Welding Defects-Welding Slag
Welding slag can be divided into two categories. The first kind is metal inclusion slag, especially refers to the metal particles such as tungsten, copper and so on remain in the weld seam, which is customarily called tungsten inclusion, copper inclusion. The second kind is non-metal inclusion slag, especially refers to the residual slag in the weld seam after welding, including electrode coating or flux, sulfide, oxide and nitride, etc. . According to the condition of its formation, it can be divided into point slag, linear slag, chain slag and dense slag.
The slag inclusion will reduce the plasticity and toughness of the weld. The slag inclusion with sharp angle will produce stress concentration at the tip, and the tip will develop into a crack source, especially in the weld with large tendency of air quenching. Under the action of stress, the crack will first appear and extend in the weld inclusion, which leads to the decrease of strength and the crack of the weld.
There are many factors for the formation of welding slag, if there are more inclusions in welding materials (including base metal, gas and welding materials, etc.) , it is bound to cause inclusions in the weld. In addition, process factors also have a great impact on the slag inclusion：1. Weld edge, welding layer and weld between the slag is not clean. 2. The welding current is too small, the molten metal and the molten slag get insufficient heat, causes its fluidity to reduce, and the molten metal solidification speed is fast, the slag didn’t float out in time. 3. Improper chemical composition of base metal and welding materials, for example, when the molten pool contains more oxygen, nitrogen, sulfur and other components, its products (oxides, nitrides, sulfides, etc.) in the molten metal solidification faster case, too late to float, will remain in the weld formed slag.
The important aspect to prevent inclusion in weld is to select welding material correctly, to make it better deoxidation, desulphurization and so on, and then to pay attention to process measures: 1. Adopt welding material with good process performance, prohibit using welding material with expired, deterioration and appearance defects. 2. Clean rust, oil and other harmful impurities in the welding area before welding, groove angle should not be too small, groove and both sides, the layer of slag must be cleaned up. 3. Selecting reasonable welding parameters through welding process assessment, selecting suitable diameter electrode according to plate thickness, adjusting electrode angle and transportation method at any time according to the direction of slag flow, the slag can float to the surface of the molten pool smoothly.
