Long-term thermal stability of a novel B2-NiAl precipitate-hardened alumina forming martensitic steel

Abstract

A new steel was designed for high-performance applications under extreme conditions by combining a martensitic microstructure and the co-precipitation of nano-sized MC and B2-NiAl precipitates, along with the ability to generate a protective Al-rich oxide scale. This study focuses solely on the long-term stability of this alloy during tempering at 550 and 650 °C for holding times exceeding 1000 h. At 550 °C, very fast precipitation of fine B2-NiAl particles occurred, providing remarkable strengthening even after annealing for 1100 h, while preventing the microstructural coarsening of the martensite blocks. In contrast, tempering at 650 °C induced the rapid coarsening of the B2-NiAl precipitates and the formation of precipitate-free zones. This accelerated microstructural evolution led to a more pronounced decrease in hardness compared to samples treated at 550 °C though the hierarchical martensitic microstructure was also retained. Nevertheless, it is observed that at 650 °C, (Cr, V) 23 C 6 carbides formed instead of the MC particles, in accordance with kinetic calculations carried out. The enhanced durability at temperatures below 650 °C suggests that this novel alumina-forming martensitic steel could be a suitable candidate for applications requiring long-term thermal stability under harsh environments.