Sub-Zero

Sub-zero treatment is a method of reducing the product’s temperature below 0℃ after carburizing quenching and before tempering. Making the temperature below 0℃ is to improve the faulty structure. After quenching, the product structure will become martensite with high hardness. As it becomes martensite, corrosion resistance will be improved due to its high harness. But about 80% of the structure becomes martensite, the remaining 20% is austenite. Normally, it is called residual austenite. This austenite has low hardness and low abrasion resistance.

Furthermore, the austenite naturally changes to martensite at above－63℃ after several years of heat treatment. Due to the metal crystal structure is changed, the product’s size will be changed and causes some troubles such as strange noises of transmission components (this is called degradation over time). Sub-zero treatment is changing the remaining 20% from residual austenite to martensite.

The austenite will be changed to martensite at －63℃ or higher, so it must be cooled to change into martensite. Structure changed to martensite by sub-zero method has increased hardness, improved abrasion resistance and is helpful in preventing further change from austenite to martensite.