In the construction machinery, the weight of the welded structural parts accounts for 60%-75% of the weight of the whole machine, and the high residual stress in the welded joint causes deformation, brittle fracture, fatigue failure and stress corrosion cracking of the steel structure, etc. It is extremely important to understand the residual stress distribution in the welded structure to ensure the safety and reliability of the welded steel structure.
The commonly used test methods for residual internal stress of welded structures can be divided into two categories according to their principles: stress release mechanical measurement and non-destructive physical measurement. Due to the combination of the electrical measurement method of the electric strain gauge and the mechanical method, the destructive property of the mechanical method is greatly reduced; the X-ray diffraction measurement method is more commonly used in the physical method. In this paper, the similarities and differences between the measurement results of the two measurement methods are compared and discussed theoretically.
1 Experimental process
The base metal is a 500MPa grade 15MnVN normalized low alloy high strength steel with a thickness of 20mm. It is semi-automatic CO2 gas shielded welding and H04Mn2SiTiA for welding wire. The joint size is shown in Figure 1. The joint is welded under the condition that the spot is restrained by the positioning welding. After grinding and electropolishing at each measuring point, the residual stress is measured by XYL-75 type X-ray stress tester, and then the stress release method is used at the same point (double Grid strain) measures stress. The measurement positions are respectively the longitudinal centerline of the weld as shown in Fig. 1, the fusion line and the B line perpendicular to the weld.
The experimental results are shown in Fig. 2, the stress parallel to the longitudinal direction of the weld is denoted by σL, and the transverse stress perpendicular to the weld is denoted by σT. It can be seen from Fig. 2(a) and (b) that the residual stress measured by the two methods is quite consistent except for the heat affected zone of the weld. This shows that the accuracy of stress measurement by X-ray diffraction is at the same level as the stress relief method. On the weld and the fusion line, there are significant differences between the two methods, as shown in Figure 2(c)(d). In the longitudinal direction of the weld, except for the two ends and its vicinity, the value of the stress and the residual stress measured by the stress release method is approximately equal to the yield limit, and the stress value measured by the X-ray method does not reach the yield limit.
3 Conclusion
The residual stress measured by the X-ray method on the welded joint base material is in good agreement with the results measured by the mechanical method, and there are some differences in the residual stress values ​​measured by the two methods in the weld zone. It can be explained that the baseline stress in the steel is the stress at which the ferrite and the pearlite are in equilibrium.
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