Effect of process variables on formation of dynamic strain induced ultrafine ferrite during hot torsion testing

Ultrafine grain sizes were produced using hot torsion testing of a 0.11C-1. 68Mn-0.20Si (wt-%) steel, with ultrafine ferrite (< 1 mum) nucleating intra granularly during testing by dynamic strain induced transformation. A syste matic study was made of the effect of isothermal deformation temperature, s train level, strain rate, and accelerated cooling during deformation on the formation of ultrafine ferrite by this process. Decreasing the isothermal testing temperature below the Ae(3) temperature led to a greater driving fo rce for ferrite nucleation and thus more extensive nucleation during testin g; the formation of Widmanstatten ferrite prior to, or early during, deform ation imposed a lower temperature limit. Increasing the strain above that w here ferrite first began (0.8 at 675 C and a strain rate of 3 s(-1)) increa sed the intragranular nucleation of ferrite. Strain rate appeared to have l ittle effect on the amount of ferrite formed. However, slower strain rates led to extensive polygonisation of the ferrite formed because more time was available for ferrite recovery. Accelerated cooling during deformation fol lowed by air cooling to room temperature led to a uniform microstructure co nsisting of very fine ferrite grains and fine spherical carbides located in the grain boundaries regions. Air cooling after isothermal testing led to carbide bands and a larger ferrite grain size.