Current situation and development trend of welding

2022-08-12
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Current situation and development trend of welding technology in boiler industry

after decades of development, especially the rapid development in recent 5 years, China's boiler industry has made brilliant achievements. According to statistics, there are more than 1400 boiler manufacturers with a boiler manufacturing capacity of nearly 100million kW. Various boiler technologies have developed rapidly, such as supercritical and ultra supercritical boilers, large CFB boilers and steam gas combined cycle boilers. Large power generation boilers are developing towards large capacity, high parameters and high efficiency. In just a few years, Chinese boilers have increased from 300MW units to 600MW and 1000MW units, the steam pressure has developed from subcritical to supercritical and ultra supercritical, and the main steam temperature has developed from 540 ℃ to 566 ℃ and 600 ℃. China's boiler design and manufacturing technology has reached the world's advanced level

welding is one of the most critical process methods in boiler manufacturing. The level of welding technology in the boiler industry directly affects the level of China's power generation equipment manufacturing industry. For a long time, the boiler manufacturing industry has been committed to the development of boiler welding technology and improve the level of boiler manufacturing technology. Especially in the past five years, the development of boiler technology has further accelerated the development of welding technology. At the same time, the improvement of welding technology has also promoted the development of the boiler industry. The following focuses on the development status of boiler welding technology, and combined with the later boiler development trend, talk about the development direction and Prospect of boiler welding technology

development status of boiler welding technology

over the years, domestic boiler plants have been actively carrying out technological transformation, introducing new equipment and implementing advanced new welding processes to adapt to the changes of boiler capacity, parameters and furnace type, and meet the manufacturing requirements of complex boiler components and new boiler materials. The following focuses on the development status of welding technology for key components of boiler

1. Welding process status of key components of boiler

the main key components of utility boiler include steam drum, header, membrane wall and serpentine tube (superheater, reheater and economizer). The current situation of welding technology of various components is as follows:

(1) welding of boiler drum

the drum plays the role of steam collection, water collection and steam water separation in the boiler, and is the key core component of subcritical boiler. The steam drum is composed of cylinder section, head, downcomer pipe seat, steam water connection pipe seat, water level gauge pipe seat and steam drum internals. Its main welds include the welding of cylinder section longitudinal seam, circumferential seam, downcomer pipe seat, steam water connection pipe and other accessories

due to the special position of the steam drum in the boiler, its welding quality has been highly valued by the manufacturer and generally concerned by the users of the power plant. China's national regulations put forward quite high requirements for its welding and inspection, so the welding process must ensure the welding quality. At the same time, due to the large wall thickness and size of the drum, the welding workload is quite large. Therefore, boiler manufacturers need to actively develop efficient welding processes and methods

① longitudinal and circumferential seam welding of boiler barrel

at present, the drum barrel is mainly welded by plate, and the forming process includes pressing and rolling processes. According to different barrel forming processes, the welding methods, processes and welding grooves adopted by various factories are also different. At present, electroslag welding, electrode arc welding + conventional submerged arc welding and electrode arc welding + narrow gap submerged arc welding are used in the longitudinal and circumferential seam welding of boiler drum. The welding groove and process are shown in the table

the longitudinal and circumferential seams of the pressed cylinder section of domestic 300MW and 600MW boilers mostly adopt narrow gap submerged arc automatic welding, which is equipped with a welding anti movement roller frame and a welding operator to form a longitudinal circumferential seam welding workstation. The welding process is widely used in boilers because of its small filling amount, good joint toughness and good hydrogen induced crack resistance. When welding longitudinal joints, pay attention to the change of groove size caused by welding deformation, and take measures in selecting groove angle and preventing deformation

narrow gap submerged arc welding can be single wire or double wire, and the efficiency of double wire is 70% - 80% higher than that of single wire. However, due to the high requirements of double wire welding on groove processing accuracy, weld tracking, flux process performance and welder operation skills, it is rarely used. Dongfang Boiler has fully mastered this technology and used it on boiler drums and heavy vessels

before the 1980s, all boiler plants used electroslag welding technology to weld the longitudinal joints of boiler drum joints below 200MW. The equipment is 3-wire electroslag welding imported from the former Soviet Union and copied domestically. Later, due to equipment scrapping, domestic production was stopped and gradually eliminated. Some manufacturers continue to use it because of its high welding efficiency, supporting forming equipment and other factors, and the comprehensive mechanical properties of electroslag welded joints have been better improved by improving welding materials, external magnetic field and heat treatment process

② welding of downcomer socket

the downcomer socket of boiler drum of large capacity unit is usually made of 15CrMo, 20MnMo, 13MnNiMo54 and other low-alloy steel, and each drum has at least 4 downcomers. Typical boiler downcomer socket and its groove form are shown in Figure 1. The commonly used welding methods are electrode arc welding and submerged arc automatic welding, in addition to flux cored wire semi-automatic gas shielded welding

Figure 1 welding groove of boiler downcomer

before the 1990s, manual arc welding was widely used in boiler plants in China. The amount of welding is quite large. For example, the welding rod used for the fillet weld of 300MW boiler downcomer is 120~140kg, which requires two welders to weld symmetrically for about 3 days. Moreover, it is difficult to remove the welding slag in the deep groove, resulting in welding defects. The preheating temperature of alloy steel welding is more than 150 ℃, and the welding labor intensity is quite large

since Beijing Jingyi company developed the saddle submerged arc welding equipment in 1993, it has been successively used in Shanghai Boiler, Dongfang Boiler, Harbin boiler, Wuhan boiler and other manufacturers, greatly improving the welding efficiency and welding quality. Now it has been widely used in the boiler industry and used for the welding of other similar structures, as shown in Figure 2. Submerged arc welding of downcomer saddle generally adopts numerical control saddle cutting machine to prepare A-type groove on the barrel, and preheat by electric heating or electromagnetic induction heating. The welding material must take into account the flow of molten pool of saddle uphill and downhill welding

Figure 2 saddle welding

③ large pipe seat welding

pipe diameter on boiler drum φ Generally, there are as many as 150 pipe sockets ≥ 108mm, and the welding workload is also quite large. Moreover, the boiler regulations require that all joints adopt the process method of full penetration structure and full penetration, and undergo 100% ut flaw detection. Therefore, the groove form shown in Figure 3 must be adopted, and the argon arc welding backing welding process must be adopted to ensure the root penetration. Each factory adopts different welding methods, including manual arc welding, semi-automatic gas shielded welding and saddle submerged arc welding. Saddle submerged arc automatic welding adopts φ 1.6~ φ 2.0 welding wire, the welding efficiency is more than 50% higher than that of manual welding, and the qualification rate is 98%. This method has been widely used in the boiler industry, but manual arrangement is required between weld beads

at present, some boiler plants have realized the new process of inner hole automatic argon arc welding + saddle submerged arc welding

④ the welding of Internals and other accessories has not yet realized mechanical or automatic welding, and semi-automatic gas shielded welding and electrode arc welding are mainly used

Fig. 3 welding groove of steam drum socket

(2) header welding

header is also one of the key pressure bearing parts in the boiler, which plays the role of steam collection or steam separation, water collection or water separation in the boiler. It is often arranged outside the furnace to connect the water-cooled wall, reheater, superheater and economizer in the furnace or flue. Therefore, its materials change with the use temperature, including carbon steel, alloy heat-resistant steel, high alloy heat-resistant steel, and even stainless steel. Header welding mainly includes barrel circumferential seam welding, large pipe seat welding and heating surface pipe seat welding. Their respective welding methods are as follows:

① when carrying out barrel circumferential seam welding, when φ< 190mm or wall thickness δ< When 14mm, manual argon arc welding + electrode arc welding are usually used; When φ When ≥ 190mm, manual argon arc welding + electrode arc welding + submerged arc automatic welding are usually adopted

② the combined welding process of manual TIG welding + manual arc welding or flux cored wire gas shielded welding is generally adopted for the welding of header large pipe base

③ heating surface socket welding. For short pipe joints with full penetration structure, inner hole TIG welding + arc welding can be adopted; Electrode arc welding is adopted for short pipe joints and long pipe joints that are not fully welded

header is the difficulty of welding in the boiler industry. Its welding structure is complex, the variety and specification of materials are diverse, and the welding requirements are high, so it is difficult to realize automatic welding. For many years, China's boiler industry has been committed to the research of automatic welding process and equipment, and has successively developed or introduced superheated wire TIG welding, welding robot workstation, robot welding, etc., which have not been successfully applied to production. In recent years, with the development of electronic technology and welding technology, some achievements have been made, such as circumferential narrow gap hot wire TIG, argon arc welding of inner hole of header short pipe seat, fine wire submerged arc welding of header pipe seat, etc

the breakthrough of hot wire TIG welding technology with narrow gap of circumferential seam can well solve the backing welding of header circumferential seam and welding to 8~3. Brake pedal force: 390n, 340n10mm, so as to directly use submerged arc welding, which solves the welding task that can be completed by the previous two methods, and has the following advantages:

① good welding quality, and the qualified rate is more than 99%

② the consumption of welding materials is low, which is 1/3 of the original

③ adapt to the welding of high alloy materials that are difficult to weld

the new process of narrow gap hot wire TIG welding (as shown in Figure 4) solves the long-term problems of spot welding and rotation of the barrel, and the channeling and tracking of the barrel, which will become the direction of header and pipe welding in the future

Fig. 4 narrow gap hot wire TIG welding

(3) membrane wall welding

the furnace and flue of the boiler adopt the fully welded sealing structure, and the pipe + flat steel is often used to weld the membrane wall. According to the different media in the tube, the tube panel is divided into water-cooled wall and wall wrapped superheater. The most distinctive feature of membrane wall welding is the welding of smooth tube + flat steel. Its welding volume is quite large. The weld length of a 300MW boiler is 180000 M. therefore, the goal of membrane wall welding is high efficiency and automation

the world's advanced membrane wall automatic welding technology mainly includes MPM welding technology developed by Mitsubishi Heavy Industries of Japan and saw automatic welding technology developed by bobcock company of Germany. China's major boiler plants began to introduce automatic welding technology in the late 1980s. Due to different channels of introduction, different welding processes have been used and developed. No matter which welding technology is adopted, it has reached the world's advanced level. These two kinds of special welding equipment have been fully localized, especially in recent years, with the arrival of the power blowout market, the boiler industry has been equipped with a large number of membrane wall automatic welding technology

mpm welding is equipped with 12 (or even 20) GMAW welding heads, which can weld from the front and back sides of the tube panel, and a stable pulse spray transition is generated during the welding process, which ensures that the upper and lower weld formation is beautiful and the welding quality is stable. At the same time, the welding deformation of the front and back sides of the weld can offset each other, and there is basically no deflection deformation after the tube panel is welded, and its welding speed is 700mm/min

the tube panel submerged arc welding technology adopts 1.6mm welding wire and high-speed flux, which can reach the welding speed of 1200~1300mm/min. The submerged arc welding has beautiful shape and large penetration. For supercritical boiler, 9mm flat steel can not be beveled. However, submerged arc welding cannot realize the simultaneous welding of both sides, so it needs to be turned over after welding the front side. The problem is that there is deformation after one side welding, which needs to be corrected, and the workshop welding environment is poor

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