On-line quenching and self tempering is the advanced technology in the production of large bars.Considering the latent heat of phase transformation and the changes of thermal physical properties of material with temperature,the temperature fields of online quenching and air cooling process after rolling of large diameter bars(more than 50 mm in diameter)of 45 steel and 42 CrMo with different specifications were simulated with the finite element software MSC.Marc.The critical cooling speeds of quenching were...