Influence of Preheating and Post Heating on Diffusible Hydrogen Content, Residual Stresses and Hydrogen Assisted Cracking (HAC) Susceptibility of Induction Assisted Gas Metal Arc Welded P91 Steel Joint

The modified 9Cr-1Mo-V steel commercially designated as P91 steel have major application in thermal and nuclear power plants for its capability of working in optimum temperature and gas pressure region. A careful joining or welding process of P91 steel is required during its application in thermal and nuclear power plants to avoid different limitation such as evolution of delta ferrite in the fusion zone and its susceptibility to type IV cracking as well as Hydrogen Assisted Cracking (HAC). HAC poses a significant challenge in P91 steel welded joint owing to the undesired formation of martensitic metallurgical transformations during welding process. This present study investigates the impact of different induction assisted heat treatment conditions on HAC of P91 welded joint. The susceptibility of HAC was evaluated through implant test, amount of diffusible hydrogen in the weld metal and numerically estimated tensile residual stress. Implant test results were analyzed to find out lower critical stress and Critical Stress Ratio. The microstructure of the coarse grain heat affected zone and weld metal under various conditions was investigated using scanning electron microscopy and transmission electron microscopy. Finally, a correlation was established between different induction assisted heat treatment condition with diffusible hydrogen content and tensile residual stress for assess the hydrogen assisted cracking susceptibility. The findings clearly indicate that P91 steel welded joint without any pre/post heating is more susceptible to HAC. However, implementation of induction assisted preheating and post heating effectively reduced the amount of diffusible hydrogen, tensile residual stress and lowers down the susceptibility of HAC. The results suggested that induction assisted GMAW followed by induction post heating could be recommended for the welding of P91 steel.