Discontinuous Creep Behaviour of 15Mo3 Type Steel

15Mo3 type steel samples were subjected to creep loading at 550°C in order to investigate the development of microstructure as well as the effect of the carbides. It was found that after reaching the secondary creep region, the slope of the creep curve decreased, the creep rate reached a minimum value and remained unchanged as the effect of strainhardening is counterbalanced by an annealing influence. After a short plateau, the slope increased. In order to explain this type of behaviour and to suggest a damage mechanism, TEM and EBSD investigations were performed. At the beginning of the process, subgrain formation started which was retarded by the carbides. At elevated temperature, however, the average distance between carbides increased, and the pinned dislocations were able to bypass them, the internal strain inside the grains (after reaching a maximum value after about 481 hours) decreased, and the creep process accelerated.

Evaluation of Gas Turbine Transitions After 20,000 and 37,000 Hours of Service

A metallurgical investigation was conducted on transitions from two land-based 105 MW gas turbines. One transition suffered catastrophic failure, and two transitions had cracking at the top panel. Two alloys, IN-617 and a modified IN-617, were used in these transitions. The investigation consisted of thickness measurements, optical and scanning electron microscope fractographic studies, metallography, EDS and Auger analysis of precipitates, chemical analysis, and hardness measurements. In both units, heavy oxidation of the top panel on the hot gas side (inside) occurred. Oxide intrusion along the grain boundaries and exfoliation of the oxide layer occurred. This caused thinning of the panel with a resultant loss of about 50% of the panel thickness. Bulk creep cavitation along the grain boundaries and multiple discontinuous creep cracks were present. Crack origins were located at the outer surface of the top panels. Creep cracks and cavities, carbide precipitates, agglomeration of grain boundary carbides, and aluminum nitride were present in the microstructure. The primary cause of cracking was the increase in net section stress, which exceeded the creep strength of the panel. The effect of microstructural changes on the formation of cracks was secondary in nature. Cracks were not associated with welds.