Autogenous laser welding investigations on modified 9Cr-1Mo (P91) steel

Modified 9Cr-1Mo steel plates of 6 mm thickness have been laser welded using CO2 laser. The effects of beam intensity and overall heat input (168-1500J/mm) on the bead characteristics, microstructure and mechanical properties of the welds have been investigated by varying the laser welding parameters such as laser beam mode, power and welding speed. The microhardness survey carried out on the welds after post-weld heat treatment did not reveal any soft zones in the intercritical heat affected zone for welds made with a heat input of up to 420 J mm(-1). The tensile strengths of the welds were comparable to that of the base material. Charpy impact tests on subsize specimens revealed that the welds have good toughness. delta-Ferrite was observed in the fusion zone of the welds made at heat input of 700 J mm(-1) and above, the content of which increased with the increased heat input.