Vitaly Dmytryk

Assessing the reliability of welded joints of steam pipelines made of heat-resistant pearlitic steels that are operated for a long time (over 270 thousand hours) under creep and fatigue conditions, has become an extremely relevant issue. The purpose of this research is to determine the avenues for the development of regulatory provisions regarding the reliability of welded joints of steam pipelines that are operated for a long time under creep and fatigue conditions. To achieve the goal, appropriate methods of research and analysis were used to study the structural-&-phase state and determine the chemical composition and physical-&-mechanical properties of the metal of welded joints, as well as the level of their damageability. It was established that the damageability of the metal of welded joints depends primarily on its structural-&-phase state. With an increase in the operating time of welded joints, a ferrite-carbide mixture is gradually formed in their initial structure, and accordingly, and the amount of it as its component is increased. The presence of such a mixture contributes to the damageability of welded joints. which follows creep and fatigue mechanisms. The feasibility of detecting a ferrite-carbide mixture in the structure of welded joints by using already improved techniques and developing new quality control methods has been substantiated. To obtain appropriate results, we propose improving the quality control methods for welded joints of steam pipelines that operate for a long time under creep and fatigue conditions. We substantiated that when developing new and improving standard methods for quality control of metal of welded joints that operate for a long time under creep and fatigue conditions, it is necessary to consider their structural and phase state, which will increase the reliability of the operation of welded joints. Improving the technologies already available as well as developing new methods and techniques for quality control of metal of steam pipelines and their welded joints, will allow us to improve the assessment of their structural and phase state and the presence of damages. Considering the results obtained in relation to new regulatory provisions will allow us to improve the assessment of the reliability of aging steam pipelines and their welded joints, as well as to determine their residual resource.

Розглянуто питання досліджень структурних змін стосовно до зварних з'єднань паропроводів. Представлені результати металографічного аналізу структурних змін в металі паропроводів. Аналіз виконувався для ряду зразків, вирізаних з різних ділянок паропроводу при різному напрацюванні в умовах повзучості і малоциклової втоми. У процесі аналізу зображень отриманих мікрошліфів виявлені відносний вміст структурно-фазових складових і їх розподіл в металі. Виконано порівняльний аналіз статистичних характеристик розподілу і відносного змісту структурних складових для різних ділянок металу зварних з'єднань з різним напрацюванням. Дано науково обґрунтований опис структурних змін в металі різних ділянок зразків зварних з'єднань, а також можливість продовження ресурсу експлуатації елементів паропроводів з деградованою структурою і наявністю пошкоджуваності. Сформульовано та обґрунтовано найбільш перспективні, на думку авторів, напрямки подальшого дослідження зразків для забезпечення умов, що дозволять продовжити ресурс експлуатації паропроводів.

The authors studied the interrelation between the type of structure and the damage rate of the welded joints of steam pipelines made of the heat-resistant pearlitic steels that were operated for a long time, i.e. more than 270 thousand hours in the conditions of creepage and low-cycle fatigue. The purpose of this research was to establish the interrelation between the structural-&-phase condition of the metal used for welded joints of the elements of steam systems and their damageability rate for the service life of welded joints exceeding 270 thousand hours. During the studies, the methods of optical and electron microscopy were used according to the requirements of the guideline documentation and also the methods that are used for the determination of mechanical properties. The level of their reliability has been substantiated and the residual life has been determined. To impart functional performances to welded joints we used well-known methods that were appropriately emended according to the structural changes of above joints. Such changes condition the conversion of the original structure of welded joints into the ferrite-carbide mixture. The availability of the conversion process of the initial structure on the thermal action zone sections (TAZ) of welded joints has essential distinctions due to a different disposition of metal to its own damageability. On the whole, the welded joints are damaged more intensively in comparison to the basic metal of steam pipelines. The analysis of the structural state of welded joints in the steam pipelines of thermal power plants as for the extension of their service life results in a considerable economic effect. Understanding the fact that the metal deterioration in welded joints adheres mainly to the fragile mechanism we managed to establish the level of their damageability that demands the renewal of damaged welded joints. We believe that the damageability level of welded joints that tots up to 0.25 or 0.35 of the volume of their TAZ section should be considered as critical for the service life exceeding 270 thousand hours. The damaged welded joints should be renewed throughout the time period of 15 to 20 thousand hours as soon as the specified damageability level is attained. Изучили связь структуры сварных соединений паропроводов из теплоустойчивых перлитных сталей, длительно эксплуатируемых в условиях ползучести и малоцикловой усталости с их повреждаемостью. Исследование структурного состояния сварных соединений паропроводов тепловых электростанций относительно продолжения срока их наработки обеспечивает получение значительного экономического эффекта. Осознавая, что разрушение металла сварных соединений реализуется преимущественно по хрупкому механизму, установили уровень их повреждаемости, по достижении которого следует производить замену поврежденных сварных соединений. Вивчили зв’язок структури зварних з’єднань паропроводів із теплостійких перлітних сталей, які довготривало експлуатувалися в умовах повзучості і малоциклової втоми з їх пошкоджуваністю. Дослідження структурного стану зварних з’єднань паропроводів теплових електростанцій, стосовно продовження терміну їх напрацювання, забезпечує отримання значного економічного ефекту. Усвідомлюючи, що руйнування металу зварних з’єднань реалізується переважно за крихким механізмом, встановили рівень їх пошкоджуваності, після досягнення якого слід виконувати заміну пошкоджених зварних з’єднань.

Automatic Welding, 2021, №09. Journal «AVTOMATICHESKAYA SVARKA» Monthly scientific-technical and production journal. ISSN: 0005-111X, Since 1948. Russian version of «The PATON Welding Journal» Text in Russian, contents and summaries in English. The Journal presents a wide range of scientific-technical and promotion information in the field of technologies, equipment and materials for welding, surfacing, brazing, and deposition of protective coatings.

Assessment and prevention of the propagation of carbon monoxide over a working area at arc welding

(2007). Relationships governing slip in the structure of the weld metal of welded joints in heat-resisting pearlitic steels. Welding International: Vol. 21, No. 7, pp. 525-528.