The martensitic crystallography and strengthening mechanisms of ultra-high strength rare earth H13 steel

Abstract

Advanced die manufacture demands hot work die steels with superior strength. Unfortunately, due to the rare understanding of the crystallographic structures and strengthening mechanisms of H13 steel, the contradictory between ultra-high strength and low ductility remains inevitably as far as authors known. In this work, we developed a strategy combining rare earth addition, electroslag remelting and pre-tempering treatment, and prepared rare earth (Re) H13 steel with ultra-high ultimate tensile strength up to 2 gigapascals (GPa) and good ductility. The Re H13 steel underwent pre-tempering treatment exhibited hardness of ca. 53.6 HRC, ultimate tensile strength of ca. 2029 MPa, yield strength of ca. 1654 MPa, total elongation of ca. 9.3% and impact energy of ca. 16.0 J (superior grade of NADCA#207–2003 standard), respectively. The good ductility of Re H13 steel mainly attributed to high density of high angle grain boundaries (45° <) of ca. 1.45 μm−1, which derived from high length fraction of V1/V2 inter-variant boundaries (66.9%) by quenching, as well as coarsen sub-grains and variants refinement by subsequent tempering. The dislocation and carbide precipitates have the dominant role in controlling the strengthening of tempered Re H13 steel. The ultra-high strength of Re H13 steel mainly attributed to its high dislocation density (5.72 × 1014 m−2) and high carbide precipitates density (10.1 μm−2), which raised by 73.3% and 21.7% compared to Re H13 steel treated by conventional twice tempering (600 °C) process. The present work not only reveals the strengthening mechanisms of ultra-high strength Re H13 steel, but also characterizes the contributing microstructure feature to its good ductility.