Effect of temperature on microstructure evolution and localized corrosion resistance of high tungsten hyper duplex stainless steel

The high tungsten hyper duplex stainless steel grade A890 7A, the newest addition to cast duplex grades has a very different composition compared to previous grades. The alloy is extremely corrosion-resistant, and it is necessary to understand the intermetallic formation of the new alloy for its proper processing and use. The high amount of tungsten, manganese, and larger chromium significantly influence the microstructural evolution and sigma formation of the material. The present work analyses the sigma phase formations of the alloy after prolonged soaking and solutionizing at various temperature ranges. Optical microscopic and x-ray diffractometry results revealed the temperature range for sigma formation to be between 770 °C–1100 °C. Electron microscopic revealed the island like a discrete formation of sigma leaving chromium deficient regions in the immediate vicinity. Highest sigma precipitation observed in the samples solutionized at temperatures in the range 850 °C–900 °C, and the sigma precipitations appeared to be very fine. As the soaking temperature increased, the quantity of sigma phase reduced and sigma appeared to be coarser. The composition analysis of sigma phase using energy dispersive spectroscopy (EDS) revealed more extensive diffusion of Cr, Mo and W into the phase with increase in soaking temperature and reduction in percentage of sigma. (paper)