Effect of Nickel on Continuous Cooling Transition Law and Impact Energy of New Hot Work Die Steel

The continuous cooling transition curve （CCT curve） and impact energy curve of QDH of the new hot work die steel with 0.8% Ni and without Ni were determined by thermal expansion phase change instrument and impact testing machine， and the influence of Ni on the continuous transformation law and impact energy of QDH steel were analyzed by combining the microstructure and hardness. The test results of thermal expansion curve show that Ni element can significantly reduce the critical phase transition point （Ac1， Ac3 and Ms） of QDH steel， expand the austenite phase region， shift the CCT curve to the right as a whole， reduce the critical cooling rate of pearlite from 0.03 ℃/s to less than 0.02 ℃/s， and reduce the critical cooling rate of martensitic from 0.4 ℃/s to 0.2 ℃/s， which improves the hardenability of steel. The test results of impact energy curves show that the first type of tempering brittleness and the second type of tempering brittleness of the two experimental steels are in the tempering temperature ra