The methods to prevent and reduce welding deformation must consider welding process design and overcome the variation of hot and cold cycles during welding. Shrinkage cannot be eliminated, but it can be controlled. There are several ways to reduce shrinkage deformation.
1 Do not weld too much
The more metal is filled in the weld, the greater the deformation force will be generated. The correct size of the weld can not only get smaller welding deformation, but also save welding material and time. The amount of welding metal to fill the weld should be minimum, and the weld should be flat or slightly convex. Excessive welding metal will not increase the strength. On the contrary, it will increase the shrinkage force and increase the welding deformation.
2 Discontinuous weld
Another way to reduce the amount of weld filling is to use more intermittent welding. For example, when welding reinforced plates, intermittent welding can reduce the amount of weld filling by 75%, while also ensuring the required strength.
3. Reduce weld passage
The welding with coarse wire and fewer passes has smaller deformation than the welding with thin wire and more passes. In the case of multiple passes, the shrinkage caused by each pass accumulatively increases the total weld shrinkage. As can be seen from the figure, the welding process with fewer passes and thick electrode has better results than that with multiple passes and thin electrode.
Note: The welding process of coarse wire, less pass welding or fine wire, multi-pass welding depends on the material. Generally, low carbon steel, 16Mn and other materials are suitable for rough wire and less pass welding. Stainless steel, high carbon steel and other materials are suitable for fine wire and multi-pass welding
4. Anti-deformation technology
Bend or tilt the parts in the opposite direction of welding deformation before welding (except for invert welding or vertical welding). The preset amount of reverse deformation should be determined by test. Prebending, presetting, or prearching welded parts is a simple way to offset welding stresses by using reverse mechanical forces. When the workpiece is preset, a deformation occurs that causes the workpiece to be opposite to the weld shrinkage stress. The preset deformation before welding cancels out with the deformation after welding, making the welding workpiece an ideal plane.
Another common way to balance the force of contraction is to place the same welders against each other and clamp them together. This method can also be used for pre-bending, where the wedge is placed in the appropriate position of the workpiece before clamping.
Special heavy-duty welders can produce the required balance force due to their own rigidity or the position of parts to each other. If these balance forces are not produced, other methods are needed to balance the shrinkage force of welding materials to achieve the purpose of mutual cancellation. The balance force can be other shrinkage force, mechanical binding force formed by fixture, binding force of assembly and welding sequence of components, binding force formed by gravity.
5 Welding Sequence
According to the structure of the workpiece to determine a reasonable assembly sequence, so that the structure of the workpiece in the same position shrink. Double-sided groove is opened in the workpiece and the shaft, multi-layer welding is adopted, and the double-sided welding sequence is determined. Intermittent welding is used in fillet welds, and the shrinkage in the first weld is balanced by the shrinkage in the second weld. The fixture can hold the workpiece in the desired position, increasing rigidity and reducing welding deformation. This method is widely used in the welding of small workpiece or small components, due to the increase of welding stress, only suitable for the plastic structure of low carbon steel.
6 Remove shrinkage force after welding
Percussion is a way of counteracting weld shrinkage, as is weld cooling. Tapping will cause the weld to extend and become thinner, thus removing stress (elastic deformation). However, when using this method, it must be noted that the root of the weld can not be knocked, which may produce cracks. In general, percussion cannot be used in cover welds.
Because, the cover layer may have weld cracks, affect the weld detection, hardening effect. Therefore, the utilization of technology is limited, and there are even instances that require the tapping only in the multi-layer pass (except the bottom welding and cover welding) to solve the deformation or crack problem. Heat treatment is also one of the methods to remove the shrinkage force, control the high temperature and cooling of the workpiece; Sometimes the same workpiece back to back clamping, welding, with this aligning condition to eliminate stress, so that the workpiece residual stress is minimal.
6. Reduce welding time
Welding produces heating and cooling, and it takes time to transfer heat. Therefore, the time factor also affects the deformation. In general, it is desirable to finish the welding as soon as possible before the bulk of the workpiece is heated and expanded. Welding process, such as the type and size of the electrode, welding current, welding speed and so on affect the degree of shrinkage and deformation of the welding workpiece. The use of mechanized welding equipment reduces the welding time and the amount of deformation caused by heat.
Second, other methods to reduce welding deformation
1 Water cooling block
Many techniques can be used to control the welding deformation of special welders. For example, in thin sheet welding, the use of water-cooled blocks can take away the heat of the welded workpiece. The copper pipe is welded to the copper fixture by brazing or soldering, and the pipe is cooled in circulation to reduce welding deformation.
2 Wedge block positioning plate
“Positioning plate” is an effective control of welding deformation of steel plate butt welding technology, as shown in the figure. One end of the positioning plate is welded on a plate of the workpiece, and the other end of the wedge block is wedged into the pressing plate. Multiple positioning plates can even be arranged to maintain the positioning and fixing of the welding steel plate during welding.
3. Eliminate thermal stress
Except in special cases, the use of heating to remove stress is not the correct method, should be done before the workpiece is welded to prevent or reduce welding deformation.
In order to reduce the influence of welding deformation and residual stress, the following points should be paid attention to when designing and welding the workpiece:
(1) No excessive welding; (2) Control the positioning of the workpiece; (3) Use discontinuous welding as far as possible, but should meet the design requirements; (4) As small as possible welding foot size; (5) For open groove welding, the welding amount of the joint should be minimized, and bilateral groove should be considered to replace single groove joint; (6) Multi-layer and multi-pass welding should be adopted as far as possible to replace single-layer and bilateral welding. Open double-sided groove welding at the workpiece and shaft, adopt multi-layer welding, and determine the double-sided welding sequence; (7) multi-layer less pass welding; (8) Adopt low heat input welding process, which means higher melting rate and faster welding speed; (9) The positioner is used to make the workpiece in the ship-shaped welding position. Ship-shaped welding position can use large diameter wire and high fusion rate welding process; (10) As far as possible in the workpiece’s neutralization shaft set weld, and symmetrical welding; (11) As far as possible through the welding sequence and welding positioning to make the welding heat evenly spread; (12) Welding to the unconstrained direction of the workpiece; (13) Use fixture, tooling and positioning plate for adjustment and positioning. (14) Prebend the workpiece or preposition the weld joint in the opposite direction of contraction. (15) Separate welding and total welding according to the sequence, the welding can keep balance around the neutralization shaft.
Post time: Dec-19-2022