Evaluation of Weld Joint Strength Reduction Factor due to Creep in Alloy 740H to P92 Dissimilar Metal Weld Joint

A study for evaluating the weld joint strength reduction factor (WSRF) due to creep was conducted for alloy 740H-P92 dissimilar metal weld joint to be used advanced ultra-supercritical fossil power plants. To this end, a series of the creep rupture tests were carried out for P92 and alloy 740H parent metals and alloy 740H-P92 dissimilar metal weld joint at elevated temperatures ranging from 600 to 700 °C. Additionally, for the creep tested specimens, hardness measurement and microstructure observation were performed. Weldment creep rupture test used specimens of transverse cross-weld type. In the creep test result, weldment creep rupture strength was almost same as that of the P92 parent metal when analyzed as a function of Larson-Miller parameter (LMP). However, when analyzed as a function of creep rupture time for individual temperatures, it showed a tendency to be lower than that of the P92 parent metal after certain times. The difference between two creep strengths increased with increasing time. Based on the analysis result for creep rupture time at each temperature, the WSRF for 100,000 h creep life was determined as 0.84 at 600  °C, 0.72 at 650  °C and 0.58 at 700  °C through extrapolation. Additionally, from the results of investigation on hardness and microstructure, it was shown that the creep life of alloy 740H-P92 dissimilar metal weld joint was dominantly determined by the characteristics of the heat affected zone (HAZ) on P92 side.