At the "2013 National Steel Structure Technology Academic Exchange Conference" held recently, Duan Bin, Director of the Welding Research Institute at China Metallurgical Group Building Research Institute, highlighted common welding defects in steel structures. He identified six main types of issues: hot cracks, cold cracks, lamellar tearing, incomplete fusion and penetration, porosity, and slag inclusions.
First, hot cracks occur during the cooling of the weld. These cracks are primarily caused by impurities like sulfur and phosphorus in the base metal or welding materials. These elements form brittle, low-melting-point eutectics that become vulnerable to cracking under tension as the weld solidifies.
Second, cold cracks—also known as delayed cracks—develop within a temperature range of 200°C down to room temperature, sometimes appearing minutes or even days after welding. They are often linked to factors such as the type of steel used, structural design, welding material handling, and the welding process itself.
Third, lamellar tearing is a defect that occurs in high-strength low-alloy steels with significant thickness and impurity levels, especially high sulfur content. When the welded area cools below 400°C, stress perpendicular to the plate’s thickness can cause stepped cracks along the rolling direction.
Fourth, incomplete fusion and penetration typically result from improper welding parameters, insufficient groove cleaning, or poor welder technique. Both issues share similar causes, including inadequate surface preparation and the presence of contaminants like scale or slag.
Fifth, porosity refers to gas pockets trapped in the weld. It can be classified into two types: gas pores (like hydrogen and nitrogen pores) and reaction pores (such as CO pores in non-ferrous metals). Hydrogen pores tend to appear as isolated bubbles, while nitrogen pores are usually more clustered. The formation of these pores is influenced by welding consumables, process settings, and the cleanliness of the joint area.
Lastly, slag inclusions occur when non-metallic materials like flux residues remain in the weld. Their shape, size, and distribution depend on the welding method, electrode composition, and flux properties.
Song Jingping
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