Isothermal stress-relief and stress-relaxation of cold-extruded mild steel rods

In the present investigation, the kinetics of stress-relief in cold-extruded mild steel rods have been obtained by the longitudinal slitting method, the fractional residual stress remaining as a function of heating time being estimated at each annealing temperature. Corrections have been applied for the time taken to reach the required annealing temperatures. It is found that stress-relief is substantially completed at approximately 500°C in one hour with little change in hardness from that of the as-extruded rod. The residual stress distribution has also been measured, using the Sach's bore-out method. Stress-relaxation and strain rate cycling tests have been conducted on the cold-extruded and annealed mild steel rods at temperatures ranging from room temperature to 500°C. Attempts are made to correlate the results of the stress-relaxation and stress-relief tests. Theoretical and experimental approaches, based on constant strain are presented. Some of the theoretical and experimental analyses, which have been successfully applied to stress-relaxation tests at low temperature (i.e. below room temperature) are found to be inadequate in analysing the stress-relaxation tests on both the cold extruded and annealed mild steel rods over the test temperature range because the assumptions, on which they are based become invalid. The apparent activation energies, as measured from the temperature dependence of the rate of reduction in residual stress or stress relaxation, for both the stress-relaxation and stress-relief tests are approximately equal over the same temperature considered and are found to increase as the residual stress or applied stress decreases. The same mechanisms are suggested to be operative in both the stress-relief and stress-relaxation tests. Dislocation-point defects interaction mechanism has been suggested to operate at the low temperature of the test temperature range, while the higher temperatures have been associated with dislocation climb-controlled processes. Hot tensile tests have been conducted on the extruded specimens while room temperature tensile tests have also been performed on specimens previously subjected to stress-relaxation tests. Very small changes in hardness, due to a precipitation phenomenon, occur during stress relief. Electron transmission examinations of the extruded and stress-relaxed extruded specimens are presented.