Spherical graphite has little effect on the fracture of the matrix, so the mechanical properties of ductile iron mainly depend on the matrix structure. Therefore, the mechanical properties of ductile iron can be significantly improved through heat treatment.
•Stress Relief Annealing The casting internal stress of ductile cast iron is about twice as large as that of gray cast iron. For ductile iron castings that are no longer subjected to other heat treatments, stress relief annealing is required.
The purpose of graphitization annealing is to decompose the free cementite in the as-cast structure and the eutectoid cementite in the pearlite to obtain a ductile iron with a high plasticity ferrite matrix, eliminate casting stress, and improve its workability.
The purpose of normalizing is to obtain a matrix structure dominated by pearlite, the stainless steel belt refines the grains, and improves the strength, hardness and wear resistance of ductile iron.
Normalizing can be divided into high temperature and low temperature normalizing. For thick-walled castings under high temperature normalizing, air cooling or even spray cooling should be used to ensure that pearlitic ductile iron is obtained. Low-temperature normalizing is to heat the casting to 840～860℃, keep it for 1～4h, and empty the furnace to obtain the nodular cast iron with pearlite + ferrite matrix. The thermal conductivity of ductile iron is poor, and the internal stress of the casting is relatively large after normalizing. Therefore, a stress relief annealing should be performed after normalizing.