Pressure vessels are widely used in various industrial sectors such as petroleum, chemical, and boilers, and are important special equipment in national economic production. According to the statistics of the State Bureau of Quality and Technical Supervision [1], in the year of 2003, a total of 144 explosions of various pressure vessels and cylinders occurred in China, and the direct economic losses amounted to 14.8633 million yuan. Therefore, improving the quality of welding and the level of mechanical automation are of great significance for pressure vessels and the entire welding industry.
The pressure vessel is a device that is subjected to internal and external pressures and has various structural forms. The basic composition is composed of a casing, a head, a flange, a joint, a support, etc., and these components need to be assembled into a whole by welding. Therefore, the welding process is the core of the pressure vessel manufacturing process. The most common types of joints for pressure vessels are butt joints, angle joints and lap joints. The welds are available in butt welds, fillet welds and combined welds [2]. At present, large-scale vertical oil tanks widely used in the petrochemical industry at home and abroad have adopted automatic welding technology _3_3 for different weld seams. The submerged arc welding automatic welding technology is mainly used for the bottom of the tank, the circumferential weld of the tank wall and the big joint of the lower joint of the tank wall. The longitudinal weld of the tank wall mainly adopts the gas vertical welding technology. As early as the 1970s, foreign countries began to study the all-position automatic welding technology of spherical tanks. At present, industrialized countries such as the United States and Japan have successively applied flux-cored wire self-shielded welding and MIG welding in the automatic welding of spherical tanks. In the 1980s, MIG welding was used for all-position automatic welding of spherical tanks, but it failed to reach the practical application level. Through the introduction and development of research, the construction of spherical tank pressure vessels in China has gradually developed 1415J in the direction of automatic welding. The research on large-scale spherical tank automatic seam welding machine has been successfully applied to the transverse seam welding of floating roof type spherical tanks with a thickness of 30mm or more. The development of universal automatic arc welding machine can be applied to the welding of ring seams in the pressure vessel industry [6]. For the vertical storage tank of 50,000 m3, the literature [7] adopts the combination of CO, gas shielded welder and submerged arc automatic welder. It can reduce production costs, improve labor efficiency, and has good application prospects.
The welding process has become an important metal processing technology in modern industrial production, which directly restricts the development of other industries. Danish scholar Jens Klastrup
The Kristensen [8] system summarizes the influencing factors of the welding process and believes that the welding field affects many disciplines such as automation technology, materials science, environment, non-destructive testing, mechanical and corrosion, and fracture mechanics. Welding automation, as an important branch of the welding field, in turn restricts the welding process. For pressure vessels, welding automation techniques can be summarized as hardware and software factors. The hardware factors mainly refer to the automatic welding equipment and methods of pressure vessels; the software factors mainly refer to the welding control technology, artificial intelligence technology and expert system formed by the computer technology, electronic technology, automatic control technology and information technology into the field of pressure vessel welding.
1 hardware factor
1.1 Automatic welding equipment
When the pressure vessel is assembled, it is necessary to weld different parts of the container. Due to the large number of welds, various automatic welding equipments are often involved in the welding production process, such as electroslag welding machine and gas shielded welding machine (CO: welding) Machine and argon arc welding machine, etc.), submerged arc welding machine, plasma welding machine, etc. How to make the welding equipment have automation and intelligent effects to improve the automation degree of pressure vessel welding is an important part of the research of welding scholars at home and abroad.
China's pressure vessel welding automation technology developed late, starting in the 1950s and 1960s, and has made great achievements so far. Various automatic welders used throughout the industry are increasingly replacing manual welders. In 2002, domestic automatic and semi-automatic welding machines accounted for 20.55% of the total number of welding machines [9], of which MIG/MAG welding machines increased by 63.12% compared with the previous year, and TIG welding machines increased by 19.24 compared with the previous year. %, submerged arc welding machine increased by 135.36% compared with the previous year. Traditional arc welding machines have already produced thyristor arc welding rectifiers in batches in China, but they lack the functions of hot start and digital display compared with foreign countries. For MIG/MAG welding machines, there are various gas shielded arc welding machines in China. Waveform control technology has been added to the developed inverter MIG welder, and computer control can be used to adapt to the requirements of all-position welding of pressure vessels; digitally controlled submerged arc automatic welding machine [10], its welding power supply The output characteristics, wire feeding speed and welding speed are all adjusted by digital control algorithm. The arc ignition, welding and arc collection are controlled by process control software. However, the current maximum rated capacity of domestic submerged arc welding machines is limited to 2000.
Below A, is in the development stage.
As a new technology and new technology, the inverter welding machine is recognized as the most advanced welding machine at home and abroad. It has excellent welding performance and electrical performance, and is suitable for various arc welding methods and multi-position welding of pressure vessels. In the late 1970s, thyristor inverter welders were mainly used in manual arc welding and TIG welding processes. The new generation inverter welding machine uses power transistors, MOSFETs, IGBTs, etc. as switching devices, while using high-frequency PWM switching technology and microelectronic technology. Inverter welding machines have been widely used abroad, and the application degree of inverter welding in the United States and Japan has reached 30%. China's inverter welding machine research began in the 1980s. At present, three generations of products have been formed. The first generation is an arc welding inverter with thyristor as the main switching device. The inverter frequency is 2000~5 000 Hz; the second The generation is an arc welding inverter with GTR and MOSFET as the main switching device. The inverter frequency is 20~50 kHz. The third generation is IGBT arc welding inverter with inverter frequency of 20~30.
kHz. Due to its unique advantages, inverter welding machine is very optimistic in the domestic pressure vessel industry. For example, Dongfang Boiler Factory and Lanzhou Petrochemical Machinery Factory have introduced foreign TIG, c02 and MMAW inverter welding machines, which have been used for all-position welding of boilers and pressure vessels.
During the “Seventh Five-Year Plan†period, all boiler plants imported full-position automatic welding machines from Sweden and other countries. For example, boiler factories in Shanghai and Wuhan introduced KOMESMA 800, 1600 and other types of membrane water wall special welding machines from Germany. In the early 1990s, a multi-head submerged arc automatic welder and a multi-headed MAG automatic special welder were developed in China. At present, many domestic boilers have used domestically produced water-cooled wall welders [12]. In 2002, domestic special sets of welding equipment increased by 67.63% compared with the same period of last year [9].
Most of the modern welding robots are flexible automation workstations or welding production lines. They are composed of welding robots, welding power supplies, welding process equipment, loading and unloading robots, and offline CAD simulation programming. The system is controlled by a microcomputer. According to relevant reports, the total value of global robot production in 2002 exceeded $8 billion, and is expected to rise at a rate of 14.7% per year. As a typical program control flexible welding system, arc welding robot has the advantages of high efficiency and stable quality, and is highly valued in the field of pressure vessel welding [13]. Flexible welding robot workstations will gradually be promoted and applied in China as their prices continue to decrease, representing a development direction of microcomputer automation control technology for complete sets of welding equipment.
In summary, submerged arc automatic welding machine, gas shielded welding machine and inverter welding machine have a good application prospect for the pressure vessel industry. Specialized automatic welding equipment is gradually being applied to various boiler and pressure vessel manufacturing industries. In addition, some process equipment improvements, such as hydraulic heads and simple assembly equipment, universal welding turret, hydraulic slag welding plate hydraulic clamping device and small diameter cylinder longitudinal seam automatic welding device, etc. To a large extent, the automation of pressure vessel welding is also improved. 1.2 automated welding method
For the assembly and welding of pressure vessels, the materials currently used for production mainly include carbon steel, low alloy steel and high alloy steel. Commonly used methods are manual arc welding, electroslag welding, gas shielded welding and submerged arc welding, etc., which are suitable for welding pressure vessels of different materials and thicknesses. As a traditional welding process, arc welding has limited improvement in welding factor and welding heat input. The key to improving the automation level of arc welding lies in the level of mechanical automation of arc welding equipment. At present, although manual arc welding is still applied to the welding of pressure vessels in a small amount, it is gradually reduced due to its low degree of automation, low production efficiency, and poor welding quality.
Gas shielded welding has the advantages of arc heat concentration, small molten pool, fast crystallization and no slag in the welding process, which is beneficial to achieve full position and thin plate welding, and can facilitate the mechanization and automation of the welding process. Co2 is a high-efficiency welding method. It is divided into solid core wire and flux cored wire CO according to different welding wires. Gas shielded welding is currently widely used in developed countries. Domestic applications and promotion began in the 1980s. Based on CO, gas shielded welding single-sided welding double-sided forming technology [14], has positive significance for the promotion of C02 gas shielded welding in pressure vessels; research and application of flux cored wire and C02 gas shielded welding [15] Low cost, high efficiency automatic welding in the manufacture of spherical tanks. China's oil and gas seventh construction company introduced a full-position automatic welding technology for ball-and-core flux-cored wire from the United States [16]. The welding deposition efficiency is high, the speed is fast, and the welding conditions are improved. TIG welding technology is mainly used for pressure vessels with high sealing performance and mechanical properties. Since pulsed TIG welding has a wide current adjustment range, the arc energy can be precisely controlled and the distribution can be precisely controlled by the adjustment of the pulse parameters. Accurate control of volume and shape enables better all-position welding of pressure vessels.
The submerged arc automatic welding began to develop in the late 1940s. It is the main process for the welding of pressure vessels. It is applied to the welding of welded plates, the joints of the barrel joints, the joints between the barrel joints, etc. Automation and mechanization become a reality. As early as in the 1980s, there were many researches on the principle of automatic arc welding control in the world. The focus of people's attention was on how to improve the automation level of arc welding, and enhance the accuracy and efficiency. However, most of the domestic submerged arc automatic welding control systems still use simple analog circuits, and the overall performance still needs to be further improved.
Surfacing technology is mainly used for welding thick-walled pressure vessels, including automatic welding of twin-wire automatic arc welding, double-wire indirect arc automatic surfacing, single-wire or double-wire lateral constant-speed oscillating gas protection, automatic surfacing, with buried Automatic arc welding and double hot wire plasma surfacing. Due to the shallower and more uniform penetration of the base metal, the SAW has a lower requirement on the surface quality of the workpiece, and is internationally known as the main method for surfacing the inner wall of the pressure vessel. With the advancement of technology, a high-speed strip surfacing method (HSW) has recently appeared, which is compared with the submerged arc surfacing (SAW) and the electroslag surfacing (ESW). Fine grain size and low impurity content are... a more economical surfacing method [17].
Narrow gap welding technology is a welding method developed to solve the problem of thick plate welding, improving the performance of welded joints and saving welding materials. The welding wire is small, the molten pool is small, and all-position welding is possible, which is easy to automate the welding process. This technology adopts flux with good slag removal property and adopts wire correction mechanism. At present, some alkaline sintering flux with good slag removal property has been developed abroad, such as 86435/DIN8857, KBl20, etc.; in Japan, when narrow gap welding is used, The arc mechanical swing device composed of the wire bending plate makes the arc generate waveform; Kobe Steel has developed the "torque arc" method, which uses two wires that are twisted each other, without special wire swinging device. Domestic research on this, Wu Xiaolong [18] and others for the welding of thick-walled containers using double-wire narrow-space submerged arc welding, the device has pre-weld preset parameters, automatic stable welding voltage, welding current and welding speed. Closed loop control and automatic height and lateral tracking system for automatic welding of welds.
Lasers are hailed as the discovery of semiconductors in the 20th century. Laser welding is a special method that uses a focused high-power density laser beam as a heat source. It has a very high energy density and can be soldered due to its small HAZ area. Almost all metals can be welded all the way. At present, the method is mainly applied to high-tech fields such as automobiles, aviation, electronics, and precision instruments. With the advent of high-power lasers, laser automatic welding technology is developing for thick-wall welding. However, due to its high price and cumbersome equipment, the application prospects in the pressure vessel industry are not optimistic.
According to statistics [19], in 2001, the domestic production of various types of welding consumables was about 1.3 million tons. Except for exports, the actual consumption was about 1.25 million tons. Among them, the welding treaty is 1 million tons, accounting for about 77% of the total amount of welding consumables; the submerged arc welding material is about 120,000 tons, accounting for 9.2%; the gas-protecting solid welding wire is about 140,000 tons, accounting for 10.8%; The welding wire is about 16,000 tons, accounting for only 1.2% of the total welding consumables. In Japan, the production of welding rods has decreased year by year in the past 20 years, mainly developing automated and semi-automatic welding consumables. For example, in 1999, the production of welding rods only accounted for about 19% of the total amount of welding consumables, submerged arc welding materials accounted for 12%, gas-shielded solid welding wires accounted for 38%, and flux-cored welding wires accounted for 31%. Its automatic welding consumables are higher than the existing level in China.
2 software factors
2.1 Welding control technology
The main hallmarks of modern welding automation are the intelligence of the welding process control system, the flexibility of the welding production system and the integration of the welding production system. In recent years, welding automatic control technology has developed rapidly at home and abroad, and it has also received high attention. It has become an independent branch of welding technology.
Due to the application of programmable controllers, microprocessors, automatic control technology, computer technology, etc. in welding production, the welding automation technology in the pressure vessel manufacturing process has been greatly developed. Modern high-precision automatic control systems, such as optimal control systems, adaptive control systems, and self-learning control systems, have been used in industrial production. In the process of specification parameter control of all-position TIG pipe welding machine, the literature [20] changed the traditional TIG pipe welding machine welding current parameter segmentation control mode, using single-chip technology, through the parabola fitting algorithm to form a smooth welding current specification parameter curve, The smooth adjustment of the welding current of the full-position TIG pipe welder is realized, and the arc energy and its distribution are precisely controlled. The feasibility of this control technology for all-position welding of pressure vessels is proved by experiments.
Weld seam tracking is an important part of the welding automation control system, and has a profound meaning for the automation of welding in the pressure vessel production process. At present, the application of the seam tracking system mainly includes contact type and non-contact type.
The contact tracking system is divided into a probe type and a mode tracking system. The probe type contact sensor detects the position of the weld by the guide rod or the guide wheel in front of the welding torch, and has the advantages of simple structure, direct operation and strong anti-interference ability. However, when welding thick plate pressure vessels, the spot welding joint will cause the sensor to block, and the ellipticity of the cylinder will cause the sensor to move axially. The model tracking system can better solve this problem. It uses the principle of synchronous motion _2lj to maintain the distance between the contact tip and the weld by horizontal tracking, longitudinal tracking and fine-tuning system. It can be applied to the welding of cylindrical pressure vessels. To achieve automation of circumferential seam welding. However, the measurement accuracy of the sensor may be affected by the uneven processing of the groove and the weld.
Non-contact tracking systems are divided into ultrasonic, acoustic emission, photoelectric, arc sensing and visual sensing tracking systems. The non-contact tracking system is closely related to other disciplines. At present, domestic and foreign scholars have carried out research on different contents and degrees. The direct arc tracking system directly extracts the weld position deviation signal from the welding arc itself, and has good real-time performance. It does not need to attach any device to the welding gun. The flexibility and accessibility of the welding gun movement is the best, especially in line with the low cost automation of the welding process. Requirements. At present, the main method adopted by foreign developed countries is to measure the welding current, the arc voltage u and the wire feeding speed. To calculate the distance H between the workpiece and the wire, and apply fuzzy control technology to achieve weld tracking. However, since arc-type tracking can only obtain local information of the weld zone, important information such as torch height matching, root gap and weld form cannot be obtained, thus limiting the scope of use. Literature [22] proposed a new type of weld seam tracking system, which can meet the needs of pressure vessel manufacturing by conducting weld seam tracking test on weld bevel on Mz-1000 submerged arc welder. The system adopts non-contact ultrasonic tracking sensor, which has the characteristics of strong anti-interference ability and low cost, and has good application prospects in low-cost welding automation. According to the survey conducted by the Japan Welding Technology Society, the sensors that lead the future weld tracking are mainly optical sensors, among which the visual sensors are the most attractive. The CCD-based weld seam tracking system can be used in various welding methods and equipment such as submerged arc welding and plasma arc welding. However, in view of the harsh application environment of the welding process, the sensor is subject to interference from arc, high temperature, smoke, splash, vibration and electromagnetic field, so that the accuracy, anti-interference performance and sensitivity of the sensor are affected to varying degrees. Although a variety of automated tracking methods have been developed to date, most are still in the laboratory stage. At present, there are researches on electron beam welding automatic penetration and weld seam tracking, co2 gas shielded weld seam tracking, and laser ultrasonic induction automatic tracking [23]. With the development of computer information technology and the research of new sensing methods, the seam tracking technology will be widely used in the pressure vessel industry, thereby further improving the automation and intelligence of the pressure vessel welding process.
2.2 Artificial intelligence technology and expert system
The introduction of artificial intelligence technology into welding equipment has formed an intelligent control system for welding equipment. The representative welding processes in this field are fuzzy control systems, Shenjing network control systems and welding expert systems. The development of intelligent welding equipment based on high-performance wave-controlled welding power supply is the development direction of welding equipment in the future. For example, Ill-Soo Kim and Joon-Sik Son et al. [24] introduced artificial neural networks into the GMA welding method to predict the width of the weld zone and broaden the application field of GMA welding. Future weld seam tracking technology will have intelligent fuzzy control and neural network penetration into the weld tracking control to enhance the accuracy of nonlinear system control.
The establishment of expert system in welding engineering has become the research foundation of intelligent welding equipment. The wide application of flexible intelligent welding automation technology with welding robot as the core has become the recognized development direction at home and abroad. The expert system research in the field of welding began in the mid-1980s. The earliest report was the welding material selection system Weldselector [25] jointly developed by the American Kordo Mining Institute (CSM) and the American Welding Research Institute (AWl). At present, some industrialized countries such as the United Kingdom, Japan and Germany have carried out this work in succession, mainly focusing on process development, defect analysis, material selection and equipment selection. Among them, about 70% of the process selection and process development software. In the real-time control of the welding process, the Camtech 100 and Adaptitech 1000 developed by Adaptive Technologies of the United States can perform parts positioning, welding operations and quality inspection functions. The system can automatically adjust the welding path based on information such as light, temperature and arc from the sensor. Line energy, wire feed speed and swing parameters, and optimize multi-pass welding parameters. The "welding parameter control expert system" developed by NKK Japan can give the optimal welding parameters and control the welding equipment to ensure constant penetration and welding height. At present, a variety of commercial products have been formed in the international market, and there are broad development and application prospects.
After years of efforts, the research and application of welding software in China has achieved gratifying results. Tsinghua University, Harbin Institute of Technology, Shanghai Jiaotong University, Tianjin University, etc. have carried out a series of research in this field, and have developed different types of application software. Among them, Tsinghua University developed the "general arc welding process expert system QHWES" because of its It is unique in its ability to adapt and redevelop. The pressure vessel welding expert system developed by Lu Bo et al [26] has achieved good results in Jinxi Chemical Machinery Plant and Jinzhou Heavy Machinery Plant.
3 Outlook
At present, pressure vessel welding automation technology has been widely used internationally. The development of pressure vessel welding technology in China mainly goes through the following stages: manual arc welding stage, automatic welding application stage, welding low-alloy high-strength steel container stage, and pressure vessel large-scale and new welding technology development stage. In the manufacturing process of pressure vessels, a lot of experience has been accumulated and the level of welding automation has been developed. However, compared with the industrialized countries in the world, there is still a certain gap in the overall technical level. In order to maximize the automatic welding of the pressure vessel production process and improve the manufacturing level of the pressure vessel, the application research of its welding automation technology needs to be further improved.
(1) Hardware:
Most welding processes require a power supply with specific static and dynamic performance. Researching new electronic power supplies that are suitable for welding process control is the development direction of current welding equipment. The automatic welding equipment used in pressure vessels has developed rapidly in the world. As a comparison, the large-scale use of manual arc welding in welding production in China is low, and the mechanization level and automation level of the welding machine are low. Domestic research and development of high-performance welding power supply, automatic welding consumables (flux cored wire, solid cored wire) should be intensified; intelligent improvement of traditional welding process equipment, such as automatic adjustment of welding method setting parameters represented by submerged arc automatic welding Functions such as automatic adjustment device, weld seam tracking system and automatic program control system to meet the basic requirements of all-position welding and intelligent control of pressure vessels; new welding methods (narrow gap welding, electron beam welding, laser welding and On-site assembly automatic welding technology, etc.) integrates micro and macro technology, providing a new way for the field of pressure vessel automatic welding technology. Welding robot is a typical mechatronics product that combines robot with modern welding technology to complete the welding task. Its development and utilization can be regarded as an important symbol of welding automation. Using advanced computer technology, automation control technology and information technology to transform a large number of old equipment, designing an advanced welding control system with high degree of automation, high efficiency and high precision is an important direction of welding automation research.
(2) Software:
Improving welding quality, improving working conditions and increasing labor productivity have become a strong desire of all welding workers. The welding process is a chemical and physical metallurgical process carried out under specific energy conversion conditions. Different welding methods mean different metallurgical processes and energy conversion processes. Welding process control is to control the energy conversion conditions by welding the characteristics of the metallurgical process. The control requirements of the pressure vessel welding process are from simple to complex, the function is getting stronger and stronger, and the precision is getting higher and higher, which reflects the progress and trend of productivity development. As a specialized science, automatic control is constantly evolving, from classical linear control to modern nonlinear control, even fuzzy control, artificial intelligence expert system control, etc., which plays a big role in modern mechanical systems. Therefore, it is necessary to study the dynamic behavior characteristics of the welding process, to find out various boundary conditions that determine the quality of the welded joint or weld, to find a more reasonable control variable, and to establish a control model and control method that can ensure the welding quality.
At present, the weld seam tracking system, artificial neural network and welding expert system are gradually applied to the welding automation process of pressure vessels, which has become an important direction for the development of pressure vessel welding automation technology. Study welding process parameters and detection sensing methods and sensors, especially those that directly or indirectly reflect the state variables of the dynamic quality of the welding process. Among them, the research of CCD visual weld seam tracking system has great application prospects.
In general, pressure vessel welding automation technology involves the comprehensive utilization of multiple disciplines. With the development and application of new materials for pressure vessels, the comprehensive mechanical properties are gradually improved. It is necessary to further develop new welding equipment and process methods, and fully realize welding process control by means of other disciplines such as electronic information technology, computer technology and automatic control technology. The intelligence of the system, the flexibility of the welding production system and the integration of the welding production system. This increases the level of automation of the pressure vessel to a new level.
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About the author: Lei Yi (1960), male, associate professor, mainly engaged in material molding and control engineering, non-destructive testing research, mailing address: Shandong Dongying Petroleum University (East China) Mechanical and Electrical Engineering College.
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