Vacuum hardening is a method generally used for the hardening of tool, mold and martensitic stainless steels. The process is on the same principle as the reclamation process, with the main differences being the equipment used, the atmospheric environment and the cooling environment. The steel to be vacuum hardened is first taken to the vacuum environment in the vacuum furnace. Thus, impurities that will diffuse from the atmosphere to the material surface during heating are prevented.
After the Vacuum process, the furnace is brought to the process temperature (850-1250) and austenitic transformation takes place at these temperatures. After the austenite transformation takes place, the cooling stage is started and the martensitic transformation (hardening) in the steel is completed. Nitrogen, hydrogen, argon or oil can be used as a cooling medium under high pressure. The martensitic transformed steel is subjected to deep cooling to remove residual austenite, if necessary, and then to the tempering process, if not required, directly to the Tempering (Blueing) process to obtain the desired hardness and stress. This process is preferably carried out in an oven with a vacuum environment.
After the tempering (blueing) process, the material is ready for the next stage in its production, having reached the desired hardness and microstructure.
Since nitrogen gas is generally used as the cooling medium in the vacuum hardening process, the cooling rate is lower than in other processes. For this reason, the steel to which the process will be applied must be highly alloyed and have high hardenability. Cold work Tool Steel, hot work Tool Steel, plastic mold steel, high speed steel and martensitic stainless steels are frequently used in vacuum hardening.