Abstract: The three types of Solder joint matching and their advantages and disadvantages are reviewed. It is pointed out that for low-strength steel grades, it is possible to use equal or super strong matching, but for high-strength steel, super-strong matching is unfavorable, and equal-strength matching is preferable. If the weld toughness is significantly reduced, then It is more advantageous to use low-strength matching, which can obtain a greater toughness reserve and improve fracture resistance. The weld toughness index varies depending on the use.
1 Strength matching of welded joints
The traditional design principle of welded structures has long been essentially strength design. In the actual welded structure, there are three kinds of matching relationship between the weld and the base material in strength: the weld strength is equal to the base metal (equal strength matching), and the weld strength exceeds the base material (super strong match, also called high strength match) and The weld strength is lower than the base metal (low strength match). In view of the safety and reliability of the structure, it is generally required that the weld strength is at least equal to the strength of the base metal, that is, the so-called "equal strength" design principle. However, in actual production, most of the welding materials are selected according to the strength of the deposited metal, and the strength of the deposited metal is not the actual weld strength. Deposited metal is not equivalent to weld metal, especially for low-alloy high-strength steel. The strength of the weld metal is often much higher than that of the deposited metal. Therefore, there will be a result of the nominal "equal strength" and the actual "super strong". Whether the super strong match is safe and reliable, the understanding is not consistent, and there are doubts. In the design of the Jiujiang Yangtze River Bridge, the "super value" of the weld is limited to not more than 98 MPa; the American scholar Pellini proposes that in order to achieve the conservative structural integrity goal, the weld or the equivalent of the base material in strength can be used. The weld is 137MPa low (ie low-strength matching); according to the research results of Japanese scholar You Tengbangyan, low-strength matching is also feasible and has been applied in engineering. However, Zhang Yufeng et al. pointed out that [3], super strong matching should be beneficial. Obviously, the design principles related to weld strength matching, which are safe and reliable for welded structures, lack sufficient theoretical and practical basis, and there is no unified understanding. In order to determine the more reasonable design principles of welded joints and provide the basis for the correct selection of welding materials, Professor Chen Bojun of Tsinghua University and others undertook the National Natural Science Foundation research project "High-strength steel weld seam toughness matching theory research". The research contents of the subject include: the breaking strength of 490MPa grade low yield ratio high strength steel joint, the breaking strength of high strength steel joint of 690～780MPa grade high yield ratio, the tensile strength of unnotched welded joint, and the notched top of deep notched specimen. Deformation behavior, NDT test of welded joints, etc. A large number of test results show that:
(1) For low-strength ratio high-strength steels with a tensile strength of 490 MPa, it is advantageous to use a welding material which has a certain toughness and is suitably super strong. If the welding processability and the adaptability of the use are comprehensive, it is more reasonable to select a welding material that has certain toughness and is actually "equal strength". The fracture strength and fracture behavior of the welded joint of this type of steel depend on the combination of strength and toughness of the weld material. Therefore, the design of the welded structure considering only the strength regardless of the toughness cannot reliably ensure the safety of its use.
(2) For high-strength ratio high-strength steel with tensile strength of 690-780MPa, the fracture properties of welded joints are not only related to the strength, toughness and plasticity of the weld, but also by the heterogeneity of the welded joint. Excessive over-extension or excessively low-strength is not ideal, and the joints that are close to the same strength have the best fracture performance. It is reasonable to design the welded joint according to the principle of actual strength. Therefore, the weld strength should be limited by the upper and lower limits.
(3) The tensile strength matching coefficient (Sγ) is the ratio of the tensile strength of the deposited metal of the welding material to the tensile strength of the base material, which can reflect the inhomogeneity of the mechanical properties of the joint. The test results show that when Sγ≥0.9, the strength of the welded joint can be considered to be very close to the strength of the base metal. Therefore, in the production practice, welding with a welding material that is 10% lower than the strength of the base metal is used to ensure the strength design requirements of the joint. When Sγ≥0.86, the joint strength can reach more than 95% of the strength of the base metal. This is because the base metal having a higher strength has a restraining effect on the weld metal, and the strength of the weld is improved.
(4) The yield ratio of the base metal has an important influence on the fracture behavior of the welded joint. The brittle fracture resistance of the base material is lower than that of the base material. This shows that the plastic reserve of the base metal also has a great influence on the brittle fracture resistance of the joint.
(5) The deformation behavior of the weld metal is affected by the matching of the mechanical properties of the weld and the base metal. Under the same tensile stress, the weld strain of the super-strength joint of the low yield ratio steel is larger, and the weld strain of the low-strength joint of the high yield-bending steel is smaller than that of the steel. The crack opening displacement (COD value) of the welded joint also exhibits the same tendency, that is, the super-matching joint of the low yield ratio steel has the advantage that the crack tip is easy to yield and the crack tip deformation is larger.
(6) The brittle fracture resistance of welded joints is closely related to the heterogeneity of joint mechanical properties. It is determined not only by the strength of the weld but also by the toughness and plasticity of the weld. The selection of welding materials not only ensures that the weld has suitable strength, but also ensures that the weld has sufficient toughness and plasticity, that is, to control the toughness matching of the weld.
For steel grades with higher strength grades, there is a great technical difficulty in matching the weld metal to the base metal. Even if the weld strength is equal, the plasticity and toughness of the weld are reduced. The degree of acceptance; the crack resistance is also significantly reduced. In order to prevent the occurrence of welding cracks, the construction conditions are extremely strict and the construction cost is greatly improved. In order to avoid this kind of pursuit of strength and damage the overall performance of the structure and improve the reliability of construction, the strength has to be reduced, and a low-strength matching scheme is adopted. For example, the submarine steel NS110 in Japan has a yield strength ≥1098 MPa; and the welding metal and the gas-shielded Welding Wire have a yield strength of ≥940 MPa, and the yield strength matching coefficient is 0.85. After using low-strength matching welding materials, the carbon content and carbon equivalent of the weld can be reduced, which will improve the plastic toughness of the weld and improve the crack resistance, which brings convenience to the welding construction and reduces the construction. The cost of the aspect.
In addition, some test data of the Japanese scholar Zoe Tengbangyan showed that [2], as long as the strength of the weld metal is not less than 80% of the strength of the base metal, the joint and the base metal can be ensured, but the joint of the low-strength weld is overall. The elongation is lower. Under the fatigue load, if the residual height of the weld is not removed, the fatigue crack will be generated in the fusion zone; however, if the weld height is removed, the fatigue crack will be generated in the low-strength weld. Therefore, regarding the application of low-strength welds, it is advisable to carry out some test work in combination with specific conditions.
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