4 Elimination of
welding stress and correction of welding deformation
The welding deformation is closely related to the welding residual stress. It is precisely because of the welding residual stress inside the welding that causes the welding deformation. If there is a large residual stress in the welding, stress concentration will occur during the use, which will cause cracks in the welding, further causing damage to the welding steel structure, causing significant safety hazards to the equipment. At the same time, with the use of the welding components, the welding residual stress will be released continuously, and new deformation will occur. Therefore, after the assembly and welding of the welding steel structure, the stress relief and deformation must be eliminated, so that it can meet the requirements and requirements of the drawings.
1）Method of eliminating welding stress
At present, the commonly used methods to eliminate stress include natural aging, vibration aging and heat treatment to remove stress, each of which has its own characteristics. Natural aging is simple and easy, without any equipment. It only needs a suitable open space, almost no cost. But the natural aging cycle is long and cannot completely eliminate residual stress, so it is not suitable for products with short production cycle and urgent delivery requirements. Vibration aging has been used more and more because of its simple equipment, simple operation, short production cycle (usually not more than 30 minutes), and better stress effect. However, the vibration aging can only reduce the peak value of residual stress and cannot eliminate the residual stress, so the effect of vibration aging on reducing the existing deformation is not significant, but it can prevent the deformation energy that will be produced in the future. Heat treatment is the best way to eliminate welding stress as a whole. When welding at suitable temperature for a suitable time, the welding residual stress can be completely eliminated. It is effective for important and small welding components; for large welding steel structure, the stress cannot be eliminated as a whole because of the size limit of the furnace. Better effect in eliminating the stress and deformation can be obtained by local heating, but the heating temperature should be strictly controlled according to the heat treatment temperature stipulated by the national standard of carbon steel and low alloy steel.
2）Correction of welding deformation
For big size of steel structure, there are two methods to correct the welding deformation which are mechanical correction and heating correction. Mechanical correction is a kind of correction method widely used in production at present, it is to achieve the purpose of correcting deformation by applying the mechanical force of reverse direction to the deformation of the welding parts. This method does not need complex equipment, it is easy to do and the result is always good, the disadvantage is that the effect of eliminating welding residual stress is not obvious, and the new deformation will occur with the release of residual stress during use. Heating correction is to do the local heating at the tensile part in the deformation area, which makes heated zoon it shrink during cooling to produce reverse deformation so as to correct the deformation. At present, two commonly used heating methods are flame heating and electric heating. The advantage of heating correction to eliminate the deformation completely, and achieving good results for any complex shape steel structure. The disadvantage is that temperature is strict, and the correction temperature is not easy to control in practice. These two methods are often used together in production, such as the vibration aging treatment before the mechanical correction, the local deformation of the place to assist in the heat treatment.
To sum up, comprehensive consideration of two aspects of design and manufacturing process, the factors that affect the welding deformation are taken into consideration and the corresponding measures are taken to effectively control the welding deformation and achieve satisfactory results.