Role of microstructural degradation in the heat-affected zone of 2.25Cr-1Mo steel weldments on subscale features during
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TRODUCTION
LOW-ALLOY ‘‘Cr-Mo’’ ferritic steels, viz., 2.25Cr1Mo and 1Cr-0.5Mo steels, are used extensively in the steam generating and handling systems of power plants (in the temperature range of 623 to 873 K) because they satisfy the required mechanical properties, weldability, formability, and corrosion resistance.[1,2] Common applications of 2.25Cr-1Mo steel include reactors for refining and processing of petroleum and high-temperature/high-pressure vessels for thermal reforming, polymerization, alkylation, and hydrocracking.[3–6] This material is also a strong candidate for the fabrication of pressure vessels used for the gasification and liquefaction of coal.[7] The microstructures of Cr-Mo ferritic steels are very susceptible to thermomechanical treatments. This microstructural susceptibility is often exploited in order to develop carbide precipitates of a required chemistry, morphology, and distribution to effect precipitation hardening. However, due to the metastable nature of the chemical composition and the morphology of the strengthening precipitates, the secondary precipitates undergo undesirable transformations during elevated temperature service and/or thermomechanical treatments experienced during fabrication, viz., welding, forging, hot rolling, etc. The strength of the weldments of these steels is generally reported to be inferior,[7,8] to the extent that the creep rupture of the welds is often the life-limiting factor. In fact, about 80 pct of the in-service failures are reported to take place in the weld region of low-Cr ferritic steel compoR.K. SINGH RAMAN, Postdoctoral Research Associate, is with the Department of Communication and Electronic Engineering, Royal Melbourne Institute of Technology (RMIT), Melbourne-3000, Australia. Manuscript submitted April 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
nents.[9] Since weldments are an indispensable part of most component fabrication, considerable efforts have been directed, in the past three to four decades, to the correlation of the in-service failure of these steels with the microstructural degradation caused during welding. A. Microstructural Features and Oxidation Resistance of 2.25Cr-1Mo Steel Microstructural changes due to welding include variations in the grain size in the area adjoining the weld metal (i.e., the heat-affected zone (HAZ)) and enrichment of Cr in the secondary precipitates and/or additional Cr-rich precipitate formation.[1,2,7,9–11] Trapping of ‘‘free’’ chromium (in the matrix) through Cr-rich precipitate formation and variations in grain size is reported to alter the oxidation resistance of low-Cr alloys.[12,13,14] For example, the heat treatments which cause depletion of chromium due to Crrich carbide formation impair the oxidation resistance of 2.25Cr-1Mo steel by forming a less protective scale. In a similar way, the HAZ of Cr-Mo steel weldments, in which extensive formation of Cr-rich precipitates occurs,[10,11] may exhibit a greater oxidation rate than other weld regions. Low-chromium steels are also known to
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