AUSTENITE FLOW-STRESS BEHAVIOR DURING CONTINUOUS COOLING THROUGH THE METASTABLE AUSTENITE REGION

Studies of the austenite flow stress during cooling have revealed chan ges in the flow stress behaviour as the temperature decreases from the single phase austenite region into the metastable region. This sugges ts 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 temperatu re, Ae(3), and the non-equilibrium transformation start temperature, t he Ar-3), austenite has yet to transform to ferrite. Several steel com positions with different Ae(3) temperatures were first subjected to co ntinuous deformation by compression during cooling to follow the varia tions in flow stress with temperature. In these tests, the metastable region appeared to be associated with an increase in the rate of incre ase 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 indi cate a Faster rate of contraction of the austenite lattice as the temp erature decreases through the metastable state, compared with that obs erved as the temperature decreases through the stable austenite region . The possible relationship between this observation and the Bow stres s behaviour will be addressed in this paper.