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

J. Phase Equilib. Diffus. https://doi.org/10.1007/s11669-024-01118-7 ORIGINAL RESEARCH ARTICLE Interdiffusion Studies in Alloy 617 and 10Cr Steel Joints Using Diffusion Couple Approach and Simulations S. Haribabu1 • C. Sudha1,2 • P. Ganesh3 • Abhay Kumar4 Submitted: 11 November 2023 / in revised form: 6 February 2024 / Accepted: 6 May 2024  The Author(s) 2024 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 (150200 lm at 1050 C, 8 h) than in 10Cr steel (15-16 lm 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 This invited article is part of a special tribute issue of the Journal of Phase Equilibria and Diffusion dedicated to the memory of Thaddeus B. ‘‘Ted’’ Massalski. The issue was organized by David E. Laughlin, Carnegie Mellon University; John H. Perepezko, University of Wisconsin–Madison; Wei Xiong, University of Pittsburgh; and JPED Editor-in-Chief Ursula Kattner, National Institute of Standards and Technology (NIST). & C. Sudha 1 Metallurgy and Materials Group, IGCAR, Kalpakkam, Tamilnadu, India 2 Department of Atomic Energy, Homi Bhabha National Institute, Mumbai, India 3 Laser Materials Processing Division, RRCAT, Indore, India 4 Design and Manufacturing Technology Division, RRCAT, Indore, India calculated using Dayananda’s approach. While the diffusivities of substitutional solutes were similar in alloy 617 (0.31-0.42 9 10-15 m2/s at 1050 C), they differed in 10Cr steel in the following sequence: D~Cr [ D~Fe &D~Ni [ Further, multicomponent interdiffusion profiles were predicted using homogenization model in DICTRA and an integrated approach combining DICTRA with ThermoCalc helped in understanding the experimental observations on the interface microstructure. Keywords 10Cr steel  alloy 617  diffusion couple  DICTRA  interdiffusion coefficients 1 Introduction Due to increased concern over climate change, advanced clean coal technologies with lower CO2 emissions and higher thermal efficiency are being developed to replace operational subcritical plants.[1–3] Advanced Ultra Super Critical (AUSC) technology has a target thermal efficiency of 50%[2] and the Indian programme is aimed at the design, construction and operation of 800 MW AUSC thermal power plant with steam parameters in the range of 30-31 MPa and 700-710 C.[4,5] Hence, selection of structural materials capable of withstanding such high temperature and pressure is of paramount importance.[6] Prime candidates for components experiencing temperatures up to 620-650 C in rotor and piping of AUSC plants are creep strength enhanced 9-12wt.%Cr Ferritic-Martensitic (F/M) steels.[7] For higher temperatures in superheaters, headers and steam turbine rotor, Ni-based super alloys like alloy 617, 625 and 263 are chosen for their 123 J. Phase Equilib. Diffus. exceptional creep and oxidation resistance.[8–10] Hence in large components, Dissimilar Materials Joints (DMJ) of F/M steels and Ni-based alloys are inevitable due to cost considerations. Most of the experimental investigations on DMJs of F/M steels and Ni-based alloys are on mechanical properties and associated microstructural changes and have identified Heat Affected Zone (HAZ) of the steel and interdiffusion zones as regions of failure.[11–18] Formation of compositionally modified ‘transition’ regions, enhanced precipitation and diffusion controlled phase transformation affect the impact and fracture toughness and creep strength of the dissimilar joints prepared by fusion welding processes such as Narrow Gap Submerged Arc Welding (NGSAW) and Gas Tungsten Arc Welding (GTAW).[17,18] Important observations based on these studies are: (a) formation of a ‘transition’ region in the weld zone of Shielded Metal Arc Welded (SMAW) 12Cr steel/ENiCrFe[11] and GTAW 12Cr/alloy 617[12] DMJ (b) enhanced Ni and Cr concentration resulting in c-austenite formation during Post Weld Heat Treatment (PWHT) and c ? a0 transformation during cooling in SMAW welded P92/alloy 82 or 182[13] and 16CrMo/alloy 625 Cold Metal Transfer (CMT) joints[14] (c) partial and localized c transformation during creep tests in GTAW alloy 617B/COSTE graded transition joints[19] (d) d-ferrite formation at the interface of GTAW and SMAW P92 weldments[20,21] (e) formation of partially melted/unmixed zones containing Ni deficient and rich ferrite and martensite phases, respectively, at the interface of NG-GTAW alloy 617/high Cr steel[15] and alloy 617B/ COSTE[22] welds and Fe rich FCC phase in the weld metal of laser welded alloy 617/9Cr[23] (f) formation of fine M23C6 carbides in aged alloy 617 which prevents migration of grain boundaries in NG-GTAW alloy 625/9Cr[16] (g) formation of lamellar carbides in the HAZ of alloy 617 that lead to intergranular fracture in NG-SAW alloy 617/9Cr[17] (h) diffusion of carbon and nitrogen across NGGTAW alloy 617/10Cr interface resulting in the precipitation of M23C6, M6C, AlN and TiN along the grain boundaries leading to cavity formation and creep failure[18] and (i) soft zone formation in GTAW alloy 617/P92 weldments due to carbon transport after PWHT.[20] Thus fusion welded joints exhibit a wide variety of microstructures which transform during PWHT or testing. Unlike experimental investigations, simulation of multicomponent diffusion profiles in DMJs is a challenge and such efforts over a period of time shifted from analytical to thermo-kinetic approach. The CALPHAD method addressed the difficulties faced in multicomponent systems by extrapolating the experimental information on phase diagrams, enthalpy, chemical potential, diffusion coefficients etc. on binary and ternary to higher order systems.[24,25] It was proposed by Ågren and Anderson that by assuming a 123 vacancy exchange mechanism for diffusion in crystalline solids the intrinsic diffusion coefficient of component k with respect to gradient in component j in a multicomponent system can be expressed as a product of mobility  function ‘Mk ’and thermodynamic factor ‘olk oxj ’ as follows.[24,26,27] DLkj ¼ xk Mk olk oxj ðEq 1Þ where xk is the mole fraction of component k. With this formalism and on the assumption of sharp interface and local equilibrium, the DICTRA software package was deve (...truncated)