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 in the case of other production processes, marriage sometimes occurs. It refers to defects in the weld, which can drastically reduce the quality of the finished product, or even make its operation deadly.
Classification
By the way, how can they be separated? All weld defects are divided into three large groups:
- Outdoor.
- Domestic.
- Through.
External weld defects are often the most numerous category. It includes: excessively small dimensions, as well as a displacement of the seam line, various sags, “cuts”, shrinkage shells and craters that have not been sealed during the welding process, porosity or cracks. Uneven seam width also applies to this variety. It is believed thatExternal weld defects are classified as Least Concern.
Accordingly, the internal ones include: pores, numerous inclusions of slag, incompletely welded 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.
Main causes of welding defects
- They almost always appear when they try to use exclusively cheap and low-grade materials.
- The same can be said for low quality welding equipment. In addition, the frequency of defects often increases after poor-quality repairs of devices used by specialists.
- Of course, this happens all the time when the work technology is violated.
- Serious weld defects are not uncommon for inexperienced, low-skilled technicians.
It is easy to understand that the highest quality products are obtained when using fully automated equipment. Do not forget about the convenience of the workspace. So, large scaling of the seam and violations of its width are very common in cases where a welder (even an experienced one) works in an uncomfortable position.
Actually, it is not by chance that the requirements for the performance of welding work contain items that specifically stipulate the full-fledged equipment of the workplace, providing for its high-quality ergonomics.
Important note
Even novice welders are well aware that in order to ensure maximum strength, the seam should have a slight reinforcement with a height of about 1-2 mm. At the same time, the same welders often make a gross mistake when they make reinforcement 3-4 mm high. 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 likelihood of breakage.
Undercuts
As we have already said, defects in welds and joints are extremely dangerous. It is not difficult to imagine what would happen if they were present in a part intended for installation, for example, in the supporting structure of a railway bridge. They are especially dangerous in the case of welding parts made of alloy steel, which will be operated in conditions of constant temperature changes.
The most dangerous are undercuts, as they are a natural "accumulator" of stresses that will be concentrated in the weakest point of the seam. In addition, they significantly reduce its working section, which also has an extremely negative effect on the strength of the entire connection.
As a rule, these external defects of welds in most cases are not corrected. 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 happen?
The main reason is setting too much current. ATin combination with a long arc, this factor gives an almost one hundred percent probability of their occurrence. In addition, in some cases, undercuts occur when the heat source moves too quickly over the metal surface.
If the design is in any way important, even the smallest defects in welds and joints of this type are completely unacceptable. They are corrected by carefully welding a thin seam. If this is possible, it is still better to completely replace the part (note that the last remark applies to all defects).
Plots of uncooked metal
If such a site is located right in the thickness of the seam, it is extremely dangerous. Firstly, such a defect can be found only with the help of a flaw detector. Secondly, they are again accumulating places of natural stresses in the metal. In combination with the 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 localization) sharply 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 a slight porosity are several times more likely to fail under load even at the beginning of operation. Pores appear due to the fault of gases that simply do not have time to leave the layer of molten metal.
Like all kinds of weld defectsseams, they extremely often occur in the case of using low-quality raw electrodes. It often happens that porosity occurs due to some foreign impurities in the protective gases. As in the previous case, this type of defects can also be observed at an excessively high welding speed, when the integrity of the gas protective "bath" is tritely violated.
Slag inclusions
Slag inclusions greatly spoil the uniformity of the metal structure. The classic cause of formation is careless cleaning of the seam surface from rust and scale residues. 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 pose a danger, products with them can pass the Quality Control Department.
Note that if a tungsten electrode was used during welding, then particles of this metal can be detected in the parts. The degree of their danger is the same as in the previous case (i.e., these are permissible defects in welds).
Cracks
There are transverse and longitudinal, going 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 to be welded, the crack can either retain its original localization or spread to the entire length of the workpiece in a very short time.
Unsurprisingly, these are the most dangerous weld defects. GOST in most cases requiresimmediate rejection of such parts, regardless of its purpose (with the exception of very unimportant products).
Uneven seams
This is the name of a gross discrepancy between the geometric parameters of the joints and the characteristics required in the regulatory documents. Simply put, if welding goes “snake”, obliquely, etc., we are talking about this type of defect.
Most often they appear during the work of inexperienced welders, as well as with significant power surges, poor-quality equipment and banal haste. This defect is dangerous because it is often combined with undercooking, which is already much more dangerous. If the deviation from the center line of the connection is insignificant and does not cause a decrease in the strength of the product, the part may be allowed for service.
In this case, you should always remember one simple thing: the smaller the transition angle from the base metal to the deposited layer, the worse the mechanical strength of the welded product becomes. Of course, in the manufacture of some household structures (the frame of a greenhouse, for example), under conditions of insufficient stress, it is simply unrealistic to do without uneven seams. However, in this case, they are not particularly dangerous.
Basic troubleshooting, fixing defects
Let's say the following right away: in most cases, it makes no sense to discuss ways to eliminate defects in welds, since under conditions of a more or less strict quality control department, all products with some kind of flaws are simply rejected. But sometimes it really doesso 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 unsuccessful connection is carefully cleaned, and then the attempt is repeated. If there are minor external defects in the welds (unevenness of the connection, shallow pockmarks), then they can simply be sanded. Of course, you should not get carried away and remove too much metal.
Important note
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 ° С.
Correction of other varieties
The easiest way to correct the sagging and mechanical unevenness of the seam. In this case, the junction is simply cleaned (which we have already written about). We have already talked about the correction of undercuts above, but we note again that with such defects it is more advisable to immediately discard the part, since its operation can be dangerous!
If there is a burn (which is not so common), then the elimination of defects in welds is quite simple: first, the surface is properly cleaned, and then it is re-boiled. Approximately the same is done with craters.
Basic conditions for "redecoration"
When eliminating defects, certain technological conditions must be observed. First, you needfollow 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 its re-welding should not exceed twice its size before the start of work. Do not be lazy before correcting flaws to prepare the surface well. Firstly, it will prevent pieces of slag from getting into 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 planted area. If you are using an angle grinder ("grinder"), then it is better to take a disk of the smallest diameter. The side edges of the sample should be made as even as possible, without burrs and other protruding parts, which during the welding process can turn into the same slag.
If we are talking about compounds of aluminum, titanium, as well as alloys of these metals, then the matter should be approached even more responsibly. Firstly, when eliminating defects in this case, it is allowed to use only (!) Mechanical methods, while the use of arc welding is unacceptable. It is best to cut down the damaged area, clean and re-weld the seam.
Note on corrected defects
Places with corrected - re-welded joints must again undergo the OTC procedure. If the defect persists to some extent, you can try to eliminate it again. Important! The number of corrections depends on the steel grade and the characteristics of the product itself, but under normal conditionsyou can redo the work no more than two or three times, otherwise there is a sharp decrease in the strength qualities of the part.
Here we discussed the main types of weld defects.
