Investigation on hot deformation of 20Cr-25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

The deformation behaviour of a 20Cr-25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000-1200 degrees C and strain rates of 0.01-10 s(-1). It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516.7 kJ mol(-1). Based on the dynamic materials model and the Murty's instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025-1120 degrees C and 0.01-0.03 s(-1) or 1140-1200 degrees C and 0.08-1 s(-1), where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.