Low-Temperature Sensitization Behavior of Base, Heat-Affected Zone, and Weld Pool in AISI 304LN
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STAINLESS steels, despite having good mechanical and electrochemical properties, are susceptible to sensitization when exposed to temperatures in the range of 500 C to 800 C. Sensitization results in the precipitation of chromium carbide along the grain boundary with the simultaneous depletion of chromium from areas near the grain boundaries; this ultimately reduces the working life of the components in service.[1–5] Chromium depletion in stainless steels in this temperature range progresses through both the nucleation of fresh carbides and the growth of existing carbides, if any. However, Povich et al.[6,7] showed that AISI 304 at or below 500 C may get sensitized. It has also been reported that sensitization at £500 C occurs through the growth of pre-existing carbide and that fresh chromium carbide nucleation was ruled out unless specimens are coldworked to certain critical limits.[8] This was concluded based on the authors’ investigation, which showed that, for T £ 500 C, the number of carbide particles remained unchanged, even though their RAGHUVIR SINGH, GAUTAM DAS, and I. CHATTORAJ, Scientists, are with the National Metallurgical Laboratory, Jamshedpur-831007, India. P.K. SINGH, Scientist, is with the Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Contact e-mail: [email protected] Manuscript submitted September 19, 2008. Article published online March 19, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
size increased significantly.[7] The sensitization below the classic sensitization temperature range is termed lowtemperature sensitization (LTS); this is extremely relevant to the several industrial processes using stainless steels that operate at above ambient temperatures. A nuclear power plant is one such example; in nuclear power plants, stainless steel pipes are being used at an operating temperature of ~300 C. If the components operated at this temperature contain chromium carbide nuclei, they may be sensitive to the LTS phenomenon. Chromium carbide nucleation in austenitic stainless steels is possible during fabrication processes such as welding, solution annealing, stress relieving, etc., when temperatures in the range of 500 C to 800 C are encountered. Stainless steels thus precipitated with carbide nuclei do not necessarily contain a chromiumdepleted zone (CDZ), which is critical for sensitizationinduced failures in corrosive environments. It is possible that the stainless steels with tiny carbide nuclei with an insignificant CDZ would not encounter failures at low temperature and that, therefore, this is not a serious concern. Such carbide nuclei may, however, proliferate at operating temperatures such as ~300 C and broaden the CDZ after prolonged service exposure, to cause sensitization-related failures. This was verified by several failures encountered in the nuclear industries and the resulting analyses. The pre-existing tiny carbide particles were observed to grow in size when failed components were investigated; this was accompanied by severe VOLUME 40
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