Effect Of Heat Treatment Process On Microstructure And Properties Of 34CrNiMo6

The 34CrNiMo6 steel is an important kind of tempering steel for Beiman special steel.But in production,grain size of the steel was often too large,resulting in the high unqualified rate of mechanical properties.In this paper,the different heat-treatment states of the 34CrNiMo6 steels were investigated by using microstructural observation,tensile test,impact test,and hardness test,to exploring the effects of the conventional heat treatment and the intercritical heat treatment on the microstructures and mechanical properties.The main results were as follows:After normalizing,the matrix of 34CrNiMo6 steel was martensite and the feather like upper bainite.With increasing nonnalizing temperature and time,the volume fraction of martensite increases whilst that of bainite decreases,accompanying a decreasing grain size.Hardness and toughness of steel increase first and then decrease.This phenomenon is resulted from a decreasing upper bainite and an increasing grain size.The optimum normalizing temperature is 870?,and the thermal insulation coefficient is 1.91 mm/min.After conventional heat treatment,the matrix of the 34CrNiMo6 steel is tempered sorbite and the dispersed fine carbides.With increasing quenching temperature,hardness and toughness are increase first and then decrease.The main reason is due to an increase in grain size and the increasing supersaturation degree of martensite,promoting the precipitate of carbides during the subsequent tempering processAfter quenching,the strength and hardness of the 34CrNiMo6 steel decrease and toughness increase with the increase of tempering temperature.This is mainly due to the recovery of dislocations and the precipitation of granular carbide during tempering.The optimum quenching temperature and tempering temperature are 860? and 560?After intercritical heat treatment,the matrix of the 34CrNiMo6 steel is ferrite,martensite and bainite.With the increase of the quenching temperature,the volume fraction of ferrite decreases and grain size increases,accompanying with the first increasing and subsequently decreasing hardness and toughness.The main reason is because the strength and hardness of ferrite are lower than martensite.Therefore,the variation of volume fraction and grain size of ferrite with the increase of quenching temperature will affect the strength and hardness However,the effect of quenching temperature on toughness is slightly different due to the fact that only acicular ferrite phase can improve the toughness of the steel.As the quenching temperature is low,the ferrite is dominant and decreases with the increasing quenching temperature,leading to an increase in the toughness.Subsequently,the toughness decreases because of the increasing grain size.The strength and hardness of the steel decrease and toughness increases after the intercritical heat treatment,due to the effect of acicular ferrite.The optimum quenching temperature and tempering temperature for the 34CrNiMo6 steel are 770? and 580?.