The most effective method to reduce CO₂ gas emission from the steam power plant is to improve its performance by elevating the steam temperature to more than 700 °C. For this, it is necessary to develop applicable materials at high temperatures. Ni-based Alloy 617 and 12Cr steel are used in steam power plants, due to their remarkable mechanical properties, high corrosion resistance, and creep strength. However, since Alloy 617 and 12Cr steel have different chemical compositions and thermal and mechanical properties, it is necessary to develop dissimilar material welding technologies. Moreover, in order to guarantee the reliability of dissimilar material welded structures, the assessment of mechanical and metallurgical properties, fatigue strength, fracture mechanical analysis, and welding residual stress analysis should be conducted on dissimilar material welded joints. In this study, first, multi-pass dissimilar material welding between Alloy 617 and 12Cr steel was performed under optimum welding conditions. Next, mechanical properties were assessed, including the static tensile strength, hardness distribution, and microstructural analysis of a dissimilar material welded joint. The results indicated that the yield strength and tensile strength of the dissimilar metal welded joint were higher than those of the Alloy 617 base metal, and lower than those of the 12Cr steel base metal. The hardness distribution of the 12Cr steel side was higher than that of Alloy 617 and the dissimilar material weld metal zone. It was observed that the microstructure of Alloy 617 HAZ was irregular austenite grain, while that of 12Cr steel HAZ was collapsed martensite grain, due to repeatable heat input during multi-pass welding.

The reliability of welded structure can be evaluated through welding residual stress analysis and fatigue strength assessment. In this study, welding residual stresses of multi-pass dissimilar material welded joint between alloy 617 and 12Cr steel were analyzed numerically and experimentally. Fatigue strength was then assessed in the air. Based on results of welding residual stress analysis and fatigue strength assessment, a fatigue design method considering welding residual stress was investigated. Welding residual stresses at the weld of dissimilar welded joints distributed complicatedly on longitudinal and transverse directions, showing differences but a very similar distribution tendency between numerical and experimental results. Numerical and experimental peak values of welding residual stresses at HAZ of the weld on the 12Cr steel side were predicted to be 333 MPa and 282 MPa HAZ, respectively. The fatigue limit of dissimilar material welded joint between alloy 617 and 12Cr steel was assessed to be 306.8 MPa, which was 40% of tensile strength (767 MPa) of dissimilar material welded joint. However, the stress range including welding residual stress was assessed to be 206.9 MPa, which was 14% lower than that calculated by including the effect of residual stresses.

Welding alloy 617 with other metals and alloys has been receiving significant attention in the last few years. It is considered to be the benchmark for the development of economical hybrid structures to be used in different engineering applications. The differences in the physical and metallurgical properties of dissimilar materials to be welded usually result in weaker structures. Fatigue failure is one of the most common failure modes of dissimilar material welded structures. In this study, fatigue life prediction of dissimilar material weld was evaluated by the accelerated life method and artificial neural network approach (ANN). The accelerated life testing approach was evaluated for different distributions. Weibull distribution was the most appropriate distribution that fits the fatigue data very well. Acceleration of fatigue life test data was attained with 95% reliability for Weibull distribution. The probability plot verified that accelerating variables at each level were appropriate. Experimental test data and predicted fatigue life were in good agreement with each other. Two training algorithms, Bayesian regularization (BR) and Levenberg–Marquardt (LM), were employed for training ANN. The Bayesian regularization training algorithm exhibited a better performance than the Levenberg–Marquardt algorithm. The results confirmed that the assessment methods are effective for lifetime prediction of dissimilar material welded joints.

In this study, dissimilar material welding between Alloy617 and 12Cr steel was performed using the buttering welding technique on the 12Cr steel side in order to increase the weldability. After multi-pass welding, post weld heat treatment (PWHT) was performed in order to reduce the welding residual stresses, and metallurgical microstructures were observed on dissimilar material weld. Additionally, the corrosion fatigue crack growth characteristics of dissimilar material weld was assessed according to Fracture Mechanics. Based on the results, the fatigue and corrosion fatigue strength of dissimilar material weld between Alloy617 and 12Cr steel did not show a big difference prior to and after post weld heat treatment. Fatigue and corrosion fatigue crack growth of dissimilar material weld were slightly faster than those of similar material weld of Alloy617. However, the characteristics of fatigue and corrosion fatigue crack growth did not show a big difference prior to and after PWHT.