Precipitated Phases in a Weldment of P92 Steel

The microstructural stability of ferritic-martensitic heat resistance steels is mainly controlled by the thermal stability of precipitates that are able to retard the growth rate of subgrains. The welded zones appear to be a weak region compared to the base material (BM) and creep rupture takes place in the intercritical or the fine-grained heat affected zones (ICHAZ and FGHAZ). In the present work, precipitates of a single-pass weld performed by the flux-cored arc welding process in a P92 steel were identified by means of a transmission electron microscope on carbon replicas extracted from the BM and different regions generated during welding. In the BM, M₂₃C₆ carbides were the major precipitates, followed by VN, ^wings and NbCN. The dissolution of these precipitates took place during welding at different peak temperatures. M₂₃C₆ carbides were observed in the BM, ICHAZ and FGHAZ, VN were found in all zones, ^wings in the BM, ICHAZ and FGHAZ near the ICHAZ and NbCN were observed in all the HAZ. It was observed that increasing the peak temperature (from the BM to the FZ) the size and quantity of precipitates decreased and they acquired a more spherical morphology. Moreover, the higher the peak temperature reached the greater the number of W-rich M₂₃C₆ carbides found. (author)