On study of effect of varying tempering temperature and notch geometry on fracture surface morphology of P911 (9Cr-1Mo-1W-V-Nb) steel

Before putting in service, the creep strength enhanced ferritic (CSEF) steels are subjected to varying tempering treatment to produce a vast array of mechanical properties and to achieve the optimum combination of microstructure and mechanical properties by controlling normalizing and tempering temperature/time. In the present investigation, P911 steel was subjected to a fixed normalizing temperature of 1040 °C for 1 h and then, subjected to varying tempering temperature (720 °C-800 °C) for 2 h of tempering time. The effect of notch geometry on the tensile properties with respect to the varying tempering temperature was also investigated. The microstructural characterization was studied using optical microscope and field-emission scanning electron microscope (FESEM). A decrease in grain size, hardness and strength were noticed with increase in tempering temperature initially but after 740 °C, the grain size and strength were found to be increased continuously. The area fraction of precipitates was found to be varied inversely with the grain size. For a particular tempering temperature, an increase in notch depth resulted in increase in strength and decrease in ductility. The best combination of hardness, ductility and strength were achieved after normalized at 1040 °C for 1 h and tempered at 760 °C for 2 h.