Mechanical Properties of Gas Main Steels after Long-Term Operation and Peculiarities of Their Fracture Surface Morphology (original) (raw)
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Effect of long-term operation on steels of main gas pipeline: Structural and mechanical degradation
Journal of King Saud University - Engineering Sciences, 2016
Based on the results of experimental studies of 17MnSi steel the regularities of the inservice degradation influence into its deformation and strength properties were established with the use of full strain diagrams. The important role of the hydrogen absorption that takes place under operation and its negative influence onto the mechanical properties of 17MnSi steel are shown. The latter is manifested through the microdefect growth in the gas pipeline material wall (in the form of dispersed damages) and reduction of its resistance to the brittle fracture.
Impact Strength of Main gas Pipeline Steel After Prolonged Operation
Metallurgist, 2015
The main features of crack initiation in main gas pipeline steel after forty years of operation with impact loading are studied. The energy capacity of gas pipeline material breakdown is studied for different directions of specimen cutting. Each stage of the impact loading diagram is specifi ed by self-organization of deformation levels, and the shape of the diagram makes it possible to describe the stages of breakdown.
Influence of in-service degradation on strain localization in steel of main gas pipelines
2014
General regularities in the failure kinetics of steel of main gas pipelines (17GS) are established using the method of complete stress-strain curves, meanwhile in-service degradation of metals is taken into account. The influence of material degradation on material properties under static tensioning is considered using two independent approaches: the phenomenological model of damage accumulation in metals, and the fractographic analysis method. The accumulation of in-service damage is found to increase a degree of material opening and lead to partial "embrittlement" of the steel matrix due to the microdefects accumulation in the vicinity of metal cracking caused by hydrogenation.
Feature of stress corrosion cracking of degraded gas pipeline steels
Procedia Structural Integrity, 2019
Stress corrosion cracking (SCC) of steels can reduce the structural integrity of gas pipelines. To simulate in-service degradation of pipeline steels in laboratory the method of accelerated degradation consisted in subjecting specimens to electrolytic hydrogenation, to loading up the certain plastic deformation and heating of specimen at 250°C was recently developed. The purpose of this paper was to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC. Three pipeline steels of the different strength (17H1S, which is equivalent of API X52, API X60 and API X70) were investigated. The characteristics of the as-received pipeline steels with different strength were compared with the properties of pipeline steels after in-service and in-laboratory degradation. An influence of the NS4 solution on SCC resistance of 17H1S and API X60 steels in the as-received state and after the accelerated degradation, using slow strain rate tension method, was analysed. The noticeable decrease of plasticity for 17H1S and API X60 steels after longterm operation was shown. Deep microdelaminations revealed in the central part of fracture surfaces for the operated steels can be considered as the signs of dissipated damaging in the metal caused by texture and hydrogen absorbed by metal. Comparison of the SCC tests results showed that the characteristics of both steels in the as-received state were insignificantly changed under the influence of the environment. At the same time, the degraded steels were characterized by a high sensitivity to SCC. It was shown fractographically that it associated with cracking along interfaces of ferrite and pearlite grains with secondary deep intergranular cracks formation and also by delamination between ferrite and cementite inside pearlite grains. The similar fracture mechanism at SCC tests was revealed for near the outer surface of the specimens and in the central part of the fracture surfaces of in-laboratory degraded specimens. These results demonstrated the key role of hydrogen during SCC and in-bulk cracking as well.
Fracture and Structural Integrity, 2021
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.
In-Service Degradation of Pipeline Steels
Lecture Notes in Civil Engineering, 2020
Long-term operation of structural steels causes an essential decrease of the mechanical properties, especially characteristics of brittle fracture and SCC resistance. General regularities of in-service degradation of pipeline steels are analysed in the chapter. On these base two stages of pipeline steels degradation are distinguished in the chapter. The first one is deformation aging which is characterized by improvement of characteristics of strength and hardness but from the other hand a decrease of plasticity and brittle fracture resistance. The stage II is the stage of in-bulk steel dissipated microdamaging, which is more dangerous with regard to a loss of pipeline integrity. Operational degradation of the mechanical properties of the steels is accelerated by their hydrogenation from the inner surface of the pipe, which indicates the hydrogenating ability of transported hydrocarbons. The accelerated method of pipeline steels degradation is substantiated. It is based on the common method of deformation ageing of steels by plastic strain with subsequent heat treatment up to 250°C, however, it additionally involves preliminary hydrogen charging.
Low temperature impact toughness of the main gas pipeline steel after long-term degradation
Central European Journal of Engineering, 2014
The correlation of microstructure, temperature and Charpy V-notch impact properties of a steel 17G1S pipeline steel was investigated in this study. Within the concept of physical mesomechanics, the dynamic failure of specimens is represented as a successive process of the loss of shear stability, which takes place at different structural/scale levels of the material. Characteristic stages are analyzed for various modes of failure, moreover, typical levels of loading and oscillation periods, etc. are determined. Relations between low temperature derived through this test, microstructures and Charpy (V-notch) toughness test results are also discussed in this paper.
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States
Strength of Materials, 2004
Вязкость разрушения и трещиностойкость стали паропровода в исходном состоянии и после эксплуатации М. Зри л и ч а, 3. Б урзи ч 6, 3. Ц ви й ови ч а а Белградский университет, факультет технологии и металлургии, Белград, Сербия и Черногория б Военно-технический институт, Белград, Сербия и Черногория Исследуется проблема преждевременного разрушения паропроводов (сталь 14MoV6 3) с расчетным ресурсом 100 тыс. ч при температуре 540°С. При изготовлении эксперимен тальных образцов использовалась сталь в исходном состоянии и после 117 тыс. ч эксплу атации. Применение локального подхода механики разрушения и металлографического ана лиза наряду с классическими методами (испытания на растяжение, трещиностойкость, усталостную прочность) позволило более точно оценить деградацию свойств стали под действием высоких температур и напряжений. Обоснована необходимость дальнейшего развития локального подхода к прогнозированию условий разрушения материалов при дли тельной эксплуатации в условиях высоких температур. К лю чевы е слова: паропровод, разрушение, локальный подход механики разрушения, металлографический анализ, испытания на растяжение, трещино стойкость, усталостная прочность. Introduction. In late seventies, application of 14MoV6 3 steel (DIN) for highly loaded steam pipelines (temperature up to 540oC and pressure as high as 45 bar for service life of 100,000 hours) offered significant benefits compared to the steels o f previous generation [1, 2] and allowed reduction of pipe wall thickness. However, frequent premature failures o f steam lines produced o f this steel, sometimes after only 30,000 service hours, imposed the necessity to retrofit damaged steam pipelines, e.g., steam lines o f thermoelectrical power plants in Greece [3]. This unexpected repair cost hampered further application o f 14MoV6 3 steel and the designers preferred to replace it by other, highly alloyed steels (e.g., alloyed steel X12 CrMoV 1, according to DIN, and low alloyed steel 10CrMo9). There is no clear explanation for failure occurrence, and steel
In-service degradation of gas trunk pipeline X52 steel
2008
A workability assessment of the trunk pipelines exploited for a long time has been mostly based on the monitoring of the corrosion damage of the outer surface of the pipes. As a result, special attention has been paid to the reliability and durability of the protective coatings and to the external electrochemical protection. However observations of the inner surface of the oil and gas trunk pipelines being in service for long time, revealed the pits, especially numerous on the bottom of the pipe cross-section. The main cause for this corrosion damage has been the presence of condensed water. In the case of oil trunk pipelines exploited for about 30 years, the substantial decrease in the corrosion resistance, mechanical (especially resistance to the brittle fracture) and corrosion-mechanical in-bulk material properties has been established in comparison to the as-received material [1, 2]. The above findings strongly suggest that not only the pipe surface, but also the bulk material underwent degradation during service. The conclusions drawn from the studies [1 -4] have stated that the long time service of the oil trunk pipelines and the control of the pipelines, limited only to the detection of the surface defects and damage was not sufficient to ensure the safety of the operation of the oil lines. The stated degradation of the pipes being in service might be caused by the material long term aging promoted by the low cyclic stresses erasing from the flowing media or from external reasons. On the other hand, a much more pronounced metal deterioration was observed at the bottom than at the upper part of the same pipe [2 -4]. This suggests the aggressive role of the residual (deposited on the bottom) water. Although the corrosion occurring at the bottom of the pipe cannot itself cause the degradation of the bulk material, the hydrogen evolved in the corrosion processes and entering the metal might be an important factor producing metal deterioration. In the case of the the wet gas pipelines, the condensed water accumulates at the bottom [5]. However, gas flow splashes the water deposited at the pipe bottom, thus affecting the other parts of the pipe inner surface . Therefore, a similar processes of metal degradation during the long term exploitation should proceed in the metal of the crude oil and of the gas pipe lines. During the long term exploitation the stress assisted aging and the hydrogen induced degradation modify the metal properties and its resistance to the stress, corrosion and hydrogen cracking. The state of the material degradation of gas pipeline cannot be usually established by routine material testing. However, the application of other methods, such as fracture mechanics, stress and fatigue corrosion tests in selected electrolytes, hydrogen permeation measurements, should allow to evaluate the state of the metal degradation. This is especially important to predict the residual life time of the old Eni, Milano, Italy Received Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv, Ukraine,
Engineering Failure Analysis, 2019
The use of effective non-destructive evaluation techniques for assessing degradation degree of pipeline steel would allow planning actions in order to correctly manage required emergency response at gas pipeline incident. In this paper, a new non-destructive evaluation technique based on electrochemical analysis of fracture surface for assessing in-service degradation degree of operated pipeline steels was developed. A significant difference between open-circuit potentials of the fracture surface (brittle fracture) and the polished surface of specimens made of operated pipeline steels observed after their long-term operation was associated with increased content of carbon compounds on the fracture surface. Embrittlement mechanism of ferrite-pearlite pipeline steels under operation was explained by carbides enrichment both grain boundaries and intragranular defects. The correlation dependence between operational changes in brittle fracture resistance of a metal and in open-circuit potential of its fracture surface being the basis of the developed electrochemical method for evaluation of operational degradation of pipeline steels was determined. 1. Introduction Long-term operation of natural gas transit pipelines implies aging, stress corrosion and hydrogen-induced cracking and it causes hydrogen embrittlement of steels, degradation of mechanical properties associated to a safe serviceability of the pipelines, and failure risk increase [1-9]. The implementation of effective diagnostic measures of degradation degree of pipelines steels would allow planning actions in order to correctly manage required emergency response at gas pipeline incident. In-service degradation of pipeline steels can be evaluated by destructive and non-destructive techniques [10-14]. Taking into account the fact that significant decrease in brittle fracture resistance lead to increase of failure risk, it is very important to evaluate it for long-term operated metal. The most sensitive mechanical characteristics for an evaluation of in-service degradation by destructive methods are impact toughness, fracture toughness (J-integral), resistance to stress corrosion cracking [15,16]. Among non-destructive methods, the special attention is paid to the electrochemical one, which is based on a good correlation between relative changes in electrochemical and mechanical characteristics of structural steels caused by their operational degradation [12,13,15]. A metal is influenced by a number of operational factors; in particular, changes in a metal condition are intensified under longterm mutual effect of mechanical stresses and corrosion and hydrogenating media [3,17]. Corrosion-hydrogen degradation of longterm operated pipeline steels is manifested, first of all, in hydrogen embrittlement and decrease in resistance to brittle fracture and to stress corrosion cracking, as it was demonstrated in numerous issues reported in [1-3]. It is known that process of in-service degradation of a metal occurs non-uniformly at a micro scale. Therefore fracture propagates along path with minimum energy