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

Methodology of hydrogen embrittlement study of long-term operated natural gas distribution pipeline steels caused by hydrogen transport

A methodology of experimental research on hydrogen embrittlement of pipe carbon steels due to the transportation of hydrogen or its mixture with natural gas by a long-term operated gas distribution network is presented. The importance of comparative assessments of the steel in the as-received and operated states basing on the properties that characterize plasticity, resistance to brittle fracture and hydrogen assisted cracking is accentuated. Two main methodological peculiarities are pointed out, (i) testing specimens should be cut out in the transverse direction relative to the pipe axis; (ii) strength and plasticity characteristics should be determined using flat tensile specimens with the smallest possible thickness of the working part. The determination of hydrogen concentration in metal, metallographic and fractographic analyses have been supplemented the study. The effectiveness of the proposed methodology has been illustrated by the example of the steel research after its 52-year operation

The fractography features of fracture of the 2,25CR-1MO steel after creep test

Виявлено, що швидкість усталеної повзучості нижча для металу у вихідному стані порівняно з металом після експлуатації в технологічному процесі гідрокрекінгу нафти. Крім того, ця швидкість у газоподібному водні вища ніж на повітрі, причому для сталі 2,25Cr-1Mo у вихідному стані й після експлуатації. Це свідчить, по-перше про втрату металом під час експлуатації його початкових властивостей, а по-друге про вплив водню й на деградацію, й на процес повзучості. Проаналізовано фрактографічні особливості руйнування сталі 2,25Cr-1Mo після випроб на повзучість у вихідному стані та після експлуатації. Встановлено, що руйнування за повзучості відбувається за класичним в’язким механізмом шляхом зародження, зростання та об'єднання пор. Фрактографічною особливістю деградованої сталі є великі глибокі ямки, спричинені декогезією неметалевих включень від матриці. Крім того, виявили великі плиткі дископодібні ямки з дрібними мікропорами на їх дні, що виникли навколо карбідів. Обидва типи ямок спричинені воднем, який накопичився в об’ємі металу під час експлуатації.The unstable properties of steels for the large construction during long term operation are one of the reasons of loss of oil equipment workability. High operating temperature, strain and hydrogenating technological environment are among the main factors of their degradation. The reactor damage causes huge destructions due to high processing parameters (temperature – 450 °C and pressure – 15 MPa) and large its dimensions (diameter up to 4 m, wall thickness up to 0.25 m and height ~20 m). Because that the reactor damage is very dangerous for service staff and environment. Stability of the mechanical properties of the steel of the oil hydrocracking reactor body during long-term exploitation is very important to guarantee it save workability. That’s why it is very significant to know metal properties after different service life for prediction of the material workability. The 2,25 Cr–1Mo steel in the initial state and after high-temperature degradation in hydrogenated environment under working conditions of hydrocracking reactor service for 6•104 hours (as the specimen-witnesses) were creep tested. The steady creep rates for both variants of heat-resistant steel were estimated in hydrogen and in air. It was revealed that steady creep rate of virgin metal is lower compared with metal exploited into oil hydrocracking process. The degradation of the metal in operating conditions increases creep strain rate in air and hydrogen compared with unexploited metal. More over the steady creep rate of the 2,25Cr-Mo steel in virgin state and serviced one creep tested in hydrogen is higher than in air. It firstly means that initial properties of the steel are exhausted during service, and secondly that hydrogen simultaneously effects on the degradation and creep processes too. The hydrogen intensities of the diffusion process and consequently accelerate of the steel degradation and makes easy of the creep process. The fractography features after creep test of the 2,25Cr-1Mo steel in the initial and after the operation during 6•104 hours in the process of oil hydrocracking process were analyzed. It was found the creep fracture occurs according to the classic ductile mechanism of nucleation, growth and association of pores. Special fractographical feature of degraded steel is large and at the same time deep holes caused by decohesion nonmetallic inclusions from the matrix. The next feature is the big and at the same time plate disc-like dimples with the small micro pores around carbides on their bottom. Both types of holes were initiated under the influence of hydrogen accumulated by bulk metal during operation. They become more visible at the creep test in hydrogen because their initiation is additionally stimulated by hydrogen, absorption by metal from the tested environment

Fatigue crack growth resistance of the 12Kh1MF steel from different zones of steam pipeline bends of TPP after long term operation

Гини головних парогонів ТЕС характеризуються неоднаковою деформацією металу різних зон (розтягненої, нейтральної, стисненої) на етапі їх виготовлення. Як результат, механічні характеристики металу цих зон після експлуатаційної деградації відрізняються. Досліджено втомний ріст тріщини сталі 12Х1МФ різних зон гину після 1,3∙105 год її експлуатації на парогоні ТЕС. Показано, що найменш деградованим є метал прямої ділянки гину. Метал зі стисненої зони виявив найнижчий опір поширенню втомної тріщини, що є ознакою найсильнішої деградації, яку зв’язали з виникненням дефектів термічної втоми. Закриття втомної тріщини в найбільшій мірі проявилася у стисненій зоні гину, що зв’язали з виникненням розсіяної пошкодженості металу внаслідок термічної втомиHard temperature-power operating condition of the steam pipeline (vapor pressure up to 14 MPa, temperature – up to 565 oC and a large number of shutdowns of the technological process), and long- term influence of an aggressive hydrogenating environment on stressed metal promote degradation of its structure and worsening of the mechanical properties, which ensured their workability at the beginning of operation. Therefore, the diagnostics of the technical state of steam pipeline bends directed on providing the reliability of their operation does not lose relevance to the power system of Ukraine with critically wear of equipment. The 12Kh1M1F steel from different zones of pipe bend from main steam pipelines after ~13104 h operation was investigated. During bend manufacturing the deformation of metal from its different zones (stretched, neutral and compressed) is not the same. As a result the properties of degraded metal from these zones are different too. The nominal and effective fatigue crack growth diagrams of the metal of stretched, neutral and compressed zones and straight pipe were received. Thresholds fatigue crack growth resistances were defined too. It is shown that the effective threshold fatigue crack growth resistance of metal with compressed zone is lower compared with other zones. High sensitivity of local fracture mechanics parameters to the metal degradation and their ambiguous changes in various zones at the same service duration and conditions were proved. It was shown that metal from compressed zone was the most degraded because its fatigue threshold levels were the least. It was explained by an appearance of dissipated intergranular damages in metal under the influence of thermal fatigue. Such intergranular elements on the fracture surface facilitate the fatigue crack growth and cause the strongest crack closure effect under fatigue test of degraded metal from compressed zone of bend

Intensification of high-temperature hydrogen degradation of heat-resistance steels caused by heat changes during exploitation on the main steam pipeline of a heat power plant

Запропоновано підхід для оцінювання технічного стану деградованого металу парогонів з урахуванням впливу пусків-зупинок енергоблоків ТЕС. Цей підхід дозволяє встановлювати можливість подальшого використання обладнання після довготривалої роботи. Моделювання пусків-зупинок проводилося в лабораторних умовах термоциклуванням зразків у чистому водні від робочої температури парогону до кімнатної температури. Зреалізовано цей підхід на сталях 12Х1МФ та 15Х1М1Ф за результатами випроб на циклічну тріщиностійкість після різної їх експлуатації на головних парогонах ТЕС і після термоциклування у водні. Визначили ефективний поріг циклічної тріщиностійкості й ефективний час експлуатації, враховуючи різну історію експлуатації. Згідно з цими даними побудовано базову діаграму для сталі 12Х1МФ та 15Х1М1Ф.An approach for evaluation of technical state of degraded metal of steam pipelines taking into account shutdowns of the power units of power plant has been proposed. This approach enables to establish the possibility to use the equipment after long term service. Shutdowns were simulated in laboratory conditions by thermocycling of specimens in pure hydrogen from service temperature of steam pipelines to room temperature. This approach was realized by fatigue fracture testing of 15Kh1M1F and 12Kh1MF steels after different service time and number of shutdowns during exploitation on the heat power plant stem pipelines and after thermocycling in hydrogen. The effective fatigue crack growth resistance thresholds have been determined and the effective operation time has been calculated using the service history data. According to this data the basic diagram for assessment of the 15Kh1M1F and 12Kh1MF steels state has been developed

Operational damage to steam turbine rotor elements as a result of contact fatigue

Проаналізовано причини експлуатаційних пошкоджень в дисках роторів парової турбіни на двох об’єктах енергетики. Показано, що сумісний вплив концентраторів напружень та технологічного середовища сприяли зародженню в них тріщин контактної втоми та їх поширенню внаслідок корозійної втомиThe causes of operational damage in steam turbine rotor disks at two energy facilities are analyzed. It was shown that the combined influence of stress concentrators and the technological environment contributed to the initiation of contact fatigue cracks in them and their propagation due to corrosion fatigu

Peculiarities of Fatigue Crack Growth in Steel 17H1S after Long-Term Operations on a Gas Pipeline

This work presents the results of metallographic studies and the tensile, impact, and fatigue crack growth (FCG) resistance tests of 17H1S main gas pipeline steel in the as-received (AR) state and after a long-term operation (LTO). A significant number of non-metallic inclusions forming chains stretched along the direction of pipe rolling were found in the microstructure of the LTO steel. The lowest values of elongation at break and impact toughness of the steel were determined for the lower part of the pipe close to its inner surface. FCG tests at a low stress ratio (R = 0.1) did not reveal a significant change in its growth rate in degraded 17H1S steel compared to steel in the AR state. During tests at a stress ratio R = 0.5, the effect of degradation was more pronounced. The Paris’ law region of the da/dN—∆K diagram for the LTO steel corresponding to the lower part of the pipe close to its inner surface was higher than those for the steel in the AR state and the LTO steel corresponding to the higher part of the pipe. Fractographically, a significant number of delaminations of non-metallic inclusions from the matrix were recognized. Their role in the embrittlement of steel, especially steel from the lower part of the pipe close to its inner surface, was noted

Study of the Fatigue Crack Growth in Long-Term Operated Mild Steel under Mixed-Mode (I + II, I + III) Loading Conditions

The paper presents an analysis of mixed-mode fatigue crack growth in bridge steel after 100-years operating time. Experiments were carried out under mode I + II configuration on Compact Tension Shear (CTS) specimens and mode I + III on rectangular specimens with lateral stress concentrator under bending and torsion loading type. Due to the lack of accurate Stress Intensity Factor (SIF) solutions, the crack path was modelled with the finite element method according to its experimental observation. As a result, the Kinetic Fatigue Fracture Diagrams (KFFD) were constructed. Due to the change in the tendency of higher fatigue crack growth rates from KI towards KIII dominance for the samples subjected to bending and torsion, it was decided to analyze this phenomenon in detail using electron-scanning microscopy. The fractographic analysis was carried out for specimens subjected to I + III crack loading mode. The mechanism of crack growth in old bridge steel at complex loads was determined and analyzed

Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel