Interdiffusion Studies in Alloy 617 and 10Cr Steel Joints Using Diffusion Couple Approach and Simulations

Abstract

In the steam turbine circuit of advanced ultra supercritical power plants dissimilar joints of alloy 617 and 10Cr steel are unavoidable due to economic reasons. In these joints diffusional interaction causing change in microstructure is identified as possible reason for failure during service. To investigate the interdiffusion driven structural changes, alloy 617/10Cr steel diffusion couples were fabricated. To achieve good metallurgical bond between Fe- and Ni-based alloys and to study diffusional transformations under accelerated conditions, diffusion couples were prepared by annealing in the temperature range of 1000-1100 °C for 3-8 h. For all conditions heat treatment interaction zones were wider in alloy 617 (150-200 μm at 1050 °C, 8 h) than in 10Cr steel (15-16 μm at 1050 °C, 8 h) and the phase stability at the interface was studied using electron microprobe and x-ray diffraction. Average effective interdiffusion coefficients were calculated using Dayananda’s approach. While the diffusivities of substitutional solutes were similar in alloy 617 (0.31-0.42 × 10⁻¹⁵ m²/s at 1050 °C), they differed in 10Cr steel in the following sequence: \(\tilde{D}_{{{\text{Cr}}}}\) > \(\tilde{D}_{{{\text{Fe}}}}\)≈\(\tilde{D}_{{{\text{Ni}}}}\) > \(\tilde{D}_{{{\text{Co}}}}.\) Further, multicomponent interdiffusion profiles were predicted using homogenization model in DICTRA and an integrated approach combining DICTRA with Thermo-Calc helped in understanding the experimental observations on the interface microstructure.