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Welding stress and Internal stress 2/3

Adjustment of residual welding stress

Welding stress is always present and constantly changing throughout the welding process. For metals with poor weldability, stress and strain are often one of the causes 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 will occur. To this end, should try to reduce the welding stress, the commonly used methods are as follows.

Take a reasonable welding sequence

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 lateral (especially the lateral) shrinkage of the weld is not subject to greater constraints. 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 first welded seam is less hindered when shrinking, the stress is smaller. For example, when a structure has both butt and fillet welds, the butt weld should be welded first because the shrinkage of the butt weld is greater.

  • 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-shaped welds and crisscross welds should be welded in the order shown in Figure Welding sequence for cross welds, so that the lateral shrinkage of the welds is relatively free and helps to avoid cracks at the intersections of the welds. At the same time, the arc starting and ending of the weld should avoid the intersection, or although it is on the intersection, 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.
Welding sequence for cross welds

Leave a margin in advance to ensure that the weld can shrink freely

Welding of holes in structural parts

On the hull or vessel, the existing holes are often plugged and 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 produces a large welding stress. In the weld area, especially when welding the first and second layers of welds, it is easy to generate thermal stress that is torn by stress. crack. This kind of crack occurs in the process of temperature drop and always cracks along the weak section. One of the ways to overcome it is to press the edge of the patch into a certain concave drum shape, as shown in Figure Patch Welding Method. 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.

Patch Welding Method

Method for reducing stress by opening alleviation grooves

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. The circular head shown in Figure Boiler head repair welding-a 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. Take the method of opening a relief groove near the welding seam (Figure Boiler head repair welding-b) to reduce the rigidity of the place, and cracks can be avoided during welding. Figure Welding of steam turbine rotors shows a forged welded structure, the forged ring is sleeved on the shaft, and the fillet weld is welded. Because the material is alloy steel and it is very thick, it is difficult to preheat during welding. After the relaxation groove is opened, local preheating is easy to carry out, and cracks can be avoided during welding. Figure Measures to reduce the rigidity of the end of the round bar-a shows the structure of a round bar welded to a thick plate. The closed weld has high rigidity and is easy to crack after welding. Cracks can be avoided by taking the measures shown in Figure b or Figure c.

Boiler head repair welding
Welding of steam turbine rotors
Measures to reduce the rigidity of the end of the round bar

Cold welding method

The principle behind 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 higher the overall temperature of the structure, the better. 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 resulting thermal stress cracking.

The specific operations are as follows:

  • Use a small diameter electrode and select welding parameters with low welding current.

  • Only weld a very short seam at a time. For example, welding cast iron is only 10 ~ 40mm per pass. Welding rigid components, as shown in Figure Patch Welding Method to patch the plate, welding half of the electrode each time. After each weld is welded, the next one can be welded only when the temperature drops to a point where it is not hot to the touch.
  • At the same time, the method of hammering the weld is used. During 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 Patch Welding Method, 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 the section of the first layer of weld should be as thick as possible on the premise that only half-to-root electrodes are welded in each pass. The cold welding method has also successfully repaired a large number of cylinders and other cast iron castings. 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.

Overall preheating method

The principle of reducing welding stress by this method is similar in nature to that of cold welding, that is, the difference between the temperature of the welding zone and the temperature of the structure as a whole is also reduced. The smaller the difference, the smaller the welding stress after cooling and the smaller the tendency to crack. One of the purposes of the overall preheating in the thermal welding of iron castings and the surfacing of many wear-resistant alloys is to reduce this temperature difference and reduce the welding stress, thereby preventing cracks. Preheating can also play other roles. For different metals, this role is also different. For example, hot welding is used for cast iron repair welding to avoid white cracks, and for wear-resistant surfacing welding, it helps to improve the structure and properties of the surfacing layer metal and base metal. Since the function of the overall heating is different from that of the heated part, the heating temperature is also different.

Using the heated "reduced zone" method

As we all know, the welding process is a non-uniform heating process. The expansion and contraction of the welding part when heated 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.

The method of heating the “stress reduction zone” is to heat those parts (“stress reduction zone”) that hinder the free expansion and contraction of the welding zone, so that it expands and contracts at the same time with the welding zone, which reduces the welding stress. This method is also known as the heating removal constraint method.

Schematic diagram of the heating reduced strain zone method

Figure Schematic diagram of the heating reduced strain zone method is a schematic diagram of the heating “stress reduction zone” method. In the figure, the fracture in the middle of the “sun”-shaped frame needs to be repaired by welding. During repair welding, cracks are often generated due to the large lateral shrinkage of the weld. The “stress reduction zone” is heated at the same time (usually with a gas torch), this zone is heated to expand and stretch, and the butt gap of the welded part that has not been heated is pulled apart. The distance depends on the extension of the “stress reduction zone” when heated. value of . At this time, repair welding is carried out immediately. At the end of welding, the welding area and the “stress reduction area” are at high temperature at the same time.Then the two zones cool and shrink at the same time (Fig. Schematic diagram of the heating reduced strain zone method-b), and there is no mutual obstruction, which reduces the welding stress.

Figure Schematic diagram of the heating reduced strain zone method is a schematic diagram of the heating “stress reduction zone” method. In the figure, the fracture in the middle of the “sun”-shaped frame needs to be repaired by welding. During repair welding, cracks are often generated due to the large lateral shrinkage of the weld. The “stress reduction zone” is heated at the same time (usually with a gas torch), this zone is heated to expand and stretch, and the butt gap of the welded part that has not been heated is pulled apart. The distance depends on the extension of the “stress reduction zone” when heated. value of . At this time, repair welding is carried out immediately. At the end of welding, the welding area and the “stress reduction area” are at high temperature at the same time.Then the two zones cool and shrink at the same time (Fig. Schematic diagram of the heating reduced strain zone method-b), and there is no mutual obstruction, which reduces the welding stress.

This method is the most widely used and most effective in cast iron repair welding. The key to the success or failure of the application of this method lies in the correct selection of the heating part. The general principle is to select those parts that hinder the expansion and contraction of the welding zone. Due to the complex shape of the actual structure, it is unavoidable that sometimes the wrong heating part is selected. The correct way to check whether the heating part is selected is to use a gas welding torch to try heating the place.Open, it means the selection is correct, otherwise it is incorrect. Several examples of selecting the “reduction zone” are described below.

As shown in Figure The heating part is not on the same cross section as the part to be welded, the heating part and the part to be welded are not on the same cross section, but on the cross section of other parts. Most of these weldments belong to the frame or rod structure, and their heating parts are more than two, and they must be heated at the same time. The combined result of the thermal expansion everywhere is the opening displacement of the place to be welded. Welding in this condition reduces stress.

A method of heating the "reduced zone" using the entire cross-section

As shown in Figure A method of heating the “reduced zone” using the entire cross-section, a certain cross section of the entire weldment is selected as the heating part. The place to be welded is also on this cross section. Doing so is equivalent to dividing the entire weldment into left and right or upper and lower sections. When the section is heated to expand, the two sections are free to move to both ends; when cooled after welding, they contract back freely, resulting in a reduction in stress.

The heating part is not on the same cross section as the part to be welded

Figure Choose heating in areas that have little contact with other areas shows the selection of local areas that have little connection with other parts and have little influence on other parts even if they are thermally expanded as heating “stress reduction areas”, such as edges, corners, edges, ribs, and bosses of the workpiece Wait. Due to the localized heating properties of this method quality, so the effect of reducing stress is not as good as the first two methods.

Choose heating in areas that have little contact with other areas

Figure Local heating zones at different fractures of locomotive rocker handle is a practical example of heating and stress reduction for repairing the fracture of the rocker arm handle of a locomotive. Crack 1 is located at the end of the rocker arm and is completely broken. The end can expand and contract freely during repair welding, so there is no need to take heating stress reduction measures before welding; Before welding, heating must be carried out along the section where the defect is located (heating zone A in the figure), so that the end can be obtained from the defect.To reduce the stress at the weld by expansion and contraction, when repairing the crack 3, it is only necessary to heat in the B heating zone, and ensure that this zone is heated through, otherwise the effect will be opposite.

Local heating zones at different fractures of locomotive rocker handle