Design, Fabrication and Characterization of Graded Transition Joints

Abstract Carbon diffusion in dissimilar metal welds (DMWs) at elevated temperatures leads to a microstructure that is susceptible to premature failure. Graded transition joints (GTJs) can potentially provide a viable replacement to prolong the service life of these components. In the current investigation, the use of thermodynamic modeling was used to identify candidate alloys that reduce the chemical potential gradient, which is the driving force for carbon diffusion. Additionally, kinetic modeling was used to determine an optimal grade length for a graded transition joint to further reduce the extent of carbon diffusion. A graded transition joint was fabricated using three candidate filler metals (Inconel 82, EPRI P87, and 347H), aged to understand the microstructural evolution, and characterized for a direct comparison with the observed trends from the model simulations. Microhardness measurements were performed on the GTJs in the as-welded and aged conditions to understand the initial strength gradients throughout the graded region, and how they evolve with aging time. Additionally, energy dispersive spectrometry was performed to measure the compositional gradients, which were input into thermodynamic and kinetic calculations to understand the carbon diffusion behavior and phase stability. Enhanced carbon diffusion occurred at the layer interfaces in the graded region of the GTJ, which indicated important regions that undergo microstructural evolution. The hardness results also revealed hardness changes at the layer interfaces. The analyzed interfaces demonstrated that carbon diffusion and corresponding carbide redistribution occurred that accounted for the observed hardness gradients. Additionally, the transition from a martensitic to austenitic region was observed in each GTJ that contributed to the hardness variations in the graded region. Finally, the formation of a nickel-rich martensitic constituent was observed in the graded region of all filler metals after aging. This constituent was originally austenite at the aging temperature, and transformed to martensite with no change in composition upon cooling. The morphologies of the constituent in the three filler metals are presented and discussed.