Fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted at 800ºC in air. Fatigue results reveal that the endurance limits for the TBC and bond coated substrate was substantially higher than that of the base alloy, while the opposite was found for high stress, low cyclic life times. It appears that the increase in endurance limit for the TBC and bond coated superalloy is due to load shifting to the bond coat, and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. In addition to fatigue, accelerated creep properties of thermal barrier coated (TBC) Superni C263 were evaluated. Creep results reveal that the life of the TBC composite under accelerated creep condition is substantially high compared to that of the bare substrate. The mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular. Delamination of bond coat, oxidation of the substrate and spallation of the ceramic layer were evident at very high stress. It was evident that the substrate has negligible estimated rupture strength after 10,000 hours of service exposure.
A radiant heater lube in an oil refinery made of a particular grade of austenitic steel and service exposed for 61,000 hours was investigated for health assessment and safety. Experimentally determined yield strength and ultimate tensile strength as well as estimated 10,000 hours -100,000 hours rupture strength as obtained from experimental data in the temperature range of 773 to 873K exhibit a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization etc. Analysis of hot tensile and accelerated stress rupture data revealed that the service exposed radiant heater tube could remain in service for a length of more than ten years at the operating hoop stress level and maximum allowable skin temperature (158.78 MPa / 865 K), provided no localised damage in the form of cracks or dents had developed. It is recommended that a similar health check of these tubes should be carried out after 50,000 hours of service exposure at the desired operating conditions.
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