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

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

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