Закономірності зміни механічних характеристик сталі 12Х1МФ з різних зон гину парогону ТЕС

Abstract

Гини парогонів ТЕС характеризуються неоднаковою деформацією металу різних зон: розтягненої, нейтральної, стисненої. Як результат – різні механічні властивості цих зон унаслідок деградації. Досліджено міру деградації сталі 12Х1МФ з різних зон гину після 2,5·105 год її експлуатації у системі парогонів ТЕС. Стан металу оцінювали за результатами візуального обстеження та за механічними характеристиками за одновісного розтягу та удару на повітрі. Виявили, що максимальних змін внаслідок експлуатації зазнають механічні характеристики сталі розтягненої та нейтральної зон гину.

Hard temperature-power operating mode of the steam pipeline (vapor pressure up to 14 MPa, temperature – up to 565 oC and a large number of shutdowns of the process), and long-term influence of an aggressive hydrogenating environment on stressed metal promote degradation of its structure and reduce the mechanical properties, which were ensured by their workability at the beginning of operation. Therefore, the diagnostics of the technical state of metal of the steam pipeline ensures the reliability of their operation and does not lose relevance to the power system of Ukraine with critically worn-out equipment. Features of degradation of thermal power plant steam pipeline bending were caused by non-identical conditions of deformation of the metal in different zones (tensile, compressed and neutral) of bend as on the stage of production and the follow long-term operation. Due to the technological features of production and peculiarities of the cross-sectional shape of bends the maximum tensile stresses occur in the tensile zone of bending, where the wall thickness of tube is minimal. Mechanical properties of metal from different bend zones were differed after long-term impact of stress. The level of degradation of 12Kh1MF steel from different zones of bend after 2,5·105 hours of operation in the steam pipeline system of TPP was evaluated in this paper. Technical state of metal was estimated by visual inspection of bend and using mechanical characteristics determined at uniaxial tension and impact test on the air. During visual inspection of outer surface of the pipe an axial cracks in the tensile zone of bend, caused by high temperature creep, were detected. It was established that mechanical properties of operated steel from tensile and neutral zones of bend is maximally changed. That is, the characteristics of plasticity, determined on the tangential specimens were sensitive to changes in the technical state of steel. Reduction of area of metal from tensile zone of bend is in 2 times lower than the minimum allowable value (27% and 55% respectively). Impact toughness level of the metal from tensile zone of bend determined on specimens of both orientations was lower than the regulated value for heat-resistant steels too. This confirmed that the brittle fracture resistance characteristic is also highly sensitive to changes in the technical state of the pipeline bend metal. Taking into account the fact that degradation of damaged bending has reached the maximum degree (since the bend has been damaged during operation), the level of the impact toughness of the metal can be proved expediency of stopping the exploitation the actual structural elements of steam pipeline.