According to DIN 17014:
Heating following hardening, cold-working or welding to a temperature between room temperature and the lower transition point Ac1 and maintaining this temperature with subsequent specific cooling.

After hardening, a lattice with high stresses forms. This is the reason for the hardness of steel. If martensite is now heated, the inherent carbon atoms regain a certain degree of mobility. A portion may separate and start to form finely dispersed carbides. At around 100 °C Fe2C begins to form, so-called ε-carbide.
The stresses of the lattice reduce somewhat and the hardness decreases. Above 250 °C the ε-carbide slowly becomes Fe3C-cementite and as the temperature rises the Fe3C depositions begin to coagulate. The hardness reduces, whilst the tenacity increases. In addition, in the case of non-alloyed or low-alloyed steels the residual austenite changes from around 230 °C, and reconverts into martensite upon cooling down again.
With high-alloyed steels (cold, hot and high speed steel) the residual austenite only decomposes above 500 °C. With such steels it is possible to attain an increase in hardness through annealing at a suitable temperature.

Tempering usually results in a reduction in hardness and an increase in tenacity. However, two temperature ranges are known, in which the tenacity also declines. Blue embrittlement in the range of approx. 250 to 300 °C, and in the case of chrome, nickel and manganese alloyed steels with tempering embrittlement between 450 and 550 °C. Through the addition of molybdenum and a reduction in the phosphor content to below 0.01 % it is possible to reduce tempering embrittlement or even exclude it completely. During tempering processes it is not only the temperature that plays an important role, but also the time. It is necessary to observe a minimum holding time. In the majority of tempering diagrams, the duration of tempering is therefore usually listed as a parameter.

Tempering results in a change in characteristics:

  • Change in the hardness and strength
  • Increase in deformability
  • Decrease in internal stresses
  • Decrease in risk of cracking
  • Decrease in residual austenite
  • Change in mass and possibly form