Welding is one of the most important manufacturing processes. With its help, the connection of steel parts in a wide variety of designs is performed. As with other manufacturing processes, marriage is sometimes found. It means defects in the weld that can drastically reduce the quality of the finished product, or even make its operation deadly.
Classification
By the way, how can they be divided? All weld defects are divided into three large groups:
- Outdoor.
- Internal
- Cross-cutting.
External weld defects are often the most numerous category. It includes: excessively small dimensions, as well as displacement of the weld line, various influxes, “cuts”, shrinkage shells and craters not plugged during welding, porosity or cracks. Uneven seam width also applies to this variety. It is believed that external defects in welds belong to the least dangerous category.
Accordingly, the internal ones include: pores, numerous inclusions of slag, incompletely boiled places, as well as cracks in the thickness of the welded metal. As for through defects, these are fistulas passing through the entire thickness of the crack part, as well as burnout.
The main causes of welding defects
- Almost always they appear when they try to use extremely cheap and low-grade materials.
- The same can be said for low-quality welding equipment. In addition, the frequency of occurrence of defects often increases after poor repair of devices used by specialists.
- Of course, this happens all the time with violations of the work technology.
- Serious defects in the weld are often found in inexperienced specialists with low qualifications.
It is easy to understand that the highest quality products are obtained in the case of the use of fully automated equipment. Do not forget about the convenience of the workspace. So, large flake of the seam and violations of its width are very common in cases where the welder (even an experienced one) works in an uncomfortable position.
Actually, it is no coincidence that there are items in the requirements for welding work that specifically stipulate the full equipment of the workplace, providing for its high-quality ergonomics.
Important note
Even novice welders are well aware that to ensure maximum strength, the seam should have a small reinforcement with a height of about 1-2 mm. At the same time, the same welders often make a gross mistake when making reinforcement with a height of 3-4 mm. In principle, in simple cases there is nothing wrong with this, but not when it comes to products that are constantly in a state of dynamic load. All this leads to stress concentration and a sharp increase in the probability of failure.
Undercuts
As we have already said, defects in welds and joints are extremely dangerous. It is easy to imagine what will happen if they are available in the part intended for installation, for example, in the supporting structure of the railway bridge. They are especially dangerous in the case of welding alloy steel parts , which will be used in conditions of constant temperature changes.
The most dangerous are undercuts, since they are a natural “accumulator” of stresses that will concentrate in the weakest point of the seam. In addition, they significantly reduce its working cross section, which also has an extremely negative effect on the strength of the entire joint.
Typically, these external weld defects are not corrected in most cases. This is due to the fact that the metal (most often) will still have an inconspicuous marriage, which can lead to very significant consequences.
How do undercuts occur?
The main reason is setting too much current. In combination with a long arc, this factor gives an almost absolute probability of their occurrence. In addition, in some cases, undercuts occur when the heat source moves excessively fast above the metal surface.
If the design is at least somehow important, even the smallest defects in welds and joints of this type are completely unacceptable. Correct them with the help of neat welding of a thin seam. If this is possible, the part is still better to completely replace (note that the last remark applies to all defects).
Uncooked metal sections
If such a site is located directly in the thickness of the seam, this is extremely dangerous. Firstly, to find such a defect is possible only with the help of a flaw detector. Secondly, they are again accumulating places of natural stresses in the metal. In combination with a violation of the welded structure, all this leads to the risk of premature failure of the part. Especially often, such internal defects of welds occur in the case of alloy steel and poor welding equipment.
Porosity (regardless of its location) dramatically reduces the strength characteristics to unacceptable values, leads to "stratification" of the metal, that is, to a violation of its natural structure. Parts even with low porosity are several times more likely to fail under load even at the beginning of operation. Pores appear due to the gases, which simply do not have time to leave the layer of molten metal.
Like all types of weld defects, they are extremely common when low-quality raw electrodes are used. It often happens that porosity occurs due to some extraneous impurities in the protective gases. As in the past case, this type of defect can also be observed with an excessively high welding speed, when the integrity of the gas protective “bathtub” is corrupted.
Slag inclusions
Slag inclusions significantly damage the uniformity of the metal structure. The classic reason for the formation is the careless cleaning of the surface of the seam from the remnants of rust and scale. The probability of their occurrence tends to zero under the condition of welding in a layer of protective gases. Rare inclusions of a rounded shape do not represent a danger, products with them can pass OTC.
Note that if a tungsten electrode was used in welding, particles of this metal can be detected in the parts. The degree of their danger - as in the previous case (i.e., these are permissible defects of the welds).
Cracks
They are transverse and longitudinal, running both along the seam itself, and along the metal along or near it. They are extremely dangerous in that in some cases they reduce the mechanical and vibrational strength of the product to almost zero. Depending on the properties of the material being welded, the crack can both maintain its original location and spread over the entire length of the workpiece in a very short time.
Not surprisingly, these are the most dangerous weld defects. GOST in most cases requires the immediate rejection of such parts, regardless of its purpose (with the exception of absolutely unimportant products).
Uneven seams
This is the name of a gross mismatch of the geometric parameters of the joints with the characteristics required in regulatory documents. Simply put, if the welding is "snake", obliquely, etc., we are talking about a similar type of defects.
Most often they appear during the work of inexperienced welders, as well as during significant power surges, poor-quality equipment and banal rush. This defect is dangerous in that it is often combined with undercooking, which is already much more dangerous. If the deviation from the center line of the connection is negligible and does not cause a decrease in the strength of the product, the part may be approved for use.
In this case, you should always remember one simple thing: the smaller the angle of transition from the base metal to the deposition layer, the worse the mechanical strength of the welded product becomes. Of course, in the manufacture of some household structures (the frame of the greenhouse, for example) in conditions of insufficient voltage, it is simply unrealistic to do without uneven seams. However, in this case, they do not pose a particular danger.
The main methods of elimination, correction of defects
Let's say the following right away: in most cases, it makes no sense to discuss ways to eliminate defects in welds, since in conditions of more or less strict quality control procedures, all products with some flaws are simply rejected. But sometimes it really happens that the defect is not too serious, and therefore it can be eliminated. How to do it?
In the case of steel structures, the damaged surface is cut off (plasma-arc welding), the place of the failed connection is thoroughly cleaned, and then the attempt is repeated. If there are minor external defects in the welds (uneven joints, shallow pock marks), they can be simply sanded. Of course, you should not get carried away and remove too large a layer of metal.
Important Notice
If we are talking about alloy steel products that must undergo mandatory heat treatment, then the correction of defects in welds should be made only (!) After tempering in the temperature range from 450 to 650 ° C.
Fix other varieties
The easiest way to correct the influx and mechanical unevenness of the seam. In this case, the junction is simply cleared (which we already wrote about). We have already mentioned the corrections of undercutting above, but once again we note that it is more expedient to discard the part with such defects immediately, since its operation can be dangerous!
If there is a burn through (which is not so common), then the elimination of defects in the welds is quite simple: first, the surface is cleaned properly, and then it is boiled again. The same is true for craters.
Basic conditions of “cosmetic repair”
When eliminating defects, certain technological conditions must be observed. Firstly, you need to follow a simple rule: the length of the defective area should correspond to its width, plus 10-20 mm should be left “just in case”.
Important! The width of the weld after it is re-welded should not exceed twice its size before starting work. Do not be lazy before correcting flaws to prepare the surface well. Firstly, it will prevent pieces of slag from entering the metal. In addition, this simple measure will help speed up the work and improve the quality of its results.
It is very important to prepare a sample for the newly patched area. If you use angle grinder ("grinder"), then it is better to take a disk of the smallest diameter. The side faces of the sample should be made as even as possible, without burrs and other protruding parts, which during welding can turn into all the same slag.
If we are talking about compounds of aluminum, titanium, as well as alloys of these metals, then it is worth approaching the matter even more responsibly. Firstly, when eliminating defects in this case, it is allowed to use only (!) Mechanical methods, the use of arc welding is unacceptable. It is preferable to cut down the damaged area, clean and re-weld the seam.
Corrective defects note
Places with corrected - re-welded joints must again go through the quality control procedure. If the defect is preserved to one degree or another, you can try to eliminate it again. Important! The number of corrections depends on the steel grade and characteristics of the product itself, but under normal conditions, you can redo the work no more than two or three times, since otherwise a sharp decrease in the strength properties of the part is observed.
So we discussed the main types of defects in welds.