EFFECT OF INTERNAL HEATING DURING HOT COMPRESSION ON THE STRESS-STRAIN BEHAVIOR OF ALLOY 304L

The temperature change due to the conversion of mechanical deformation to internal heat and its effect on the as-measured stress-strain behavior of alloy 304L was investigated by means of initially isothermal (compression specimen, dies, and environment at same temperature at initiation of test), constant strain rate, uniaxial compression of laboratory-sized cylindrical specimens. Strain rate was varied in the range 0.01 to 1 s(-1) where the thermal state of the test specimen varied from nearly isothermal to nearly adiabatic, respectively. Specimens were deformed in the temperature range of 750 degrees C to 1150 degrees C to a strain of 1. The change in specimen temperature with applied strain was calculated via finite-element analysis (FEA) from the as-measured stress-strain data. Selected predictions were confirmed with embedded thermocouples to verify the model employed. Temperature was found to increase monotonically with strain at a strain rate of 1 s(-1), consistent with-what,is theoretically expected for the adiabatic case. At the 0.1 and 0.01 s(-1) rates, the sample temperature initially increased; peaked, and then decreased as the sample thinned and the contact area between the sample and the cooler dies increased. As-measured stress was corrected for softening associated with deformational heating by interpolation between the various instantaneous stress-temperature behaviors. The resulting isothermal flow data are compared to those predicted by a conventional method that employs an empirical estimate of the heat retention efficiency of the test specimen, assumed dependent on strain rate but independent of strain, to reduce the increase in temperature calculated for the adiabatic case. Differences between the calculated isothermal stress-strain data from the two methods are discussed. Values for the apparent activation energy of deformation and the strain to the peak in the flow curve, which is associated with the onset of dynamic recrystalization, determined from isothermal stress-strain data differed significantly from those obtained from the as-measured compression test data.