The connection of metal parts by welding is a complex physical and chemical process, where the metal itself, atmospheric gases, and electrode combustion products interact at high temperatures. Each of the components contributes to the overall outcome of the welding process. Some of the impacts lead to poor quality welding, so-called weld defects are formed.
They can be formed as a result of various situations, for example, excessively high or too low welding current, high humidity, the presence of contamination in the welding zone. There is a certain classification of defects and their causes, a general overview of which will be given in this article. You will also learn how to eliminate defects and in what cases it is possible.
Most weld defects occur when the welding technology is violated. Only in some cases, defects are caused by unforeseen situations. In fusion welding, the following are of great importance:
- preliminary preparation and assembly of the connection;
- heat treatment mode;
- correct selection of welded materials;
- the quality of the device and consumables.
Defects are classified according to various criteria - size, shape, location in the seam line, degree of danger regarding the possibility of destruction of the joint. Normative defects are classified according to the international document - “Classification, designations and definitions. Defects in joining metals in fusion welding. All requirements of the collection are collected in GOST 30242-97.
According to this document, all defects in welded joints are divided into 6 main groups:
- cracks;
- violation of the shape of the seam;
- cavities, craters and shells;
- unmelted edges and unfinished areas;
- the presence of solid inclusions and inclusions;
- other defects not included in the first 5 groups.
Each of the defects has its own digital designation, which is affixed in the area of the weld during inspection. According to the international classification, defects in welds can also be indicated by letters. But, in any case, the definition of defects according to the Russian and international classifications coincide in their main characteristics.
cavities
Formed by the accumulation of gases in molten metal. Cavities can be both spherical and shapeless. But, in any case, they lead to a decrease in the strength of the connection. The cavities are arranged randomly, in a chain, in clusters, evenly. They are marked with the numbers 2012, 2013, etc. Open cavities formed by expanding gas that has left the seam and escaped into the atmosphere are called fistulas.
Shells and craters are formed in the zone of cavities and fistulas, when the metal has not yet cooled down, and the internal pressure of the gases has decreased below the critical one. Cooling down, the metal shrinks and, as it were, falls into the seam.
hard inclusions
The total index of solid inclusions is 300. Such defects in welded joints are particles of slag, metals or non-metals that remain in the weld zone and are fused into the metal, but do not form one whole with it. Such inclusions can be fluxes, particles of tungsten, copper, oxides, for one reason or another, found themselves in the seam.
non-fusion
Welded seam areas in which zones are observed between the metal and the seam, inside the seam or along the side and root parts, where the metal has not melted enough and has not joined into an integral structure. This phenomenon occurs as a result of too fast movement of the electrode or insufficient welding current. Defects are marked with index 400.
A type of non-fusion is lack of penetration - areas of the seam where the metal has not melted enough to penetrate the root of the seam and fill the entire gap between the parts.
Seam irregularities
- undercuts;
- linear and angular displacements;
- sagging;
- burns;
- uneven width;
- profile violation.
In flaw detection, such violations are marked with numbers from 500.
Elimination of defects
In many cases, violations of the integrity of the seam revealed during flaw detection can be eliminated. External defects, that is, those that can be seen visually, without the use of special equipment. Internal - visible when using a fluoroscopic apparatus or machining, in which part of the deposited metal is removed. Also defects are shown at ultrasonic research.
Elimination of defects is possible in most cases, except for burnout, when the structure of the metal is broken. Often, in order to eliminate the consequences of an unsuccessful welding process, part of the seam is removed mechanically, and welding is performed again.
Almost all defects can be eliminated if it is technically possible to solve and the process is justified economically. In some cases, it is better to reject the part and re-melt it than spend time fixing the weld.
Welding defects on alloyed steels are eliminated only after the tempering of parts - a special heat treatment process at temperatures of 450-650 ° C. Without this stage of preparation, the elimination of defects can lead to even greater violations of the integrity of the connection and the appearance of internal stresses in the metal.
Defectoscopy
This term defines a sequence of actions aimed at detecting defects in welded joints, which can lead to leakage, destruction of structures or their partial deformations. Flaw detection of welded seams is carried out by various methods, which in no case violate the integrity of the seam and the metal itself.
The initial stage is visual and measuring control. With it, almost all external and a number of internal defects are detected - violation of geometry, lack of penetration, burns, cracks, sagging. Often, in order to detail visual control, it is necessary to treat the surface of the seam with reagents - alcohol or nitric acid (small cracks and pores become visible).
External visual inspection also includes the use of optical means - magnifiers, microscopes, direct and side lighting lamps. Also in this process, measuring tools are used - calipers, rulers, probes, templates. With their help, the geometric dimensions of defects are determined and the possibility of their classification into acceptable and unacceptable (depending on the requirements for a particular product).
Capillary control is carried out with the help of special liquids, penetrants. Penetrating into the pores and cracks, the liquid colors them and makes them clearly visible. Surface defects are stained and are much easier to spot. Color flaw detection, as a rule, allows you to see most external defects, but it is impossible to show internal ones in this way.
For in-depth research, magnetic flaw detection, ultrasonic and X-ray diffraction are used. These studies require the use of complex instruments, but in most cases this is justified, especially at complex and critical objects.
As a rule, visual and measuring control is carried out at the very beginning. All other diagnostic methods are applied only after the correction of welding defects found during inspection has been completed, and this method does not bring more results.
Each welder has in his arsenal his own methods for determining defects and knows how to eliminate them. If you have your own experience in this area - share it on the pages of our site. Practical methods of working with welding is one of the most interesting topics for our readers.
They negatively affect the quality and durability of the manufactured metal structure, provoking its deformation and destruction over time.
Therefore, welding must be performed in such a way that the created joints are of the highest quality and accurate, without flaws.
If it didn’t work out skillfully, it’s worth asking if there are reliable ways to eliminate welding defects and study them.
Weld defects are flaws on the surface or inside the seam created by the use of welding equipment.
They can have a different degree of severity, shape, size and lead to a decrease in useful life, can affect its operational parameters, and therefore are highly undesirable in operation.
External defects in welds.
The appearance of welding flaws can be explained by various reasons:
- The created joints may be of low quality if the master does not have extensive experience in welding operations: violates the technology of electric arc, argon, beam welding, neglects the preparatory process, heat treatment of nodes, confuses the assembly scheme of parts, selects the wrong mode of operation of the welding machine during laser welding, etc. P.
- Also, poor weld performance may be the result of the use of handicraft or faulty equipment for manual arc welding, low-quality metal, and cheap consumables.
All seam defects are called differently and are conditionally divided into several groups, each of which has a certain type and features:
- outdoor;
- internal;
- through.
The features of the deficiency will determine the most appropriate way to correct it. To prevent such problems in the future, it is important for the welder to work on the mistakes and understand what led to such sad results in his work.
Important! Not every problematic section of a seam is considered an unacceptable flaw. Based on the list of requirements for welded joints and metal structures in general, there are permissible defects.
These are flaws that are not able to affect the quality of the welding joint. But their number in any case should be minimal in order to maximize the service life of the product.
Species diversity of defects
An inexperienced welder in the process of creating semi-automatic welded joints may encounter various types of welding defects. They differ in external characteristics and appear due to a violation of the welding technology: manual arc welding, automatic, etc.
Causes of defects in welds.
It is important to carefully study such problems, which will prevent damage to the parts to be welded during manual arc welding and the implementation of other technologies for creating joints in metal structures in the future.
- external: cracks, undercuts, sagging, craters, scale, welded shells;
- internal: porous structure, insufficient penetration, foreign inclusions;
- through: cracks, burns.
External flaws have this name, as they are on the front side of the connection and are visible to the eye. To detect them, it is enough to conduct a visual inspection of the part. Internal defects are located inside the welding joint, so they are not immediately noticeable.
The presence of this problem can be determined using flaw detection of welds, including ultrasonic, mechanical and X-ray processing. Cross-cutting flaws are the most catastrophic, since their elimination is not always carried out 100%.
External defects
If the welding technology is violated and consumables of poor quality are used, the following welding defects can be obtained: sagging, undercuts, unwelded craters, surface pores, burns, cracks, etc.
The sags are the result of the flowing of the molten metal of the weld wire onto the unmelted base metal of the structure or a previously made bead.
Such shortcomings can be local in nature and manifest themselves in separate zones, or they can acquire an elongated shape and occupy a decent area on the metal product.
The main reasons for the appearance of influxes are as follows:
- the welder incorrectly set the current strength with a long arc and made a mistake with the selection of the speed of the equipment;
- an excessively large slope of the plane on which the weld was applied was chosen;
- the electrode was incorrectly led, or it changed its original position when making circumferential welds under the flux;
- the welder had insufficient experience or worked in an uncomfortable spatial position: vertical or horizontal.
Undercuts are depressions on the surface of the main, running along the edges of the weld. The depth of the cut can vary between 0.1-1 mm.
The reasons for the formation of such defects in welded joints are:
- excessively high current;
- arc voltage above the norm;
- uncomfortable posture of the welder in spatial terms;
- carelessly welded.
The presence of such errors is dangerous, since undercuts can reduce the working thickness of the metal at the junctions of metal parts, provoke the appearance of local stress concentration from working loads and cause deformation of welds over time.
Names of weld defects.
We also note that undercuts of butt and fillet welds located across the forces acting on them can cause a sharp decrease in the vibration strength of the joints.
Crater - a dent that appears in the event of a sharp break in the arc at the end of welding. Very often this problem occurs when creating short seams.
The size of the crater is determined by the value of the welding current:
- with a manual welding method, its diameter is 3-20 mm;
- during automatic welding, the crater takes the form of an elongated groove.
Important! If such a defect is not welded, the strength of the welded joint and the cross section of the seam will decrease, and this will entail the appearance of foci of cracking.
Burn-throughs are the penetration of the base or deposited metal, on which through holes are sometimes formed.
The causes of these defects are:
- insufficient blunting of the edges, a large gap between them;
- overestimated welding current or burner power against the background of low welding speeds;
- insufficient compression of the flux pad, copper lining during automatic welding;
- during excessively long welding, insufficient compressive force, in the presence of contamination on the surfaces of welded parts, wire during spot and seam contact welding.
Especially often, burns can be observed when welding thin metal, when organizing the first pass of a multilayer weld. Such defects can be eliminated, but even after that, the connection does not acquire satisfactory characteristics and aesthetic appearance.
Therefore, it is worth initially making every effort to prevent the appearance of such defects on the surface of the weld.
On a note! Separately, it is worth noting such a defect as a crack in the welded joint. We will answer what is called a crack: a violation of the plane of the metal, provoked by cooling, by the action of loads. It can apply to both external and internal welding defects.
It is possible to detect external defects on welds using visual-measuring control, capillary flaw detection, as well as other non-destructive testing methods: X-ray, ultrasound.
Internal defects
Varieties of welded joints.
Internal welding defects include:
- Cold cracks.
They appear only after cooling and solidification of the welded joint due to its inconsistency with the existing loads. - Hot cracks.
They appear at the moment when the metal of the welded joint is in a state between the melting and solidification temperatures due to the use of a low-quality additive, incorrect crater filling technology, due to an abrupt stop of the welding process, due to burns during welding. Such defects can have several types of location: along and across the connection of metal parts. - Pores.
They can occur with any welding technology due to the presence of contamination on the surface of the parts to be joined, poor protection of the weld pool with gas flow, oil, paint, welding of incompatible alloys, rust and metal oxidation. The pores vary in size and often have a chaotic distribution over the weld: they are located both inside the joint and on its surface.
The elimination of identified defects in welds that are invisible to the eye must be carried out after determining the main causes that provoked the appearance of the defect, regardless of the fact whether the technology was carried out or another type of welding operations was used.
This will allow you to choose the most effective method of dealing with the problem and prevent such errors in the future.
through
Through defects are holes in a metal part, which are formed due to improper welding. The master selects the wrong operating mode of the welding machine and burns the metal through.
Also, such problems arise when welding stops suddenly, when operations are carried out in drafts, and when working with thin metal.
Through resistance welding defects are:
- undercut when welding;
- crack;
- burn through .
How to detect weld defects?
You can detect a defect in a welded joint in the following ways:
- visual inspection is carried out using a magnifying device and allows you to detect even tiny defects in spot welding;
- flaw detection of welds - a method for diagnosing the quality of a weld, based on the tendency of a special material to change its color at the moment when it comes into contact with a fluid material, for example, kerosene;
- method - performing measurements of the distortion of magnetic waves;
- Ultrasound - ultrasonic testing involves the use of special ultrasonic flaw detectors that can measure the degree of reflection of sound waves;
- The radiation method is carried out by X-raying the weld seam, obtaining a picture that describes all the details of the problem area.
Beads on the inside and outside of the weld.
Color flaw detection and ultrasonic testing of welded joints are considered the most effective methods for detecting defective welded joints, but it is almost impossible to implement them in domestic conditions.
Elimination of defects in welds
Almost all defects in welded joints, with the exception of the most insignificant in size, require elimination.
If this is not done, the operational parameters of the seams and the metal structure itself will deteriorate significantly: the presence of welding defects can lead to deformation of the metal, its rapid destruction under mechanical pressure.
Varieties of shortcomings will determine the methods of dealing with them.
Types of defects in welded joints.
Therefore, we characterize the most common welding defects and how to eliminate them:
- Deviations of the parameters of the seams from the norms in width, height, leg, constriction of joints.
They are identified by carrying out an external examination of the seams, analyzing their sizes using templates. You can eliminate the defect by cutting off excess metal, cleaning the seams, and welding the bottlenecks of the joint. - Undercuts are a recess along the line of fusion of the working and base metal.
An external examination of the seams will help to find the problem, and it will be possible to eliminate it if you perform a high-quality cleaning of the undercut and welding the seam itself. - The time of the weld is a round-shaped cavity with gas.
Sometimes several pores are connected in a chain. The method of detection and methods for eliminating defects of this kind: visual inspection, inspection of a fracture in the seam. - Fistulas in the form of a funnel-shaped depression are detected during external examination, removed by cutting, gouging with further cleaning and welding.
- Lack of penetration appears due to insufficient melting of the edges of the welded joint.
It is possible to eliminate the defect after visual detection, finding out the cause of the formation of lack of penetration. The method of control during laser welding will prevent the use of a defective part, and lack of penetration is eliminated by cutting and pulling out, cleaning and welding. - The sags on the welds have the form of leakage of the weld metal on the surface of the working metal.
They are effectively identified and eliminated by conducting an external examination, hemming and removing the influx, and unfinished areas will need to be welded. - Slag inclusions are defects in the form of slag inclusions.
Such a problem can be identified and eliminated by visual analysis of the part, X-ray and gamma control, control by an ultrasonic unit, and magnetographic equipment. Slag from the defective area will need to be removed, cleaned, welded.
Summing up
Weld defects occur when the welding technology is violated and endanger the stability of the weld and the functionality of the entire metal structure.
For this reason, it is important for a self-respecting craftsman to learn the main defects of the seams - the appearance of pores, sagging, burnouts, etc. - and the reasons for their formation during the welding process.
This will allow you to choose the most effective solution to eliminate the shortcomings of joints in spot welding, the use of a laser, an electric arc, argon, etc.
The further operation of the structure depends on the quality of welding, therefore defects in welded joints are not allowed. Many factors contribute to the occurrence of defects, for example:
- violation of work technology;
- negligence;
- low qualification of the welder;
- use of faulty equipment;
- performance of work without proper preparation, in adverse weather conditions.
Permissible and non-permissible values of defects in welds are distinguished from the degree of reduction in the technical parameters of the product in terms of strength. In case of permissible violations, welding defects are not corrected; in the second case, their elimination is necessary. The suitability of the product for operation, the determination of the compliance of the seam with the standards is carried out in accordance with GOST 30242-97.
Types of welding defects
The correct welding seam implies the uniformity of the composition of the base and filler material, the formation of its desired shape, the absence of cracks, lack of fusion, influx, the presence of foreign substances. There are the following types of defects in welded joints:
- outdoor;
- internal;
- through.
What are external defects?
External defects in welds and joints are detected visually. Violations of the welding mode, non-observance of the accuracy of the direction and movement of the electrode due to the haste or irresponsibility of the welder, fluctuations in the voltage of electricity during welding lead to the formation of a seam of the wrong size and shape.
The characteristic features of the external appearance of the marriage are: the difference in the width of the longitudinal seams and fillet legs, the sharpness of the transition from the base steel to the deposited.
With the manual method of welding, violations occur due to errors in the preparation of edges, neglect of the welding mode and speed, and the lack of timely control measurement. Defects in welds and the reasons for their formation during automatic or semi-automatic welding work lie in excessive power surges, errors in the mode. There are the following external types of defects in welds:
cracks seams are hot and cold, longitudinal, transverse, radial. The first of them take place when applying high temperatures from 1100 to 1300°C, affecting the properties of the metal in terms of reducing plasticity, the appearance of tensile strains. This type of weld defects is accompanied by an increase in the composition of undesirable chemical elements in the steel. Cold cracks can appear at temperatures up to 120°C during cooling, later - under the influence of loads during operation. The reason for this type of marriage may be a decrease in the strength of the steel due to welding stresses or the presence of dissolved hydrogen atoms.
Crack in the weld
undercut characterized by the presence of a recess between the alloyed and base steel. This type of weld defect is more common than others. An increase in arc voltage during fast welding leads to a thinning of the steel thickness, a decrease in strength. A deeper penetration of one of the edges causes the liquid steel to run off to the other surface, due to which the welding groove does not have time to fill. In this case, welding defects and ways to eliminate them are determined visually. Deficiencies in the work are eliminated by stripping, followed by overcooking.
Undercut welding seam
influx occurs when the fused metal flows onto the surface of the base steel without forming a homogeneous mass with it. This type of marriage is characterized by the formation of a seam outline without gaining sufficient strength, which affects the overall endurance of the metal. The reason for the marriage is the use of low arc voltage, the presence of scale on the edges of parts, the leakage of melted steel when welding horizontal seams, when the surface of the welded structures is vertical. Excessively slow welding also leads to the formation of sagging, due to the appearance of excess molten metal.
craters appear due to a sharp separation of the arc. They have the form of depressions, where lack of penetration, friability of the material with a shrinkage property, leading to cracks, can form. Craters occur due to welder errors. Since a crater is usually the cause of cracking, which is why it is not allowed, if it is found, it should be cleaned, then re-welded.
The resulting crater in the weld
Fistulas have the form of funnels with a recess on the body of the seam. They are formed from shells or pores of sufficiently large sizes, with insufficient preparation of the surface of the welding elements and filler wire. This type of defect can also be seen during visual inspection and must be immediately eliminated.
Typical funnel fistula
Internal defects in welds
Internal flaws in welding cannot be visually detected. Usually they appear due to a violation of the welding process and inadequate quality of the material. With internal defects, cracks may also appear, but they are not visible or small, but may open up over time. Hidden cracks are dangerous because they are difficult to detect, and the stress can increase gradually, and can lead to transient destruction of the structure, therefore they are extremely dangerous. The cause of marriage can be huge stresses, rapid cooling when using carbon and alloy steels. The most common types of this kind of defects are as follows:
Lack of fusion occurs when there is insufficient fusion of the welded parts of the seam in places. The reason lies in the improper preparation of the edges associated with the presence of rust, scale, lack of clearance and blunting of the edges. In addition, haste and fast welding speed, low current or displacement of the electrode from the axis of the seam can also lead to lack of penetration of the seam. Due to a decrease in the cross section of the weld, a stress concentration appears, which is reflected in a decrease in the strength of the joints, which is up to 40% under vibration loads, and large areas of lack of penetration - up to 70%. If the permissible values \u200b\u200bare exceeded, cleaning of the seam and its overcooking is required.
Lack of penetration and non-filling
pores are the free spaces of the weld, filled with gas, mainly hydrogen. The reason for this type of marriage is the presence of foreign impurities in the materials being welded, dampness, insufficient protection of the weld pool. If the allowable pore concentrations are exceeded, the weld seam must be overcooked.
Pores in the weld
In addition, slag, tungsten, oxide inclusions can also be noted, which also occur when the welding process technology is violated.
Through defects
This type of marriage implies the presence of pores passing through the entire thickness of the weld, which are also detected visually. Predominantly occur in the conduct of through welding. With this type of defect, burns and cracks may appear.
Burn-through occurs from the use of high current and slow welding. The reason is the excessive openness of the gap at the edges, the loose fit of the linings, as a result of which the weld pool leaks. Checking the seam for the presence of marriage is carried out visually, if it exceeds the permissible norm, cleaning and overcooking of welding is required.
Methods for detecting, controlling and eliminating defects
To detect weld defects, the following methods are used:
- visual inspection - carried out using magnifying devices;
- color flaw detection - based on a change in the color of a special material in contact with a fluid material, for example, kerosene;
- magnetic method - measurement of the distortion of magnetic waves;
- ultrasonic method - the use of ultrasonic flaw detectors that measure the reflection of sound waves;
- radiation method - X-ray transmission of welds and obtaining a picture with all the details of the defect.
To ensure the quality of the weld, marking and branding are carried out. Each welder puts his stamp on his welding area.
If a marriage is found, it is necessary to eliminate welding defects. For this, the following types of work are used:
- welding - used to eliminate large cracks, having previously prepared the crack by drilling and cleaning with a chisel or abrasive tool;
- internal small cracks, lack of penetration and inclusions are subject to complete cleaning or cutting with re-welding;
- incomplete seams and undercuts of the weld are eliminated by surfacing or welding in thin layers;
- removal of sagging is carried out mechanically using an abrasive tool;
- overheating of the metal is eliminated by heat treatment.
In the production of welded parts and structures, various types of defects are formed, which can be conditionally classified into:
- – defects in preparation and assembly;
- - defects in the shape of the seam;
- – external and internal defects.
Defects in preparation and assembly. Typical types of defects in fusion welding are:
- wrong angle of the bevel of the edges of the seam with V-, X- and U-shaped cutting;
- too large or small blunting along the length of the joined edges;
- variability of the gap between the edges along the length of the joined elements;
- mismatch of joined planes;
- too large a gap between the edges of the parts to be welded;
- delamination and edge contamination.
These defects may occur due to:
- malfunctions of the machine equipment on which the workpieces were processed;
- poor quality of raw materials;
- errors in drawings;
- low-skilled locksmiths and assemblers.
Seam defects. The shape and dimensions of welds are usually specified by specifications, indicated on drawings and regulated by standards. The structural elements of butt welds (Fig. 1) are their width e, convex height q and welding q 1 , fillet welds of tee and lap joints without beveled edges (Fig. 2) - leg TO and thickness A. The dimensions of the seams depend on the thickness s welded metal and operating conditions of structures.
Rice. 1. The main structural elements of welds: a - without preparation of edges of small thicknesses (b - gap width); b - with V-shaped cutting
Rice. Fig. 2. The main structural elements of the rollers: a - normal; b - convex; c - concave O
When making welded joints by any methods of fusion welding, the seams can have uneven width and height, bumps, saddles, uneven height of legs in fillet welds (Fig. 3 .
Rice. 3. Defects in the shape of the seams: a - uneven width of the seam in manual welding; b - the same, with automatic welding; c - uneven bulge - tubercles and saddles
Uneven seam width is formed when the electrode moves incorrectly, depending on the visual-motor coordination (VMC) of the welder, as well as as a result of deviations from the specified edge clearance during assembly. In automatic welding, this defect is caused by a violation of the wire feed speed, welding speed, etc.
Convexity irregularity along the length of the seam local hillocks And saddles are obtained by manual welding due to insufficient qualifications of the welder and are primarily due to the peculiarity of the welder's ZDK; improper methods of welding tacks; unsatisfactory quality of electrodes.
In automatic welding, these defects are rare and are the result of malfunctions in the mechanism of the machine that regulates the welding speed.
The listed defects in the shape of the seam reduce the strength of the joint and indirectly affect the possibility of the formation of internal defects.
external defects. They include
- influxes,
- undercuts,
- unsealed craters,
- burns.
influxes are formed as a result of the flow of the molten metal of the electrode onto the unmelted base metal or a previously made bead without fusion with it (Fig. 4). Influxes can be local, in the form of separate zones, as well as significant in length.
Floods occur due to:
- excessive current with a long arc and high welding speed;
- uncomfortable spatial position (vertical, ceiling);
- increased inclination of the plane on which the weld is applied;
- improper electrode guidance or incorrect displacement of the electrode wire when welding girth welds under arc welding;
- performing vertical seams upwards and insufficient experience of the welder.
Rice. 4. Sagging in the seams: a - horizontal; b - lap joint; c - tee connection; d - butt joint or when welding beads
Undercuts are recesses (grooves) in the base metal, running along the edges of the seam (Fig. 5). The depth of the cut can vary from tenths of a millimeter to several millimeters. The reasons for the formation of this defect are:
- significant current strength and increased arc voltage;
- not convenient spatial position during welding;
- negligence of the welder.
Rice. 5. Undercuts: a - in the butt weld; b - in a horizontal seam located on a vertical plane; c - in the fillet weld of the tee joint
Undercuts in the seam reduce the working thickness of the metal, cause local stress concentration from working loads and can be the cause of the destruction of the seams during operation. Undercuts in butt and fillet welds, located across the forces acting on them, lead to a sharp decrease in vibration strength; even sufficiently large undercuts, passing along the acting force, affect the strength to a much lesser extent than undercuts located across.
Crater- a depression formed at the end of the seam when welding is suddenly stopped. Especially often craters occur when making short seams. The dimensions of the crater depend on the magnitude of the welding current. With manual welding, its diameter ranges from 3 to 20 mm, with automatic welding it has an elongated shape in the form of a groove. Unsealed craters reduce the strength of the welded joint as they concentrate stresses. Under vibration load, the decrease in the strength of a joint from mild steel reaches 25%, and from low-alloyed steel - 50% in the presence of a crater in the seam.
- defects in the form of a through hole in the weld, formed when the weld pool flows out; welding metal of small thickness and weld root in multilayer welds, as well as when welding vertical welds from bottom to top (Fig. 6). The causes of burn-through are: excessively high heat input of the arc, uneven welding speed, stop of the power source, increased gap between the edges of the elements to be welded. In all cases, the hole that occurs during burns, although it is closed, however, the seam in that place is unsatisfactory in appearance and quality.
Rice. 6. Burns
Arson occur as a result of excitation of the arc ("electrode strike") at the edge of the edge. This defect serves as a source of stress concentration, it must be removed mechanically.
internal defects. These include pores, slag inclusions, lack of penetration, non-fusion and cracks.
pores(Fig. 7) in the form of a round-shaped cavity filled with gas, they are formed due to: contamination of the edges of the metal being welded, the use of wet flux or damp electrodes, insufficient seam protection when welding in carbon dioxide, increased speed and overestimated arc length. When welding in carbon dioxide, and in some cases submerged arc at high currents, through pores are formed - the so-called Withvishchi.
Rice. Fig. 7. The nature of porosity in the deposited weld metal: a - uniform porosity; b - accumulations of pores; c - chains of pores
The dimensions of internal pores range from 0.1 to 2...3 mm in diameter, and sometimes even more. The pores emerging on the surface of the seam may be larger. Fistulas during submerged arc welding and in carbon dioxide at high currents can have a diameter of up to 6 ... 8 mm. The length of the so-called "worm-like" pores is up to several centimeters.
Uniform porosity (Fig. , a) usually occurs with constantly acting factors: contamination of the base metal on the surfaces to be welded (rust, oil, etc.), variable thickness of the electrode coating, etc. The accumulation of pores (Fig. 7, b) is observed with local contamination or deviations from the established welding mode, also with a violation of the continuity of the electrode coating, welding at the beginning of the weld, arc breakage, or random changes in its length.
Chains of pores (Fig. 7, c) are formed under conditions when gaseous products penetrate the metal along the axis of the weld along its entire length (when welding over rust, air leaks through the gap between the edges, welding the weld root with poor-quality electrodes). Single pores arise due to the action of random factors (mains voltage fluctuations, etc.). Pores are most likely to occur when welding aluminum and titanium alloys, and to a lesser extent, when welding steels.
Slag vklstudies in the weld metal, these are small volumes filled with non-metallic substances (slags, oxides). The probability of formation of slag inclusions is largely determined by the brand of the welding electrode. When welding with electrodes with a thin coating, the probability of the formation of slag inclusions is very high. When welding with high-quality electrodes that produce a lot of slag, the molten metal stays in a liquid state for a longer time and non-metallic inclusions have time to float to its surface, as a result of which the seam is not significantly clogged with slag inclusions.
Slag inclusions can be divided into macro- and microscopic ones. Macroscopic inclusions have spherical and oblong shapes in the form of elongated "tails". These inclusions are formed in the weld due to poor cleaning of the welded edges from scale and other contaminants, and most often due to internal undercuts and poor cleaning of the surface of the first layers of multilayer welds from slag before welding subsequent ( Fig. 8).
Microscopic slag inclusions appear as a result of the formation during the melting process of some chemical compounds that remain in the seam during crystallization.
Rice. 8. Slag inclusions along the undercut edge in a multilayer seam
oxide films can occur in all types of welding. The reasons for their formation are the same as for slag inclusions: contamination of the surfaces of the welded elements; poor cleaning of the surface of the weld layers from slag in multilayer welding; low quality electrode coating or flux; insufficiently good qualification of the welder, etc.
Hdilutions- this is a defect in the form of local non-fusion in a welded joint due to incomplete melting of the edges or surfaces of previously made beads. Lack of penetration (Fig. 9, a) in the form of non-fusion of the base metal with the deposited metal is a thin layer of oxides, and in some cases, a coarse slag layer between the base and deposited metal. The reasons for the formation of such lack of penetration are:
- - poor cleaning of the edges of the welded parts from scale, rust, paint, slag, oil and other contaminants;
- - wandering or deflection of the arc under the influence of magnetic fields, especially when cooking with direct current;
- - electrodes made of low-melting material (when a seam is made with such electrodes, liquid metal flows onto unmelted edges to be welded);
- - excessive welding speed, at which the welded edges do not have time to melt;
- - a significant displacement of the electrode towards one of the welded edges, while the molten metal flows onto the second unmelted edge, covering the lack of penetration;
- - unsatisfactory quality of the base metal, welding wire, electrodes, fluxes, etc.;
- - poor performance of welding equipment - fluctuations in the strength of the welding current and arc voltage during the welding process;
- - low qualification of the welder.
Rice. 9. Lack of penetration: a - along the edge with the base metal; b - at the root of the seam; c – between separate layers; d - between rollers
The reasons for the formation of lack of penetration at the root of the seam (Fig. 9, b), in addition to those indicated above, may be: insufficient bevel angle; a large amount of their blunting; a small gap between the edges of the parts to be welded; a large cross section of the electrode or filler wire placed in the groove of the seam, which greatly complicates the melting of the base metal. Lack of penetration between individual layers (Fig. 9, c, d) occurs for the following reasons: due to incompletely removed slag formed during the application of the previous roller, which is possible due to the difficulty of its removal or the negligence of the welder; insufficient thermal power (low current, excessively long or short arc).
cracks- partial local destruction of the welded joint in the form of a gap (Fig. 10). The following factors contribute to the formation of cracks:
- – welding of alloyed steels in rigidly fixed structures;
- – high cooling rate when welding carbon steels prone to hardening in air;
- - the use of high-carbon electrode wire for automatic welding of structural alloy steel;
- - the use of increased welding current densities when applying the first layer of a multilayer weld of thick-walled vessels and products;
- – insufficient clearance between the edges of parts in electroslag welding;
- - too deep and narrow seams in automatic submerged arc welding;
- – performance of welding works at low temperature;
- - excessive pile of seams to strengthen the structure (the use of overlays, etc.), as a result of which welding stresses increase, contributing to the formation of cracks in the welded joint;
- - the presence in welded joints of other defects, which are stress concentrators, under the influence of which cracks begin to develop in the region of defects.
Rice. 10. Cracks in welds and joints: a - in the deposited metal; b - in the zones of reflow and thermal influence
An important factor influencing education hot cracks(GT), is the contamination of the base and filler metals with harmful impurities of sulfur and phosphorus. Xcold cracks(CT) are formed in the presence of components of martensitic and bainitic types, diffuse hydrogen concentration in the zone of crack initiation, and tensile stresses of the 1st kind. Cracks are among the most dangerous defects and, according to all current regulatory and technical documents (NTD), are unacceptable.
For electron-beam (EBW) and laser welding (LS) the most characteristic are: NotWithmelting due to beam displacement due to magnetization (ELS) or variability of the gap along the length (LS); gas cavities arising from incomplete closure of the gas-dynamic channel; weld formation defects due to metal ejection; porosity.
metal inclusions. In practice, tungsten inclusions are most common in welding aluminum alloys. They usually occur during TIG welding with a tungsten electrode. In this case, instantaneous instability of the arc and the appearance of oxide inclusions simultaneously with tungsten inclusions can be observed. Tungsten inclusions can be located inside the seam and on the surface of the joints in the form of splashes. When tungsten enters a liquid bath, it usually sinks to the bottom of the bath. Tungsten is insoluble in aluminum and has a high density. On x-ray, it gives characteristic clear images of arbitrary shape. Tungsten inclusions tend to form at arc breaks, with tungsten accumulating at the top of craters where cracks often form.
Tungsten inclusions are divided into two main groups: isolated and group. The size of the diameter of the isolated inclusions is 0.4 ... 3.2 mm. Group inclusions are described (according to the X-ray pattern) by the size of the group, the number and size of individual (isolated) inclusions in the group, while the size of the group is characterized by the size of the minimum circle into which the group of inclusions fits. If the image of several inclusions merges, then they are taken as one inclusion.
When welding various metal structures, the quality of the welded joints made on them is of particular importance.
Along with the mechanical properties and corrosion resistance of welded joints, the absence of defects in the weld, fusion zone, and heat-affected zone is one of the most important factors determining the performance of welded structures.
Weld defects in fusion welding are divided into:
Defects in preparation and assembly;
Seam shape defects;
Defects in the structure of the metal of welded joints (external and internal).
Defects in preparation and assembly are most often caused by:
Violations of the geometry of the bevel edges of the seam;
The inconsistency of the gap between the edges along the length of the joined elements;
The mismatch of the planes of the joined parts.
Defects in the shape of the seam (undercuts, sagging, burns, shrinkage grooves, etc.) are primarily due to:
Uneven width of the seams formed in violation of the technique of movement of the electrode;
The uneven gap of the edges during assembly, the unevenness of the bulges along the length of the seam, local thickenings and depressions (first of all, they depend on the unsatisfactory quality of the electrodes in manual welding and the instability of the machine mechanism in automatic welding).
For students of welding specialties, it is necessary to clearly know the characteristic types of defects (external and internal), the reasons for their formation and ways to prevent and eliminate them; the influence of various defects on the properties of the welded joint.
The given illustrations (diagrams and photographs) of defects allow you to quickly and reliably visually identify the type of defect, establish the causes of occurrence and promptly take measures to eliminate it.
Fusion welding defects are classified according to their location into surface, internal and through.
Surface defects include:
- lack of penetration at the root of the seam;
undercuts; influxes;
Craters; underestimation (weakening) of the front surface of the seam;
Concavity of the root of the seam;
Offset of welded edges;
A sharp transition from the seam to the base metal (wrong pairing of the weld);
splashes of metal; surface oxidation; surface cracks.
Internal defects include:
Pores; inclusions;
oxide films;
Internal cracks;
Lack of penetration along the edge with the base metal and between individual layers;
Through defects include cracks and burns.
In addition to defects - discontinuities, defects in fusion welding include: distortion of the shape of the joint associated with deformation, and discrepancy between the geometric dimensions of the weld or points, the regulated values established by the NTD (normative and technical documentation).
GOST 30242-97 provides a classification, designation and a brief description of defects in welded joints, a three-digit numerical designation of defects and a four-digit designation of their varieties, a letter designation of defects, the name of defects in Russian, English and French, an explanatory text, drawings that supplement the definitions.
When choosing methods and means of controlling joints made by welding, it is necessary to have a clear idea of the nature of defects and the reasons for their occurrence. The most characteristic defects that occur during fusion welding are listed in Table. 21.1.
Table 21.1. Fusion Welding Defects
| Defects | Defect definition (GOST 2601-84) | Reasons for the formation of defects | Features of the defect and ways to correct and exclude its formation |
| Lack of penetration: - at the root of the seam; - between individual layers; - along the edge with the base metal (OM). | A defect in the form of local non-fusion due to incomplete melting of the welded edges or surfaces of previously made rollers. | - low heat input; - unsatisfactory preparation of surfaces; - incorrect form of cutting; - a large amount of blunting; - small gaps; - electrode displacement; - poor-quality cleaning of the seam after the passage. | Most typical when welding aluminum alloys and under a layer of flux. It is a stress concentrator. It is difficult to detect in the annular seams of pipelines. Correction - removal of the root part of the seam, followed by welding in one or more passes. |
| Burns: - single; - extended; - discrete | Defect in the form of a through hole, formed as a result of leakage of the weld pool | - large linear energy; - increased clearance; small amount of dullness; - large offset edges; - buckling of the edges and their lagging behind the lining during welding | Invalid defect. It can be eliminated by mechanical sampling (milling cutters) and subsequent welding in a vertical position. |
Continuation of table 21.1.
| craters | Defects in the form of a funnel-shaped depression formed as a result of a sudden cessation of welding or a quick shutdown of the welding current | - the welding equipment does not have or the “crater filling” function is disabled. Low qualification of the welder, violation of welding technique. | Section weakening. It is accompanied by shrinkage and cracks of shrinkage origin. Voltage concentrator. Correction - removal of the defective area and welding. In automatic welding, technological strips are used to remove the crater or smooth current shutdown |
| Beads on a welded joint | A defect in the form of leakage of liquid metal onto the surface of the main or previously made bead without fusion with it. | - high current; - high welding speed; - long arc (high voltage); - electrode displacement; - high feed rate of filler wire; - tilt of the electrode (incorrect guidance). | Occurs on the front side of the joint or on the reverse side due to poor-quality preload to the lining and, as a rule, when welding in a horizontal and vertical position, as well as on the descent and on the ascent. Voltage concentrator. Corrected by machining. |
| Undercuts of the fusion zone: - one-sided; - double-sided | Defects in the form of an extended recess along the line of fusion of the base metal and the weld. | - high current; - high speed; - long arc; - tilt of the electrode (incorrect guidance). - Low qualification of the welder, violation of welding technique. | As a rule, it occurs when welding with concentrated sources in the deep penetration mode, as well as when welding fillet welds. Voltage concentrator. Section weakening. Correction - mechanical cleaning and welding with a "thread" seam along the entire length of the undercut. |
Continuation of table 21.1.
| Non-smooth interface of the weld with OM | Defect in the form of a sharp transition of the surface of the weld to the base metal. | - non-compliance with welding techniques; - high feed rate of the filler wire. | Voltage concentrator. Occurs when the height of the reinforcement of the outer seam is excessive. Correction - mechanical processing. |
| metal spatter | A defect in the form of hardened droplets of liquid electrode metal on the surface of a welded joint. | - non-compliance with the technique and modes of welding; - long arc; - not calcined or low-quality electrodes. | Occurs during welding with thick-coated electrodes, during MP welding in CO 2, and electron beam welding with deep penetration. Correction - mechanical cleaning. |
| Weld root concavity | Defect in the form of a recess on the reverse surface of a welded one-sided seam. | - incorrect preparation and assembly of edges for welding; - non-compliance with welding technique. | Occur when welding butt and fillet welds in the overhead position. Weakening of the seam section. Correction - welding from the side of the weakening of the seam. |
| understatement of the seam | Defect in the form of sagging of the weld. | - a large gap; - a large angle of cutting edges; - non-compliance with welding technique. | Occurs with a large heat input of welding; Correction - welding on softer modes. |
| Offset welded edges | A defect in the form of a mismatch of the welded edges in height due to poor assembly of the welded joint. | - Violation of assembly technology; - post-operational control was not carried out. | Occurs, as a rule, during welding of butt joints. Voltage concentrator. Correction - welding with a smooth transition to the base metal. |
Continuation of table 21.1.
| Fistula weld | Defect in the form of a blind recess in the weld. | - low-quality base metal; - violation of the protection of the weld pool. | Accompanies pores and cracks that come to the surface. Most often occur during MP welding in CO. Correction - cutting with subsequent welding. |
| Surface oxidation of a welded joint | A defect in the form of an oxide film with different tint colors on the surface of the welded joint. | - low consumption of shielding gas; - the presence of impurities in the protective gas; - contamination of the nozzle surface; - incorrectly selected nozzle diameter and its distance from the metal surface; - lack of additional protective peaks. | Occurs when welding high-alloy steels and active metals. Correction - mechanical cleaning and chemical treatment of the surface of the welded joint. |
| Cracks: - superficial; - internal; - through; - longitudinal; - transverse; - branched. | A defect in the form of a gap in the volume of the weld or along the line of fusion with the base metal. They can go into the near-seam zone. | - rigid product design; - welding in rigid fixtures; - long time between welding and heat treatment; - high cooling rate; - an error in the design of the weld (closely spaced concentrators); - violation of technology (heating temperature, suturing procedure); - violation of protection; - low-quality base metal (OM). | The most dangerous and unacceptable defect. Correction - preliminary drilling of the ends of the crack. Crack sampling to the full depth with the necessary edge preparation (grooving) followed by welding in one or more passes. After correction, it is necessary to carry out non-destructive testing of the repaired area. |
End of table 21.1.
| Weld seam pores: - single; - scattered; -clusters; -chain. | Weld defect in the form of a round or oblong cavity filled with gas. | - wet flux; - damp electrodes; - poor-quality preparation of the edges to be welded and the surface of the welding wire; - increased electrode diameter; - long arc; - increased welding speed; - low-quality protection; - low-quality base metal. | As a rule, it occurs when welding aluminum and titanium alloys, in deep butt welds, when degassing is difficult. Section weakening. Decreased tightness. Correction - single allowable pores are left, in all other cases, the defective area is selected to a high-quality OM, followed by welding in one or several passes. |
| Inclusions: - slag; - oxide; - nitride; - tungsten. | Defects in the form of non-metallic particles or foreign metal in the weld metal. | - poor surface preparation; - low-quality base metal; - violation of welding technology; - violation of protection. | They have a spherical or oblong shape, and are also arranged in the form of layers. Voltage concentrators. Correction - removal with subsequent welding. |
In accordance with the specified standard, defects are divided into six groups, mainly according to their shape and location in the welded joint (Table 21.2):
1. cracks;
3. solid inclusions;
4. non-fusion and lack of penetration;
5. violation of the shape of the seam;
6. other defects.
Table 21.2. Types of defects (in accordance with GOST 30242-97)
Continuation of table 21.2.
| microcrack | A crack having microscopic dimensions, which is detected by physical methods at least at 50x magnification. |
| Longitudinal crack | A crack oriented parallel to the axis of the weld. It can be located in the weld metal, at the fusion boundary, in the heat affected zone, in the base metal. |
| transverse crack | A crack oriented across the axis of the weld. It can be located in the weld metal, in the heat affected zone, in the base metal. |
| Radial cracks | Cracks that radiate from one point. They can be in the weld metal, in the heat affected zone, in the base metal. |
| Crack in the crater | A crack in the weld crater, which can be longitudinal, transverse, star-shaped. |
| Separate cracks | A group of cracks that can be located in the weld metal, in the heat-affected zone, in the base metal. |
| branched cracks | A group of cracks originating from a single crack. They can be located in the weld metal, in the heat affected zone, in the base metal. |
| Group 2. Pores | |
| gas cavity | A free-form cavity formed by gases trapped in molten metal that has no corners. |
| gas time | The gas cavity is usually spherical |
| Evenly distributed porosity | A group of gas pores distributed evenly in the weld metal. Should be distinguished from a chain of pores. |
| Clumping of pores | A group of gas cavities (more than two) located in a heap with a distance between them of less than three maximum dimensions of the largest of the cavities. |
| Pore chain | A series of gas pores arranged in a line, usually parallel to the axis of the weld, with a distance between them of less than three maximum dimensions of the largest of the pores. |
| oblong cavity | A discontinuity extended along the axis of the weld. The length of the discontinuity is at least twice its height |
| Fistula | A tubular cavity in the weld metal caused by outgassing. The shape and position of the fistula are determined by the mode of solidification and the source of the gas. Typically, fistulas are grouped into clusters and distributed in a herringbone pattern. |
| Surface pore | A gas pore that breaks the continuity of the weld surface. |
| shrink sink | Cavity resulting from shrinkage during curing. |
| Crater | Shrinkage at the end of the weld bead, not welded before or during subsequent passes. |
Continuation of table 21.2.
| Group 3. Solid inclusions | |
| Solid inclusion | Solid foreign matter of metallic or non-metallic origin in the weld metal. |
| Slag inclusion | Slag trapped in the weld metal. Depending on the formation conditions, such inclusions can be linear or separated. |
| Flux inclusion | Flux that has entered the weld metal. Depending on the formation conditions, such inclusions can be linear, separated, or others. |
| oxide inclusion | Metal oxide incorporated into the weld metal during solidification. |
| metallic inclusion | A piece of foreign metal that has entered the weld metal. There are particles of tungsten, copper or other metal. |
| Group 4. Non-fusion and lack of fusion | |
| non-fusion | No connection between weld metal and base metal or between individual weld beads. |
| Lack of penetration (incomplete penetration) | Non-fusion of the base metal along the entire length of the weld or in the area, resulting from the inability of the molten metal to penetrate into the root of the joint (lack of penetration at the root of the weld). |
| Group 5. Violation of the shape of the seam | |
| Form breaking | Deviation of the shape of the outer surfaces of the weld or the geometry of the joint from the value established by the NTD. |
| Continuous undercut | Longitudinal extended recess on the outer surface of the weld bead along its edges, formed during welding. |
| Shrink groove | Undercut on the root side of a one-sided weld caused by shrinkage at the fusion line. |
| Excess convexity of the butt weld | Excess weld metal on the front side of the butt weld in excess of the specified value. It is a stress concentrator. |
| Exceeding the convexity of the fillet weld | Excess weld metal on the front side of the fillet weld (over the entire length or in a section) in excess of the specified value. |
| Excess penetration | Excess weld metal on the reverse side of the butt weld in excess of the set value. |
| Local elevation | Local excess penetration in excess of the set value. |
| Incorrect weld profile | Deviation of the dimensions of the seam from the specified RTD values. |
| influx | Excess weld metal that has flowed onto the surface of the base metal but is not fused to it. |
| Linear offset | An offset between two elements to be welded, in which their surfaces are parallel, but not at the required level. |
End of table 21.2.
| Angular offset | An offset between two elements to be welded, at which their surfaces are located at an angle that differs from the specified one. | |
| Natek | Weld metal that has settled due to gravity and is not fused to the surface to be joined. | |
| burn | Leakage of weld pool metal, resulting in a through hole in the weld. | |
| Incompletely filled grooves | Longitudinal continuous or discontinuous groove on the surface of the weld due to insufficient filling of the required cross-sectional area with filler material. | |
| Excessive asymmetry of the fillet weld | Exceeding the size of one leg over another. | |
| Uneven seam width | Deviation Uneven width of the seam in its various sections, which differs from the values specified by the NTD. from | |
| uneven surface | Rough irregularity of the shape of the surface of the reinforcement of the seam along the length. | |
| Weld root concavity | A shallow groove on the side of the root of a one-sided weld, formed due to shrinkage of the metal of the weld pool during its crystallization. | |
| Porosity at the root of the weld | The presence of pores in the root of the weld due to the appearance of bubbles during the solidification of the metal. | |
| Resumption | Local unevenness of the surface at the place where welding is resumed. | |
| Group 6. Other defects | ||
| Other defects | All defects that cannot be included in groups 1-5. | |
| Random arc (arson) | Local damage to the surface of the base metal adjacent to the weld, resulting from accidental ignition or arcing. | |
| metal spatter | Drops of weld or filler metal formed during welding and adhering to the metal surface. | |
| Surface scuffs (pulls) | Surface damage caused by the removal of a temporarily welded fixture (technological strips, clamps, etc.). | |
| Metal thinning | Reducing the thickness of the metal to a value less than acceptable during machining or exposure to a corrosive environment. | |
Cracks. Types of cracks
Cracks are among the most dangerous defects and, according to all normative and technical documents in welded joints, they are considered an unacceptable defect.
A crack is a discontinuity in a welded joint in the form of a gap in the weld or adjacent zones.
Cracks in accordance with GOST 30242-97 are divided according to orientation to the seam into:
Longitudinal, oriented parallel to the axis of the weld and located in the weld metal, at the fusion boundary, in the heat-affected zone and in the base metal (Fig. 21.1 and 21.2);
Transverse, oriented across the axis of the weld and located in the weld metal, in the heat-affected zone, in the base metal;
Radial - radially diverging from one point and located in the weld metal, in the heat affected zone, in the base metal.
According to the temperature of crack formation, there are the following types:
Hot, arising in the temperature range of liquid metal crystallization;
Cold, arising at temperatures below the metal crystallization interval;
Reheat cracks.
Rice. 21.1. Longitudinal and transverse cracks in the weld metal
Rice. 21.2. Location of cracks along the weld cross section in electroslag welding:
A- along the axis of the seam; b– between branches of columnar crystals
Rice. 21.3. Cracks in the fracture of the seam: A- coming to the surface of the seam; b- not exposed to the surface of the seam
Rice. 21.4. Location of cracks along the weld cross section (arc welding): A- cracks that do not go to the surface of the seam; b- cracks that go to the surface of the seam
