Improving creep properties of RAFM steel with high V & Ta

Reduced activation ferritic-martensitic (RAFM) steels are promising candidates for fusion reactor blankets, and their creep properties are significantly influenced by characteristics of MX carbonitrides. In this study, a RAFM steel with high contents of V (0.27 wt%) and Ta (0.18 wt%) was fabricated to promote the formation of numerous nanoscale MX carbonitrides, designated as 9Cr-MX. The 9Cr-MX steel was subjected to creep tests at 650 °C with 100, 120, 175, 200 MPa and 700 °C with 40, 50, 60 MPa. The creep rupture life, minimum creep rate and Larson-Miller parameter (LMP) values were obtained. The microstructure of the steel, including martensitic laths, M₂₃C₆ carbides, MX carbonitrides, was characterized utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in both the tab and gauge sections before and after creep. The results indicated that 9Cr-MX steel exhibited favorable creep properties. The steel achieved a rupture life of 3566 h, a minimum creep rate of 2.83 × 10⁻⁹ s⁻¹, an LMP value of 32.6 at 700 °C with 40 MPa. These properties were related to the numerous nanoscale MX carbonitrides with a size of 35 ± 21 nm and a number density of 1.73 × 10²⁰ m⁻³. On the one hand, MX carbonitrides were found to impede laths rotation, coalescence and coarsening, thereby elevating creep deformation resistance. On the other hand, these carbonitrides enhanced the resistance to propagation of creep microcracks, and triggered microcrack deflection. Besides, an intriguing observation was made of an increase in V concentration within MX carbonitrides after creep, potentially due to the dissolution of initial ultrafine-scale intralath V-rich carbonitrides followed by precipitating onto the pre-existing Ta-rich ones.