Evolution of microstructure, microhardness, crystallographic texture, and grain size variation in dissimilar joints used in concentrated solar power plants application

Abstract

High carbon stainless steel 347H (SS347H) and Inconel 625 (IN625) are high-strength alloys, commonly used in high-temperature applications. These metals are commonly used in the fabrication of concentrated solar power energy storage tanks and pipelines. Construction of such tanks involves dissimilar welding as the primary manufacturing process. The critical factor to consider when joining two different materials is the development of microstructure, as it directly impacts the mechanical characteristics of the joints. According to the available literature, an increase in the heat input (HI) promotes alloying element segregation and susceptibility to weld cracking. Traditional arc welding processes, such as metal inert gas (MIG) welding, produce a higher HI. Therefore, there is a need to develop welding techniques that result in reduced HI. In the present study, a pulse-current MIG (PC-MIG) welding method was employed, utilizing current pulsing to minimize HI during the welding process. The welding was followed by metallographic and mechanical characterization. The microstructural examination found variations in microstructure at different regions. Electron backscatter diffraction analysis shows the crystallographic orientation at different regions. Analysis shows that both SS347H and IN625 have a face-centred cubic austenite structure. The inverse pole figure discloses strong texture formation. The average tensile strength of dissimilar weldments was found to be 479 MPa, while the average hardness value of the fusion zone was measured to be 235 HV. The research indicates that the PC-MIG technique is suitable for joining dissimilar materials with reduced HI, effectively eliminating the risk of cracking and enhancing the efficiency of the joint.