Superplasticity of Annealed H13 Steel

H13 steel is a widely used hot work die material. A new type of hot working method is imperative to develop complex and precise dies. In this paper, the heat treatment of H13 steel (AISI) was carried out by annealing, the final structure is a point or spherical pearlite, and the grain size is about 30-40 μm. The tensile properties of the annealed microstructure were investigated at 650, 750, and 850 °C with the strain rates of 1 × 10-3 s-1, 5 × 10-4 s-1, and 1 × 10-4 s-1. The tensile fracture and microstructure were analyzed by SEM and HREM. The results show that the tensile samples reach superplasticity at the strain rate of 1 × 10-4 s-1 in the temperature range of 750-850 °C. When the temperature is 850 °C, the maximum elongation rate reaches 112.5%. This demonstrates the possibility of making superplastic forming molds. During the tensile process, the refined M23C₆ and other high hardness carbides which are dispersed uniformly in the matrix, effectively inhibits grain growth and hinders dislocation movement, leading to the improvement of plasticity.