Welding stress is always present and constantly changing throughout the welding process. For metals with poor weldability, stress and strain are often the cause of weld cracks. If the weldment is very rigid, and the welding sequence and method are not appropriate, even for materials with good weldability, such as low carbon steel, cracks caused by welding stress can occur. To this end, we should try to reduce the welding stress. The commonly used methods are as follows:
Not only should the welding sequence be reasonably arranged to prevent bending and angular deformation, but also a reasonable welding sequence should be selected to reduce stress.
When welding the weld on the plane, it is necessary to ensure that the longitudinal and transverse (especially the transverse) shrinkage of the weld is not greatly restricted. For example, when welding the butt weld, the welding direction should point to the free end. Therefore, although the segmented de-welding method can reduce some deformation, the transverse shrinkage of the weld is greatly hindered, so the welding stress is large.
The weld with the largest shrinkage should be welded first. Because the weld that is welded first is less hindered when it shrinks. So less stress. For example, when a structure has both butt joints and fillet joints, the butt welds should be welded first. Due to the large shrinkage of the butt weld.
When welding joints with cross welds, a welding sequence that ensures that the intersections are not prone to defects must be adopted. For example, T-weld and cross weld should be welded in the order shown in Figure 3-2, so that the transverse shrinkage of the weld is relatively free, which helps to avoid cracks at the intersection of the weld. At the same time, the arc starting and ending of the weld should avoid the intersection or although on the intersection, but when welding another weld that intersects, the arc starting or ending has been shoveled out first. This point must be paid attention to in the welding of large-area panels such as large oil tanks and ship hull construction.
（a）Welding sequence of T-shaped welds；（b）T-shaped weld, the order on the left is incorrect, and the right one is correct;
(c)Welding sequence for crisscross welds
In the hull or vessel, the existing holes are often “plug welded” with steel plates (Figure Welding of holes in structural parts). This kind of girth weld cannot be shortened freely along the longitudinal and transverse directions, so it generates a large welding stress. stress cracks. This kind of crack occurs in the process of temperature drop and always cracks along the weak section. One of the ways to overcome this is to pre-process the patch into a concave drum shape, as shown in Figure Patch Welding. After welding, the repair plate is flattened due to the shrinkage of the weld seam, which reduces the welding stress and deformation and avoids cracks.
A workpiece with a large thickness has high rigidity and is prone to cracks during welding. Under the premise of not affecting the structural strength performance, the method of opening a relief groove near the weld can be adopted. The essence of this method is to reduce the local rigidity of the structure, and try to make the welds have the possibility of free shrinkage. Figure Boiler head repair welding-a The circular head needs to be repaired and welded with a plug. Because the steel plate is thicker and the weld is closed, it is easy to crack after welding. Taking a slot close to the weld (Fig. Boiler head repair welding-b) to reduce the rigidity there, cracks can be avoided during welding. Figure steam turbine rotor welding shows the one-by-one forging and welding structure. Forging is a ring sleeved on the shaft and welded fillet welds. Because the material is alloy steel and is very thick, it is difficult to preheat during welding. After opening the relaxation groove, local preheating is easy to carry out, and cracks can be avoided during welding. Figure Measures to reduce the rigidity of the round bar-a shows a structure in which a round bar is welded to a thick plate. The rigidity of the closed weld is easy to crack after welding, and the cracks can be avoided by taking the measures shown in Figure Measures to reduce the rigidity of the round bar-b and c.
The principle of this approach is to make the temperature distribution as uniform as possible over the entire structure. That is to say, it is required that the temperature of the “part” of the welding part should be controlled as low as possible, and the area occupied by this “part” in the “whole” of the structure should be as small as possible. At the same time, the overall temperature of the structure is as high as possible. For example, indoors are better than outdoor in winter, and the ambient temperature heated to 30~40℃ is better than normal room temperature. This method of making the temperature difference in the structure as small as possible can effectively reduce the welding stress and the thermal stress crack caused by it.
The specific operations are as follows:
Welding parameters with small diameter electrodes and low welding current.
Only weld a very short seam at a time. For example, welding cast iron is only 10 to 40 mm per pass. Welding rigid components, such as patch plate as shown in Figure Welding of holes in structural parts, weld half of the electrode each time. After each weld is finished, wait for the temperature to drop to not hot to the hand before welding the next one.
At the same time, the method of hammering the weld is used. In the cooling process of each weld, the weld is forged with a small hammer, so that the weld metal is thinned by forging and elongated around, offsetting some weld shrinkage and reducing welding stress. This “cold welding” method can also be used to avoid cracks in the patch welding shown in Figure Welding of holes in structural parts, but it is less effective than the process method of pre-processing the patch into a concave drum shape. Sometimes a combination of the two methods is used. Note that on the premise that only half-to-one electrodes are welded in each pass, the section of the first layer of welds should be as thick as possible. Repair welding of cast iron is easy to tear from the fusion line, so the section of each weld should be slightly thinner to reduce cracks.
The principle of reducing welding stress by this method is similar in nature to the “cold welding method, that is, the difference between the temperature of the welding zone and the overall temperature of the structure is also reduced. The smaller the difference, the greater the welding stress after cooling.” The smaller the temperature, the smaller the tendency to crack. One of the overall preheating purposes of hot welding of iron castings and surfacing of many wear-resistant alloys is to reduce this temperature difference and reduce welding stress, thereby preventing cracks. Preheating can also play other roles, and these roles are different for different metals. For example, thermal welding is used for cast iron repair welding, which helps to avoid white spots; thermal welding is used for hardfacing, which helps to improve The structure and properties of the surfacing layer metal and base metal, etc. Since the overall heating effect is different from that of the heated part, the heating temperature is also different.
As we all know, the welding process is a non-uniform heating process. The expansion and contraction of the welding part when heated and cooled are hindered (or constrained) by other parts of the weldment, and stress must be generated. Stress can be avoided or reduced only if it is free to expand and contract.