Microstructure and high-temperature mechanical properties of RAFM steel processed by friction stir processing combined with tempering heat treatment

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M3C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M23C6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M23C6 precipitates. Meanwhile, the homogeneous distribution and refinement of M23C6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel.