Classification of Cast Irons

There are six basic types of cast irons and several varieties of each. The types of iron are classified as to how the excess carbon occurs in the microstructure. The correspondence between commercial and microstructural classification, as well as the final processing stage in obtaining common cast irons, is given in Table 1.

Table 1 Classification of cast iron by commercial designation, microstructure, and fracture

Commercial designation

Carbon-rich phase

Matrix(a)

Fracture

Final structure after

Gray iron

Lamellar graphite

P

Gray

Solidification

Ductile iron

Spheroidal graphite

F,P,A

Silver-gray

Solidification or heat treatment

Compacted graphite iron

Compacted vermicular graphite

F,P

Gray

Solidification

White iron

Fe3C

P,M

White

Solidification and heat treatment1^"1

Mottled iron

Lamellar Gr + Fe3C

P

Mottled

Solidification

Malleable iron

Temper graphite

F,P

Silver-gray

Heat treatment

Austempered ductile iron

Spheroidal graphite

At

Silver-gray

Heat treatment

(a) F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite).

(a) F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite).

(b) White irons are not usually heat treated, except for stress relief and to continue austenite transformation.

White iron is essentially free of graphite, and most of the carbon content is present as separate grains of hard Fe3C. White iron exhibits a white, crystalline fracture surface because fracture occurs along the iron carbide plates. White cast iron contains 2.0 to 3.6% C and 0.5 to 2.0% Si with high-alloy grades containing as much as 7% Ni, 28% Cr, and 3.5% Mo.

White cast irons have high compressive strength and good retentions of strength and hardness at elevated temperature, but they are most often used for their excellent resistance to wear and abrasion. The massive carbides in the microstructure are chiefly responsible for these properties.

Malleable iron contains compact nodules of graphite flakes. These are called "temper carbon" because they form during an extended annealing of white iron of a suitable composition. Malleable cast iron contains 2.2 to 2.9% C and 0.9 to 1.9% Si. Tensile strengths can range from 275 MPa (40 ksi) to 725 MPa (105 ksi).

Gray iron, the most commonly used cast iron, contains carbon in the form of graphite flakes. Gray iron exhibits a gray fracture surface because fracture occurs along the graphite plates (flakes). Gray irons usually contain 2.5 to 4.0% C, 1 to 3% Si, and 0.1 to 1.2% Mn. Tensile strengths range from 140 to 415 MPa (20 to 60 ksi) with higher strengths possible in high-alloy gray irons.

Gray cast iron has several unique properties that are derived from the existence of flake graphite in the microstructure. Gray iron can be machined easily at hardnesses conducive to good wear resistance. It resists galling under boundary-lubrication conditions (conditions wherein the flow of lubricant is insufficient to maintain a full fluid film). It has outstanding properties for applications involving vibrational damping or moderate thermal shock.

Ductile iron, also known as spheroidal graphite or nodular iron, contains spherulitic graphite in which the graphite flakes form into balls as do cabbage leaves. Ductile iron is so named because in the as-cast form it exhibits measurable ductility. Ductile iron typically contains 3 to 4% C, 1.8 to 2.8% Si, and 0.1 to 1.0% Mn. Alloying additions are sometimes made to ductile irons to improve heat and corrosion resistance. These alloyed grades may contain 1 to 6% Si, 0.7 to 2.4% Mn, 18 to 36% Ni, and up to 5.5% Cr.

The chief advantage of ductile iron over gray iron is its combination of high strength and ductility--up to 18% minimum elongation for ferritic ductile iron with a tensile strength of 415 MPa (60 ksi) as opposed to only about 0.6% elongation for a gray iron of comparable strength. Martensitic ductile irons with tensile strengths of about 830 MPa (120 ksi) exhibit at least 2% elongation.

Austempered Ductile Iron. If ductile iron is austenitized and quenched in a salt bath or a hot oil transformation bath at a temperature of 320 to 550 °C (610 to 1020 °F) and held at this temperature, transformation to a structure containing mainly bainite with a minor proportion of austenite takes place (Fig. 1). Irons that are transformed in this manner are referred to as austempered ductile irons. Austempering generates a range of structures, depending on the time of transformation and the temperature of the transformation bath. The properties are characterized by very high strength, some ductility and toughness, and often an ability to work harden, giving appreciably higher wear resistance than that of other ductile irons. Austempered ductile irons exhibit in excess of 5% elongation at tensile strengths exceeding 1000 MPa (145 ksi).

Fig. 1 Comparison of ductile iron microstructures. (a) Microstructure of normalized ductile iron showing pearlite matrix. (b) Microstructure (tempered martensite) of hardened-and-tempered ductile iron. (c) Microstructure of austempered ductile iron showing matrix of upper bainite and retained austenite. All etched in picral. 500x

Fig. 1 Comparison of ductile iron microstructures. (a) Microstructure of normalized ductile iron showing pearlite matrix. (b) Microstructure (tempered martensite) of hardened-and-tempered ductile iron. (c) Microstructure of austempered ductile iron showing matrix of upper bainite and retained austenite. All etched in picral. 500x

Compacted graphite (CG), or vermicular graphite, irons have structures and properties that are in between those of gray irons and ductile irons. The graphite forms interconnected flakes as in a gray iron, but the flakes are shorter and thicker. The CG irons contain 2.5 to 4.0% C, 1.0 to 3.0% Si, with manganese contents varying between 0.1 and 0.6%, depending on whether a ferritic or pearlitic structure is desired.

High-alloy iron contains over three percent alloy content and is commercially classified separately. As indicated above, high-alloy irons may be a type of white iron, gray iron, or ductile iron. The matrix may be ferritic, pearlitic, martensitic, or austenitic, depending upon which alloying element dominates the composition. Table 2 lists approximate ranges of alloy content for various types of alloy cast irons used for abrasion-resistant, corrosion-resistant, and heat-resistant applications.

Table 2 Ranges of alloy content for various types of alloy cast irons

Description

Composition, wt%(a)

Matrix structure, as-cast(c)

TC(b)

Mn

P

S

Si

Ni

Cr

Mo

Cu

Abrasion-resistant white irons

Low-carbon white iron1®

2.22.8

0.20.6

0.15

0.15

1.0-1.6

1.5

1.0

0.5

(e)

CP

High-carbon, low-silicon white iron

2.83.6

0.32.0

0.30

0.15

0.3-1.0

2.5

3.0

1.0

(e)

CP

Martensitic nickel-chromium iron

2.53.7

1.3

0.30

0.15

0.8

2.7-5.0

1.1-4.0

1.0

M,A

Martensitic nickel, high-chromium iron

2.53.6

1.3

0.10

0.15

1.0-2.2

5-7

7-11

1.0

M,A

Martensitic chromium-molybdenum iron

2.03.6

0.51.5

0.10

0.06

1.0

1.5

11-23

0.53.5

1.2

M,A

High-chromium iron

2.33.0

0.51.5

0.10

0.06

1.0

1.5

23-28

1.5

1.2

M

Corrosion-resistant irons

High-silicon iron(f)

0.41.1

1.5

0.15

0.15

14-17

5.0

1.0

0.5

F

High-chromium iron

1.24.0

0.31.5

0.15

0.15

0.5-3.0

5.0

12-35

4.0

3.0

M,A

Nickel-chromium gray iron*8"1

3.0

0.51.5

0.08

0.12

1.0-2.8

13.536

1.5-6.0

1.0

7.5

A

Nickel-chromium ductile iron®

3.0

0.74.5

0.08

0.12

1.0-3.0

18-36

1.0-5.5

1.0

A

Heat-resistant gray irons

Medium-silicon iron®

1.62.5

0.40.8

0.30

0.10

4.0-7.0

F

Nickel-chromium iron®

1.83.0

0.41.5

0.15

0.15

1.02.75

13.536

1.8-6.0

1.0

7.5

A

Nickel-chromium-silicon iron®

1.82.6

0.41.0

0.10

0.10

5.0-6.0

13-43

1.8-5.5

1.0

10.0

A

High-aluminum iron

1.32.0

0.41.0

0.15

0.15

1.3-6.0

20-25 Al

F

Heat-resistant ductile irons

Medium-silicon ductile iron

2.83.8

0.20.6

0.08

0.12

2.5-6.0

1.5

2.0

F

Nickel-chromium ductile iron(h)

3.0

0.72.4

0.08

0.12

1.755.5

18-36

1.75-3.5

1.0

A

Heat-resistant white irons

Ferritic grade

1-2.5

0.31.5

0.5-2.5

30-35

F

Austenitic grade

1-2.0

0.31.5

0.5-2.5

10-15

15-30

A

(a) Where a single value is given rather than a range, that value is a maximum limit.

(b) Total carbon.

(c) CP, coarse pearlite; M, martensite; A, austenite; F, ferrite.

(d) Can be produced from a malleable-iron base composition.

(e) Copper can replace all or part of the nickel.

(f) Such as Duriron, Durichlor 51, Superchlor.

(g) Such as Ni-Resist austenitic iron (ASTM A 436).

(h) Such as Ni-Resist austenitic ductile iron (ASTM A 439).

(j) Such as Nicrosilal

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