1. Mechanical surface scars
This kind of crack is generally not deep, and the rolling process itself cannot completely avoid such mechanical scars.
2. Cracks during quenching
One is the further expansion of cracks caused by stress concentration during heat treatment of mechanical scars; the other is that new cracks are generated due to improper selection of quenching process and quenching medium.
(1) The quenching process is not correct, and the quenching is not promptly tempered after quenching, and it is easy to cause cracks without annealing before repeated quenching.
(2) Water quenching is more likely to cause hot cracks than oil quenching. Under normal circumstances, in order to ensure permeability, carbon steel generally uses water quenching, and alloy steel generally uses oil quenching. However, oil quenching can be used for workpieces with a diameter of less than 4 mm, especially in the case of high carbon steel, oil quenching should be more appropriate.
