Bainitic reaction under continuous cooling conditions and tempering behaviour of a bainitic microstructure in a low alloyed steel

The use of bainitic steels in the automotive industry constantly increases, because of the excellent combination of toughness and strength. However, due to economic reasons it is not possible to realize long isothermal holding times to achieve a bainitic microstructure. Therefore, the aim of the present work is to analyse the bainitic reaction during continuous cooling conditions and to describe the microstructural changes that occur during a subsequent tempering treatment in a low alloyed steel. It was found that the decomposition of austenite to bainite occurs between cooling rates higher than 0.3°Cs-1 and lower than 6°Cs-1. The transformation kinetics of the bainitic reaction can be described on basis of an adopted T0’ concept, which incorporates elastic strain energy values between 950 and 1100 Jmol-1. However, the phase transformation remains incomplete and amounts of 10 to 17 vol.-% of retained austenite in the form of blocks and films are present in the final bainitic microstructure. A further reheating leads to the decomposition of retained austenite, whereas the block morphology is thermally more stable than the films of austenite. An isothermal tempering treatment at 620°C for 4 hours lead to the precipitation of M2C type secondary hardening carbides, which increases the hardness around 100HV1.