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 beha vior of alloy 304L was investigated by means of initially isothermal ( compression specimen, dies, and environment at same temperature at ini tiation of test), constant strain rate, uniaxial compression of labora tory-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 fr om 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 stra in was calculated via finite-element analysis (FEA) from the as-measur ed stress-strain data. Selected predictions were confirmed with embedd ed thermocouples to verify the model employed. Temperature was found t o increase monotonically with strain at a strain rate of 1 s(-1), cons istent with-what,is theoretically expected for the adiabatic case. At the 0.1 and 0.01 s(-1) rates, the sample temperature initially increas ed; peaked, and then decreased as the sample thinned and the contact a rea between the sample and the cooler dies increased. As-measured stre ss was corrected for softening associated with deformational heating b y interpolation between the various instantaneous stress-temperature b ehaviors. The resulting isothermal flow data are compared to those pre dicted 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 te mperature calculated for the adiabatic case. Differences between the c alculated isothermal stress-strain data from the two methods are discu ssed. Values for the apparent activation energy of deformation and the strain to the peak in the flow curve, which is associated with the on set of dynamic recrystalization, determined from isothermal stress-str ain data differed significantly from those obtained from the as-measur ed compression test data.