High strain deformation of steels

The changes in mechanical properties and structure which occur when metals are deformed to high strains by cold working are summarised from the published literature, with particular emphasis on the behaviour of iron. In addition to this, the effects of second phase particles, the deformation behaviour during machining and the influence of strain history on ductile fracture are also outlined. The results of mechanical tests, carried out at various stages of deformation, are compounded to show the overall changes in mechanical properties occurring during the process, which are then interpreted in terms of the results of structural investigations and fractography. Investigations have included a number of different steels, which were selected and heat treated in order to investigate the fundamental changes which occur, the affect on deformation and subsequent fracture of free machining additions and the affect of a dispersion of hard particles. It is concluded that deformation is microscopically heterogeneous, that changes in mechanical properties are associated with the formation and subsequent behaviour of an increasing number of microbands in the substructure, that ductile fracture is nucleated at pre-existing fracture centres which are formed early in the deformation process and that these play an important part in the mechanism of machine chip formation. An important, previously unreported strengthening mechanism is seen to be operating in one of the steels