Microstructural evolution during isothermal heat treatment of super duplex stainless steels as a function of tungsten content
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Duplex stainless steels(DSS) and super duplex stainless steels(SDSS) can be described as austenitic-ferritic stainless steels of 22%Cr and 25%Cr content, respectively. The DSS and SDSS are characterised by their combination of high mechanical strength and good toughness with superior corrosion resistance in chloride containing environments at a competitive cost. They have good formability and are used for various applications in chemical-, paper- and pulp industry, as well as the offshore industry.During production and manufacturing, the DSS and SDSS are frequently exposed to high temperature treatments such as welding or hot-rolling.The high fraction of alloying elements in DSS and SDSS makes them sensitive to precipitation of deleterious intermetallic phases, such as sigma- and chi-phase, when exposed in the temperature range 800C to 900C. These intermetallic phases makes the DSS and SDSS prone to localised corrosion by chromium and molybdenum-depletion of the matrix material, and causes embrittlement of the steel. Partial replacement of molybdenum with tungsten has been proposed to delay precipitation of the sigma-phase, by promoting precipitation of the less deleterious chi-phase.To examine the effect of W-substitution on the precipitation kinetics of SDSS during ageing, three commercially available SDSS have been investigated as a function of their W-content. The SDSS were solution annealed at 1100C, and the microstructural evolution was studied after isothermal heat treatment in the temperature range 810C to 920C. Finally the amount of intermetallic phases was measured by the use of light optical microscopy, scanning electron microscopy and electron backscatter diffraction. The results were summarised in time-temperature-transformation-diagrams. The precipitation rate and amount of intermetallic phases was found to increase with temperature and hold time. The rate of precipitation was found to increase in the order 2.7%W, 0%W, 0.5%W-SDSS. The sigma-phase was the dominating intermetallic phase in both the 0%W- and 0.5%W-material, whereas in the 2.7%W, the precipitation was delayed towards longer ageing times, and the chi-phase was the dominating constituent after prolonged ageing. The results indicated that below a critical amount of W-addition, precipitation of intermetallic phases is promoted and stabilised towards higher temperatures. Above this critical value, the precipitation is delayed, and chi-phase is stabilised on behalf of sigma-phase.