Revealing the failure mechanism of industrial Fe-25Cr-20Ni stainless steel plate: Fine-grained segregation band and brittle phase induced delamination cracking

Abstract

The use of heat-resistant stainless steel thick plates in industrial production often presents challenges due to the presence of uneven microstructure and segregation, which can pose a persistent challenge for the prolonged safe operation of critical components made from such materials. This study aimed to investigate the fracture mechanism of an industrial Fe-25Cr-20Ni stainless steel plate at room temperature through uniaxial tensile experiments and fully reversed strain-controlled tension-compression fatigue experiments. Delamination cracks were observed on the fracture surfaces of both experiments. The specimens with fracture delamination were systematically studied by detailed microstructural evolution, and the failure mechanism and main reasons of fracture delamination were analyzed. The results show that the fracture delamination is related to the fine-grained segregation band (FGSB) consisted of fine grains and sigma phase located at the center of the thickness of the stainless-steel plate. The FGSB demonstrates low transgranular and intergranular fracture modes and exhibits bamboo knobs crack during plastic deformation, leading to a mixed fracture mode of ductile and brittle fracture. Furthermore, FGSB, as an anisotropic microstructure, which has a high stress concentration with nearby grains during plastic deformation. The sigma phase fracture and the weakening of the interface strength of the microstructure in FGSB provide a potential initiation site for fatigue crack initiation, thereby contributing to the possibility of aggravating fatigue damage and deteriorating fatigue performance. It is concluded that FGSB, as a metallurgical defect in stainless steel plates, is responsible for delamination failure.