Low-Temperature Sensitization Behavior of Base, Heat-Affected Zone, and Weld Pool in AISI 304LN RAGHUVIR SINGH, GAUTAM DAS, P.K. SINGH, and I. CHATTORAJ Present investigations were focused on low-temperature sensitization (LTS) behavior of 304LN stainless steels considered from pipes of two different thicknesses. The specimens for the present study were taken from solution-annealed pipes (of varying thicknesses) and welded pipes (including the heat-affected zone (HAZ)). The specimens were subjected to thermal aging at 400 °C and 450 °C for different durations ranging from 125 to 8000 hours, to evaluate their sensitization susceptibility. The aging durations were worked out to simulate the 30-to-100-year life of the studied stainless steel at 300 °C using the Arrheneous equation and considering the activation energy of 150 kJ/mol. The thermally aged specimens were characterized for their degree of sensitization (DOS) and susceptibility to intergranular corrosion (IGC) by double- loop (DL) electrochemical potentiokinetic reactivation (EPR) and by methods given in the ASTM A262 practices A and E. It has been clearly shown that the weld pool is more sensitive to IGC than are the base and the HAZ at both the aging temperatures (LTS), because they showed IGC cracks during the bending subsequent to the boiling in H 2 SO 4 -CuSO 4 solution. Both the base and the HAZ of the thicker pipe material showed susceptibility to sensitization, as indi- cated by the increasing DOS and ‘‘dual-type’’ microstructure during electrolytic oxalic acid (EOA) etching; however, they were found safe from IGC for the studied sensitization times. The susceptibility to sensitization and IGC in the weld pool is related to the presence of copious delta ferrite with chromium diffusivity that is accelerated compared to the austenite phase. The time- temperature-sensitization (TTS) curves were prepared accordingly, based on these results. DOI: 10.1007/s11661-009-9803-7 Ó The Minerals, Metals & Materials Society and ASM International 2009 I. INTRODUCTION STAINLESS steels, despite having good mechanical and electrochemical properties, are susceptible to sensi- tization when exposed to temperatures in the range of 500 °C to 800 °C. Sensitization results in the precipita- tion 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] Chro- mium 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 size increased significantly. [7] The sensitization below the classic sensitization temperature range is termed low- temperature sensitization (LTS); this is extremely rele- vant 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 chromium- depleted zone (CDZ), which is critical for sensitization- induced 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 RAGHUVIR SINGH, GAUTAM DAS, and I. CHATTORAJ, Scientists, are with the National Metallurgical Laboratory, Jamshed- pur-831007, India. P.K. SINGH, Scientist, is with the Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Contact e-mail: raghujog@yahoo.co.in Manuscript submitted September 19, 2008. Article published online March 19, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 40A, MAY 2009—1219