How many common defects are there in welds?
ALAND WELDING Let you feel the most sincere welding service
Defects in welds can be divided into two categories: internal defects and external defects according to their location in the weld.
Internal defects: mainly include incomplete welding, internal pores, internal cracks, slag inclusions, etc. Internal defects located inside the weld can only be discovered by non-destructive testing or destructive testing.
Welded Seam
The weld size does not meet the requirements. Mainly refers to the uneven height of the weld, different widths, excessive or insufficient reinforcement, etc.; the height of the weld during inspection is too large or too small; or the width of the weld is too wide or too narrow, as well as the weld and the motherboard. The transition between the materials is not smooth, the surface is rough, the weld is irregular vertically and horizontally, and the weld is too concave in the fillet weld.
If the weld size is too small, it will reduce the load-bearing capacity of the welded joint; if the weld size is too large, it will increase the welding workload, increase the welding residual stress and welding deformation, and cause stress concentration.
Improper welding groove angle or uneven assembly gap, too large or too small welding current, improper strip transportation method or speed and improper welding angle will cause the weld size to not meet the requirements. defects in welds
Reason
It is caused by the poor straightness of the weld groove processing, improper angle of the groove or uneven assembly gap size.
If the current during welding is too large, the welding rod will melt too quickly, making it difficult to control the weld formation. If the current is too small, the welding rod will cause “adhesion” during arc ignition, resulting in incomplete welding or welding.
The welder’s operating proficiency is insufficient, the method of transporting the rod is improper, such as too fast or too slow, and the angle of the welding rod is incorrect.
In the submerged arc automatic welding process, the welding process parameters are improperly selected.
Prevention
Process the weld bevel according to the design requirements and welding specifications. Try to use mechanical processing to ensure that the bevel angle and straightness of the bevel edge and the straightness of the bevel edge meet the requirements. Avoid using manual gas cutting and manual shoveling to process the bevel. mouth. When assembling, ensure the uniformity of the weld gap to lay the foundation for ensuring welding quality.
Through welding process qualification, select appropriate welding process parameters.
Welders must hold certificates to work, and trained welders have a certain theoretical foundation and operational skills.
The last layer of multi-layer welding on the welding surface should be welded with a smaller current than the welding current between the layers and a small diameter (φ2.0mm~3.0mm) electrode to cover the welding while ensuring fusion with the bottom layer. The speed of transporting the strip must be uniform, rhythmically advance longitudinally, and make a certain width of horizontal swing, so that the surface of the weld seam can be neat and beautiful.
Undercut
During welding, the groove formed at the interface between both sides of the weld and the base metal is called undercut (or undercut). Undercutting will reduce the effective cross-section of the base metal, weakening the strength of the welded joint. At the same time, stress is easily concentrated at the undercut. After loading, cracks may occur at the undercut, or even cause structural damage.defects in welds
Three reasons
The main reasons are that the welding current is too large, the arc is too long, the angle of the welding rod is not properly controlled, the speed of the rod transport is inappropriate, and the length of the electrode left at the end of the welding is too short, etc., which causes undercuts. It is a common defect in vertical welding, horizontal welding and overhead welding.
Prevention
When welding, the current should not be too large, and the arc should not be drawn too long or too short. Use short arc welding as much as possible.
Master the appropriate angle of the welding rod and skillful handling of the rod. The welding rod should swing slightly slower to the edge so that the molten rod metal fills the edge, while it should be slightly faster in the middle.
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. Once the depth or yield exceeds the above tolerance, the defective area should be cleaned up, and a welding rod with a smaller diameter and the same brand should be used, and the welding current should be slightly larger than normal to fill it up.
Overlap
During the welding process, the metal nodules formed by the molten metal flowing onto the unmelted base metal outside the weld are called weld nodules. Welding nodules not only affect the appearance of the weld, but there are often incomplete welding defects under the welding nodules, which can easily cause stress concentration.defects in welds
Reason
Excessive weld gap, incorrect welding rod position and rod transport method, excessive welding current or too slow welding speed will cause welding nodules.
Burn-through
During the welding process, the molten metal flows out from the back of the groove, forming a perforation defect called burn-through.defects in welds
Reason
The main reasons for burn-through are that the welding current is too large and the welding speed is too slow. Burn-through can also occur when the assembly gap is too large or the blunt edge is too thin.
Not fused
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.defects in welds
Reason
The reasons for lack of fusion include: the welding line energy is too low; arc blow occurs; there is rust and dirt on the side wall of the groove; the welding layer is not completely cleaned of slag, etc.
Pits, collapses and underfills
Dimples refer to local depressions formed on the surface of the weld or on the back of the weld that are lower than the surface of the base metal.
Collapse refers to the phenomenon that during single-sided fusion welding, due to improper welding technology, excess metal of the weld passes through the back, causing the front of the weld to collapse and the back to bulge. Continuous or intermittent grooves formed on the surface of the weld due to insufficient filler metal are not filled.defects in welds
Slag
The slag remaining in the weld after welding is called slag inclusion.
There are many reasons for slag inclusion, such as the edges of the weldment, the weld layer, and the weld bead are not cleaned cleanly; the welding current is too small, causing the melting and solidification speed to accelerate, and the slag has no time to surface; improper transportation of the rod, the slag and the molten iron The separation is not clear, which hinders the floating of the slag; the chemical composition of the weldment and welding rod is inappropriate; the molten pool contains too much oxygen and nitrogen, etc.
Air hole
During welding, the cavities formed by the bubbles in the molten pool that fail to escape during solidification are called pores. Stomata can be divided into dense stomata, worm-like stomata and needle-like stomata. The gases that form pores in welds are mainly hydrogen, nitrogen and carbon monoxide.
Porosity has a great influence on the performance of the weld. It not only reduces the effective area of the weld and reduces the mechanical properties of the weld, but also destroys the density of the weld and easily causes leakage.
The causes of pores are: the good protection of the welding zone is damaged during the welding process; there are oil stains, rust and water-absorbing pollutants on the surface of the base metal welding zone and the welding wire; the welding rod is damp and insufficiently baked; the welding current is too large or too small , The welding speed is too fast; the welding arc is too long and the arc voltage is too high.
Cracks
The temperature at which welding cracks form can be divided into hot cracks and cold cracks. According to the location where the crack occurs, it can be divided into cracks in the weld metal and cracks in the heat-affected zone. During the welding process, the weld cracks produced when the weld and heat-affected zone metal cool to a high temperature zone near the solidus line are called hot cracks; the weld cracks produced when the welded joint cools to a lower temperature are called cold cracks.
Welding cracks are the most dangerous welding defects, which seriously affect the performance, safety and reliability of welded structures. In addition to reducing the strength of the welded joint, cracks also have a sharp notch at the end of the crack, which will cause severe stress concentration and promote the development and destruction of the crack.
