Microstructure and mechanical properties of the new cladding austenitic stainless steel of 15-15Ti-Y after aging at 500-700 °C for up to 1000 h

As a promising material for fuel cladding, 15Cr-15Ni-Ti (15-15Ti) austenitic stainless steel is widely used in core materials of nuclear reactors due to its excellent creep and oxidation resistance. To further enhance the stability of 15-15Ti steel in ultra-long service time at high temperature, the yttrium-modified 15Cr-15Ni-Ti (15-15Ti-Y) was aged at 500 & DEG;C, 600 & DEG;C and 700 & DEG;C for up to 1000 h. The grain size and precipitation behavior including precipitates species, morphology, quantity, formation sequence and location were systematically investigated. Through the detection and analysis of electron backscatter diffraction, scanning electron microscope, X-ray diffraction and transmission electron microscope, the results indicates that Y is a strong solute segregation element, which hinders the formation and coarsening of M23C6 phase along the grain boundary, thus facilitating the formation of fine intragranular precipitation. At the same time, the addition of Y inhibits recrystallization, grain growth and intergranular precipitation, and promotes intragranular precipitation. Therefore, after aging at 500 & DEG;C for 1000 h, the Chib particles only precipitated in 15-15Ti-Y steel, while the size of precipitates in grains interiors and area fraction of M23C6 phase along grain boundaries in 15-15Ti-Y steel were always smaller than that of 15-15Ti steel. Besides, the effect mechanism of Y addition on precipitation behavior and mechanical properties of 15-15Ti-Y after aging is also discussed. The comparison shows that the hardness of the two steels has little change after aging, but the Charpy impact toughness and tensile properties have decreased to varying degrees. As the main reason affecting the mechanical properties, the intergranular precipitation in the chain after low temperature aging weakens the impact toughness of 15-15Ti, while the fine intragranular precipitation after high temperature aging can improve the strength of 15-15Ti-Y. Therefore, compared with 15-15Ti steel, 1515Ti-Y steel shows higher hardness, impact toughness and uniform elongation after aging at 500-700 & DEG;C for 1000 h.