When welding, a black loose crust is formed over the place where the metal is joined (seam) - welding slag. It consists of molten flux or electrode coating, oxidized metal. In the process of melting and joining the materials to be welded and the electrode, it closes the melting bath. Thanks to the slag, the seam cools slowly, without oxygen, and does not oxidize. Once molten waste gets directly into the seam, it causes rapid destruction of the metal. You can prevent oxides from getting into the seam by changing the welding modes and position of the part.
Welding slag
What are slag inclusions
Welding slag is a glassy by-product material formed from the molten electrode coating or welding flux. The presence of slag inclusions is considered a very serious defect that can reduce the quality of the welded joint and the entire structure.
All slag inclusions arising on a metal surface during welding are divided into two types:
- macroscopic . Formed when the edges being welded are insufficiently cleaned or absent;
- microscopic . As a rule, they arise as a result of chemical reactions occurring during welding and crystallization of the metal.
Both types of inclusions have a negative impact on the mechanical characteristics of a metal product.
Residues and cinders of steel welding electrodes
Any production process is accompanied by the formation of a secondary by-product, which can be reused to obtain raw materials or must be disposed of. The only question that remains open is specifying the type of scrap according to the type of work.
See also the article: Electrodes - recycling and scrap metal.
Welding waste
The modern level of access to information allows you to navigate in any aspect of interest. There are two ways to determine what waste is generated from welding with welding wire. The first option involves a search query, the second is to visit the waste codifier 2022 welding on the corresponding web resource.
The information collected in the FKKO online catalog identifies the following waste from welding and soldering work:
- residues and cinders of steel welding electrodes;
- remains of steel wire;
- decomposition products of calcium carbide.
This is what welding slag looks like
Electrode stubs
Each of the points is worth considering in more detail, especially for the first category.
Welding slag – FKKO classification
The waste codifier assigns its own number to each by-product resulting from human production activities. In particular, for welding slag, the FKKO code has three variations. This:
- 9 19 100 02 20 4 – directly the slag formed during the electric welding process;
- 9 19 111 21 20 4 – slag waste with a predominance of silicon dioxide;
- 9 19 111 24 20 4 – welding slag, mainly containing titanium dioxide.
The last two options allow us to determine the main component of this type of welding waste. A somewhat different situation arises if we consider welding slag in general. The composition of this type of waste will be determined by the type of electrodes used.
Chemical composition of welding slag
The melting process characteristic of electric welding is always accompanied by oxidation of the metal. This explains the entry of predominantly oxides into the slag crust. The hazard class of this type of waste is IV, which requires issuing a waste passport for welding slag. The chemical composition of such waste, as mentioned earlier, contains oxides of the following elements:
- silicon;
- titanium;
- manganese;
- iron;
- calcium;
- sodium;
- aluminum;
- potassium.
However, in some cases, calcium fluoride keeps company with the oxides. This is explained by the inclusion of the compound directly in the composition of salt fluxes, as well as certain coatings of welding electrodes.
The presence of basic oxides inside the slag crust is similarly related. In particular, manganese plays the role of a deoxidizer, removing sulfur from the metal, while simultaneously improving the quality of the weld. Silicon also has a similar effect. It allows you to avoid gas pores inside the weld seam, which are formed due to carbon monoxide that has not had time to release.
Weld
Thus, slag acts as a full-fledged “participant” in the welding process, determining the structure and quality of the seam, and not just a production waste. Therefore, the physical and chemical properties of welding slags are an important characteristic.
Basic parameters of the slag layer
All chemical properties of the slag are directly related to the weld seam. They include: the ability to deoxidize the seam, alloy it, form fusible compounds from oxides, as well as dissolve them and sulfides. From the physical side, important criteria for slag are:
- Thermophysical parameters: heat capacity, threshold melting and softening temperatures.
- Viscosity.
- Specific gravity of liquid slag melt.
- Properties of the hardened crust, causing its easy separation from the metal being processed.
- Gas permeability.
The melting temperature allows us to divide slags into two groups: “short” with a range of 1100 – 1200 0C and “long” with large threshold values. Today, preference is given to short slags, therefore, in the production of electrodes, the composition of coatings and fluxes is mixed at lower melting temperatures.
Another important characteristic of slag is its viscosity. The mobility of individual layers of molten slag increases its chemical activity and, therefore, contributes to the refining of the weld metal. As a result, harmful impurities, in particular: manganese and iron sulfides, phosphorus anhydride, as well as oxygen and other gases; removed from the metal before the seam hardens.
Welding slag from a plasma cutter
The next criterion that attracts attention is waste density. Welding slag must have a low specific gravity in order to quickly rise to the surface of the bath. However, an excessively liquid slag melt is not able to evenly close the metal seam. Moreover, a higher density of welding slag (kg/m3 - unit of measurement) is especially important for vertical welding - ceiling, for example.
Steel cinders, other waste from the welding process
It is easy to determine the specific type of slag by knowing the composition of the electrodes: their coating, as well as the flux, if used. On the other hand, it is another type of arc welding waste. It is defined by FKKO as residues and cinders of steel welding electrodes.
This type of product is the main consumable material of the welding process. Despite the relatively small size of the waste: a part of the rod remains from the electrode, fixed in the holder fork; the total mass of cinders is quite large.
In some industries it amounts to hundreds of kilograms of scrap metal. Such waste is extremely rarely thrown away. Moreover, for the remains and cinders of steel welding electrodes, recycling is also extremely unprofitable.
It is more promising to use TX as a material for recycling.
Remains of welding electrodes
Indeed, most electrode cinders no longer have a coating and are ordinary metal wire of a certain diameter. In this case, the density of residues and cinders of steel welding electrodes is equivalent to a similar parameter of the metal. Thus, such waste can be melted down to produce new consumables.
Of course, the composition of welding electrode residues remains an important characteristic. Therefore, it is necessary to sort the cinders according to their type in order to obtain steel during the remelting process that is already alloyed with the required chemical composition and does not require further purification.
Sale of welding waste
The size of the cinders depends directly on the welder, more precisely on the place where he finished the work and is 50 - 100 mm. Thus, electrode waste remains a promising market for scrap metal. However, you should differentiate between web ads. Often, the phrase: let's buy the remains of electrodes means illiquid goods, and not a steel cinder, as such.
Disposal of welding waste, especially electrode residues, is becoming a regulated procedure. As a result, steel cinders are collected directly at the welding site and sorted according to the brand of the product. Next, the scrap metal is weighed and can be sold to a recycling site.
Alternatively, you can sell welding slag. The price for this type of waste will be significantly lower, and it is more difficult to find a buyer for it.
Source: https://xlom.ru/recycling-and-disposal/ostatki-i-ogarki-stalnyx-svarochnyx-elektrodov/
Reasons why slag inclusions form
Quite often, only specialists mastering welding technologies wonder why a lot of slag is formed at the connecting joints during welding. The appearance of such inclusions is due to various factors:
- the metal cools faster than usual and the slag simply does not have time to leave the weld pool;
- low quality electrodes used in welding. In this case, melting occurs unevenly and electrode particles enter the weld pool;
- at low values of metal deoxidation, a lot of slag is formed during welding. This is a process in which oxygen molecules are eliminated from an already soft metal. They worsen the mechanical properties of the metal and destroy its structure;
- poor-quality preparation and cleaning of welded edges from dirt, rust and oils;
- high values of surface tension of the slag prevent it from floating to the surface;
- the use of flux or electrodes made of refractory metals and with a high specific gravity;
- non-compliance with welding modes and technology, for example, incorrectly selected angle of inclination or inappropriate speed of electrode movement.
In order to weld without slag or with a minimum amount of it, it is advisable to seek help from experienced welders. If you want to weld yourself, then you should learn how to weld the simplest elements and only then proceed to more complex ones.
How to distinguish slag from metal
Welders, especially beginners, face various problems and questions when creating metal products by welding. For example, many find it difficult to distinguish slag from metal when welding.
In reality, it is not difficult to distinguish between metal and slag inclusions. To do this, you should pay attention to the following factors:
- color . Under the influence of high temperature, the metal melts during welding, acquiring a reddish tint. As the metal cools, the color of the reddened metal darkens. Slag behaves completely differently. It has a dark color directly during the welding process, and when it cools it becomes lighter;
- cooling rate . Metal, unlike slag, hardens much faster;
- the structure of the cooled metal alloy is more dense, and the slag inclusions form a loose crust;
- fluidity _ When melted, the metal is more liquid, which contributes to its greater mobility. During the welding process, it is easy to see how it boils. The slag is more viscous and does not heat up as well.
You can distinguish slag from metal during welding directly at the moment when it appears in the weld pool. If you watch how the metal melts, you can see the appearance of a bright light under the tip of the electrode, and behind its outlines you can see the clear contours of the butt joint and the weld pool itself. Metal is identified by its light shade, slag by its dark shade.
Basics of electric welding
Having understood the types and types of weld joints, you can move on to the basics of electric welding. If you plan to study on your own, then you will need to stock up on all the necessary materials. Since you will most likely experience a slight failure the first time you practice, it is better to stock up on a large amount of material.
It is also important to know some terms and symbols before starting work:
- A welding arc is a gas heated to incredibly high temperatures (about 5-7 thousand), so you need to handle welding with extreme care, otherwise you risk causing serious burns to yourself or others.
- Arc Welding – This type of welding is done using an electric arc that heats gas to incredibly high temperatures. Upon contact with the surface of a metal product, the metal begins to melt, resulting in the formation of a so-called “weld pool”. After the metal cools, a weld appears.
- Argon arc welding is practically the same as an arc type of welding, only argon must be used as the heated gas. This type of welding is very suitable for various reinforcement rods with a thickness of no more than 5 millimeters.
Why does slag need to be removed?
Slag inclusions mainly consist of oxides due to their porous structure and significantly reduce the strength properties of the metal. During operation of a welded structure, oxides from the slag are capable of entering into a chemical reaction with iron, which leads to its destruction. Therefore, immediately after cooling, when the slag turns black, it must be removed.
At the initial stage of welding, the slag with oxides formed above the pool protects the metal from rapid cooling. Since the temperature of the metal decreases much more slowly, when removing slag after welding, the seams are more even and uniform.
There are other reasons why it is recommended to remove the slag formed at the joints after welding parts:
- it is much easier to check the quality of the welded joint when there are no slag inclusions on it;
- paint and varnish coatings are often applied to finished products, and the presence of slag inclusions significantly worsens the appearance of structures;
- If it is necessary to perform a shock in several layers, you must first remove the slag and only then create the next layer.
Note! If the welding slag is not removed, it may be impossible to use the finished product due to defects present in the form of tops and non-metallic inclusions. This is especially important for structures that will be subject to high external loads during operation.
Typical welding errors and defects
When depositing the first bead along the root of a butt weld, metal penetration into the root may be insufficient due to insufficient welding current or excessive welding speed. The reason may also be that the electrode diameter is too large for the groove. If the welding current is too strong, the root of the weld may burn through.
Fusion defects
If the current is insufficient or the welding speed is too high, lack of fusion between the weld metal and the base metal may occur.
Failure of fusion can also occur if too small an electrode is used when welding over a large area of cold base metal. In this case, you should use a larger diameter electrode and heat the base metal.
Seam edge defects
Edge defects can occur due to excessive welding current. However, such defects can also occur with the correct current, if the arc is too long or if the electrode is not moved correctly.
When welding from bottom to top in a vertical position with an oscillating movement of the electrode, the latter must be pressed momentarily against each side of the roller so that the metal penetrates well into the seam and to avoid defects in the seam edge.
Edge defects can be indicators of a break in the weld.
Pores in the weld
Porosity in the weld can form due to moisture content in the electrode coating, especially when welding with base coated electrodes. In addition, the cause of pore formation may be wet or damp base metal of the workpiece being welded. Such a seam impairs the strength of the connection.
Thermal cracks
Thermal cracks can form during and immediately after the weld cooling period for two main reasons:
Due to inclusions in the base metal which tend to segregate and may form a layer in the middle of the weld. This layer prevents the crystals from merging. These substances primarily include carbon and sulfur.
In cases where thermal cracks are caused by such substances, switch to a basic coated electrode. If cracks appear during welding with an electrode with a base coating, then this metal is unweldable.
Stress through the weld can cause thermal cracks to appear, even if the base metal does not segregate in the weld. At a certain critical temperature interval, immediately after coagulation of the bead, the seam has very little ability to deform and, if the shrinkage of the metal is greater than the tensile strength of the seam, a crack will form. This can be avoided by clamping the part to be welded with a special clamping device, which limits the shrinkage of the metal.
Thermal cracks appear in the middle of the bead and are a straight crack on the surface.
Shrinkage cracks
Shrinkage cracks form when the ability of a weld to deform (toughness) is less than the actual shrinkage. Such cracks are usually transverse and are caused by significant longitudinal shrinkage. To avoid the formation of such cracks, it is best to use electrodes with a basic coating.
Hydrogen cracks
Cracks in weld metal are caused by hydrogen and can occur in any type of steel that has been hardened or is being hardened during welding.
High yield strength steel will contain a certain amount of strengthened structure, usually martensite.
The higher the melting point, the higher the risk of hydrogen cracks; they form in the base metal, which is directly adjacent to the melting zone, and, combined with welding voltage, form cracks in the weld metal.
For welding hardened steel, only dry electrodes with a basic coating can be used, since in this case very little hydrogen is released. Wet electrodes release a lot of hydrogen. Other sources of hydrogen include rust, oil, paint, or condensation along the weld groove. Heating the groove to, say, 50°C will significantly reduce the amount of hydrogen.
Slag inclusions in the weld
Slag consists of non-metallic particles from the electrode coating. After each roller, all slag must be carefully removed. To do this, use a chipping hammer and a wire brush. Slag particles trapped in the seam will seriously reduce the strength of the joint. Try to avoid burning out the recesses, as slag trapped there is difficult to remove.
When preparing the weld groove, ensure that it has sufficient clearance to ensure good fusion and facilitate slag removal. Remove scale and rust from the surface being welded and check whether you have selected the correct electrode for this welding position.
How to minimize slag inclusions when welding metals
Many novice craftsmen are concerned about the question “why is there a lot of slag when welding with an inverter?” Typically, such problems are observed during welding when the elements are in the lower position. In cases where the part is located on a slope, the slag drains much faster than the liquid metal mixture from the weld pool. Due to the fact that the slag did not have time to come out, it remains in the weld seam.
Also, slag formations appear when gaps are excessively large or when the current is insufficient in relation to the thickness of the metal. Much less often, problems with slag arise when creating vertical seams, while the seam remains on top and the slag flows down.
Some professional welders advise placing the workpiece on an incline and welding from top to bottom, while others suggest using slag-free electrodes with a dark coating for welding.
To prevent slag particles from getting inside the weld pool, the direction of the electrode should be coordinated. It must be positioned in such a way that when the electrode coating evaporates, the gas flow “blows” such a defect onto the outer surface of the connecting joint. You cannot leave slag in the weld pool. It should crystallize quickly, allowing you to remove it without much effort.
How to get rid of slag
To get rid of slag when welding, you can try to increase the arc. This will prevent slag formations from flowing under the weld pool.
By changing the polarity of the current when welding with an inverter and moving the electrode from minus to plus, you can prevent the accumulation of slag in the weld. You can’t stay in one place for too long; you need to move the arc quickly and evenly.
If the dimensions of the product allow, you can try to “drive” the slag back by changing the angle of inclination of the part. Inverter welding with reverse currents produces less slag. Such a device is best used by novice welders, as they prevent sticking of the electrode and greatly simplify the welding process.
