High-Temperature Oxidation of Cr-Mo Steels and Its Relevance to Accelerated Rupture Testing and Life Assessment of In-Se
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CHROMIUM-MOLYBDENUM steels are used extensively in the manufacture of piping, heat exchangers, pressure vessels, and several other high-temperature components in processing and power plants, due to their considerable resistance to creep, oxidation, and stress corrosion cracking. These steels are designed for a service life up to 30 years. Often, the remaining life of these high-temperature components hinges on accurate life assessment procedures such as accelerated creep rupture tests. In these tests, new or aged materials such as Cr-Mo steels are tested under accelerated conditions, viz. higher temperature or greater stress than those encountered in service. The results of these tests are then extrapolated to in-service conditions to estimate the remaining life of the components. Oxidation resistance of Cr-Mo steels, such as 1.25Cr0.5Mo and 2.25Cr-1Mo steels, is generally acceptable at commonly employed service temperatures (~450 C), but not at higher temperatures where accelerated creep testing is carried out. Oxidation can be a life limiting factor as a result of the direct metal wastage or, indirectly, by local temperature gradients as a result of scale thickening.[1,2] R.K. SINGH RAMAN, Associate Professor, and A. AL-MAZROUEE, Graduate Student, are with the Department of Mechanical Engineering, Monash University, Melbourne, Vic 3800, Australia. Contact e-mail: [email protected] Manuscript submitted December 27, 2006. Article published online July 14, 2007. 1750—VOLUME 38A, AUGUST 2007
Different parameters of oxidation of Cr-Mo steels at high temperatures have been studied. Oxidation studies on 2.25Cr-1Mo steel, for example, include the role of the oxidation temperature (550 C to 700 C),[3,4] the effects of cold work,[5] the use of acoustic emission monitoring for detection of the spallation of oxide scales developed at 600 C to 950 C,[6] the role of alloy grain size,[7–10] oxidation behavior of different zones of weldments,[9,11] and general specimen thinning kinetics at 600 C to 800 C.[12,13] Most of these experimental investigations have been carried out using rectangular specimen coupons. However, in the study by Marino and Bueno,[12,13] specimens of cylindrical geometry, i.e., similar to the specimens used in high-temperature creep testing, were used. The use of specimens of cylindrical geometry is important for studying the effects of oxidation on the creep properties of steels during accelerated creep testing. Different alloying elements are added to Cr-Mo steels in order to improve their creep strength, corrosion resistance, and weldability.[14] Alloying elements cause improvement in mechanical strength through solid solution strengthening or precipitation strengthening. However, the presence of certain alloying elements in the alloy matrix (namely, chromium, aluminum, and silicon) is also significant for oxidation resistance of steels. When sufficient amounts of these elements are present in the alloys, a layer of their oxides (Cr2O3, Al2O3, or SiO2) is developed, which confers protec
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