The subcritical spheroidization annealing

ordinary spheroidizing annealing

and isothermal spheroidizing annealing heat treatment process experiment on 20CrNiMo steel are carried out. By means of optical microscope and brinell hardness tester is carried out to observe respectively after the spheroidizing microstructure. The results show that the hardness value in 20CrNiMo steel can be controlled below 160HBW through ordinary spheroidizing annealing and isothermal spheroidizing annealing ； when the subcritical spheroidizing annealing at 710 °C

the spheroidization rate has increased with the extension of time

when the spheroidizing annealing time more than 25 h

the spheroidization rate may be more than 65% ； the normal spheroidizing annealing process

which is held at 750 °C for 6 h and slow cooling

can obtain 83% spheroidizing rate； by using isothermal spheroidizing annealing

when at 750 °C for 6 h

the in 30 min furnace cooling to 650 °C for 6 h

the spheroidizing rate is 93% and the hardness value is 145HBW.