The effects of annealing temperature and cooling rate on carbide precipitation behavior in H13 hot-work tool steel

The precipitation behavior of H13 hot-work tool steel was investigated as a function of both annealing temperature and cooling rate through thermodynamic calculations and microstructural analyses using transmission and scanning electron microscope and a dilatometer. The V-rich MC carbide and Cr-rich M7C3 and M23C6 carbides were observed in all annealed specimens regardless of annealing and cooling conditions, as expected from an equilibrium phase diagram of the steel used. However, Mo-rich M2C and M6C carbides were unexpectedly precipitated at a temperature between 675 °C and 700 °C during slow cooling at a rate of below 0.01 °C/s from the annealing temperatures of 830 °C and below. The solubility of Mo in both M7C3 and ferrite reduces with decreasing temperature during cooling. Mo atoms diffuse out of both M7C3 and ferrite, and accumulate locally at the interface between M7C3 and ferrite. Mo carbides were form at the interface of M7C3 carbides during the transition of Cr-rich M7C3 to stable M23C6.