05_cast iron.pdf

7/26/2019 05_Cast Iron.pdf

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Outline

Properties of cast irons

Microstructure

Mechanical properties

Types

Gray cast iron

White cast iron

Nodular cast irons

Malleable cast irons


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Properties

Cast Iron

C = 2 ~ 6.67%

Properties depend on

1) Carbon content

2) Cooling rate of the casting

3) Alloying elements


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Carbon content

1) Degree of solution

= %

4.30.3(%+%)

T>1 Hypereutectic Ledeburite + Pr. Cementite

T=1 Eutectic Ledeburite

T>1 Hypoeutectic Ledeburite + Perlite


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Cooling rate

2) Cooling rate

Slow cooling rate Iron + Graphite

section size > 10 mm

Quick Cooling rate Iron + Cementite

section size < 10 mm


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Cooling rate

2) Cooling rate Fe - graphite

Fe - cementite


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Alloying elements

Graphite producing elements

Co, P, Cu, Ni, Ti, Si, C, Al

Carbide producing elementsW, Mn, Mo, S, Cr, V, Mg, Ce

The microstructure depends on:- Carbon and Silicon content

- Section size (cooling rate)


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Maurer diagram

Ferrite + Graphite

White cast iron(metastable)

Gray cast iron

(Stable)


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Microstructure

Hypoeutectic white cast iron

Perlite and ledeburite

Hypereutectic white cast iron

Primer cementite and ledeburite

Gray cast iron

Ferrite and graphite

Gray cast iron

Ferrite, perlite and graphite


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Greiner - Klingenstein diagram

C+Si%

Wall thickness (mm)

10 20 30 40 50 60 70

Ferrite + Graphite

Ledeburite+Perlite

Perlite +

Graphite

L + P + G

At a given C+Si% the graphite

producing elements effectsincreases with increasing section size

Typical chamical compossition for gray cast iron:

C % Si % Mn % S % P %2.5-3.5 1-3 0.5-1


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Mechanical properties of cast iron

Tensile

Strength

PerliticmicrostructureFerriticmicrostructure

100

400

Graphites effect on Tensile strength

- graphite produces notch effect

- graphite excludes parts in the matrix


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Disadvantage of cast iron

- gray cast iron has low strength- gray cast iron has no plastic strain = brittle

Graphite forms in gray cast iron


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Advantage of cast iron

- Good compressive strength

- high damping capability (tool machines)

- good machinability

- good wear resistance (graphite as lubricant)

- lower cost


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Utilization of gray cast iron

Machine stands, engines, etc


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Increasing the strength of cast iron

a) Increase the perlite amount in the matrixb) Modify the shape and distribution of the graphite

flakes

c) Alternating the graphites geometry from flake to

spheroidal graphite


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Increase the perlite amount in the matrix

Ferrite + Graphite

ASTM A438 TS (psi) TS (MPa) T

Class 20 150 1

30 200 0.94

35 250 0.88


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Alternating the graphites geometry from flake to

spheroidal graphite

Ductile or Nodular cast iron

Mg and Si alloying

Mg alloying by Fe-Cu-Mg and Fe-Ni-Mg

F + P +Sph +

Graphite flakes

Si%

Mg %

100

400

F + Sph. + Carbides

F + P + Sph.

Ferrite+spheroidalgr.

+Graphiteflakes

Ferrite+Grap

hiteflakes C=3.5%F + Sph +

Graphite flakes


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Utilization of ductile cast iron

Machine parts, gears, pipes, crankshaft, etc


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Ductile or nodular cast irons

ASTM A395 TS (MPa) YS(Mpa) El (%) structure

Grade 60-40-18 400 250 18 Ferrite

Grade 80-55-06 600 370 6 F + PGrade 100-70-03 700 420 3 P (AQ)Grade 120-90-02 800 480 2 M (Q+T)

Elongation (%)Yield Stress (psi)

Tensile strength (psi)


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Malleable cast iron

Convert iron-carbibe to tempercarbon

increases the ductility


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Ferritic structure with temper carbon

ASTM A47 TS (MPa) YS(Mpa) El (%)Grade 325-10 400 130 10

Black heart malleable cast iron

time

Ledaburitic-perlitic structure

Iron carbide disociates to Fe and CAustenite transforms to ferrite and

graphite

Ferrite + temper carbon

T(C)

100

723

12 48 hours

940 C

neutralatmosphere


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Microstructure

Black heart cast iron

Temper-carbon in ferrite matrix


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05_cast iron.pdf

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