Duplex stainless steels (DSS) are the dual phase steels with approximately equal volume fractions of ferrite and austenite, and characterized by an unusual combination of mechanical strength, toughness and corrosion resistance. The phase fractions of δ ferrite and γ austenite in DSS have the considerable effects on the most properties of DSS, and are mainly determined by compositions and heat treatments. The most preferrable δ / γ ratio to any property has been suggested to be 50 : 50 and the usual heat treatment method for the optimum phase fraction has been reported to be holding at 1000~1100℃ for 2 hours and then water-quenching.
In this study, the effects of heat treatments and alloying additions such as Si and Mn on the δ and γ phase fractions in super duplex stainless steels (SDSS) were investigated using thermodynamic calculations and experiments. The thermodynamic calculations were performed by using Thermo-Calc program and SGTE solution database. Based on the phase equilibria calculations, specific alloy compositions were selected and heat treated, and the phase fractions were analyzed through the results of Feritscope, XRD and SEM. The calculated phase fractions and experimentally analyzed results were compared with each other.
The basic chemical composition of SDSS used in this study was Fe-25Cr-7Ni-3W-1.5Mo-xSi-yMn-0.25N (in wt%), and the contents of Si (ferrite former) and Mn (austenite stabilizer) were varied. First, it was expected from the calculation of the phase fraction according to temperature that the temperature at which δ and γ fractions are 50 : 50, was decreased gradually by increase of Si and Mn. However, the excess addition of Si and Mn stablized the second phases that are harmful to the both mechanical and corrosion-resistant properties of SDSS up to the usual heat treatment temperature range. The above thermodynamic calculations agreed well with the experimental results.
Based on the investigation through the thermodynamic calculations and experiments, it was suggested that the proper contents of Si and Mn in SDSS should be about 0.8wt% and 1.0wt% respectively and at this composition, the optimum temperature of the heat treatment to maintain the δ, γ ratio as 50 : 50 and to avoid the second phases precipitation should be ranged in 1050~1100℃.
