Quenching and Tempering: The workpiece is heated up to the austenitic region under high-vacuum environment (<10-3 torr), and then it is quenched by high-pressure nitrogen to generate the martensite phases, which will increase the hardness of the materials. If the workpiece is further tempered under the protective atmosphere of the mixture of nitrogen and hydrogen gas, the martensite structures will be stabilized and the toughness of the parts is increased.
Precipitation Hardening: The workpiece is heated up to the austenitic region under high-vacuum environment (<10-3 torr) where the alloying elements and carbides are allowed to dissolve into the matrix. It is then quenched by high-pressure nitrogen gas to gain supersaturated solid solution. Upon subsequent aging, specific carbides or precipitates will form in the microstructures and thereby strengthen the materials.
Benefits:
Since the aforementioned processes are carried out under high-vacuum environment, it will prevent the components from oxidation. This is why the workpiece can still retain its original color after this heat treatment.