Characterization of As-Welded Microstructure in a P91 Steel

The microstructure of the heat affected zone (HAZ) and fusion zone (FZ) in the as-welded condition of a single-pass weld performed by the flux-cored arc welding (FCAW) process was studied in a P91 steel using microhardness measurements, field-emission scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The evolution of precipitates in each region of the single-pass weld was analyzed, and particular attention was paid to the presence of retained austenite (RA). Experimental observations indicated that M23 C6 carbide played a major role in the thermal cycle associated with the weldment. On one hand, the dissolution of M23 C6 led to the precipitation of spherical NbCN in the finegrained HAZ (FGHAZ) near the coarse-grained HAZ (CGHAZ). On the other hand, the total or partial dissolution of M23 C6 carbides changed the chemical composition of the initially formed austenite. In the regions where the M23 C6 carbides were completely dissolved (i.e., the CGHAZ and FZ), retained austenite was observed as a thin, continuous film along primary austenite grains and lath boundaries. Instead, a shell of retained austenite was observed around some partially dissolved M23 C6 of the intercritical HAZ (ICHAZ) and FGHAZ.