Evaluation of caustic embrittlement susceptibility of steels by slow strain rate testing
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I. INTRODUCTION
THE extraction of alumina from mineral bauxite using the Bayer process is a major industry in Australia, where the world’s largest alumina refineries are located. A large aluminum company (namely, Comalco) is in the process of erecting a new alumina refinery and cogeneration plant, with an annual capacity for over 1 million tonnes of alumina. The Bayer process makes extensive use of aggressive caustic solutions at elevated temperatures/pressures, and the effective containment of these solutions during processing presents a serious challenge in infrastructure maintenance. Cracking and failures in the reaction vessels, digesters, and caustic cleaning tanks, and in pipework linking major vessels in the processing plant, are ongoing problems, and plant integrity is a major concern with regard to occupational health, safety, and environment. Mild steel vessels, digesters, and pipework used in pulp-and-paper processing (by Kraft process) also encounter aggressive caustic solutions. Caustic cracking is often the first suspect whenever a major failure occurs in a component operating in caustic environment. In spite of the range of studies[1–22] on the role of the key process variables in caustic cracking of mild/carbon steels and cast iron, caustic cracking continues to be a major maintenance and safety concern. In the face of the industrywide thrust for improving processing efficiency of Bayer process by increasing processing temperature, it is becoming increasingly significant to develop improved understanding of safe/unsafe liquor concentrations and temperatures with respect to caustic cracking susceptibility of the currently used materials (i.e., mild steel). Stress corrosion cracking (SCC) is the premature cracking of materials under the synergistic action of a tensile stress and corrosive medium,[23] neither of which would cause cracking, when acting alone. Caustic cracking is a form of SCC that results from embrittlement of material exposed to caustic environment. Given the required combination of stress and environment, stress corrosion cracks may propagate undetected to leak or even to a sudden catastrophic failure. It requires a delicate balance between the R.K. SINGH RAMAN, Senior Research Fellow, is with the School of Physics and Materials Engineering, Monash University, Melbourne, Victoria 3800, Australia. Contact e-mail: [email protected] Manuscript submitted May 25, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
tensile stress and the corrosive environment at the crack tip for the stress corrosion crack to propagate. In this context, it may be important to emphasize that an increase either in the corrosiveness of the environment or the tensile stress does not necessarily mean an increase in the susceptibility to SCC, meaning thereby that SCC occurs within a rather narrow window of synergistic interaction of stress and corrosion. It is not surprising therefore that Champion[24] has reported caustic cracking under a given Bayer liquor chemistry but no cracking in many areas of
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