Effect of δ-ferrite decomposition on the tensile properties of one modified 316H stainless steel: Experimental investigations and crystal plastic finite element simulations

It has been widely reported that the formation of σ phase due to the decomposition of δ-ferrite at specific conditions plays a negative impact on the mechanical properties of steels, but the affecting mechanism has not been fully explored. In this study, one modified 316 stainless steel was designed to obtain the high strength by Ti alloying, which contains some δ-ferrite. To determine the effect of δ-ferrite decomposition, this steel was aged at 750 °C for 100 h. The microstructure observations indicate that δ-ferrite has decomposed into σ phase and secondary austenite phase (γ2), and σ phase particles are also formed at grain boundaries. The tensile test results indicate that the aged samples possess higher yield strength, but poorer ductility than the as-received ones. The fracture observations show that the secondary cracks preferentially occur in the decomposed δ-ferrite rather than the grain boundaries with σ phase, which also indicates that the decomposition of δ-ferrite is the major cause of ductility loss. To deepen the understanding of this common phenomenon, the crystal plasticity finite element method was used to simulate the development of strain distribution during the tensile deformation process. It was found that the strain incompatibility occurs at the interface between the σ phase and the γ2 phase, which becomes more severe in the position with dense σ phase. Our results provide a clearer perspective on the role of the decomposition of δ-ferrite based on the combination of experiments and simulations.