Solution for pipe bending defects

29/nov/2019 09:04:41 beautyyuan Contatta l'autore

In order to prevent or reduce the occurrence of bending pipel defects and obtain satisfactory bending pipe quality, corresponding actions should be taken in the process of bending pipe. First of all, in the scope allowed by the product design structure, the bent pipe fittings should be designed with a large bending radius as much as possible. At the same time, when selecting equipment, the pipe bending machine with side boosting and tail pushing mechanism should be the first choice. Under normal circumstances, for the several common defects mentioned above, measures should be taken in a targeted manner. The specific methods can be as follows:

For pipe fittings with severe flatten outside the arc, the compacting die (wheel) can be designed to have an anti-deformation groove in the form of a coreless bend to reduce or eliminate the degree of flatten when the pipe is bent. 

For cored bends, when the diameter of the mandrel is too small or with serious abrasion, the appropriate mandrel should be replaced. The unilateral clearance between the mandrel and the inner wall of the pipe should be no more than 0.5mm, and pre-install the mandrel appropriately. In addition, when installing the mold, it is necessary to ensure that the tube groove axes of the pieces are on the same level.

The thinning of the outer side of the arc when the small radius bends is determined by the characteristics of the bending process. It is unavoidable, but measures should be taken to overcome the situation of excessive thinning. The commonly used effective method is to use the side with a booster or tail. There is a pusher or a combination of both, so that the auxiliary push or push mechanism pushes the pipe forward, counteracts part of the resistance when the pipe is bent, improves the stress distribution on the pipe section, and makes neutral The layer is moved outward to achieve the purpose of reducing the amount of thinning of the tube wall outside the tube. The boost and push speeds are determined based on the actual conditions of the bends to match the bend speed. At the same time, it should be checked whether the advance amount of mandrel installation is appropriate, and the necessary adjustments should be made at the time of discomfort.

When bending outside the arc of the pipe, the reason should be carefully analyzed. Firstly, the pipe should have a good heat treatment condition. The weld of the seamed steel pipe should not be in the direction of the force of F1 and F2, that is, do not face the clamp. Tightening and bending the wheel mold; after removing the factors of the pipe, check whether the pressure of the pressing die is too large and adjust the pressure to be appropriate. For the newly used mandrel, check whether the diameter is too large. If the diameter is too large, it is necessary to carry out the necessary grinding, and ensure that the mandrel and the inner wall of the pipe are well lubricated to reduce the bending resistance and the friction between the inner wall of the pipe and the mandrel. And take appropriate measures to avoid machine shake, etc.

For the inner side of the arc, the corresponding measures should be taken according to the wrinkle position. If the front cut point is wrinkled, the position of the mandrel should be adjusted forward so that the advancement of the mandrel is appropriate to achieve reasonable support of the pipe when the pipe is bent; after the cut point is wrinkled, anti-wrinkle block should be added, and the anti-wrinkle block should be installed in the correct position. It can be well attached to the bending die, and the pressure of the pressing die (wheel) should be adjusted to make the pressure proper; the inside of the arc is all wrinkles. In addition to adjusting the compression mold (wheel) to make the pressure appropriate, check the diameter of the mandrel and the pitch between the joints of the ball joint. If the diameter is too small or the wear is serious, the mandrel should be replaced.

Welding deformation
After the workpiece is welded, it will generally be deformed. If the amount of deformation exceeds the allowable value, it will affect the use. A few examples of weld distortion are shown in Figure 2-19. The main reason for this is that the weldment is unevenly heated and cooled locally. Because of the welding, the weldment is only heated to a high temperature in a local area, the closer to the weld, the higher the temperature and the greater the expansion. However, the metal in the heating zone is prevented from being freely expanded by the metal having a low ambient temperature, and cannot be freely contracted due to the pinning of the surrounding metal during cooling. As a result, the partially heated metal has a tensile stress, while the other portions of the metal have a compressive stress balanced therewith. When these stresses exceed the yield limit of the metal, weld deformation will occur; when the strength limit of the metal is exceeded, cracks will occur. 

External defects of welds 
Weld reinforcement too high 
As shown in Figure 2-20, this phenomenon occurs when the angle of the weld bevel is too small or the welding current is too small. The dangerous plane of the weldment weld has transitioned from the M-M plane to the N-N plane of the fusion zone. Due to the stress concentration, the fatigue life of the pressure vessel is required to increase the weld seam. 
Weld undercut 
The depression formed along the edge of the weld on the workpiece is called the undercut, as shown in Figure 2-22. It not only reduces the working section of the joint, but also causes severe stress concentration at the undercut. 
Welded beading
The molten metal flows to the unmelted workpiece at the edge of the bath and accumulates to form a solder joint that is not fused to the workpiece, see Figure 2-23. The weld has no effect on the static load strength, but it will cause stress concentration and reduce the dynamic load strength.

Burn through 
Burn through means that part of the molten metal leaks from the reverse side of the weld and even burns through the hole, which reduces the strength of the joint. 

The above five defects exist in the appearance of the weld, which can be found by the naked eye and can be repaired in time. If the operation is skilled, it can generally be avoided.

Internal defects of welds 
Incomplete penetration
Incomplete penetration is a defect that is not partially fused between the workpiece and the weld metal or weld layer. Incomplete penetration reduces the working section of the weld, causing severe stress concentration and greatly reducing joint strength, which often becomes the source of weld cracking. 
Slag
The slag is welded with non-metal slag, which is called slag. The slag inclusion reduces the working section of the weld, resulting in stress concentration, which reduces weld strength and impact toughness. 
Porosity
At the high temperature, the stomatal weld metal absorbs too much gas (such as H2) or gas generated by the metallurgical reaction inside the bath (such as CO), which cannot be discharged when the bath is cooled and solidified, but forms inside or on the surface of the weld. The hole is the stomata. The presence of pores reduces the effective working section of the weld and reduces the mechanical strength of the joint. If there are penetrating or continuous pores, it will seriously affect the sealability of the weldment. 
Cracks 
During or after welding, local cracking of the metal that occurs in the area of the welded joint is called cracking. Cracks may be generated on the weld and may also create heat affected zones on both sides of the weld. Sometimes it occurs on metal surfaces, sometimes inside metal. Generally, according to the mechanism of crack generation, it can be divided into two types: hot crack and cold crack. 
Hot cracks 
Hot cracks are generated during the crystallization from liquid to solid in the weld metal and are mostly produced in the weld metal. The main reason for this is the presence of low-melting substances (such as FeS, melting point of 1193 ° C) in the weld, which weakens the relationship between the grains, and when subjected to large weld stress, it is easy to cause cracks between the grains. When there are many impurities such as S and Cu in the weldment and the welding rod, hot cracks are likely to occur. 

Thermal cracks are characterized by a distribution along the grain boundaries. When the crack penetrates the surface and communicates with the outside, there is a clear tendency to hydrogenate. 

Cold cracks 
Cold cracks are generated during post-weld cooling and are mostly produced on the fusion line between the base metal or the base metal and the weld. The main reason for this is that the quenched structure is formed in the heat-affected zone or the weld, and the internal crack of the grain is caused by the high stress. When welding the easily hardened steel with high carbon content or more alloying elements, the most It is prone to cold cracks. Excessive hydrogen is melted into the weld and can also cause cold cracks. 
Crack is one of the most dangerous defects. In addition to reducing the load-bearing cross-section, it also causes severe stress concentration. In use, the crack will gradually expand and eventually cause damage to the member. Therefore, such defects are generally not allowed in the welded structure, and once found, it is necessary to shovel the re-welding. 

Welding inspection
Performing the necessary inspection of the welded joint is an important measure to ensure the quality of the weld. Therefore, after the workpiece is welded, the weld should be inspected according to the technical requirements of the product. Any defects that do not meet the technical requirements must be repaired in time. Inspection of welding quality includes three aspects: visual inspection, non-destructive testing and mechanical performance testing. These three are complementary to each other, and non-destructive testing is the main one.

A ppearance inspection 
Visual inspection is generally based on visual observation, sometimes with a magnifying glass of 5-20 times. Through visual inspection, weld surface defects such as undercuts, welds, surface cracks, pores, slag inclusions, and weld penetration can be found. The dimensions of the weld can also be measured using a weld detector or template. 

Nondestructive test
Inspection defects such as slag inclusions, pores, cracks, etc. hidden inside the weld. At present, the most common use is X-ray inspection, as well as ultrasonic flaw detection and magnetic flaw detection.
The X-ray inspection uses X-rays to take pictures of the weld, and judges whether there are defects, the number and type of defects in the interior based on the image of the film. According to the technical requirements of the product, the weld is qualified.
The ultrasonic beam is emitted by the probe and transmitted to the metal. When the ultrasonic beam is transmitted to the metal-air interface, it is refracted and passed through the weld. If there is a defect in the weld, the ultrasonic beam is reflected to the probe and accepted, and a reflected wave appears on the screen. Based on the comparison and discrimination of these reflected waves with normal waves, the size and position of the defects can be determined. Ultrasonic flaw detection is much simpler than X-ray photography and is therefore widely used. However, ultrasonic flaw detection often can only be judged based on operational experience, and can not leave inspection basis. 
For internal defects that are not deep from the surface of the weld and extremely small cracks on the surface, magnetic flaw detection can also be used.

Hydraulic test and air pressure test 
For pressurized containers requiring sealing, a hydrostatic test and/or a pneumatic test shall be carried out to check the sealability and pressure bearing capacity of the weld. The method is to inject 1.25-1.5 times working water or a working pressure gas (mostly air) into the container for a certain period of time, then observe the pressure drop in the container and observe whether there is leakage outside. According to these, it can be assessed whether the weld is qualified. 

Mechanical test of welded test panels
Non-destructive testing can reveal the inherent defects of the weld, but it does not indicate the mechanical properties of the metal in the heat affected zone of the weld. Therefore, the welded joints are sometimes subjected to tensile, impact, bending and other tests. These tests were performed by the test panels. The test panels used are preferably welded together with the longitudinal joints of the cylinder to ensure consistent construction conditions. The test panels were then tested for mechanical properties. In actual production, only welded joints of new steel grades are generally tested in this respect.

 

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