Abstract: The mechanical properties and microstructures of tube sheet of steam generator for the small modular reactor use are estimated to examine the uniformity of material in different sections. The composition analysis shows that the C content varies between 0.18wt.%-0.19wt.%, and S and P contents are less than 0.002 wt.% and 0.005 wt.%, respectively. The contents of other alloying elements vary in a very small and reasonable range. The strength at room (23℃) or elevated temperatures (350℃) is generally homogenously distributed in circumferential direction after quench-temper treatment, whereas the corresponding impact toughness has a relatively larger fluctuation with the average and minimum adsorbed energy of 76 J and 47 J for sample E tested at -10℃, which just meets the required value. The uniformly distributed short rod-like carbides in tempered martensite are responded for the improved combination of strength and toughness, whereas the unevenly distributed carbides with irregular shape at grain boundaries formed due to insufficient hardenability induces unexpected mechanical properties, especially the degraded toughness. Generally, the uniform mechanical properties can be achieved throughout the integral forged tube sheet of steam generator, but it seems that the integral forging and heat treatment processing can be further modified after more meticulous consideration on heat treatments.

Key words: heat treatment, heavy forging, impact toughness, SA508 Gr.3 Cl.2 steel, tensile strength, tube sheet