Welding deformation
Types of welding deformation
The overall deformation of the welded structure varies from component to component. Common deformations are: the butt welding of steel plates will cause the deformation of shortened length and narrowed width; angular deformation will occur when V-shaped grooves are used; when the steel plate is thin, wave deformation may occur; the welding of special-shaped steel beams will produce distortion deformation, etc. The basic form of welding deformation is shown in Figure welding deformation. Generally speaking, several deformations may occur at the same time after the components are welded.
(a) longitudinal shortening and transverse shortening; (b) angular deformation; (c) bending deformation; (d) wave deformation; (e) twisting deformation
Welding deformation longitudinal shortening and lateral shortening
- Vertically shortened. The shortening of the weldment along the length of the weld after deformation is called longitudinal shortening. The longitudinal shrinkage deformation value of the weld increases with the increase of the length of the weld and the cross-sectional area of the deposited metal. decrease as the cross-sectional area increases. For weldments of the same thickness, the longitudinal shrinkage deformation produced by multi-layer multi-pass welding is less than that of single-layer welding. The longitudinal deformation shrinkage of butt and fillet welds is shown in Tables Longitudinal deformation shrinkage of butt and fillet welds.
- Horizontal shortening. The shrinkage of the weldment perpendicular to the direction of the weld after welding is called lateral shortening, and the amount of lateral shortening deformation increases with the increase of welding heat input, and increases with the increase of plate thickness.
Corner deformation
The angular deformation is the rotational deformation caused by the uneven lateral shrinkage of the weld area along the thickness of the plate during welding. The magnitude of the angular deformation is measured by the deformation angle α , as shown in Figure Corner deformation. Corner deformation often occurs during surfacing, lap and T-joint welding.
The deformation of the welding angle is not only related to the shape of the cross-sectional area of the weld and the form of the groove, but also to the welding operation method. For the same plate thickness and groove form, multi-layer welding has larger corner deformation than single-layer welding, and the more welding layers, the greater the corner deformation.
（a）Corner deformation of V-groove butt joint after welding；（b）Corner deformation of double V-groove butt joint after welding
Bending deformation
The bending deformation is mainly the deformation caused by the asymmetrical arrangement of the welds on the structure or the asymmetrical shape of the cross-section of the weldment, and the shrinkage of the welds.
The size of the bending deformation is measured by the deflection f. Deflection f refers to the maximum distance that the central axis of the weldment deviates from the original central axis after welding, as shown in Figure Bending deformation.
Distortion
If the deformation of the weld corner is unevenly distributed along the length direction, the longitudinal direction of the weldment is wrong, or the assembly is poor, and the welding sequence is unreasonable, resulting in no certain rules for the longitudinal shrinkage and lateral shrinkage of the weld. Will cause distortion of the component.
Wave deformation
Due to the small structural rigidity, the deformation caused by the large compressive stress caused by the combined action of the longitudinal shrinkage and the transverse shrinkage of the weld. Thin plates are prone to wave deformation. In addition, when several welds are very close together, wave deformation will also be formed due to the angular deformation of the fillet welds.
Through the above analysis, it is explained that the longitudinal shrinkage and transverse shrinkage of the weld after welding are the root causes of various deformations and welding stresses. At the same time, it also shows that whether the shrinkage of the weld can be transformed into various forms of deformation is directly related to the position of the weld in the structure, the welding sequence and repeatability, and the rigidity of the structure.
Control and correction of welding deformation
Welding deformation design measures
Reasonable design of welding structure
Design a reasonable welding structure, including reasonably arranging the position of the welding seam, reducing unnecessary welding seams; reasonably selecting the shape and size of the welding seam, etc. For structures such as beams and columns, in order to reduce the bending deformation, the symmetrical arrangement of the welds should be adopted as far as possible.
Select a reasonable weld size
The shape and size of the seam are not only related to the welding deformation, but also determine the workload of the welder. When the size of the weld increases, the welding deformation also increases. However, if the size of the weld is too small, the bearing capacity of the structure will be reduced, and the cooling rate of the joint will be accelerated, resulting in a series of welding defects, such as cracks and heat-affected zones. increase in hardness, etc. Therefore, on the premise of satisfying the bearing capacity of the structure and ensuring the welding quality, the minimum weld size possible in the process is selected according to the plate thickness. For fillet welds commonly used for rib and web connection, the size of the fillet should not be too large, so there are generally corresponding regulations for the size of the fillet. Table Low carbon steel minimum fillet size is the recommended size of small fillet for mild steel welds. When welding low alloy steel, the size of the weld leg can be slightly larger than the recommended value in the table because it is sensitive to the cooling rate.
Minimize the number of welds
Appropriate selection of the thickness of the plate can reduce the number of rib plates, thereby reducing the amount of welding seam and post-weld deformation correction. For structures that do not require strict self-weight, it is economical to do so even if the weight is slightly larger. For the thin plate structure, the profiled structure can be used instead of the rib structure to reduce the number of welds and prevent welding deformation.
Reasonable arrangement of welding seam position
The welding seam is symmetrical to the central axis of the component section, or the welding seam is close to the central axis, which can reduce the bending deformation, the welding seam should not be dense, and the cross welding seam should be avoided as much as possible. The distance between the longitudinal weld and the distance between the head joint and the longitudinal weld of the adjacent tube section shall be greater than 3 times the wall thickness, and shall not be less than 100 mm.
Welding deformation process measures
Appropriate process measures are taken during welding, including anti-deformation method, heat treatment method for controlling welding deformation by assembly sequence and welding sequence, symmetrical welding method, rigid fixing method and hammering method, etc., which can control or correct welding deformation.
Choose a reasonable assembly sequence
- Choose a reasonable assembly sequence. The structure with high rigidity has small deformation, and the structure with small rigidity has large deformation. The rigidity of a welded structure is gradually increased during the assembly and welding process, and the assembly and welding sequence have a great influence on the deformation of the welded structure. Therefore, reasonable assembly is often used to control deformation in production. For structures with symmetrical cross-section and symmetrical welds, first assemble them into a whole, properly divide the structural parts into components, assemble and weld them separately, and then tailor-weld them into a whole to make asymmetric welds or welds with larger shrinkage. The seam can shrink relatively freely without affecting the overall structure. Then welding in a reasonable welding sequence can reduce the deformation. According to this principle to manufacture complex welded structures, it is beneficial to control welding deformation and shorten the production cycle.
- Choose a reasonable welding sequence. If there is only a reasonable assembly sequence without a reasonable welding sequence, deformation will still occur. Therefore, there must be a reasonable welding sequence when splicing large-area plates.
- Symmetrical welding is used when the weld is symmetrical. When the structure has symmetrically arranged welds, if the welding is performed sequentially by a single person, since the welds welded first have a large deformation, the entire structure will still have a large deformation after welding. Welds with a symmetrical arrangement are welded symmetrically (preferably by two welders symmetrically) so that the deformations caused by the individual welds cancel each other out. If the welding can not be carried out completely symmetrically at the same time, the welds are allowed to be welded successively, but the welding sequence should be as symmetrical as possible, which can also reduce the deformation of the structure.
- When the weld is asymmetric, weld the side with less weld first. Because the deformation of the welding seam welded first is large, the total deformation caused by the first welding on the side with less welding seam is not large, and then the deformation caused by the more welding seam on the other side can be used to offset it, so that the deformation of the whole structure can be reduced. deformation, so that the deformation after welding is minimal.
- Welding of complex structures. For a complex structure, it can be divided into several simple parts and welded separately, and then assembled and welded. This allows those welds with asymmetrical or large shrinkage to shrink as freely as possible. It will not affect the overall structure, so as to control the overall welding deformation.
- Long seam welding. For long welds on weldments, different welding directions and sequences are used according to the length. If the weld is more than 1 m, the segmented welding method and jump welding method can be used; for the weld of medium length (0.5~1 m), the divided butt welding method can be used. In addition, when welding the welds of important components (such as pressure vessels, etc.), it is necessary to carefully operate according to the process requirements to ensure the quality of the welds.
- Welding of different seams. If there are several shapes of welds on the structure, the butt welds should be welded first, followed by fillet welds and other welds. When combined into a circular simple weldment, the longitudinal weld should be welded first, and then the transverse weld should be welded.
Inverse deformation method
In order to counteract the welding deformation, the weldment is artificially deformed in the opposite direction to the welding deformation during assembly before welding. This method is called the inverse deformation method. For example, the corner deformation of V-shaped groove one-sided butt welding is basically eliminated after reverse deformation, as shown in Figure Anti-Deformation Control. Sometimes in order to eliminate deformation, the weldment can be bent before welding.
For large and rigid workpieces, the components can be made into anti-deformation of predetermined size and direction during blanking. This kind of component is usually solved by the method of making the upper arch on the top of the web. When the bridge crane girder is blanking, the webs on both sides are pre-welded into an upper arch with a span greater than 1/1000 of the bridge crane.
rigid fixation
Before welding, the welded parts are restrained by external rigidity, and the welded parts cannot be deformed freely during welding. This method of controlling deformation is called rigid fixing method. However, this method will generate large residual stress in the welded joint, and it should be used with caution for some materials that are easy to crack after welding.
Layered welding
When welding parts with larger thickness, layer welding can be used to reduce welding stress and deformation. In order to reduce the internal stress, it is best to weld each layer into a wave-shaped weld, as shown in Figure Layered Welds and Wave Welds. When welding, the second layer of welding seam should cover the first layer of welding seam, and its welding seam should be twice as long as the first layer; mm, and finally fill up the short weld. This method can use the heat of the back layer welding to heat preservation and slow cooling of the front layer weld, eliminate stress, reduce deformation and prevent weld cracks.
Preheat and post heat treatment before welding
Preheating the weldment before welding can not only reduce the temperature difference between the heated part and the unheated part of the weldment, but also reduce the cooling rate of the weldment, so as to reduce the internal stress and the deformation of the weldment.
The preheating temperature is generally determined by the carbon content of the weldment. In general: the preheating temperature of carbon steel is 250 ~ 450℃; the preheating temperature of aluminum is 200 ~ 300℃
After welding, it can be slowly cooled (annealed) in the furnace or in the insulation material, or tempered, and the internal stress is greatly reduced. The holding time in the furnace after welding is generally 12 to 20 hours, and it can also be held for more than 24 hours. Tempering is carried out in the furnace, the tempering heating temperature is 600~650℃, and the furnace is cooled after holding for a certain period of time.
heat dissipation
During welding, forced cooling is used to dissipate heat in the welding area, and the purpose of deformation is achieved due to the reduction of the heated area. Forced cooling can immerse the weld around the weld, or use a copper cooling block to increase the heat dissipation of the weldment. The heat dissipation method is more effective in reducing the welding deformation of thin-plate weldments, but the heat dissipation method is not suitable for welding materials with high hardenability.
Correction method of welding deformation
In the production process of welded structures, although a series of measures have been taken, welding deformation is always inevitable. When the residual deformation value generated by welding exceeds the technical requirements, measures must be taken to correct it.
There are two ways to correct the deformation of the welded structure: mechanical correction method and flame correction method.
mechanical correction
Using mechanical methods such as manual hammering, presses and other mechanical methods to produce new plastic deformation of the material of the component, which extends the original multi-segment part, thereby correcting the deformation. The welding seam is relatively regular thin shell structure, and the disc-shaped rolling seam of the narrow wheel rolling machine and its two sides are often used to extend it to eliminate deformation.
flame correction
The local compressive plastic deformation produced by the flame heating causes the longer metal material to shorten after cooling to eliminate the deformation. The heating temperature and location should be controlled during use.
For low carbon steel and ordinary low alloy steel, the heating temperature of 600 ~ 800°C is often used. Since this method requires reheating the component to a high temperature, it should be used with caution for materials such as alloy steel.
There are three methods of flame correction:
- The spot heating method is mostly used for thin plate structures. The heating diameter d ≥ 15 mm, the center distance α of the heating point is 50~100 mm;
- The linear heating method is mostly used to correct angular deformation, twisting deformation and excessive diameter or ovality of the cylinder.
- The triangular heating method is mostly used to correct bending deformation.
Notes on using the flame correction method:
Before correcting the deformation, carefully analyze the deformation, formulate a corrective work plan, determine the heating position and corrective steps.
Carefully understand the material properties of the corrected structure. Materials with good weldability have little change in material properties after flame correction. For high-strength steels that have been heat treated, the heating temperature should not exceed their tempering temperature.
Carefully understand the material properties of the corrected structure. Materials with good weldability have little change in material properties after flame correction. For high-strength steels that have been heat treated, the heating temperature should not exceed their tempering temperature.
To correct the deformation of the sheet, use a wooden hammer when hammering is required.
The heating flame generally adopts neutral flame.