Microstructural and mechanical characterization of Stellite-hardfaced coatings with two types of buffer layers

摘要

In recent years, a major challenge facing the power generation industry is delamination of Stellite-hardfaced coatings from high-temperature valves. In order to meet this challenge, IN82 buffer layer was evaluated to replace the problematic S21 buffer layer. S6 and either S21 or IN82 hardfacing alloys were deposited onto F91 steel substrates using plasma-transferred arc hardfacing. Aging was conducted at temperatures between 550 and 650 degrees C for three exposure durations within a one-year period. A comparative study was carried out on the microstructure and mechanical properties of IN82 and S21 hardfaced specimens. The F91/S21 interface is unstable during aging because a hard and brittle interfacial layer grows, which significantly reduces the impact energy of S21 hardfaced specimens. In contrast, the F91/IN82 interface is microstructurally much more stable than the F91/S21 interface; an interfacial band, part of the F91 steel, and some discrete M23C6 carbides along this band grow during aging at 650 degrees C for 8760 h. The impact energy loss of the IN82 specimens is much less significant than that of the S21 specimens. Moreover, the toughness degradation is not related to the F91/IN82 interface, but rather to coarsening and precipitation of the intergranular/interdendritic carbides in the IN82 bulk material during aging. To conclude, the IN82 buffer layer is a good alternative to replace the problematic S21 buffer layer, enhancing the durability of the S6 coating components. However, the hardness of the S6 top layer is compromised due to dilution of Fe and Ni. To maintain its hardness, more S6 layers can be deposited and process optimization can be undertaken to reduce dilution in the applications where wear resistance is critical.