Abstract
The importance of dynamic recrystallisation in restoring ductility at the high temperature end of the hot ductility trough has been examined in an austenitic stainless steel. Compression testing was used to establish the critical strain for dynamic recrystallisation ɛc for the temperature range 1000 to 850°C using the same strain rates of 3 × 10-2, 3 × 10-3, and 3 × 10-4 s-1 as had been used in previous work to establish the hot tensile ductility curves. Specimens were heated first to 1175 or 1000°C to give a coarse (240 μm) and finer (80 μm) grained steel respectively. The flow stress data from the compression tests on the coarser grained material were used to obtain the strain to the peak stress ɛp which could then be used to calculate the curve of ɛp versus temperature for use in establishing the temperature at which dynamic recrystallisation would first occur in a tensile test. For the coarse grained steel, the hot tensile tests had given ductility troughs for each strain rate with minimum ductility occurring at 850°C, the trough deepening and widening with decreasing strain rate. The trough was found to be associated with the presence of coarse carbides situated at the boundaries. Below 850°C, ductility recovered because grain boundary sliding was reduced. Above 850°C, ductility improved since fewer carbides were precipitated at the boundaries, facilitating dynamic recrystallisation. Recovery in ductility at the high temperature end of the trough in the coarse grained 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 ɛc versus temperature, i.e. when dynamic recrystallisation was possible but only for a strain rate of 3 × 10-3 s-1. At higher strain rates, the reduction in the rate of grain boundary sliding was sufficient to improve ductility without the necessity for dynamic recrystallisation. At lower strain rates cracks were able to develop to such a degree that dynamic recrystallisation was not effective in improving ductility. Refining the grain size eliminated the trough for all the strain rates examined. In this case the calculated curve of ɛp or ɛc versus temperature intersected the hot ductility curves at temperatures below the range examined, indicating that dynamic recrystallisation always occurred. It was concluded that dynamic recrystallisation can have a major influence in restoring ductility at the high temperature end of the trough. However, it must often be well advanced to be effective and ductility can recover without the necessity for dynamic recrystallisation, by increasing the strain rate, thus reducing the amount of grain boundary sliding.
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