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 °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 °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.

为了提高SA508 Gr1.A的机械性能，以便在核电厂的主蒸汽管道中应用裂前泄漏（LBB）设计，冷却速率对组织形成，拉伸性能，夏比冲击性能和JR的影响分析了SA508 Gr.1A钢的断裂韧性。随着冷却速度的增加，粗大的铁素体/珠光体组织转变为细小的贝氏体和马氏体，粗大的晶界碳化物消失，细小的碳化物沉淀在晶粒内部。当冷却速度更快时，由于形成了精细的低温转变相，因此在286°C时的强度和伸长率均得到了改善。冷却速度的提高对286°C时的韧性-脆性转变温度（DBTT）和JR断裂韧性也有积极影响。从对JR测试样品的分形图分析可以看出，在粗晶界碳化物上形成的大空隙和裂纹沿着铁素体/珠光体界面传播。冷却速度的提高抑制了粗大碳化物和铁素体/珠光体的形成，并促进了具有有效晶粒度的贝氏体和马氏体的形成，从而改善了SA508 Gr.1A钢的强度和韧性。