Microstructure degradation and creep failure study of the dissimilar metal welded joint of heat-resistant steel and Inconel 617 alloy tested at 650 °C and applied stress range of 100–150 MPa

Abstract

The advanced ultra-supercritical (A-USC) power plant system is anticipated to become India's next-generation base-load power station. To adopt AUSC technology, dissimilar welded joints (DWJs) between heat-resistant steels and the nickel-based alloys, using the nickel-based fillers, will need to be implemented. However, failure of dissimilar welded joints from P92 steel base metal or the heat affected zone (HAZ) has been commonly observed under high-temperature creep conditions. In the present study, the creep rupture behaviours and rupture mechanisms of DWJ between the Ni-based alloy Inconel 617 and heat-resistant P92 steel with Inconel 617 (ERNiCrCoMo-1) filler metal were investigated. Creep tests were conducted at 650 °C in the stress range of 100–150 MPa. To examine the creep rupture behaviour of the DWJ samples, optical microscopy (OM), scanning electron microscopy (SEM) and microhardness tests were performed. Cross-sectional images of the fractured creep specimens tested under various operating conditions revealed failures originating from distinct locations, including the P92 base metal and the inter-critical heat affected zone (HAZ). The specimen tested at 650 °C/150 MPa exhibited failure originating from the P92 base metal, whereas the specimen tested at 650 °C under the stress range of 100–130 MPa showed failure from the inter-critical heat affected zone (ICHAZ). The failure from P92 BM was primarily governed by plastic deformation, with the growth and coalescence of dimples ultimately resulting in trans-granular fracture. The specimens tested at 650 °C/100–130 MPa, which failed from the ICHAZ, exhibited a typical Type IV inter-granular failure. This failure mode is primarily attributed to matrix softening in HAZ, weakening of the boundaries, coarsening of the precipitates, and the evolution of intermetallic Laves phases. The specimen that failed in the stress range of 100–130 MPa exhibited a high density of microvoids in the ICHAZ, along with a few microvoids in the FGHAZ. The weld metal showed negligible degradation in microstructure, while the hardness study revealed a significant increase in hardness with an increase in rupture time, i.e., a decrease in applied stress and it was attributed to evolution of the new carbide phases in weld metal. Coarsening of the carbide precipitates was observed in each zone of the HAZ of P92 steel as well as in the base metal. The EDS study of the fracture tip and the FGHAZ/ICHAZ of the specimen that failed under stresses of 100 MPa and 120 MPa confirmed the evolution of intermetallic Laves phases. High magnification SEM images confirmed that triple boundaries are preferential locations for microvoid nucleation. The failed specimen showed the presence of microvoids near the carbide precipitates, with a large density of both coarse and fine precipitates confirmed all around and inside the microvoids. The ICHAZ and FGHAZ confirmed the formation of fine prior austenite grain boundaries (PAGBs) during the welding thermal cycle, which exhibited a lower density of carbide precipitates and this played a major role in Type IV failure.