Selection of welding methods for carbon structural steel and low-alloy high-strength steel

The important structures currently under construction of the Three Gorges Project mainly include power station penstocks, hydraulic turbine seats, and ship gates. The quality of the construction process of the turbine seats is undertaken by foreign companies. The other two are contracted by domestic manufacturers and construction units. Most of the gates are manufactured by well-known domestic manufacturers. The shipyard assumes that the welding process is relatively mature, and it is not difficult to prepare and process the hull; because the material is low-alloy steel with a lower strength level (Q345), the main problem in the future is how to improve efficiency during site installation ,reduce costs. The production and installation of penstocks will become the main contradiction. In the early stage of the project, there are 14 penstocks, about 22500t. Due to the complex materials (16MnR in the upper section and 610U2 low-alloy high-strength steel in the lower section), the plate thickness is large (the maximum thickness is 58mm), especially The pipe diameter is large (φ12499mm) and the installation location is complicated, so it is different from the production and installation of conventional pipes. This time I was fortunate to participate in the “Three Gorges Project Metal Structure Welding Technical Expert Consultation Meeting” organized by the Engineering Construction Department of the Three Gorges Development Corporation. I benefited a lot. However, due to the short period of time, the work and test data of several contractors were not available before the meeting. Study carefully, so some opinions are not allowed to be expressed. Now I will explain some points. 1 The material selection idea and practice of the penstock of the Three Gorges Project is successful. The upper section uses 16MnR and the lower section selects the 60kg grade 610U2 of Japan’s NKK. They are all steels with good weldability, especially the Japanese 610U2, which belongs to the low-carbon quenched and tempered steel. Welding crack-free steel (CF steel) is characterized by low carbon content (≤0.09), low total carbon equivalent (CEQ2=0.39%), and low crack sensitivity coefficient (PCM≤0.19). Due to the use of new technologies in the steel production process, such as online waste heat quenching, when the carbon equivalent is not large, its hardenability is increased, and a variety of trace elements are added, so it can improve its plasticity and toughness while ensuring high strength (At -40℃, its AKv> 200J or even more than 300), which increases the possibility of obtaining high-quality welds while reducing weight. 2 Starting from the welding design, the principle of selecting welding consumables 16MnR is the most widely used steel grade for welding structures. Generally, the welds are designed according to the same strength. The domestic welding materials and welding methods of this steel grade are very mature. Regarding 610U2 type low carbon quenched and tempered steel, its weldability is also good originally, but if it is not handled properly during welding, defects such as cold cracking in the fusion zone and embrittlement and softening in the affected zone may also occur. In special cases Particularly in site installation, there are certain requirements for welding heat input and preheating. For welding non-cracked steel grades, low-H or ultra-low-H welding consumables are used, and no preheating is allowed in the environment of plate thickness below 50mm or above 0℃. This steel smelting technology is superior, and its mechanical indicators are outstanding, especially in the impact performance of the yield ratio (such as the 610U2 selected this time), but when welding, if the impact performance of the weld is required to meet the requirements of the base metal, this Obviously it is inappropriate. The mechanical index of weld design is mainly based on work requirements, not lower than the guaranteed value of the base material mechanical index, and a proper margin is left. The actual measured value of the base metal should not be used as the standard, sometimes in order to improve The plastic toughness of the weld can appropriately reduce the design strength index of the weld. Practice has proved that low-strength matching welds can often improve weld toughness and crack resistance. 3 Regarding the welding method The main processing technology of penstocks is welding. In principle, manual arc welding, submerged arc welding, gas shielded solid wire and flux-cored wire welding, self-shielded flux-cored wire, etc. can be selected, and should be based on construction conditions and structure Comprehensive consideration of form, efficiency, cost accounting, and welding quality, the selection principle should be: Under good working conditions, low-cost completion of high-quality welds. The basic conclusions of this demonstration meeting are: the implementation of automatic solid wire gas shielded welding in factory prefabrication; manual welding for site installation; development of all-position automatic welding equipment. Although most participants can accept this conclusion, there are still some doubts. (1) From the perspective of ensuring welding quality, the welding metallurgical process is complete (such as controlling the weld composition and H value content through alloying); good protection; welding heat source energy is concentrated, easy to control heat input and welding deformation; welding can be controlled by welding equipment Quality, etc., welding methods with these capabilities are the best. For these two steels, especially 610U2, gas shielded welding should be the first choice, because the main problem of welding quality of low-alloy high-strength steel is welding cracks and embrittlement and softening of the heat-affected zone. The biggest feature of gas shielded welding is low H welding and easy welding. Control heat input, such as measuring the average value of diffusion H content: acid electrode 21 for manual arc welding. 9. Alkaline electrode 3.15; CO2 protection welding 1, MAG welding 0.03, submerged arc welding 2.17, unit: ml/100g. Welding rust resistance test: submerged arc welding produces pores at 0.3g/10mm, while CO2 welding produces pores at 1g/10mm. Therefore, C02 welding is a kind of low H welding. In addition, the energy density of gas shielded welding is large. Under normal specifications, its heat input is only 1/2 to 1/3 of manual welding (especially pulsed MAG welding) and its deformation is small. This is extremely important for high-strength steels with certain thermal sensitivity. important. The advantages of gas shielded welding are high efficiency and low cost because of its high melting efficiency, no need to clean the slag to replace the welding rod, small groove, less deposited metal, and less groove processing. (2) Gas shielded welding is divided into solid wire and flux-cored wire. They have some common characteristics, such as heat concentration, high efficiency, and differences (see later). Gas shielded welding has become the main process method for welding carbon steel and low-alloy high-strength steel. The steel used in China’s shipbuilding industry is basically similar to the 16MnR and 610U2 of the Three Gorges, and the proportion of molten electrode gas protection has reached more than 60%. It accounts for more than 50% of gas shielded welding), and other industries such as petrochemical, electric power, machinery, etc. are basically the same. It shows that this welding method is the housekeeping method of metal structure manufacturing enterprises. 4 Regarding the efficiency and quality of gas shielded welding, gas shielded welding, especially CO2 welding, has certain limitations. On the other hand, the promotion of gas shielded welding is a systematic project. From equipment and welding materials to welding seam design, all must adapt to new methods. . Therefore, it is necessary to formulate regulations and reward systems when promoting. 4. 1 The efficiency of gas shielded welding The efficiency of a welding method is determined by its penetration depth, energy density, melting speed, deposition efficiency and other factors. In addition, the groove type of the welded workpiece and its filling amount are also Directly affect efficiency. Although the heat sources of manual welding and gas shielded welding are both arcs, because of the different arcing rates and different arc zone media, they will affect the penetration depth and energy density, resulting in great differences in melting speed and deposition efficiency. ————————————————– —————————— Arcing rate, deposition speed, deposition efficiency, average penetration ——– ————————————————– ———————- Hand welding 30% 35~50g/min 55% 3mm CO2 45% average 90g/min 90~95% 6mm MIG/MAG> 50% 60~140g/min 96~99% 4~6mm cored wire>50% 140~200g/min 83~87% 4~6mm —————— ————————————————– ———— Judging from the deposition speed and deposition efficiency in the table, the amount of metal deposited on the weld per unit time of gas shielded welding should be more than twice that of manual welding. When promoting gas shielded welding, the increase in actual efficiency is often not up to the theoretical data. The important reason is that the groove pattern of the weldment has not been changed accordingly; in addition, the gas shielded welding equipment materials are not matched or used improperly, which greatly increases In addition to the auxiliary time, according to the statistics of China’s shipyards, gas shielded welding consumes 10-15kg of welding material (up to 50kg in Japan) and 6-7kg for manual welding. 4. 2 The quality of gas shielded welding. Gas shielded welding can not only be used for welding low-alloy high-strength steels, but also can be said to be the preferred method of welding. This is not only because it is at least twice as efficient as manual welding, but also because it is the easiest to ensure the welding quality of high-strength steel. As mentioned in 1, the material selection is very good, the carbon equivalent and the crack sensitivity coefficient are very small, and the weldability is good, which does not require a lot of complicated processes and can guarantee the quality. Of course, for such a cross-century project, it still needs to be foolproof. 16MnR belongs to the C-Mn series of hot-rolled normalized steel, and 610U2 belongs to the ultra-low carbon multi-element quenched and tempered steel, which is generally used in the welded state. The main problem of the quality of these two types of steel welded joints is to ensure the high comprehensive performance of the weld, prevent the embrittlement and softening of the affected zone, and ensure that the fusion zone and the heat-affected zone do not crack and have a certain degree of toughness. Since 610U2 is a heat-treated strengthened steel used in the welded state, how to ensure the overall performance of the weld and the toughness of the heat-affected zone at the same time, although it is not very difficult in practice, it is necessary to ensure that the weld metal is chemically selected when selecting welding materials and processes. Composition, select the appropriate heat input and the appropriate preheating and interlayer temperature to get the appropriate t8/5 to ensure that the part between AC1 and AC3 is affected by the heat to obtain the appropriate organization (the most acicular ferrite Body, the least MA component) and product granularity. In addition, the H content should be controlled to further prevent the occurrence of cold cracks. Low-C quenched and tempered steel, especially CP steel, has extremely low C content, and only low-C martensite can be formed in the heat-affected zone. Since the Ms point is high, self-tempering can occur, so the cold cracking tendency is not large, and because The content of C and S are low, and the Mn/S is large, so the hot cracking tendency is small. As long as the selection of the process is paid attention to, the welding quality can be guaranteed whether it is manual welding, submerged arc welding, solid or flux-cored gas shielded welding. It can be seen that the selection of welding consumables can ensure the composition of the weld, but it is more important to select the appropriate process. It should be said that gas shielded welding is the most suitable method for welding the above two kinds of steel, because it has concentrated heat, easy to control heat input, and is a low-H welding method; under the same allowable heat input, its welding efficiency is great. Compared with manual welding, the welding deformation is small, and it is not easy to cause stress concentration and correcting man-hours. But why has it not been recognized in some units so far? The first reason is the limitation of the method itself: gas shielded welding has inert gas non-melting electrode (T1G ), CO2 gas solid welding wire and flux-cored welding wire, oxidizing mixed gas solid and flux-cored welding wire. Except TIG, it can be used for these two steels. Compared with manual welding and submerged arc welding, solid wire shielded welding is not a gas-slag combined protection, and the composition is mainly adjusted by welding wire. In terms of metallurgical reaction, it is simple, so in order to ensure the quality , The smelting special matching welding wire is very important; in addition, because the gas plays a protective role and participates in the thermal reaction, it has many advantages (such as the formation of charged ions and compressed arcs, increased arc energy density, and low H) but also its disadvantages (such as increased C, Therefore, in the welding wire commonly used for welding carbon structural steel and hot-rolled normalized low-alloy steel, reduce the C content and increase the Mn and Si content to ensure the metal composition and performance of the weld, especially the toughness. In addition to the limitations of gas shielded welding itself, China’s matching welding wire is extremely imperfect (H08Mn2Si and H08Ma2SiA), which is the reason why many departments cannot achieve the desired weld performance after gas shielded welding. Especially when welding low-C quenched and tempered steel, it is necessary to select a suitable welding wire for the steel type. In addition, the selection process is unreasonable, the equipment is improperly used, the gas is improperly selected and handled, and the workers switch to manual welding. Naturally, there is a certain resistance to the promotion of gas shielded welding.

    5 Concerns about toughness and toughness stem from the low-stress failure of welded joints, and the reason for low-stress failure is the plastic-brittle transition of the material at a certain temperature and the expansion of defects in the joints. Therefore, it is used separately from the quality assurance system. Impact toughness and fracture toughness indicators to control the brittle failure caused by the above two reasons. Although the examples of brittle fracture in the past were all produced when there were defects, it is difficult to detect small defects in the joints, and the fracture toughness has been tested.

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