Austenite flow stress behaviour during continuous cooling through the metastable austenite region

Studies of the austenite flow stress during cooling have revealed changes in the flow stress behaviour as the temperature decreases from the single phase austenite region into the metastable region. This suggests that the onset of metastability is associated with a change in the microstructure even though, in the metastable temperature region (i.e. between the equilibrium austenite-to-ferrite transformation temperature, Ae₃, and the non-equilibrium transformation start temperature, the Ar₃), austenite has yet to transform to ferrite. Several steel compositions with different Ae₃ temperatures were first subjected to continuous deformation by compression during cooling to follow the variations in flow stress with temperature. In these tests, the metastable region appeared to be associated with an increase in the rate of increase in flow stress with decreasing temperature. Neutron diffractometry at high temperatures was used to monitor any crystallographic changes associated with the metastable region. The results of the latter indicate a faster rate of contraction of the austenite lattice as the temperature decreases through the metastable state, compared with that observed as the temperature decreases through the stable austenite region. The possible relationship between this observation and the flow stress behaviour will be addressed in this paper.