Malleable cast iron

Abstract:
Malleable cast iron is a heat-treated iron-carbon alloy, which solidifies in the as-cast condition with a graphite-free structure, i.e. the total carbon content is present in the cementite form (Fe3C).
Two groups of malleable cast iron are specified (whiteheart and blackheart malleable cast iron), differentiated by chemical composition, temperature and time cycles of the annealing process, the annealing atmosphere and the properties and microstructure resulting therefrom.

Malleable cast iron is a heat-treated iron-carbon alloy, which solidifies in the as-cast condition with a graphite-free structure, i.e. the total carbon content is present in the cementite form (Fe3C).

Two groups of malleable cast iron are specified, differentiated by chemical composition, temperature and time cycles of the annealing process, the annealing atmosphere and the properties and microstructure resulting therefrom.

Whiteheart malleable cast iron

The microstructure of whiteheart malleable cast iron depends on section size. Small sections contain pearlite and temper carbon in ferritic substrate. In the large sections exists three different zones:

  • surface zone which contains pure ferrite,
  • intermediate zone which has pearlite, ferrite and temper carbon,
  • core zone containing pearlite, temper carbon and ferritic inclusions.
The microstructure shall not contain flake graphite.

Blackheart and pearlitic malleable cast iron

The microstructure of blackheart malleable cast iron has a matrix essentially of ferrite. The microstructure of pearlitic malleable cast iron has a matrix, according to the grade specified, of pearlite or other transformation products of austenite.

Graphite is present in the form of temper carbon nodules. The microstructure shall not contain flake graphite.

Malleable cast iron designation system

The designation according to ISO 5922 (1981) of malleable cast iron consists of one letter designating the type of iron, two figures designating the tensile strength and two figures designating the minimum elongation.

  • Letters designating the type of malleable cast iron can be:

    • W for whiteheart malleable cast iron,
    • B for blackheart malleable cast iron,
    • P for peariitic malleable cast iron.
    This letter is followed by a space.
  • The first two figures designating the minimum tensile strength, in Newtons per square millimetre, of a 12 mm diameter test piece, divided by ten. For example if the minimum tensile strength were 350 N/mm², the designation would be 35.

  • The next two figures designating the minimum elongation (L0 = 3d) as a percentage of a 12 mm diameter test piece. A nought (0) shall be the first figure when the value is less than 10%, for example if the minimum elongation is 4%, the designation is 04, and if the minimum elongation is 12%, the designation is 12.
For example: The designation of a whiteheart malleable cast iron having a minimum tensile strength of 400 N/mm² and minimum elongation of 5% when measured on a 12 mm diameter test piece, would be W 40-05.

Chemical composition of malleable iron

The chemical composition of malleable iron generally conforms to the ranges given in the Table 1. Small amounts of chromium (0.01 to 0.03%), boron (0.0020%), copper (≤ 1.0%), nickel (0.5 to 0.8%), and molybdenum (0.35 to 0.5%) are also sometimes present.

Table 1. Chemical composition of malleable iron
Element Composition %
Carbon 2.16-2.90
Silicon 0.90-1.90
Manganese 0.15-1.25
Sulfur 0.02-0.20
Phosphorus 0.02-0.15


Mechanical properties of malleable iron

Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and low-carbon metallic matrix. Because of the way in which graphite is formed in malleable iron, however, the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates.

Malleable iron and ductile iron are used for some of the applications in which ductility and toughness are important. In many cases, the choice between malleable and ductile iron is based on economy or availability rather than on properties. In certain applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings:

  • for parts that are to be pierced, coined, or cold formed,
  • for parts requiring maximum machinability,
  • for parts that must retain good impact resistance at low temperatures, and
  • for parts requiring wear resistance (martensitic malleable iron only).
Ductile iron has a clear advantage where low solidification shrinkage is needed to avoid hot tears or where the section is too thick to permit solidification as white iron (Solidification as white iron throughout a section is essential to the production of malleable iron). Malleable iron castings are produced in section thicknesses ranging from about 1.5 to 100 mm and in weights from less than 0.03 to 180 kg or more.

The mechanical properties of test pieces of malleable cast iron shall be in accordance with the values listed below:

Table 2. Mechanical properties of whiteheart malleable cast iron
Designation Diameter of test piece
mm 		Tensile strength
N/mm² 		0,2% proof stress
N/mm² 		Elongation
(L0 = 3d)
% min 		Hardness
HB
W 35-04 9 - 15 340 - 360 - 5 - 3 230
W 38-12 9 - 15 320 - 380 170 - 210 15 - 8 200
W 40-05 9 - 15 360 - 420 200 - 230 8 - 4 220
W 45-07 9 - 15 400 - 480 230 - 280 10 - 4 220

Table 3. Mechanical properties of blackheart and pearlitic malleable cast iron
Designation Diameter of test piece
mm 		Tensile strength
N/mm² 		0,2% proof stress
N/mm² 		Elongation
(L0 = 3d)
% min 		Hardness
HB
B 30-06 12 - 15 300 - 6 150 max
B 32-12 12 - 15 320 190 12 150 max
B 35-10 12 - 15 350 200 10 150 max
P 45-06 12 - 15 450 270 6 150-200
P 50-05 12 - 15 500 300 5 160-220
P 55-04 12 - 15 550 340 4 180-230
P 60-03 12 - 15 600 390 3 200-250
P 65-02 12 - 15 650 430 2 210-260
P 70-02 12 - 15 700 530 2 240-290
P 80-01 12 - 15 800 600 1 270-310


Melting Practices

Melting can be accomplished by batch cold melting or by duplexing. Cold melting is done in coreless or channel-type induction furnaces, electric arc furnaces, or cupola furnaces. In duplexing, the iron is melted in a cupola or electric arc furnace, and the molten metal is transferred to a coreless or channel-type induction furnace for holding and pouring.

Charge materials (foundry returns, steel scrap, ferroalloys, and, except in cupola melting, carbon) are carefully selected, and the melting operation is well controlled to produce metal having the desired composition and properties. Minor corrections in composition and pouring temperature are made in the second stage of duplex melting, but most of the process control is done in the primary melting furnace.

Molds are produced in green sand, silicate CO2 bonded sand, or resin bonded sand (shell molds). Equipment ranges from highly mechanized or automated machines to that required for floor or hand molding methods, depending on the size and number of castings to be produced. In general, the technology of molding and pouring malleable iron is similar to that used to produce gray iron. Heat treating is done in high-production controlled-atmosphere continuous furnaces or batch-type furnaces, again depending on production requirements.

After solidification and cooling, the metal is in a white iron state, and gates, sprues, and feeders can be easily removed from the castings by impact. This operation, called spruing, is generally performed manually with a hammer because the diversity of castings produced in the foundry makes the mechanization or automation of spruing very difficult. After spruing, the castings proceed to heat treatment, while gates and risers are returned to the melting department for reprocessing.


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