Pulsed GTAW joint of P92 steel and Inconel 625: microstructure and mechanical properties

Abstract

In the present work, the relationship between microstructure and mechanical properties has been investigated for the dissimilar welded joint of P92 steel and Inconel 625, fabricated using the pulsed current gas tungsten arc welding (GTAW) process. Microstructural investigation revealed that the pulsed current resulted in finer equiaxed dendrites in the bulk weld metal, while columnar dendrites were observed in the weld metal near the interface. A more uniform distribution of the fine secondary phases was observed in FESEM study. The EDS study of the inter-dendritic areas showed alloying element segregation throughout the weld metal, with higher density near the interface. This segregation led to the formation of secondary phases, specifically MC-type carbides (NbC, TiC), which was confirmed by the EDS analysis. The characterization of the interface between P92 steel and ERNiCrMo-3 filler weld revealed the presence of a filler-deficient zone, marked by features such as islands, peninsulas, and unmixed zones. Elemental diffusion and segregation of Nb, Mo, and Ti at the interface were also confirmed through EDS analysis. Tensile testing demonstrated acceptable tensile properties of the welded joint at room temperature, with a tensile strength of 764 ± 8 MPa and elongation of 33 ± 1%, with the sample failing from the P92 base metal. Significant hardness variations were observed along the welded joint, with the most notable changes occurring in the P92 heat-affected zone (HAZ). A maximum hardness of 420 HV was recorded in the coarse-grained HAZ of P92, while the inter-critical HAZ of P92 showed a minimum hardness of 215 HV. In addition, considerable hardness variation was noted within the weld metal, corresponding to each welding pass as well as across the transverse direction of the welded joint. The maximum and minimum hardness values in the weld metal were 261 ± 9 and 239 ± 13 HV, corresponding to the center and capping passes, respectively, with an average hardness of 250 HV. The Charpy toughness test also indicated acceptable results, with an impact energy value of 176 ± 8.5 J. The study also provides a detailed discussion on the relationship between microstructure and mechanical properties, highlighting how microstructural features influence the mechanical performance of the welded joint.