Hot workability of stainless steels: influence of deformation parameters,microstructural components,and restoration processes

The literature on the hot ductility of stainless steels is surveyed. The role of different fracture mechanisms is analysed with the aid of the fracture-map concept, proposed recently by Ashby and co-workers. A low hot ductility is practically always associated with a large portion of intercrystalline cracking. The hot ductility is, in general, lower in the temperature interval 900°-1000°C than 1100°-1200°C owing to the presence of dynamic recrystallization in the latter interval, which prevents grain -boundary cracks from growing. At still higher temperatures, the ductility decreases with increasing temperature owing to a loss of grain -boundary cohesion. In addition to a general tendency of reducing the ductility, the influence of a large grain size and high contents of the alloying elements Mo, N, and Ni is characterized by an increased ‘transition temperature’ from the low- to the high-ductility regime. The dependence of ductility on strain rate is less pronounced than the effect of temperature but, in general, the ductility increases with increasing strain rate owing to the suppression of inter crystalline cracking mechanisms. The strong influence of Mo and N can be explained in term s of their effect on flow stress, recovery, and static and dynamic recrystallization, or the presence of eutectic phases and δ-ferrite. This latter phase gives rise to cracking in γ/δ grain boundaries, and its presence, in most cases, has been reported to reduce the ductility.