The austenitized steel balls were heated to 320 ° C and maintained in a salt bath of potassium nitrate for a sufficient period of time to convert them into lower bainite structure, which was then taken out and cooled in air. The lower bainite structure has higher strength and hardness and good toughness, which improves the overall mechanical properties of the steel ball.
Austempering reduces the temperature difference between the steel ball and the quenching medium, thereby reducing the quenching stress. Since the volume of bainite is smaller than that of martensite, and the temperature inside and outside the steel ball is uniform, the quenching structure stress is also small. Austempering can significantly reduce the deformation and cracking tendency of the steel ball.