In the casting process of ductile iron, this ordinary gray cast iron has a large tendency to white mouth and large internal stress. It is difficult to obtain pure ferrite or pearlite matrix for iron castings. In order to improve the ductility or toughness of iron castings, cast iron is often used. The parts are reheated to 900-950°C and kept for sufficient time for high temperature annealing, and then cooled to 600°C and then released from the furnace. During the process, the cementite in the matrix decomposes graphite, and graphite is precipitated from austenite. These graphites are gathered around the original spherical graphite, and the matrix is ??completely converted to ferrite.