chapter 5. metal-casting processes and equipment; heat

M2794.001800 M A T E R I A L A N D M A N U F A C T U R I N G P R O C E S S E S

Chapter 5. Metal-Casting

Processes and Equipment;

Heat Treatment

Prof. Ahn Sung-Hoon ( )

School of Mechanical and Aerospace Engineering

Seoul National University

© Prof. Ahn, Sung-Hoon

Historical casting parts

Korean bronze dagger( ( ))

& molds( )

Bronze bell( )

2


Casting

Casting is a manufacturing process by which a molten material such as metal or

plastic is introduced into a mold made of sand or metal, allowed to solidify within

the mold, and then ejected or broken out to make a fabricated part.

Advantages

Making parts of complex shape in a single piece.

Producing large number of identical castings within specified tolerances.

Good bearing qualities and jointless product.

Disadvantages

Limitations of mechanical properties because of the polycrystalline grain structure.

Poor dimensional accuracy due to shrinkage of metal during solidification.

If the number of parts cast is relatively small, the cost per casting increases rapidly.

Fundamental aspects in casting operations

Solidification of the metal from its molten state.

Flow of the molten metal into the mold cavity.

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

Mold material and its influence on the casting process.

3


Solidification of Metals4


Solid solution Solute( )

Solvent( )

When the particular crystal structure of the solvent is maintained during alloying,

the alloy is called solid solution. Substitutional solid solution( )

Interstitial solid solution( )

5.2.2 Intermetallic compound( ) Complex structures in which solute atoms are present among solvent atoms in certain specific

proportions.

5.2.3 Two-phase system( ) Phase: a homogeneous portion of a system that has uniform physical and chemical characteristics

5


Polycrystalline alpha brass 6


Phase diagram ( )

Graphically illustrates the relationships among temperature, composition,

and the phase present in a particular alloy system.

LS

OS

LS

LO

CC

CC

LS

L

CC

CC

LS

S

RuleLever

7


Lever-Rule ( )8


Eutectic system, Pb-Sn9


Types of 3-phase invariant reactions10


Iron-carbon system (1)

Pure iron( ) : 0.008% C

Steels( ) : 2.11% C

Cast irons( ) : ~6.67% C

a-ferrite( ): BCC, soft and ductile

d-ferrite: BCC, stable only at very high temperatures

Austenite( ) : FCC, ductile

Cementite( ): Fe3C, C 6.67%, iron carbide( ), brittle

11


a-ferrite & austenite

a-ferrite (x 90) Austenite (x325)

12


Iron-carbon system (2)13


Eutectoid steel

a- ferrite: white

Fe3C: dark

Lamellar structure

(pearlite)

(x 500)

14


1% carbon (hypereutectoid) pearlite steel

a- ferrite - white

eutectoid - cementite -

blue

proeutectoid -

cementite - violet

(x 500)

15


Classification of ferrous alloys16


Composition and naming steels17


Amount of phases in carbon steel

Casting 1040 steel 10kg, calculate a phase and g phase at (a) 900 C,

(b) 728 C and (c) 726 C

(a) Austenite:100% g

(b)

(c) kg

kgCC

CC

kgCC

CC

o

o

4.9 is that %,94100022.067.6

40.067.6

,5 is that %,50100022.077.0

022.040.0100(%)

,5 is that %,50100022.077.0

40.077.0100(%)

a

g

a

ag

a

ag

g

18


Cast irons

Fe, C 2.11~4.5%, Si ~3.5%

According to solidification

morphology :

Gray cast iron( ) Flake graphite( )

Gray fracture surface( )

Damping( )

Ductile(nodular) iron( ) Ductile

White cast iron( ) Large amount of Fe3C

Brittle White fracture surface( )

Malleable cast iron( ) Obtained by annealing white cast iron

Compact graphite iron( )

19


Cast irons20


Cast irons21


Ternary phase diagram

Fe-Cr-Ni

22


Cast structures

Pure metal vs. alloys

23


Dendrites ( )24


Dendrites25


Fluid flow

vD

h

h

A

A

vAvAQ

fg

v

g

ph

g

v

g

ph

g

v

g

ph

ghcv

Re

22

constant2

2

1

2

2

1

2211

2

222

2

111

2

26


Solidification time & shrinkage

Chvonrinov’s rule

Solidification time

= C(volume/surface area)2

Shrinkage occurs at

Molten metal

Phase change

Solid metal

Cast iron expands

Graphite has high volume/mass

Net expansion during

precipitation

Similarly Bi-Sn alloys expand

27


Defects/DFM28


Casting alloys29


Applications30


Properties31


Casting processes

Expendable mold,

permanent pattern

Sand casting

Shell-mold casting

Plaster mold casting

Ceramic mold casting

Vacuum casting

32


Casting processes (2)

Expendable mold, expendable pattern

Evaporative-pattern casting (lost foam)

Investment casting (lost wax)

33


Investment casting34


Casting processes (3)

Permanent mold

Slush casting

Pressure casting

Die casting

Centrifugal casting

Squeeze casting

Semisolid metal forming

Casting for single crystal

Rapid solidification

In permanent-mold casting, a mold are

made from materials such as steel,

bronze, refractory metal alloys, or

graphite. Because metal molds are better

heat conductors than expendable molds,

the solidifying casting is subjected to a

higher rate of cooling, which turn affects

the microstructure and grain size within

the casting.

Cooling methods : water, air-cooled fin

Used for aluminum, magnesium, and

copper alloys due to their lower melting

points

Pros : good surface finishing, close

dimensional tolerances, and uniform and

good mechanical properties

Cons : not economical for small

production runs, not good for intricate

shapes

35


Pressure casting/centrifugal casting36


Die casting

Hot-chamber process

Cold-chamber process

37


Squeeze casting/single crystal38


Casting for single crystal

floating-zone

method

Crystal-pulling method(Czochralski process)

39


Heat treatment-ferrous alloys

Pearlite

Spheroidite

Bainite

Martensite

Quenching( )

Body Centered Tetragonal(BCT)

Retained austenite

Tempered martensite

40


Transformation-ferrous alloys

Austenite

Pearlite

(a+Fe3C)

(+proeutectic a) Bainite

(a+Fe3C)

Martensite

Tempered

martensite

reheat

Quenching

Moderate

cooling

Slow cooling

41


Ferrous alloys42


Shape memory alloy (SMA)43


Quenched AISI 9310 steel

The white strikes are

excess proeutectoid

cemetite

Cream color is

retained austenite

Gray area is bainite

Blue/brown regions

are martensite

(x 320)

44


Nonferrous alloys/stainless steel (1)

Precipitation hardening

( ), Al-Cu alloy

Age hardening( )

45


Nonferrous alloys/stainless steel (2)

Solution treatment

Precipitation hardening

Aging

Maraging(martensite + aging)

46


Case hardening

Surface hardening

Carburizing ( )

Carbonitriding ( )

Cyaniding ( )

Nitriding ( )

Boronizing ( )

Flame hardening ( )

Induction hardening ( )

47


Annealing ( )/ tempering ( )

Normalizing( )

48


Design consideration (1)49


Economics of casting52


Case study53


Bridge design54


Material of the bridge

Another bridge

55

http://www.youtube.com/watch?v=3mclp9QmCGs

chapter 5. metal-casting processes and equipment; heat

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