Svirska L. Structural and mechanical regularities of operational degradation of heat-resistant steels of fastening fittings and steam pipelines bends of TPPs

The object of the study is the high-temperature degradation of heat-resistant steels of the main steam bends and the fixing fittings of the high-pressure cylinder of the steam turbine TPP under the combine condition of influence of technological and operational factors. The purpose of the research was to evaluate the mechanical condition and to identify the structural and fractographic features of degradation of heat-resistant low alloy steels from the main steams bends and elements of the fixing fittings of the high pressure cylinder of the steam turbine TPP after the long-term exploitation and to justify the possibility of their lifetime prolongation. Research methods: metallographic analysis of the structure; fractographic studies of fracture mechanisms; mechanical tests with determination of hardness, strength and ductility characteristics, toughness, static and cyclic crack resistance, restorative thermal treatment of fixing fittings. For the first time, the possibility of using a repeated heat treatment for restoring the structure and mechanical characteristics of the exploited 25Kh2M1F steel to values accepted for fastening fittings in the initial state was substantiated. It was experimentally proved the more intensive degradation of the 12Kh1MF steel in all zones of the steam pipelines bends of TPPs compared to their straight sections. It was confirmed by the structural features (carbide sizes along the grain boundaries), and the characteristics of resistance to brittle fracture (impact strength, static and cyclic fracture toughness) of degraded steel. It was found at the first time that not only the metal of the stretched zone of the bend can become critically degraded (due to creep), but also the metal of the compressed zone (because of the peculiarities of its deformation during a pipe bending).The results of this PhD work are used on TPPs of Ukraine, namely, for the assessment of the technical state of the operated 25Kh2M1F steel of fastening fittings of HPC of the steam turbine and for a selection of restorative heat treatment of nuts and pins after their operation. The results of the technical state assessment of the 12Kh1MF steel from both bends of the main steam pipelines after their operation at TPPs of Ukraine were used to substantiate decisions about a priority of elements replacement or terms for their next surveys.