What are the causes of microscopic cracks in self-tapping screws?

Micro cracks are divided into mechanical surface scars and cracks produced during quenching.
1. Scars on the mechanical surface
This kind of crack is generally not deep, and the thread rolling process itself cannot completely avoid this kind of mechanical scar.
2. Cracks produced during quenching
One is the further expansion of cracks caused by stress concentration during heat treatment of mechanical scars; the other is new cracks caused by improper selection of quenching process and quenching medium.
(1) The quenching process is incorrect and it is not tempered in time after quenching, and it is easy to cause cracks if the annealing treatment is not carried out before repeated quenching.
(2) Quenching with water is easier to cause hot cracks than quenching with oil. Under normal circumstances, in order to ensure permeability, carbon steel is generally selected for water quenching, and alloy steel is generally used for 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 is more appropriate.