To weld well, controlling the molten pool is something you must learn

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The pool temperature has a direct impact on the quality of welding. A higher pool temperature, along with a larger pool and better fluidity of the molten iron, facilitates fusion. However, when the temperature is too high, the molten iron tends to flow downward, which can cause the back side of the weld to burn through during single-sided welding with double-sided formation. This leads to the formation of weld bulges, makes it difficult to control the shape of the weld, and reduces the plasticity of the joint, increasing the risk of cracking under bending stress. When the pool temperature is low, the molten pool is smaller, the molten iron appears darker, its fluidity is poor, and defects such as lack of penetration, lack of fusion, and slag inclusions are likely to occur. The pool temperature is closely related to factors such as welding current, electrode diameter, electrode angle, and arc burning time. The following measures are taken to control the pool temperature by addressing these factors. 1. Welding current and electrode diameter: The welding current and electrode diameter are selected based on the position of the weld area and the welding layer; a larger welding current and electrode diameter are used when starting welding, while smaller values are used in vertical, horizontal, and overhead welding positions. For the root pass of butt welding of 12mm thick plates, a φ3.2mm electrode is used, with a welding current of 80-85A ； For filling and the surface coating, use welding rods with a diameter of φ4.0mm; the welding current should be 165–175A ； Choosing the welding current and electrode diameter appropriately to easily control the molten pool temperature is the foundation for weld shape formation. 2. Strip movement method: The temperature of the molten pool when using circular strip movement is higher than that when using crescent-shaped strip movement; the temperature obtained with crescent-shaped strip movement is in turn higher than that with zigzag-shaped strip movement. For the root pass in flat welding at 12 mm thickness, zigzag-shaped strip movement should be used ； Furthermore, by controlling the amplitude of the swing and the pauses on both sides of the groove, the temperature of the molten pool was effectively regulated, resulting in weld pores of roughly uniform size. The likelihood of weld bulges and burn-through at the root of the groove was reduced, and the problem of incomplete penetration was improved, thereby making single-sided welding with double-sided formation in butt joint flat welding no longer a challenge. 3. Electrode angle: When the angle between the electrode and the welding direction is 90 degrees, the arc is concentrated and the temperature of the molten pool is high; when the angle is smaller, the arc is dispersed and the temperature of the molten pool is lower. This is the case for 12mm flat welding root passes ； Welding rod angle: 50-70 degrees, to reduce the temperature of the molten pool and prevent the formation of weld beads or bulges on the back side. For example, after changing the welding electrode for the root pass in vertical welding of 12mm thick plates, a welding electrode angle of 90-95 degrees is used during joint formation, which helps to rapidly increase the temperature of the molten pool, allowing the molten hole to open smoothly. This results in a smoother backside shape and effective control over the indentation at the joint area. 4. Arc combustion time: In the practical teaching of horizontal and vertical welding of φ57×3.5 pipes, the arc interruption method is used for welding ； During the welding of the sealing layer, the arc interruption frequency and arc combustion time directly affect the temperature of the molten pool ； Due to the thin wall thickness, the capacity to withstand arc heat is limited; if the arc interruption frequency is reduced to lower the temperature of the molten pool, shrinkage cavities are likely to form. Therefore, only the arc combustion time can be used to control the temperature of the molten pool. If the pool temperature is too high and the melting hole is large, the arc combustion time can be reduced to lower the pool temperature; this will result in a smaller melting hole, and an appropriate forming height inside the tube, thereby preventing the weld inside the tube from being too high or causing weld bumps.