Formation of Delta-Ferrite in the Weld Metal of 9-12Cr Steels

Formation of delta-ferrite in the weld metal, during autogenous bead-on-plate welding of 9-12Cr steels, using gas tungsten arc welding (GTAW) process has been studied. The microstructure of 9-12Cr steel base metal consists of tempered lath martensite, where precipitates decorate the boundaries. In single pass bead-on-plate, the area fraction of delta-ferrite was found to be higher at the weld interface as compared to center of the weld metal at all preheat temperatures. Decrease the area fraction of delta-ferrite, with an increase in preheat temperature were observed. Area fraction of delta-ferrite was found to be 7.6 in P91B weld metal at room temperature compared to that of 6.1 at preheat temperature of 250°C, whereas it was 5.7 in AISI 410 weld metal at room temperature and 3.9 at pre heat temperature of 250°C. High cooling rate caused stability of delta ferrite at room temperature as well as preheat condition in the weld metal.