The main defects in the trial production of hook body and hook tongue parts are crack, air hole, sand inclusion and mechanical sand sticking. Cracks are mainly thermal cracks, that is, the cracks caused by the obstruction of solid-state shrinkage of steel castings when the steel castings are still in a state of low strength and plasticity at the end of solidification or shortly after final solidification. Combined with the production characteristics of hook body and tongue, the main causes of thermal cracks may include:
1) The setting of pouring and riser system is unreasonable, which hinders the normal shrinkage of steel castings, and the cooling speed difference of each part of steel castings is too large;
2) The transition fillet at the connection between pouring riser and parts is too small;
3) As the formed thermal insulation riser is adopted for the pouring riser, the cooling speed of liquid steel near the connection between the pouring riser and the steel casting body is inconsistent, the cooling is fast in contact with the sand mold, slow in contact with the thermal insulation riser pipe, internal stress is generated during solidification, and finally hot cracks are formed;
4) The steel castings are unpacked and desanded too early, and the cooling is too fast;
5) If the pouring temperature is too high, the primary crystal structure formed in the solidification process of steel castings is coarse, the high temperature strength is low, the forming elements (s, P) of low melting point phase in the alloy are too high, and the linear shrinkage of the alloy is too large;
6) The sand box belt is too dense, and the formed fire-resistant insulation pipe is used at the connection part of the pouring riser, which has poor concession;
7) The cooling is too fast or uneven after heat treatment processes such as pre normalizing.