Shilei Li

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.

Effects of thermal aging temperature and Cr content on phase separation kinetics in Fe-Cr alloys simulated by the phase field method

Phase field simulations of phase separation in Fe-Cr binary alloys were performed by using the Cahn-Hilliard diffusion function. A new mobility model in relation to aging temperature and Cr content was used in the simulations. Two alloys of Fe-30at%Cr and Fe-35at%Cr were investigated at two different aging temperatures of 573 and 673 K. The phase separation kinetics was found to consist of three stages: wavelength modulation, amplitude increase, and coarsening of Cr-enriched regions. A higher thermal aging temperature accelerated the phase separation and increased the wavelength of concentration fluctuation. While the effect of Cr content on the phase separation kinetics was slight, Fe-Cr alloys with a higher Cr content were found to generate a larger number and a finer size of Cr-enriched regions. The simulation results provide consultation for design and safe operation of duplex stainless steel pipes in nuclear power plants.

Effect of thermal aging on the fatigue crack growth behavior of cast duplex stainless steels

The effect of thermal aging on the fatigue crack growth (FCG) behavior of Z3CN20?09M cast duplex stainless steel with low ferrite content was investigated in this study. The crack surfaces and crack growth paths were analyzed to clarify the FCG mechanisms. The microstructure and micromechanical properties before and after thermal aging were also studied. Spinodal decomposition in the aged ferrite phase led to an increase in the hardness and a decrease in the plastic deformation capacity, whereas the hardness and plastic deformation capacity of the austenite phase were almost unchanged after thermal aging. The aged material exhibited a better FCG resistance than the unaged material in the near-threshold regime because of the increased roughness-induced crack closure associated with the tortuous crack path and rougher fracture surface; however, the tendency was reversed in the Paris regime because of the cleavage fracture in the aged ferrite phases.

Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD

The effect of thermal aging on phase transformation and impact toughness of an as-cast duplex stainless steel was investigated at room temperature. After long-term thermal aging, the impact toughness decreases significantly and the cracks initiate and propagate more easily. The plastic deformation ability of the ferrite phase decreases after thermal aging, which leads to the degradation of impact toughness. High stress concentration occurs on the grain boundaries of the austenite phase in the aged materials. Meanwhile, high-stress concentration areas are also observed in the austenite phase near the grain boundaries. After long-term thermal aging, pinned dislocations in ferrite and along phase boundaries lead to the high stress concentration. Micro-cracks preferentially initiate in the ferrite phase and propagate via separation of phase boundaries. The blocking influences of spinodal decomposition precipitates and G-phase precipitates are stronger than the effect of grain boundaries and phase boundaries on the dislocation movement.

Study on LBB Behavior of Nuclear Primary Pipes After Long-Term Thermal Aging

The leak-before-break (LBB) analysis is a high level safety assessment method for pressurized vessels and pipes containing defects. The key technology of LBB safety assessment is the crack stability analysis and the determination of critical crack length. In this paper, on the basis of experimental data of a pipe material, the evolution of the fracture toughness after long-term thermal aging at 280 and 330 °C were estimated using the Argonne National Laboratory (ANL) procedure. Three-dimensional finite element analysis models were built to make elastic-plastic fracture mechanics analysis for the postulated circumferential through-wall cracks with different sizes in the nuclear primary pipe. According to the J-integral tearing modulus (J-T) method, the critical crack length were obtained and LBB curves of circumferential through-wall cracked pipes were constructed. Effects of the fracture toughness and operating temperature of nuclear primary pipe on critical crack length and LBB curves were also investigated. The results show that the pipe material is sensitive to thermal aging embrittlement during long-term service, which results in the critical crack length and LBB curve significantly decreasing due to degradation of fracture toughness. The non-conservative (unsafe) results will be produced if LBB analysis does not take into account of the effect of thermal aging. Compared to the results at 280 and 330 °C, the pipes suffer more serious thermal aging embrittlement and the degree of non-conservative in LBB analysis become larger when operating at a higher temperature.