Tested steel 18CrNiMo7-6 (/%: 0. 17C

0. 59Mn

0. 24Si

1.56Ni

1.71Cr

0. 28Mo) is a case-hardening steel for gear

it is required that the structure of normalized steel is ferrite + pearlite and has lower HB hardness value. The higher hardness bainite structure is easily obtained in steel 18CrNiMo7-6 after continuous cooling. The tested results by high temperature chamber electric resistance furnace in laboratory show that with 870 〜900 °C 1 h-640 ~ 660°C 4 h furnace cooling to 300 °C air cooling the structure of steel is ferrite + pearlite + bainite with HB hardness value 340 ~ 350

while with 870 -900 X. 1 h

30 Y/h cooling to 640 ~660 °C furnace cooling to 300 °C air cooling the structure of steel is ferrite + pearlite with HB hardness value 190 ~ 210. The commercial production test results show that with 900 °C 10 h

≤30°C /h cooling to 650 °C 25 h

then 30 °C/h cooling to 500 °C air cooling for 30 t Φ180 mm finished forged products the structure of steel 18CrNiMo7-6 has ferrite + pearlite structure.