Why do stresses and deformations occur during welding?

The uneven temperature field of the welding process and the local plastic deformation and the different specific volume of the structure caused by it are the root causes of welding stress and deformation.

When the uneven temperature field caused by welding has not disappeared, this stress and deformation in the weldment is called transient welding stress and deformation; The stress and deformation after the disappearance of the welding temperature field is called residual welding stress and deformation. In the absence of external forces, the welding stress is balanced inside the weldment.

Welding stress and deformation can affect the function and appearance of weldments under certain conditions, so they are issues that must be considered in design and manufacturing.

As long as welding is involved, there is no such thing as no deformation at all, because it is caused by the effect of thermal expansion and cold contraction, so it can also be said that temperature is the main factor causing deformation. Welding: also known as welding, bonding, is a manufacturing process and technology that joins metals or other thermoplastic materials such as plastics by heating, high temperature or high pressure.

Because the weldment is subjected to local and uneven heating and cooling during the welding process, the degree of thermal expansion and contraction of the metal in each part of the welded joint is different. Because the weldment itself is a whole, each part is interconnected and mutually restricted, and cannot be freely elongated and shortened, so stress and deformation will occur during the welding process!

The basic forms of welding deformation include shrinkage deformation, angular deformation, bending deformation, wave deformation and twisting deformation. During the welding process, uneven heating and cooling of the weldment is the root cause of welding stress and deformation. The process measures to reduce welding stress and deformation mainly include:

1. Reserve shrinkage deformation According to theoretical calculation and practical experience, the shrinkage margin is considered in advance when preparing and processing weldments, so that the workpiece can reach the required shape and size after welding.

2. Anti-deformation method According to theoretical calculation and practical experience, the direction and size of the structural welding deformation are estimated in advance, and then a preset deformation with the opposite direction and equal size is given during welding and assembly to offset the deformation generated after welding.

3. Rigid fixing method The weldment is rigidly fixed during welding, and the rigid fixing is removed after the weldment cools to room temperature after welding, which can effectively prevent angular deformation and wave deformation. This method will increase the welding stress, and is only suitable for low-carbon steel structures with good plasticity.

Fourth, choose a reasonable welding sequence and try to make the weld shrink freely. When welding structural parts with many welds, the staggered short welds should be welded first, and then the straight through long welds should be welded to prevent cracks at the weld junction. If the weld is long, the step-by-step desoldering method and skip welding method can be used to make the temperature distribution more uniform, thereby reducing the welding stress and deformation.

5. Hammer weld method In the cooling process of the weld, the weld is hammered evenly and quickly with a round-headed hammer to produce plastic extension deformation of the metal and offset a part of the welding shrinkage deformation, thereby reducing the welding stress and deformation.

6. Heating "subtraction zone" method Before welding, the area near the welding part (called the subtraction zone) is heated to make it elongate, and when it is cooled after welding, the heating zone shrinks together with the weld, which can effectively reduce the welding stress and deformation.

7. Preheating before welding and slow cooling after welding The purpose of preheating is to reduce the temperature difference between the weld area and other parts of the weldment, reduce the cooling rate of the weld area, and make the weldment cool down more uniformly, thereby reducing the welding stress and deformation.

Generation and prevention of welding stress.

The main cause of welding stress and deformation is uneven heating.

Reduction method: reasonable design of poor welded components Under the condition of ensuring that the structure has sufficient bearing capacity, the number of welds, the length of the weld and the cross-sectional area of the weld should be minimized; It is necessary to make the welds in the structure as symmetrical as possible. When welding thick and large parts, grooves on both sides should be opened for welding to avoid cross-cutting or dense welds.

Try to use large-size sheet metal and suitable shaped steel or stamping parts instead of tailored welding, so as to reduce the number of welds and reduce deformation.

Take the necessary technical measures: anti-deformation method Anti-deformation method refers to the calculation or pre-judgment of the deformation size and direction after welding, or when assembling before welding, the weldment is placed in the opposite position to the direction of welding deformation. Or deform the workpiece in the opposite direction before welding to counteract the deformation that occurs after welding.

Margin method The margin method is to add 0 1 -0 2 shrinkage to the weldment before welding to supplement the shrinkage after welding.

Rigid clamping method The rigid clamping method uses fixtures or spot welding fixation to restrain welding deformation. This method can effectively prevent angular deformation and wavy deformation of thin plate structures. The rigid clamping method can only be applied to some welding materials with good plasticity, and the celery skin should be quickly annealed to eliminate internal stress, and the materials with poor plasticity, such as steel and cast iron with greater hardening can not be used, otherwise, cracks are easy to occur after welding.

Choose a reasonable welding sequence A reasonable selection of welding sequence can greatly reduce the deformation. If there are welds on both sides of the symmetry of the component, try to make the shrinkage of the welds on both sides cancel each other out or weaken.

Welding stress is caused by uneven heat input from welding.

To put it simply, it is the principle of thermal expansion and cold contraction.

Different areas have different heat and different deformations, resulting in stress.

Reduction measures: 1. In the case of meeting the requirements, try to use small current to reduce heat input; Staring at Zen.

2. Try to weld symmetrically, from the middle to both sides, and weld in sections;

3. Pre-do anti-deformation before welding;

4. Post-welding heat treatment to eliminate the dust force of Yingkaizhou;

5. Mechanical correction method arch flat.

Answer]: Such as pretending to be 1Adopt appropriate welding procedures, such as segmented slag stove welding and layered welding; 2.

Symmetrical welds are used as much as possible to offset the deformation in the opposite direction; 3.Before welding, the structure has a pre-deformation opposite to the welding deformation; 4.For small components, preheating before welding, tempering after welding, and then slowly cooling to eliminate welding stress.