For a long time, the traditional design principle for welded structures has been based primarily on strength design.
In actual welded structures, there are three types of matching relationships between the weld and the base material in terms of strength: weld strength equals base material strength (equal strength matching), weld strength exceeds base material strength (superior strength matching, also known as high strength matching), and weld strength is lower than base material strength (low strength matching).
From the perspective of structural safety and reliability, weld strength is generally required to be at least equal to base material strength, which is known as the “equal strength” design principle.
However, in actual production, welding materials are often selected based on the strength of the deposited metal, which is not equivalent to the actual weld strength.
The deposited metal is not equivalent to the weld metal, especially when welding materials are used for low-alloy high-strength steel, where the strength of the weld metal is often much higher than that of the deposited metal.
As a result, there can be a nominal “equal strength” but an actual “superior strength” outcome.
There is no consensus on whether superior strength matching is always safe and reliable, and there are some doubts.
In the design of the Jiujiang Yangtze River Bridge in China, the “superior strength value” of the weld is limited to no more than 98MPa. American scholar Pelini proposed that in order to achieve conservative structural integrity goals, welds with strength equivalent to or lower than the base material by 137MPa (i.e. low strength matching) can be used. According to the research results of Japanese scholar Sato Kunihiko and others, low strength matching is also feasible and has been applied in engineering.
However, Belgian scholar Soete and Chinese scholar Zhang Yufeng hold the view that superior strength matching should be advantageous.
Clearly, there is still a lack of sufficient theoretical and practical basis for the design principles related to weld strength matching that affect the safety and reliability of welded structures, and there is no unified understanding.
In order to determine more reasonable design principles for welded joints and provide a basis for the correct selection of welding materials, Professor Chen Bolin and others from Tsinghua University undertook the National Natural Science Foundation research project “Theoretical Research on High-strength Steel Weld Toughness Matching.
The research content of the project includes the fracture strength of 490MPa grade low yield strength ratio high-strength steel joints, the fracture strength of 690~780MPa grade high yield strength ratio high-strength steel joints, the tensile strength of unwelded joints, the deformation behavior of the top of deep notched specimens, and NDT tests on welded joints.
A large number of experimental results have shown that:
For high-strength low-yield ratio steel with a tensile strength of 490MPa, it is advantageous to use welding materials with a certain toughness and appropriate superior strength.
If considering factors such as welding processability and usage adaptability, it is more reasonable to select welding materials with a certain toughness and actual “equal strength”.
The fracture strength and behavior of welded joints of this type of steel depend on the combined effect of the strength and ductility of the welding material.
Therefore, welding structure design based solely on strength considerations without taking into account toughness cannot reliably guarantee their safety of use.
For high-strength steel with a yield strength ratio of 690~780MPa, the fracture performance of its welded joints is not only related to the strength, toughness, and plasticity of the weld, but also constrained by the heterogeneity of the welded joint.
Excessive superior or low strength of the weld is not ideal, while joints that are close to equal strength matching have the best fracture performance. Therefore, designing welded joints according to the actual equal strength principle is reasonable. Thus, there should be upper and lower limits on weld strength.
The strength matching coefficient (Sr) is the ratio of the tensile strength of the deposited metal of the welding material to the tensile strength of the base material, and it can reflect the heterogeneity of the mechanical performance of the joint.
Experimental results show that when Sr≧0.9, the strength of the welded joint can be considered close to the strength of the base material. Therefore, in production practice, using welding materials that reduce the strength by 10% compared to the base material can ensure that the joint meets the equal strength design requirements.
When Sr≧0.86, the strength of the joint can reach more than 95% of the strength of the base material. This is because the higher strength of the base material restricts the weld metal, thereby improving the strength of the weld.
The yield strength ratio of the base material has an important influence on the fracture behavior of welded joints. J
oints with lower yield strength ratios of the base material have better resistance to brittle fracture than joints with higher yield strength ratios of the base material. This indicates that the plasticity reserve of the base material also has a significant impact on the resistance to brittle fracture of the joint.
The deformation behavior of the weld metal is influenced by the matching of mechanical properties between the weld and the base material.
Under the same tensile stress, the weld strain of the superior strength matching joint of low yield strength ratio steel is larger, while the weld strain of the low strength matching joint of high yield strength ratio steel is smaller. The crack opening displacement (COD value) of the welded joint also shows the same trend, indicating that the superior strength matching joint of low yield strength ratio steel has the advantage of easy yielding at the crack tip and larger deformation at the crack tip.
The resistance to brittle fracture of welded joints is closely related to the heterogeneity of the mechanical performance of the joint. It is not only determined by the strength of the weld but also constrained by the toughness and plasticity of the weld. The selection of welding materials should not only ensure that the weld has suitable strength but also ensure that the weld has sufficient toughness and plasticity. That is, the strength-toughness matching of the weld should be controlled well.
For high-strength steel, achieving equal strength matching between the weld metal and the base material presents significant technical difficulties. Even if the weld strength reaches equal strength, the plasticity and toughness of the weld may be reduced to an unacceptable level, and the resistance to cracking may also significantly decrease. To prevent welding cracks, the construction conditions must be extremely stringent, and the construction cost will be greatly increased.
In order to avoid sacrificing the overall performance of the structure by pursuing strength alone and to improve the economic reliability in construction, it is necessary to reduce the strength and adopt a low strength matching scheme.
For example, Japan’s submarine steel NS110 has a yield strength greater than or equal to 1098MPa, and the yield strength of the deposited metal of the matching welding rod and gas shielded welding wire is required to be greater than or equal to 940MPa, with a yield strength matching coefficient of 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 toughness and cracking resistance of the weld, making welding construction more convenient and reducing construction costs.
In addition, some test data from Japanese scholar Kunihiko Sato shows that as long as the strength of the weld metal is not lower than 80% of the base material strength, the joint can still be guaranteed to be equal in strength to the base material.
However, the overall elongation of the joint with low-strength welds will be slightly lower. Under fatigue loading, if the excess height of the weld 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, when using low-strength welds, it is necessary to conduct some experimental work based on specific conditions.