Effect of large-strain warm deformation prior to austenitization on gamma grain size in 0.3%C-9%Ni steel

gamma grain size refinement is one of the most important factors for micros tructural control in heat treatment. Cold or warm deformation prior to aust enitization is known to have a beneficial effect on gamma grain size reduct ion, and in this paper, the effect of large-strain deformation at a tempera ture range just below AcI transformation temperature on gamma grain size af ter austenitization was studied using 0.3%C-9%Ni steel. Cylindrical specime ns were deformed up to 70% in a single pass by compression using laboratory deformation simulator. Increase in reduction ratio up to 50% continuously refined gamma grain size down to 2 mu m after subsequent reheating to auste nitization temperature: 700 degrees C. Drastic change in alpha --> gamma tr ansformation behavior was found for the reduction above 70%. Spontaneous re verse transformation was induced by such a large strain deformation even wi thout subsequent reheating. It was revealed by TEM observation that gamma g rain size of the specimen was remarkably refined down to around 0.5 mu m. A diabatic heating due to deformation likely contributes to such a spontaneou s transformation. Transformation mechanism for the spontaneous transformati on was discussed.