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7METALLOGRAPHIC EXPERTISE OF WELDED BELLOWS FROM AUSTENITIC STEEL
Morgay F.
PhD student Ivanov V.
D.Tech.Sc., Associate Professor Ivashchenko V.
PhD, Associate Professor
State Higher Educational Institution «Pryazovskyi State Technical University»
Abstract
Welded bellows expansion joints operate under a complex stress state due to the presence of their own welding (ow, Sw), assembly (oa, sO) and operational (oo, So) elastoplastic deformations and stresses. In the general case ow, Sw > 0; oa, Sa > 0; oo, So > 0, and strains and stresses can have different values, signs, concentrations, gradients, etc. The analysis performed of the chemical composition of materials used for the manufacture of welded bellows expansion joints and metallographic examination in order to identify the causes of damage to the bellows in the process of manufacturing and operation. Based on the research carried out, technological recommendations have been developed that will ensure the required manufacturing quality of bellows expansion joints.
Keywords: welded bellows, metallographic expertise, austenitic steel, microstructure, austenite grains.
A bellows expansion joint is a device in the form Bellows can be single-layer, i.e., consisting of one layer
of a flexible insert used in piping systems and serves to compensate for changes in the length of piping sections caused by thermal expansion of the pipe material or due to installation work. The main part of this type of product is the bellows itself, which is a corrugated elastic asymmetric shell, made most often of stainless steel.
of material, and multilayer - made of two or more layers. In bellows used in pipe fittings, the number of layers is usually 2 to 12. Table 1 shows the mass fraction of steel elements 12Cr18Ni10Ti for different delivered batches.
Chemical composition of the test batches of samples ;
Table 1
N Element, mass % Sample batch number
Batch 1 Batch 2 Batch 3
1 C** 0,04 ± 0,002 0,05 ± 0,002 0,03 ± 0,002
2 Si 0,334 ± 0,026 0,387 ± 0,024 0,614 ± 0,029
3 Mn 0,765 ± 0,034 0,115 ± 0,035 1,005 ± 0,033
4 Cr 17,551 ± 0,049 17,921 ± 0,053 17,288 ± 0,046
5 Ni 9,520 ± 0,049 10,447 ± 0,053 9,473 ± 0,046
6 Mo 0,186 ± 0,004 0,206 ± 0,004 0,215 ± 0,004
7 Ti 0,167 ± 0,018 0,509 ± 0,022 0,192 ± 0,017
8 Fe 71,476 ± 0,071 70,414 ± 0,075 71,212 ± 0,067
* The content of sulfur, phosphorus, copper, vanadium and tungsten was not determined ** Carbon content determined by chromatography
Bellows expansion joints are widely used in power generation, shipbuilding, chemical, oil and many other industries to perceive static and dynamic movements of pipelines and mechanisms, compensate for thermal expansion of pipelines, and also to reduce the level of vibrations transmitted through pipelines. The reliable operation of these devices to a decisive extent depends on the normal trouble-free functioning of heating systems of settlements, the operability of pipelines, units and life support systems of nuclear power plants, industrial enterprises, ships and aircraft.
The manufacturing process of bellows is as follows: the material in rolls weighing from 15 to 50 kg goes to the cutting area, where it is cut into cards of appropriate sizes using guillotine shears. After that, the process of rolling of pipe billets takes place, which are then transferred to the welding section, where the longitudinal seams are performed by automatic argon-arc welding with a non-consumable tungsten electrode in the same mode parameters. The welded workpieces are
collected in a package (three layers) symmetrically moving away from each other. The assembled package is subjected to hydraulic corrugation forming in the following modes. The flaring process takes place with an emulsion 10% solution.
One of the important elements of the technological process of manufacturing bellows expansion joints is the quality control system at all stages of production, starting with the incoming inspection of materials for compliance with technical requirements, as well as measures for controlling technological parameters when performing individual operations. Failure to comply with these rules leads to irreparable defects in the manufacture of bellows, which entails colossal losses for enterprises.
Studies of damage to welded bellows expansion joints during their operation demonstrate the possibility of the formation of areas with a high level of residual stresses as a result of ongoing technological operations [1]. In addition, areas with chemical and, accordingly,
structural inhomogeneity in welds can cause damage at the stage of product manufacturing.
Since the influence of welding modes on the quality of welded joints of bellows expansion joints was studied earlier [2], in this work the main attention was paid to metallographic examination of bellows specimens.
Analysis of the chemical composition showed (see table. 1) that all batches comply with the standard, with the exception of batch 2, which is characterized by an increased carbon content. This batch of samples also contains slightly more chromium, nickel and titanium than the batch. It is known that chromium provides the self-passivation phenomenon, nickel stabilizes the aus-tenitic structure, and titanium provides fine grain. Thus, none of these elements should have adversely affected the structure and mechanical properties. On the other hand, among the undetermined elements, strong carbide-forming vanadium and tungsten remained, which
could affect the uniformity of carbon and chromium distribution during the crystallization of the weld and cause dendritic segregation.
A metallographic examination was carried out to study the causes of damage to welded bellows expansion joints. Investigated the macro- and microstructure of the base metal, weld metal and heat-affected zone. Used an optical microscope "Neophot 21", microstructure samples were examined in a polished state without etching in order to detect inclusions, as well as after etching with a magnification of x 200 and x 400.
Studies have shown that the base metal is characterized by a uniformly etched austenite structure with small inclusions of manganese sulfides, which can be observed in Fig. 1a, there are also spheroidal and row-shaped, which are distributed over the volume of the base metal (Fig. 1b).
a) 6)
Fig. 1. Microstructure of the base metal containing elongated manganese sulfides (a) and other non-metallic
inclusions (b) (x200)
The size of the austenite grains in the heat affected zone was estimated. For this purpose, the secant method was used, and in the samples with defect-free seams, these indicators were: 0.182 mm (2nd grain point) and 0.342 mm (grain point 0), while in the specimen with defects along the seam this indicator was 0.155 mm - t .e. the grain size was the smallest. Taking into account the fact that the parameters of the yield point and the ability of the metal to deform are inversely proportional to the grain size, it seems that it is necessary to explain the formation of defects by other reasons.
A fundamental difference in microstructures was observed near the fusion zones in the studied series of
specimens with the lowest and highest number of fractures along the seam during corrugation. Thus, the microstructure of the former (Fig. 6a) had a columnar structure without pronounced chemical heterogeneity, whereas in the samples of the second batch, pronounced dendritic segregation was found on both sides of the seam (Fig. 2b). In the samples with the largest number of defects (Fig. 2c), hot subsurface cracks were found that originated directly above the fusion zone and propagated towards the heat-affected zone, i.e. in the zone of the largest grain, in which less energy is required for the development of destruction.
Fig. 2. Microstructure near fusion zones: a) columnar structure of the zone (x200); b) pronounced dendritic segregation in the fusion points on both sides of the seam (x400); pronounced subsurface hot crack over the heat-affected zone (x200)
Based on the results of the studies carried out, the following technological measures are recommended:
- to introduce into the technological process of welded bellows expansion joints the operation of incoming inspection of incoming materials for compliance with the chemical composition, structure and mechanical properties of regulatory requirements;
- to weld using the mode parameters that ensure the optimal plastic properties of welded joints;
- to reduce stresses and prevent the appearance of weld defects, use recrystallization annealing after rolling, which will reduce the stresses and ultimate strength of the material, which will lead to a redistribution of thermal stresses after welding.
References
1. Ivanov, V., Morgay, F. & Lavrova, E. (2020) Influence of the deformation degree on the corrosion resistance of AISI 304 and AISI 316 steels in various environments, Paradigmatic view on the concept of world science: Proc. Int. Sci. and Pract. Conf. (V.1), 75-76. doi: 10.36074/21.08.2020.v1.30
2. Solidor, N.A., Ivanov, V.P., Morgay, F.V. & Nosovsky, B.I. (2015) Investigation of corrosion resistance welds metal hose made of steels AISI 304 and AISI 316, Eastern-European Journal of Enterprise Technologies, 76(4/5), 33-39. doi: 10.15587/ 17294061.2015. 47035.
ФАКТОРЫ, ВЛИЯЮЩИЕ НА ПЕРИОДИЧНОСТЬ ОЧИСТКИ ВОДОСБОРНИКОВ В СИСТЕМАХ ВОДООТЛИВА ПОДЗЕМНЫХ КИМБЕРЛИТОВЫХ РУДНИКОВ
Овчинников Н.П.
Северо-Восточный федеральный университет им. М.К. Аммосова,
директор Горного института, к.т.н.
Чемезов Е.Н.
Северо-Восточный федеральный университет им. М.К. Аммосова, зав. кафедрой техносферной безопасности Горного института, д.т.н.
Аммосова М.Н.
Северо-Восточный федеральный университет им. М.К. Аммосова, старший преподаватель кафедры техносферной безопасности Горногоинститута FACTORS AFFECTING ON THE FREQUENCY OF WATER RESERVOIRS CLEANING IN THE DRAINAGE SYSTEMS OF UNDERGROUND KIMBERLITE MINES
Ovchinnikov N.
North-Eastern federal university, director of Mining institute,
candidate of technical science Chemezov E. North-Eastern federal university, head of department of technosphere safety of Mining institute, doctor of technical science
Ammosova M. North-Eastern federal university, senior lecturer of department of technosphere safety of Mining institute
