Low-cycle fatigue behavior and life prediction of fine-grained 316LN austenitic stainless steel
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Low-cycle fatigue behavior and life prediction of fine-grained 316LN austenitic stainless steel Zhe Zhang1, An Li1, Yanping Wang2, Qiang Lin1,a), Xu Chen1 1
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China School of Chemical Engineering, Inner Mongolia Polytechnic University, Hohhot 010051, China a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 9 August 2020; accepted: 28 October 2020
Grain refinement has been applied to enhance the materials strength for miniaturization and lightweight design of nuclear equipment. It is critically important to investigate the low-cycle fatigue (LCF) properties of grain refined 316LN austenitic stainless steels for structural design and safety assessment. In the present work, a series of fine-grained (FG) 316LN steels were produced by thermo-mechanical processes. The LCF properties were studied under a fully reversed strain-controlled mode at room temperature. Results show that FG 316LN steels demonstrate good balance of high strength and high ductility. However, a slight loss of ductility in FG 316LN steel induces a significant deterioration of LCF life. The rapid energy dissipation in FG 316LN steels leads to the reduction of their LCF life. Dislocations develop rapidly in the first stage of cycles, which induces the initial cyclic hardening. The dislocations rearrange to form dislocations cell structure resulting in cyclic softening in the subsequent cyclic deformation. Strain-induced martensite transformation appears in FG 316LN stainless steels at high strain amplitude (Δe/2 = 0.8%), which leads to the secondary cyclic hardening. Moreover, a modified LCF life prediction model for grain refined metals predicts the LCF life of FG 316LN steels well.
Introduction 316LN austenitic stainless steel is a prospective structural material for primary circuit piping systems in pressurized-water reactor (PWR) nuclear power plants (NPPs) due to its good high-temperature mechanical properties as well as excellent pitting and crevice corrosion resistance [1, 2, 3]. The primary circuit pipelines of NPPs are often subjected to cyclic loading and alternating temperature, especially during start/stop operation [4, 5, 6, 7]. Therefore, it is imperative to investigate the low-cycle fatigue (LCF) properties of 316LN steels for structural design, safety assessment, and life extension of primary circuit piping systems. Nowadays, the conventional grain size of wrought 316LN steels for NPPs main pipelines ranges from 90 to 180 μm, with yield strengths of approximately 250 to 400 MPa [8]. Usually, some fine grains (FG) can be formed in the primary circuit pipelines due to the heterogeneous distributions of temperature and thermo-mechanical deformation [6]. The relatively low yield strength of 316LN steels restricts miniaturization and lightweight design of nuclear equipment. Therefore, it is of great interest to enhance the yield strength of 316LN steels for extensive
applications by grain refinement. Recent developments in the
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