Tempering temperature and strengthening and toughening correlation for subcritical quenched 45 steel

The relationships between the tempering temperature and strengthening and toughening of sub-critical quenched 45 steel were investigated by metallographic examination,hardness testing,impact and tensile experiments and related factors were discussed.The results show that when the sub-critical quenching temperature is higher than 770 ℃,quenching hardness is not lower than conventional quenching hardness at 840 ℃.The effect of the tempering temperature on the hardness of sub-critical quenching microstructure shows the tempering hardness at temperature below 400 ℃ is higher than that of conventional quenching at 840 ℃ following the tempering at same temperature.Whereas high temperature tempering is performed,tempering hardness changes in contrast.At the tempering temperature range from 200 ℃ to 600 ℃,the strength of the sub-critical quenching and tempering is lower than that of conventional quenching at 840 ℃ and tempering,but there is the contrary trend for tempering toughness.The strength of the sub-critical quenching following tempering at 400 ℃ reaches the extremum.The improvement on tempering hardness and toughness by residual ferrite exceeds that on the tempering strength.