Judgment of Welding Defects in Auto Parts

There are many types of welding defects, which can be divided into external defects and internal defects according to their positions. Common defects include pores, slag inclusions, welding cracks, incomplete penetration, incomplete fusion, weld dimensions and shapes that do not meet the requirements, undercuts, welding bumps, arc craters, etc.

1. Stomata

     Pores refer to the cavities formed when the bubbles in the molten pool fail to escape when they solidify during welding. The main causes of pores are: the edge of the groove is not clean, there is water, oil and rust; the welding rod or flux is not baked according to the regulations, the welding core is corroded or the coating is deteriorated and peeled off.

2. Slag inclusion

  Slag inclusions are slag that remains in the weld. Slag inclusions also reduce the strength and compactness of the weld. The main reason for slag inclusion is that there is slag left by oxygen cutting or carbon arc gouging on the edge of the weld; the groove angle or welding current is too small, or the welding speed is too fast. When using acid electrodes, "paste slag" is formed due to too small current or improper transportation; when using basic electrodes, slag inclusions will also be caused due to excessively long arc or incorrect polarity. When submerged arc welding is used for bottom sealing, the welding wire deviates from the center of the weld seam, and slag inclusions are also easy to form

3.Undercut

     The depression left by the edge of the weld is called undercut. The cause of the undercut is due to excessive welding current, fast transport speed, too long arc or improper electrode angle. The welding speed of submerged arc welding is too fast or the track of the welding machine is unequal, which will cause the weldment to be melted to a certain depth, and the filler metal cannot be filled in time, resulting in undercut. The undercut reduces the working section of the base metal joint, thus causing stress concentration at the undercut. Therefore, in important structures or structures subject to dynamic loads, the undercut is generally not allowed to exist, or the depth of the undercut is limited. .

4.no penetration, no fusion

During welding, the phenomenon of incomplete penetration at the root of the joint is called incomplete penetration; there is a partial incomplete penetration between the weldment and the weld metal or the weld layer, which is called incomplete fusion. Incomplete penetration or lack of fusion is a relatively serious defect. Due to incomplete penetration or lack of fusion, the weld will appear intermittent or abrupt, the strength of the weld will be greatly reduced, and even cracks will occur. Therefore, incomplete penetration and fusion are not allowed in important structural parts of the hull. The causes of incomplete penetration and incomplete fusion are that the assembly gap or groove angle of the weldment is too small, the blunt edge is too thick, the electrode diameter is too large, the current is too small, the speed is too fast, and the arc is too long.

5. Welding cracks

Weld cracks are a very serious defect. The damage of the structure mostly starts from the cracks. During the welding process, all necessary measures should be taken to prevent cracks. After welding, various methods should be used to check whether there are cracks. Once a crack is found, it should be completely removed and then repaired.

6. Other defects

Welding is also common to see some weld bead, arc crater and weld size and shape defects. The main reason for the generation of welding nodules is that the unevenness of the strips causes the temperature of the molten pool to be too high, and the liquid metal solidifies and slowly falls, thus forming metal nodules on the surface of the weld. When correcting weld defects, attention should be paid to:

1.When repairing defects, it is advisable to use low current, no swing, multi-layer and multi-pass welding, and excessive current repair welding is prohibited;

2.When repairing welding for a rigid structure, except for the first and last layers of weld bead, hammering can be carried out in the hot state after welding. The arc start and end arc of each layer of weld bead should be staggered as much as possible;

3. For materials that require preheating, when the working environment temperature is lower than 0°C, corresponding preheating measures should be taken;

4.For weldments requiring heat treatment, defect correction shall be carried out before heat 5.For weld defects of grade D, grade E steel and high-strength structural steel, when manual arc welding is used to repair welds, the controlled line energy welding method should be used. Each defect should be repaired at one time, and no stoppage is allowed. Both the preheating temperature and the interlayer temperature should be kept above 60°C.

6.Welding repairs to eliminate weld defects are not allowed to be carried out under pressure and backwater;

7.The corrected weld should be re-inspected according to the flaw detection requirements of the original weld. If defects exceeding the allowable limit are found again, it should be corrected again until it is qualified. The number of welding repairs shall not exceed the specified number of repairs.