Although there is no so-called technical secret in the welding process, there are many welding technologies, methods and processes in the actual welding process, which can make the welding process easier.
These processes are called technical know-how.
Welding know-how can save time, cost and labor, and even determine the success and failure of welding, profit and loss.
Most of the welding processes are mainly based on scientific research, and some are based on actual welding experience.
This article introduces the synthesis of welding experience in practice.
1. Welding process problems and Solutions
1.1 Welding of thick plate and thin plate
1.1.1 When welding steel workpiece with GMAW and FCAW, if the thickness of the workpiece exceeds the maximum welding current of the welder, how to deal with it?
The solution is to preheat the metal before welding.
Propane, standard gas or acetylene torch are used to preheat the welding area of the workpiece. The preheat temperature is 150 ~ 260 ℃, and then the welding is carried out.
The purpose of preheating the metal in the welding area is to prevent the weld area from cooling too fast, so as not to make the weld crack or incomplete fusion.
1.1.2 If it is necessary to weld a thin metal cover on a thick steel pipe by GMAW or FCAW, if the welding current can not be adjusted correctly, two situations may occur:
- First, in order to prevent the thin metal from burning through and reduce the welding current, the thin metal cover can not be welded to the thick steel pipe at this time;
- Second, the welding current will burn through the thin metal cover.
How to deal with it?
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 welding torch, and then weld the two metal structures with thin plate welding process.
② Adjust the welding current to fit the welding of thick steel pipe. During welding, the residence time of welding arc on thick steel pipe should be kept at 90%, and the residence time on thin metal cover should be reduced. It should be pointed out that only by mastering this technique can good welded joints be obtained.
1.1.3 When a thin-walled pipe or rectangular thin-walled pipe is welded to a thick plate, the electrode is easy to burn through the thin-walled pipe. Besides the above two solutions, is there any other solution?
Yes, mainly in the welding process using a cooling rod. If a solid round bar is inserted into a thin-walled tube or a solid rectangular bar is inserted into a rectangular tube, the solid bar will take away the heat of the thin-walled workpiece and prevent it from burning through.
Generally speaking, solid round bar or rectangular bar is tightly installed in most hollow pipe or rectangular pipe materials. When welding, pay attention to keep the weld away from the end of the pipe, the end of the pipe is the most vulnerable area to burn through. The schematic diagram of using the built-in cooling rod to avoid burn through is shown in Figure 1.
Fig1. Use the built-in cooling rod to avoid burning through
1.1.4 When it is necessary to weld a galvanized or chromium containing material to another part, what should be done?
The best process is to file or grind the area around the weld before welding, because galvanized or chromium containing metal sheet will not only pollute and weaken the weld, but also release toxic gas during welding.
1.2 Welding of vessel and frame structure
1.2.1 If a welding process (such as brazing) is used to seal a buoy or the end of a hollow structure, what will be done to prevent hot air from entering the vessel and causing the vessel to burst during the final sealing of the weld?
③ Firstly, a pressure relief hole with a diameter of 1.5mm is drilled on the pontoon to facilitate the circulation of hot air and external air near the weld, then the sealing welding is carried out, and finally the sealing pressure relief hole is welded. The schematic diagram of sealed welding pontoon or closed vessel is shown in Figure 2.
Fig.2 Schematic diagram of narrow welding pontoon or closed vessel
When welding the gas storage vessel structure, the pressure reducing hole can also be used. It should be noted that it is very dangerous to weld in the closed container. Before welding, the inside of the container or pipe should be clean, and flammable and explosive materials or gases should be avoided.
1.2.2 When it is necessary to weld the screen, wire mesh or extended metal to the steel structure frame by means of GMAW, FCAW or TIG, the wire mesh is easy to burn through and the weld is not fused during welding. How to deal with it?
① The non-metallic washer shall be placed on the wire mesh or extension metal, and the washer, wire mesh and frame shall be clamped together. Chromium containing or galvanized washer is not allowed. The washer shall be uncoated, as shown in Fig. 3 (a).
Fig3. Welding sketch of wire mesh and frame structure
② A larger washer is placed on the top of the washer at the position to be welded as a heat sink. The upper washer should have a larger hole than the lower washer to avoid the upper washer being welded together. Plug welding shall then be carried out through the two holes of the gasket so that the weld is in the lower part of the gasket. The operator can use some other methods to get enough heat and carry out welding, and pay attention to prevent the surrounding grid or wire mesh from burning through, as shown in Fig. 3 (b) and (c).
③ Another method is to use a metal strip with a hole, align the hole with the position to be welded, place the heat sink washer, and then plug weld, as shown in Figure 3 (d).
1.3 Repair of welded components
1.3.1 In addition to the commonly used screw opener, what other methods can be used to remove damaged or rusted screws?
Two methods are introduced here:
① If the installed screw will not be damaged during heating, the nut and its assembly can be heated with an oxygen acetylene torch until it is red hot, and then water quenched rapidly to facilitate the removal of the screw. In this process, several heating and cold quenching cycles may be required.
② If the screw groove, nut or alveolar is damaged or lost, a nut can be placed on the upper part (or residual part) of the screw head, the nut can be tightened, and then the metal can be filled inside the nut and screw by any welding method.
In this way, the nut and the remaining part of the screw will be connected, and then a wrench or pliers will be placed on the nut to quickly pull out the screw. This method is beneficial to provide a new grip point, and heat can be used to fasten the screw. The residual part of the fixed screw can be removed by welding, as shown in Fig. 4.
Fig.4. Removal of the remaining part of the set screw by welding
1.3.2 If there is a worn crankshaft, what is the best way to repair and reinforce it by welding?
When repairing worn crankshaft, the methods of GMAW, FCAW or TIG can be used. However, in order to obtain satisfactory bead shape, the following four requirements must be paid attention to.
① Make the bead direction parallel to the crankshaft axis.
② Firstly, a weld bead is overlaid on the lower part of the crankshaft, and then the next weld bead is overlaid by rotating the crankshaft 180 degrees, which can balance the welding stress and significantly eliminate the welding thermal deformation. It should be noted that sequential surfacing on the first pass will cause crankshaft warpage. The surfacing process is suitable for repairing and welding roller crankshaft.
③ 30% – 50% overlap of deposited metal must be kept between two welding passes to ensure smooth surface of welding pass during machining 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 by brush or cutting method.
In addition to the above crankshaft repair method, a surfacing bead can be added at every 90 ° position of the crankshaft to further reduce the welding deformation. In the repair of bronze or copper parts, the addition of brazing metal is more advantageous than that of surfacing in relieving stress and deformation. Figure 5 shows how to repair worn crankshaft by welding.
Fig.5 Schematic diagram of repairing worn crankshaft by welding method
1.3.3 If a steel bearing is stuck in the equipment and does not want to scrap the equipment, how to remove the bearing by welding?
Firstly, a weld bead is welded on the inner surface of the bearing. The diameter of the bearing is reduced by the stretch force of the weld bead, and the heat in the welding process can make the bearing move. If the inner surface of 10cm diameter pipe is covered with weld bead, the diameter of steel pipe will shrink by 1.2mm. The schematic diagram of removing stuck bearing by welding method is shown in Fig. 6.
Fig.6 Schematic diagram of removing stuck bearing by welding method
1.3.4 Cracks often occur in the structure of oil tank or ship plate. How to prevent them?
Firstly, a small hole is drilled at the end of the crack to disperse the stress at the end of the crack in a large range, and then a series of multi-channel welds with different lengths are welded to increase the strength of the steel plate at the front of the crack. Fig. 7 shows how to prevent crack propagation of steel plate.
Fig.7 Prevention of crack propagation in steel plate
2. Welding of reinforcing plate
2.1 Positioning and thickening of reinforcing plate
2.1.1 The welding reinforcing plate is often welded to the surface of the steel plate (base plate). The fillet weld on the outer edge of the reinforcing plate is easy to make the central part of the reinforcing plate tilt up, leave the surface of the steel plate and produce 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 groove welding in the middle part of the reinforcing plate, so that the surface of the reinforcing plate is close to the surface of the base plate, and the deformation is eliminated to facilitate machining. The schematic diagram of positioning the reinforcing plate by plug welding or groove welding is shown in Fig. 8 (b).
Fig.8 Schematic diagram of positioning reinforcing plate by plug welding or groove welding
2.1.2 Sometimes it is necessary to thicken the substrate in a small area, but the thicken area should not exceed the area of the whole substrate. How to solve this problem?
A thick metal plate is embedded in the part of the base plate to be thickened, and then fixed by welding. Fig. 9 shows a thick plate embedded in the substrate. This can provide enough thickness for subsequent machining, boring or drilling, and can replace the large thickness parts or castings in the equipment.
Fig.9 Schematic diagram of inserting thick plate into base plate
2.1.3 What is the standard method to enhance the rigidity of the slab to carry the load?
The standard method to enhance the rigidity of the plate to carry the load is to weld a series of angle steel vertically on the plate and add angle steel stiffeners to enhance the rigidity of the plate, as shown in Figure 10.
Fig.10 Schematic diagram of adding angle steel reinforcement to enhance the rigidity of flat plate
2.2 Noise and vibration control
2.2.1 What technical measures can be used to reduce the noise and vibration of metal plate?
The problem of noise is the same as that of vibration, which can also be solved by reducing the resonance frequency of metal plate. The main methods used are as follows:
① The rigidity is increased by folding, crimping or groove strengthening;
② The flat plate is cut into a series of small parts to enhance the support;
③ Surface spray coating is used;
④ A layer of damping fiber material is bonded on the surface of the flat plate.
Four methods of increasing resonance frequency to reduce noise are shown in Figure 11. Vibration at relatively low frequencies is usually reduced by increasing metal stiffness, as shown in Figure 12.
Fig.11 Increasing resonance frequency to reduce noise
Fig.12 Schematic diagram of increasing metal stiffness to reduce vibration
2.2.2 When a flat plate is to be fillet welded with another flat plate in the vertical direction, if there is only C-shaped fixture, how to work?
When welding, a steel block or a rectangular object is used as an auxiliary tool, and C-shaped clamp and rectangular block are used to clamp the fillet weld, as shown in Figure 13.
Fig.13 Schematic diagram of using C-shaped clamp and rectangular block to clamp fillet weld
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