Cast structure and properties of duplex stainless steels

Currently, duplex stainless steels are increasingly used in industry. Austenite and ferrite in these steels are in approximately equal proportions. During manufacture of cast products from these steels, a chemical and structural heterogeneity is formed in the castings, for the elimination of which heat treatment is carried out. In practice, within the framework of one class or even one steel grade, the chemical composition and, as a consequence, the phase ratio can vary over a wide range without reaching their optimal values. In this paper, the authors investigated the influence of chemical composition and solidification conditions on the structure and properties of cast duplex stainless steels and developed thermodynamic criteria for the selection of casting alloys, taking into account the temperature of beginning of the polymorphic transformation of δ-ferrite into austenite and the average equilibrium rate of this transformation. It was found that in the studied steels with 21 – 26 % of chromium, crystallization proceeds with the formation of δ-ferrite dendrites, and austenite is formed in the solid metal at the places of the former interdendrite spaces. It is shown that at cooling rates, which are realized in practice when obtaining, for example, casings of centrifugal pumps or other products of a similar size, the transformation of δ-ferrite into austenite is practically suppressed when the temperature reaches 1180 – 1200 °C. On the basis of this, a criterion for the development of compositions with the required phase ratio without heat treatment was proposed. The evolution of the structure during heat treatment at temperatures of 1050 – 1250 °C was studied and it is shown how by choosing the optimal temperature of annealing and quenching, depending on the actual chemical composition of 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-doped alloy leads to a catastrophic decrease in corrosion resistance. It is shown that in the steels under consideration optimal properties are achieved at 70 % of δ-ferrite.

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