The main research contents of welding residual stress welding residual stress include the distribution of stress, the influence and the method of elimination and adjustment.
The distribution of welding residual stress is in the weldment with small thickness, the welding residual stress is basically plane stress, and the stress in the thickness direction is small. In the flat plate welded in the free state, the longitudinal residual stress [6] X along the weld direction is generally tensile stress in the weld and its vicinity, and compressive stress is away from the weld. For low carbon steel and low-strength low-alloy structural steel (yield strength less than 400 MPa), the residual stress on the weld [6]X can reach the yield strength of the material [6]S (Fig. 1 [longitudinal weld Residual stress distribution] distribution "class=image>). The distribution of transverse residual stress [6] perpendicular to the weld direction is related to the welding sequence and direction. The post-weld section is generally tensile stress, but the butt weld is butt welded. [6] at both ends is often compressive stress (Fig. 2 [lateral residual stress in welds]] "class=image>[cloth]). The residual stress in the thickness direction of the thick plate weld [6] is related to the welding method. [6] in the electroslag weld is tensile stress. Multi-layer welds are lower [6]. [6] The distribution in thickness is the highest at the center and gradually transitions to zero at the surface. [6] The distribution of X and [6] in the thickness of the weld is also uneven. The values ​​of the central portion [6] X and [6] of the electroslag weld are larger than the surface layer. In contrast to the multi-layer weld, the surface stress is greater than the center (Fig. 3 [Distribution of residual stress in the thickness of the multi-layer weld in the thick plate]). When welding is carried out under restraint conditions (such as closed welds), higher tensile stresses [6] X and [6] may occur in a much larger range than in the free state, and thus are more dangerous internal stresses. .
Since the welding residual stress is affected by many factors, it is often necessary to experimentally determine the magnitude and distribution of residual stress in practice.
The influence of welding residual stress on welding residual stress has six aspects on the weldment.
1 Effect on strength: If there are serious defects in the high residual tensile stress zone and the weldment is working below the brittle transition temperature, the welding residual stress will reduce the static load strength. Under the cyclic stress, if there is residual tensile stress at the stress concentration, the welding residual tensile stress will reduce the fatigue strength of the weldment. In addition to the residual stress, the fatigue strength of the weldment is also related to the stress concentration factor of the weldment. The stress cycle characteristic coefficient [6][min]/[6][max] and the maximum value of the cyclic stress [6][max ] The influence is weakened with the decrease of the stress concentration factor, and is aggravated with the decrease of [6][min]/[6][max] (for example, the effect on the alternating fatigue strength is greater than the pulse fatigue), with [6][ The increase in max] is weakened. When [6][max] is close to the yield strength, the effect of residual stress gradually disappears.
2 Influence on the stiffness: The welding residual stress is superimposed on the stress caused by the external load, which may cause the weldment to partially yield in advance to produce plastic deformation. The stiffness of the weldment will therefore be reduced.
3 Influence on the stability of the pressed weldment: When the welded rod is under pressure, the residual stress of the weld is superimposed on the stress caused by the external load, which may cause the rod to partially yield or partially destabilize the rod, and the overall stability of the rod Sex will be reduced as a result. The effect of residual stress on stability depends on the geometry and internal stress distribution of the rod. The effect of residual stress on a non-closed section (such as an I-shaped section) is greater than that of a closed section (such as a box section).
4 Influence on machining accuracy: The existence of welding residual stress has different effects on the machining accuracy of the weldment. The smaller the stiffness of the weldment, the greater the amount of machining and the greater the impact on accuracy.
5 Influence on dimensional stability: The welding residual stress changes with time, and the size of the weldment also changes. The dimensional stability of the weldment is in turn affected by the stability of the residual stress.
6 Effect on Corrosion Resistance: Welding residual stress and load stress can also cause stress corrosion cracking.
Elimination and Adjustment of Welding Residual Stress In order to eliminate and reduce welding residual stress, a reasonable welding sequence should be adopted to weld a weld with a large amount of shrinkage. Appropriately reduce the rigidity of the weldment during welding and locally heat it at the appropriate part of the weldment so that the weld can shrink relatively freely to reduce residual stress. Heat treatment (high temperature tempering) is a common method to eliminate welding residual stress. The overall heat treatment effect of stress relief is generally better than local heat treatment. The welding residual stress can also be removed or adjusted by mechanical stretching method (preloading method). For example, the pressure vessel can be hydrostatically tested, or it can be locally heated to 200 °C on both sides of the weld to create a temperature field for welding. The seam area is stretched to reduce residual stress.
Welding deformation The deformation of the weldment caused by the welding process directly affects the performance and use of the weldment. Therefore, different welding processes are needed to control and prevent the deformation of the weldment and to correct the components that produce weld deformation.
There are seven forms of deformation and welding deformation. 1 Longitudinal shrinkage deformation: shrinkage along the length of the weld. 2 transverse shrinkage deformation: transverse contraction perpendicular to the direction of the weld. 3 angular deformation: angular displacement around the axis of the weld. 4 Flexural deformation: bending deformation caused by the asymmetrical asymmetry of the neutral axis of the member. 5 instability deformation: thin wall structure under the action of welding residual compressive stress, local instability and wave shape; 6 wrong side deformation: the welding edge in the welding process, due to the expansion inconsistency caused by the thickness direction of the wrong side. 7 Distortion: Deformation caused by irregular longitudinal and transverse shrinkage of the weld due to poor assembly and unreasonable welding procedures.
Prevention and Control of Welding Deformation The size of the welding deformation is related to the size, number and arrangement of the weld. First of all, it is important to determine the number of welds, the shape and size of the groove, and properly arrange the position of the weld. The use of high energy density welding methods and small line energy process parameters, such as multilayer welding, is advantageous in reducing the longitudinal and transverse shrinkage of the weld and the resulting deflection and instability. However, multi-layer welding is not suitable for diagonal deformation. Welding deformation can be reduced by reasonable assembly, welding sequence, reverse deformation and rigid fixing.
Correction of welding deformation Welding deformation is often corrected by mechanical means. For the deformation of the thin plate shell structure caused by the long and regular butt weld, the steel ball is used to press the weld and its both sides to obtain a good corrective effect. The flame-correcting method of generating compression plastic deformation by local heating to shrink the long weldment after cooling has the advantages of strong maneuverability and simple equipment, and is widely used.
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