Thermal scaling behavior of weldments of 9Cr-1Mo steel and its relevance to the life assessment of fossil fuel power pla
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RAMAN, Research Fellow, is with the Key Centre for Advanced Materials Technology, School of Physics and Materials Engineering, Monash University Melbourne 3800, Australia. Contact e-mail: [email protected] Manuscript submitted April 11, 2002. 3296—VOLUME 33A, OCTOBER 2002
components,[10,11,12] and a reliable life assessment of the welded components should ensure an accurate life assessment of weldments. (7) The validity of nonuniform oxide scaling of weldments of a steam-generator steel (as observed in the air oxidation of 2.25Cr-1Mo steel weldments[6–9]) has never been examined for the weldments of other steam-generator steels, such as 9Cr-1Mo steel, which is a serious contender for wider applications. The 9Cr-1Mo steels were originally developed for steam generators and superheaters of U.S. fast breeder nuclear reactors and were subsequently studied extensively also for their similar use in European fast breeder reactors.[13] In comparison to their popular predecessors (viz., 1Cr-0.5Mo and 2.25Cr-1Mo steels), 9Cr-1Mo steels possess a superior combination of elevated temperature mechanical properties and resistance to stress corrosion cracking and other forms of corrosion. This steel, also commercially known as T91, is ideally suited[14] to new supercritical boilers at metal temperatures of 873 to 898 K and steam temperatures of up to 838 K. However, the microstructural changes in 9Cr-1Mo steel and their influence on mechanical properties have not been studied as thoroughly as in the case of other popular steam generator steels. In fact, there is little information available on the oxide scaling of weldments of 9Cr-1Mo steel. This article presents the results of the first investigations on the scaling behavior of weldments of 9Cr-1Mo steel and compares the recent findings of nonuniform scaling observed during air oxidation of 2.25Cr-1Mo steel weldments. The article also discusses the bearing of the nonuniform scaling on the life assessment. Normalized-and-tempered plates of 9Cr-1Mo steel were butt welded by the standard manual metal arc welding technique, using an appropriate electrode, so the chemical compositions of the base metal and the deposited weld metal were similar (which was ascertained upon the chemical analysis). Polished transverse sections of the weldments were oxidized at 923 K in air for different durations up to 500 hours. The thickness of the oxide scales over the HAZ region of the oxidized section of the weldment was visibly greater. The difference in thickness of the scales formed over the weld metal and the HAZ, after different durations of oxidation of the weldment, was measured using a sensitive surface profilometer. This surface profile measuring system can be employed to measure the marked and abrupt changes in the thickness of the oxide scales developed over the different regions of the specimen. Details of the surface profilometer can be found elsewhere.[7] There was a marked difference in the thickness of the scales (0.45 m) formed over the weld metal and the HAZ even in 4
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