No matter how cool the quenching medium may be, if the heat inside a large piece cannot escape faster than a certain critical rate, there is a definite limit to the inside hardness.

The method has considered the effect of difference in material hardness on the fatigue limit, which causes the shaft diameter design to become different depending on heat treatment difference.

With no limit of viscosity, hardness, and fiber content of the material, it is functioned with air cooling, screen-free, etc.