Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

Dissimilar metal welds between ferritic and austenitic alloys are used extensively in power plants. Premature failure of such welds can occur below the expected creep life of either base metal. This article reviews microstructural evolution in dissimilar welds and describes factors that contribute to premature failure. The microstructure in the as welded condition consists of a sharp chemical concentration gradient across the fusion line. Martensite forms within this gradient due to high hardenability and rapid cooling rates from welding. Upon aging, carbon diffuses down the chemical potential gradient from the ferritic steel toward the austenitic alloy. This can lead to formation of a soft carbon denuded zone in the ferritic steel, and nucleation and growth of carbides in the austenitic steel that produce high hardness. These differences in microstructure and hardness occur over distances of about 50–100 μm. Failure is attributed to the steep microstructural and mechanical property gradients, the large difference in coefficient of thermal expansion, formation of interfacial carbides that promote creep cavity formation, and preferential oxidation of the ferritic steel. Information is also provided on available creep rupture properties, remaining life estimation techniques, current best practices and research in progress.