Computationally optimize microstructural investigation of austenitic steels for high strength–toughness properties

Austenitic steel has been the backbone of the nuclear reactor industry for power generations for many decades. 304 Steel alloy with the minimal amount of nitrogen and carbon was designed based on the different alloying composition systems for traditional high strength–toughness combinations and longevity for high-temperature applications. At elevated temperatures, the combination of strength–toughness deteriorated by metallic phases and coarsening of carbides. Chromium increases oxidation and corrosion resistance; however, brittle intermetallic compounds can develop in the resultant alloys when the Cr level rises, resulting in low ductility. First, Calphad strategical optimization was used for alloying composition and processing parameters for high temperature and pressure applications using Thermocalc and Jmat-Pro software packages. Second, to increase strength–toughness combination, isothermal deformation for different stain rates (0.001–1 s−1) with different deformation temperatures (900–1200 °C) was applied. It was also proven that different dynamic recrystallization (DRX) mechanisms caused different grain size evolutions, and the dividing point was estimated to be 0.1 s−1 strain rate at high deformation temperatures.