Stress and deformation control of the hottest weld

2022-09-23
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Welding stress and deformation control

Abstract: in order to effectively control the welding deformation of steel structure caused by uneven expansion and contraction of weldments, various influencing factors of welding deformation and welding stress are analyzed, and the corresponding control measures are put forward

key words: welding deformation, welding stress, thermal process, welding process

today, with the development of building steel structures in full swing, welding machines and welding methods in different forms are changing with each passing day, and welding technology has become a key topic. However, in the construction process, the welding residual stress and residual deformation caused by welding seriously affect the quality of the project, installation progress and structural bearing capacity (i.e. use function). Therefore, it is urgent to adopt reasonable methods to control

the welding process of steel structure is actually a thermal process in which the local area of the weldment is heated and then cooled and solidified. However, due to the uneven temperature field, the weldment expands and contracts unevenly, resulting in welding stress inside the weldment and welding deformation. Common welding stresses are: 1) longitudinal stress; 2) Transverse stress; 3) Thickness direction stress. Common welding deformations include: 1) longitudinal shrinkage deformation; 2) Transverse shrinkage deformation; 3) Angular deformation; 4) Bending deformation; 5) Distortion and deformation; 6) Wave deformation. For these different types of welding deformation and stress distribution, trace the root causes, and conduct specific research and control

1 control measures for welding deformation

comprehensively analyze the influence of various factors on welding deformation, and declare to the world that China will grasp its influence law under its leadership, so as to take reasonable control measures

1.1 influence of weld cross-sectional area

weld cross-sectional area refers to the metal area within the fusion line. The larger the weld area is, the greater the plastic deformation caused by shrinkage during cooling. The influence trend of weld area on longitudinal, transverse and angular deformation is consistent, and it plays a major role. Therefore, when the plate thickness is the same, the larger the groove size is, the greater the shrinkage deformation is

1.2 influence of welding heat input

generally, when the heat input is large, the range of high-temperature area heated is large, and the cooling speed is slow, which increases the plastic deformation area of the joint

1.3 influence of welding methods

the heat input of various welding methods varies greatly. Among the several commonly used welding methods for the welding of building steel structures, except electroslag, submerged arc welding has the largest heat input. Under other conditions, such as the same weld cross-sectional area, the shrinkage deformation is the largest, manual arc welding is in the middle, and CO2 gas shielded welding is the smallest

1.4 influence of joint form

when the welding heat input, weld cross-sectional area, welding and other factors are the same, different joint forms have different effects on longitudinal, transverse and angular deformation. Common weld forms include surfacing, fillet welding and butt welding

1) during surface surfacing, the transverse deformation of the weld metal is not only constrained by the longitudinal and transverse base metal, but also the heating is limited to a certain depth of the workpiece surface, so that the shrinkage of the weld is constrained by the plate thickness, depth and base metal at the same time. Therefore, the deformation is relatively small

2) when T-shaped fillet joint and lap joint are used, the transverse shrinkage of the weld is similar to that of surfacing, and its transverse shrinkage value is directly proportional to the area of fillet weld and inversely proportional to the plate thickness

3) in the case of single pass (layer) welding, the transverse shrinkage of the butt joint is greater than that of surfacing and fillet welding. In the case of single-sided welding, the groove angle is large, and the difference between the upper and lower shrinkage of the plate thickness is large, so the angular deformation is large

the situation is different when welding on both sides. With the reduction of groove angle and gap, the transverse shrinkage decreases and the angular deformation also decreases

1.5 influence of the number of welding layers

1) transverse shrinkage: when the butt joint is multi-layer welded, the transverse shrinkage of the first layer of weld conforms to the general conditions and deformation law of butt welding. After the first layer, it is equivalent to seamless butt welding, which is similar to the conditions and deformation law of surfacing when it is close to the cover weld bead. Therefore, the shrinkage deformation is relatively small

2) longitudinal shrinkage: when multi-layer welding is encouraged to use domestic instruments according to the actual needs, the heat input of each layer of weld is much smaller than that of single-layer welding, the heating range is narrow, the cooling is fast, and the shrinkage deformation is much smaller. Moreover, after the welding of the previous layer of weld is completed, the longitudinal shrinkage deformation of multi-layer welding is much smaller than that of single-layer welding, and the number of welding layers is more, The smaller the longitudinal deformation is

in engineering welding practice, due to the comprehensive action of various conditions and factors, the law of welding residual deformation is relatively complex. Understanding the influence of each factor alone is convenient for making a specific comprehensive analysis of the specific situation of the project. Therefore, to understand the causes and influencing factors of welding deformation, the following measures can be taken to control the deformation:

1) reduce the cross-sectional area of the weld, and try to use a smaller groove size (angle and gap) on the premise of obtaining a complete weld without excessive defects

2) for steels with yield strength below 345mpa and weak hardenability, adopt small heat input, and try not to preheat or appropriately reduce the preheat and interpass temperature; Welding methods with low heat input, such as CO2 gas shielded welding, are preferred

3) multi layer welding shall be used to replace single-layer welding as far as possible for thick plate welding

4) under the condition of meeting the design requirements, intermittent welding method can be adopted for the welding of longitudinal reinforcing ribs and transverse reinforcing ribs

5) when both sides can be welded, the double-sided symmetrical groove shall be used, and the welding sequence symmetrical to the neutral axis of the member shall be used in multi-layer welding

6) when the thickness of T-joint plate is large, the groove fillet weld shall be used

7) the anti deformation method before welding is used to control the angular deformation after welding

8) adopt rigid fixture fixation method to control post welding deformation

9) the method of reserved length of components is used to compensate the longitudinal shrinkage deformation of welds. For example, 0.5mm ~ 0.7mm can be reserved per meter length of H-shaped longitudinal welds

10) for the distortion of long members, it mainly depends on improving the flatness of plates and the assembly accuracy of members, so that the groove angle and gap are accurate, and the direction or alignment of the arc is accurate, so that the angular deformation of welds and the longitudinal deformation of flanges and webs are consistent with the length direction of members

11) when assembling and welding components with numerous welds or structural installation, a reasonable welding sequence should be adopted

12) in the design, the number and size of welds should be reduced as much as possible, and the welds should be arranged reasonably. In addition to avoiding dense welds, the position of welds should also be replaced by qualified steel balls selected by micrometer as far as possible; (3) The error of the indentation measuring device is too large, close to the neutral axis of the component, and make the arrangement of the weld symmetrical with the neutral axis of the component

2 control measures of welding stress

it is an inevitable phenomenon that instantaneous internal stress is generated during component welding, residual stress is generated after welding, and residual deformation is generated at the same time

the correction of welding deformation is time-consuming and labor-consuming. The component manufacturing and installation enterprises first consider the control of deformation, and often ignore the control of residual stress. Some fixtures and supports are often used to increase rigidity to control deformation, which actually increases the residual stress after welding

for some components with high rigidity, such as large plate thickness and large moment of inertia of the section itself, although the deformation will be small, it will also produce large internal stress and even cracks

therefore, for some important structures with large section thickness, complex welded joints, high restraint, high steel strength level and poor service conditions, attention should be paid to the control of welding stress. The goal of stress control is to reduce its peak value and make it evenly distributed. Its control measures include the following:

1) reduce the weld size: the internal stress in welding is caused by local heating cycle. Therefore, under the condition of meeting the design requirements, the weld size and layer height should not be increased, and the idea that the larger the weld is, the safer it is should be changed

2) reduce the welding restraint: the greater the restraint, the greater the welding stress. First, try to weld the weld under a small restraint, and try not to control the deformation by rigid fixation, so as not to increase the welding restraint

3) adopt a reasonable welding sequence: under the assembly condition of more welds, the welds with large shrinkage shall be welded first, and then the welds with small shrinkage shall be welded according to the shape of components and the layout of welds; The principle of first welding the weld with large restraint and no free shrinkage, and then welding the weld with small restraint and free shrinkage

4) reduce the rigidity of weldments and create conditions for free shrinkage

5) hammering method to reduce welding residual stress: after each layer of weld bead is welded, use a round head slag hammer or electric hammering tool to evenly knock the weld metal to produce plastic extension deformation and offset the local tensile stress borne by the weld after cooling

however, the root weld bead, the weld bead in the groove and the weld bead on both sides adjacent to the base metal groove surface should not be hammered, so as to avoid the hardening or cracks of the fusion line and the near seam area. For high-strength low-alloy steel, if the yield strength level is greater than 345mpa, hammering method should not be used to eliminate welding residual stress

6) derusting by shot blasting machine: the welding stress of the component is offset by evenly striking the steel shot

to sum up, during the construction process, we must understand the welding process and adopt reasonable welding methods and control measures to reduce and eliminate the residual stress and residual deformation after welding. Constantly summarizing and accumulating welding experience in practice and comprehensively analyzing various factors to be considered can ensure the welding quality in the project. Extensometer or strain gauge shows the elongation of standard tensile specimen (end)

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