Strength matching of welded joints

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For a long time, the traditional design principle for welded structures has been strength-based design. In actual welded structures, there can be three types of strength relationships between the weld and the base material: the weld strength is equal to that of the base material (equal-strength matching), the weld strength exceeds that of the base material (superior-strength matching, also known as high-strength matching), and the weld strength is lower than that of the base material (lower-strength matching). From the perspective of structural safety and reliability, it is generally required that the strength of welds be at least equal to that of the base material, which is the \"equal strength\" design principle. However, in actual production, welding materials are often selected based on the strength of the deposited metal, whereas the strength of the deposited metal is not the actual strength of the weld. The deposited metal is not identical to the weld metal; especially in the case of welding materials for low-alloy high-strength steels, the strength of the weld metal is often much higher than that of the deposited metal. As a result, there is a situation where it is nominally “equally strong” but actually “super strong”. There is no consensus, and doubts exist, regarding whether ultra-strong matching is necessarily safe and reliable. In the design of the Jiujiang Yangtze River Bridge in our country, the \"ultimate strength\" of welds is restricted to be no more than 98 MPa ； American scholar Pelini proposed that, in order to achieve conservative structural integrity goals, welds with strength comparable to that of the base material can be used, or welds with a strength 137 MPa lower than that of the base material (i.e., low-strength matching) ； According to the research findings of Japanese scholars such as Kunihiko Sato, low-strength matching is also feasible and has been applied in engineering. However, Belgian scholar Soete and Chinese scholars such as Zhang Yufeng argue that ultra-strong matching should be advantageous. Clearly, the design principles related to weld strength matching for ensuring the safety and reliability of welded structures still lack sufficient theoretical and practical support, and there is no unified understanding on this matter. In order to establish more rational design principles for welded joints and provide a basis for the proper selection of welding materials, Professor Chen Boli from Tsinghua University and his colleagues undertook the **Natural Science Foundation research project titled ‘Theoretical Study on the Matching of Strength and Toughness in High-Strength Steel Welds’**. The research topics include: the fracture strength of joints made of high-strength steel with a low yield ratio at 490 MPa, the fracture strength of joints made of high-strength steel with a high yield ratio in the 690–780 MPa range, the tensile strength of weld joints without notches, the deformation behavior at the tip of notches in deeply notched specimens, and NDT testing of weld joints. Numerous test results show that: 1 for low-yield ratio high-strength steels with a tensile strength of 490 MPa, it is advantageous to use welding materials that possess a certain degree of toughness along with appropriate strength. If factors such as welding processability and usability are taken into account, it is more reasonable to choose welding materials that possess a certain degree of toughness and are practically equivalent in strength. The fracture strength and fracture behavior of welded joints of this type of steel depend on the combined effects of the strength and ductility of the welding material. Therefore, welding structure design that considers only strength and not toughness cannot reliably ensure its safety in use. 2 For high-strength steels with a high yield ratio and a tensile strength in the range of 690–780 MPa, the fracture behavior of their welded joints depends not only on the strength, toughness, and plasticity of the weld seam but also on the heterogeneity of the welded joint. It is undesirable for the weld seam to be either excessively strong or excessively weak; joints with a strength level that is close to that of the base material exhibit the best fracture performance. Therefore, it is reasonable to design welded joints based on the principle of actual strength matching. Therefore, the weld strength should be limited by upper and lower bounds. 3 The tensile strength matching coefficient (Sr) is the ratio of the tensile strength of the weld metal to that of the base material; it can reflect the heterogeneity in the mechanical properties of the joint. The test results show that when Sr≧0.9, the strength of the welded joint can be considered to be very close to that of the base material. Therefore, in production practice, using welding materials with a strength 10% lower than that of the base material ensures that the joint meets the design requirements for equal strength. When Sr≧0.86, the joint strength can reach over 95% of the base metal strength, as the higher strength of the base metal exerts restraint on the weld metal, thereby enhancing the weld strength. 4 The yield-to-tensile strength ratio of the base material has a significant impact on the fracture behavior of welded joints; joints with a lower yield-to-tensile strength ratio in the base material exhibit better resistance to brittle fracture compared to those with a higher ratio. This indicates that the plasticity reserve of the base material also has a significant impact on the crack resistance of the joint. 5 The deformation behavior of the weld metal is influenced by the matching of the mechanical properties between the weld and the base material. Under the same tensile stress, the weld strain in the super-matched joints of steel with a low yield ratio is greater, while that in the low-strength matched joints of steel with a high yield ratio is smaller. The crack opening displacement (COD value) of the welded joints shows a similar trend: the super-matched joints of steel with a low yield ratio have the advantage of easier yielding at the crack tip and greater deformation at that location. 6 The resistance of welded joints to brittle fracture is closely related to the heterogeneity of the joint’s mechanical properties; it is determined not only by the strength of the weld but also by its toughness and plasticity. The selection of welding materials must not only ensure that the weld possesses appropriate strength, but also guarantee that it has sufficient toughness and ductility; in other words, it is necessary to achieve an optimal balance between the strength and toughness of the weld. For steel grades with high strength levels, it is extremely difficult to achieve equal strength between the weld metal and the base material; even if the weld strength does reach that of the base material, the plasticity and toughness of the weld are reduced to unacceptable levels ； The crack resistance also decreases significantly; to prevent welding cracks, the construction conditions must be extremely strict, which **increases** the construction costs. To avoid this situation where only strength is pursued at the expense of the overall performance of the structure, and to improve economic reliability in construction, it is necessary to reduce the strength and adopt a low-strength matching approach. Take Japan’s submarine-grade steel NS110 as an example; its yield strength is 1098 MPa or higher. For the welding electrodes and gas shielded welding wires used with it, the yield strength of the deposited metal is required to be 940 MPa or higher, with a yield strength matching coefficient of 0.85. By using welding materials with a low strength match, both the carbon content and carbon equivalent of the weld can be reduced. This improves the plasticity and toughness of the weld as well as its crack resistance, facilitates welding operations, and reduces the costs associated with such work. Furthermore, some experimental data from Japanese scholar Kunihiko Sato show that as long as the strength of the weld metal is not less than 80% of the strength of the base material, it is still possible to ensure that the joint has the same strength as the base material; however, the overall elongation rate of joints with low-strength welds is somewhat lower. Under fatigue loading, if the weld bead height is not removed, fatigue cracks will occur in the fusion zone ； However, if the excess height of the weld is removed, fatigue cracks will occur in the low-strength weld. Therefore, regarding the use of low-strength welds, it is appropriate to conduct some testing based on specific conditions.