Effect of cooling rate on mechanical properties of SA508 Gr.1A steels for main steam line piping in nuclear power plants

摘要

To improve mechanical properties of SA508 Gr1.A steels for application of leak-before-break (LBB) design in main steam line piping in nuclear power plants, the effect of cooling rate on microstructure formation, tensile properties, Charpy impact properties, and J-R fracture toughness of SA508 Gr.1A steels was analyzed. As cooling rate increased, coarse ferrite/pearlite microstructure changed to fine bainite and martensite, and coarse grain boundary carbides disappeared and fine carbides precipitated inside grains. When the cooling rate is faster, both strength and elongation at 286 degrees C improved owing to the formation of fine low-temperature-transformation phases. The increase in cooling rate also have positive effect to the ductile-brittle transition temperature (DBTT) and J-R fracture toughness at 286 degrees C. From the fractograph analysis of J-R tested specimens, it is revealed that large voids formed at the coarse grain boundary carbides and crack propagated along the ferrite/pearlite interface. The increase in the cooling rate suppressed the formation of coarse carbides and ferrite/pearlite and promoted the formation of bainite and martensite having fine effective grain size, which improved the strength and toughness of the SA508 Gr.1A steels.