While welding process, thermal stress, the phase transformation stress, stress and other processing is over the yield limit (Yield strength), so that after cooling the weldment left failed to eliminate the stress. Thus, the welding residual after cooling in the macroscopic stress weldment called residual welding stresses. Welding process uniform temperature field and caused by it localized plastic deformation and hematocrit different organizations, which is to produce welding stress and deformation of the underlying causes.
Welding residual stress is not load bearing member and the member section already exists on the initial stress in the component during service, and the other suffered due to work stress loads are superimposed to produce secondary deformation and residual stress redistribution will not only reduce the structural stiffness and stability but also in temperature and media together, but also seriously affect the fatigue strength, resistance to brittle fracture ability to resist stress corrosion cracking and high temperature creep cracking capacity. generated when the external load stressδa region with structure superposition of residual stress reaches the yield point fy when the area: the domain material will produce local plastic deformation, the loss of the ability to further withstand external loads, resulting in effective cross-sectional structure of the plot less small stiffness of the structure is also reduced. The structure with longitudinal and transverse welds ( eg word beam welding rib ), or after the flame correction section may on the larger residual tensile stress, while the length of the member and the distribution not too large, but they are still able to have a greater rigidity of. Especially after correction using a large flame welding beam stiffness when loaded and unloaded when the amount of springback may have a more significant decline, for dimensional accuracy and high structural stability requirements can not be ignored.
If the material is brittle, since the material can not be plastically deformed, with the external force increases, the stress member not uniform. Peak stress will continue to increase until it reaches the material yield limit, partial damage, leading to the entire component fracture. The presence of residual stress of brittle materials, load capacity will decrease, resulting in breakage. For the plastic material, there are three in the low temperature to the tensile residual stresses, would impede the generation of plastic deformation, which will greatly reduce the carrying capacity of components. For welded components, as long as the member and the weld bead itself has good plastic deformation ( no temperature, dynamic loads such as the steel brittle unfavorable factors ), does not reduce the residual stress component of static strength. Because there is residual stress member to bear the axial force increases, the tensile stress caused by external loads will be superimposed on the residual stress section. In the loading process, the stress increased, when superimposed on the total stress reaches the material yield limit fy, there are residual tensile stress component cross-sectional and advance into the plastic zone, after growing only by the external load and the elastic region cut commitments, with load increases, the elastic region decreases plastic zone increases, the internal stress is constantly superimposed stress redistribution occurs until the entire cross-section of the material yield stress reaches the limit stops. Since residual stress is self- sectional stress distribution equilibrium, so static load equal, that does not reduce the residual stress component static strength. However, under certain conditions, the plastic material loses ductility, become brittle or a member of plastic materials is low, the residual stress will affect the static strength member. Because the member does not have sufficient plastic deformation in the loading process, the peak stress increased until it reaches the ultimate strength of the material occurs after damage. Thus the residual stress affecting them.