Static Mechanics Response Characteristic Of Under-matching Elcc Joint Under Residual Stress

Although high strength steel is used more and more widely in many fields, due to welding cold cracking of high strength steel, its excellent performance is not fully developed. The introduction of under-matching joint of high strength steel relieves the welding cold cracking of high strength steel. No residual stress under-matching“equal load-carrying capacity”(ELCC) joint is aided design by finite element simulation at present, which has meet the same requirement of carrying capacity as base metal. But it is widely accepted that the effects of residual stress on the failure of welded structure should be non-ignorable, multifaceted and different mechanism of action. During high strength steel welded structure loads, the extent of welding residual stress homogenization still has to be examined and assessed.In this thesis, 590 MPa grad high strength steel 10 Cr Ni3Mo V base metal matched the base metal, J422 electrode material properties, respectively, forming the1.00, 0.57 yield strength matching ratio joints. Static ELCC design of under-matching joint was done, following the principle that weld joints and base metal have equal static load-carrying capacity and according to the tensile strength matching ratio of 0.674. By means of finite element method(FEM), the internal stress change of butt joint of high strength steel equal-matching, under-matching conventional and static ELCC on the condition of tensile load after welding was investigated. And the effect of the welding residual stress on the static load carrying is analyzed. The conclusions of the study were as follows.During transverse and longitudinal tensile loaded to the critical load of failure,the stress level of both weld and base metal in equal-matching and under-matching conventional butt joint had continued to increase after welding. But compared with distal base metal, the stress of the welding zone and their adjacent zone increased more slowly on the basis of welding residual stress, there was the process of stress homogenization due to the significant difference; In the end, the stress near the seambase metal zone was significantly higher than that in the distal base metal zone. And the difference between the peak stress(1044, 1020 MPa) and the average load stress(820 MPa) of equal-matching butt joint was 224, 216 MPa, respectively, but the difference between the peak stress(898.9, 905.9 MPa) and the average load stress(720 MPa) of under-matching conventional butt joint was 178.9, 185.9 MPa,respectively, which didn’t exhibit the feature of internal stress total homogenization.This meant that by reason of the existence of welding residual stress, the static load strength of equal-matching welded structure of high strength steel might reduce a little, and under-matching welded structure of high strength steel reduced more under the low strength weld seam. From the contrast of stress homogenization extent of equal-matching and under-matching butt joints, the effect of geometrical stress concentration, material performance and residual stress alone on stress homogenization was 23.9, 45.1 and 200.1 MPa, respectively. So the influence of geometrical stress concentration, material performance and residual stress to the extent of stress homogenization increased gradually.Compared with previous ELCC design scheme, the area of cosmetic bead of a single circular arc transition curve of static load ELCC design scheme was small,which saved weld material. It proved that ELCC joint with no residual stress met the requirement of equal static load-carrying capacity, base metal prior to weld fractured,which belonged to the general yield safety fracture mode. During transverse and longitudinal tensile loaded to the critical load of failure, stress homogenization process of under-matching ELCC butt joint with residual stress was similar to under-matching conventional joint, but the difference between peak stress(885.8,897.9 MPa) and average stress(820 MPa) was 65.8, 77.8 MPa, respectively.Compared with the extent of stress homogenization of under-matching conventional butt joint, which reduced 113.1, 108.1 MPa; Compared with the extent of stress homogenization of equal-matching conventional butt join, which reduced 158.2,122.2 MPa. So the extent of stress homogenization of under-matching ELCC joint had been improved obviously.