Ultra-high strength and toughness of a novel hot-work tool steel achieved through a powder metallurgy approach

This study employs self-developed, third-generation gas atomization technology in combination with a powder metallurgy (PM) route to fabricate hot-work tool steels. A high-alloy hot-work tool steel was designed, and trace nitrogen was introduced during gas atomization, yielding a novel tool steel that simultaneously exhibits ultra-high strength and good toughness. The microstructures and phase compositions of the steel in both the PM and forged conditions were characterized in terms of optical microscopy, X-ray computed tomography, X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. Tensile and impact tests were conducted, and the relationships between microstructural features and mechanical behavior in the two conditions were comparatively analyzed. The results show that the PM steel exhibits uniform chemical composition, fine martensitic laths (width: 0.62–0.86 μm), and uniformly dispersed nanoscale M(C,N) carbonitrides, together with M 3 C and M 7 C 3 carbides. Nitrogen addition promotes the formation of stable M(C, N) phases and refines the prior-austenite grains through a Zener pinning effect. After forging, the porosity is further reduced to below 0.03%, accompanied by a marked enhancement in mechanical properties: a yield strength of 1604.5 MPa, tensile strength of 1881.7 MPa, elongation of 9.8%, and impact energy of 20.5–22.5 J. The strength improvement of forged steel is primarily attributed to precipitation strengthening from fine, dispersed nanoscale M(C, N) carbonitrides and M 3 C carbides, complemented by fine-lath martensite strengthening and dislocation strengthening. This work demonstrates that the combined PM-forging route offers substantial potential for manufacturing hot-work tool steels with superior microstructural uniformity, low defect content, and high performance.