Weld undercutRefers to the depression or groove formed along the weld toe, base metal part, which is a gap left by the arc after melting the base metal at the edge of the weld without being adequately replenished by the deposited metal. Weld undercut not only affects the appearance of the weld, but also reduces the strength and durability of the weld, and even leads to the formation of cracks. Therefore, the weld undercut is a defect that must be avoided in the welding process. So, how does a weld undercut come about? And how can we prevent and fix it? This article will give you a detailed answer.
The main reasons for welding undercut are as follows:
Improper selection of welding parameters. If the welding current is too large, the arc is too long, or the welding speed is too slow, it will cause the base metal to melt excessively and form a undercut. Conversely, if the welding current is too small, the arc is too short, or the welding speed is too fast, it will also cause the deposited metal to be insufficient to fill the weld pool, and undercuts will also occur.
The welding operation is incorrect. If the angle between the electrode and the workpiece is not suitable, or the way of transporting the rod is not standardized, it will also affect the formation of the weld pool and the flow of metal, resulting in undercut. For example, if the angle between the electrode and the workpiece is too large or too small, it can cause the droplets to fall unevenly into the weld pool, or the width of the weld pool is inconsistent, resulting in undercuts. If the arc stays on both sides of the weld for too long or too short when transporting the bar, it will also make the depth of the weld pool uneven and form a undercut.
The welding position and sequence are not reasonable. If the welding position is not conducive to the stability of the weld pool, or if the welding sequence is not conducive to the uniformity of the weld, the risk of undercut will also be increased. For example, if you weld at a vertical, horizontal, or vertical weld, the shape and position of the weld pool will change due to gravity, which can easily cause undercuts. If the weld is not carried out in a reasonable order and direction when welding multi-layer welds, the weld will also be subjected to uneven thermal influence, resulting in undercuts.
In order to avoid the occurrence of weld undercuts, we can take precautions from the following aspects:
Select the appropriate welding parameters. According to the nature of the welding material, the requirements of the welding method, and the characteristics of the welding position, select the appropriate welding current, voltage, welding speed and other parameters, so that the shape and size of the weld pool are suitable for the requirements of the weld, and avoid the excessive melting of the base metal or the lack of deposited metal.
Standardize welding operations. Master the correct angle between the welding rod and the workpiece, generally 60° 80°, and adjust it appropriately according to the different welding positions. Adopt a reasonable transportation method, generally straight or slightly swinging, to keep the length of the arc and the width of the weld uniform, so that the molten droplets fall smoothly into the molten pool, and avoid the fluctuation and sag of the molten pool.
If the weld undercut has been generated, we can fix it with the following methods:
Grinding method. If the depth of the undercut is shallow, not more than 10% of the thickness of the base metal, a grinder or grinder can be used to polish the base metal at the undercut to make a smooth transition with the weld and eliminate the hazard of stress concentration. However, this method reduces the effective cross-sectional area of the base metal and reduces the strength of the weld, so it is not suitable for structures subjected to large loads.
Repair welding. If the depth of the undercut is large, more than 10% of the thickness of the base metal, or there are high requirements for the strength of the weld, the base metal at the undercut can be filled with the method of repair welding, so that it forms a whole with the weld and restores the strength and durability of the weld. However, this method increases the amount of work and cost of welding, as well as the heat-affected zone of the weld, which can lead to deformation and stress of the weld.
In conclusion, weld undercut is a common welding defect that can affect the quality and performance of the weld and even lead to the failure of the weld. Therefore, we should take effective preventive measures to avoid welding undercut from the aspects of selecting appropriate welding parameters, standardizing welding operations, and determining reasonable welding positions and sequences. If the weld undercut has been generated, we can choose the grinding method or repair welding method to repair according to the depth of the undercut and the requirements of the weld to ensure the quality and safety of the weld.