Krishna Guguloth

Fatigue damage of a thermal barrier coated Ni-base superalloy

High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy were conducted in air. Results reveal that the endurance limits for the TBC and bond coated substrate were 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, interdiffusion of A] from coating to substrate and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. The mode of fracture in the substrate at very high fatigue stress was intergranular whereas that at low stress was transgranular. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5400107 cycles during fatigue testing at 1073 K in air, due to a continuous alumina scale growth at the top coat (TBC) / bond coat interface.

Remnant Life Assessment and Microstructural Studies on Service Exposed Primary Reformer Tubes of a Catalytic Converter of an Ammonia Plant

Abstract 13.5 year service exposed (SE) catalyst primary reformer tube material made of H39WM micro-paralloy grade used in feritilizer plant was assessed for remaining life. The investigation includes mechanical properties evaluation; microstructural analysis and accelerated stress rupture tests. Failed tube portions showed coarsening primary carbides of Chromium and Niobium along the grain boundaries. Degradation of Niobium carbide (NbC) into Ni-Nb-Si phase and partial conversion this phase back to NbC was observed. Secondary carbides within grains were almost absent. Degradation in tensile strength for a range of temperature from 1123 to 1223 K was also observed but they were within the specified limits. Premature failures within 3–5 years service exposure are more common in reformer tubes. The failure was attributed to localized overheating leading to creep damage. The cast tube material may undergo microstructural changes during service exposure which is the main cause of degradation in strength and hardness changes. Accelerated stress rupture tests were performed in the range of 1173–1248 K on samples machined from 13.5 years at 1191 K and 15.4 MPa exposed reformer steel tubing, did not reveal any degradation of rupture behavior compared to that of the virgin alloy. An additional life of at least 10.6 years is predicted at the operating stress-temperature conditions.

Characterization of Microstructure and Mechanical Properties of 9.2Cr0.45V0.7C Heat-Resistant Steel

Microstructure and mechanical properties of 9.2Cr0.45V0.7C steel have been evaluated in this work. Differential scanning calorimetry study indicating the various phase changes during heating and cooling is consistent with high-temperature XRD analysis. Carbide precipitates are observed during tempering in the temperature range of 750–800xa0°C. Two types of precipitates are identified, MC-type precipitates with an average size of 50xa0±xa015xa0nm and M23C6-type precipitates with an average size of 150xa0±xa050xa0nm. The tensile properties of the steel are dependent on tempering and processing conditions. Room-temperature tensile properties increase with tempering temperature. The hot rolling is more effective than hot forging to improve tensile properties of the steel at room temperature. High-temperature tensile strength decreases, whereas elongation increases with increasing temperature. Conventional creep curve shows three stages of behavior for this steel at 60xa0MPa and 600xa0°C.