Adhesive Clamps metal beads

  4. Pattern and shell

  5. Mold halves joined together

  6. Mold placed in flask removed from dump box and metal poured FIGURE 5.17 Schematic illustration of the shell-molding process, also called the dump-box technique.

Caramic Mold Manufacture

  Transfer bowl Green mold

  Torch Ceramic slurry Pattern Pattern Plate Flask

  Flask Mold

  1. Pouring slurry

  2. Stripping green mold

  3. Burn-off FIGURE 5.18 Sequence of operations in making a ceramic mold. Vacuum-Casting Process Vacuum Mold Casting Gate Molten metal

  Induction furnace (a) (b) 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.

Evaporative Pattern Casting

  1. Pattern molding

  2. Cluster assembly

  3. Coating Cluster Parts

  4. Compacted in sand

  5. Casting

  6. Shakeout

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

Investment Casting

  Mold to make pattern Wax pattern

  1. Injection wax or

  2. Ejecting

  3. Pattern

  4. Slurry coating

  5. Stucco coating plastic pattern pattern assembly (tree) Autoclaved Molten metal Heat

  Heat Casting Molten wax or plastic

  6. Completed mold

  7. Pattern meltout

  8. Pouring

  9. Shakeout

  10. Pattern

FIGURE 5.21 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.

Rotor Microstructure

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

  ASM International.

  

Pressure & Hot-Chamber Die Casting

Hydraulic shot

  Railroad wheel cylinder Nozzle

  Graphite mold Plunger rod Gooseneck

  Die cavity Plunger

  Air pressure Ejector die

  Molten metal Pot Cover die

  Airtight chamber Molten metal

  Furnace

Refractory tube Ladle

FIGURE 5.23 The pressure casting process,FIGURE 5.24 Schematic illustration of the hot-chamber utilizing graphite molds for the production

  die-casting process. of steel railroad wheels. Source: Griffin Wheel Division of Amsted Industries Incorporated.

Cold-Chamber Die Casting

  Cavity Stationary platen Ejector platen Hydraulic (Moves)

  Ladle cylinder Ejector die half Ejector box

  

Stationary Shot Plunger

die half sleeve rod Pouring hole Plunger Plunger rod

FIGURE 5.25 Schematic illustration of the cold- chamber die-casting process. These machines are large

  compared to the size of the casting, because high forces are required to keep the two halves of the die closed under pressure.

  Closing Ejector Cover Metal Shot cylinder box disc sleeve cylinder

Properties of Die-Casting Alloys

  Ultimate Elonga- Tensile Yield tion Strength Strength in 50 mm Alloy (MPa) (MPa) (%) Applications

  Aluminum 380 320 160

  2.5 Appliances, automotive (3.5 Cu-8.5 Si) components, electrical motor frames and housings, engine blocks. 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, automotive AZ91B (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, business equipment Source:

  The North American Die Casting Association TABLE 5.6 Properties and typical applications of common die-casting alloys.

Centrifugal Casting

  Mold Molten metal Mold

  Ladle Drive roller Free roller Spout Drive shaft

  Rollers (a) (b)

FIGURE 5.26 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped hollow parts can be cast by this process.

Semicentrifugal Casting

  Casting Pouring basin and gate

  Molten metal Cope Mold Casting Flasks

  Drag Holding fixture

  Revolving table (a)

  (b)

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 the

  machine, and the molten metal is forced into the molds by centrifugal forces.

Squeeze-Casting

  Die Finished casting

  Cavity Ejector pin

  3. Close die and

  4. Eject squeeze casting,

  1. Melt metal metal into die apply pressure charge melt stock, repeat cycle

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

Turbine Blade Casting

  Radiant heat Heat baffles

  Radiant heat Columnar Columnar

  Constriction crystals crystals

  Chill plate Chill plate (a)

  (b) (c)

FIGURE 5.29 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) After B.H. Kear, (c) Courtesy of ASM International.

  

Crystal Growing

m/s µ feed m/s

  µ

  10 ~1 rev/s

  Polycrystalline

  Seed coil Liquid

  Single crystal (c) (a)

  (b)

FIGURE 5.30 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) floating-zone method. Crystal growing is especially important in the semiconductor industry. (c) A single-crystal silicon ingot

  produced by the Czochralski process. Source: Courtesy of Intel Corp.

Melt-Spinning Process

  Gas Crucible Induction coil Melt Strip Copper disk (a)

  (b)

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: Courtesy

  of Siemens AG.

Austenite-Pearlite Transformation

  3

FIGURE 5.32 (a) Austenite to pearlite transformation of iron-carbon alloys as a function

  Eutectoid temperature Austenite (unstable)

  M (start) Critical cooling rate

  Transformation

begins

  

1

  10

  10

  50% Completion curve Pearlite Completion curve (~100% pearlite)

  °F Pearlite Martensite

  4

  10

  5 Transformation ends

  Austenite (stable)

  10

  2 Time (s) T emperature ( °C)

  200 100 200 400 600 800 1000 1200 1400

  300 400 500

  10

  Austenite (%) °F

  50 100 600 400 500 700

  50 100 Austenite (%) Pearlite (%)

  of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675°C (1247°F). (c) Microstructures obtained for a eutectoid iron- carbon alloy as a function of cooling rate. Source:

  (a)

  25

  75

  50 100

  75

  25

  Time (s) 600°C

  600 700 800 Eutectoid temperature

  650° 675°

  1

  10

  10

  2

  10

  3

  50 100 Percent of austenite transformed to pearlite

  35°C/s 140 °C/s

Courtest of ASM International

  Begin curve (~0% pearlite) Transformation temperature

675°C

  800 1000 1200 1400

  Martensite

  T emperature ( °C)

• pearlite Austenite pearlite
• liquid

• !

  90 Aluminum (Al)

  T emperature (°C) °F

  (b) (a) 1300 Composition (% by weight)

  20 200 500 600 700

  Liquid

  70 400 900 1100

  10 Copper (Cu)

  5

  95

  Phase Diagram for Aluminum-Copper

  100

  AC —over-aging, precipitate agglomerates

   AB —age-hardened, precipitation starts (submicroscopic)

   XA —quenched, solid solution retained

   X —solid solution

  X T emperature

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

  A B C Time ฀

  

Outline of Heat Treating

  Bolts, nuts, screws, small gears.

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

  Axles, crankshafts, piston rods, lathe beds, and centers. Induction hardening

  Surface hardness 50-60 HRC. Case depth 0.7-6 mm (0.030- 0.25 in.). Little distortion.

  None Surface is heated with an oxyacetylene torch, then quenched with water spray or other quenching methods.

  Flame hardening Medium-carbon steels, cast irons

  Tool and die steels.

  Extremely hard and wear- resistance surface. Case depth 0.025-0.075 mm (0.001-0.003 in.).

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

  Boronizing Steels B

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

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

  F) in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No fur- ther treatment.

  N Heat steel at 500-600 ^◦ C (925- 1100 ^◦

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

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

TABLE 5.7 Outline of heat treatment processes for

  F) in a molten bath of solutions of cyanide (e.g., 30% sodium cyanide) and other salts.

  C and N Heat steel at 760-845 ^◦ C (1400-1550 ^◦

  Cyaniding Low-carbon steel (0.2% C), alloy steels (0.08-0.2% C)

  Bolts, nuts, gears.

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

tion than in carburizing.

  F) in an atmo- sphere of carbonaceous gas and ammonia. Then quench in oil.

  Gears, cams, shafts, bearings, piston pins, sprockets, clutch plates Carbonitriding Low-carbon steel C and N Heat steel at 700-800 ^◦ C (1300-1600 ^◦

  Some distortion of part dur- ing heat treatment.

  A hard, high-carbon surface is produced. Hardness 55-

  F) in an at- mosphere of carboaceous gases (gas carburizing) or carbon-containing solids (pack carburizing). Then quench.

  C Heat steel at 870-950 ^◦ (1600-1750 ^◦

  Carburizing Low-carbon steel (0.2% C), alloy steels (0.08-0.2% C)

  Element Metals added to General Typical Process hardened surface Procedure characteristics applications

  surface hardening.

  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

  0.2

  0.4

  0.6

  0.8

  1.0

  1.2

  1.4

  1.6 T emperature ( °C )

  600 700 800 900

  1000 1200 1400 1600 1800

  

Normalizing

Full annealing

  A cm

  Spheroidizing 738°C A

  3 A

  1 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 good quality; high production rate.

  Equipment is expensive; part shape limited.

Chills

  (a) Sand Chill Casting Sand (b)

  Chill Porosity (c) Casting

Boss Chill

FIGURE 5.35 Various 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 metal, as

  shown in (c).

  

Hydrogen Solubility in Aluminum

Liquid

  Fusion Hydrogen solubility Solid

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

Elimination of Porosity in Castings

  Shrinkage cavity Poor

  Poor Poor Good

Good Good (a) (b) (c) (d)

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 maintaining

  uniform cross-sections in castings to avoid hot spots and shrinkage cavities.

Design Modifications

  Poor Good Poor Good Core in Core in cover half ejector half

  Use radii or fillets to avoid corners Deep cavities should be on one and provide uniform cross-section. side of the casting where possible.

  Poor Good Poor Good Wall sections should be uniform. Ribs and/or fillets improve bosses. Poor Good

FIGURE 5.38 Suggested

  Poor Good

  design modifications to avoid defects in castings. Source: Courtesy of The North

American Die Casting Association

  Sloping bosses can be designed for Side cores can be eliminated straight die parting to simplify die design. with this hole design.

  

Economics of Casting

  8 Die cast

Plaster cast

  7

  6

  5

  4 Sand cast

  3

  2 Permanent-mold Cost per piece (relative)

casting

  1

  1

  2

  3

  4

  5

  6

  10

  10

  10

  10

  10

  10

  10 Number of pieces

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

  

Lost-Foam Casting of Engine Blocks

(a)

  (b)

FIGURE 5.40 (a) An engine block for a 60-hp 3-cylinder marine engine, produced by the lost-foam casting process; (b) a robot pouring molten aluminum into a flask containing a polystyrene pattern. In the pressurized

  lost-foam process, the flask is then pressurized to 150 psi (1000 kPa). Source: Courtesy of Mercury Marine