Features of damage to welded joints of steam pipes made of steel 12X1M1F and 15X1MF

A significant number of heat and power equipment, including welded connections of steam pipelines made of 15x1m1f and 12x1mf Steels, have now reached the design service life. Their practical experience is about 200,000 hours at an estimated temperature of 545°C, and the park resource according to regulatory documentation is 250,000 hours. Note that aging welded joints are operated in conditions of start-ups and stops, which have become more frequent, which contributes to an increase in the intensity of their damage. In this connection, the question arose about the possibility of their operation longer than the estimated period in the new conditions. Goal. The goal is study of the features of damage to welded elements of steam pipelines operating in conditions of creep and low cycle fatigue. Methodology. The study of samples from existing damaged steam pipelines was carried out by the method of structural analysis, based on the consideration of the processes of structural and phase changes occurring in the areas of welded joints and conclusions were drawn regarding their resistance to destruction during operation. Results. The mechanisms of crack formation, their causes and the main provisions of metallographic analysis on the study of damaged welded joints operating under creep conditions and representing a certain number of types of cracks are presented. Such cracks are cold-breaking cracks, which are brittle cracks at moderate temperatures (up to 100–150 0C) during hydraulic tests and start-ups; long-term ductility failure cracks, which are brittle cracks caused by the failure of long-term ductility creep temperatures and rated stresses below permissible; cracks on exhausted areas, which are cracks when operating in the range of creep temperatures and rated stresses above permissible and fatigue cracks, which are cracks caused by the action of variable stresses with an amplitude greater than permissible values. Scientific novelty and practical significance. It is established that the mechanism of aging of long-term operated welded joints of steam pipelines is associated with structural transformations that cause the appearance of cracks in the welded joints.

Innovative Approach to Analysis Drilling Tool Works

Information technologies have long been an integral part of our lives, and the oil and gas industry has also undergone natural IT evolution. Modern technologies have allowed to automate the basic processes and structure the existing order of work, but there are still unresolved problems, one of which is monitoring the full life cycle of drill pipes and predicting the accumulation of fatigue damage. In most cases, the failure of the drill pipes is associated with fatigue destruction, which begins with microcracks as a result of exposure to variable stresses during the construction of the well (drilling). Currently, there are no effective methods to control accumulated fatigue damage or residual durability of the pipe at a given level of stress. In this regard, a system is required for a more reliable assessment of the condition of the drill pipes, which will take into account the whole list of factors influencing the rate of accumulation of fatigue damage in the body of the pipe and will allow to calculate (predict) the accumulated fatigue of the drilling pipes, using data from drilling regimes and well parameters. Understanding the mechanism of accumulation of fatigue wear, which leads to the failure of drilling pipes, makes it possible to manage this process, significantly reduce the cost of maintenance of the drilling pipe fund and reduce incidents with drilling pipes.

Fatigue Life Prediction of AA6063-T6 under Erosion Condition

The study of fatigue behavior of aluminum alloy 6063 exposed to periodic fatigue stresses was studied in laboratory conditions under the two conditions of the presence of the first indentation in the first test, then the presence of the phenomenon of erosion in the second test resulting from the projection of pure water Jet on samples of the same metal used in The first test. The purpose of these tests was to estimate the practical life of these samples and the resulting accumulation by using upward and downward variable stresses. A mathematical model was built to calculate the life of the samples in the above conditions, and the results of the estimated life of the samples calculated by the model showed a large convergence with the results of the estimated life of the samples practically calculated. So this mathematical model can be used to estimate the life of samples made of different minerals under these same conditions, after knowing the (S-N) curve for each metal and the amount of the value of the fatigue notch factor (Kf), which can be calculated from special tables without referring to performing practical tests for them.

Features of the kinetics of cyclic elastoplastic deformation diagrams at dwells in cycles and superimposition of variable stresses on them

The goal of the study is determination of the regularities of changes in cyclic strains and related deformation diagrams attributed to the existence of time dwells in the loading modes and imposition of additional variable stresses on them. Analysis of the obtained experimental data on the kinetics of cyclic elastoplastic deformation diagrams and their parameters revealed that in contrast to regular cyclic loading (equal in stresses), additional deformations of static and dynamic creep are developed. The results of the studys are especially relevant for assessing the cyclic strength of unique extremely loaded objects of technology, including nuclear power equipment, units of aviation and space systems, etc. The experiments were carried out on the samples of austenitic stainless steel under low-cycle loading and high temperatures of testing. Static and dynamic creep deformations arising under those loading conditions promote an increase in the range of cyclic plastic strain in each loading cycle and also stimulate an increase in the range of elastoplastic strain due to active cyclic deformation. At the same time the existence of dwells on extrema of stresses in cycles without imposition of additional variable stresses on them most strongly affects the growth of plastic strain ranges in cycles. Imposition of additional variable stresses on dwells also results in the development of creep strains, but their growth turns out to be somewhat less than in the presence of dwells without stresses imposed. The diagrams of cyclic deformation obtained in the experiments are approximated by power dependences, their kinetics being described in terms of the number of loading cycles using corresponding temperature-time functions. At the same time, it is shown that increase in the cyclic plastic deformation for cycles with dwells and imposition of additional variable stresses on them decreases low cycle fatigue life compared to regular loading without dwells at the same stress amplitudes, moreover, the higher the values of static and dynamic creep, the greater decrease in low-cycle fatigue life. This conclusion results from experimental data and analysis of conditions of damage accumulation for the considered forms of the loading cycle using the deformation criterion of reaching the limit state leading to fracture.

Fatigue Study of the Pre-Corroded 6082-T6 Aluminum Alloy in Saline Atmosphere

This work studies the influence of the saline atmospheric corrosion on the fatigue strength of 6061-T6 aluminum alloy. For this purpose, this alloy was subjected to tests in a salt spray corrosion chamber at different exposure times (1, 2, and 3 months) according to ASTM B117 standard. The morphological study of the pits was carried out by confocal microscopy. Subsequently, fatigue tests were performed at variable stresses whose maximum stress (Smax) was between 30% and 95% of the yield strength (S0) in order to keep them within the zone of elastic behavior of the material. Data were analyzed using the Basquin equation and the maximum likelihood function method. The results show a similar decrease in the conventional fatigue limit (2 × 106 cycles) after one month (98 MPa) and two months (91 MPa) of corrosion. After three months of corrosion, the material showed a very important reduction in the fatigue limit (68 MPa) with respect to the uncorroded material (131 MPa). The data of Se/S0 (fatigue limit/yield strength) versus the ratio Pm/Dm (pit average depth/pit diameter at zero depth) can be fitted to a logarithmic curve.

Deformational Assessment of Shallow Footings Founded on Carbonate Sand

Carbonate (calcareous) sandy soils are well known to be prone to crushability and to have higher compressibility compared to siliceous sands. This study assess the deformational behavior of carbonate (calcareous) sand under shallow footings by carrying out back analyses load tests of twelve full-scale footings founded on improved carbonate (calcareous) sand fill and uniformly loaded up to the allowable bearing pressure. The footings are reinforced concrete isolated pads ranging from 1.5×1.5 m up to 3×3 m in size. These full-scale loading tests are combined with extensive in-situ static cone penetration tests carried out under the location of each footing to test the foundation soil. The recoded measurements of these full-scale load tests are utilized to measure the immediate settlements of the sand fill under variable stresses and to extrapolate long-term (creep) settlement as wells. Based on the analyzed results, comprehensive back analyses were carried out to validate several commonly-used settlement prediction formulae, including Schmertmann’s method (Schmertmann et al., 1978) and Meyerhof (1965 and 1974) for utilization validation with carbonate (calcareous) coarse-grained materials.

THE INFLUENCE OF STRUCTURE FORMING FACTORS ON LOW-CYCLE FATIGUE OF A POLYMER COMPOSITE MATERIAL BASED ON FURFURAL ACETONE MONOMER (FAM)

Research studies of polymer concretes are frequently carried out under the action of static loads while the material is affected by cyclic loads, most often, low-cycle as a result of its work in the structure. In the case when the level of stresses caused by these loads exceeds a certain limit, irreversible damage accumulation processes begin to occur in the material, which lead to the formation of cracks. In the future, the stress concentration at the edge of the crack contributes to its development. Most often cracks occur at the surface of the part, but sometimes in the thickness of the material. This process weakens the section and after some time, when the crack reaches a critical length, the part or structure is destroyed. As a rule, they are destroyed without visible residual deformations, even in cases when they are made of plastic materials. It was suggested that under the influence of variable stresses material gradually degenerates over time, as if "gets tired" (fatigued). Material fatigue is a process of gradual accumulation of damage under the influence of variable stresses, leading to the formation of cracks and destruction of the material. The peculiarities influencing low-cycle stresses on strength characteristics of polymer concretes based on furfural acetone monomer (FAM) are considered in the article. The regression equation has been obtained as a result of experimenting on a polymeric composite material which allowed constructing the response surface of the low-cycle fatigue of the polymeric composite material based on the resin of furfural acetone monomer. A second-order plan for three factors studying low-cycle fatigue of a tested composite material is also presented in the paper. It has been established that the ratio of the polymer component to the filler and the coarse aggregate spreading factor are the fundamental ideas influencing the cyclic durability of polymer concretes while the thickness of the polymer bonding layer is an insignificant factor.

Liquid Nitriding of Stainless Steel 316L to improve fatigue properties for Orthopedic Screws

Liquid nitrate is an important method used to improve mechanical properties. One of these properties is resistance to fatigue. The aim of this study was to improve the fatigue resistance of the stainless steel 316L. The rotational bending method was used with constant and variable stresses at different times of (1, 3, 5) hours and at (530, 630) C0. These tests were performed before and after nitration. The results showed that the depth of the nitride layer was (0.21, 0.33, 0.45) mm, increasing with time nitriding when the temperature was 530 C0. While the depth of this layer at a temperature of 630 C0 (0.26, 0.39,0.5) mm with increasing time. As a result of these processes, a layer of solid chromium nitrides and other phases of iron nitride were formed on the outer surface. These layers helped to inhibit the growth of the cracks and their progress in addition to the generation of pressure stresses on the surface leading to obstructing the progress of the cracks. This study showed that the fatigue resistance was directly proportional to the increase in nitrate time due to the increased depth of the hardened layer, but this resistance decreased when the temperature was 630 C0 due to the formation of brittle phase with low resistance.