The final structure formed after the Tempering (Blueing) quenching process is very hard and brittle, and has internal stresses that occur during sudden cooling. Therefore, Tempering (Blueing) is the reheating of the material, keeping it at the same temperature for a while and cooling it in order to improve the toughness of the material. Tempering (Blueing) process can be done at different temperatures depending on the desired toughness ratio, hardness and final structure. Tempering (Blueing) done after waiting for the quenched part to cool completely may cause cracking. For this reason, Tempering (Blueing) should be done immediately after the part drops to 60-80 ˚C.
Tempering (Blueing) can be divided into three main groups:
Tempering (Blueing) at low temperatures is applied to surface hardened and cold work tool steels at 160-300°C. It is mostly suitable for 60 HRC hardness level.
Spring steel Tempering (Blueing) 300-500°C, used in spring steels or similar applications. Rather, it is for a hardness level of 45 HRC.
Tempering (Blueing) at high temperatures is done at 500°C or higher, applied to improvement steels, hot work tool steels and high speed steels. Hardness levels range from 300HB to 65HRC depending on the material used.