Aiming at the welding problems of structural steel, some common welding materials and successful experiences in welding practice are highlighted; in terms of oxy-acetylene cutting, tips for solving welding problems are provided, cutting applications and the performance of oxygen spears and combustion rods are discussed ; for fasteners frequently used in welded structures, this article mainly introduces commonly used bolts, nuts and how to apply them.

Welding process problems and solutions

1.1 Welding of thick plates and thin plates

1. When welding steel workpieces with gas metal arc welding (GMAW) and flux cored gas arc welding (FCAW), if the thickness of the workpiece exceeds the maximum welding current that the welding machine can achieve, how will it be handled?

The solution is to preheat the metal before welding. Use propane, standard gas or acetylene torch to preheat the welding area of the workpiece. The preheating temperature is 150~260℃, and then weld. The purpose of preheating the metal in the welding area is to prevent the welding area from cooling too quickly and causing the welding seam to crack or fail to fuse.

2. If it is necessary to use gas metal arc welding or flux cored wire gas shielded welding to weld a thin metal cover to a thicker steel pipe, if the welding current cannot be adjusted correctly during welding, two situations may result: First, in order to prevent The thin metal will burn through and reduce the welding current. At this time, the thin metal cover cannot be welded to the thick steel pipe; secondly, excessive welding current will burn through the thin metal cover. How should this be handled?

There are two main solutions:

①Adjust the welding current to avoid burning through the thin metal cover, preheat the thick steel pipe with a welding torch, and then use the thin plate welding process to weld the two metal structures.

②Adjust the welding current to suit the welding of thick steel pipes. When welding, keep the dwell time of the welding arc on thick steel pipes at 90%, and reduce the dwell time on thin metal covers. It should be noted that good welding joints can only be obtained when this technique is mastered.

3. When welding a thin-walled round pipe or rectangular thin-walled pipe to a thick plate, the welding rod easily burns through the thin-walled pipe. In addition to the above two solutions, are there any other solutions?

Yes, mainly using a heat sink rod during the welding process. If a solid round rod is inserted into a thin-walled round tube, or a solid rectangular rod is inserted into a rectangular pipe fitting, the solid rod will take away the heat of the thin-walled workpiece and prevent burn-through.

Generally, most supplied hollow or rectangular tube materials have solid round or rectangular rods tightly fitted into them. When welding, care should be taken to keep the weld away from the end of the pipe, which is the weakest area where burnout is most likely to occur.

4. When galvanized or chromium-containing materials must be welded to another part, how should it be done?

The best method is to file or grind the area around the weld before welding, because galvanized or chromium-containing metal plates will not only contaminate and weaken the weld, but also release toxic gases during welding.

1.2 Welding of containers and frame structures

1. If a welding process (such as brazing) is used to seal a pontoon or the end of a hollow structure, what will be done during the final sealing of the weld to prevent hot air from entering the container and causing it to burst?

First, drill a pressure relief hole with a diameter of 1.5mm on the float to facilitate the circulation of hot air near the weld and external air, then perform closed welding, and finally weld and seal the pressure relief hole.

When welding the gas storage container structure, pressure relief holes can also be used. It should be noted that welding in a closed container is very dangerous. Before welding, make sure the inside of the container or pipe is clean and avoid the presence of flammable and explosive substances or gases.

2. When it is necessary to use gas metal arc welding, flux-cored wire gas shielded welding or tungsten arc welding to weld the screen, metal mesh or extended metal to the steel structure frame, the metal mesh is prone to burn-through during welding. How to deal with the phenomenon of weld failure and lack of fusion?

① Place non-metallic washers on the wire mesh or extended metal and clamp the washers, wire mesh and frame together. Chrome-containing or galvanized washers are not allowed. The washers should be uncoated.

② Place a larger washer on top of the washer at the welded location as a heat sink. The upper gasket should have a larger hole than the lower gasket to avoid the upper gasket also being welded together. Then plug weld through the two holes of the gasket so that the weld is in the lower gasket part. The operator can use some other methods to obtain enough heat and perform welding, and be careful to prevent the surrounding screen or metal mesh from burning through.

③Another method is to use a metal strip with holes, align the holes with the parts to be welded, place a heat dissipation gasket, and then perform plug welding.

1.3 Repair of welded components

1. In addition to using a common nail opener, what other methods can be used to remove damaged or rusty screws?

There are two main methods introduced here:

① If the installed screws will not be damaged when heated, you can use an oxygen-acetylene torch to heat the nut and its assemblies until they are red hot, and then quickly quench with water to facilitate removal of the screws. This process may require several times of heating , cold quenching cycle process.

② If the screw groove, nut or tooth socket is damaged or missing, you can place a nut on the upper part (or remaining part) of the screw head, tighten the nut, and then use any welding method to fill the inside of the nut and screw with metal.

This will connect the nut and the remainder of the screw, then place a wrench or pliers on the nut and quickly pull out the screw. Using this method will help provide a new grip point and use heat to tighten the screw, and use welding to remove the remaining portion of the set screw.

2. If you have a worn crankshaft, what is the best way to repair and strengthen it with welding?

When repairing a worn crankshaft, gas metal arc welding, flux cored wire gas arc welding or tungsten arc welding can be used. However, in order to obtain a satisfactory cladding bead shape, the following four requirements must be paid attention to.

①make the surfacing bead direction parallel to the crankshaft axis.

② first build up a weld bead on the lower part of the crankshaft, and then rotate the crankshaft 180° to build up the next weld pass. This can balance the welding stress and significantly eliminate welding thermal deformation. It should be noted that sequential overlay welding on the first weld pass will cause crankshaft warping. This surfacing process is suitable for repairing and welding repairs on roller crankshafts.

③ 30% to 50% overlap of deposited metal must be maintained between the two welding beads to ensure that the surface of the welding bead remains smooth during machining after welding repair.

④ when manual arc welding and flux-cored wire gas shielded welding are used, the residual flux between the weld beads must be cleaned by brushing or cutting.

In addition to the above crankshaft repair methods, you can also add a surfacing weld bead at every 90° position of the crankshaft to further reduce welding deformation. In the repair of bronze or copper parts, adding braze metal is more beneficial than using cladding in relieving stress and deformation.

3. If there is a steel bearing stuck in the equipment and you do not want to scrap the equipment, how should you use welding to remove the bearing?

First, a weld bead is welded on the inner surface of the bearing, and the bearing diameter is reduced by the tensile force of the weld bead. The heat of the welding process should be added to make the bearing move. If a pipe with a diameter of 10cm is covered with weld beads on the inner surface, the diameter of the steel pipe will shrink by 1.2mm. Use welding to remove stuck bearings

4. Cracks often occur in oil tank or ship plate structures. How to prevent them?

First, a small hole is drilled at the end of the crack to help disperse the stress at the end over a larger range, and then a series of multi-pass welds of varying lengths are welded to increase the strength of the steel plate at the front end of the crack. Prevent crack growth in steel plates

Enhanced version of welding

2.1 positioning and thickening of reinforcing plates

1. Welded reinforcement plates are often welded to the surface of the steel plate (base plate). The fillet welds on the outer edge of the reinforcement plate can easily cause the center of the reinforcement plate to lift, leaving the surface of the steel plate and causing angular deformation. 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 of the reinforcing plate to close the surface of the reinforcing plate to the surface of the base plate to eliminate deformation and facilitate mechanical processing. Use plug welding or slot welding method to position reinforcement plate

2. Sometimes the substrate needs to be thickened in a small area of the substrate, but the thickened area cannot exceed the area of the entire substrate. How to solve this problem?

Insert a thick plate of metal into the part of the substrate that needs to be thickened, and then fix it by welding. Thick plates are embedded in the base plate. This can provide sufficient thickness for subsequent machining, boring or drilling, and can replace large-thickness parts or castings in equipment.

3. What is the standard method of increasing the rigidity of a slab to carry loads?

The standard way to stiffen a slab to carry loads is to weld a series of angles vertically to the slab and add angle steel reinforcements to stiffen the slab.

2.2 controlling noise and vibration

1. What technical measures can be used to reduce the noise and vibration of metal plates?

Noise problems, like vibration problems, can also be solved by reducing the resonant frequency of the metal plate. The main methods used are as follows:

①increase rigidity by folding, curling or trough-shaped reinforcement;

② cut the slab into a series of small parts to enhance support;

③ use surface spray coating;

④a layer of vibration-absorbing fiber material is bonded to the surface of the flat plate.

4 ways to reduce noise by increasing resonant frequency. Vibrations caused at relatively low frequencies are usually reduced by increasing metal stiffness.

2. When you want to perform fillet welding between one flat plate and another flat plate in the vertical direction, how should you work if you only have a c-shaped clamp?

When welding, use a steel stop or a rectangular object as an auxiliary tool, and use a c-shaped clamp and a rectangular stop to clamp the fillet weld.

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