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

Historical casting parts

Korean bronze dagger( ( ))

& molds( )

Bronze bell( )

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.

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

Polycrystalline alpha brass 6

Phase diagram ( )

Graphically illustrates the relationships among temperature, composition,

and the phase present in a particular alloy system.

RuleLever

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

a-ferrite & austenite

a-ferrite (x 90) Austenite (x325)

Iron-carbon system (2)13

Eutectoid steel

a- ferrite: white

Fe3C: dark

Lamellar structure

(pearlite)

(x 500)

1% carbon (hypereutectoid) pearlite steel

a- ferrite - white

eutectoid - cementite -

proeutectoid -

cementite - violet

(x 500)

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

(c) kg

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(%)

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( )

Cast irons20

Cast irons21

Ternary phase diagram

Fe-Cr-Ni

Cast structures

Pure metal vs. alloys

Dendrites ( )24

Dendrites25

Fluid flow

constant2

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

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

Casting processes (2)

Expendable mold, expendable pattern

Evaporative-pattern casting (lost foam)

Investment casting (lost wax)

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

Pressure casting/centrifugal casting36

Die casting

Hot-chamber process

Cold-chamber process

Squeeze casting/single crystal38

Casting for single crystal

floating-zone

method

Crystal-pulling method(Czochralski process)

Heat treatment-ferrous alloys

Pearlite

Spheroidite

Bainite

Martensite

Quenching( )

Body Centered Tetragonal(BCT)

Retained austenite

Tempered martensite

Transformation-ferrous alloys

Austenite

Pearlite

(a+Fe3C)

(+proeutectic a) Bainite

(a+Fe3C)

Martensite

Tempered

martensite

reheat

Quenching

Moderate

cooling

Slow cooling

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)

Nonferrous alloys/stainless steel (1)

Precipitation hardening

( ), Al-Cu alloy

Age hardening( )

Nonferrous alloys/stainless steel (2)

Solution treatment

Precipitation hardening

Maraging(martensite + aging)

Case hardening

Surface hardening

Carburizing ( )

Carbonitriding ( )

Cyaniding ( )

Nitriding ( )

Boronizing ( )

Flame hardening ( )

Induction hardening ( )

Annealing ( )/ tempering ( )

Normalizing( )

Design consideration (1)49

Economics of casting52

Case study53

Bridge design54

Material of the bridge

Another bridge

chapter 5. metal-casting processes and equipment; heat

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