Experimental and Thermokinetic Simulation Studies on the Formation of Deleterious Zones in Dissimilar Ferritic Steel Wel
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erritic and high-Cr ferritic-martensitic steels are used as structural materials in the steam generator (SG) circuits of nuclear- and fossil-fired power plants because of their adequate creep resistance in the temperatures ranging from 673 K to 823 K (400 C to 550 C) and resistance to stress corrosion cracking.[1] Different types of Cr-Mo steels are used for the components in the SG circuit depending on the service temperature that will be experienced by it, making dissimilar weld joints unavoidable. When dissimilar weldments between Cr-Mo steels are exposed to high temperatures either during service or postweld heat treatment (PWHT), significant microstructural and microchemical changes occur near the weld fusion line.[2–6] Diffusion of carbon from low- to high-Cr steel results in simultaneous dissolution and precipitation of carbides near the weld interface, consequently forming carbon-depleted and -enriched ‘‘soft’’ and ‘‘hard’’ zones in low- and high-Cr steels, respectively. This long-range R. ANAND, Research Engineer, formerly with the Microscopy and Thermophysical Properties Division, Physical Metallurgy Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, India, and is now with the Sandvik Materials Technology (SMT) R&D, Sandvik Asia Pvt Ltd, Pune, India. C. SUDHA, Scientific Officer ‘F’, S. SAROJA, Head, and M. VIJAYALAKSHMI, Associate Director, are with the Microscopy and Thermophysical Properties Division, Physical Metallurgy Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research. Contact e-mail: [email protected] Manuscript submitted July 27, 2012. Article published online January 5, 2013 2156—VOLUME 44A, MAY 2013
diffusion of carbon was identified to be because of the steep carbon activity gradient existing across the weld interface, the magnitude of which is proportional to the relative chromium content of the steels.[5] Sopousek and Foret[7] established a strong correlation between carbon activity and chromium content in a variety of engineering steels and model alloys, even when several other alloying additions like V, Mo, W, N, Mn, and Si were present. Carbon activity calculated using CALPHAD method was observed to decrease exponentially with increase in the concentration of chromium. Problems associated with the diffusion of carbon across dissimilar ferritic/ferritic and ferritic/austenitic weldments are well studied using both experimental[3–11] and computational methods.[12–22] The authors have systematically studied the formation of hard zone (HZ) and soft zone (SZ) in 2¼Cr-1Mo and 9Cr-1Mo dissimilar joints.[8,10] Direct experimental evidence was obtained for the formation of chromium-rich M23C6 carbides in the HZ and depletion of molybdenum-rich M6C carbides in the soft zone (SZ). In addition, it was also demonstrated that at 1023 K (750 C), diffusion of substitutional alloying elements across the weld interface changes the precipitation sequence in 9Cr-1Mo to resemble that of medium/low-Cr steels. Further, a single-phase
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