Thermomechanical processing of ferrite-martensite steels for power plant applications

Steel, combining strength, high-temperature performance, and corrosion resistance when derived from suitable alloying design, remains among the most widely utilized materials in the energy sector, including oil and gas, nuclear power, and renewables. Ensuring reliable operation in aggressive environments requires a deeper understanding of its metallurgy and the behavior of advanced steel adaptations. Despite extensive studies on supermartensitic stainless steels, a limited quantitative understanding persists of how deformation temperature, strain amount, and alloying additions collectively govern the evolution of the austenitic microstructure, interfacial area per unit volume, and transformation behavior. This research aims to understand the role of the prior austenite microstructure in supermartensitic stainless steel on the final transformed microstructure. Particular emphasis is placed on the influence of controlled nickel and manganese additions on critical processing temperatures and transformation behavior. Specifically, this research aims to investigate the influence of austenite being deformed either above the recrystallization-limit temperature T95% or below the recrystallization-stop temperature T5%, providing a comprehensive understanding of the influence of the difference in the microstructure on the difference in the interfacial area per unit volume Sv which will affect the transformation behavior and accordingly, the transformation product. Successful achievement of this research has been accomplished through carrying out plane strain compression (PSC) tests to address the identified purpose of this work. The principal findings demonstrated distinct compositional effects arising from variations in nickel and manganese content, with manganese increasing total Sv and strength across the studied conditions, while nickel contributed to higher ductility and a greater suppression of the martensitic transformation temperature. In addition, deformation below T5% resulted in significantly higher total Sv values, more refined martensitic structures, and enhanced mechanical strength. Conversely, deformation above T95% produced equiaxed austenitic grains with lower total Sv values, enhancing ductility and offering a controllable pathway for balancing strength and toughness via thermomechanical processing.