Papers by Keyword: Lean Duplex Stainless Steel

Abstract: Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and formed by ferrite (30-70%) and austenite. They have high tensile strength, good toughness and weldability and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion. The increase of the raw materials of the last years, there has been a motivation to develop new stainless steels with lower contents of nickel and molybdenum. Lean duplex stainless steels (LDSSs) are almost Mo free and nickel content lower than 4%. The lean duplex grades are expected to substitute not only 304/316 grades but also other duplex stainless steel grades. LDSSs are used for structural applications and for the less corrosion conditions such as liquor tanks and suction rolls. The aim of the present work was to study the kinetics of recovery and recrystallization of the lean duplex stainless steel 1.4362 during annealing treatment. The material was subjected to hot rolling and cold rolling of 70%, annealing treatment for different times at temperatures from 1000 to 1100°C and subsequently water quenched. Optical microscopy and electron back scattering diffraction (EBSD) were employed to study the evolution of the microstructure during the annealing treatment. After cold rolling austenite exhibited more strain hardening than ferrite. Consequently, the driving force of the austenite for recrystallization is higher. During annealing, recovery took place in ferrite, while the austenite remained nearly unrecovered until beginning of recrystallization. The layered grain morphology produced during cold rolling remained after the annealing treatment. The volume fraction of the phases did not show significant changes with the annealing time. Nevertheless, the volume fraction of austenite decline with an increasing of annealing temperature. After 60 seconds at 1100°C, annealing primary recrystallization had progressed in both phases, which show a bamboo-structure where the grain boundaries ran perpendicular to the phase boundaries. Grain growth progressed for longer annealing times. After 600 seconds, the bamboo-structure started to change for a more globular grain structure, pearl-structure. It continued and became more pronounced at longer annealing times. At lower temperatures, the recrystallization behavior is similar; however, the structure was refined. Recovery is favored by the high stacking fault energy of ferrite and the layered grain morphology. The lean duplex stainless steel 1.4362 shows a similar recrystallization kinetics compare with standard duplex stainless steels.

Abstract: Lean duplex stainless steels have been developed in Korea for the purpose of being used in the seawater systems of industries. The flow velocity of some part of seawater systems in nuclear power plants is high and damages of components from corrosion are severe. Therefore, this environment requires using high strength and high corrosion resistant steels. The newly developed lean duplex stainless steels STS329LD(20.3Cr-2.2Ni-1.4Mo) and STS329J3L(22.4Cr-5.7Ni-3.6Mo) are evaluated for the compatibility in seawater systems of nuclear power plants. In this study, the physical & mechanical properties and corrosion resistance of two alloys were quantitatively evaluated in comparison with commercial stainless steel 316L. Microstructures and mechanical properties of them were analyzed and the electrochemical properties related to corrosion resistance were measured such as pitting potential, passive current density, and corrosion rates from Tafel analysis. Critical pitting temperatures were measured in accordance with ASTM G48E method. The pitting initiation time and lifetime for replacement were predicted from the PRE values of test alloys and empirical equations that have been formulated from the condenser tubes of a nuclear power plant.