High-temperature tensile properties and interface structure of Ni–Fe dissimilar butt joints welded using the cold metal transfer process

Abstract

Ni–Fe dissimilar butt joints (Inconel 718 and SUS316 sheets) were welded with an Ni-based filler metal using the cold metal transfer (CMT) process. The morphology of the joints was analyzed to elucidate the formation characteristics of the low-dilution-rate welding joints. The tensile strength of the joints was tested at different temperatures and with different macro morphologies to locate the weak area. The finite element method (FEM) was utilized to calculate the stress concentration of the Ni–Fe fusion line (FL) region at high temperatures. It was found that the stress was reduced and was not concentrated in the Ni–Fe FL region, if the WM reinforcement width was wider than 8 mm (original top gap of welding butt joint). The micro morphology and element distribution of the fracture surface and the interface cross section demonstrate that the interface of the Ni–Fe FL with insufficient diffusion is the region where the solidification crack occurs. Thus, this interface is the source of the FL crack and can substantially undermine the high temperature tensile properties of the joints. The metallurgical mechanism of solidification crack formation was also discussed by the phase diagram and solidus-liquidus temperature calculation. The surface of the solidification crack is Fe-based, indicating that it was formed during the melting and re-solidification of the SUS316 base metal surface. Furthermore, the formation temperature was higher than the initial solidification temperature of the welding metal.

Keywords

• Cold metal transfer
• Inconel 718
• SUS316
• High temperature strength
• Solidification cracks