Although there are no so-called technical secrets in the welding process, there are many welding technologies, methods and processes in the actual welding process that can make the welding process easier. These technical methods are called know-how. Welding technical know-how can save time, cost and labor, and even determine the success or failure of welding. Most welding processes are based on scientific research, and some welding processes are based on actual welding experience. This chapter is a synthesis of some practical welding experience in practice.
Understanding the common welding problems and solutions in production can help solve some common welding problems. In the part of excellent design criteria, the key factors that should be considered when designing welds are expounded. Aiming at the problem of controlling welding deformation, the causes of deformation and the actual correction of welding deformation are introduced. Among other design problems, the size of fillet weld and how to avoid fracture are discussed. The concept of simple design mainly introduces some common welding application examples; The concept of advanced design discusses the elastic matching of welds and the arrangement of welded joints. Aiming at the welding problem of structural steel, some commonly used welding materials and successful experience in welding practice are emphatically introduced. In the cutting of oxyacetylene, the skills to solve welding problems are provided, and the cutting application and performance of oxygen lance and fuel rod are discussed. For the fasteners commonly used in welding structures, this paper mainly introduces the commonly used bolts and nuts and how to apply them.
First, the welding process problems and solutions
1. 1 Welding between thick plate and thin plate
1. When welding steel workpieces with GMAW and FCAW, what should I do if the workpiece thickness exceeds the maximum welding current that can be achieved by the welder?
The solution is to preheat the metal before welding. Preheat the welding area of the workpiece with propane, gas specified in the standard or acetylene torch at 150 ~ 260℃, and then weld. The purpose of preheating the welding zone metal is to prevent the welding zone from cooling too fast, so as to avoid cracking or incomplete fusion of the weld.
2. If it is necessary to weld the thin metal cover on the thick steel pipe by gas shielded metal welding or flux-cored wire gas shielded welding, if the welding current cannot be adjusted correctly during welding, it may lead to two situations: one is to reduce the welding current to prevent the thin metal from burning through, and the thin metal cover cannot be welded on the thick steel pipe at this time; Second, excessive welding current will burn through the thin metal cover. What should I do at this time?
There are two main solutions.
① Adjust the welding current to avoid burning through the thin metal cover. At the same time, preheat the thick steel pipe with a welding torch, and then weld two metal structures by using the thin plate welding process.
② Adjust the welding current to adapt to the welding of thick steel pipes. During welding, the residence time of welding arc on thick steel pipe is kept at 90%, and the residence time on thin metal cover is reduced. It should be pointed out that only by mastering this technology can a good welded joint be obtained.
3. When a thin-walled circular tube or rectangular thin-walled pipe fitting is welded with a thick plate, covered electrode is easy to burn through the thin-walled pipe fitting. Besides the above two solutions, are there any other solutions?
Yes, cooling rods are mainly used in the welding process. If a solid round bar is inserted into a thin-walled circular tube or a solid rectangular bar is inserted into a rectangular pipe fitting, the solid bar will take away the heat of the thin-walled workpiece and prevent burning through. Generally speaking, solid round bars or rectangular bars are closely installed in most supplied hollow or rectangular tube materials. When welding, attention should be paid to keep the weld away from the pipe end, which is the weak area where burning through is most likely to occur. The schematic diagram of using the built-in cooling rod to avoid burning through is shown in figure 1.
4. What should I do when I have to weld galvanized or chromium-containing materials with another part?
The best process is to file or polish the area around the weld before welding, because galvanized or chromium-containing metal plates will not only pollute and weaken the weld, but also release toxic gases during welding.
1.2 welding of container and frame structure
1. If the end of pontoon bridge or hollow structure is sealed by welding process (such as brazing), how to prevent hot air from entering the container and causing the container to burst when the weld is finally sealed?
(3) First, drill a pressure relief hole with a diameter of 1.5mm on the buoy to facilitate the circulation of hot air and outside air near the weld, then conduct closed welding, and finally weld the sealed pressure relief hole. The schematic diagram of sealed welded pontoons or closed containers is shown in Figure 2. Pressure relief holes can also be used when welding gas storage container structures. It should be noted that welding in a closed container is very dangerous. Before welding, ensure that the inside of the container or pipeline is clean to avoid flammable and explosive substances or gases.
2. When gas shielded arc welding, flux-cored wire gas shielded arc welding or TIG welding are needed to weld the screen grid, wire mesh or extended metal to the steel structure frame, the wire mesh is prone to burn-through and weld fusion during the welding process. What should I do?
① Place the nonmetallic gasket on the metal mesh or extended metal, and clamp the gasket, metal mesh and frame together. Chromium-containing or galvanized washers are not allowed, and the washers should be uncoated, as shown in Figure 3(a).
② Place a larger washer as a radiator on the upper part of the washer at the welding position. The upper gasket should have a larger hole than the lower gasket to prevent the upper gasket from being welded together. Then plug weld through the two holes of the gasket, and the weld should be at the lower part of the gasket. The operator can take some other methods to get enough heat and welding, and pay attention to prevent the surrounding screen or wire mesh from burning through, as shown in Figure 3(b) and (c).
③ Another method is to use a metal strip with a hole, align the hole with the part to be welded, place the radiator gasket, and then plug weld, as shown in Figure 3(d).
1.3 Repair of welded parts
1. Besides the common nail opener, what other methods can be used to take out damaged or rusty screws?
There are two main methods.
(1) If the installed screw will not be damaged during heating, you can add the love nut and its components with an oxygen-acetylene torch until it burns red, and then quickly quench it with water to facilitate the disassembly of the screw. In this process, several heating and cold quenching cycles may be needed.
② If the screw groove, nut or sleeve is damaged or lost, a nut can be placed on the upper part (or the remaining part) of the screw head, and then the nut and screw can be filled with metal by any welding method. This will connect the nut with the rest of the screw, and then put a wrench or pliers on the nut to quickly pull out the screw. This method is beneficial to provide a new grip point and use heat to fasten screws. The schematic diagram of removing the remaining part of the fixing screw by welding is shown in Figure 4.
2. If there is a worn crankshaft, what is the best way to repair and reinforce it by welding?
When repairing worn crankshaft, MIG, FCW or TIG methods can be used. However, in order to obtain a satisfactory bead shape, we must pay attention to the following four requirements.
① Make the direction of surfacing weld bead parallel to the crankshaft axis.
(2) First build up a weld on the lower part of the crankshaft, and then rotate the crankshaft180 to build up the next weld, which can balance the welding stress and obviously eliminate the welding thermal deformation. It should be noted that the first continuous surfacing will cause the crankshaft to warp. This surfacing process is suitable for repairing and welding roller crankshafts.
(3) 30% ~ 50% of deposited metal overlap must be maintained between the two welding passes to ensure the smoothness of the welding pass surface during processing after welding repair.
(4) When manual arc welding and flux-cored wire gas shielded welding are used, the residual flux between welding passes must be cleaned up by brush or cutting.
In addition to the above-mentioned crankshaft repair methods, a surfacing weld can be added to the crankshaft every 90 degrees to further reduce welding deformation. In the repair of bronze or copper parts, adding brazing seam metal is more conducive to eliminating stress and deformation than surfacing. Views on repairing worn crankshaft by welding.
3. If a steel bearing is stuck in the equipment and you don't want to scrap the equipment, how can you remove the bearing by welding?
Firstly, a weld bead is welded on the inner surface of the bearing. The tension of the weld bead reduces the diameter of the bearing, and the heat in the welding process makes the bearing move. If the inner surface of the pipe with a diameter of 10cm is covered with weld bead, the diameter of the steel pipe will shrink by 1.2 mm, and the schematic diagram of removing the clamp bearing by welding is shown in Figure 6.
4. Cracks often appear in the structure of oil tanks or ship plates. How to prevent it?
Firstly, a small hole is drilled at the crack end to disperse the stress at the end in a large range, and then a series of multi-pass welds with different lengths are welded to increase the strength of the steel plate at the crack front. Views on preventing crack propagation in steel plate Figure 7.
2. 1 Positioning and thickening of stiffening plate
1. Welded reinforcing plates are often welded on the surface of steel plate (bottom plate). The fillet weld at the outer edge of the reinforcing plate easily inclines the center of the reinforcing plate away from the surface of the steel plate, causing angular deformation, as shown in Figure 8(a). This phenomenon will increase the difficulty of machining and turning. How to solve this problem?
The solution is to use plug welding or slot welding in the middle part of the reinforcing plate, so that the surface of the reinforcing plate is closely attached to the surface of the substrate to eliminate deformation and facilitate mechanical processing. The schematic diagram of positioning the reinforcing plate by plug welding or slot welding is shown in fig. 8(b).
2. Sometimes it is necessary to thicken the substrate in a small area, but the thickened area cannot exceed the whole area of the substrate. How to solve this problem?
A thick metal plate is embedded in the part of the substrate that needs to be thickened, and then welded and fixed. On the view of embedding a thick plate on the substrate, Figure 9. This can provide sufficient thickness for subsequent processing, drilling or drilling, and can replace large-thickness parts or castings in the equipment.
3. What is the standard method to enhance the stiffness of flat plate under load?
The standard method to enhance the load-bearing stiffness of flat plate is to weld a series of angle steel vertically on the flat plate and increase angle steel bars to enhance the stiffness of the flat plate, as shown in figure 10.
2.2 Control noise and vibration
1. What technical measures can reduce the noise and vibration of metal plates?
Noise problem, like vibration problem, can also be solved by reducing the vibration frequency of metal plate. The main methods adopted are as follows:
① Increase rigidity by folding, crimping or groove reinforcement;
(2) cutting the flat plate into a series of small parts to enhance the support;
(3) adopting a surface spraying layer;
(4) sticking a layer of damping fiber material on the surface of the flat plate.
See figure 1 1 for four ways to reduce noise by increasing vibration frequency. Vibration caused by relatively low frequency is usually reduced by increasing metal stiffness, as shown in figure 12.
2. When a flat plate is to be fillet welded with another flat plate in the vertical direction, how can it work if there is only a C-clamp now?
When welding, use a steel stopper or a rectangular object as an auxiliary tool, and clamp the fillet weld with a C-clamp and a rectangular stopper, as shown in figure 13.
3. 1 layout design
1. What are the main contents of design requirements during welding?
① When designing, the design scheme should meet the requirements of the strength and hardness of the parts, but not exceed the safety design standards, and allow the welding engineer to check the safety of the design of each part. If the hardness required by the design is set too high, the design will exceed the safety design standard, and the cost of the whole process will also increase due to the addition of additional materials, welding operation and transportation. Exceeding the safety design standards may also increase users' long-term expenses in fuel, energy and maintenance, so experienced engineers and technicians should be invited to strictly check the rationality of the design scheme.
(2) The appearance requirements of welds in the structure shall be determined to avoid unnecessary increase. Sometimes, the welds on many equipment parts are completely hidden, which can reduce the cost of polishing and trimming the welds and improve the appearance quality of the welds. Therefore, in order to let the operator know which welds need to be polished and trimmed to obtain a good appearance, these parts should be marked.
(3) If the product must be welded according to certain process regulations, the relevant process regulations should be checked to decide to adopt an economical and reasonable welding method.
(4) Using thicker structural parts can prevent welding bending and deformation.
⑤ It is more effective to adopt symmetrical structure in welding to prevent welding bending and deformation.
⑥ Welding rigid supports at the end of beam structure can increase the strength and rigidity of the structure. In the beam structure with the same material, width and load, the bending deformation caused by rigid support is smaller than that of welded structure without rigid support, as shown in figure 14.
⑦ Using closed structure or diagonal bracing structure can prevent torsional deformation. The bending angle of closed structure is much smaller than that of open structure, as shown in table 1. At the same time, proper reinforcement can also reduce the structural quality and improve the structural stiffness, as shown in figure 15 ~ 17.
In figure 15, the torsional deformation capacity of the frame structure is almost equal to the sum of the individual torsional deformation capacity of each part, and the torsional deformation capacity of the whole structure can be improved by adopting the closed C-shaped frame structure. In Figure 16, the circular structure has better torsional load resistance than the rectangular structure, mainly because the shear stress distribution around the rectangular structure is uneven, while the bearing stress of the circular structure is concentrated, and the circular structure also has the ability to resist bending deformation in all directions. In Figure 17, the welded structure with inclined ribs can often replace the heavy casting of the base to improve the strength of the structure. As far as compressive stress load is concerned, transverse stiffening ribs and longitudinal stiffening ribs have different functions. Transverse stiffeners are generally used for casting structures, while longitudinal stiffeners are often used for welding structure design.
⑧ The diagonal bracing structure is more effective than the vertical structure in resisting torsional load. Figure 18 shows the structural schematic diagram of two kinds of steel structure bearings. In figure 18(a), the base is composed of a steel plate with a thickness of 25mm, and in figure 18(b), the base is composed of a steel plate with a thickness of 10mm. Their torsional deformation resistance is almost the same, but the reinforced design of diagonal brace structure can save 60% of structural quality, reduce 78% of welding workload and 54% of total manufacturing cost compared with longitudinal reinforced structure.
Pet-name ruby determine the position of the low-grade steel that may be used in the structure. In the actual welding operation, the welding of high carbon steel and alloy steel needs preheating and post-weld heat treatment, but this will increase the cost of welding structure. Therefore, in welded structures, high-grade steel is only used when necessary, and low-carbon steel can be used in other structures.
Attending high-grade steel and other expensive materials are not supplied with standard shape workpieces.
⑾ If expensive materials or refractory materials with good color and surface wear resistance are needed in the structure, carbon steel structure can be considered as the matrix, and satisfactory surface performance requirements can be obtained through surfacing or surface hardening treatment.
⑿ In order to save cost and shorten delivery time, plates, bars or other structural parts with standard shapes are generally used for welding.
[13] If the plate or rod must be machined, ground or case hardened, the structural dimension requirements of the original plate or rod can be quickly obtained from the workshop or supplier.
[14] The equipment components shall be guaranteed necessary repair and maintenance, and the maintenance of bearing seats or other important easily worn components in the closed structure shall not be neglected, which is also applicable to the maintenance requirements of power and pressure pipelines or components.
⒂ In order to carry out automatic welding, sometimes the structural parts are designed into circular structures, which is beneficial to the subsequent welding, processing, assembly and other links, as shown in figure 19.
[14] Before welding design, experienced technicians in the factory should be consulted in order to obtain a better design scheme and save costs. These works must be carried out before the welding design scheme is determined.
⒄ Before welding design, we should check the tolerance range and the stress of each part of the structure. The actual operator may not master more economical and reasonable operation specifications, because sometimes more accurate tolerance requirements may not be needed.
2. What factors should be considered in the layout design of parts?
① The minimization of the number of parts should be considered first, which will reduce the assembly time and welding workload of the equipment, as shown in Figure 20.
② Optimizing the structural layout and design scheme can save materials and welding time. Before deciding to adopt the scheme shown in Figure 2 1(a) and Figure 2 1(b), the cost of materials, cutting and welding should be considered, and the effective utilization of scraps should also be considered. In Figure 2 1(a), the scraps cut from the frame structure can be directly used in the subsequent process, which is more economical than the splicing process. Fig. 2 1(b) is an imaginary optimization scheme. The frame structure is divided into several parts for welding, which can replace cutting and blanking from large plates.
(3) The annular structural components can be cut from a single plate or welded into nested structural components. As with the selection of the above layout and design scheme, the dimensional tolerance of parts, materials, cutting and welding costs and the effective utilization of scraps should be fully considered before determining the best process scheme. Considering transportation factors, cutting nested parts from thick plate material and welding them into ring parts can save material cost and transportation time, as shown in Figure 22.
④ Within the range of dimensional tolerance, we can consider rolling the steel plate into a ring structure and then welding it into a hollow ring structure instead of cutting the ring structure directly from the thick plate, which can reduce the material cost, as shown in Figure 23.
⑤ If there are requirements for the number of annular structural parts in the welding structure, consider rolling the flat plate into a cylindrical structure and then seam welding. Flame cutting can also be used to cut the cylinder into a series of annular members, as shown in Figure 24.
⑥ Some very complicated structural parts can be assembled by welding all parts, which can save the quality, materials and processing time of the whole structure, as shown in Figure 25.
⑦ Curled flat plate structure can increase the rigidity of steel plate and save material cost, as shown in Figure 26.
⑧ When two plates are butt welded, bending and curling the edge of one plate can provide reinforcing ribs for the welding structure, and the cost is not high, as shown in Figure 27.
Pet-name ruby corrugated plate can be considered to increase the rigidity of the plate, or the surface of the plate can be dented to increase the rigidity of the plate, as shown in Figure 28.
Attending before each process step, you should carefully check the design scheme to see if you can save materials and make the welding process adopted not affect the strength requirements of the final product, as shown in Figure 29.
⑾ In the welding manufacturing process, check whether the weld position is in the best position. Changing the weld position as shown in Figure 30 can reduce the waste of welding materials and is more suitable for the use of automatic welding technology.