Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

Abstract

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics.