Investigation on post-weld heat treatment (PWHT) inputs for controlling average fusion zone hardness in fibre laser-welded modified 9Cr–1Mo steel

Abstract

Post-weld heat treatment (PWHT) generally relieves stress and enhances weldments’ mechanical properties. Optimisation of PWHT parameters is very much needed to control the process parameters for achieving the desired property requirements. In this research, the PWHT parameters for laser-welded P91 steel were optimised using the response surface methodology-based Box–Behnken methodology. Tempering temperature, heating rate and holding time are considered the input parameters and using analysis of variance (ANOVA), their impact on average fusion zone hardness has been analysed. The results indicate that the hardness values are highly influenced by temperature followed by holding time. A parametric combination of 780°C tempering temperature, 6 hours holding time, and 200°C/h heating rate have evolved as the optimal solution. The autogenous nature of the welding process resulted in a heterogeneous microstructure with untempered lath martensite structure showing precipitate dissolution in the as-welded fusion zone, ultimately resulting in a high average fusion zone hardness of 502 HV0.3, whereas PWHT with the optimal solution resulted in an average fusion zone hardness value of 252 HV0.3. The decrease in hardness value followed by heat treatment is primarily due to the tempering process, which resulted in precipitation within and over the grain boundaries. The validation experiment result using the optimum combination of parameters has been observed to be aligned significantly with the expected values, indicating the adequacy of the developed model.