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Section 3. Materials Science
DOI: http://dx.doi.org/10.20534/AJT-17-1.2-26-30
Bukleshev Dmitry Olegovich, Federal State Budgetary Educational Institution of High Education
«Samara State Technical University», graduate student, SamSTU E-mail: [email protected]
Sharaukhova Anastasia Grigoryevna, graduate student, SamSTU
Buzuev Igor Ivanovich, Candidate of technical science, SamSTU
Industrial danger reduction of the piplines operation by assesment of the weld-affected zones in a stress-strain state
Abstract: Reliability of the pipelines operation directly depends on the performed works quality on technical diagnosing, which assumes analytical procedures of technical condition assessment and forecasting. The result of the technical diagnosing to a great extent depends on completeness and quality of the analysis results received at the inspection. That, in turn, depends on the relevant normative documents existence, assessment techniques and other materials which allow estimating negative influence of all revealed defects at the maximum degree. A lot of the corresponding standard documentation is developed, but the cases of pipelines accidents and breakages, under the condition of essential amounts of the performed work on technical diagnosing, nevertheless can confirm problems regarding negative influence assessment of some defects types.
One of the reasons of accidents on pipelines is the stress-corrosive destruction (SCD). The problem of the main pipelines SCD came to the world's forefront as one of the main reasons for pipe body destructions. However because of the significant complexity of SCD emergence process the mentioned phenomenon is insufficiently studied today and therefore not all factors, which influence it, are enough considered when determining potentially dangerous segments. Along with the general regularities SCD process has considerable number of specific features inherent in the particular studied pipeline (a particular steel brand resistance to the stress corrosion process, chemical characteristics of the pipe surrounding environment, regulations of the pipeline operation and its construction features, stress in the pipelines joints and weld-affected zone).
In literature and according to the research works results one of the main reasons for a stress — corrosive cracks incubation is a stress which arises in a pipe body near to the concentrators (welded seams are referred to them as well) and exceeds the material fluidity limit. According to the authors' opinion, besides other factors (the corrosion environment, a relief, mounting conditions, set-on weight presence etc.), inherent in any pipe irrespective of the way and place of its production, emergence of rather essential local additional stress is caused by two-joint pipes geometrical form defects which are generally formed at a stage of their production [6, 10].
Keywords: pipeline, stress-corrosive destruction, weld-affected zone, stress, crack.
It is known that the centers of metal construction destruction, including the main pipelines, are most often located near the welded connections. The reason of it besides the defects arising in metal during welding because of various deviations from the established norms and technical requirements, and considerable internal stress which are formed in metal during welding near the welded joints, is also the fact that the structure of material and its physical and mechanical properties respectively change in metal during welding [3, 3].
Stress conditions and corrosion are the main reasons for accidents on the main and distributing steel pipelines. In such segments during the time at the metal stress condition development, a change of residual magnetization usually occurs [2, 4].
Practice of gas pipelines operation shows that the main sources of damages at the gas-main pipelines operation are stress local zones — local corrosion, cracks on the corrosion cracking under the stress basis (CCS), and deformations from joints mounting which are formed under the working loadings influence [4].
Corrosion cracking under the stress (CCS) currently is the most frequent reason for failures on a line part of the gas-main pipeline. Corrosion and active environment influence, temperature fluctuation, working loadings and stress during the time change structure and properties of the operated pipe metal in comparison with the initial characteristics. Repeatedly static loadings along with the geometrical (a welded joint, mechanical damages of a pipe surface, corrosion damages) and structural heterogeneity (grains boundary, nonmetallic inclusions) cause the inevitable metal damages owing to irreversible microplastic deformations accumulation.
Dislocation density increase and damages accumulation is the first stage of destruction process the subsequent stages of which are microcracks incubation, their stable growth and spontaneous destruction. Destruction processes are intensified in zones of double plastic deformation caused by a technique of pipe manufacturing (edges turn-up for welding and subsequent calibration), by sections of cold bending, by pipeline laying with a compulsory bend at the mounting, and by pipeline deformations caused by geophysical processes [1,8].
At load influence on the pipe metal there is a prone-ness to the stress concentration zones (SCZ) accumulation and formation. Stress concentration is understood as a local stress increase in a deformable body's zones with abrupt change in section [9]. Defects of orifice welding production, pores, inclusions, notching and others can be such concentrators in the weld-affected zones. Stress
concentration in the welded connections is defined by the connected elements general structure, by geometrical form of the pipeline joint welded with the base metal and by transition way and energy power of welding process as well. Circular butt joints are such concentrators for pipelines undoubtedly. And the fact of their existence and counterbalancing in a pipe material without being influenced by the external factors is a residual stress characteristic. SCZ are determined by a total contribution of all inheritance forms accumulated at the stages of rolling sheet manufacturing, pipe production, performance of the pipeline construction assembly, welding and installation work and also metal structure and properties changes which are accumulated in it during the time.
With the stress gradient growth as it is near to the crack-like defects, stress distribution is influenced by separate structural elements, such as grain blocks, separate grains which are variously focused in relation to a power flow, grains boundary, etc. All this causes the metal inhomogeneity contribution to the stress growth. Low-volume real constructional materials of a crystal structure, isotropy conditions, homogeneity and material continuity are violated. Because of various orientation of separate structural components stress distribution in low-volume real material can't be smooth. Therefore real material structural microinhomogeneity is revealed in the form of its deformation heterogeneity.
Currently a lot of works are actively carried out on nondestructive methods elaboration of a metal structural condition determination and assessment of a product's intense deformed state (IDS) changes at operational loadings, but the majority of these works don't consider the fact that the pipe can undergo an additional plastic deformation at a stage of production, and when transporting a pipe to the mounting place and directly at the pipe mounting.
In the welded constructions, as a rule, three following zones are distinguished: base metal, welded joint and weld-affected zone (WAZ). Fig. 2 displays the scheme of thermal influence zone structure in the welding of a single-layer butt weld at the constructional steel [8]. At the same time, as well as the mentioned zones differ on structure, physical and mechanical properties, and residual stress level, material of various welded constructions segments in the course ofproduction, transporting, mounting and operation will react differently to loadings activity. Respectively, preliminary plastic deformation will affect differently the behavior of metal magnetic characteristics of various welded connection segments of a pipe at their subsequent elastic deformation [3, 3].
Fig.1. Sample of a gas-main pipeline fragment (a welded joints fragment of the Central Asia-Center gas-main pipeline, steel 09GSF, diameter of 1 420 mm, ball hardness HRB = 110)
Fig. 2. Structural scheme of the weld
It is known that welding stress and deformation emergence is caused:
1) irregular temperature distribution in welded connection at a stage of its heating and subsequent cooling;
2) metal casting shrinkage of a j oint — metal volume reduction of a welding puddle when hardening;
3) metal volume changes of a joint and weld-affected zone at phase transformations of heating and cooling stages [7].
The following reasons of internal residual stress formation can be distinguished in the weld-affected zones:
1. Local irregular metal heating. It is known that all metals expand when heating, and compress when cooling. In the course of welding, as a result of local metal heating and its subsequent cooling, irregular temperature field is formed in the welded connection. Thus, in the welded detail there is a squeezing and (or) stretching thermal internal stress. Stress magnitude depends mainly on heating temperature, coefficient of linear expansion
-affected zone in welded connection and the welded metal thermal conductivity. At the
firmly fixed construction welding, thermal stress magnitude can increase because of its transfer limitation in the course of heating and cooling. At the same time in the heating-up construction, because of its expansion, there will be a squeezing internal stress at first, and at the subsequent cooling in the course of its shortening there will be a stretching stress. When the stress internal magnitude reaches fluidity limit level, plastic deformations causing a change of the welded product form size will occur in metal. After the end ofwelding process, residual stress will occur in the areas which have undergone irregular plastic deformation.
2. Irregular structural transformations in metal. At the gas-main pipelines joints welding, when heating the temperature above critical, there can be stress caused by phase transformations with a change in a crystal lattice type and formation of the phase having a large specific volume and other linear expansion coefficient.
Structure transformations in the pipe steel cause a formation of the so-called quenching structures (martensite) having a large specific volume, higher hardness, fragility and lowered plasticity. Such transformation is followed by increase in volume; metal adjoining to it will undergo the stretching stress, and martensite-structured segments will have a phase fluidity limit. In non-plastic alloys it can cause cracks formation.
3. Casting shrinkage of the weld metal. There is a metal shrinkage at the process of its cooling and hardening in the welded joint and weld-affected zone of a joint. This results from the fact that the metal density increases when hardening therefore its volume decreases. According to the welded metal indissoluble connection with the
base metal, remaining in its invariable volume and counteracting shrinkage, longitudinal and cross internal stress, causing the corresponding welded connection deformations, occur in the welded connection.
In fig. 3 gives "geometrical" multitudes of elastic stress concentration at the pure gas pipeline bending. In the segments with the largest mechanical heterogeneity of joint properties SCZ is shown, which results in the form of deformations stress in thermal influence zone in testing. It is possible to notice that extreme damages begin to be shown from the inside of a pipe body at dynamic loadings, forming defective areas. This effect can be explained with pipeline metal compression and stretching process in testing [5, 25].
2)
Fig. 3. Stress concentration of a sample at dynamic loading: 1) — load of 80 kN; 2) — load of 120 kN; 3) — without loading
3)
When calculating the of welded construction durability it is important to consider a stress and deformations existence at a thermal and deformation welding cycle. On the basis of the approximate calculations, used in the theory of welding deformations and stress, welded elements deformations for the establishment of admissions and allowances for elements of the bearing and protecting construction are usually defined. At the same time with a certain probability, calculation of the residual welding stress and deformations generally is a quite complex challenge as it has to consider all reasons causing their emergence, and material heat and physical properties as well.
Conclusion
The main sources of damages at the gas-main pipelines operation are local stress zones — local corrosion, cracks on the corrosion cracking under the stress, and also deformations from joint assembly mounting which are formed under the influence of working loadings. Reliable pipeline operation can be provided only in the absence of defects of various nature: chemical and structural homogeneity of a pipeline body. In turn, lack of defects will guarantee the main pipeline reliability and
service life, maintaining operating abilities, pipe material qualitative characteristics which will be as close as possible to their theoretical (calculated) values.
Stress in the weld-affected zone is an indicator of the stress and strain state. Metal stress in the weld-affected zone is a source of defects emergence. This stress adds up to the working stress, accelerating a process of crack formation in the weld-affected zones of pipe connections, and causes continuous corrosion process, and also contributes to a crack propagation until the pipeline destruction. Stress in the weld-affected zone is a result of internal stress existence which can be caused by various reasons. Among the main reasons for their emergence can be referred: irregular heating and a welded joint shrinkage, structural changes in metal and the weld-affected zone. Also among the reasons for their emergence is an inappropriate application of welding equipment and technology (electrode diameter is incorrectly chosen, the welding modes aren't observed etc.), low welder qualification, violation of the welded joints sizes, etc. One of the reasons for a stress in the weld-affected zone is a pressure created by a transportation product.
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DOI: http://dx.doi.org/10.20534/AJT-17-1.2-30-34
Maksudova Nasima Atkhamovna, Senior Lecturer, department: Strength of Materials, faculty: Mechanical Engineering Tashkent State Technical University Iskandarov Asilbek Akrom ugli, bachelor, department: Thermal Engineering, faculty: Energetics,
Tashkent State Technical University named after Abu Raykhon Beruni, Tashkent, Republic of Uzbekistan E-mail: [email protected]
Research project of metal oxide nanofluids reaching an increase of heat transfer rate capacity in solar absorption refrigerator
Abstract: Solid metallic materials, such as silver, copper and iron, and non-metallic materials, such as Alumina, CuO, SiC and carbon nanotubes, have much higher thermal conductivities than heat transfer fluids (HTFs). It is thus an innovative idea trying to enhance the thermal conductivity by adding solid particles into HTFs and can be used as heat transfer media in the solar absorption refrigeration system. AgO nanofluid with weights percent of 0.1, 0.2, 0.3 and 0.4 %, which compared in the ability of transfer and storage the heat with distilled water, it is found that the suitable weight percent was 0.1 wt %. The flow rate required supplying heat input to generator and the volume of hot fluid storage required to operate the refrigerator for 24 hours has been calculated. Experimental and theoretical results obtained from the present work show a good improvement by comparing with literatures.
Keywords: nanofluid, absorption refrigeration system, energy storage, heat transfer, heat capacity.
Generally, nanofluids are formed by dispersing large surface area to volume ratio, dimension-dependent nanometer-sized particles (1-100 nm) or droplets into physical properties, and lower kinetic energy, which can HTFs. Nanoparticles have unique properties, such as be exploited by the nanofluids. At the same time, the large
