Changes in Precipitate Distributions and the Microstructural Evolution of P24/P91 Dissimilar Metal Welds During PWHT
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FERRITIC creep resistant steels are widely used in modern power plant. The strength of creep resistant martensitic and bainitic Cr-Mo alloys is afforded by a combination of dispersion strengthening, solution strengthening, dislocation hardening, and subgrain boundary hardening.[1,2] The creep strength of new materials, which have been developed out of classical alloys such as 2.25Cr-1Mo (ASTM A335-P22) and 9Cr1Mo (ASTM A335-P9), has been enhanced by making micro-alloying additions.[3,4] Steels containing vanadium, niobium, titanium, tantalum, and/or other strong carbide and nitride forming elements form a dispersion of fine, stable, MX precipitates.[4–6] These particles, which display only low coarsening rates and remain stable at service temperatures, are formed during the
KARL E. DAWSON, Post Doctoral Research Associate, and GORDON J. TATLOCK, Henry Bell Wortley Professor of Materials Engineering, are with the Centre for Materials and Structures, School of Engineering, University of Liverpool, Liverpool L69 3GH, U.K. Contact e-mail: [email protected] KUANGNAN CHI, Senior Engineer, and PETER BARNARD, Principal Engineer, are with the Boiler Product Development Department, Doosan Power Systems, Porterfield Road, Renfrew, U.K. Elements of this work were presented at ASME PVP 2011 and were published in the conference proceedings: Dawson, K. and G.J. Tatlock. The Stability of Fine Sub-Grain Microstructures within Carbon Depleted Regions of Dissimilar Metal, Ferritic, Creep Resistant Welds. in ASME Pressure Vessel and Piping Conference. 2011. Baltimore, US: paper no. 57868. Manuscript submitted November 30, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
application of the necessary post-weld heat treatment (PWHT). The selection of materials which are used to construct power plant is governed by factors including cost, service environments, design life, and fabrication issues. These constraints dictate that a variety of alloys are used in the construction of boiler systems; hence transition joints need to be made between alloys which differ in chemical composition. Problems can arise from the use of dissimilar metal welds (DMW) in which the mechanical properties fall below those expected of either parent material. Lundin’s review[7] condenses the findings of numerous open source articles and internal industrial reports, to provide a broad overview of the issues surrounding DMW. The report, which covers a wide range of types of DMW including joints made between ferritic alloys, austenitic stainless steels, and nickelbased alloys, indicates the majority of reported failures occurred in ferritic material close to the fusion line. The mechanisms of failure, although it is stated that they may be oversimplified, could be traced to the formation of a soft, low strength zone of carbon-depleted ferritic material. Works by Christoffel and Curran,[8] and Livshits,[9] showed that ferritic–ferritic fusion welded transition joints, made between alloys which differed in concentrations of carbide forming elements, had a tenden
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