MME 345, Lecture 36

Cast Iron Foundry Practices3. Metallurgy of grey irons

Ref: Heine, Loper and Rosenthal. Principles of Metal Casting, Tata McGraw-Hill, 19670

Topics to discuss today …

1. Graphite morphology

2. Metastable nature of iron – iron carbide system

3. Solidification of Fe-C-Si alloy

4. Chemical composition effects

5. Properties of grey iron

6. Heat treatment of grey iron

Graphite Morphology

Shape

Size

Distribution

ASTM A247

ISO R-945

classification of graphite flake size and shape

The properties of grey iron castings are influenced by the shape

and distribution of the graphite flakes.

The standard method of defining graphite forms is based on the

system proposed by the American Society for the Testing of Metals,

ASTM Specification A247, which classifies the form, distribution

and size of the graphite.

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Summary and description of ASTM and equivalent

ISO classification of graphite shapes

ASTM

Type(a)

Equivalent

ISO Form(b) Description

I VI Nodular (spheroidal) graphite

II VI Nodular (spheroidal) graphite, imperfectly formed

III IV Aggregate, or temper carbon

IV III Quasi-flake graphite

V II Crab-form graphite

VI(c) I Flake graphite

(a) As defined in ASTM A 247;

(b) As defined in ISO/R 945-1969 (E);

(c) Divided into five subtypes base on graphite distribution; uniform flakes; rosette grouping;

superimposed flake size; interdendritic, random orientation; and interdendritic, preferred orientation.

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reference diagrams for different graphite form / shape as specified in ISO/R 945

Six forms:

I flake graphite

II crab-form graphite

III quasi-flake graphite

IV aggregate or tempered carbon

V nodular graphite, imperfectly formed

VI nodular graphite

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longest flakes 4 in.

or more in lengthlongest flakes 1 to 2 in.

in length

longest flakes 2 to 4 in.

in length

longest flakes 1/2 to 1 in.

in length

longest flakes 1/4 to 1/2 in.

in length

longest flakes 1/16 to 1/8 in.

in length

longest flakes 1/8 to 1/4 in.

in length

longest flakes 1/16 in.

or less in length

graphite flake sizes as specified in ASTM A247

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reference diagrams for the distribution of graphite (Form 1) as specified in ASTM A247

Type A: Random orientation, uniform distribution

The preferred type for engineering applications. This type of

graphite structure forms when a high degree of nucleation

exists in the liquid iron, promoting solidification close to the

equilibrium graphite eutectic.

Type B: Rosette grouping

The eutectic cell size is large because of the low degree of

nucleation. Fine flakes form at the centre of the rosette

because of undercooling, these coarsen as the structure

grows.

Type C: Superimposed flake sizes, random orientation

Structures occur in hypereutectic irons, where the first

graphite to form is primary kish graphite. It may reduce tensile

properties and cause pitting on machined surfaces.

Type D: Interdendritic segregation, random orientation

Type E: Interdendritic segregation, preferred orientation

Both are fine, undercooled graphites which form in rapidly

cooled irons having insufficient graphite nuclei. Although the

fine flakes increase the strength of the eutectic, this

morphology is undesirable because it prevents the formation

of a fully pearlitic matrix. Occurs in hypoeutectic alloys.

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chemical composition, structure and properties of grey iron vary over broad limits

range of alloy composition and properties produced are better understood by

considering grey iron metallurgy, particularly the metastable nature of iron carbide

The Metastable Nature of Fe-Fe3C System

iron carbide becomes unstable

1. in contact with graphite at elevated temperature

2. at prolonged exposure to high temperature

3. in presence of certain elements in the alloy

under normal conditions,

• a hypoeutectic Fe-C alloy (>4.3%C) freezes with austenite dendrite and ladeburite (austenite-carbide

eutectic), which at room temperature transform into pearlite dendrite and transformed ladebutite

(pearlite-carbide mixture)

• a eutectic Fe-C alloy consists only the transformed eutectic

conversely, nucleation of graphite is prevented and metastable carbide persisted if

1. the cooling is rapid

2. the alloy contains certain elements8/26

Solidification of Fe-C-Si Alloy

• presence of Si in the alloy is the single most

important composition factor that promote

graphitisation in grey iron

• Three important stages of graphitisation:

1. During solidification

2. By carbon precipitation from austenite (solid state)

3. During eutectoid transformation (solid state)

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Graphitisation During Solidification

• Size, shape and distribution of graphite flakes developed.

• Segregation, undercooling and rapid cooling promotes type D/E-type graphites.

• Suppression of eutectic freezing (by chilling for example) form white iron.

• Factors to consider: Section size, Superheat, Inoculation.

Section Size

• Large, randomly nucleated flakes (type A/B) low nucleation rate, slow cooling rate, rapid graphitization

• Small flakes moderate undercooling, moderate nucleation with still time for diffusion and graphitization

• No flakes (chilled / white iron) severe undercooling (prevents graphitization)

Superheating (heating liquid above 1510 C)

• Undercooling would most likely to occur

• Produce type D/E flakes

• Chill / mottled iron would also occur if not inoculate properly.

Inoculation (additions to molten iron)

• Produce marked change in graphite type by preventing undercooling

• The effect is the most pronounced when added to superheated liquid

• Only 0.05 – 0.25% FeSi or other graphitizing agent addition produces type A graphite10/26

Graphitisation in the Solid State

• On slow cooling, graphite precipitates on previously existing flakes.

• On very slow cooling, austenite completely transforms into ferrite and graphite.

• Fine graphite flakes (formed during freezing) promote solid-state graphitisation.

• The commercial practice is to retain 100% or some portion of pearlite

• Proper balance between Mn and S assists to obtain pearlitic structure even when

cooled in sand moulds

• Rapid solid-state cooling and presence of carbide-forming elements increase

retention of combined carbon

• Fine graphite flakes (developed during solidification), regardless of type,

promotes solid-state graphitisation

• The flakes serve as the precipitation centre for carbon

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Grade (BS 1452: 1990) 150 200 250 300 350

Total carbon, % 3.1 – 3.4 3.2 – 3.4 3.0 – 3.2 2.9 – 3.1 3.1 max.

Silicon, % 2.5 – 2.8 2.0 – 2.5 1.6 – 1.9 1.8 – 2.0 1.4 – 1.6

Manganese, % 0.5 – 0.7 0.6 – 0.8 0.5 – 0.7 0.5 – 0.7 0.6 – 0.75

Sulphur, % 0.15 0.15 0.15 max. 0.12 max. 0.12 max.

Phosphorous, % 0.9 – 1.2 0.1 – 0.5 0.3 max. 0.01 max. 0.10 max.

Molybdenum, % – – – 0.4 – 0.6 0.3 – 0.5

Copper or Nickel, % – – – – 1.0 – 1.5

cast iron compositions

Chemical Composition Effects

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• Mechanical-property specifications are usually considered far more important

than chemical specifications

• So, composition and foundry practice must be adjusted to obtained the desired

strength class of grey iron

Element Effect

Carbon • Reported as total carbon: % TC = % Graphitic C + % Combined C

• For graphitisation, TC must have a minimum value (2.2% , value depends on Si content)

Silicon • Shifts eutectic and eutectoid points to the left.

Eutectic %C = 4.3 - %Si / 3

CE = %C + %Si / 3

• Promote graphitisation after carbon; a certain minimum level of Si is necessary to cause

sufficient graphitisation during solidification and develop a satisfactory grey iron

• Low Si is not sufficient to causes graphitisation during solidification, but promote nucleation

and graphitisation at high temperature in the solid state (malleableisation treatment)

Sulphur and

Manganese

• Both act as carbide stabiliser; presence in low level will cause complete graphitisation

• S alone form FeS and segregates along grain boundary, but with Mn, form MnS and

precipitated throughout the matrix; the effect as carbide stabiliser is lost

• Relationship between S and Mn:

%Mn = 1.7 %S form MnS

%Mn = 1.7 %S + 0.15; Highest limit of Mn to promote ferrite & graphite

%Mn = 3.0 %S + 0.35; Lowest limit of Mn to develop 100% pearlite

Phosphorous • Forms steadite and segregated along grain boundary

• Forms iron – iron phosphide eutectic, thereby promoting eutectic formation

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Properties of Grey Irons

foundry properties

for several reasons, grey irons are among the most easily cast of all alloys

1. Pouring Temperature

• wide working temperature (1200 – 1700 °C)

• permits easy manipulation, re-ladling, adequate time for pouring

• typical pouring temperature: 1250 – 1550 °C

2. Shrinkage and Feeding

• favourable freezing mechanism and low shrinkage characteristics

• higher yield (60 – 70% or even higher)

• feeding is not always easy; some casting designs are easily cast

with commercially acceptable soundness with low CE value

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3. Fluidity

• most fluid of ferrous alloys;

intricate and thin sections can

be produced

• the eutectic composition has

the most fluidity

• the hypereutectic composition

suffers extreme loss of fluidity

due to graphite precipitation

Composition Factor (CF)

= %C + %Si / 4 + %P / 2

(for highest fluidity, CF = 4.55)

Fluidity (inch)

= 14.9 x CF + 0.05 T - 155

(T = pouring temperature in °F)

Fluidity related to pouring temperature and composition

of grey and malleable cast iron

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engineering properties

Country SpecificationDesig-

nation

Minimum Tensile Strength (MPa)

100 150 180 200 220 250 260 300 350 400

France NFA 32-101-1987 FGL 150 200 250 300 350 400

Germany DIN 1691 -1985 GC 10 15 20 25 30 35

India IS 210 -1978 FG 150 200 250 300 350 400

Italy UNI 5007 -1969 G 10 15 20 25 30 35

Japan JIS G5501-1989FC 100 150 200 250 300 350

Class 1 2 3 4 5 6

Netherlands GOST 1412 -1979 Sch 10 15 18 20 25 30 35 40

UK BS 1452 1990 Grade 100 150 180 200 220 250 300 350

USA ANS/ASTM A48-83 Grade 20A 25A 30A 35A 40A 45A 50A 60A

International ISO 185 -1988 Grade 100 150 200 250 300 350

Equivalent Tonf/in2 6.5 9.7 12.9 16.2 19.4 22.7

Specification of grey irons

Grade 150 200 250 300 350 400

BHN (10/3000) 136 – 167 159 – 194 180 – 222 202 – 247 227 – 278 251 – 307

Hardness ranges for grades of grey iron

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from metallurgical standpoint, grey irons are viewed as

microstructurally-sensitive alloys

microstructure, chemical composition and mechanical properties

are intimately related

the processing parameters that influence structure, chemical

composition variations and cooling rate also influence properties

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relationship between tensile strength and

carbon equivalent value for various bar diameters

composition and structure effect

tensile strength of 1.20-in.-diameter gray-iron

bars as affected by carbon equivalent.

• C and Si are the most important composition factors

affecting mechanical properties

• maximum strength obtained with a pearlitic matrix

• maximum limit of strength by decreasing CE value is about

45000 psi; higher strength requires special alloy additions

• type A graphite produces maximum strength

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• Addition of alloying elements has two effects:

1. effect on microstructure, metal matrix, and graphitisation process

2. effect on properties (increased strength, resistances to wear, corrosion, oxidation/scaling and abrasion)

relationship between section size, CE value and structure

cooling rate (section size)

• effect of cooling rate on

properties is profound because

of its influence on microstructure

• rapid cooling increased

hardness and tensile strength

(as long as no white or chilled

iron or D-type graphite is

produced)

• slow cooling coarsening of

graphite flakes and lamellar

pearlite and appearance of ferrite,

causing softening and weakening

of grey iron with reduced wear

resistance

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variation of tensile strength with section thickness for several grades of iron

22/26

dependence of grey iron properties on section thickness

• thin section casting has the possibility of

misruns and chilled iron surface or hard spot

• so, certain minimum section thicknesses are

desirable in grey iron castings

ASTM

Class Iron

Suggested Wall Thickness,

min. (inch)

20 1/8

25 1/4

30 3/8

35 3/8

40 5/8

50 1/2

60 3/4

summary of relationships of CE, section size and properties of unalloyed grey iron24/26

Heat Treatment of Grey Irons

Since grey irons may be heated to austenite zone, heat treatments

similar to steels can be applied

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Principal purposes to heat treat grey irons:

1. improve machinability – sub-critical annealing to 650-675°C for 2-4 hrs, followed by

slow cooling; spherodisation of pearlite with some graphitisation

2. improve wear resistance – hardening at 900-925°C, followed by oil or water

quenching and then tempering to suit the need

3. improve strength – rarely used; hardening followed by tempering at 425-535°C

produces optimum tensile strength; may cause warpage or cracking

4. dimensional stability and stress relief – often desirable; annealing or normalising

can be used; a specific stress-relief anneal consists in heating to 480-595°C for 1 hr or

more, followed by slow cooling

Next ClassMME 345, Lecture 37

Cast Iron Foundry Practices4. Grey irons foundry practice