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/MS371/ Structure and Properties of Engineering Alloys

Chapter 8

Cast Irons

/MS371/ Structure and Properties of Engineering Alloys/MS371/ Structure and Properties of Engineering Alloys

Cast irons

• Family of ferrous alloys with a wide diversity of properties

• % C, 1~3% (normal steel: <1% C)

• Properties

– low cost

– versatile engineering property

– cast into the desired shape (good )

– low MP

– high capacity (see Fig. 8-17)

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Classification of cast irons

Chemical composition ranges for typical unalloyed cast irons



• White cast iron (백주철)– solidification rate is enough

– C to combine with iron in the form of meta-stable iron carbide or cementite

– therefore, relatively hard and

– showing crystalline fractured surface

– high compressive strength and excellent wear resistance

• Gray cast iron (회주철)– solidification rate is enough

– stable C separates or graphitizes during solidification to form separate flakes

– most among ferrous alloys, excellent machinability to provide good wear resistance

– fractured surface showing a sootish color

White cast iron

(cementite + pearlite)

Gray cast iron

(ferrite + pearlite + graphite flake)



• Malleable cast iron (가단주철) from white

– white cast iron → annealing treatment ( ) → malleable cast iron

– graphite nucleates and grows from the cementite of the white iron to form

– a wide range of mechanical properties can be obtained

• Ductile cast iron (유연주철) from gray

– free C in the form of instead of flakes

– other name: nodular cast iron, spherulitic graphite iron (SG iron)

– adding a very small amount of to molten iron before casting for spheroidal graphite

– similar comp with gray but Mg is added for spheroidization

• High-alloy cast iron

– special property (ex. high abrasive, wear resistance, and heat resistance)

Malleable cast iron

(ferrite + graphite nodules)

Ductile cast iron

(ferrite + spherical graphites)


Fe-C-Si alloys• Si: graphitization element

– eutectic comp to shift toward C

– Carbon Equivalent:

CE = %C + 1/3 (%Si + %P)


Gray cast iron ( % of all castings)

Slow solidification of a hypoeutectic gray

cast iron

• (1) to (2)

– austenite begin to form and grow

until the temp at (2) is reached

• (2) to (3)

– eutectic freezing occurs

– eutectic mixture of austenite and graphite

• (3) to (4)

– austenite + carbide region

– C will be rejected from the austenite as

graphite and precipitate on the graphite flakes

in the eutectic

• (4) to (5)

– equilibrium cooling through the eutectoid range

– transformation of austenite to and the

precipitation of remaining C on the graphite

flakes


Gray cast iron

Chemical composition

• C, Si

– promote the formation of in gray cast iron

– increased C & Si contents, formation of gray cast iron to be favored over white

– % eutectic C (Fe-C-Si alloy) = 4.3 – 0.33 x % Si (in alloy)

• S, Mn

– FeS→

– S content must be very low in order to allow formation of spheroidal graphite

– FeS: segregate into GB during freezing

• P (steadite)

– Fe-Fe3P-Fe3C, steadite is an of Fe – Fe3P

– low melting point (954 ~ 980°C)

– solidify at a relatively low temp and segregate at boundaries of solidification cells

– 0.2% P: steadite solidifies at the junction of three cells to form concave triangular-

shaped constituents

– above 0.3% P: increased hardness and brittleness and decreased machinability


Gray cast iron

Graphitization during solidification

5 types of graphite flakes

• type A

– uniform distribution and random orientation

• type B

– rosette grouping and random orientation

• type C

– superimposed flake size and random orientation

• type D (due to undercooling effect)

– interdendritic segregation and random orientation

• type E

– interdendritic segregation and preferred orientation


Ductile cast irons

• Graphite dispersed in a matrix of ferrite, pearlite, or both

• Consisting of graphite which are surrounded by free (“Bull’s eye”

structure) in a matrix of pearlite [free ferrite = proeutectoid ferrite]

• High strength, ductility, toughness, and hot workability

• 3~4% C, 1.8~2.8% Si, ~ % Mg

Bull’s eye structure


Ductile cast irons

Solidification of ductile cast iron

a) Spheroidizing treatment

• spheroidizing treatment was done with Ce when first discovered

• addition of alloy after melting (~0.1% Mg)

• residual Mg, 0.03~0.05% (enough to determine the extent of spheroidizing)

• spheroidizing treatment method:

– ladle transfer method

– plunging (phosphorizer)

– in-mold process

b) Fading: time dependent loss of treatment effect

• Consumption of Mg

– Mg + O ↔ MgO

– Mg + S ↔ MgS

– 2Mg + SiO2 ↔ Si + 2MgO


Ductile cast irons

Chemical composition

• C, Si

– 3~4% (narrower limits 3.6~3.9%)

– higher C content necessary to develop higher density of graphite nodules

– Si is increased, so are the number of nodules

– Si also strengthens the ferrite in ductile cast iron

• S

– usually kept below 0.03%

– an increase in S content means additional Mg must be added for spheroidal graphite

• P

– max 0.10%, usually kept below 0.05%

– P adversely affects impact properties and ductility because of formation

• other elements

– Pb, Ti, Al, Sb, Zr (promote graphite flakes)


Ductile cast irons

Heat treatment and microstructure

• Stress relieving

– removal of internal stress in castings by heating at 538 to 675°C for 1h, plus 1h per

inch of thickness

• Annealing

– increases ductility and produces machinability

– single-step anneal: heated to 788°C for 6h and then furnace-cooled

– two-stage anneal: heated to 900°C at 130°C per h, held 4h, cooled at 22°C per hour to

691°C, held 6h, then furnace-cooled

• Normalizing and tempering

– normalizing is carried out by holding for about 1h at 900°C followed by air cooling

– after normalizing, the iron can be tempered for 1h at 566°C

• Quenching and tempering

– oil-quenched

– austempering and martempering


Ductile cast irons

Mechanical properties


Malleable cast irons

• white cast iron → annealing (malleablizing) → malleable cast iron

• ferritic (or standard) malleable iron and pearlitic malleable iron

Typical comp for malleable cast irons




a) Ferritic malleable cast irons

• annealing heat treatment (called malleablization)

‒ 1st, graphite is nucleated, this occurs principally in heating stage and in the early part

of holding at temp

‒ 2nd, held at 870 to 954°C, involves elimination of massive carbides and conversion of

their C to graphite

‒ 3rd, involves slow cooling of iron through allotropic transformation range so that

a ferritic matrix completely free of pearlite and carbide is created

• nucleation of graphite

‒ chemical comp: high Si content favors nucleation

‒ heating rate: rapid heating to holding temperature decreases the number of nuclei

developed

‒ size selection: thin sections develop more nuclei

‒ pretreatment: a preliminary holding at 315 to 650°C increases nucleation upon

subsequent malleablization




a) Ferritic malleable cast irons continued

• 1st-stage graphitization (decomposition of eutectic Fe3C to 3Fe + C)

- after quenching: martensite matrix + tempered C

- after air-cooling: pearlite matrix + tempered C

• 2nd-stage graphitization (decomposition of eutectoid Fe3C to 3Fe + C)

- after air-cooling: ferrite matrix + tempered C

b) Pearlitic malleable cast irons

- increased strength and wear resistance with reduced ductility

- producing a structure of a matrix of pearlite or tempered martensite

with tempered graphite nodules




Ferritic malleable cast iron

(slow cooling)

Pearlitic malleable cast iron

(air-cooling)

Pearlitic malleable cast iron

(air-cooling at faster rate)

Tempered martensite malleable cast iron

(quenching in oil)


Abrasion-resistance alloy cast iron

• Chilled cast iron

– hard white-cast-iron surface and a softer gray-cast-iron core

– usually produced by casting the molten metal against a metal or graphite chill to

provide rapid cooling of the solidifying cast iron

– duplex structure by using “chills”

– Ni-Cr abrasive-resistance cast iron

• White cast iron

– excess C occurs as iron carbides instead of graphite flakes or nodules

– addition of alloying elements (Ni, Cr, Mo)

• Cr white cast iron

– 12~35% Cr, increase hardness and improve abrasion resistance

• Ni-Cr white cast iron

– Ni & Cr both increase strength, oxidation and corrosion resistance


Abrasion-resistance alloy cast iron

Ni-Cr abrasion-resistance cast iron: as-cast against a chill

Cr abrasion-resistance cast iron: as-cast against a chill


Corrosion-resistance alloy cast iron

• High-Si irons

– 12~18% Si, very resistant to corrosive acids

– because of their high Si content, mechanical properties (low thermal and

mechanical shock resistance)

• High-Cr irons

– 15~30% Cr

– mechanical properties are better than those of the high-Si cast irons

• High-Ni irons

– 14~30% Ni

– excellent machinability, good foundry properties, although their tensile strengths

are relatively low


Corrosion-resistance alloy cast iron

• High-Si irons

High-Cr irons

High-Ni irons


Heat-resistant alloy cast iron

• Cr cast irons

– even a small addition of Cr (0.5 to 2%) to reduce in volume in gray irons

subjected to cyclic heating at 800°C

– assisting in stabilizing carbides and forming a protective oxide on metal surface

• High-Si cast irons

– up to 3.5% Si, increase rate of growth of gray cast iron by promoting graphitization

– however, 4 to 8% Si greatly reduce both oxidation (scaling) and growth

– Si also raises ferrite-to-austenite transformation temp to about 900°C

• High-Ni cast irons

– 18% or more Ni up to 7% Cu, and 1.75 to 4% C

– greater toughness and shock-resistance than other cast iron

– nodular graphites are considerably stronger and have higher ductility than the flake-

graphite nickel alloy irons


Austempered ductile cast iron• Unique materials which combine strength and wear resistance comparable to some

wrought steels

• Low cost and design flexibility of cast irons

Schematic diagram illustrating

the various phases

Comparison of the tensile strength and

ductility of austempered ductile iron and

other grades of ductile cast iron

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