Development of duplex stainless steels compositions

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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Increasing the corrosion properties of duplex steel with REM modification

Karasev,

Kodzhaspirov,

Fedorov

et al. 2024

Izv. vysš. učebn. zaved., Čern. metall.

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Duplex stainless steels are a modern class of materials with a unique combination of high corrosion and mechanical properties. Due to this, they can be widely used in machine parts and aggregates in fields with aggressive oil and gas production conditions. One of the disadvantages of these materials is their tendency to local corrosion damage on non-metallic inclusions, other things being equal, formed during smelting and casting. To control the purity of steel in conditions of open induction smelting, it is effective to use modification with rare earth metals (REM). Therefore, the purpose of this work was to determine the optimal content of REM in duplex steel to increase corrosion properties. Thermodynamic modeling of the formation of nonmetallic inclusions in duplex corrosion-resistant steel S32750 was carried out and the results of calculations were compared with the experimental data. It is shown that there is an optimal concentration of REM at which contamination with inclusions is minimal due to favorable conditions for their removal, and with a further increase in consumption it increases due to coagulation of a large number of refractory oxides. Electrochemical tests were performed and parameters such as corrosion potential, pitting formation potential and the basis of pitting resistance of experimental steels were determined. Therefore, the corrosion properties of the investigated duplex steel are significantly improved when treated with REM. The electrochemical potentials of different types of inclusions are evaluated on a qualitative level. Based on the obtained results on corrosion resistance and contamination of the studied castings, the optimal amount of REM introduced for modifying inclusions is 0.05 % (0.65Ce + 0.35La).

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Development of duplex stainless steels compositions

Cited by 7 publications

References 23 publications

Effect of Vanadium Alloying on the Phase Composition of Super Duplex Steel

Effect of Vanadium Alloying on the Phase Composition of Super Duplex Steel

Cast structure and properties of duplex stainless steels

Increasing the corrosion properties of duplex steel with REM modification

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