As an indispensable part of modern manufacturing, the quality of welding process is directly related to the overall performance and safety of the product. However, in the actual production process, due to the influence of various factors, a series of quality defects often occur in the welding process. This article will discuss in detail the common quality defects in the welding process, analyze the causes of their occurrence, and propose corresponding prevention and control measures.
1. Overview of welding process quality defects
Welding process quality defects are mainly divided into two categories: external defects and internal defects. External defects refer to defects on the surface of the weld that can be observed by the naked eye or low-power magnifying glass, such as undercut, weld bead, arc pit, surface pores, slag inclusions, surface cracks, and unreasonable weld position. Internal defects must be discovered through destructive tests or special non-destructive testing methods, such as internal pores, slag inclusions, internal cracks, incomplete penetration and incomplete fusion.
2. External defects and their prevention and control measures
(1). Undercut
The undercut refers to the gap left by the concave or groove melted by the arc at the edge of the weld during welding without being supplemented by the molten metal. Too deep an undercut will weaken the strength of the weld joint, causing local stress concentration, and cracks are likely to form at the undercut after loading. The undercut is mainly caused by factors such as excessive welding current, too long arc, improper electrode angle, inappropriate electrode feeding speed, and too short electrode retention length at the end of welding.
Preventive measures include: the current should not be too large during welding, the arc length should be moderate, and short arc welding should be used as much as possible; master the appropriate electrode angle and skilled electrode feeding techniques, and the electrode should swing to the edge slowly so that the molten electrode metal fills the edge; the depth of the weld undercut should be less than 0.5mm, the length should be less than 10% of the total length of the weld, and the continuous length should be less than 10mm.
(2). Weld nodules
Weld nodules refer to metal nodules that appear on the surface of the weld. They are usually caused by excessive welding current, too fast melting of the electrode, difficulty in controlling the weld formation, or unskilled welder operation. Weld nodules not only affect the aesthetics of the weld, but may also reduce the strength and toughness of the weld.
Preventive measures include: choosing appropriate welding current and welding rod, ensuring that the welder has skilled operation skills; when welding multi-layer welds, the flatness and uniformity of each layer of welds should be ensured to avoid weld nodules.
(3). Arc pits
Arc pits are the sliding phenomenon produced at the end of the weld, which not only weakens the strength of the weld, but also may cause cracks during the cooling process. Arc pits are mainly caused by too short arc extinguishing time at the end of welding or too large current used when welding thin plates.
Preventive measures include: when the weld is finished, make the welding rod stay for a short time or make several circular movements, do not stop the arc suddenly, to ensure that there is enough metal to fill the molten pool; ensure appropriate current during welding, and add arc starting plates to the main components to lead the arc pit out of the weldment.
(4). Surface pores
Surface pores refer to tiny holes on the surface of the weld, which are mainly caused by damp welding rods, oil or rust on the surface of the weldment, too fast welding speed, too strong current or inappropriate arc length. Surface pores will reduce the strength and sealing of the weld.
Preventive measures include: select qualified welding rods and ensure that the welding rods are dried before use; clean the welded parts before welding to remove impurities such as oil, rust, etc.; reduce the welding speed to make it easy for internal gas to escape; use the appropriate current and arc length recommended by the manufacturer.
(5). Slag inclusion
Slag inclusion refers to non-metallic inclusions such as oxides, nitrides, sulfides and phosphides remaining in the weld. Slag inclusion will reduce the plasticity and toughness of the weld, increase stress concentration, and cause cold and hot brittleness to crack easily.
Preventive measures include: use welding rods with good welding process performance and ensure that the welded steel meets the requirements of the design documents; select reasonable welding process parameters through welding process assessment; pay attention to the cleaning of welding grooves and edge ranges, and the welding rod groove should not be too small; for multi-layer welds, carefully remove the welding slag of each layer of welds; adopt measures such as preheating before welding, heating during welding and insulation after welding to reduce the generation of slag inclusions.
3. Internal defects and their prevention and control measures
(1). Incomplete penetration
Incomplete penetration refers to the phenomenon that the weld metal has not completely penetrated into the parent metal, resulting in partial infusion. Incomplete penetration will significantly reduce the strength and toughness of the weld.
Preventive measures include: selecting a more penetrating electrode; using appropriate current and welding speed; increasing the groove degree, increasing the gap and reducing the root depth; ensuring the correct weld design and combination.
(2). Internal cracks
Internal cracks refer to the metal rupture phenomenon inside the weld, which may occur inside the weld or in the heat-affected zone. Internal cracks will seriously affect the strength and safety of the weld.
Preventive measures include: eliminating cracks caused by uneven heating and cooling of welds due to thermal stress; selecting materials that meet the requirements of the design drawings; strictly controlling the source of hydrogen, and drying the welding rod before use and cleaning the oil, moisture and other impurities on the groove; selecting reasonable welding parameters to control the input heat between appropriate cooling temperatures; when the welding environment temperature is low and the material is thin, in addition to increasing the operating environment temperature, it should also be preheated before welding; after welding, try to keep warm and slow cool and post-weld heat treatment to eliminate delayed cracks caused by residual stress in the weld during the cooling process.
4. Summary
Welding process quality defects are an important factor affecting product performance and safety. In order to ensure welding quality, various parameters and conditions in the welding process must be strictly controlled, and effective preventive measures must be taken to prevent and eliminate various quality defects. At the same time, strengthening the training and management of welders is also an important way to improve welding quality. Through continuous technological innovation and process improvement, we can further improve the quality level of welding technology and make greater contributions to the development of the manufacturing industry.
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