Views: 2 Author: Site Editor Publish Time: 2023-09-28 Origin: Site
Methods to prevent and reduce welding distortion must consider welding process design and overcome changes in hot and cold cycles during welding. Shrinkage cannot be eliminated, but it can be controlled. The ways to reduce shrinkage deformation are as follows.
1. Do not over-solder
The more metal filling at the welding point, the greater the deformation force will be. Properly sizing the weld seam can not only achieve smaller welding deformation, but also save welding materials and time. The amount of weld metal filling the weld should be minimal, the weld should be flat or slightly convex, and excess weld metal will not increase strength. On the contrary, it will increase the shrinkage force and increase the welding deformation.
2. Interrupted welds
Another way to reduce the amount of weld fill is to use more interrupted welding. For example, when welding reinforcement plates, intermittent welding can reduce the amount of weld filling by 75% while ensuring the required strength.
3. Reduce weld beads
Welding with thick welding wire and few welding passes will have less deformation than welding with thin welding wire and multiple welding passes. When there are multiple weld passes, the shrinkage caused by each weld pass cumulatively increases the total shrinkage of the weld. The welding process with fewer welding passes and thicker electrodes has better results than the welding process with multiple welding passes and thinner electrodes.
Note: The welding process of thick welding wire, few welding passes or fine welding wire, multiple welding passes depends on the material. Generally, low carbon steel, 16Mn and other materials are suitable for thick welding wire and few welding passes, while stainless steel, high carbon steel and other materials are suitable for fine welding. Welding wire, multi-pass welding
4. Anti-deformation technology
Before welding, the parts should be bent or tilted in the opposite direction of the welding deformation (except for overhead welding or vertical welding). The preset amount of anti-deformation needs to be determined through testing. Pre-bending, pre-setting or pre-arching welded parts is a simple way to counteract welding stresses by counteracting mechanical forces. When the workpiece is preset, deformation occurs that causes the workpiece to oppose the shrinkage stress of the weld. The pre-weld deformation and post-weld deformation cancel each other out, making the welding workpiece an ideal plane.
Another common method of balancing shrinkage forces is to place identical welded workpieces opposite each other and clamp them. This method can also be used for pre-bending, where the wedge is placed in the appropriate position on the workpiece before clamping.
Special heavy-duty welding workpieces can generate the required balancing force due to their own rigidity or the mutual position of the parts. If these balancing forces are not generated, other methods need to be used to balance the shrinkage force of the welding material to offset each other. The balancing force can be other shrinkage forces, mechanical restraints formed by tooling fixtures, restraints formed by the assembly and welding sequence of components, and restraints formed by gravity.
Determine a reasonable assembly sequence according to the structural form of the workpiece so that the workpiece structure shrinks at the same position. Make double-sided grooves on the center and axis of the workpiece, use multi-layer welding, and determine the double-sided welding sequence. Intermittent welding is used in fillet welds, and the shrinkage in the first pass is balanced by the shrinkage in the second pass. The tooling fixture can fix the workpiece in the required position, increase rigidity and reduce welding deformation. This method is widely used for welding small workpieces or small components. Due to the increased welding stress, it is only suitable for low carbon steel structures with good plasticity.
6. Remove shrinkage force after welding
Tapping is a method of counteracting the shrinkage forces of the weld, as the weld cools. Tapping will cause the weld to extend and become thinner, thus relieving stress (elastic deformation). However, when using this method, attention must be paid to not tapping the root of the weld, as cracks may occur when tapping. In general, tapping also cannot be used on cap welds.
Because the cover layer may have weld cracks, which will affect the weld detection and produce a hardening effect. Therefore, the utilization of the technology is limited, and there are even instances where it is required to tap only within the multi-layer weld bead (except bottom welding and cover welding) in weld bead tapping to solve the problem of deformation or cracks. Heat treatment is also one of the methods to remove shrinkage force by controlling the high temperature and cooling of the workpiece; sometimes the same workpiece is clamped and welded back to back, and this alignment condition is used to eliminate stress and minimize the residual stress of the workpiece.
7. Reduce welding time
Heating and cooling occur during welding, and it also takes time to transfer heat. Therefore, the time factor also affects deformation. Usually, you want the welding to be completed as quickly as possible before the large workpiece expands due to heat. The welding process, such as the type and size of the welding rod, welding current, welding speed, etc., affect the degree of shrinkage and deformation of the welded workpiece. The use of mechanized welding equipment reduces welding time and the amount of distortion caused by heat.
Other methods to reduce welding deformation
1.Water cooling block
Many techniques are available to control welding deformation of special welding workpieces. For example, in thin plate welding, a water cooling block can be used to remove heat from the welding workpiece. The copper pipe is welded to the copper fixture using brazing or soldering, and is circulated and cooled through water pipes to reduce welding deformation.
2. Wedge block positioning plate
"Positioning plate" is an effective technology for controlling welding deformation during butt welding of steel plates, as shown in the figure. One end of the positioning plate is welded to a plate of the workpiece, and the other end wedges the wedge block into the pressure plate. Multiple positioning plates can even be arranged to maintain the positioning and fixation of the welded steel plate during welding.
3. Eliminate thermal stress
Except for special circumstances, using heating to relieve stress is not the correct method. Welding deformation should be prevented or reduced before the workpiece is welded.
In order to reduce the effects of welding deformation and residual stress, the following points should be noted when designing and welding workpieces:
(1) Do not perform excessive welding; (2) Control the positioning of the workpiece; (3) Use intermittent welding as much as possible, but it should meet the design requirements; (4) Use the smallest welding leg size possible; (5) For slope openings For welding, the welding amount of the joint should be minimized, and double-sided grooves should be considered to replace single-sided groove joints; (6) Multi-layer multi-pass welding should be used as much as possible instead of single-layer double-sided alternating welding. Make double-sided groove welding in the workpiece and on the axis, use multi-layer welding, and determine the double-sided welding sequence; (7) Use multi-layer welding with fewer weld passes; (8) Use low heat input welding technology, which means higher Deposition rate and faster welding speed; (9) Use a positioner to place the workpiece in the boat welding position. Large-diameter welding wire and high deposition rate welding process can be used for the ship welding position; (10) Set the weld on the neutral axis of the workpiece as much as possible, and weld symmetrically; (11) Try to use the welding sequence and welding positioning as much as possible Make the welding heat spread evenly; (12) Weld in the unconstrained direction of the workpiece; (13) Use fixtures, tooling and positioning plates for adjustment and positioning. (14) Pre-bend the workpiece or pre-set weld joint in the opposite direction of shrinkage. (15) Partial welding and total welding in sequence can keep the welding balanced around the neutral axis.
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