Superaustenitic Stainless Steels: Metallurgy and Chemistry

Superaustenitic stainless steels are one of the families of high-performance stainless steels, the so-called “super” grades. Although they share the face-centered cubic lattice with standard austenitics, these alloys have a more complex chemical composition and are able to provide a unique combination of outstanding corrosion resistance and high mechanical properties. The chemical composition is certainly the basis from which the analysis of the characteristics of superaustenitic stainless steels has to start. In fact, it allows them to be distinguished from each other and classified into two different subgroups: 6Mo superaustenitics and high-N superaustenitics. The chemical composition of each alloy defines it in all its characteristics; it also plays a fundamental role in defining its possible uses while also conditioning its market share. In addition to this, the chemical composition also determines the fundamental metallurgical aspects related to the alloy. Among these, key ones appear to be those related to the solidification process, the evolution of the microstructure and the possibility of secondary phases formation. Complex compositions rich in certain alloying elements such those of the superaustenitics necessarily have to deal with the phenomenon of segregation and related consequences. The study of the thermodynamics and kinetics associated with these materials becomes critical for understanding the types of secondary phases that can form. Finally, understanding the nature of secondary phases is critical to address the consequences of their presence and strategies for their elimination, as well as the possibilities for preventing their formation.