Sub-critical annealing

Sub-critical annealing consists of heating the steel to below the lower critical temperature. This type of annealing is mainly carried out in the temperature range 630° – 700°C to reduce hardness by allowing recrystallisation of the microstructure to occur. Alternatively, if a temperature in the range 690°C to 719°C is used, it is possible to spheroidise the cementite phase instead of forming lamellar pearlite consisting of platelets of ferrite and cementite. This technique is particularly useful with high carbon steels to optimise machinability.

The lower temperature sub-critical annealing treatments (550° – 600°C), are specifically used for stress relief of welded fabrications and to stabilise rough machined components which are to be ultimately hardened and tempered, case hardened, or nitrided and whose dimensional stability is critical.