Evolution of Carbide Precipitates in 1.25Cr-0.5Mo Steel During Simulated Postweld Heat Treatment
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rrent industrial trend calls for intensive welding practice due to the ever-increasing demand on pressure vessel volume, invariably leading to residual stress accumulation on welding joints and, as a consequence, deteriorated microstructure as well as downgraded mechanical properties. One of the practical strategies is to apply postweld heat treatment (PWHT) where necessary.[1–4] However, mechanical properties of the base metal will be negated to a certain extent after long periods of PWHT.[5,6] In practice, in order to guarantee the performance of postweld heat treated steels, it is necessary to estimate the performance of the delivery state steels by applying simulated postweld heat treatment (SPWHT). Consequently, it is of great significance to elucidate the changes of the microstructure after SPWHT, which dictate the ensuing properties of end products.
YANG SHEN and CONG WANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] HIROYUKI MATSUURA is with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Manuscript submitted April 15 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Possessing an array of highly desirable properties, such as elevated temperature strength, creep resistance, and antisulfur corrosion performance, 1.25Cr-0.5Mo steel (ASTM A387 Grade 11 Class 2) was extensively employed as plates for boiler and pressure vessel applications.[7–10] Many studies have focused on creep rupture analysis and the prediction of service lifetime. Wei et al.[11] investigated the effect of heat treatment on microstructure and mechanical properties of 2.25Cr-1Mo steel and revealed that coarsening of carbides led to degraded impact toughness. Yang et al.[10] demonstrated carbide evolution in 2.25Cr-1Mo after long-term aging, which caused significant growth and coarsening of carbides. Jayan et al.[12] revealed coarsening of nanosized M23C6 particles reflected in diffraction pattern after extended service. Nevertheless, only limited documents are available describing the evolution of microstructure and mechanical properties of Cr-Mo steel after SPWHT. Chen et al.[13] demonstrated that deformation promotes the precipitation of M3C, M2C, and M23C6 during creep rupture testing on the simulated postweld heat treated 2.25Cr-1Mo steel. Kim et al.[14] investigated the complete history of carbide evolution in the 8 pct Cr steel along its full processing route and discovered the final microstructure was tempered martensite with MC, M7C3, and M23C6. However, evolution behavior of carbide of 1.25Cr-0.5Mo steel during SPWHT was entirely left blank. From this point of view, it is imperative to systematically investigate the evolution of microstructure of 1.25Cr-0.5Mo pressure vessel steels under different heat treatment conditions, which can offer a general suggestion for selecting PWHT holding time. The chemical composition of 1.25Cr-0.5Mo steel is shown in Table I. Five 110-mm-thick steel plates were held at 1203 K for 220 mi
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