Types of weld defects and methods for eliminating them. Definitions of the main welding defects Welding defects
The reliability of the structure depends on the quality of the work performed. Defects in welded fasteners are not allowed, otherwise the product may fail at the most inopportune moment. Negligence during work and low qualifications of the master can lead to various problems; work technologies and equipment must be used in accordance with GOST standards. The values of the welds may be within tolerance or, on the contrary, in the latter case, it is necessary to redo the work until the required result is achieved.
When exposed to various factors during work, some unacceptable seam inconsistencies are formed. Welding defects are divided into several groups, the tolerances of which are described in detail in GOST:
- External defects have an uneven shape of the structure, a consequence of non-compliance with the formation technology.
- According to GOST-23055, non-metallic or slag deposits, lack of fusion and lack of penetration of metal products are accepted for internal defective parts. Welding production monitoring devices are used to identify this category of defects.
The correction is made by drilling through the entire cavity, in order to prevent development, the defect is removed and a new connection is welded.
Cavities
An arbitrary shape that appears as a result of exposure to gases is called a cavity. Occurs when the metal melts, the cycle of expulsion of excess gases is not complete, or the weld pool is not formed correctly. Discontinuities are formed in the form of oblong cavities; this category includes craters and shells. The main type of welding rejection is fistulas that arise due to the following circumstances:
- the plasticity of the metal does not meet the requirements;
- hardening structures;
- uneven heating.
Defects differ in shape, depth and location, which can be located both on the inside and on the outside of the seam. Fistulas have an oblong tubular shape and are caused by gases. Failure to comply with technical regulations, namely the presence of oil, oxidation and other contaminants at the welding site, leads to a design defect.
A low-quality tool can cause irreparable damage, as well as the use of auxiliary materials in the form of fluxes. Increased production speed and non-standard flow of protective gases adversely affect the characteristics of the seam. Pores are formed due to the use of faulty tools, wire, or an excessively ventilated room.
When the arc breaks, or the final section is performed incorrectly, craters are formed. The appearance is determined by the type of funnel that needs to be welded upon detection. Modern welding machines can eliminate the discrepancy by reducing the current at the end of the connection.
Solid inclusions
Foreign substances of any origin are a serious problem in the welding process. The main mistakes are high welding speed, low current, and dirty edges. Weld defects are caused by:
- flux residues;
- slag or oxide inclusions.
Oxide ones are formed as a result of the lack of metal stripping and chemical exposure. The slag, if the technology is followed, floats to the surface, but in some situations remains inside the seam. Protective gases create an environment in which it is impossible to include a foreign body. Metallic inclusions can be dangerous because... have dimensions up to several tens of millimeters.
Conditions for occurrence depend on the type of formation:
- disunited;
- linear;
- other education.
The area of welding operations in which the content of slag additives is exceeded is digested. Often, inclusions appear at the junction of a static and an added seam when making multilayer products.
Lack of fusion and lack of penetration
The lack of joining of the base metal, or between individual elements, is called non-fusion. They are distinguished into superficial ones, consisting between the forks, located at the base of the seam. The main reasons for non-fusion are determined:
- increased arc length;
- inadequate edge cleaning;
- reduced welding current;
- increased welding speed.
The presence of a defect using static connections can be compensated for by additional welding. As a result, strength decreases and stress concentration occurs in the non-fusion zone.
Insufficient fusion of the joint during the welding zone is called lack of fusion. The main reasons are residues of rust, oxidation, scale, and other adverse effects. As a result of a decrease in concentration, the possibility of stresses that negatively affect the structure as a whole increases. In case of deviations from the tolerances, areas with an unwelded seam are cleaned down to the base, and the welding operation is repeated.
Deviation from the specified parameters of the shape of the weld surface and the geometric state of the connection is caused by a violation of the shape.
There are various disorders, each of which occurs due to certain conditions.
- Undercuts - a defect in the form of indentations is formed along the edges of the weld when positioned longitudinally. It is often formed at an increased welding speed, as a result of which the weld pool hardens faster than expected. The increased arc distance promotes the spread of the weld width across the metal, because with this scheme, the heat transfer of the arc remains at the same level, the power is not enough to melt the entire metal cavity.
- Excess deposited material found on the inside of the weld is excess penetration. Linear displacement defects are a condition in which the parts being connected are located at different levels, and there is a difference in height between the joints. There is an angular type of defect, in situations where the angle is asymmetrical to the butt element.
- Overlay is an excess amount of material formed during the process of joining a seam. The defect is formed due to an excessively long arc, incorrect tilt of the electrode, or increased welding current.
- A burn-through is a through hole formed as a result of the leakage of the metal component of the weld pool. Defects are formed as a result of using a high current at a low speed of movement of the electrode, poor lining or incorrect edge clearance.
There are also other problems associated with the shape, for example, concave edges of the seam formed on the root side of the joint. Other nonconformities include surface-type scuffing, accidental arcing, metal spatter, and others.
Detection and control methods
A high-quality seam has appropriate markings. At large enterprises, each specialist places a certain mark on the joined area. The following methods are used to detect defects:
- visual inspection;
- color flaw detection;
- ultrasonic method for identifying defective areas;
- radiation;
- magnetic method.
After a defect is discovered, an employee of the quality department determines the further fate of the part; in most cases, they are sent for revision. Sagging is removed using an abrasive tool through mechanical action. Welding is used for flaw detection of large cracks, with the residual welding site previously cleaned.
When welding various metal structures, the quality of the welded joints made on them is especially important.
Along with the mechanical properties and corrosion resistance of welded joints, the most important factors determining the performance of welded structures include the absence of defects in the weld, fusion zone and heat-affected zone.
Defects in welded joints during fusion welding are divided into:
Defects in preparation and assembly;
Seam shape defects;
Defects in the metal structure of welded joints (external and internal).
Defects in preparation and assembly are most often caused by:
Violations of the geometry of the bevel of the seam edges;
Inconsistency of the gap between the edges along the length of the joined elements;
Mismatch of planes of joined parts.
Defects in the shape of the seam (undercuts, sagging, burns, shrinkage grooves, etc.) are primarily due to:
Uneven width of seams formed when the electrode movement technique is violated;
Unevenness of the edge gap during assembly, unevenness of convexities along the length of the seam, local thickenings and depressions (primarily they depend on the unsatisfactory quality of the electrodes during manual welding and the instability of the machine mechanism during 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 methods of prevention and elimination; the influence of various defects on the properties of the welded joint.
The provided illustrations (diagrams and photographs) of defects allow you to quickly and reliably visually identify the type of defect, establish the causes of its 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; surges;
Craters; understatement (weakening) of the front surface of the seam;
Concavity of the root of the seam;
Displacement of welded edges;
A sharp transition from the seam to the base metal (incorrect weld mating);
Metal splashes; 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 joint shape associated with deformation, and non-compliance of the geometric dimensions of the weld or points with the regulated values established by the NTD (normative and technical documentation).
GOST 30242-97 provides a classification, designation and 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, explanatory text, drawings that supplement the definitions.
When choosing methods and means for monitoring welded joints, it is necessary to have a clear understanding of the nature of the defects and the reasons for their occurrence. The most typical defects that occur during fusion welding are listed in Table. 21.1.
Table 21.1. Defects that occur during fusion welding
| Defects | Definition of defect (GOST 2601-84) | Reasons for the formation of defects | Features of the defect and methods for correcting and eliminating its formation |
| Lack of penetration: - at the root of the seam; - between individual layers; - along the edge with the base metal (BM). | A defect in the form of local lack of fusion due to incomplete fusion of the welded edges or surfaces of previously made beads. | - low heat input; - unsatisfactory surface preparation; - incorrect cutting form; - large amount of dullness; - small gaps; - electrode displacement; - poor-quality cleaning of the seam after completing the pass. | Most common when welding aluminum alloys and submerged arcs. They are stress concentrators. Difficult to detect in circumferential welds of pipelines. Correction - removal of the root part of the seam followed by welding in one or several passes. |
| Burns: - single; - extended; - discrete | A defect in the form of a through hole resulting from leakage of the weld pool | - high heat input; - increased clearance; small amount of dullness; - large displacement of edges; - warping of edges and their separation from the lining during welding | Inadmissible defect. Can be eliminated by mechanical sampling (with 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 stop in welding or a rapid shutdown of the welding current | - the welding equipment does not have the “crater filling” function or is turned off. Low qualification of the welder, violation of welding technique. | Weakening the section. Accompanied by shrinkage and shrinkage cracks. Voltage concentrator. Correction - removal of the defective area and welding. When automatic welding, technological strips are used to remove the crater or smoothly turn off the current |
| Sagging on a welded joint | A defect in the form of leakage of liquid metal onto the surface of the main or previously made roller without fusion with it. | - high current; - high welding speed; - long arc (high voltage); - electrode displacement; - high feed speed of filler wire; - tilt of the electrode (incorrect guidance). | It occurs on the front side of the connection or on the back side due to poor-quality pressing to the lining and, as a rule, when welding in a horizontal and vertical position, as well as downhill and uphill. Voltage concentrator. Corrected by mechanical treatment. |
| Undercuts of the fusion zone: - one-sided; - double-sided | Defects in the form of an extended depression along the fusion line 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. Weakening the section. Correction – mechanical stripping and welding with a “thread” seam along the entire length of the undercut. |
Continuation of Table 21.1.
| Unsmooth interface of the weld with the OM | A defect in the form of a sharp transition of the surface of the weld to the base metal. | - non-compliance with welding technique; - high feed speed of filler wire. | Voltage concentrator. Occurs when the height of the external seam reinforcement is excessive. Correction - mechanical processing. |
| Metal splashes | A defect in the form of solidified drops of liquid electrode metal on the surface of a welded joint. | - non-compliance with welding techniques and modes; - long arc; - unheated or low-quality electrodes. | Occurs when welding with thickly coated electrodes, during MT welding in CO 2, and electron beam welding with deep penetration. Correction - mechanical cleaning. |
| Concavity of the root of the seam | A defect in the form of a depression on the back surface of a single-sided weld. | - improper preparation and assembly of edges for welding; - non-compliance with welding technique. | Occurs when welding butt and fillet welds in an overhead position. Weakening of the seam section. Correction - welding from the side of the weakened seam. |
| Lowering the seam | Defect in the form of sagging weld. | - large gap; - large cutting angle; - non-compliance with welding technique. | Occurs at high heat input of welding; Correction: welding at softer settings. |
| Offset of welded edges | A defect in the form of a mismatch of welded edges in height due to poor quality assembly of the welded joint. | - violation of assembly technology; - operational control was not carried out. | It usually occurs when welding butt joints. Voltage concentrator. Correction - welding to ensure a smooth transition to the base metal. |
Continuation of Table 21.1.
| Weld fistula | A defect in the form of a blind depression in the weld. | - low-quality base metal; - violation of the weld pool protection. | Accompanies pores and cracks emerging to the surface. Most often they occur during MT welding in CO. Correction: cutting followed by cooking. |
| Surface oxidation of a welded joint | A defect in the form of an oxide film with different tarnish colors on the surface of the welded joint. | - low consumption of protective gas; - presence of impurities in the shielding gas; - contamination of the nozzle surface; - incorrectly selected nozzle diameter and its distance from the metal surface; - lack of additional protective visors. | 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; - end-to-end; - longitudinal; - transverse; - branched. | A defect in the form of a break in the volume of the weld or along the fusion line with the base metal. They can go into the heat-affected zone. | - rigid design of the product; - welding in rigidly fixed fixtures; - long time between welding and heat treatment; - high cooling rate; - error in the design of the weld (closely located hubs); - violation of technology (heating temperature, order of sutures); - violation of protection; - low-quality base metal (BM). | The most dangerous and unacceptable defect. The fix is to pre-drill the ends of the crack. Extraction of the crack to its full depth, ensuring the necessary preparation of the edges (grooving), followed by welding in one or several passes. After correction, non-destructive testing of the repaired area is necessary. |
End of table 21.1.
| Pores of the weld: -single; - absent-minded; -clusters; -chain. | A weld defect in the form of a round or oblong cavity filled with gas. | - wet flux; - damp electrodes; - poor quality preparation of the welded edges and surface of the welding wire; - increased electrode diameter; - long arc; - increased welding speed; - poor 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. Weakening the section. Reduced tightness. Correction - single acceptable 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 followed by welding. |
In accordance with this 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. lack of 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 that has microscopic dimensions, which is detected by physical methods at at least 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, or in the base metal. |
| Transverse crack | A crack oriented transverse to the axis of the weld. It can be located in the weld metal, in the heat-affected zone, or in the base metal. |
| Radial cracks | Cracks that radiate from one point. They can be in the weld metal, in the heat-affected zone, or in the base metal. |
| Crack in the crater | A crack in the weld crater, which can be longitudinal, transverse, or 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 arising from a single crack. They can be located in the weld metal, in the heat-affected zone, or in the base metal. |
| Group 2. Pores | |
| Gas cavity | A cavity of arbitrary shape formed by gases trapped in molten metal, which has no corners. |
| Gas time | The gas cavity is usually spherical in shape |
| Evenly distributed porosity | A group of gas pores distributed evenly in the weld metal. Should be distinguished from a chain of pores. |
| Pore accumulation | A group of gas cavities (more than two), arranged in a cluster with a distance between them of less than three maximum sizes of the larger of the cavities. |
| chain of pores | A series of gas pores arranged in a line, usually parallel to the axis of the weld, with a distance between them less than three times the maximum size of the larger pore. |
| 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 the release of gas. The shape and position of the fistula are determined by the hardening mode and the gas source. Typically, fistulas are grouped into clusters and distributed in a herringbone pattern. |
| Superficial pore | A gas pore that disrupts the continuity of the weld surface. |
| Shrinkage shell | A cavity formed due to shrinkage during hardening. |
| Crater | A shrinkage hole at the end of a weld bead that is not sealed before or during subsequent passes. |
Continuation of Table 21.2.
| Group 3. Solid inclusions | |
| Solid inclusion | Solid foreign substances of metallic or non-metallic origin in the weld metal. |
| Slag inclusion | Slag trapped in the weld metal. Depending on the conditions of formation, such inclusions can be linear or disconnected. |
| Flux inclusion | Flux trapped in the weld metal. Depending on the conditions of formation, such inclusions can be linear, disconnected, or other. |
| Oxide inclusion | Metal oxide introduced into the weld metal during solidification. |
| Metal inclusion | A foreign metal particle embedded in the weld metal. There are particles of tungsten, copper or other metal. |
| Group 4. Lack of fusion and lack of penetration | |
| Non-fusion | Lack of connection between the weld metal and the base metal or between individual weld beads. |
| Lack of penetration (incomplete penetration) | Failure of fusion of the base metal along the entire length of the weld or in a section, resulting from the inability of the molten metal to penetrate the root of the joint (lack of fusion at the root of the weld). |
| Group 5. Violation of the seam shape | |
| Violation of form | Deviation of the shape of the external surfaces of the weld or the geometry of the connection from the value established by the technical documentation. |
| Continuous undercut | A long longitudinal depression on the outer surface of the weld bead along its edges, formed during welding. |
| Shrinkage groove | Undercut from the root side of a single-sided weld caused by shrinkage along the fusion boundary. |
| Excess of convexity of butt weld | Excess of deposited metal on the face of the butt weld in excess of the specified value. Is a stress concentrator. |
| Excess of fillet weld convexity | Excess of deposited metal on the front side of a fillet weld (over the entire length or in a section) in excess of the specified value. |
| Exceeding penetration | Excess of deposited metal on the back side of the butt weld in excess of the specified value. |
| Local excess | Local excess penetration in excess of the established value. |
| Incorrect weld profile | Deviation of seam dimensions from the specified technical documentation values. |
| influx | Excess weld deposit metal that has flowed onto the surface of the base metal but has not fused to it. |
| Linear displacement | An offset between two welded elements in which their surfaces are parallel, but not at the required level. |
End of table 21.2.
| Angular offset | Displacement between two elements to be welded, in which their surfaces are located at an angle different from the specified one. | |
| Natek | Weld metal that has settled due to gravity and does not have fusion with the surface being joined. | |
| Burn-through | The flow of metal from the weld pool, which results in the formation of a through hole in the weld. | |
| Incomplete edge groove | A longitudinal continuous or discontinuous groove on the surface of a weld due to insufficient filler material filling the required cross-sectional area. | |
| Excessive fillet weld asymmetry | Exceeding the size of one leg over the other. | |
| Uneven seam width | Deviation Uneven width of the seam in its various sections, which differs from the values specified in the technical documentation. from | |
| Uneven surface | Rough unevenness in the shape of the seam reinforcement surface along its length. | |
| Concavity of the root of the seam | A shallow groove on the root side 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 at the root of the weld due to the formation of bubbles during the solidification of the metal. | |
| Resumption | Local surface roughness at the site 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 burning of the arc. | |
| Metal splashes | Drops of weld metal or filler metal formed during welding and adhered to the surface of the metal. | |
| Surface scuffs (tears) | Surface damage caused by the removal of a temporarily welded fixture (process strips, clamps, etc.). | |
| Metal thinning | Reducing the thickness of the metal to a value less than permissible during machining or exposure to a corrosive environment. | |
Cracks. Types of cracks
Cracks are among the most dangerous defects and, according to all regulatory 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 their orientation to the seam into:
Longitudinal, oriented parallel to the axis of the weld and located in the metal of the weld, at the fusion boundary, in the heat-affected zone and in the base metal (Fig. 21.1 and 21.2);
Transverse, oriented transverse to the axis of the weld and located in the metal of the weld, in the heat-affected zone, in the base metal;
Radial - radially diverging from one point and located in the metal of the weld, in the heat-affected zone, in the base metal.
Based on the temperature at which cracks form, there are the following types:
Hot, occurring in the temperature range of crystallization of liquid metal;
Cold, occurring at temperatures below the crystallization range of the metal;
Reheat cracks.
Rice. 21.1. Longitudinal and transverse cracks in the weld metal
Rice. 21.2. Location of cracks along the cross-section of the weld during electroslag welding:
A– along the axis of the seam; b– between the branches of columnar crystals
Rice. 21.3. Cracks in a seam break: A– extending to the surface of the seam; b– not extending to the surface of the seam
Rice. 21.4. Location of cracks along the weld cross-section (arc welding): A– cracks that do not extend to the surface of the seam; b– cracks extending to the surface of the seam
Today, welding is used everywhere to connect various metal parts. It is successfully used both in industry and in private households. is called permanent connection of parts by welding. As a result of this, various areas are formed, which are characterized by a certain set of properties. It all depends on the degree of heating. They may differ in physical, chemical and mechanical properties. The main defects of welded joints have been known for a long time. They should be avoided while doing work.
Welding is used to join metal parts in industry and domestic settings.
Characteristics and types of welded joints
Before we start talking about defects in welded joints, it is worth talking in more detail about their main types and characteristics. The principle of welding is quite simple. The molten metal forms a seam, which crystallizes. The material that is partially melted constitutes the fusion zone. Near this zone, one is formed in which the heated metal experiences additional stress. It is called the heat-affected zone. After this comes the base metal. Its structure and properties do not change in any way during the work.
Classification of welds by position in space.
There are several main types of welded joints. The most common among them are butt, overlap, tee and corner. They all differ in the installation of basic materials and the location of the seam. The quality of the seam is directly influenced by many different factors. Both internal and external defects can form. The quality of the seams is directly affected by the degree of contamination of the metals that are to be joined.
A wide variety of oxides, fatty films, and so on may be present here. That is why the surfaces to be welded must be cleaned before work. By the way, during the process it is necessary to combat the oxides formed on the surface. In any case, the strength of the final connection directly depends on the absence of defects. The seam can sometimes have exactly the same strength as the base material, but this is quite difficult to achieve.
About defects in welded joints
As noted earlier, defects in welded joints can be of a very diverse nature. They must be remembered during the work process. If a person has a wealth of knowledge on them, then he will be able to weld parts that will have perfect seams. This is exactly what we should strive for.
Table of the main types of welded joints.
- Undercut This is one of the types of defects in welded joints. It is a groove that is formed at the point of fusion of the base metal and the weld. Most often, such defects appear when there are large weld pools. This means that a large amount of metal is melted due to the use of high current values.
- Float. This defect is characterized by leakage of the weld material onto the base metal. A very unpleasant drawback.
- Lack of penetration. Such a defect in welded joints can occur in cases where insufficient melting of the base metal occurs at the joints of structural elements. This place is most often filled with slag, which, due to its structure, forms porosity and voids in the seams. It is unacceptable. The design immediately loses its properties. When arc welding is used, lack of fusion may occur due to the use of insufficient current. This is one of the most dangerous defects. This is primarily due to the fact that additional stresses begin to form in this place during the subsequent operation of the structure. This very often leads to its rapid destruction. You can get rid of this defect. To do this, lack of penetration is identified, and then surfacing is carried out in difficult areas.
- Cracks. This is partial destruction of the material at the seam or in the area located near it. They can form for several reasons. If we talk about the process when the metal is still hot, then cracks appear as a result of crystallization of the metal. In the solid state, a wide variety of structural transformations can also occur with it. This is the second reason for the appearance of such defects.
Defects in welds: lack of fusion, uneven shape, sagging, cracks, fistulas, overheating.
The mechanism of hot crack formation is quite simple. During welding, the metal heats up. Once the heat source is removed, it begins to gradually cool. Of course, crystallization zones begin to form. They begin to float among the still molten metal. If there were no microzones that allow the interaction of hot and cold material, then all welded joints would contain defects. However, this does not happen. Thus, it can be assumed that the higher the crystallization interval, the more likely the occurrence of hot cracks. Carbon directly affects this indicator. There is a direct relationship here. The more carbon there is in steel, the wider the crystallization interval becomes.
Cold cracks can form at the seam. They appear when the material is cooled to a temperature of approximately 200-300 degrees Celsius. They may not appear immediately, which makes them more dangerous. The appearance of cold cracks is associated with the fact that various structural transformations begin to occur in the material due to certain chemical transformations. There is a direct dependence on the amount of carbon in the material. The more it is, the more likely it is that cold cracks will appear. This tendency to form cold and hot cracks determines such a parameter as the weldability of metals. This parameter characterizes the ability to obtain a welded joint that is no different from the base materials.
Pores and non-metallic inclusions
Defects in welds: craters, undercuts, pores, lack of penetration, slag, burn-through.
Pores. These defects in welded joints are quite common. Pores are voids that are filled with gas. They can be microscopic in size, or they can form defects several millimeters in size in the structure. In this case, they most often form at the junction of the seam with the base material. This defect is influenced by many different parameters.
The most important of these is the concentration of gas in the cooking bath. Gas is released from the metal during its melting process. This process cannot be prevented in any way. Carbon monoxide is not able to dissolve in iron; accordingly, it is released in the form of bubbles.
Non-metallic inclusions. These defects in the welded joints themselves are associated with the entry of foreign inclusions into the weld structure as a result of the work.
Cracks in the welded joint.
There is a huge variety of such inclusions. Slag, for example, can form as a result of insufficient cleaning of the materials that are to be joined.
Their cause may be insufficient complete removal of slag during multilayer welding. When work is carried out by melting, a material is formed in the weld, which differs in physical and chemical properties from the base metal. In this regard, similar defects can also form. Foreign inclusions can be of a very diverse nature.
Defect Study
A welding defect is pores, which is the filling of voids with gases.
Of course, if there are defects in various welded joints, then they must be studied. Macroanalysis is often used for this purpose. It lies in the fact that the structure of the metal is studied using the naked eye or a magnifying glass. Unlike microscopic analysis, macroanalysis does not allow one to properly study the structure of the material. Its main task is to control the quality of the parts being joined during the welding process. It allows you to determine the type of fracture, fibrous structure, violations of the continuous structure, and so on. In order to carry out such an analysis, it is necessary to subject the part being studied to etching with special elements and processing on grinding machines. This sample is called a macrosection. There should be no unevenness or foreign inclusions on its surface, including oil.
All those defects that were described above can be studied and identified using macroanalysis.
To reveal the structure of a material, surface etching methods are most often used.
Types of beads in seams.
This approach is best suited for low and medium carbon steels. Macrosection, which is prepared in advance, must be immersed in the reagent with the part that is being analyzed. In this case, its surface must be cleaned with alcohol. As a result of the interaction of elements, a chemical reaction occurs. It allows you to displace copper from solution. Materials are being replaced. As a result, copper is deposited on the surface of the probe. Those places where the copper has not completely covered the base material are etched. These places contain any defects. After this, the sample is removed from the aqueous solution, dried and cleaned. All these actions must be done as quickly as possible so that an oxidation reaction does not occur. As a result, it is possible to identify those areas where large amounts of carbon, sulfur and other materials are present.
Etching of areas that contain these materials does not occur in the same way. Where there is a high concentration of carbon and phosphorus, copper is not released intensively on the surface. This is the minimum degree of metal protection. As a result, these places are subject to the greatest etching. As a result of the reaction, these areas become darker in color. It is better to use this method for steels that contain a minimal amount of carbon. If there is a lot of it, then it will be very difficult to remove copper from the surface of the sample.
Types of undercuts in seams.
There are other methods for macroanalysis of the structure of materials in a welded joint. For example, the photoprint method is often used to determine the amount of sulfur. The photographic paper is moistened and held in the light for some time. After this, it is dried between sheets of foil paper. The solution in which it is initially placed contains a certain amount of sulfuric acid. Then, of course, this paper is laid in an even layer on a macrosection.
It should be smoothed with a roller so that all its deformations are completely eliminated. Any air bubbles that may remain between the photo paper and the metal must be completely removed. Only in this case will the research be objective. It needs to be held in this position for approximately 3-10 minutes. The time depends on the original thickness of the probe, as well as other factors.
Types of lack of penetration.
Sulfur inclusions that are located in the deposited metal will definitely react with the acid that was applied to the surface of the photographic paper. In areas where hydrogen sulfide is released, a substance called photographic emulsion will be formed. The areas of silver sulfide that will form as a result of the reaction clearly show the distribution of sulfur in the metal.
Of course, these areas will be observed on paper. The photographic paper that was used for the experiment must be washed and then kept in a hyposulfite solution. After this, it is washed again in liquid and dried. If fluoride inclusions are present in the weld, they will definitely come out in the form of dark-colored areas.
Summarizing
Thus, there are currently many methods for identifying defects in welded joints. They all have a specific purpose. Each method allows you to find out how much of a particular material is contained in the structure of the seam, which can adversely affect its structure.
In addition to macroanalysis methods, microanalysis methods have recently been introduced quite often. They have the same purpose as the previous ones. However, they additionally allow one to study the structure of the material. Here the work is carried out at the molecular level of the structure of the crystal lattice.
The further operation of the structure depends on the quality of welding, so 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;
- carrying out work without proper preparation, in adverse weather conditions.
There are acceptable and unacceptable values of weld defects depending on the degree of reduction in the technical parameters of the product in terms of strength. In case of acceptable violations, welding defects are not corrected; in the second case, their elimination is necessary. The suitability of the product for use and determination of whether the seam meets the standards is carried out in accordance with GOST 30242-97.
Types of welding defects
A correct welding seam implies homogeneity of the composition of the base and filler material, the formation of its desired shape, the absence of cracks, lack of penetration, overflow, and the presence of foreign substances. The following types of defects in welded joints are distinguished:
- external;
- internal;
- end-to-end.
What are external defects?
External defects in welds and joints are detected visually. Violations of the welding regime, failure to maintain the accuracy of the direction and movement of the electrode due to haste or irresponsibility of the welder, fluctuations in electrical voltage during welding work lead to the formation of a seam of the wrong size and shape.
Characteristic signs of the external appearance of defects are: the difference in the width of longitudinal welds and corner legs, the sharpness of the transition from the base steel to the deposited one.
With the manual welding method, violations occur due to errors in edge preparation, neglect of the welding mode and speed, and lack of timely control measurements. Defects in welds and the reasons for their formation when carrying out automatic or semi-automatic welding methods lie in excessive surges in electrical voltage and operating errors. The following external types of weld defects are distinguished:
Cracks seams are hot and cold, longitudinal, transverse, radial. The first of them occur when high temperatures from 1100 to 1300°C are used, affecting the properties of the metal in terms of reducing ductility and the appearance of tensile deformations. This type of weld defects is accompanied by an increase in undesirable chemical elements in the steel composition. Cold cracks can appear at temperatures up to 120°C during cooling, and later under the influence of loads during operation. The reason for this type of defect 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 depression 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 and a decrease in strength. Deeper penetration of one of the edges causes liquid steel to flow onto the other surface, which is why the welding groove does not have time to fill. In this case, welding defects and methods for eliminating them are determined visually. Defects in work are eliminated by stripping, followed by overcooking.
Weld undercut
influx occurs when the fused metal flows onto the surface of the base steel without forming a homogeneous mass with it. This type of defect 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 defect is the use of low arc voltage, the presence of scale on the edges of parts, and the leakage of fused steel when welding horizontal seams when the surface of the structures being welded 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 and looseness of the material with shrinkage properties can form, leading to the appearance of cracks. Craters occur due to welder errors. Since the crater is usually the cause of cracks, which is why it is not allowed, if it is found, it should be cleaned out, then re-welded.
Formed crater in the weld seam
Fistulas They look like funnels with a depression 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 corrected immediately.
Typical fistula funnels
Internal weld defects
Internal welding defects cannot be visually detected. Usually 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 over time. Hidden cracks are dangerous because they are difficult to detect, and stress can increase gradually, and can lead to rapid destruction of the structure, and therefore are extremely dangerous. The cause of defects can be enormous stresses and rapid cooling when using carbon and alloy steels. The most common types of this type of defect are as follows:
Lack of penetration occurs when there is insufficient fusion of the welded parts of the seam in place. The reason lies in improper edge preparation due to rust, scale, lack of clearance and dull 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 weld penetration. Due to the reduction in the cross-section of the weld, a stress concentration appears, which is reflected in a decrease in the strength of the joints, which under vibration loads is up to 40%, and large areas of lack of penetration - up to 70%. If the permissible values are exceeded, the seam must be cleaned and rewelded.
Lack of penetration and filling
Pores– these are the free spaces of the weld filled with gas, mainly hydrogen. The reason for this type of defect is the presence of foreign impurities in the materials being welded, dampness, and insufficient protection of the weld pool. If the permissible pore concentrations are exceeded, the weld is subject to overcooking.
Pores in the weld
In addition, one can note slag, tungsten, and oxide inclusions, which also arise when the welding process technology is violated.
Through defects
This type of defect implies the presence of pores passing through the entire thickness of the weld and is also detected visually. Mostly occur during 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 excessive openness of the gap at the edges, loose fit of the pads, resulting in leakage of the weld pool. Checking the seam for defects is carried out visually; if it exceeds the permissible norm, the weld must be cleaned and re-welded.
Methods for detecting, monitoring 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 upon contact with a fluid material, for example, kerosene;
- magnetic method - measuring the distortion of magnetic waves;
- ultrasonic method - the use of ultrasonic flaw detectors that measure the reflection of sound waves;
- radiation method - X-raying welds and obtaining an image with all the details of the defect.
To ensure the quality of the weld, marking and branding are carried out. Each welder puts his mark on his welding area.
If a defect is detected, 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 an abrasive tool;
- internal small cracks, lack of fusion and inclusions must be completely cleaned or cut out with re-welding;
- incomplete seams and weld undercuts are eliminated by surfacing or welding in thin layers;
- Sagging is removed mechanically using an abrasive tool;
- Overheating of the metal is eliminated by heat treatment.
It is mainly assumed that the weld metal must be solid. And all formations that make the weld inhomogeneous are considered to be defects. The following are distinguished: types of weld defects: micro- and macrocracks (hot and cold), lack of penetration, pores, various inclusions.
Internal and external defects in welds
The most common method of classifying welding defects is by their location. According to this classification, a distinction is made between internal and external welding defects. The external ones go to the surface of the seam and the heat-affected zone, and the internal ones are located inside the joint without coming to the surface. It follows from this that the same type of defect (for example, cracks or pores) can be both internal (if located inside) and external (if it comes to the surface).
External weld defects
External defects of welded joints include uneven shape of the weld due to improper formation, undercuts of the seam, burns of the metal being welded, sagging, cracks, pores and other defects that are located on the surface of the metal. All of them are revealed during external visual inspection of the welded joint. Common types of external defects are listed and shown below in the text.
Internal weld defects
Internal defects of welded joints, according to GOST 23055, include non-metallic, slag and oxide inclusions, lack of penetration and lack of fusion of metal, as well as pores and cracks that do not extend to the surface of the metal. In order to identify such defects, non-destructive welding testing methods are used in practice. The text below describes common types of internal defects.
Seam formation defects
Defects in the formation of welds are manifested in the unevenness of their shape (see figure on the right). They are formed due to inconsistent welding conditions, variable gap between the welded edges and uneven bevel angle of the edges. A discrepancy between the actual shape of the seam and the required one may occur due to incorrect welding, due to incorrect positioning of the electrode relative to the welded edges.
A similar defect may appear in others. For example, during automatic welding, the cause of such a defect may be slippage of the welding wire in the feed mechanism, a voltage drop in the network, molten metal entering the gaps, etc.
Lack of weld penetration
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Most often, lack of penetration in welds occurs in cases where there are small gaps between the welded edges, when the edges are heavily blunted, as well as when they are contaminated, when the electrode or welding wire is incorrectly positioned relative to the edges being welded, when the welding current is insufficient, and when increased welding speed.
Very often, lack of penetration is formed at the root of the seam (diagram a) and b) in the figure on the left and diagrams c) and d) in the figure). In automatic submerged arc welding, lack of penetration, in most cases, forms at the beginning of the weld. To prevent their occurrence, it is recommended to carry out welding on special pads. Lack of penetration is one of the most dangerous defects for a welded joint.
Weld undercuts
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Weld undercuts are formed on the joint surface. Undercuts are depressions in the base metal located along the edges of the weld. They appear due to excessively high welding current and due to the long length of the electric arc, because in this case, the width of the weld increases and the edges of the welded edges melt more strongly.
There are several types of weld cracks:
Type of weld defect. As well as its size and place of origin.
Mechanical properties of the welded joint. These are tensile strength, fluidity, impact strength, ductility, corrosion resistance, fatigue resistance, etc.
Conditions under which the product is used. Basically, it is the nature of the environment.
Functions that the product must perform. There is even a term: “fit for purpose.” Those. the same defect in a weld may be acceptable for one task, but unacceptable for another.
To make a decision about the admissibility of defects of a particular type and size, it is necessary that the measuring ability of the device for monitoring defects is higher than the permissible value of the defect. That is, if defects of no more than 2 mm in size are allowed in the weld, then a device with a measuring capacity of 5 mm cannot be used to control this seam.
In order to determine the maximum value of an allowable defect, it must be borne in mind that weld defects mainly increase the steel’s ability to undergo fatigue and brittle fracture.
For destruction of this type, the greatest danger is posed by plane defects (microcracks, macrocracks, lack of penetration). If they are identified, you need to pay attention not only to the maximum sizes of individual defects, but also to their relative location and their number.
The danger of planar defects lies in the fact that they are concentrators of high stresses due to the lack of a radius of curvature at the cracks. Spatial defects, such as pores, gas bubbles or any inclusions have a certain radius of curvature, therefore, they pose a lesser danger, even if they are larger in number.
With a small rounding at the base of the crack, in order to evaluate the stresses acting in it, the stress intensity factor K1 is used, which allows one to evaluate the fracture mechanics. The stress intensity factor can be determined if the stress required for failure is less than the yield strength of the material. It is determined by the formula:
where a is the size (height) of the external defect, or half the size of the internal defect;
bm - tensile stress;
bv - bending stress;
Мm and Мв are coefficients, the value of which is determined by the ratio of the size of the defect to the thickness of the part and the location of the defect;
Q is a coefficient depending on the shape of the defect.
For welded joints that are not subjected to post-weld annealing, in order to reduce internal stresses, critical crack opening (COD) calculations must be used to assess the acceptability of weld defects. Calculating the coefficient K1, or finding the value of the critical opening, makes it possible to determine with high accuracy the value of a possible permissible weld defect.
