How can we ensure that the welding does not come off when welding ventilation pipes?

In the ventilation pipe welding process, to ensure that the welding does not come off, we need to start from many aspects. The following are some key points:

I. Selection and preparation of welding materials

Matching welding material and base metal

Select the appropriate welding material according to the material of ventilation pipe. For example, if the ventilation pipe is made of ordinary carbon steel, a matching carbon steel covered electrode can be selected, such as E4303 covered electrode, whose chemical composition and mechanical properties are suitable for the carbon steel pipe and can provide good welding joint strength. For stainless steel ventilation pipes, the corresponding stainless steel covered electrode, such as E308-16, should be used to weld the common 304 stainless steel pipes, so as to ensure that the welding materials are similar to the parent materials in composition, which will help to form a firm welded joint and reduce the risk of desoldering.

Welding material quality inspection

Before using welding materials, quality inspection should be carried out. Check the appearance of covered electrode, and there should be no obvious defects such as coating shedding and eccentricity. For welding wire, check whether its surface is smooth, greasy and rusted. For example, the covered electrode with the coating falling off may lead to unstable arc during welding, which will affect the welding quality, thus increasing the possibility of desoldering. At the same time, we should pay attention to the shelf life of welding materials. The expired welding materials may lead to the decline of welding performance due to the change of chemical composition.

Second, the pipeline preparation before welding

Pipeline cleaning

Before welding, the welding parts of ventilation pipes must be clean. Remove oil, rust, dust and other impurities on the surface of the pipeline. Sandpaper, wire brush or chemical cleaning agent can be used for cleaning. For example, for pipelines with oil stains, it can be wiped with organic solvent (such as acetone) first, and then polished with sandpaper to expose the metallic luster on the welding surface. Because impurities will enter the molten pool during the welding process, which will affect the quality of the weld, reduce the welding strength and easily lead to weld detachment.

Precision control of pipeline docking

Ensure the accuracy of ventilation pipe docking. The butt-joint clearance of pipes should meet the requirements of welding process. Generally, for manual arc welding, the butt-joint clearance of carbon steel pipes is between 1 and 3 mm.. The offset of the butt joint should also be controlled within the allowable range, usually not exceeding 10% of the pipe wall thickness. If the butt gap is too large or the misalignment exceeds the standard, the welding stress will be concentrated, the weld filling will be uneven, and the hidden danger of welding will be increased. Special pipeline butt tools (such as butt splicer) can be used to ensure the accurate butt joint of pipelines.

Third, the control of welding process parameters

Welding current and voltage regulation

Adjust welding current and voltage reasonably according to welding materials, pipe thickness and other factors. For example, in manual arc welding, the welding current is too small, which leads to insufficient penetration and weak combination between weld and base metal; Excessive current may cause weld burn-through and undercut, which will increase the risk of desoldering. For carbon steel ventilation pipes with a thickness of 3mm, when manual arc welding is used, the welding current can generally be adjusted between 90 and 120 A. At the same time, according to the characteristics of welding equipment and welding methods (such as manual arc welding, gas shielded welding, etc.), the corresponding voltage should be matched to ensure arc stability and good weld formation.

Welding speed control

Maintain proper welding speed. Too fast welding speed will make the weld cool too fast, which may lead to defects such as porosity and incomplete fusion in the weld; If the speed is too slow, the weld will be overheated, resulting in coarse metal structure of the weld and reducing the strength and toughness of the weld. In the actual welding process, the appropriate welding speed should be determined according to the size, shape and welding method of the weld. For example, when welding long and straight ventilation pipe welds, the welding speed of manual arc welding can be controlled at about 10-15cm per minute to ensure the weld quality and reduce the possibility of weld detachment.

Four, welding operation specification

Skilled use of welding techniques

Welders should master the correct welding techniques skillfully. For example, in manual arc welding, suitable strip conveying methods should be adopted, such as straight strip conveying, zigzag strip conveying or crescent strip conveying, etc., which should be selected according to the position and requirements of the weld. In the process of transporting bars, it is necessary to keep the covered electrode angle correct. Generally, the angle between the covered electrode and the welding surface of the pipeline is 70-80 degrees, which can ensure the good heating of the base metal and the weld metal by the arc, fully fuse the weld with the base metal and avoid desoldering.

Application of multi-layer and multi-pass welding (if necessary)

For thicker ventilation pipes, multi-layer and multi-pass welding process is adopted. In the process of welding, attention should be paid to controlling the thickness and width of each weld in each layer. The thickness of each layer of weld is generally not more than 0.8-1.2 times the diameter of covered electrode, so that the weld metal can be fully cooled, the welding stress can be reduced, and the internal quality of weld can also be ensured, and defects such as incomplete fusion and slag inclusion can be prevented, thereby reducing the probability of welding detachment. Moreover, the welding seams between each layer should be cleaned, and the welding slag and spatter should be removed before the next layer is welded.

Five, after welding quality inspection and treatment

observational check

After welding is completed, the appearance inspection shall be carried out first. Check whether there are defects such as porosity, crack, undercut and incomplete fusion on the weld surface. Porosity will reduce the effective cross-sectional area of weld and reduce the welding strength; Cracks are serious quality problems, which will directly lead to desoldering. Small defects found in visual inspection can be repaired, such as small air holes, which can be re-welded after grinding with a grinding wheel.

Non-destructive testing (if necessary)

For some important ventilation pipes or welding parts with strict quality requirements, nondestructive testing methods can be used. Such as ultrasonic flaw detection, X-ray flaw detection and other technologies, to check whether there are defects in the weld. Through nondestructive testing, problems such as incomplete fusion and slag inclusion can be found in time, and defective welds should be repaired in time to ensure welding quality and prevent weld detachment.