Temperature cycling and the rate of the bainite transformation

There is a novel range of steels under development, based on bainite forming at exceptionally low temperatures. The advantage of suppressing the transformation temperature is that incredibly fine platelets of bainitic ferrite are generated which are only 20–40 nm in thickness. This makes the steel strong and in combination with the austenite that is retained, gives a useful range of properties which have been exploited in the manufacture of armour. There are other applications in the aircraft industry that are at a critical stage of development. One characteristic of transformation at low temperatures is that the rate of reaction is slow, sometimes taking days in order to achieve the maximum fraction of bainitic ferrite. The reaction can be accelerated by refining the austenite grain size and by adding solutes which increase the free energy change accompanying transformation. Sista et al. have suggested an alternative method, that the reaction can be speeded up by cycling the austenite between two temperatures within the range Ms–Bs, the martensite and bainite start temperatures respectively. Single step changes in the temperature during the course of the reaction has been investigated before, to show indirectly that the stresses generated by phase transformation at low temperatures can stimulate transformation at higher temperatures. As will be described later, there are some difficulties in interpreting the published data. The purpose of the present work was therefore to investigate the influence of thermal cycling on the kinetics of the low temperature bainite reaction. The paper begins with an assessment of the previous work and goes on to present and interpret new experiments.