Deciphering the Indentation Creep Characteristics and the Metallurgical Aspects of the 9Cr1MoVNb(P91) and SS316LN Dissimilar Butt Joint

9Cr1MoVNb(P91) steel and SS316LN dissimilar butt joints were successfully fabricated without any flaws by utilizing the process parameters (i.e., Welding current (I) of 193 A, Arc Voltage of 15.2 V and Welding speed of 70 mm/min). The weldments were subjected to the post-weld heat treatment and indentation creep test processes, and the metallurgical features were evaluated. The zone-wise creep characteristics were evaluated at 600 °C, 625 °C and 650 °C. Creep degradation was drastically increased from 600 to 625 °C, but at 650 °C, the development of unstable microstructures that formed during primary and steady-state creep became significantly more obvious, leading to high deformation. Reduced creep resistance was noticed at the 9Cr1MoVNb (P91) heat-affected zone (HAZ) due to the quantitative transformation of micro-constitutional strengthening precipitates in the softened lath and grain structures. The creep resistance and hardness in the plastic deformed FZ were improved due to the formation of strain-free resolidified α′ + γ + cellular structures. Although the hardness fluctuated at various microstructural zones due to unstable M₂₃C₆ precipitate formation, the creep resistance remained unaffected in the FZ and the SS316LN side HAZ.