Since the first publication of the narrow gap welding technology developed by the American Battelle Institute in Iron Age magazine in December 1963 (the term "narrow gap welding" first appeared in the "British Welding Journal" magazine in May 1966. Published in the article by American Matterelle P Meister and DC Matin [1]), narrow gap welding technology as a more advanced welding technology, immediately attracted the attention of welding experts from all over the world, and A large number of studies have been invested [2]. VY Marin has compiled the following features of narrow gap welding from many publications [3]: 1 is a special technique that utilizes the existing arc welding method; 2 most adopt I-shaped groove, the angle of the groove is considered Depending on the amount of deformation in the weld; 3 multi-layer welding; 4 the number of welds from bottom to top is the same (usually 1 or 2); 5 is welded with small or medium heat input; 6 has all-position welding possibility.
The Eighth Special Committee of the Construction Committee of the Japan Pressure Vessel Committee reviewed the definition of narrow gap welding and made the following provisions [4]: ​​The narrow gap welding is to place the steel plate with a thickness of 30 mm or more, and the groove is placed opposite to the gap of the plate thickness. , then mechanized or automated arc welding method (thickness less than 200mm). After more than half a century of research and development, people have done a lot of research and development on their 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
The narrow gap welding technique is classified according to the process adopted [5]. The diameter of the wire for NG-SAW is between 2 and 5 mm, and the wire having a diameter of less than 2 mm is rarely used. The best wire size is reported to be 3mm. 4mm diameter wire is recommended for steel plates with a thickness greater than 140mm, while 5mm diameter wire is used for steel plates with a thickness greater than 670mm.
The choice of NG-SAW weld bead welding scheme is related to many factors.
Single pass welding is only used when using self-degreasing flux developed for easy slag removal in narrow bevels. However, despite the higher groove fill speeds, the single pass weld scheme has some deficiencies compared to the multipass weld plan. In addition to the need to use non-standard fluxes, it also requires the wire to be positioned very accurately within the groove, with tighter limits on the change in clearance. The sensitivity to welding parameters, especially voltage fluctuations and solidification cracks, limits the adaptability of this process. Single pass welding is used more in Japan.
Multi-channel welding is widely used in other national treasures outside Japan, which is characterized by a relatively low filling speed of the groove, but its adaptability, high reliability, and few defects. Despite the high cost of welding, the most important aspect of this solution is the use of standard or slightly improved flux, as well as common SAW welding processes.
1.1.2 Welding characteristics of narrow gap submerged arc welding
Narrow gap welding is a special technique based on the application of existing welding methods and processes, plus special welding wire, shielding gas, electrode introduction technology into narrow bevel and automatic tracking of welds. The advantages and limitations of submerged arc welding are directly inherited to the narrow gap submerged arc welding technology, and largely determine the technical characteristics, economic characteristics, application characteristics and reliability of narrow gap welding [7]:
(1) When the submerged arc welding has a large arc diffusion angle, a large weld bead shape coefficient, and a large arc power, and with appropriate wire-wall spacing control, it is not necessary to use a more complicated arc side bias as in the case of gas metal arc welding. The technology, that is, the arc heat source of the submerged arc welding method and its action characteristics can directly solve the fusion problem on both sides, which is an important reason for the highest application ratio of the submerged arc welding method in the narrow gap technology.
(2) The fluctuation of the energy parameters during the welding process is less sensitive to the influence of the weld geometry. This is because the arc power of the submerged arc welding method is high, and the same current fluctuation amount ΔI is much smaller than that caused by submerged arc welding.
(3) During the submerged arc welding process, the droplets are the transition of the slag wall, and the high-efficiency “blocking†effect of the liquid slag cover and the solid flux does not cause splashing at all. This is the submerged arc welding method unique to all the fusion arc welding methods. The characteristics of the narrow gap welding technology are all pursued. Because of the large particle spatter in the deep narrow groove, it is difficult to guarantee the stability of the wire, the effectiveness of the protection or the relative movement reliability of the narrow gap welding gun.
(4) In the multi-layer multi-pass welding method, the ratio of the coarse grain zone and the fine grain zone in the heat affected zone and the weld zone of the base metal can be effectively controlled by adjusting the shape coefficient of the single pass weld. Generally, the larger the weld shape coefficient, the larger the proportion of the fine grain zone in the heat affected zone and the weld zone. This is because the thinner the weld of the weld bead, the more complete the cumulative heat treatment of the subsequent weld bead to the previous weld bead, and the partial coarse-grained zone in the weld and heat affected zone is transformed into one, two or even three solid phase transitions. Fine grain zone, which is of great significance for improving the uniformity of the welded joints and the uniformity of mechanical properties in the narrow gap welding technology.
The submerged arc welding method relies on the characteristics of the arc itself to solve the problem of sidewall fusion under the condition of extremely small groove surface angle (0o~7o) without special technology; the weld geometry is not sensitive to the fluctuation of arc energy parameters; no welding splash The technical characteristics are unconditionally inherited to the narrow gap welding technology, which greatly improves the reliability of the wire feeding, air supply and the movement of the welding torch in the groove during the narrow gap submerged arc welding, which ensures the fusion quality and process reliability of the narrow gap welding. It has played a decisive role. However, the limitations of the submerged arc welding method were originally inherited locally to the narrow gap technique.
(1) Due to the difficulty in clearing the slag during single-pass welding in a narrow groove, it is necessary to use two (or three) layers of welding per layer for narrow gap welding, which will bring NG-SAW technology impossible. The filling gap is reduced to be as small as NG-TIG, NG-GMAW (about 10mm), and the minimum gap is generally about 18mm, which is the fundamental reason why NG-SAW is difficult to be more idealized technically and economically.
(2) Many technical advantages of submerged arc welding method originate from large arc power, which will increase the welding heat input of NG-SAW, and it is difficult to improve the welding state and toughness of welded joints. Important NG-SAW joints often need welding. Post-heat treatment can meet the performance requirements.
(3) It is difficult to perform welding at other spatial locations than flat welding.
1.1.3 Industrially mature NG-SAW technology
Submerged arc welding is one of the most widely used welding methods in the industrial field. It is also the most mature, reliable and highest application welding method applied to narrow gap technology. So far, the industrially mature narrow gap submerged arc welding technology has the following:
(1) NSA technology It is an NG-SAW developed by Kawasaki Steel Co., Ltd. for carbon steel and low carbon steel pressure vessels, offshore drilling platforms and machine manufacturing. Straight wire technology and special flat contact tips coated with ceramic. This technology uses a single weld bead and uses a single or tandem double wire. The wire diameter is 3.2mm. The specially designed KB-120 neutral flux transition with MgO-BaO-SiO2-Al2O3 as the basic component can cause thermal expansion, so that it has better slag removal.
(2) Subnap technology It was developed by Nippon Steel Welding Products Engineering Co., Ltd. for carbon steel and low alloy steel Ng-SAW. It uses straight wire, single bead and single or tandem twin wire. The wire diameter is 3.2mm. In order to obtain better slag removal properties, two kinds of fluxes with main components of TiO2-SiO2-CaF2 and CaO-SiO2-Al2O3-MgO were specially designed.
(3) ESAB technology It was developed by Swedish NG-SAW equipment and welding material manufacturer ESAB for the welding of carbon steel and low alloy steel for pressure vessels and large structural parts. The design uses a double weld bead and uses a fixed wire bend.
(4) Ansaldo technology It was developed by NG-SAW equipment manufacturers and users of the Ansaldo TPA Breda boiler plant in Milan, Italy. It uses a fixed curved single wire, and each layer is welded with multiple weld beads.
(5) MA N-GHH technology It was developed by West Germany MA N-GHH Sterkrade for the manufacture of nuclear reactor interior components. It uses a single wire double bead.
1.2 Narrow gap gas metal arc welding 1.2.1 Introduction to narrow gap gas metal arc welding
Narrow-gap MIG welding was developed after 1975. This process was achieved after a special wire bending structure was used to keep the wire bent to solve the penetration problem of the bevel side wall [8].
Narrow-gap MIG welding is a method of using arc oscillating to reach both sides of a welded steel plate. In the flat welding method, in order to make the two sides of the I-shaped groove sufficiently penetrate and make the arc point to the two side walls of the groove, various methods are adopted: 1) a method of bending the wire before the wire enters the groove; 2 making the wire A method of swinging perpendicular to the welding direction; 3 a twisted wire method; an AC arc welding method of a flux cored wire; 5 an AC arc welding method using a large diameter solid wire. In addition, there is also a gas shielded welding method using φ(Ar)30%+φ(CO2) 70% as a shielding gas in combination with a ф1.6mm solid wire for welding a joint having a complicated shape. In the transverse welding method, in order to prevent the molten metal from snagging in the I-shaped groove to obtain a uniform weld bead, the following welding method is proposed: using the periodic variation of the welding current to cause the welding wire to oscillate or to divide the groove into upper and lower layers. The method, a welding method in which two methods are combined, and the like. In the vertical welding narrow gap MAG welding method, in order to ensure the penetration of both sides of the groove, a welding method of the oscillating welding wire and a welding method in which the welding current and the welding wire are synchronously changed are developed.
1.2.2 Industrially mature NG-GMAW technology
The Suface Tension Transfer technology uses more than 20 patents in seven countries. It was first published in the Welding Journal by Lincoln, a senior engineer at Lincoln in the United States in 1993. The surface tension transition technology is derived from the short-circuit transition technology, but is different from the traditional short-circuit transition technology. It mainly realizes the droplet transfer by the action of surface tension on the droplets. According to the theory of surface tension transition, there is no force such as plasma flow force, arc thrust, spot force, metal vapor reaction force, etc. during the arc extinguishing period from the contact of the droplet to the molten pool until the neck of the neck is broken. If the effect of gravity and electromagnetic force is not considered, the droplet completely completes the spreading, necking and fracture to the molten pool under the surface tension of the fusion interface between the droplet and the molten pool. During the short circuit, the neck is narrowed. The small welding current and arc voltage are output during formation and during the presence, which greatly reduces the degree of explosion of the short-circuit liquid bridge, thereby reducing the splash.
The surface tension transition process is a huge technological advancement in the short-circuit transition process technology of the gas metal arc welding method. It has the following technical advantages: 1 the spatter rate is very low, the droplet is axially transitioned; 2 the welding dust amount is small; 3 the working environment More comfortable (low smoke, low splash, low light radiation); 4 good fusion under low heat input conditions; 5 has good bottoming weld all-position single-sided welding double-sided forming ability; 6 easier to operate, higher operating efficiency.
1.3 narrow gap tungsten argon arc welding
This welding process basically does not produce splashing and slag, and due to the stability of the arc, there are few obvious welding defects, and the application to all-position welding has also been established [9]. However, the disadvantage of this method is that the working efficiency is low. In order to improve the working efficiency, while the filler wire is energized and heated, the thermal resistance wire bonding method should also be adopted. The advantageous aspect of this method is that the welding current and the filler wire can be individually selected. Give the amount. However, if an excessive amount of energization of the filler wire is given, a magnetic shock of the tungsten inert gas shielded welding is caused, and the arc formed is unstable. Therefore, measures such as pulsing or staggering the arc current and the wire current, or alternating the unidirectional current are taken.
The use of ultra-high strength steel promotes the application of TIG welding in narrow gap welding. TIG welding is generally considered to be one of the most reliable processes for welding quality [5]. Due to the protective effect of argon, TIG welding can be used to weld non-ferrous metals and alloys thereof, stainless steel, superalloys, titanium and titanium alloys, and refractory active metals (such as molybdenum, niobium, zirconium). Good toughness, low hydrogen content in the weld metal. Due to the low current carrying capacity of the tungsten electrode, the deposition rate is not high, and the application field is relatively narrow, and is generally used for bottom welding and important structures.
1.4 narrow gap electrode arc welding
Since narrow gap welding is mainly for mechanized and automated production, the application of electrode arc welding in narrow gap welding is not much, and the welding quality is not well controlled. However, in actual production, narrow gap electrode arc welding has advantages that other welding methods cannot replace (such as ease of use, flexibility, simple equipment, etc.), so in some fields, such as narrow gaps for reinforcing steel in dam buildings. Welding solves the problem of eccentric stress of steel bars caused by the connection technology of steel bars. The cost is only 1/11 of the tie-bar welding; it is applicable to the I, II and III steel bars of ф18～40mm [5].
Compared to other NG technologies, narrow gap electrode arc welding applications are very limited.
1.5 narrow gap electroslag welding
In addition to welding various steels and cast irons, narrow-gap electroslag welding can also weld aluminum and aluminum alloys, magnesium alloys, titanium and titanium alloys, and copper. It is widely used in boiler manufacturing, heavy machinery and petrochemical industries. In recent years, bridge construction, narrow gap electroslag welding has been used to weld 25-75mm flat structure [5]. The consumption of flux, welding wire and electric energy is lower than that of submerged arc welding, and the greater the thickness of the workpiece, the more obvious the effect, the less tendency of the welded joint to produce quenching cracks, compared with the traditional electroslag welding, the weld and heat affected zone The higher metal properties allow for the elimination or simplification of post-weld heat treatment. However, the equipment is relatively large, and at the same time, the slag removal requirement of the used slag agent is high.
1.6 narrow gap laser welding
Since the thickness of the laser welding is more than 6 mm, it is included in the thick plate welding, and the width of the laser welding has a small groove width, which can be considered as a narrow gap laser welding. High-power CO2 lasers are commonly used for thick-plate laser welding. Currently, the thickness can be up to 50mm and the aspect ratio is as high as 12:1. Laser welded welds have a high hardness in the as-welded state and mainly contain martensite structure and should be post-weld heat treated. Since laser welding requires high-power lasers and high equipment requirements, applications in the production field are limited.
2 Application status of narrow gap welding
The good mechanical properties, low residual stress and residual deformation of narrow gap welds, high welding productivity and low production cost of narrow gap welds determine the objective application potential of this technology in the field of steel structure welding. And a wide range of applications. From a technical point of view, its many technical superiorities determine the technology is extremely attractive. However, from an economic point of view, the narrow gap welding technology does have a problem of the economic thickness range, that is, while enjoying its technical superiority, it can significantly increase the range of economic benefits. In general, the greater the plate thickness, the greater the economic benefits. The minimum plate thickness with obvious economic advantages can be referred to as the lower plate thickness of the narrow gap weld. The thickness of the lower limit varies depending on the structural steel type, structural reliability requirements, structural size, and spatial position, but is generally 20 to 30 mm. The upper plate thickness depends only on the depth of the torch that can be developed by the narrow gap welding technique. Theoretically, there is no upper limit thickness limiting torch. Existing narrow gap welding, welding 500 ~ 600mm plate thickness without any technical obstacles. At present, narrow gap welding has been successfully applied to many aspects of industrial production. The specific distribution structure is shown in Table 1, 2<10>.
NGW application area utilization (%)
Pressure vessel and boiler 52.5
Industrial Machinery 25
Marine structure and shipbuilding 12.5
Pressure water pipe 10
Table 2 NGW utilization distribution table
NGW method GMAW GTAW GSAW
Utilization rate (%) 75 5 20
In economic construction, large steel structures, bridges, ships, and nuclear reactors are required to be connected by large-thickness steel plates [10]. China's welded steel structure basically stays at the level of electrode arc welding, and there are few applications for narrow gap welding, which is not only difficult to improve labor productivity, but also the welding quality level is not high.
At present, large-thickness steel plates are more and more widely used in production. In China, on the basis of traditional electrode arc welding, the pace of narrow gap welding can be accelerated. China's narrow gap welding technology can be developed on the basis of foreign emphasis on mechanical, using advanced computer control technology to the combination of machinery and control, and thus become a direction for its future development.
3 Development direction of narrow gap welding and its new progress
Narrow gap welding has a very high welding productivity, better joint mechanical properties, smaller welding residual stress and residual deformation, lower welding production cost and other significant technical and economic advantages, which is classified as advanced manufacturing technology, deserved. However, so far, the promotion and application of this technology in the field of thick plate welding is still extremely limited, and many industries in China still have no zero breakthrough in application. In order to make the narrow gap welding technology more mature, more practical, and more obvious in technical and economic advantages, it should also accelerate technological development and technological progress mainly from the following aspects:
(1) Develop arc welding technology with lower heat input to meet the needs of high-strength steel and even high-alloy steel, and wider spatial position adaptability;
(2) Develop ultra-low spatter rate control technology (including power supply) of GMAW method to meet the needs of high reliability and high stability of narrow gap automatic welding process;
(3) Develop automatic tracking technology with high anti-interference ability, high reliability and high precision to meet the needs of the safe and reliable operation of the welding torch in the narrow groove, and the position of the arc in the space inside the groove is highly accurate.
In the past 10 years, the development of new technologies for narrow gap welding seems to have slowed down in the world. The reason may be that ultra-low spatter rate control technology and high-reliability real-time tracking control have not yet produced a technological leap. Absolutely not the narrow gap welding technology has reached the perfect state. It is very gratifying that the welding experts of various countries have not been disheartened. In the unremitting research on the qualitative leap of arc welding technology since the 1990s, exciting new progress has been made, which is narrow. The rapid development of gap welding technology laid the foundation. Some of the progress in the past 10 years is as follows:
(1) Using pulsed jet flow transition technology to enhance the fusion of the two side walls while reducing the spatter rate; using a magnetic field to control the arc swing in the narrow gap groove;
(2) Ultra-low spatter rate (<3%) surface tension transition welder has been developed (Lincoln, USA) and has been commercialized; (3) Various tracking systems such as optoelectronics and lasers, which are controlled by computer, have been developed. (such as Sweden ESAB company, American Jellin company and several domestic universities, etc.);
(4) New power sources such as constant current CO2 welding machine and fuzzy control semi-automatic GMAW welding machine (such as Japan) have been developed (some have been commercialized);
(5) The development of high-fusion rate, low spatter rate, flux-cored wire without interlayer slag, and the possibility of application of narrow-gap flux-cored arc welding;
(6) High-stability wire feeding mechanism (such as dual motor, four-wheel drive, etc.) has been successfully applied to the conventional GMAW method.
In short, many new technologies, new equipment, new equipment, new equipment, and continuous improvement of industrial technology developed in the field of GMAW in recent years have provided new ideas, new ways and new technology reserves for the technological advancement of narrow gap welding. . It is believed that in the near future, narrow gap welding technology with higher efficiency, higher quality, lower cost, more reliability and more practicality will continue to emerge.
The Eighth Special Committee of the Construction Committee of the Japan Pressure Vessel Committee reviewed the definition of narrow gap welding and made the following provisions [4]: ​​The narrow gap welding is to place the steel plate with a thickness of 30 mm or more, and the groove is placed opposite to the gap of the plate thickness. , then mechanized or automated arc welding method (thickness less than 200mm). After more than half a century of research and development, people have done a lot of research and development on their 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
The narrow gap welding technique is classified according to the process adopted [5]. The diameter of the wire for NG-SAW is between 2 and 5 mm, and the wire having a diameter of less than 2 mm is rarely used. The best wire size is reported to be 3mm. 4mm diameter wire is recommended for steel plates with a thickness greater than 140mm, while 5mm diameter wire is used for steel plates with a thickness greater than 670mm.
The choice of NG-SAW weld bead welding scheme is related to many factors.
Single pass welding is only used when using self-degreasing flux developed for easy slag removal in narrow bevels. However, despite the higher groove fill speeds, the single pass weld scheme has some deficiencies compared to the multipass weld plan. In addition to the need to use non-standard fluxes, it also requires the wire to be positioned very accurately within the groove, with tighter limits on the change in clearance. The sensitivity to welding parameters, especially voltage fluctuations and solidification cracks, limits the adaptability of this process. Single pass welding is used more in Japan.
Multi-channel welding is widely used in other national treasures outside Japan, which is characterized by a relatively low filling speed of the groove, but its adaptability, high reliability, and few defects. Despite the high cost of welding, the most important aspect of this solution is the use of standard or slightly improved flux, as well as common SAW welding processes.
1.1.2 Welding characteristics of narrow gap submerged arc welding
Narrow gap welding is a special technique based on the application of existing welding methods and processes, plus special welding wire, shielding gas, electrode introduction technology into narrow bevel and automatic tracking of welds. The advantages and limitations of submerged arc welding are directly inherited to the narrow gap submerged arc welding technology, and largely determine the technical characteristics, economic characteristics, application characteristics and reliability of narrow gap welding [7]:
(1) When the submerged arc welding has a large arc diffusion angle, a large weld bead shape coefficient, and a large arc power, and with appropriate wire-wall spacing control, it is not necessary to use a more complicated arc side bias as in the case of gas metal arc welding. The technology, that is, the arc heat source of the submerged arc welding method and its action characteristics can directly solve the fusion problem on both sides, which is an important reason for the highest application ratio of the submerged arc welding method in the narrow gap technology.
(2) The fluctuation of the energy parameters during the welding process is less sensitive to the influence of the weld geometry. This is because the arc power of the submerged arc welding method is high, and the same current fluctuation amount ΔI is much smaller than that caused by submerged arc welding.
(3) During the submerged arc welding process, the droplets are the transition of the slag wall, and the high-efficiency “blocking†effect of the liquid slag cover and the solid flux does not cause splashing at all. This is the submerged arc welding method unique to all the fusion arc welding methods. The characteristics of the narrow gap welding technology are all pursued. Because of the large particle spatter in the deep narrow groove, it is difficult to guarantee the stability of the wire, the effectiveness of the protection or the relative movement reliability of the narrow gap welding gun.
(4) In the multi-layer multi-pass welding method, the ratio of the coarse grain zone and the fine grain zone in the heat affected zone and the weld zone of the base metal can be effectively controlled by adjusting the shape coefficient of the single pass weld. Generally, the larger the weld shape coefficient, the larger the proportion of the fine grain zone in the heat affected zone and the weld zone. This is because the thinner the weld of the weld bead, the more complete the cumulative heat treatment of the subsequent weld bead to the previous weld bead, and the partial coarse-grained zone in the weld and heat affected zone is transformed into one, two or even three solid phase transitions. Fine grain zone, which is of great significance for improving the uniformity of the welded joints and the uniformity of mechanical properties in the narrow gap welding technology.
The submerged arc welding method relies on the characteristics of the arc itself to solve the problem of sidewall fusion under the condition of extremely small groove surface angle (0o~7o) without special technology; the weld geometry is not sensitive to the fluctuation of arc energy parameters; no welding splash The technical characteristics are unconditionally inherited to the narrow gap welding technology, which greatly improves the reliability of the wire feeding, air supply and the movement of the welding torch in the groove during the narrow gap submerged arc welding, which ensures the fusion quality and process reliability of the narrow gap welding. It has played a decisive role. However, the limitations of the submerged arc welding method were originally inherited locally to the narrow gap technique.
(1) Due to the difficulty in clearing the slag during single-pass welding in a narrow groove, it is necessary to use two (or three) layers of welding per layer for narrow gap welding, which will bring NG-SAW technology impossible. The filling gap is reduced to be as small as NG-TIG, NG-GMAW (about 10mm), and the minimum gap is generally about 18mm, which is the fundamental reason why NG-SAW is difficult to be more idealized technically and economically.
(2) Many technical advantages of submerged arc welding method originate from large arc power, which will increase the welding heat input of NG-SAW, and it is difficult to improve the welding state and toughness of welded joints. Important NG-SAW joints often need welding. Post-heat treatment can meet the performance requirements.
(3) It is difficult to perform welding at other spatial locations than flat welding.
1.1.3 Industrially mature NG-SAW technology
Submerged arc welding is one of the most widely used welding methods in the industrial field. It is also the most mature, reliable and highest application welding method applied to narrow gap technology. So far, the industrially mature narrow gap submerged arc welding technology has the following:
(1) NSA technology It is an NG-SAW developed by Kawasaki Steel Co., Ltd. for carbon steel and low carbon steel pressure vessels, offshore drilling platforms and machine manufacturing. Straight wire technology and special flat contact tips coated with ceramic. This technology uses a single weld bead and uses a single or tandem double wire. The wire diameter is 3.2mm. The specially designed KB-120 neutral flux transition with MgO-BaO-SiO2-Al2O3 as the basic component can cause thermal expansion, so that it has better slag removal.
(2) Subnap technology It was developed by Nippon Steel Welding Products Engineering Co., Ltd. for carbon steel and low alloy steel Ng-SAW. It uses straight wire, single bead and single or tandem twin wire. The wire diameter is 3.2mm. In order to obtain better slag removal properties, two kinds of fluxes with main components of TiO2-SiO2-CaF2 and CaO-SiO2-Al2O3-MgO were specially designed.
(3) ESAB technology It was developed by Swedish NG-SAW equipment and welding material manufacturer ESAB for the welding of carbon steel and low alloy steel for pressure vessels and large structural parts. The design uses a double weld bead and uses a fixed wire bend.
(4) Ansaldo technology It was developed by NG-SAW equipment manufacturers and users of the Ansaldo TPA Breda boiler plant in Milan, Italy. It uses a fixed curved single wire, and each layer is welded with multiple weld beads.
(5) MA N-GHH technology It was developed by West Germany MA N-GHH Sterkrade for the manufacture of nuclear reactor interior components. It uses a single wire double bead.
1.2 Narrow gap gas metal arc welding 1.2.1 Introduction to narrow gap gas metal arc welding
Narrow-gap MIG welding was developed after 1975. This process was achieved after a special wire bending structure was used to keep the wire bent to solve the penetration problem of the bevel side wall [8].
Narrow-gap MIG welding is a method of using arc oscillating to reach both sides of a welded steel plate. In the flat welding method, in order to make the two sides of the I-shaped groove sufficiently penetrate and make the arc point to the two side walls of the groove, various methods are adopted: 1) a method of bending the wire before the wire enters the groove; 2 making the wire A method of swinging perpendicular to the welding direction; 3 a twisted wire method; an AC arc welding method of a flux cored wire; 5 an AC arc welding method using a large diameter solid wire. In addition, there is also a gas shielded welding method using φ(Ar)30%+φ(CO2) 70% as a shielding gas in combination with a ф1.6mm solid wire for welding a joint having a complicated shape. In the transverse welding method, in order to prevent the molten metal from snagging in the I-shaped groove to obtain a uniform weld bead, the following welding method is proposed: using the periodic variation of the welding current to cause the welding wire to oscillate or to divide the groove into upper and lower layers. The method, a welding method in which two methods are combined, and the like. In the vertical welding narrow gap MAG welding method, in order to ensure the penetration of both sides of the groove, a welding method of the oscillating welding wire and a welding method in which the welding current and the welding wire are synchronously changed are developed.
1.2.2 Industrially mature NG-GMAW technology
The Suface Tension Transfer technology uses more than 20 patents in seven countries. It was first published in the Welding Journal by Lincoln, a senior engineer at Lincoln in the United States in 1993. The surface tension transition technology is derived from the short-circuit transition technology, but is different from the traditional short-circuit transition technology. It mainly realizes the droplet transfer by the action of surface tension on the droplets. According to the theory of surface tension transition, there is no force such as plasma flow force, arc thrust, spot force, metal vapor reaction force, etc. during the arc extinguishing period from the contact of the droplet to the molten pool until the neck of the neck is broken. If the effect of gravity and electromagnetic force is not considered, the droplet completely completes the spreading, necking and fracture to the molten pool under the surface tension of the fusion interface between the droplet and the molten pool. During the short circuit, the neck is narrowed. The small welding current and arc voltage are output during formation and during the presence, which greatly reduces the degree of explosion of the short-circuit liquid bridge, thereby reducing the splash.
The surface tension transition process is a huge technological advancement in the short-circuit transition process technology of the gas metal arc welding method. It has the following technical advantages: 1 the spatter rate is very low, the droplet is axially transitioned; 2 the welding dust amount is small; 3 the working environment More comfortable (low smoke, low splash, low light radiation); 4 good fusion under low heat input conditions; 5 has good bottoming weld all-position single-sided welding double-sided forming ability; 6 easier to operate, higher operating efficiency.
1.3 narrow gap tungsten argon arc welding
This welding process basically does not produce splashing and slag, and due to the stability of the arc, there are few obvious welding defects, and the application to all-position welding has also been established [9]. However, the disadvantage of this method is that the working efficiency is low. In order to improve the working efficiency, while the filler wire is energized and heated, the thermal resistance wire bonding method should also be adopted. The advantageous aspect of this method is that the welding current and the filler wire can be individually selected. Give the amount. However, if an excessive amount of energization of the filler wire is given, a magnetic shock of the tungsten inert gas shielded welding is caused, and the arc formed is unstable. Therefore, measures such as pulsing or staggering the arc current and the wire current, or alternating the unidirectional current are taken.
The use of ultra-high strength steel promotes the application of TIG welding in narrow gap welding. TIG welding is generally considered to be one of the most reliable processes for welding quality [5]. Due to the protective effect of argon, TIG welding can be used to weld non-ferrous metals and alloys thereof, stainless steel, superalloys, titanium and titanium alloys, and refractory active metals (such as molybdenum, niobium, zirconium). Good toughness, low hydrogen content in the weld metal. Due to the low current carrying capacity of the tungsten electrode, the deposition rate is not high, and the application field is relatively narrow, and is generally used for bottom welding and important structures.
1.4 narrow gap electrode arc welding
Since narrow gap welding is mainly for mechanized and automated production, the application of electrode arc welding in narrow gap welding is not much, and the welding quality is not well controlled. However, in actual production, narrow gap electrode arc welding has advantages that other welding methods cannot replace (such as ease of use, flexibility, simple equipment, etc.), so in some fields, such as narrow gaps for reinforcing steel in dam buildings. Welding solves the problem of eccentric stress of steel bars caused by the connection technology of steel bars. The cost is only 1/11 of the tie-bar welding; it is applicable to the I, II and III steel bars of ф18～40mm [5].
Compared to other NG technologies, narrow gap electrode arc welding applications are very limited.
1.5 narrow gap electroslag welding
In addition to welding various steels and cast irons, narrow-gap electroslag welding can also weld aluminum and aluminum alloys, magnesium alloys, titanium and titanium alloys, and copper. It is widely used in boiler manufacturing, heavy machinery and petrochemical industries. In recent years, bridge construction, narrow gap electroslag welding has been used to weld 25-75mm flat structure [5]. The consumption of flux, welding wire and electric energy is lower than that of submerged arc welding, and the greater the thickness of the workpiece, the more obvious the effect, the less tendency of the welded joint to produce quenching cracks, compared with the traditional electroslag welding, the weld and heat affected zone The higher metal properties allow for the elimination or simplification of post-weld heat treatment. However, the equipment is relatively large, and at the same time, the slag removal requirement of the used slag agent is high.
1.6 narrow gap laser welding
Since the thickness of the laser welding is more than 6 mm, it is included in the thick plate welding, and the width of the laser welding has a small groove width, which can be considered as a narrow gap laser welding. High-power CO2 lasers are commonly used for thick-plate laser welding. Currently, the thickness can be up to 50mm and the aspect ratio is as high as 12:1. Laser welded welds have a high hardness in the as-welded state and mainly contain martensite structure and should be post-weld heat treated. Since laser welding requires high-power lasers and high equipment requirements, applications in the production field are limited.
2 Application status of narrow gap welding
The good mechanical properties, low residual stress and residual deformation of narrow gap welds, high welding productivity and low production cost of narrow gap welds determine the objective application potential of this technology in the field of steel structure welding. And a wide range of applications. From a technical point of view, its many technical superiorities determine the technology is extremely attractive. However, from an economic point of view, the narrow gap welding technology does have a problem of the economic thickness range, that is, while enjoying its technical superiority, it can significantly increase the range of economic benefits. In general, the greater the plate thickness, the greater the economic benefits. The minimum plate thickness with obvious economic advantages can be referred to as the lower plate thickness of the narrow gap weld. The thickness of the lower limit varies depending on the structural steel type, structural reliability requirements, structural size, and spatial position, but is generally 20 to 30 mm. The upper plate thickness depends only on the depth of the torch that can be developed by the narrow gap welding technique. Theoretically, there is no upper limit thickness limiting torch. Existing narrow gap welding, welding 500 ~ 600mm plate thickness without any technical obstacles. At present, narrow gap welding has been successfully applied to many aspects of industrial production. The specific distribution structure is shown in Table 1, 2<10>.
NGW application area utilization (%)
Pressure vessel and boiler 52.5
Industrial Machinery 25
Marine structure and shipbuilding 12.5
Pressure water pipe 10
Table 2 NGW utilization distribution table
NGW method GMAW GTAW GSAW
Utilization rate (%) 75 5 20
In economic construction, large steel structures, bridges, ships, and nuclear reactors are required to be connected by large-thickness steel plates [10]. China's welded steel structure basically stays at the level of electrode arc welding, and there are few applications for narrow gap welding, which is not only difficult to improve labor productivity, but also the welding quality level is not high.
At present, large-thickness steel plates are more and more widely used in production. In China, on the basis of traditional electrode arc welding, the pace of narrow gap welding can be accelerated. China's narrow gap welding technology can be developed on the basis of foreign emphasis on mechanical, using advanced computer control technology to the combination of machinery and control, and thus become a direction for its future development.
3 Development direction of narrow gap welding and its new progress
Narrow gap welding has a very high welding productivity, better joint mechanical properties, smaller welding residual stress and residual deformation, lower welding production cost and other significant technical and economic advantages, which is classified as advanced manufacturing technology, deserved. However, so far, the promotion and application of this technology in the field of thick plate welding is still extremely limited, and many industries in China still have no zero breakthrough in application. In order to make the narrow gap welding technology more mature, more practical, and more obvious in technical and economic advantages, it should also accelerate technological development and technological progress mainly from the following aspects:
(1) Develop arc welding technology with lower heat input to meet the needs of high-strength steel and even high-alloy steel, and wider spatial position adaptability;
(2) Develop ultra-low spatter rate control technology (including power supply) of GMAW method to meet the needs of high reliability and high stability of narrow gap automatic welding process;
(3) Develop automatic tracking technology with high anti-interference ability, high reliability and high precision to meet the needs of the safe and reliable operation of the welding torch in the narrow groove, and the position of the arc in the space inside the groove is highly accurate.
In the past 10 years, the development of new technologies for narrow gap welding seems to have slowed down in the world. The reason may be that ultra-low spatter rate control technology and high-reliability real-time tracking control have not yet produced a technological leap. Absolutely not the narrow gap welding technology has reached the perfect state. It is very gratifying that the welding experts of various countries have not been disheartened. In the unremitting research on the qualitative leap of arc welding technology since the 1990s, exciting new progress has been made, which is narrow. The rapid development of gap welding technology laid the foundation. Some of the progress in the past 10 years is as follows:
(1) Using pulsed jet flow transition technology to enhance the fusion of the two side walls while reducing the spatter rate; using a magnetic field to control the arc swing in the narrow gap groove;
(2) Ultra-low spatter rate (<3%) surface tension transition welder has been developed (Lincoln, USA) and has been commercialized; (3) Various tracking systems such as optoelectronics and lasers, which are controlled by computer, have been developed. (such as Sweden ESAB company, American Jellin company and several domestic universities, etc.);
(4) New power sources such as constant current CO2 welding machine and fuzzy control semi-automatic GMAW welding machine (such as Japan) have been developed (some have been commercialized);
(5) The development of high-fusion rate, low spatter rate, flux-cored wire without interlayer slag, and the possibility of application of narrow-gap flux-cored arc welding;
(6) High-stability wire feeding mechanism (such as dual motor, four-wheel drive, etc.) has been successfully applied to the conventional GMAW method.
In short, many new technologies, new equipment, new equipment, new equipment, and continuous improvement of industrial technology developed in the field of GMAW in recent years have provided new ideas, new ways and new technology reserves for the technological advancement of narrow gap welding. . It is believed that in the near future, narrow gap welding technology with higher efficiency, higher quality, lower cost, more reliability and more practicality will continue to emerge.
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