ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 28 consisted of a gas turbine (GT) and a heat recovery steam generator (HRSG). The GT, which was fuelled with pipeline natural gas, drove an electrical generator. The hot exhaust gases from the GT were directed into the HRSG, where they flowed through banks of water-filled tubes to generate steam. The casing of the HRSG was made from plates of carbon structural steel (ASTM A36), with a maximum carbon content of 0.25%. The plates, which were 0.25 in. (6 mm) thick, had been butt-welded together. The welds did not receive a post-weld heat treatment (PWHT) to relieve stresses. A thick layer of porous insulation material covered most of the internal walls of the HRSG, and this layer significantly reduced the temperature of the casing. The GT was The pipe flange, which was located on an offshore oil and gas platform, broke apart when the studs failed in service (Fig. 1). The failed studs were forwarded to a laboratory for cross-sectioning and examination under the microscope. Transgranular stress corrosion cracks were observed (Fig. 2), with most originating at the roots of the threads (stress-raisers). To keep the flanged joint tightly closed, the studs were subjected to considerable tensile stress. Operating in a marine environment meant that the pipe flange was subjected to sea spray and mist. In addition, being located in the hot sun without thermal insulation caused the metal surface temperature to reach levels well above ambient. These temperatures may have accelerated the cracking process and may well have increased the chloride concentration (by evaporation, wetting and drying, etc.). Common austenitic stainless steels (like Type 302, 304, and 316) are wellknown for their tendency to fail via SCC in chloride environments. Recognition of this problem led to the development of more resistant materials such as low-carbon ferritic alloys, austenitic- ferritic (duplex) alloys, and austenitic alloys containing high levels of molybdenum (e.g., 6% Mo alloys). Because of their chemical compositions and microstructures, these alloys are highly resistant to SCC in most common operating conditions. At one time, some scientists and engineers believed that SCC of common austenitic stainless steels did not occur unless the metal surface temperature was at least 60°C (140°F)[2]. This belief was largely based on experience with in-service equipment such as vessels, piping, and heat exchangers, as well as with metal surfaces exposed to hot, direct sunshine. About 30 years ago, failures of austenitic stainless steel fixtures in the ceilings and roofs of swimming pools were reported to be due to stress chloride cracking[3,4]. These failures had occurred at ambient temperatures of 20-30°C (68-86°F). It seems likely that SCC may occur at relatively low temperatures if the tensile stresses are high and/or the environments are particularly aggressive, such as high chloride concentrations or low pH (Fig. 3). Numerous metallurgical factors can also influence the initiation and propagation of stress cracking, including grain size and shape; grain boundary precipitation and segregation; composition, amount, and distribution of minor phases and inclusions; dislocation interactions; grain orientation; degree of phase stability; and de-alloying[5,6]. CARBON STEEL: NITRATE STRESS CRACKING This example involves a co-generation power plant that produced electricity and steam (Fig. 4). The plant Fig. 3 — Factors influencing initiation and propagation of stress chloride cracking. Fig. 4 — Diagram of a typical co-generation plant producing electricity and steam.
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