White Heart Malleable Iron
White-heart malleable iron was successfully developed in 1722 by the Frenchman R.A.F. de Romaud. White cast iron with low carbon and silicon content is sealed in an oxidizing medium and held at a temperature of 950 to 1050°C for dozens of hours for decarburization and annealing. The outer layer is ferrite and a small amount of pearlite remains in the center. and microstructure of flocculent graphite. Its core fracture is white, so it is called white-core malleable iron.
Black Heart Malleable Iron
Black-core malleable iron was developed by American S. Boyden in 1826 based on the introduction of the manufacturing process of white-heart malleable iron. White cast iron with low carbon and silicon content is placed in a neutral medium for graphitization, and is maintained at a temperature of 850 to 950°C for dozens of hours. The furnace is cooled to 720 to 740°C and kept warm for more than ten hours. Finally, Ferrite black-core malleable cast iron with a ferrite matrix and flocculent graphite is obtained; or it can be kept at 850-950°C for more than ten hours and then released from the furnace and cooled in the air to obtain pearlite black-core malleable cast iron with a pearlite matrix and flocculent graphite. Properties and uses: Graphite in malleable cast iron is in the form of floccules, with less content. The stress concentration phenomenon is not obvious. The effective load area of the cast iron is not reduced much. The tensile strength can reach 300 to 700 MPa, and the elongation can reach 2 ~ 12%, with good cutting performance, oxidation resistance and corrosion resistance. The as-cast structure of malleable cast iron is white, the molten iron has poor fluidity, is prone to shrinkage cavities, and has a greater tendency to hot crack, so it is generally only suitable for castings with less complex shapes. In addition, since the annealing time increases as the wall thickness increases, and it is difficult to achieve complete annealing in the center part of an overly thick casting, the wall thickness of white-core malleable iron castings generally does not exceed 12 mm, and the wall thickness of black-core malleable iron does not exceed 25 mm. Ferritic malleable cast iron is widely used in rims, differential cases and chassis parts of automobiles and tractors, wrenches in machine tool accessories, porcelain bottle iron caps, wire clamps, bowl head plates in power transmission lines, and slubbing machines in textile machinery. And printing machine pan head as well as elbows, tees, joints, medium pressure valves in water and oil pipelines, etc. Pearlitic malleable cast iron is used in valve rockers, stoker parts, high-pressure joint valve bodies, automobile industry shift forks, differential gear boxes, etc. White-core malleable iron is used in automobile parts hangers, steering wheel pillar crowns, textile machine parts, etc. Research on the development of malleable cast iron mainly focuses on stable carbides as cast, no free graphite flakes appearing in castings, and shortening the annealing time to improve mechanical properties and serviceability. In addition, the thickness and weight limits of castings have been expanded, and modern malleable iron castings with wall thicknesses of 2 to 80 mm or weights of up to 150 kg have been produced.
