Importance of filler material chemistry for optimising weld metal mechanical properties in modified 9Cr-1Mo steel

Although modified 9Cr-1Mo (grade 91) steel is considered weldable using con ventional procedures, the attainment of optimum weld metal properties often causes concern. In the current work, plates of grade 91 were manual metal arc welded using electrodes with different compositions provided by two man ufacturers Post-weld tempering was performed at 730 and 760 degreesC for 2 and 6 h in each ease. The weld fusion zones were metallographically examine d in detail and hardness, tensile properties, and toughness were estimated The results showed that the weld metal produced from one of the two electro des was distinctly superior in terms of ductility and toughness for any gh, en welded or heat treated condition. This was attributed to the slightly hi gher silicon, niobium, and chromium contents in the inferior electrode, whi ch resulted in both retention of some high temperature ferrite and a greate r degree of precipitation. In this electrode, additionally, the alloying el ements were introduced through the flux coating, which produced inhomogenei ties in the microstructure that degraded the mechanical properties.