Application status and development trend of the ho

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Application status and development trend of narrow gap welding

since the narrow gap welding technology developed by Battelle Research Institute in the United States was first published in Iron Age magazine in December 1963 (the term "narrow gap welding" first appeared in the article jointly written by Battelle schinker  P Meister and D C matin published in British Welding Journal in May 1966 [1]), As a more advanced welding technology, narrow gap welding technology has immediately received high attention from welding experts all over the world, and has invested a lot of research [2]. V y Marin sorted out the following characteristics of narrow gap welding from many publications [3]: ① it is a special technology using the existing arc welding methods; ② Most of them adopt I-shaped groove, and the groove angle depends on the deformation during welding; ③ Multi layer welding; ④ The number of weld passes of each layer from bottom to top is the same (usually 1 or 2 passes); ⑤ Welding with small or medium heat input; ⑥ Possibility of all position welding

the eighth special committee of the construction branch of the Japan pressure vessel Commission has considered the definition of narrow gap welding and made the following provisions [4]: narrow gap welding is a method of placing steel plates with a thickness of more than 30mm, beveling them according to the gap less than the plate thickness, and then carrying out mechanized or automatic arc welding (the plate thickness is less than 200mm). After more than half a century of research and development, people have carried out a lot of development and Research on its welding methods and welding materials. At present, narrow gap welding plays a huge role in industrial production in many countries

1 classification and principle of narrow gap welding technology

narrow gap welding technology is classified according to its adopted process [5]. It can be divided into narrow gap submerged arc welding (ng-saw), narrow gap gas metal arc welding (ng-gmaw), narrow gap argon tungsten arc welding (ng-gtaw), narrow gap electrode arc welding, narrow gap electroslag welding, and narrow gap laser welding. Each welding method has its own characteristics and scope of application

1.1 narrow gap submerged arc welding

1.1.1 introduction to narrow gap submerged arc welding

narrow gap submerged arc welding appeared in the 1980s and was soon applied to industrial production. Its main application fields are low alloy steel thick walled vessels and other heavy welded structures. The welded joint of narrow gap submerged arc welding has higher resistance to delayed cold cracking, and its strength performance and impact toughness are better than the traditional wide groove submerged arc welding joint. Compared with the traditional submerged arc welding, the total efficiency can be increased by 50% - 80%; It can save 38% - 50% of welding wire and 56% - 64.7% of flux. Narrow gap submerged arc welding has a variety of single wire, double wire and multi wire complete sets of equipment, which are mainly used for horizontal or near horizontal welding, and the flux is required to have the required flux carrying capacity and slag removal effect during welding, so that the weld has appropriate mechanical properties. Generally, multi-layer welding is used. Due to the narrow groove gap and the difficulty of slag removal between layers, the requirements for the slag removal performance of the flux are high, and suitable flux needs to be developed

although saw process has the following advantages: high deposition speed, low splash and arc magnetic bias blowing, can obtain welds with good weld bead shape and high quality, simple equipment, etc., due to the latest progress in filler metal, flux and technology, Japan, Europe, Russia and other countries and regions widely adopt ng-saw process when welding carbon steel, low alloy steel and high alloy steel

the diameter of welding wire used in ng saw is between 2 ~ 5mm, and welding wire with diameter less than 2mm is rarely used. It is reported that the best welding wire size is 3mm. 4mm Diameter welding wire is recommended for steel plates with thickness greater than 140mm, while 5mm diameter welding wire is recommended for steel plates with thickness greater than 670mm

ng-saw weld bead deposition scheme selection is related to many factors

single pass welding is only used when using self deslagging flux developed specifically for easy deslagging in narrow grooves. However, despite the use of higher groove filling speed, the single pass welding scheme still has some shortcomings compared with the multi pass welding scheme. In addition to using non-standard flux, it also requires that the welding wire be positioned very accurately in the groove, and there are strict restrictions on the change of gap. It is sensitive to welding parameters, especially voltage fluctuations and solidification cracks, which limits the adaptability of this process. Single pass welding is widely used in Japan

multi pass welding is widely used in other national treasures outside Japan, which is characterized by relatively low groove filling speed, but it has strong adaptability, high reliability and few defects. Although the welding cost is high, the most important point of this scheme is to allow the use of standard or slightly improved flux and ordinary saw welding process

1.1.2 welding characteristics of narrow gap submerged arc welding

narrow gap welding is a special technology formed on the basis of applying the existing welding methods and processes, plus special technologies such as special welding wire, shielding gas, electrode introduction into narrow groove and automatic seam tracking. The advantages and limitations of submerged arc welding are directly inherited from narrow gap submerged arc welding technology, and to a large extent determine the technical characteristics, economic characteristics, application characteristics and reliability of narrow gap welding [7]:

(1) during submerged arc welding, the diffusion angle of arc is large, the shape coefficient of weld is large, and the arc power is large. Combined with appropriate wire wall spacing control, it is not necessary to be like consumable electrode gas shielded welding, It is necessary to adopt more complex arc side deflection technology, that is, the arc heat source and its action characteristics of submerged arc welding method, which can directly solve the fusion problem on both sides, which is an important reason why submerged arc welding method has the highest application proportion in narrow gap technology

(2) the fluctuation of energy parameters in the welding process has a low sensitivity to the impact of Weld Geometry. This is because the arc power of submerged arc welding method is high, and the fluctuation amplitude caused by the same current fluctuation △ I during submerged arc welding is much smaller

(3) in the process of submerged arc welding, the droplet is the transition of slag wall, and the efficient "blocking" effect of liquid slag cover and solid flux will not produce splash at all. This is the unique characteristic of submerged arc welding in all consumable electrode arc welding methods, which is what narrow gap welding technology is trying to pursue. Once large particles splash in the deep and narrow groove, it will be difficult to ensure the stability of wire feeding, the effectiveness of protection and the relative movement reliability of the narrow gap welding gun

(4) in multi-layer and multi pass welding, the proportion of coarse-grained zone and fine-grained zone in base metal welding heat affected zone and weld zone can be effectively controlled by adjusting the shape coefficient of single pass weld. Generally, the larger the shape coefficient of the weld is, the larger the proportion of the fine grain zone in the heat affected zone and the weld zone is. This is because the thinner the weld bead is deposited, the more complete the cumulative heat treatment effect of the subsequent weld bead on the previous weld bead. Through the primary, secondary or even tertiary solid-state phase transformation, some coarse-grained regions in the weld and heat affected zone are transformed into fine-grained regions, which is of great significance to improve the uniformity of microstructure and mechanical properties of welded joints in narrow gap welding technology

the submerged arc welding method can solve the problem of side wall fusion under the condition of very small groove surface angle (0 ~ 7) by relying on the characteristics of the arc without special technology; Weld geometry is not sensitive to the fluctuation of arc energy parameters; The technical characteristic of no welding spatter is unconditionally inherited from narrow gap welding technology, which greatly improves the reliability of wire feeding, air feeding and the movement of welding gun in the groove during narrow gap submerged arc welding, which plays a decisive role in ensuring the fusion quality and process reliability of narrow gap welding. However, the limitations of submerged arc welding are also inherited from narrow gap technology

(1) due to the extremely difficult slag removal during single pass welding in narrow groove, it is necessary to adopt the deposition method of 2 (or 3) passes per layer during narrow gap welding, which will make it impossible to reduce the filling gap to as small (about 10mm) as ng-tig and ng-gmaw in ng-saw technology, and the minimum gap is generally about 18mm, which is the fundamental reason why ng-saw is difficult to be more ideal in technology and economy

(2) many technical advantages of submerged arc welding method originate from high arc power, which will increase the welding heat input during ng-saw, and it is difficult to improve the plasticity and toughness of welded joints. Important ng-saw joints often need post weld heat treatment to meet the service performance requirements

(3) it is difficult to carry out welding at other spatial positions except flat welding

1.1.3 industry mature ng-saw technology

submerged arc welding is one of the most widely used welding methods in the industrial field at present, and it is also the most mature, reliable and most widely used welding method in the narrow gap technology. So far, there are several kinds of relatively mature narrow gap submerged arc welding technologies in industry:

(1) NSA technology, which is developed by Kawasaki Steel Company of Japan for carbon steel and low carbon steel pressure vessels, offshore drilling platforms and machine manufacturing. Special flat conducting nozzle with straight wire technology and ceramic coating. This technology adopts single pass welding and single wire or tandem double wire. The diameter of welding wire is 3.2mm. The specially designed kb-120 neutral flux with mgo-bao-sio2-al2o3 as the basic component can cause thermal expansion, so it has good slag removal

(2) subnap technology is developed by Japan steel welding products engineering company for carbon steel and low alloy steel ng saw. It adopts straight wire, single pass and single wire or tandem double wire. The diameter of welding wire is 3.2mm. In order to obtain better slag removal, two kinds of flux with main components of tio2-sio2-caf2 and cao-sio2-al2o3-mgo were specially designed

(3) ESAB technology is developed by ESAB, a Swedish manufacturer of ng-saw equipment and welding materials, for the welding of carbon steel and low alloy steel for pressure vessels and large structural parts. The design adopts double weld beads and fixed bent wires

(4) Ansaldo technology is developed by the manufacturer and user of ng-saw equipment of Ansaldo TPA Breda boiler plant in Milan, Italy. It adopts fixed bending single welding wire, and each layer is deposited with multiple weld passes

(5) m a n-ghh technology was developed by m a n-ghh sterkrade in West Germany for the manufacture of indoor components of nuclear reactors. It adopts single welding wire and double welding passes

1.2 narrow gap gas metal arc welding

1.2.1 introduction to narrow gap gas metal arc welding

narrow gap gas metal arc welding was successfully developed after 1975. This process was realized after adopting a special welding wire bending structure to keep the welding wire bent, so as to solve the penetration problem of the side wall of the slope [8]

narrow gap gas metal arc welding is a method of using arc swing to reach both sides of the welded steel plate. In the flat welding method, in order to fully weld both sides of the I-shaped groove and make the arc point to the walls on both sides of the groove, various methods are adopted: ① before the welding wire enters the groove, bend the welding wire; ② The method of making the welding wire swing perpendicular to the welding direction; ③ Fried dough twist twist method; ④ AC arc welding method of flux cored wire; ⑤ AC arc welding method with large diameter solid welding wire, etc. In addition, it is also used φ (Ar)30%+ φ (CO2) 70% as protective gas and ф The gas shielded welding method with 1.6mm solid welding wire is used to weld joints with special and complex shapes. In the horizontal welding method, in order to prevent the molten metal from flowing down in the I-shaped groove and obtain a uniform weld bead, the following welding methods are proposed: the welding method of using the periodic change of welding current to swing the welding wire or divide the groove into upper and lower layers, and the welding method of combining the two methods. Vertical welding of narrow gap mag

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