Some studies on dissimilar welds joint P92 steel and Inconel 617 alloy for AUSC power plant application

Abstract

The Cr–Mo steel containing 9% Cr is generally preferred in producing high-temperature operating components (lower than 620 °C) like steam pipes and headers due to their outstanding combination of elevated temperature mechanical properties and creep strength. The components with an operating temperature of more than 620 °C (superheater and reheaters tubes) are mainly produced using stainless steel or Ni-based superalloy. Hence due to economic aspects of boilers, joining of P92 steel with Ni-based superalloy is inevitable in advanced ultra-supercritical (AUSC) plants. Thus the development of joining technology without diminishing the high-temperature mechanical properties of the weldments becomes essential. The present work introduces the microstructure evolution in gas tungsten arc welded (GTAW) dissimilar joint of P92 steel and Inconel 617 alloy produced by using the P92 filler. The microstructure study along weldments was examined using an optical microscope (OM) and field emission scanning electron microscope (FESEM). The mechanical properties of welded joint and joint integrity were determined by using the microhardness measurement, cross-weld tensile tests, and impact toughness test. The effect of the post-weld heat treatment (PWHT) on mechanical properties and microstructure evolution was also performed. The residual stresses were also examined using the blind hole drilling methods, and the effect of PWHT on the nature and magnitude of the residual stresses were also conducted. The microstructure observation showed the martensitic lath structure in the weld metal, which resulted in poor impact toughness (36 ± 5 J) and high hardness (458 ± 25 HV) of weld metal in the as-welded condition. A considerable increase in impact toughness (90 ± 4 J) and a decrease in hardness (371 ± 11 HV) of weld metal were observed after the PWHT. A wide region of the unmixed zone at the interface of weld metal and Inconel 617 was formed due to the difference in chemical composition and structure. The unmixed zone formation led to the poor tensile properties of the welded joint, and failure was observed both in P92 BM and at the interface of weld metal and Inconel 617 interface. The tensile tested sample failed from the interface region and showed the presence of Ti(C, N) particles, while secondary phase M23C6 particles were obtained from the tensile sample fractured from P92 BM. The impact test results also showed the variation in impact toughness along the weldments. The P92 BM was found the strongest zone, while weld metal was noticed as the weakest zone in terms of impact toughness. The variation in residual stresses was also measured along with the thickness of the plate, and the peak magnitude of the residual stresses was measured in the capping pass, which was compressive in nature.