Materials Science Tutorial paper no. 5

STRUCTURE OF GRAY CAST IRONS (with flake graphite, malleable and nodular cast irons)

Gray iron, or grey iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material.

It is used for housings (carcase) where tensile strength is non-critical, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors.

Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good wear resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping ( amortizare ) capacity because it absorbs the energy. It also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increase fluidity when casting. Gray iron is generally considered easy to weld. Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost non-existent.

A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon. Silicon is important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the tendency for graphite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed. Rapid cooling partly or completely suppresses graphitization and leads to formation of cementite, which is called white iron.The types of gray cast irons are:

1. Grey cast irons with flake graphite.2. Malleable cast iron.3. Nodular cast iron (or ductile iron).

1. Grey cast iron with flake graphiteThe graphite takes on the shape of a three dimensional flake. In two dimensions, as a

polished surface will appear under a microscope, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips (extremităţi) of the flakes act as preexisting notches (crestături); therefore, it is brittle. The presence of graphite flakes makes the grey iron easily machinable as they tend to crack easily across the graphite flakes. Grey iron also has very good damping capacity and hence it is mostly used as the base for machine tool mountings.

The factors favouring their appearance are the low cooling rate on solidification and the presence of Si and Al in the melt. Grey cast irons are ternary Fe-C-Si alloys, the crucial element in obtaining free carbon as graphite being the silicon.

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Fig.1. Iron - graphite equilibrium phase diagram

As a first approach, grey cast irons with flake graphite are classified according to their matrix structure: a) ferritic gray cast irons.b) ferritic - perlitic gray cast iron.c) perlitic gray cast iron.

→ To improve castability a high phosphorous content is necessary in cast iron. In their composition one can find some phosphorus contents (0.1 … 0.2%) from the charred coal used on casting and also from first melting white cast irons used to obtain grey cast irons. These are the grey cast irons with phosphorous eutectic.

Phosphorus lead to the appearance of a new constituent, the phosphorous eutectic, composed of ferrite (containing carbon and phosphorus), cementite Fe3C and iron phosphide Fe3P : E( α + Fe3C+ Fe3P).

This eutectic forms on 1.96%C and 6.89%P, solidifying at 953°C. It is seen as a microscopic mottled feature, lacy loops – like. At higher phosphorus concentrations, the eutectic is distributed as a network, rising the hardness characteristics and diminishing the plasticity ones.

At lower concentrations, it improves castability by rising the fluidity.→ Depending on the cooling rate and Mn and Si contents, between white cast irons and grey cast irons one cand find an intermediary grade of cast irons, named mottled cast irons, named after the fracture surfaces macroscopic aspects.

Even if its hardness is not very high (200280 HB) its machinability is poor because of the cementite presence in the structure or even ledeburite (sample G14 microstructure).

Thus, according to the matrix constitution, one can discern the following grey cast irons with flake graphite:

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a. Ferritic gray cast irons having a structure made of ferrite and graphite (see sample G4

microstructure).b. Ferritic - pearlitic gray cast iron having a structure made of ferrite, pearlite and

graphite (see sample G10 microstructure).c. Pearlitic gray cast iron having a structure made of pearlite and graphite (see sample G7

microstructure).d. Phosphorous gray cast iron, having a structure made of pearlite, phosphorous eutectic

and graphite (see sample G13 microstructure).e. Mottled cast iron, having a structure which reveals structural aspects found in white

cast irons (pearlite, cementite, ledeburite) as well as in grey cast irons (pearlite, graphite) (see sample G14 microstructure). It is a cast iron produced when both cementite and graphite precipitate during solidification. It is a structure in the transition zone of the rolls for plate - rolling mills. The rolls have a superficial hard crust which is formed of a white cast iron. One can assume that the structure of a mottled iron is composed by the structure of a hypoeutectic white cast iron (having P + Fe3C" + Led) and graphite.

Usually, grey cast irons as cast alloys have low prices. They exhibit good castability and poor plasticity. They have very low solidification contraction (<1.5%) hence low shrinkage, being at the same time very fluid. Such materials can be cast as very thin wall parts (≈ 2 mm).

A hardness of 160200 HB, as well as graphite’s presence in the structure lead to an optimal machinability.

Their tensile strength is quite low (100350 N/mm²), elongation and toughness next to zero. Grey cast irons have good compressive strength and damping capacity, being used for machine tools frameworks (batiuri pentru maşini unelte).

Grey cast irons are the cheapest Fe-C alloys.

2. Malleable cast iron is produced by heat treating (malleabilization heat treatment) of a hypoeutectic white cast iron, to produce a rounded graphite shape. During this heat treatment, the cementite formed during solidification is decomposed and graphite clumps (bulgări) are produced. This graphite is also called temper carbon.The types of malleable cast iron considering the nature of the metallic matrix are:

Ferritic malleable cast iron.Ferritic - pearlitic malleable cast iron. Pearlitic malleable cast iron.

According to the malleabilization process, two types of malleable cast irons are obtained:a. Black-heart malleable cast irons. Their name is due to the fracture surface’s dark grey colour. They are obtained by annealing the hypoeutectic white cast irons in neutral atmosphere (sample G1). During the annealing, cementite decomposes according to the following reaction:

Fe3C T ° C↔ 3 Fe + C graphite

Process called graphitization, that takes place in two steps: Step I – decomposition of the eutectic cementite, and Step II – decomposition of the secondary and eutectoid cementite.

Graphite precipitates in rounded agglomerations, more or less compact, in a ferrite metallic matrix (see sample G6).

When step II is suppressed, the matrix will be a pearlitic one (see sample G9).

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When step II is incomplete, meaning not all the eutectoid cementite is decomposed and also in the temperature interval 750-680°C the cooling rate is higher, a ferrite „ring” appears arround the graphite clump, the rest of the matrix being made of pearlite – bull’s eye malleable iron (see sample G12).

b. White-heart malleable cast iron, so called because of its fracture surfaces silverly aspect. They are obtained by heating in oxidizing atmosphere (the common, normal one) of the hypoeutectic white cast irons. Thus, workpieces will exhibit a ferrite with no graphite separations in the superficial zones; unto the center the structure may be made of pearlite or ferrite + pearlite containing graphite clumps. Hardness varies between 125215 HB from the surface to the centre.

Malleable cast irons show better mechanical and plasticity characteristics as against grey cast irons. The tensile strength values, according to the matrix structure and the graphite’s dispersion and fineness, may be between 300600 N/mm2.

3. Nodular or ductile cast iron are obtained by inoculation of Mg (or Ca, Ce, Ba, etc.) from liquid iron before casting in order to change the shape of graphite, resulting in spheroidal graphite (nodule). The formation of nodules is thus achieved by addition of nodulizing elements. These graphite nodules are much more compact than temper carbon obtained by malleabilization.

The matrix structure depends on the cooling rate on solidification (on the wall thickness of the workpiece), on the modifying element’s quantity and also on its nature and influence;

Nodular cast irons may be exploited as such, in the cast state, or after a stress relief annealing. When severe acceptance conditions are requested, workpieces are submitted to final heat treatments, such as normalizing (pearlitizing), quenching and tempering to sorbite or globular pearlite (axes, bent axles), bainitic isothermal quenching (gears, axes, bent axles), superficial quenching (machine tools frameworks, etc.). Thus, nodular cast iron can reach characteristics similar to steels: tensile strength 600800 N/mm2 and elongation 48%.

According to the metallic matrix, the types of nodular cast iron are:1. Ferritic nodular cast iron (sample G5).2. Ferritic - pearlitic nodular cast iron (bull’s eye nodular cast iron, see sample G11).3. Pearlitic nodular cast iron (see sample G8).

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G3. Grey cast iron with flake graphite- C=3,5%; Si=2,4%; Mn=0,7%; S=0,083%;

P=0,10%- Cast- Unetched- Observations: Graphite flakes in a metallic

matrix

G4. Ferritic gray cast irons with flake graphite Fc 100- C=3,51%; Si=2,42%; Mn=0,70%;

S=0,083%; P=0,10%- Cast- C: ferrite + graphite- Ph: ferrite + graphite- Rm=100 N/mm2; Hardness: 100-120 HB- Applications: Workpieces with low

mechanical strength, with no plasticity; grinding tools for optical lenses, parts having some corrosion resistance (valves, water pipes, etc.)

G5. Nodular cast iron with ferritic matrix Fgn 370-17- C=3,12%; Si=2,91%; Mn=0,32%;

P=0,04%; S=0,013%- Inoculated before casting with master alloy

SiCaMgAlTiCe- Cast- Nital 2%- C: ferrite + graphite- Ph: ferrite + graphite- Rm=370 N/mm2; Rp0,2=230 N/mm2;

A5=17%; hardness: 160 HB- Applications: Workpieces with medium

mechanical strength, with high plasticityG6. Black - heart malleable cast irons Fmn 300- C=2,68%; Si=1,11%; Mn=0,45%;

P=0,05%; S=0,02%- Cast and malleabilized under the

conditions: 1050°C/15h + 730°C/30h/furnace

- Nital 2%- C: ferrite + graphite- Ph: ferrite + graphite- Rm=300N/mm2; A5=8%; hardness: 160HB- Applications: Workpieces with medium

mechanical strength, with high plasticity workpieces for agricultural machinery

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G7

- Pearlitic grey cast iron Fc 350 - C=3,28%; Si=2,08%; Mn=0,80%;

P=0,14%; S=0,094%- Cast- Nital 2%- C: pearlite + graphite- Ph: ferrite + cementite + graphite- Rm=350 N/mm2; hardness: 210-280 HB- Utilizări: Workpieces with low mechanical

strength, with no plasticity: waste water pipes, machine tools frameworks.

G8

- Nodular cast iron with pearlitic matrix Fgn 700-2

- C=3,24%; Si=2,38%; Mn=0,83%- Inoculated before casting with master alloy

SiCaMgAlTiCe- Cast- Nital 2%- C: pearlite + graphite- Ph: ferrite + cementite + graphite- Rm=700 N/mm2, Rp0,2=450 N/mm2; A5=2%;

hardness: 260 HB- Applications: Workpieces with medium

mechanical strength, with medium toughnesss (workpieces for agricultural machinery)

G9

- Pearlitic malleable cast iron Fm 700 - C=2,74%; Si=1,08%; Mn=0,53%;

P=0,07%; S=0,02%- Cast and malleabilized under the

conditions: 1050°C/20h/air- Nital 2%- C: pearlite + graphite- Ph: ferrite + cementite + graphite- Rm=700 N/mm2; Rp0,2=500 N/mm2; A5=2%;

hardness: 280 HB- Applications: Workpieces with high

mechanical strength and toughnesss (workpieces for agricultural machinery, tractors, etc.)

G10

- Ferritic-pearlitic grey cast iron with flake graphite Fc 250

- C=3,43%; Si=2,30%; Mn=0,80%; P=0,13%; S=0,09%

- Cast- Nital 2%- C: ferrite + pearlite + graphite- Ph: ferrite + cementite + graphite- Rm=250 N/mm2; hardness: 170-200 HB

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- Applications: Workpieces with medium mechanical strength, with no plasticity: boxes, frames, waste water pipes, valves, etc.

G11

- Ferritic-pearlitic nodular cast iron Fgn 500-7 - C=3,32%; Si=2,46%; Mn=0,76%- Inoculated before casting with master alloy

SiCaMgAlTiCe- Nital 2%- C: ferrite + pearlite + graphite- Ph: ferrite + cementite + graphite- Obs. Structure "bull’s eye"- Rm=500 N/mm2; Rp0,2=350 N/mm2; A5=7%;

200 HB- Applications: Workpieces with medium

mechanical strength, with medium toughnesssG12

- Ferritic- pearlitic malleable cast iron Fmp 500 - C=2,65%; Si=1,10%; Mn=0,60%; P=0,07%;

S=0,02%- Cast and malleabilized under the conditions:

1050°C/25h/cuptor- Nital 2%- C: ferrite + pearlite + graphite- Ph: ferrite + cementite + graphite- Obs. Structure "bull’s eye"- Rm=500 N/mm2; Rp0,2=300 N/mm2; A5=5%;

240 HBG13

- Grey cast iron with flake graphite and phosphorous eutectic

- 3,43%C; 2,42% Si; 0,66% Mn; 0,28% P; 0,086% S

- Cast- Nital 2%- C: pearlite + graphite + phosphorous eutectic

(ferrite + cementite + iron phosphide -Fe3P)- Ph: ferrite + cementite + graphite + iron

phosphide- Rm=330 N/mm2; 250-300 HB- Applications: Workpieces with thin walls and

intricate shapeG14

- Mottled iron - 3,88% C; 1,96% Si; 1,22% Mn; 0,12% P;

0,062% S;- Cast- Nital 2%- C: pearlite + graphite + cementite + ledeburite- Ph: ferrite + cementite + graphite- Hardness: 280-360 HB- Obs. Mottled aspect in fracture surfaces. It is a

casting defect, remediable by softening

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annealing. It is a classical structure in the transition zones of the rolls for plate-rolling mills.

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