Development of Lean Duplex Stainless Steels (LDSS) with Superior Mechanical and Corrosion Properties on Laboratory Scale

Abstract

Low-Ni and Ni-free varieties of duplex stainless steels (DSSs) have been successfully developed for the first time on laboratory scale in SAIL. The alloy compositions for Ni-free and low-Ni (<1.5 wt%) DSSs were evolved through formulation of appropriate chromium and nickel equivalents to achieve an optimum phase balance of 55 vol% austenite and 45 vol% ferrite in stainless steel microstructures. Laboratory heats were made achieving target chemistries and cast into 100 mm square cross-sectioned ingots. The ingots were subsequently soaked at 1150 °C for 3 hrs for thermal/ compositional homogenization and hot rolled in number of passes to 6 and 16 mm strips in Hillé experimental rolling mill with finish rolling temperatures of 950-980 °C. The hot rolled strips were eventually conferred solution annealing treatment by soaking them at 1060 °C for 2 hours followed by rapid quenching in water to prevent precipitation of deleterious intermetallic compounds (IMCs) and secondary phases. The new steels revealed an exceptional combination of properties: higher yield strength (488 MPa for low-Ni DSS and 501 MPa for Ni-free DSS), higher tensile strength (664 MPa for low-Ni DSS and 677 MPa for Ni-free DSS) and superior Charpy V-notch impact toughness (230 and 129 Joules at room temperature and -20°C, respectively, for low-Ni DSS as against 80 and 52 Joules at similar temperatures for Ni-free DSS). The yield strength of the steels was assessed to be about 1.6-1.8 times that of conventional austenitic grades AISI 304 L and AISI 316 L in annealed condition. The low-Ni and Ni-free DSS showed remarkable corrosion resistance and have been found to exhibit passivity, corrosion rates as low as 0.08-0.11 mpy, pitting potentials in the range of 482-596 mV and charge transfer resistances of the order of 106 W.cm2 in highly corrosive environment of 3.5% NaCl. The steels have been thus found to be superior in pitting/ localized corrosion resistance to AISI 304 L with pitting potential of 437 mV and comparable in performance with AISI 316 L with its high pitting potential of 602 mV. Even in the strongly reducing environment of 0.1 N H2SO4, the new steels have revealed tendency to passive film formation, breakdown potentials of 1127-1153 mV and passive film impedances of 104 W.cm2, comparable to the standard austenitic grades AISI 304 L and AISI 316 L. In boiling MgCl2 solutions, the low Ni DSS has been found to resist stress corrosion cracking (SCC) up to 24 h; the time for crack initiation being intermediate to that for AISI 304 L (3 h) and AISI 316 L (32 h). The Ni-free DSS, on the other hand, was found to exhibit no signs of SCC failure even after 72 h of exposure to the test solution. The degree of sensitization (DoS) for both low Ni and Ni-free DSSs has been quantified to be £ 0.05 in 0.5 M H2SO4 + 0.01 M KSCN, which has revealed their insusceptibility to intergranular corrosion (IGC). The steels have been found to be free from deleterious intermetallic phases such as sigma (s), chi (c), etc. and this has been ascertained from corrosion rates of <10 mdd in ferric chloride corrosion testing as per ASTM A 923 Method C.