Effect of microstructural homogeneity on ultra-low temperature impact fracture mechanism of high-strength 9%Ni steel

• Segregation of Ni and Mn elements formed tempered martensitic banded structure. • The heterogeneous microstructure exhibited a mixed fracture mode consisting of micro-voids coalescence ductile fracture and transgranular cleavage fracture. • A uniform microstructure with fine austenite, high-angle grain boundaries, and low kernel average misorientation hindered crack propagation. • The uniform microstructure exhibited ultra-high crack propagation energy, achieving excellent shear-fracture percentage (100%). The influence of microstructural homogeneity on the ultra-low temperature impact fracture mechanism of 9 %Ni steel was studied through microstructure characterization, ultra-low temperature impact toughness testing and analysis of crack propagation. The results indicated that for the tested steel with inhomogeneous microstructure, the segregation of Ni and Mn elements formed tempered martensitic banded structure at the segregation zone, while other non-segregated zones were uniformly tempered sorbite. At the tempered martensitic banded structure, the residual austenite was coarse in size and distributed in a strip shape, with fewer high angle grain boundaries and higher KAM values, resulting in banded structures being unable to effectively hinder the propagation of impact cracks. For the tested steel with uniform microstructure, its microstructure was a single and uniform tempered sorbite, which exhibited ultra-high crack propagation energy, resulting in excellent ultra-cryogenic impact absorption energy (245 J) and shear-fracture percentage (100 %). The tested steel with uniform microstructure reflected a single micro-voids coalescence ductile fracture mode. The residual austenite in the uniform microstructure area without element segregation was uniformly dispersed in a spherical shape or thin film shape, with more high angle grain boundaries and lower KAM values, which can effectively hinder crack propagation.