Impact toughness of a modified HR3C austenitic steel after long-term thermal exposure at 650 °C

Abstract

The impact toughness of a modified HR3C austenitic heat-resistant steel (M-HR3C), a promising candidate for the application of next generation power plants beyond 650 °C, has been investigated systemically after long-term thermal exposure at 650 °C. This steel exhibits excellent impact toughness, which is almost 3 times as high as that of a commercial HR3C steel (C-HR3C) after exposure for 10,000 h. The microstructure observations by scanning electron microscope (SEM) and transmission electron microscope (TEM) indicate that the more addition of Nb and C and the fewer addition of Cr in M-HR3C promote the precipitation and growth of primary MX-phase during the final stage of solidification and inhibit the precipitation and growth of M23C6 carbide at grain boundaries (GBs) during thermal exposure. The fine and discontinuous M23C6 carbides in M-HR3C slow down the decline rate of impact toughness by decreasing the weakening rate of GBs strength. The tensile tests were conducted to further analyze the impact toughness for thermal exposure up to 500 h. During this period, the change of impact toughness of M-HR3C is significantly different from that of C-HR3C. The results show that the more and larger primary MX-phase in M-HR3C cause its tensile strength and plasticity to be lower than those of C-HR3C during tensile tests. However, for impact tests with a higher load and a faster strain rate, the plasticity of M-HR3C for contribution to impact deformation is greater than that of C-HR3C, which is the main reason why impact toughness of M-HR3C is higher than that of C-HR3C during thermal exposure. The mechanisms related to impact toughness are discussed.