Temper embrittlement refers to the decrease in the notch toughness which often occurs in alloy steels when heated or cooled slowly through the temperature range of 450 to 600 degree Celsius. This can be a very dangerous for thicker sections which can not be cooled rapidly across the entire cross section. Temper embrittlement can also be produced by isothermal treatments in the temperature range of 450 to 600 degree Celsius.

The presence of temper embrittlement is determined by measuring the transition temperature with the notch-bar impact test. Hardness and tensile strength are independent of the embrittlement. The fracture in the temper-embrittled steel is intergranular in nature and propagates along the grain boundaries.

The primary reason for temper embrittlement is the presence of certain impurity elements and their co-segregation. The chief elements responsible for embrittlement are Antimony, Tin, Phosphorous and Arsenic. Plain carbon steel with less than 0.5wt% of Mn are not susceptible to temper embrittlement. Susceptibility to temper embrittlement is greatly enhanced by the presence of Cr and Mn. Small amount of Mo and W inhibit embrittlement but larger amount enhance it.

Sb, Sn, P and As diffuse to the grain boundaries to reduce the grain boundary energy. One of these elements react with one or more elements like Mn, Ni, Cr etc. Co-segregation like Ni-Sb, Ni-P, Ni-Sn at the grain boundaries have been confirmed. Co-segregation reduces the grain boundary cohesion to promote intergranular fracture.

The effect of temper embrittlement can be minimized by keeping the impurities such as SB, Sn, As and P as low as possible. Use of alloy steels having 0.5-0.75% Mo. Avoid the tempering range of 450 to 600 degree Celsius or do tempering below 450 degree Celsius. If tempering process requires heating above 600 degree Celsius, use quenching to avoid slow cooling within the temperature range of 450 – 600 degree Celsius.