1, the reason of deformation

(1) Too many welding layers.

(2) Improper welding sequence.

(3) Inadequate construction preparation.

(4) The substrate is cooled too fast.

(5) The parent metal is overheated. (thin plate)

(6) Improper weld design.

(7) Too much welding metal.

(8) the way of restraint is not true.

2. Methods to reduce deformation

(1) Use covered electrode with larger diameter and larger current.

(2) Correct the welding sequence

(3) Before welding, fix the weldment with a fixture to avoid warping.

(4) Avoid cooling too fast or preheating the parent material.

(5) Choose welding materials with low penetration.

(6) Reduce the gap between welds and the degree of grooving.

(7) Pay attention to the welding size, so as not to make the weld bead too large.

(8) Pay attention to the fixed measures to prevent deformation.

Second, methods to prevent and reduce the stress of welded structures

1, choose a reasonable assembly and welding sequence.

(1) Let the recovery energy contract freely as much as possible.

1) For large welded structures, welding should be carried out from the middle to the periphery. Only in this way can the recovery shrink from the middle to the outside in turn and reduce the welding stress.

2) For I-beams with ribbed plates, if the cover plate and the web plate are welded first, and then the rib plate and the web plate are restored, the lateral contraction caused by angle recovery will generate great stress between the cover plate and the web plate. If the weldments are welded one by one from the middle and the two sides are symmetrical, the welding stress will be greatly reduced.

(2) When the shrinkage is maximum, the welding should be resumed first.

1) The recovery of the first welding is little hindered, so there is some deformation but less stress after welding.

2) Welds with large shrinkage are prone to generate large welding stress. Therefore, the welding stress can be reduced by welding the weld with the largest shrinkage on the weldment first. If there are both butt welds and fillet welds on the weldment, as the shrinkage of the butt weld is greater than that of the weld, try to weld the butt weld first.

(3) When the plane crosses, the transverse weld should be welded first.

1) will produce a large welding stress at the intersection of welds, so if the design is inevitable, a reasonable welding sequence should be adopted.

2) The reasonable sequence of T-weld and cross weld should ensure that the transverse weld is welded first to make it shrink freely, so as to reduce the welding stress.

Note: the arc starting point and arc closing point should be avoided at the intersection of welds.

2. Choose reasonable welding parameters

When welding, covered electrode (welding wire) with small diameter and small heat input should be used as far as possible according to the specific conditions of weldments, so as to reduce the heating range of weldments and thus reduce welding stress.

3, preheating method

(1) Before welding, heat the whole (or part) of the weldment, generally at 150 ~ 350℃ to reduce the temperature difference of the whole weldment in the welding area. The smaller the temperature difference, the more uniform the cooling of the weld zone and the whole structure, thus reducing the internal stress.

(2) This method is usually used for materials with high hardening tendency or for repairing weldments with high hardness. The preheating temperature varies according to the physical properties of metal materials, structural rigidity and heat dissipation conditions.

4, heating "stress reduction zone" method

Select an appropriate part of the weldment and heat it to make it elongate. Welding after heating can greatly reduce the original gold of weldments with high rigidity. It can greatly reduce the temperature difference between the parts that hinder the free contraction of the joint in the welding area of the weldment, and can uniformly cool and contract to carry out welding stress.

5, hammering method

(1) Welded metal is subjected to tensile stress because it is blocked during cooling and shrinkage. If the welding metal is struck with a hammer or power hammer during post-weld cooling, it will cause plastic deformation of the welding metal, which can offset a certain amount of welding shrinkage and reduce welding stress.

(2) Practice has proved that tapping the first layer of weld metal can almost completely eliminate the internal stress. In order to prevent cracks, hammering should be carried out when the weld has good thermal plasticity; However, it is not advisable to hammer the cover weld, because it will destroy the appearance of the weld.