Types of Malleable Cast Iron – Whiteheart, Blackheart and Pearlite
🔗What is Cast Iron? Different types of cast iron and their properties
According to the cycle of the annealing process and chemical composition, the malleable cast iron can be classified into
(i) Whiteheart malleable cast iron
(Ii) Blackheart malleable cast iron
(iii) Pearlitic malleable cast iron
Whiteheart malleable cast iron/ whiteheart processWhite-heart malleable cast iron obtained when the white cast iron is annealed at oxidizing atmosphere to remove carbon from the surface. The white cast iron is packed in an iron or steel boxes and surrounded by a decarburizing agent (a mixture of new and used hematite ore). Then the box is slowly heated to 900-950°C and kept this temperature for several days (100hrs). During this period some quantity of carbon oxidized out of the cast surface and remaining carbon dispersed throughout the structure. Casting then cooled down slowly for several days. The resulting cast iron has silvery-grey fracture and microstructure do not contain flake graphite. The resulting whiteheart cast iron will be tough and it can withstand great compression loads without fracture.
The microstructure of white-heart malleable cast iron depends on the thickness of the cast. For small section, the microstructure is mainly ferrite with some pearlite. For large section, the surface zone has ferrite structure; the core and intermediate zone has pearlite, ferrite, and temper carbon. This causes non-uniform mechanical properties across the cross-section.
Whiteheart malleable cast iron is suitable for the manufacturing the section require high ductility. Eg: Motorcycle frames, cast iron pipe fittings.
Blackheart malleable cast iron/ Blackheart processBlackheart malleable cast iron obtained when the white cast iron is annealed at an inert/ neutral atmosphere. The white cast iron is packed with neutral substance like sand, then heated to 850-900°C in the continuous-type furnace. The cast is maintained at this temperature for three to four days. The prolonged heating causes the carbon present the white cast iron (cementite) breakdown and transforms into graphite globules. The resulting cast iron has a black fracture. The casting produced blackheart process are more malleable.
The microstructure of blackheart malleable cast iron has matrix of ferrite with temper carbon and it does not contain flake graphite. The mechanical properties are uniform throughout the section of casting.
Black-heart malleable cast iron is widely used in automobile industries because of their combination shock resistant, castability, and machinability. Eg: Brake shoes, wheel hub, axle housing, marine industries for ship component.
🔗Quick difference between Whiteheart process and Blackheart process
Pearlite malleable cast ironPearlite malleable cast iron is an improved version of blackheart cast iron, it prepared by controlled heat treatment. Initially, it is similar to the blackheart cast iron, then it is reheated to about 900°C. The reheating process causes some of the carbon dissolved in austenite, then on cooling, background pearlite is formed. The mechanism of this process is similar to the normalizing of steel. The final casting has a homogeneous matrix of pearlite and other transformation product of austenite.
The microstructure Pearlite malleable cast iron has graphite in the form of temper carbon nodules and it does not contain flake graphite. The resulting casting is tougher, harder, have high tensile strength, wear resistant; but reduce the ductility and malleability.
Pearlite cast iron is used for manufacturing of gears, camshafts, coupling, axle housing, universal joint yokes