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Section 6. Technical sciences
Bukleshev Dmitry Olegovich, graduate student of Life Safety department, Federal State Budgetary Educational Institution of High Education "Samara State Technical University"
Sumarchenkova Irina Aleksandrovna, PhD., candidate in Chemical Sciences, associate professor of Life Safety department Federal State Budgetary Educational Institution of High Education "Samara State Technical University" Buzuyev Igor Ivanovich, PhD., candidate in Technical Sciences, associate professor of Life Safety department E-mail: bukleshev_dima@mail.ru
ANALYSIS OF METAL STRESS AND DEFORMATION DISTRIBUTION IN WELD-AFFECTED ZONES AT MAIN GAS PIPELINES
Abstract: Pipeline operational integrity directly depends on the quality of technical diagnosis the scope ofwhich includes accounting and analytical estimation and technical state prediction. Technical diagnosis result mainly depends on analysis completeness and quality obtained in the course of result inspecting. This, in turn, depends on the existing relevant regulations, evaluation techniques and other materials which allow to thoroughly estimate negative influence of all the defects revealed. Keywords: main pipeline, stress, weld-affected zones, cracking, a stress-corrosion damage.
Introduction
The most important component of the power industry in the Russian Federation is pipeline systems. Their basic goal is to ensure faultless and safe oil and gas transit from supplying countries to import countries and to supply domestic consumers with the products. Main pipelines are the most economic type of hydrocarbon transport. Their total length in Russia is more than 200 thousand km. They are
high-risk facilities the accidents at which can lead to irrecoverable losses of a transported product, to pipeline, fitting and equipment damage, farmland, forest destruction owing to the fires and explosions at gas pipelines, thermal injuries for people and excessive pressure of an air shock wave. Accident hazards are defined by the factors accompanying the pumping process and dangerous properties of pumping environment. Dangerous production factors include:
pipeline breakdown or breakdown of its elements which is followed by metal and soil fragment dispersion; product ignition at pipeline breakdown, open flame and fire thermal impact; air-gas mixture explosion; collapse and damage of buildings, constructions, installations; lowered concentration of oxygen; smoke, emission toxicity of product constituents.
The major factor defining freedom from accidents and pipeline operational safety is its technical condition. Monitoring technical condition and timely defect detection is carried out by the line maintenance service.
One of the reasons for pipeline accidents is stress-corrosion failure (SCF). SCF problem at main pipelines entered into the world agenda as one of the main reasons for pipe body breakdown. However, due to essential complexity of SCF emergence, the mentioned phenomenon is insufficiently studied today and therefore not all the factors influencing it are thoroughly considered when determining potentially hazardous areas. Along with general regularities, SCF has considerable number of specific features which are common for a specific pipeline being studied (resistance of a concrete steel type to stress corrosion, chemical characteristics of external pipe environment, pipeline operation regulations and its structural features, stress in pipeline seams and a weld-affected zone).
According to the literature sources and the research results, one of the main conditions for stress corrosion crack formation is stress which occur in a pipe body near concentrators (these include welded seams) and exceed material yield limit. According to the authors, apart from other factors (corrosion environment, relief, assembly conditions, weights, etc.) which are common to all pipes irrespective of the way and place of their production, emergence of rather essential local additional stresses is caused by geometrical form weaknesses of double-seam pipes which are generally laid at the production stage [6].
Failure causes analysis of pipeline welded designs
It is known that sites of metal structure breakdown, including main pipelines, most often occur
near the welded joints [1]. Apart from the defects in metal resulting from welding due to various deviations from the set standards and specification requirements and considerable internal stresses which are created in metal in the course of welding near the welded seams, the reason for it is the fact that the metal changes its structure and its physical and mechanical properties accordingly in the course of welding [3].
Stress states and corrosion are the main reasons for accidents at the main and distribution steel pipelines. Eventually, there is a change of remanent magnetization, usually occurring in these sites along with stress state formation in metal [2].Pipeline operation practice shows that the main sources of damage during main pipeline operation are local stress zones - local corrosion, stress-crack corrosion (SCC) and deformation resulting from joint assembly, which are formed under the working loads [4].
Corrosion cracking of pipe steel
Currently, stress-crack corrosion (SCC) is the most common cause of failures at line segments of main pipelines. Corrosive medium influence, temperature, work load fluctuation and stresses gradually change the structure and properties of the operated pipe metal in comparison with initial properties. Repeatedly static loads in case of geometrical (a welded seam, mechanical damages of a pipe surface, corrosion damages) and structure heterogeneity (grain boundaries, nonmetallic inclusions) lead to inevitable metal damages due to accumulation of irreversible microplastic deformations.
Increase in dislocation concentration and damage accumulation is the first stage of failure, the subsequent stages of which are microcrack initiation, their stable growth and spontaneous breakdown. Failure processes are intensified in double plastic deformation zones caused by pipe production technologies (welding crimping and subsequent calibration), cold bending sites, pipelaying with compulsory assembly bending, pipeline deformations caused by geophysical processes [1].
Failure causes analysis ofpipeline welded designs at pipeline operation
When loads influence a pipe metal, there is predisposition to accumulate and form stress concentration zones (SCZ) [9]. Stress concentration implies local stress increase in zones of sudden alternation of cross-section of a deformable body. In WAZ, such concentrators include welding production defects, openings, pores, inclusions, cuts and others. Stress concentration in welded joints is defined by the overall structure of joint elements, geometrical form of the pipeline seam welded to basic metal, and by transmission mode and welding energy. Regarding pipelines, such concentrators by all means include ring butt joints. In addition, the peculiar fact is that residual stress exists and is counterbalanced in pipe material without extra external loads. SCZs are caused by the total contribution of all heredity forms accumulated at rolled sheet production, pipe production, assembly welding installation and welding at a pipeline construction and changes in structure and metal properties which gradually accumulate them.
With the raise of stress gradient, as it happens near crack-like defects, separate structural elements, such as subgrains, separate grains which have various focuses in relation to power flow, grain boundaries, etc. begin influencing stress distribution. It leads to the fact that metal inhomogeneity contributes to stress raise. In small volumes of real constructional materials with a crystal structure, material isotropy, uniformity and continuity conditions are violated. Due to various orientations of separate structural components, stress distribution can't be smooth in case of small volumes of a real material. Therefore, structural mi-croinhomogeneity of a real material is shown in the form of its deformation heterogeneity.Nowadays, development of nondestructive methods to define a metal structural condition and assess the changes in stressed-deformed state (SDS) of a product at operational loads is actively carried out, but the majority of these works do not consider the fact that even at the manufacturing stage and when transporting a pipe to the assembly site and at assembling immediately the pipe can be in a state of additional plastic strain.
Figure 1. A sample of the main gas pipeline section (a section of welded joints at Central Asia-Center main gas pipeline, 09GSF steel, DN is 1420 mm, Brinnel hardness number, HRB = 110)
Three zones are usually distinguished in welded designs: basic metal, welded seam and weld-affected zone (WAZ). Fig. 2 presents the scheme of a heat-affected zone structure when welding a single-layer butt joint in structural steels [4]. At the same time,
as the listed zones differ in structure, physical and mechanical properties and residual stress level, material of various welded design sites will differently react to the applied load effect in the course of production, transporting, assembling and operation.
Respectively, prior plastic deformation will differ- istics at various sites of a pipe welded joint at their ently affect behavior of metal magnetic character- subsequent elastic deformation [4; 5].
Figure 2. Structure scheme of a weld-affected zone of a welded joint The cause for stress initiation and growth in pipeline welded elements
It is known that initiation of welding stress and deformations is caused by: 1) temperature contrast in welded joints at a heating stage and subsequent cooling; 2) casting shrinkage of seam metal - reduced volume of molten crater metal when hardening; 3) volume changes of seam metal and a weld-affected zone during the phase transformation at heating and cooling stages [5].
The reasons for internal residual stress to be formed in weld-affected zones include [6; 7]:
1. Local uneven metal heating. All metals are known to extend when heating, and to compress when cooling. In the course of welding, as a result of local metal heating and its subsequent cooling, a contrast temperature field is created in a welded joint. Thus, there is squeezing and (or) stretching thermal internal stress in a welded part. The magnitude of this stress depends mainly on heating temperature, linear expansivity and heat conductivity of the welded metal. When welding a rigidly fixed design, the magnitude of thermal stress can increase owing to the limited free movement in the course of heating and cooling. At the same time, at first there
will be squeezing internal stress in a heating-up design due to its expansion, and then when cooling subsequently in the course of its shortening - there will be stretching stress. When the magnitude of internal stress reaches yield limit, plastic deformations leading to form and size changes of the welded product will begin to happen in metal. After welding, there will be residual stress in the areas exposed to uneven plastic deformation.
2. Uneven structural transformations in metal. In the course of main gas pipeline joint welding when heating above critical temperatures there can be stress caused by phase transformations with a crystal lattice change and phase formation of larger specific volume and different linear expansivity. In pipe steel, structure transformation is followed by the formation of the so-called hardening structures (martensite) having larger specific volume, higher hardness, fragility and lower plasticity. Such transformation is followed by the increase in volume; adj oining metal will be exposed to stretches, and sites with martensite structure will have phase yield limit. In nonplastic alloys it may lead to crack formation.
3. Casting shrinkage ofweld deposit. When cooling and hardening, metal shrinks at a welded seam and a seam weld-affected zone [7]. This results from the fact that when hardening, metal density increases, and its volume therefore decreases. Owing to indissoluble contact between weld deposit and basic metal, the latter remaining of the invariable volume and counteracting shrinkage, there is longitudinal and cross internal stress in a welded joint resulting in the corresponding welded joint deformations.
Figure 3 presents geometric variables of elastic stress concentration at pipeline pure bending. SCZs initiate in the places with the greatest mechanical heterogeneity of seam properties, which is shown in the form of stress diagram deformations in a heat-affected zone while testing. It should be noted that in case of dynamic loads, ultimate damages start to show in the inner side of a pipe body, forming defective regions. This effect can be explained by pipeline metal tensile and compression while testing.
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Figure 3. Stress concentration of a sample under dynamic load: 1 - load of 80 kN; 2 - load of 120 kN; 3 - without load
It is important to consider stress and deformations due to thermal and deformation welding cycle when calculating welded design durability. On the basis of approximate calculation used in the theory of welding deformations and stresses, to establish allowances and oversize for elements of bearing and enclosing structures, deformations of welded elements are usually defined. At the same time, it is generally a rather difficult task to calculate residual welding stress and deformations with a certain probability, as it has to consider all reasons causing their emergence and material thermal-physical properties as well.
Conclusion. The main sources of damages at main gas pipeline operation are local stress zones - local corrosion, fractures based on stress corrosion cracking and deformations as a result of joint assembly which are formed under working loads. Pipeline reliable operation can be provided only in case there are no defects of different nature: chemical and structural homogeneity of a pipeline body. In turn, the lack of defects will ensure main pipeline reliability and service life, maintaining operational
properties, pipe material qualitative characteristics which will be as close as possible to their theoretical (calculated) values.
Stress in a weld-affected zone is an indicator of a stressed-deformed state, metal stress in a weld-affected zone is a source of defects. These stresses are added to operating stresses, accelerating a crack formation in weld-affected zones of pipe joints, cause continuous corrosion process and contribute to a crack formation before pipeline breakdown. Weld-affected zone stress is a result of internal stress which can be caused by various reasons. Basic reasons for stress include welded seam uneven heating and shrinkage, metal and weld-affected zone structural changes. In addition, the reasons for stress include inappropriate equipment and welding technique use (incorrect electrode diameter, welding condition violation, etc. aren't observed), low welder qualification, violation of welded seam sizes, etc. One of the reasons for weld-affected zone stress also includes the pressure created by a transportation product.
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