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How to avoid bubbles and deformation of welded pipes?

Date:2024-02-20View:241Tags:welded pipe
How to avoid bubbles and deformation of welded pipes?

The pores in the welds of welded steel pipes not only affect the sealing of the pipe welds and cause pipeline leakage, but also become corrosion induction points, seriously reducing the strength and toughness of the welds.

Factors causing weld porosity include: moisture, dirt, oxide scale and iron filings in the flux, welding components and covering layer thickness, surface quality of the steel plate, side plate treatment of the steel plate, welding process and steel pipe forming process, etc.


welded pipe


Related precautions:

1. Flux composition. When the welding contains an appropriate amount of CaF2 and SiO2, a reaction will occur, absorbing a large amount of H2, and generating HF with high stability and insoluble in liquid metal, thereby preventing the formation of hydrogen holes.

2. The flux accumulation thickness is generally 25-45mm. The flux particle size is large and the density is small. The maximum accumulation thickness is taken, and vice versa. For high current and low welding speed, the stacking thickness takes the maximum value, and vice versa takes the minimum value. In addition, in summer or when the air humidity is high, the recycled flux should be dried before use.

3. Steel plate surface treatment. In order to prevent uncoiling, leveling and falling iron oxide scale and other debris from entering the molding process, a board surface cleaning device should be installed

4. Steel plate edge treatment. Rust and deburring devices should be installed on the edges of steel plates to reduce the possibility of pores. The location of the removal device is preferably installed behind the edge caster and disc shear. The structure of the device is that there are two driving wire wheels with adjustable clearance on one side, which press the edge of the plate up and down.

5. Weld morphology. The weld forming coefficient is too small, the weld shape is narrow and deep, gas and inclusions are difficult to escape, and pores and slag inclusions are easily formed. Generally, the weld forming coefficient is controlled at 1.3-1.5, with the maximum value for thick-walled welded pipes and the minimum value for thin-walled welded pipes.

6. Reduce the secondary magnetic field. In order to reduce the influence of magnetic deflection, the connection position of the welding cable on the workpiece should be as far away from the welding terminal as possible to avoid the secondary magnetic field generated by part of the welding cable on the workpiece.

7. Process. The welding speed should be appropriately reduced or the current should be increased to delay the completion of the molten pool metal and allow gas to escape. At the same time, if the strip conveying position is unstable, it should be adjusted in time to avoid frequent fine-tuning of the front or rear axles to maintain the shape, which will cause difficulty in gas escape.

How to prevent welding pipe deformation

There are many measures to prevent welding deformation of welded pipes. Because welded pipes inevitably produce welding deformation during welding production. In order to achieve the purpose of controlling the amount of deformation, one or more methods should be selected according to the specific type of welded structure during production.

1. Design measures. Reasonable structural design and weld layout play an important role in preventing and reducing welding deformation. in design. On the basis of considering material saving, convenient manufacturing and safe use, we should also consider reducing the number of welds as much as possible. Shorten the weld length. The welds should be arranged as symmetrically as possible and the welds should be symmetrical with respect to the neutral axis of the structural section. Smaller weld grooves and sizes should be used as much as possible; simply assembled welding molds and fixtures should be used during production.

2. Leave a shrinkage allowance for the weld seam when blanking. In order to compensate for the linear shortening of the weld after welding, a shrinkage allowance can be reserved in advance through experimental methods or by estimating the shrinkage of the weld, so that it can be controlled during material preparation and processing.

Since the shrinkage of the weld is related to many factors, it is difficult to calculate. We can only roughly estimate the amount of deformation based on process testing and the accumulation of large amounts of data. The following factors can be considered when estimating.
①Materials with a large linear expansion coefficient will have a large linear shrinkage after welding. Stainless steel and aluminum have larger coefficients of linear expansion than mild steel. Therefore, the welding deformation is also larger.
② The longitudinal shrinkage of the weld increases with the increase of the weld length, and the transverse shrinkage of the weld increases with the increase of the weld width. Generally, the longitudinal shrinkage is measured by the shrinkage rate per meter of welding seam, and the transverse shrinkage rate is measured by the shrinkage rate of each welding seam. When the welded pipe is in a free state, the transverse shrinkage of the same weld seam of manual arc welding is equivalent to the longitudinal shrinkage of a 2-4m long weld seam. Therefore, when the weld is not too long, the lateral shrinkage of the weld dominates.
③The lateral shrinkage of fillet welds is smaller than that of butt welds.
④ The shrinkage rate of intermittent welds is smaller than that of continuous welds.
⑤ When multi-layer welding, the first layer causes the largest shrinkage, the second layer increases the shrinkage by about 20% of the first layer, the third layer increases by 5%-15%, and the last few layers increase even less.

3. Methods to prevent deformation. In order to offset the welding deformation, when assembling the weldment, the weldment is artificially deformed in the opposite direction to the welding deformation. This method is called the anti-deformation method.

4. Choose a reasonable assembly and welding sequence. The structure is appropriately divided into parts, assembled and welded separately, and then welded into a whole. It allows asymmetric welds or welds with large shrinkage to shrink more freely without affecting the overall structure. According to this principle, complex and large-scale welding structures are not only beneficial to controlling welding deformation, but also can expand the working area and shorten the production cycle.

5. Rigid fixation method. Generally speaking, welding parts with high rigidity have less welding deformation. The method of using external rigid constraints to reduce welding deformation is called the rigid fixation method or the constraint method.
The rigid fixation method can use welding fixtures to place heavy objects on the welded pipe or fix the welded pipe on a rigid platform, which can effectively reduce welding deformation. However, it should be pointed out that after rigid fixation is used for welding, large internal welding stress will often occur in the weldment. Therefore, for workpieces or welding materials with a greater tendency to crack, it is not appropriate to use rigid fixation methods to control welding deformation.

6. Thermal conditioning method. Thermal adjustment method is to achieve the purpose of reducing welding deformation, using the reduction of welding line energy to reduce the heating area or make the heating uneven or the cooling as uniform as possible.

What is the weldability of welded pipes?

Weldability refers to the difficulty of obtaining excellent welded joints for welded steel pipes (such as ERW pipes) under certain welding process conditions, including welding methods, welding materials, welding specifications and welding structures.

The welding performance of welded pipes includes two aspects:

1. Connection performance: Under certain welding process conditions, welded pipes are very sensitive to welding defects.
Factors that determine joint performance include: physical properties of seamless pipes, such as melting point, thermal conductivity and expansion rate, chemical properties and metallurgical effects of welded pipes and welding materials during welding, etc. A material is considered to have the ability to withstand physical, chemical and metallurgical effects during the welding process, form a welded joint free of welding defects, and have good connection properties.
2. Usage performance: The adaptability of the welded pipe welded joint to the use requirements under certain welding process conditions, that is, the ability of the welded joint to withstand load. Such as bearing static load, impact load and fatigue load, as well as low temperature resistance, high temperature resistance, oxidation resistance, corrosion resistance and other properties of welded joints.

Factors affecting the weldability of welded pipes

The welding performance of welded pipes mainly depends on its chemical composition. Among them, the carbon element has the greatest influence, which means that the carbon content in the metal determines its weldability. Most of the other alloying elements in steel pipes are also not conducive to welding, but their effects are generally much smaller than carbon. As the carbon content in steel increases, the hardening tendency increases, the plasticity decreases, and welding cracks are prone to occur. The sensitivity to cracks during seamless pipe welding and the changes in mechanical properties of the welded joint area are usually used as the main indicators to evaluate the weldability of materials. Low carbon steel and low alloy steel with a carbon content of less than 0.25% have excellent plasticity and impact toughness, and the plasticity and impact toughness of post-weld welded joints are also very good. There is no need for preheating and post-weld heat treatment during welding. The welding process is universal and simple, so it has good weldability. As the carbon content increases, the tendency of welding cracks increases significantly. Therefore, steel with a carbon content greater than 0.25% is not suitable for manufacturing steel pipes for boilers and pressure vessels.

Estimation method

Since carbon's effect is most pronounced, the effects of other elements can be translated into carbon's effects.
Empirical formula for carbon equivalent of carbon steel and low alloy structural steel:
w=w(C)+1/6[w(Mn)]+1/5[w(Cr)+w(Mo)+w(V)]+1/15[w(Ni)+w(Cu) ]
Tip: Based on experience:
When w<0.4%, the steel pipe has good weldability and preheating should be considered.
When w=0.4%~0.6%, the weldability is poor.

When w>0.6%, the solderability is very poor and must be preheated to a higher temperature.


  
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