Cold-rolled low-carbon rigid strip is prone to cracking during welding. This phenomenon is closely related to material properties, welding process, and environmental factors. The main types of cracking are hot cracking and cold cracking. The former is caused by shrinkage stress during weld metal crystallization, while the latter is related to hydrogen embrittlement and microstructural stress.
From a material perspective, although cold-rolled low-carbon rigid strip has a low carbon content, excessive levels of impurities such as sulfur and phosphorus in the raw materials, or insufficient deoxidation during smelting, can lead to the formation of low-melting-point eutectics during weld crystallization. These eutectics preferentially solidify during weld cooling, forming a liquid film that weakens the intergranular bonding. When welding stress exceeds the material strength, cracks are more likely to initiate in this liquid film, manifesting as a bluish or black oxidized color on the weld surface and without a metallic cracking sound. Furthermore, using carbon arc gouging to bevel the weld, can cause localized carbonization in the weld zone, forming a brittle pearlite structure and further increasing cracking susceptibility.
Improper welding process parameters are another key factor in cracking. Excessive current and high welding speeds can lead to excessive weld penetration and high weld pool temperatures, coarsening the grains in the heat-affected zone (HAZ) and reducing the material's plasticity. Rapid cooling can also exacerbate hydrogen supersaturation and precipitation in the weld. Hydrogen atoms diffuse and aggregate at grain boundaries and in the martensite region, forming high-pressure hydrogen bubbles that can induce microcracks. Furthermore, improper welding sequences, such as asymmetrical welding, can lead to uneven weld shrinkage, generating multi-directional stresses and exacerbating crack propagation. For example, welding the outside first and then the inside can subject the inside weld to excessive tensile stress, initiating cracks.
Among environmental factors, the impact of humidity and oil contamination cannot be ignored. Moisture, oil, or rust on the weld surface decomposes under the high arc temperature to produce atomic hydrogen, which dissolves into the weld pool and precipitates upon cooling, becoming the source of hydrogen-induced cracks. Furthermore, low temperatures can reduce the material's plasticity and increase crack susceptibility. For example, welding in winter or in humid environments without preheating measures can lead to rapid weld cooling, which can easily lead to cold cracking.
To prevent cracking, three key areas must be addressed: material control, process optimization, and environmental management. Regarding materials, cold-rolled low-carbon rigid strip with low impurities and adequate deoxidation should be selected. Welding rod quality should be strictly inspected, and acidic rods with poor desulfurization and dephosphorization capabilities should be avoided. Process parameters should be adjusted based on strip thickness, such as using low current and slow welding speeds to control penetration and heat input. Furthermore, the welding sequence should be optimized, such as using symmetrical welding and welding in layers, to reduce welding stress. Furthermore, preheating before welding can reduce weld cooling, promote hydrogen diffusion, and reduce the formation of hard martensite. The preheating temperature should be determined based on material thickness and process evaluation. Generally, the preheating range on both sides of the groove is three times the plate thickness.
Regarding environmental management, the weld surface must be thoroughly cleaned of oil, rust, and moisture before welding to prevent the generation of hydrogen sources. When welding in low temperatures or humid environments, the preheating temperature and range should be increased, and preheating should be performed along the entire longitudinal seam to avoid uneven temperatures caused by preheating in sections. During the welding process, continuous welding must be maintained and interruptions must be avoided. If interruptions are necessary, insulation measures should be taken to prevent rapid cooling of the weld. Before re-welding, confirm that no cracks are present before resuming welding.
A comprehensive approach of material control, process optimization, and environmental management can effectively reduce the tendency to crack during cold-rolled low-carbon rigid strip welding and improve weld quality.
