Microstructural changes in 16-8-2 weld metal during exposure to 750 degrees C for extended times

Abstract

Ever-leaner compositions are targeted for nominal type 16–8-2 weld metals in attempts to limit sigma-phase formation during elevated temperature operation. Three variants of type 16–8-2 weld metals were exposed to aging at 750 °C for up to 3500 h. Evaluation of the resultant structures by magnetic measurements, neutron diffraction (at ambient and cryogenic temperatures), and electron backscatter diffraction established that the leaner variants were susceptible to forming martensite after aging. This is ascribed to a local increase in the martensite start temperature due to sensitization taking place during elevated temperature aging. The extent of martensite formation diminished during aging treatments exceeding 1000 h owing to diffusion of solute elements from the austenitic matrix to the sensitized regions (recovery). Subsequent elevated temperature aging of the microstructures containing martensite also resulted in a decrease in martensite content: the mechanism, however, differs from that for single-cycle exposure. This observation is explained by martensite reversion to austenite. Martensite formation was completely absent from the higher-alloyed variant. This variant experienced intergranular carbide precipitation and delta ferrite decomposition into secondary austenite and carbides. This work demonstrates significantly different aging responses for composition variants within the allowed ranges for type 16–8-2 weld metals.