Unveiling the relationship of fatigue behavior with the microstructure of 321 stainless steel with gradient structure at 550 °C

Abstract

In this work, stress-controlled tension-compression fatigue and uniaxial tensile experiments were conducted at 550 °C on gradient-structured (GS) 321 austenitic stainless steel fabricated by surface mechanical rolling treatment (SMRT). The microstructural evolution, residual compressive stress (RCS) effect, cyclic deformation behavior, fracture and fatigue behavior were unraveled. Uniaxial results indicate that the yield strength and tensile strength of the GS321 specimen are improved by 112 % and 18.6 %, respectively, which is attributed to the thicker GS layer and SMRT-induced martensite strengthening. Fatigue results confirm that the fatigue strength of GS321 is improved and the fatigue life is increased by at least ∼ 10 times. During fatigue loading at 550 °C, the grain size within the surface layer of the GS321 specimen remains stable, but a significant reverse transformation of martensite to ultrafine austenite is observed. Additionally, the RCS within the GS surface relaxes by 67.2 % prior to fatigue, and it further rapidly relaxes with cyclic loading. These results suggest that the RCS had a limited effect on the fatigue properties of the GS321 specimen at 550 °C. The excellent fatigue property of the GS321 specimen is attributed to the thicker GS layer, martensitic phase transformation strengthening, and the synergistic effect. This study can provide guidance for the design and application of stainless steel with superior fatigue performance at elevated temperature.