1. Defects of steel castings Because steel castings are directly cooled by high-temperature liquid in molds, there are many quality defects in themselves. (1) often appears coarse flake pearlite, surrounded by coarse ferrite net or coarse ferrite grains. ② The ferrite of widmanstatten structure is acicular and fragile. ③ Dendritic segregation components are not uniform. 3. The elimination of method defects in steel castings will greatly reduce its mechanical properties and make its machinability worse. In the subsequent heat treatment process, deformation and even cracking will occur, resulting in insufficient hardness. Only by preheating treatment can the structural defects be eliminated, the machinability be improved and the final quenching be prepared. (1) normalizing the low carbon steel to obtain uniform ferrite and flaky pearlite; ② Complete annealing or isothermal annealing is adopted for medium carbon steel and medium carbon alloy steel to obtain ferrite+flaky pearlite structure; ③ In order to eliminate casting stress, low temperature annealing should be chosen; (4) Diffusion annealing is adopted for dendrite segregation of large castings. However, complete annealing or normalizing should be carried out to refine grains, improve mechanical properties and improve machinability.