Mechanism of crack initiation and propagation of 316LN stainless steel during the high temperature tensile deformation

Due to the difficult deformability and cracking in forging process, the mechanism of crack initiation and propagation of 316LN stainless steel during the high-temperature tensile deformation are investigated. The thermal tension experiment is carried out on the Gleeble-1500D machine to reveal the relation between the crack propagation path and the degree of the dynamic recrystallization. By the microstructure analysis of the scanning electron microscopy (SEM) on the fracture behavior of various strains and strain rates, the dynamic recovery and dynamic recrystallization occur simultaneously, the formation of cracks was accompanied by recrystallization, and the crack propagation show both scale effect and interface effect. The intergranular ductile fracture perpendicular to the principal stress direction is found to be closely relative to the recrystallization microstructure, where the core position for crack initiation is around the intersection of the tricrystal boundaries. It is found that the tricrystal boundary cracking from the inclusion of aluminum oxide (Al _2 O _3 ) is critical to the decrease of the fracture properties and plasticity, while a certain degree of dynamic recrystallization and the dynamic recovery happened simultaneously from 1000 °C to 1200 °C benefits for the plasticity. Thus, the interface between the austenite matrix and the alumina-based brittle inclusions is easy to form local stress concentration on the tricrystal boundary due to the stacking of dislocations, which becomes the core of crack initiation and thus increases the tendency of forging cracking at high temperature.