Effect of fillers on the microstructure, mechanical properties, and hot corrosion behavior of Nb stabilized austenitic s
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This research article addresses the effect of fillers on the high-temperature corrosion behavior of AISI 347 weld joints. Multi-pass pulsed current gas tungsten arc welding was carried out on 6.67 mm thick plates of AISI 347 using three different fillers namely ER347, ER2553, and ERNiCrMo-3. The fusion zone microstructures of AISI 347 employing ER2553 and ERNiCrMo-3 exhibited columnar and dendritic grain growth; whereas vermicular delta ferrite was observed at the fusion zone of ER347 welds. Tensile studies showed that the weld employing ERNiCrMo-3 exhibited better tensile strength than the parent metal. High-temperature corrosion studies were carried out on the fusion zones by exposing the coupons to an aggressive, synthetic molten-salt incinerator environment containing 40% Na2SO4–40% K2SO4–10% NaCl–10% KCl at 650 °C for 50 cycles. The studies attested that the fusion zone employing ERNiCrMo-3 exhibited better corrosion resistance than the other two fillers used in the study. Spallation of oxides was witnessed due to the dissolution of Cr2O3 in the ER347 and ER2553 fusion zones. The hot corroded samples were characterized using surface analytical techniques.
I. INTRODUCTION
Stabilized austenitic stainless steel, AISI 347 is widely used in high-temperature applications like nuclear reactors, boilers, superheaters and chemical reactors and also in the oil and gas industry, refineries, electric power stations and because of their good resistance to sensitization and intergranular corrosion in many corrosive environments.1–4 The addition of Nb leads to the formation of NbC precipitates, which in turn stabilizes AISI 347 against grain boundary precipitation of M23C6 type of carbides, which would otherwise lead to chromium depletion and loss of corrosion resistance of the material. However, multi-pass welding or exposure to high operating temperatures, would lead to precipitation of chromium carbides at the grain boundaries in AISI 347 and the alloy becomes vulnerable to intergranular corrosion in highly oxidizing acidic media.5 Although Nb/Ti addition plays a favorable role as stated earlier, these additions also tend to bring about austenite instability due to the formation of intermetallic phases while exposure to high temperature and also during multi-pass welding.6 It was reported by various researchers that the formation of an intermetallic phase known as r-phase is a severe problem while using standard austenitic stainless steels at elevated temperatures. It is attested that this r-phase not only Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.492
reduces corrosion resistance of the stainless steels by removing chromium and molybdenum from the austenitic matrix, but also deteriorates its mechanical properties.7–9 Kaishu Guan et al.10 investigated the carbide precipitation of 12 mm thick, manual metal arc welded AISI 347 and AISI 321 samples which were exposed to an aging temperature of 700 °C for 6000 h. It is inferred from this study tha
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