Importance of deformation induced ferrite and factors which control its formation

Abstract

The influence of strain, strain rate, temperature, and grain size on the formation of deformation induced ferrite has been examined. Deformation induced ferrite forms very readily in both fine and coarse grained steels and much more rapidly than the ferrite from strain free austenite. Very small strains are sufficient to induce the production of such ferrite and the temperature range over which it appears spans from just below the Ae3 temperature down to the undeformed Ar3 temperature. Although it forms readily in both coarse and fine grained steels, the volume fraction produced is sensitive to the austenite grain size. In coarse grained steels, deformation at low strain rates is concentrated along the grain faces; extensive dynamic recovery occurs, which is why the ferrite remains soft, so that only thin ferrite films are able to form. At higher strain rates, work hardening takes place so that the strength of the ferrite at high strains approaches that of the austenite. Under these conditions, the deformation is propagated towards the centres of the austenite grains and larger volume fractions of deformation induced ferrite are able to form. In fine grained steels, the flow stress in the austenite grain boundary region is increased, so that when ferrite first forms, a considerable amount of work hardening takes place, which strengthens the ferrite. When combined with the increased number of triple points present in the material, the increased work hardening promotes spreading of the deformation, with the result that larger volume fractions of ferrite are produced, even at low strains and strain rates.