In steel structure construction, welding is an indispensable process. Improper welding methods can bring major hidden risks to steel structure projects and affect project quality. We have summarized the common defects in steel structure welding for you. Let's take a look!
　　01. Cracks
　　(1) Hot cracks. These occur during the cooling process of the weld. Because sulfur and phosphorus impurities in the steel or welding consumables form various brittle, hard low-melting-point eutectics with the steel, during weld cooling the last-solidifying low-melting-point eutectics are in a tensile state and are highly prone to cracking.
　　(2) Cold cracks. These occur in the range from 200°C to room temperature and have delayed characteristics, appearing from a few minutes to several days after welding, so they are also called delayed cracks. Their occurrence is closely related to the selection of steel, structural design, storage and use of welding materials, and welding process.
　　Preventive measures:
　　For cold cracks, steel with good crack resistance and low-hydrogen or ultra-low-hydrogen, low-strength electrodes should be selected, and preheating temperature and heat input should be controlled to reduce the tendency for cold cracks to occur; for hot cracks, steel with high nickel content should be selected, refining methods should be adopted to improve steel purity and reduce impurity content, and the weld concavity d should be controlled to less than 1 mm and heat input reduced to lower the tendency for hot cracks to occur.
　　02. Lack of fusion and incomplete penetration
　　These two phenomena are mainly caused by improper process parameters, measures, and groove dimensions, insufficient cleanliness of the groove and weld bead surface or the presence of scale and slag and other debris, and poor welder skill.
　　Preventive measures:
　　Before welding, ensure that there is no debris on both sides of the groove edge, and determine the groove form and assembly gap. During welding, reasonably select the welding current, electrode angle, and travel speed; if the workpiece conducts heat quickly and has a large heat dissipation area, preheating before welding or flame heating during welding may be used, and at the start and joint of the weld, long-arc preheating may be selected before welding.
　　For welds requiring full penetration, single-side welding with double-side forming should be adopted as much as possible. In addition, to avoid magnetic arc blow, the arc should not be biased to one side so as to ensure uniform heating everywhere.
　　03. Lamellar tearing
　　When the welding temperature cools below 400°C, due to the relatively large plate thickness and relatively high impurity content, especially when the sulfur content is high, low-alloy high-strength steel with strong segregation along the direction parallel to plate rolling will, when subjected to force perpendicular to the thickness direction during welding, produce "step-like" lamellar cracking along the rolling direction.
　　Preventive measures:
　　Reduce the fillet weld size to reduce the volume of weld metal; correctly select Z-direction steel structure components; low-hydrogen welding methods and low-strength matching welding consumables may be used. Appropriately low heat input should be used to reduce thermal effects and thereby reduce shrinkage strain, but at this time the occurrence of cold cracks must be prevented.
　　Strictly control weld dimensions, use small beads and multiple passes, and avoid excessively large weld legs. Appropriate preheating is beneficial, but the resulting increase in shrinkage strain must be prevented. Intermediate annealing to relieve stress may also be adopted.
　　04. Porosity
　　(1) Precipitation-type porosity. This is mainly hydrogen porosity and nitrogen porosity, mostly surface porosity. The main difference between hydrogen porosity and nitrogen porosity is that hydrogen porosity is mostly individual pores, while nitrogen porosity is mostly dense porosity.
　　(2) Reaction-type porosity. In the welding of steel, i.e., nonferrous metals, carbon monoxide porosity is predominant. The main causes of porosity in welds are related to the selection, storage, and use of welding consumables, the selection of welding process parameters, the cleanliness of the groove base material, and the degree of protection of the molten pool.
　　Preventive measures:
　　Before welding, strictly clean oil stains and rust from the surface of the base material and welding consumables, and dry the welding materials; correctly select welding materials and strictly control gas sources, and strengthen protection of the welding zone; to remove gases already dissolved in the molten pool, appropriate welding process parameters should be used and the welding process optimized.
　　05. Slag inclusion
　　The type, morphology, and distribution of nonmetallic inclusions are mainly related to the welding method, electrodes and flux, and the chemical composition of the weld metal.
　　Preventive measures:
　　Before welding, rust and oil stains within 120 mm of the welding area should be cleaned thoroughly. Use electrodes with good process performance, and prohibit the use of expired, deteriorated, or cracked-coating electrodes. The groove angle should not be too small, and slag in the groove, on both sides, and between layers must be cleaned thoroughly. When selecting welding parameters, the current must not be too low and the welding speed must not be too fast. During welding, adjust the electrode angle and weaving angle at any time.