Hydrogen embrittlement of low carbon structural steel

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials

Hydrogen embrittlement of low carbon structural steel

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20- St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials

Prediction and prevention of boiler tubing systems erosion in thermal plant

The problem of boiler tube erosion is a very complex problem and present in practically all fossil fuel power plants burning low heating value coals irrespective of their particular design and operating characteristics. Predicting the rate of material degradation processes due to operating conditions is the essential feature for assessing the remaining lifetime. This clearly includes the knowledge of the type and kinetics of erosion phenomenon. In order to follow the erosion process and provide a reliable prediction of erosion rate and thus tube time to failure, models taking into account all relevant effects have to be developed. In this paper the is presented results of testing carried out on the evaporator hopper tubes of 350MW unit exposed to erosion and were used to assess their remaining life

Remaining life assessment of a high pressure turbine casing in creep and low cycle service regime

Thick walled components such as high pressure (HP) steam turbine casings operating under high parameter conditions are subjected to a complex stress state. As a result of that stress state, some parts of HP turbine casing undergo to the creep fatigue caused by the combination of thermal fatigue resulted from repeated start/stop operation and the creep which occurs during long-term operation at high temperature and high-pressure. It is well known that domestic thermal power plants have been in use over 100000 h which means that significant cost is required not only for maintenance, but often for renewal of equipment. Based on comprehensive investigation, the results of residual life assessment of one high pressure steam turbine casing, which belongs to the older turbine generation, taking into account simultaneous action of thermal fatigue and creep, are presented in this paper. Also, the critical flaw crack size of HP turbine casing is determined because this parameter has a strong influence on casing integrity and residual life. The results of residual life assessment provide not only a basis for further maintenance, but also estimated time for reparation or renewal

Remaining life assessment of a high pressure turbine casing in creep and low cycle service regime

Thick walled components such as high pressure (HP) steam turbine casings operating under high parameter conditions are subjected to a complex stress state. As a result of that stress state, some parts of HP turbine casing undergo to the creep fatigue caused by the combination of thermal fatigue resulted from repeated start/stop operation and the creep which occurs during long-term operation at high temperature and high-pressure. It is well known that domestic thermal power plants have been in use over 100000 h which means that significant cost is required not only for maintenance, but often for renewal of equipment. Based on comprehensive investigation, the results of residual life assessment of one high pressure steam turbine casing, which belongs to the older turbine generation, taking into account simultaneous action of thermal fatigue and creep, are presented in this paper. Also, the critical flaw crack size of HP turbine casing is determined because this parameter has a strong influence on casing integrity and residual life. The results of residual life assessment provide not only a basis for further maintenance, but also estimated time for reparation or renewal

New methodology for monitoring and prevention of rotating parts failures

Rotacioni delovi su vrlo rasprostranjeni u mašinskim sklopovima i sistemima i obavljaju vrlo širok spektar funkcija u gotovo svim industrijskim granama. Nepredviđeni i momentalni prestanak obavljanja osnovne funkcije rotacionih delova - prenošenje rotacionog kretanja, uglavnom kao rezultat velikih inercijalnih sila, ima za posledicu pojavu havarijskih oštećenja često praćenih kompletnim razaranjem pojedinih delova ili celog mašinskog sklopa. Značaj prevencije havarije rotacionih delova se ne ogleda samo kroz sprečavanje velikih šteta, već i kroz obezbeđenje sigurnosti osoblja u pogonu. Stoga je definisanje adekvatne nove metodologije za uspešno praćenje ponašanja i prevenciju havarije rotacionih delova od izuzetnog značaja sa aspekta preventivnog inžinjeringa.Rotating parts are very widely used as a part of mechanical systems in different industries to perform a large number of functions. Unexpected and instantaneous interruption in performing the main function of rotating part i.e. transmission of rotating movement very often caused catastrophic failures of particular parts or complete mechanical systems mainly due to inertial forces. Great importance in prevention of rotating parts failures is associated not only with suppressing the injuries, but also with assuring the safety of people. Thus, to establish the adequate methodology for monitoring and prevention of rotating parts failures it is of great importance to define the optimal preventive maintenance and engineering. In this paper the chronological classification of flaws of parts which were built-in mechanical rotation-based systems was presented. New methodology for monitoring and failure prevention of rotating parts was presented through the examples of shaft, as a part which was usually possible to repair, and bearing, as a part which was not possible to repair. In addition, this paper elucidates some possible additional influences which contribute to operating stability of rotation-based systems, and which were not covered by common operating indicators, as well as some measures for prevention of severe failures

The development and application of the new methodology for conveyor idlers fits testing

The proper interference fit between the joined parts is a prerequisite for an effective pressure joint. The main purpose of the pressure joint is to transfer tangential, radial and axial loads between the joined parts. In order to provide proper functioning of the machine assembly (whose component parts are connected by the pressure joints), i.e. the transfer of loads without skidding, it is essential to determine the pressure joints interference fit parameters. The new methodology for the conveyor idlers pressure joints quality control is presented in this paper. The procedure for the analytical determination of the expected disassembling force (limiting value) in the pressure joints between the shaft rolling bearing and the bearing idler shell is described in detail. The analytically calculated boundary values are compared with the experimental ones. According to the presented criteria, the evaluation of the conveyor idler fits quality was performed and reliable conclusions were successfully adopted

New methodology for monitoring and prevention of rotating parts failures

Rotacioni delovi su vrlo rasprostranjeni u mašinskim sklopovima i sistemima i obavljaju vrlo širok spektar funkcija u gotovo svim industrijskim granama. Nepredviđeni i momentalni prestanak obavljanja osnovne funkcije rotacionih delova - prenošenje rotacionog kretanja, uglavnom kao rezultat velikih inercijalnih sila, ima za posledicu pojavu havarijskih oštećenja često praćenih kompletnim razaranjem pojedinih delova ili celog mašinskog sklopa. Značaj prevencije havarije rotacionih delova se ne ogleda samo kroz sprečavanje velikih šteta, već i kroz obezbeđenje sigurnosti osoblja u pogonu. Stoga je definisanje adekvatne nove metodologije za uspešno praćenje ponašanja i prevenciju havarije rotacionih delova od izuzetnog značaja sa aspekta preventivnog inžinjeringa.Rotating parts are very widely used as a part of mechanical systems in different industries to perform a large number of functions. Unexpected and instantaneous interruption in performing the main function of rotating part i.e. transmission of rotating movement very often caused catastrophic failures of particular parts or complete mechanical systems mainly due to inertial forces. Great importance in prevention of rotating parts failures is associated not only with suppressing the injuries, but also with assuring the safety of people. Thus, to establish the adequate methodology for monitoring and prevention of rotating parts failures it is of great importance to define the optimal preventive maintenance and engineering. In this paper the chronological classification of flaws of parts which were built-in mechanical rotation-based systems was presented. New methodology for monitoring and failure prevention of rotating parts was presented through the examples of shaft, as a part which was usually possible to repair, and bearing, as a part which was not possible to repair. In addition, this paper elucidates some possible additional influences which contribute to operating stability of rotation-based systems, and which were not covered by common operating indicators, as well as some measures for prevention of severe failures

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MVV TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MVV TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed