Hot ductility curve of an austenitic stainless steel and importance of dynamic recrystallisation in determining ductility recovery at high temperatures

The importance of dynamic recrystallisation in restoring ductility at the h igh temperature end of the hot ductility though has been examined in an aus tenitic stainless steel. Compression testing was used to establish the crit ical strain for dynamic recrystallisation epsilon(c) for the temperature ra nge 1000 to 850 degrees C using the same str ain rates of 3 x 10(-2), 3 x 1 0(-3), and 3 x 10(-4) s(-1) as had been used in previous work to establish the hot tensile ductility curves. Specimens were heated first to 1175 ol 10 00 degrees C to give a coarse (240 mu m) and finer (80 mu m) grained steel respectively. The flow stress data from the compression tests on the coarse r grained material were used to obtain the str ain to the peak stress epsil on(p) which could then be used to calculate the curve of epsilon(p) versus temperature for use in establishing the temperature at which dynamic recrys tallisation would first occur in a tensile test. For the coal se grained st eel, the hot tensile tests had given ductility troughs for each strain rate with minimum ductility occurring at 850 degrees C, the trough deepening an d widening with decreasing strain rate. The trough was found to be associat ed with the presence of coarse carbides situated at the boundaries. Below 8 50 degrees C, ductility recovered because grain boundary sliding was reduce d. Above 850 degrees C, ductility improved since fewer carbides were precip itated at the boundaries, facilitating dynamic recrystallisation. Recovery in ductility at the high temperature end of the trough in the coarse graine d condition was shown to occur at a temperature close to that at which the base of the trough in the ductility curve intersected the curve of epsilon( c) versus temperature, i.e. when dynamic recrystallisation was possible but only for a strain rate of 3 x 10(-3) s(-1). At higher strain rates the red uction in the rate of grain boundary sliding was sufficient to improve duct ility without the necessity for dynamic recrystallisation, At lower strain rates cracks were able to develop to such a degree that dynamic recrystalli sation was not effective in improving ductility. Refining the grain size el iminated the trough for all the strain rates examined. In this case the cal culated curve of epsilon(p) or epsilon(c) versus temperature intersected th e hot ductility curves at temperatures below the range examined, indicating that dynamic recrystallisation always occur red. It was concluded that dyn amic recrystallisation can have a major influence in restoring ductility at the high temperature end of the trough. However, it must often be well adv anced to be effective and ductility can recover without the necessity for d ynamic recrystallisation, by increasing the strain rate, thus reducing the amount of grain boundary sliding.