The influence of thermomechanical processing on recrystallisation and precipitation in austenitic alloys with particular reference to the effects of deformation and ageing conditions

Abstract

The recrystallisation and precipitation behaviour of an austenitic stainless steel has been studied with particular emphasis on the role of the experimental parameters in controlling the precipitation mechanisms of sigma phase. The alloy, containing 23 wt% chromium and 23 wt% nickel and with negligible carbon (0.004 wt%), was aged isothermally at different temperatures following hot-, warm-, or cold-working. Three distinct behaviours during ageing were found: static recrystallisation on ageing at 900°C; conventional precipitation of sigma phase after static recrystallisation at 800°C; and ‘discontinuous recrystallisation’ (conjoint recrystallisation and precipitation) at 700°C and 600°C. The conditions of thermomechanical processing interacted strongly with ageing history, hot-working having least influence and cold-working the greatest significance, a consequence of the combination of the amount of strain energy available for recrystallisation and the degree of supersaturation for sigma phase. Recrystallisation at adequate supersaturation caused discontinuous recrystallisation, yielding very rapid and heavy precipitation, apparently approaching thermodynamic equilibrium after only a few hours of ageing. By contrast, conventional precipitation after static recrystallisation gave only light precipitation by the same time. The distribution, shapes, and sizes of precipitates were influenced by the mechanism.