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.