Shutdowns effect during operation of steam pipeline of tpp on the fracture toughness of 15Kh1M1F steel

Показано, що локальний параметр механіки руйнування – статична тріщиностійкість (JIC), чутлива до зміни стану металу внаслідок деградації в експлуатаційних умовах. Виявлено, що кількість зупинок технологічного процесу під час експлуатації сталі, орієнтація зразків (осьова й тангенціальна) та місце їх розташування в перерізі стінки труб (біля зовнішньої та внутрішньої поверхонь труб) впливають на рівень статичної тріщиностійкості. За однакової тривалості експлуатації на головних парогонах ТЕС, але різної кількості зупинок блоків, які супроводжуються значними перепадами температури в стінці труб, найбільше зниження рівня JIC сталі 15Х1М1Ф зафіксували для металу, що переніс більшу кількість зупинок. Цим доказано, що показник JIC високочутливий до зміни стану експлуатованого металу під впливом теплозмін. Проаналізовано механізми руйнування металу у вихідному стані та після експлуатації, виділено вплив кількості пусків-зупинок технологічного процесу на інформативні фрактографічні ознаки.The main steam pipelines of TPP are subjected to a combined effect of number of technological factors causing the degradation of steel. Lot of operational factors affect the workability of energy equipment. Among these factors are hard temperature-force operating conditions (temperature of the steam pipelines is up to 570oC and the steam pressure – 24 MPa) and hydrogenated environment. One more reason of the loss workability of equipment was revealed recently. This is a frequent shutdown of the process caused by exploitation of the blocks in a maneuvering mode. It is known that fracture toughness (JIC) is a local parameter of fracture mechanics sensitive to changes in the state of the metal as a result of its degradation in service conditions. It was shown that JR-curves significantly differ for the steel 15Kh1M1F in virgin state and used for a same time (~2⋅105 h), but at different numbers of process shutdowns on the steam pipeline. It was revealed that number of shutdowns during longtime operation of steel and orientation of the specimens (axial and tangential) and its location in the cross section of tube wall (near the outer or inner surface of the tube) affect the value of fracture toughness. At the same operation time of steel on the main steam pipeline of TPP, but at different numbers of shutdowns the maximum reduction of the JIC level was revealed for the 15Kh1M1F steel after more shutdowns. Consequently, JIC level is the sensitive indicator of changes in the state of the metal under the influence of thermal cycles caused by shutdowns. The fractographic features of the steel in virgin state and after operation during the same time on the main steam pipeline of TPP, but at different numbers of shutdowns were analyzed. The wide stretch zone with following ductile fracture by forming of dimple structure was revealed on the fracture surface of steel in virgin state. A few fractographic features were revealed on the fracture surfaces of long-time operated steel. That is, the stretch zone on the fracture surface of the metal operated at smaller number of shutdowns is significantly less than in the non-operated metal, but in the steel operated at larger number shutdowns - it disappears. Ductile increment of cracks during static active loading of specimens of operated steel are essentially decreased and the continuity of the crack fronts disappeared. At the stage of spontaneous fracture the steel operated at smaller number shutdowns was destroyed by the mixed inter- and transgranular fracture, but the steel after more shutdowns – practically only by intergranular fracture. Intergranular fracture was considered as an indicator of the weakening the boundaries between the neighbouring grains