The steel structure is widely used for welding, and the weld is generally used for some important welds.In the process of heating and melting of the metal, the temperature of the metal in the heating zone is very different from that of the surrounding material, resulting in the instantaneous stress in the welding process.
After cooling to the original temperature, the tension zone and the mother material in the weld and the near joint of the whole joint area reach a balance in the compressive stress zone, resulting in the welding residual stress of the structure itself.
At this time, the welding stress is under the action of welding the structure of various forms of deformation.The existence of residual stresses and the generation of deformation are mutual transformation, and it is not difficult to find ways to prevent and correct the deformation.
1. Form and cause of welding deformation:
The deformation of steel structure after welding can be divided into two kinds: the deformation of the whole structure and the deformation of the structure.The deformation of the whole structure includes the longitudinal and transverse shortening and bending of the structure.Local deformations are convex, wavy and angular.
1.1 common basic forms of deformation
There are several basic forms of welding deformation: the shortened length (longitudinal contraction) and the deformation of width (transverse contraction) after the butt welding of plate.Angle deformation after butt welding of plate groove;
The angular deformation of the member after welding is different from that of the longitudinal direction of the longitudinal axis of the member and the longitudinal contraction of the component flange and the web.
The pressure zone of the laminate is caused by the buckling of the plate.Due to the longitudinal and transverse contraction of the weld, the integral bending of the component is caused by the asymmetry of the neutral axis of the component.
Ii. Factors affecting the deformation of welded structure:
There are many factors influencing the deformation of welding, sometimes the same factors have opposite effect on longitudinal deformation, transverse deformation and angular deformation.The influence of various factors on various deformations is comprehensively analyzed, and the influence rule is the basis of taking reasonable measures to control the deformation.Otherwise it is difficult to achieve the desired effect.
1) influence of the weld area: the area of the weld is the area of metal in the range of the fuse, the larger the weld area, the larger the plastic deformation caused by shrinkage during cooling.
2) the influence of the welding heat input: in general, the large heat input, large range of heat of high temperature and cooling speed slow, make joint plastic deformation area increases, both of the vertical, horizontal, or Angle of the influence of deformation are increased.However, when the thermal input is increased to a certain extent when the thermal input is increased to a certain extent, the angular deformation no longer increases, but decreases, even if the thermal input continues to increase.
3) the heating of the workpiece and the temperature of the interlayer: the higher the temperature of the preheating temperature and the temperature of the interlayer, the higher the heat input, the slower the cooling rate and the increase of shrinkage deformation.
4) the influence of welding methods: several methods commonly used in construction steel structure welding, besides electroslag welding, submerged arc welding heat input, the largest in the case of other conditions such as weld area of the same, the biggest contraction deformation.The manual arc welding heat input center, shrinkage deformation is less than submerged arc welding.The CO2 gas shielded welding heat input is minimal, and the contraction deformation response is also minimal.
5) influence of weld position on deformation: due to asymmetry of weld position in structure, asymmetry of weld position will cause various deformation.
6) the effect of rigid structure on welding deformation: the rigid size of the structure depends on the shape of the structure and its cross-section size, the rigid structure and the large deformation of welding;Rigid structure, less deformation after welding.
7) influence of assembly and welding specifications on welding deformation: due to different assembly methods adopted, the deformation of the structure can also be affected.The whole assembly is finished before welding, and its deformation is generally less than welding by side assembly.
In engineering welding time, due to various conditions, and all kinds of factors of welding deformation law of residual is more complex, the understanding of the factors to separate effects on engineering the comprehensive analysis of the specific situation in details.
Iii. Measures to prevent and reduce structural deformation:
1) reduce the cutting area of the weld: in the condition that the weld is intact and without excessive defect, adopt the smaller groove size (Angle and clearance) as far as possible.
2) under the yield strength of 345MPa, the hardened steel is adopted with smaller thermal input, and the preheating or interlayer temperature is not preheated or properly reduced.Heat input is preferred for small welding methods such as CO2 gas shielded welding.
3) as far as possible use of multi-layer welding instead of single layer welding.