Forecasting the perspective compositions of duplex stainless steels

Duplex Stainless Steels (DSS) are currently widely used in petrochemical industry, in which austenite and ferrite are in equal fractions, thus providing for a favorable combination of mechanical properties along with high corrosion resistance of these steels. The development of new DSS compositions is searching of a compromise. Increasing the chromium content in steel provides its higher corrosion resistance, but in order to maintain phases balance, nickel concentration have to be raised, that causes the formation of undesirable о-phase. To exclude its formation, nickel is replaced with nitrogen which induces the formation of unfavorable chromium nitrides. In the present work, thermodynamic criteria for the reasonable choice of chemical compositions of advanced DSS were developed: 1) austenite and ferrite should be in equal fractions at the beginning of hot plastic deformation; 2) temperature of the о-phase formation beginning should be below the temperature of the end of hot plastic deformation; 3) onset temperature for the beginning of chromium nitrides formation should be minimal. The success of these criteria was studied under the influence of primary and secondary alloying elements for DSS, and the results of these studies have been generalized using multiple regression equations. It has been revealed that the chemical composition optimization of well-known commercial DSS grades, as well as the development of new grades with specified properties can significantly benefit from the use of these equations. Hot physical experiments have shown that thermodynamic modeling well describes the ferrite content found in experimental steels of different compositions at different quenching temperatures.

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