Microstructure and high-temperature tensile behavior of spray-formed modified 2000MPa H13 hot work die steel with 0.5wt % carbon

Abstract

The rapid development in the advanced manufacturing industry asks for better service performance of hot work die steels, especially their high-temperature strength. In this work, a modified SF01 steel based on H13 steel was spray-formed with the carbon content increased to 0.5 wt %. The yield strength (YS) and elongation of SF01 steel at 25°C are about 1824 MPa and 8.6%, respectively, with YS improving by 14.6% and elongation decreasing by 9% compared to conventional H13 steel (about 1591 MPa, 9.6%). It was also found that the YS of SF01 steel was at least 10% larger than H13 steel over the temperature range from 25 to 650°C. The average grain size of tempered SF01 steel is smaller, the dislocation density is lower, and the volume fraction of precipitations is larger than conventional H13 steel (CH13). Furthermore, M3C and VC were observed in tempered SF01 steel, which were not found in CH13. The finer grain size is ascribed to the combined effect of spray forming and the pinning of grain boundaries by VC during 1040°C austenitization process in quenching. A mathematical model was utilized to correlate microstructural characteristics with yield strength. Calculations revealed that the superior YS of SF01 is ascribed to a higher contribution of grain boundary strengthening and precipitation strengthening. The unsatisfied toughness of SF01 steel results from the presence of VC with an average size of 440 nm, which could promote cracks nucleation in the brittle temperature range when subjected to tension loading, and help micropores initiation during plastic deformation. The brittle-to-ductile transition temperature of SF01 is about 200°C, and the deformation mechanism from room temperature to 650°C is briefly discussed.