Abstract: 30CrNiMo8 steel small ring forgings used in rail vehicles were produced by the conventional processes including ash-cooling or pit-cooling after forging,then quenching and tempering.The corresponding mechanical properties are excellent,but it is found according to the metallographic examination that the grain size is coarse,even is grade 0 for individual batches,which cannot be eliminated even adding a normalizing pretreatment before quenching and tempering.The research results show that 30CrNiMo8 steel has strong microstructure heredity,so the coarse grains are inherited to the subsequent process after forging and cannot be easily refined by the subsequent heat treatment processes.So in the conventional process flow an isothermal annealing process is added before quenching and tempering,which eliminates the coarse grains after forging,and effectively solves the problem of coarse grains in the small ring forgings of 30CrNiMo8 steel.

Key words: 30CrNiMo8 steel, small ring forgings, heat treatment process, grain size, annealing