Cast Structure and Properties of Duplex Stainless Steels

Abstract Duplex stainless steels (DSSs), in which austenite and ferrite are in equal proportion, are increasingly used in mechanical engineering. Upon receipt of cast products, chemical and structural heterogeneity is formed in the castings, to eliminate which heat treatment is carried out. In practice, within one class or even one steel grade, the chemical composition and, as a result, the phase ratio can vary over a wide range, not reaching their optimal values. In this paper, the influence of the chemical composition and crystallization conditions on the structure and properties of cast DSSs has been studied, and thermodynamic criteria for the selection of cast alloys have been developed, considering the temperature of the beginning of the polymorphic transformation of δ-ferrite into austenite and the average equilibrium rate of this transformation. It has been established that in the studied steels with 21–26% chromium, crystallization proceeds with the formation of δ-ferrite dendrites, and austenite is formed in the solid metal at the places of the former interdendritic spaces. It is shown that at the cooling rates implemented in practice when obtaining, for example, centrifugal pump casings or other similar products, the transformation of δ-ferrite into austenite is practically suppressed when the temperature reaches 1180–1200°C. Based on this, it is possible to develop DSSs compositions that make it possible to obtain the required ratio of austenite and ferrite without additional heat treatment. The evolution of the structure during heat treatment at 1050–1250°C is studied and it is shown how, by choosing the optimal solution annealing and quenching temperature depending on the actual chemical composition of the steel, it is possible to achieve an acceptable level of pitting potential in steel with a lower alloying, and vice versa, non-optimal heat treatment of a high-alloyed steel leads to a catastrophic reduction in corrosion resistance. It is shown that in the studied steels, the optimal properties are achieved already at 70% δ-ferrite.