Delta (δ) Ferrite Formation in the Welds of Aluminized 9Cr-1Mo Steels

Delta (δ) ferrite phase has several detrimental effects on mechanical properties such as toughness, creep, impact and ductility. It is necessary to examine δ-ferrite content in the weld zone of aluminized coated steels. This manuscript presents the investigation of δ-ferrite formation in the weld of aluminized P91 steels. The welds were prepared with different heat inputs by varying the weld speed at constant current by autogenous Tungsten Inert Gas (TIG) welding process. The concentration of Al in the weld zone may favor δ-ferrite formation during the welding as Al is a ferrite former. Hence, the weld zone prepared at maximum heat inputs was compared for both coated and bare P91 samples. Microstructures were examined and correlated with the temperatures and cooling curve measured during welding. The various empirical formulae such as Schneider, Schaeffler, Newhouse, and Kaltenhauser were used to predict the δ-ferrite formation based on the chemical composition of weld metal obtained with the help of spark emission spectroscopy. Among these, Kaltenhauser’s equation is more accurate for aluminized coated P91 steels. X-ray diffraction, optical microscopy, scanning electron microscopy, and microhardness measurement were done to analyze and predict the shape and morphology of δ-ferrite for coated steel and bare P91 steels. The investigation indicates that higher heat input (~ 2.12 kJ/mm) for coated steel and Al concentration (~ 0.19%) in the weld zone caused the formation of δ-ferrites in various shapes such as polygonal and isolate islands and it has an average volume fraction of ~ 5.09%. The average microhardness values were ~ 396–410 Hv for martensitic laths, while it was ~ 192–198 Hv for delta ferrite which is 52% lower.