IMPORTANCE OF DEFORMATION-INDUCED FERRITE AND FACTORS WHICH CONTROL ITS FORMATION

The influence of strain, strain rate, temperature, and grain size on t he formation of deformation induced ferrite has been examined. Deforma tion induced ferrite forms very readily in both fine and coarse graine d steels and much more rapidly than the ferrite from strain free auste nite. Very small strains are sufficient to induce the production of su ch ferrite and the temperature range over which it appears spans from just below the Ae(3) temperature down to the undeformed Ar-3 temperatu re. Although it forms readily in both coarse and fine grained steels, the volume fraction produced is sensitive to the austenite grain size. in coal se grained steels, deformation at low strain rates is concent rated along the grain faces; extensive dynamic recovery occurs, which is why the ferrite remains soft, so that only thin ferrite films are a ble to form. At higher strain rates, work hardening takes place so tha t the strength of the ferrite at high strains approaches that of the a ustenite. Under these conditions, the deformation is propagated toward s the centres of the austenite grains and larger volume fractions of d eformation induced ferrite are able to form. In fine grained steels, t he flow stress in the austenite grain boundary region is increased, so that when ferrite first forms, a considerable amount of work hardenin g takes place, which strengthens the ferrite. When combined with the i ncreased number of triple points present in the material, the increase d work hardening promotes spreading of the deformation, with the resul t that larger volume fractions of ferrite are produced, even at low st rains and strain rates. (C) 1997 The Institute of Materials.