The aim of the present work was to establish quantitative relationships between the ¯ow stress and the volume fraction of dynamic recrystallisation (DRX) as a function of processing variables such as strain rate, temperature, and strain for AISI type 4140 medium carbon steel, by means of torsion tests. Torsion tests were carried out in the temperature range 900 ± 1100°C and the strain rate range 5 . 0610 2 ± 5 . 0610 0 s 1 to study the high temperature softening behaviour. For the exact prediction of ¯ow stress, the effective stress ± effective strain curves were divided into two regions, the work hardening and dynamic recovery region and the DRX region. The ¯ow stress of the DRX region could be expressed in terms of the volume fraction of DRX. It was found that the calculated results were in agreement with the experimental ¯ow stress and microstructure of the steel for any deformation condition.MST/4895

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of ow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the ow curve at 1000°C and 0 . 5 s 1 , the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 mm). The calculation of the grain size was based on X DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a ne grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour. MST/5437The authors are in the