Why is “low-strength matching” often used when welding thick plates?
“Low-strength matching” refers to using welding materials (electrodes, wires, etc.) where the strength of the weld metal is lower than the standard strength value of the base metal. This is especially common in thick plate welding, and the underlying principles can be understood from both mechanical and metallurgical perspectives.low-strength matching
一. Mechanical Principles: Strain Compatibility and Stress Redistribution
❶ Residual Stress and Restraint:
⑴ During thick plate welding, the weld and its surrounding area (heat-affected zone) undergo intense thermal cycling during heating and cooling. Due to the high rigidity of the thick plate in the thickness direction, the weld shrinkage is strongly restrained.
⑵ This strong restraint leads to high residual stress within the weld joint, especially in the thickness and length directions of the weld, where the peak stress may approach or even reach the yield strength of the material.
❷ Strength “Short Plank” Effect and Safety Valve Function:
⑴ In low-strength matching design, the yield strength of the weld metal is lower than that of the base metal. When a joint is subjected to load (especially dynamic loads or stress concentration), the weaker weld area will undergo plastic deformation first.
⑵ This plastic deformation acts like a “safety valve,” effectively releasing and redistributing high residual stress and localized stress peaks in the joint, preventing excessive stress concentration in more dangerous areas (such as the typically less tough heat-affected zone).
⑶ A simple analogy: Imagine a chain where one link (the weld) is intentionally made slightly weaker. When the chain is overloaded, this weak link will deform and elongate first, issuing a “warning” instead of allowing other more critical and harder links (the base material or the heat-affected zone) to suddenly fracture brittlely.
❸ Improved resistance to brittle fracture:
⑴ For thick plate structures, brittle fracture is one of the main failure modes. Brittle fracture often originates in high-stress areas with defects (such as cracks or incomplete penetration) and stress concentration.
(2) Low-strength matched welds exhibit stronger plastic deformation capacity, enabling them to “passivate” crack tips through yielding, significantly reducing the risk of unstable crack propagation and thus improving the overall joint’s resistance to brittle fracture.
二. Metallurgical Principles: Improving the Comprehensive Performance of Welds
❶ Balance of Strength and Toughness in Weld Metal:
(1) In metallurgy, strength and toughness are often contradictory. Increasing strength usually requires increasing carbon content or alloying element content, but this often sacrifices toughness and plasticity and increases susceptibility to cold cracking.
(2) Low-strength matched welding materials typically have a lower carbon equivalent, which means:
● Better weldability: Lower preheating temperature requirements, making welding simpler.
● Higher toughness: The weld metal exhibits superior low-temperature impact toughness.
● Lower susceptibility to cold cracking: Less sensitive to hydrogen-induced cold cracking, which is crucial for thick plate welding.
❷ Tolerance for Minor Defects:
(1) Any weld may contain minor defects such as porosity and slag inclusions. Low-strength matching welds with good toughness are less sensitive to these defects because their good plasticity allows for a more uniform stress distribution, avoiding extremely high stress concentrations at the defect sites.
In summary, the advantages of low-strength matching are: by sacrificing a small amount of nominal strength, it achieves higher overall joint safety, crack resistance (especially resistance to cold cracking and brittle fracture), and ease of manufacturing. This is a design philosophy of “using flexibility to overcome rigidity.”
When is “equal-strength matching” or “high-strength matching” necessary?
Equal-strength matching (weld strength equal to base metal) or high-strength matching (weld strength higher than base metal) must be used in the following situations:
一. Situations where “equal-strength matching” or “high-strength matching” must be used:
❶ Structures dominated by dynamic loads and fatigue loads:
● For example: bridges, crane booms, and the booms of construction machinery.
● Principle: In these structures, the weld needs to withstand repeated cyclic stresses. If low-strength matching is used, the lower-strength weld area will prematurely accumulate plastic strain under cyclic loading, leading to a significant reduction in fatigue life. Equal-strength or high-strength matching ensures that the weld has fatigue resistance comparable to the base metal, making fatigue cracks more likely to initiate in the base metal rather than the weld.
❷ Thin-plate structures based on strength design:
● When the plate thickness is thin, the welding restraint stress is low, and residual stress issues are not prominent. In this case, the load-bearing capacity of the structure is often directly determined by the material strength. To fully utilize the material and reduce structural weight, equal-strength matching must be used to ensure that the weld does not become a weak point in the overall structure.
❸ Situations where the weld metal needs to withstand the full strength of the base metal:
● In some designs, the load is entirely transferred through the weld, and the cross-sectional dimensions of the base metal are maximized. In this case, the weld must have at least the same strength as the base metal; otherwise, it will become the starting point of failure.
❹ Requirements of specific materials and processes:
● For example, when welding quenched and tempered steel (quenched + tempered steel), to maintain the performance of the heat-affected zone, it is usually required to use equal-strength or slightly higher-strength welding materials, along with strict welding processes, to prevent softening and performance degradation in the heat-affected zone.
● In stainless steel and heat-resistant steel welding, besides strength, the most crucial requirement is that the corrosion resistance or high-temperature performance of the weld bead must match that of the base metal. This usually means that equal composition matching is necessary, often resulting in equal or high strength welds.
❺ Regulations and Standards:
● Some industry standards (such as those for pressure vessels, nuclear power plant components, and critical components in aerospace) may explicitly stipulate the use of equal or high strength matching to ensure absolute safety under extreme conditions.
Special Note Regarding “High Strength Matching”:
High strength matching generally does not mean “the stronger the better.” Excessively high weld strength may lead to:
⑴ Decreased toughness.
⑵ Incompatibility with the plasticity of the base metal; under excessive restraint, deformation may concentrate in adjacent base metal or the heat-affected zone.
Therefore, even with high strength matching, the strength is generally only one grade higher than the base metal, and it is essential to ensure that its toughness and plasticity meet requirements.
Conclusion
The selection of strength matching principles for welded joints is a comprehensive engineering decision:
● For thick plates, static loads, and structures where resistance to brittle fracture is emphasized, “low-strength matching” should be prioritized, utilizing their excellent stress redistribution capabilities and high toughness to ensure safety.
● For thin plates subjected to dynamic loads, fatigue loads, or structures with special performance requirements based on strength design, “equal-strength matching” or “high-strength matching” must be used to ensure the joint’s load-bearing capacity and service performance.
In practical engineering, the final selection must be based on specific design requirements, service conditions, followed standards and specifications, and sufficient process qualification tests.