The effect of the intercritical annealing process on the microstructure evolution, strength, and toughness of cryogenic 5.5Ni steel

This study investigates the influence of intercritical annealing time on the evolution of the microstructure of cryogenic 5.5Ni steel's strength and toughness under QLT heat treatment conditions. Testing revealed that the microstructure of the cryogenic 5.5Ni steel primarily consists of martensite (lath/block) and ferrite (lath/polygon). The impact toughness of the tested steel is influenced mainly by the ratio of high to low-angle grain boundaries and the distribution of soft and hard phase microstructures. The mechanical properties are affected by fine-grain strengthening and dislocation motion. The findings indicate that fine-grain strengthening is the primary strength of the tested steel during the short intercritical annealing treatment. Conversely, during the extended intercritical annealing process, dislocation motion becomes predominant. Addressing the negative impacts of grain growth during extended intercritical annealing treatment is essential to improve yield and tensile strength while minimizing plasticity and impact toughness. Additionally, dislocations were found to be dispersed along the edges of grain boundaries and aggregated at these boundaries in the tested steels. The microstructure features parallel dislocations and dislocation jogs, which increase the material's deformation resistance. When subjected to external stress, dislocation jogs can successfully restrict the mobility of some dislocations and prevent them from expanding. Under external stress, dislocation jogs effectively limit the mobility of inevitable dislocations, preventing their propagation. This enhances both the mechanical properties and impact toughness of the tested steel.