Metallurgical characterization and diffusion studies of successively buttered deposit of Ni-fe alloy and inconel on SA508 ferritic steel

Abstract

The present work aims to investigate the major problem of carbon diffusion in dissimilar metal weld (DMW) between ferritic and austenitic stainless steel used in Nuclear Power Plants. For such DMW joints, Inconel 82 is often deposited on ferritic steel with Gas Tungsten Arc Welding (GTAW) process, but the problems associated with carbon diffusion persist. In the present study, Ni-Fe alloy (ERNiFe-CI) and Inconel 82 (ERNiCr-3) have been successively deposited with Gas Metal Arc Welding (GMAW) process. The substrates of SA508Gr.3Cl.1 ferritic steel were used for deposition. First buffer layer of Ni-Fe alloy was deposited with GTAW in order to have low heat input initially and the subsequent deposition was carried out using GMAW over the buffer layer. The effect of operating temperature of DMW joints on carbon migration from the ferritic steel to buttering zone was studied by carrying out the thermal ageing 450°C for 240 h. Diffusivity of carbon at different ageing temperatures was quantified. The effectiveness of buttering deposit was addressed with respect to metallurgical properties and carbon diffusion by carrying out the heat flow analysis, Electron Probe Micro Analysis (EPMA), Optical Emission Spectroscopy (OES), martensite formation analysis at fusion interfaces, micro-hardness variation across the fusion interfaces and the microstructure evolution. Significant amount of martensite was observed to be formed at the fusion interface and the subsequent effect of stress relieving was addressed. Momentous reduction in carbon diffusion and favourable metallurgical properties owing to successive buttering deposit could increase the life of DMW joints with cost effective GMAW over the GTAW process at Nuclear Plants.