What is called inclusion during welding. Types of defects in welded joints
The reliability of the design 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 at work, low qualification of the master, can lead to various problems, work technologies and equipment must be used in accordance with GOSTs. The values of the welds may be within tolerance or vice versa, in the latter case, it is necessary to re-do the work until the desired result is achieved.
Under the influence of various factors during operation, some unacceptable inconsistencies in the seam 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, as a result of non-compliance with the formation technology.
- According to GOST-23055, non-metallic or slag deposits, non-fusion and lack of penetration of metal products are accepted as internal defective parts. To identify this category of marriage, welding production control devices are used.
The correction is made by reaming over the entire cavity, in order to exclude development, the marriage is removed and a new connection is welded.
cavities
An arbitrary shape, which appeared as a result of exposure to gases, is called a cavity. Occurs when the metal is melted, not a full cycle of expelling excess gases, not the correct formation of the weld pool. Discontinuities are formed in the form of oblong cavities; craters and shells belong to the category. The main type of welding rejection are fistulas that have appeared due to:
- 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, the cause of the appearance is gases. Failure to comply with the technical regulations, namely the presence of oil, oxidation and other contaminants in the welding area leads to a design defect.
A poor-quality tool can cause irreparable damage, as well as the use of auxiliary materials in the form of fluxes. Increased manufacturing speed, non-standard course of protective gases adversely affect the characteristics of the seam. Pores are formed due to the use of a faulty tool, wire, excessively ventilated room.
When the arc breaks, or the wrong way to complete the final section, craters are formed. The appearance is determined by the type of funnel that needs to be welded upon detection. Modern units that make a welding seam are able to eliminate the discrepancy by reducing the current at the end of the connection.
Solid inclusions
Foreign matter of any origin is a serious problem in the welding process. The main mistakes are high welding speed, low current, dirty edges. Weld defects are produced due to:
- flux residues;
- slag or oxide inclusions.
Oxide, are formed as a result of the lack of metal stripping, chemical exposure. Slag, subject to technology, 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 as they are up to several tens of millimeters in size.
The conditions for occurrence depend on the type of formation:
- disunited;
- linear;
- other education.
The section of the welding operations, in which the content of slag additives is exceeded, is digested. Often, inclusions appear at the junction of a static and added seam, when making multilayer products.
Non-fusion and lack of penetration
The lack of joining of the base metal, or between individual elements, is called non-fusion. They differ in surface, consisting between forks, located at the base of the seam. The main causes of non-fusion are determined by:
- increased arc length;
- inadequate edge trimming;
- reduced welding current;
- increased welding speed.
The presence of a defect with static joints can be made up for by additional welding. As a result, there is a decrease in strength, there is a concentration of stresses in the zone of non-fusion.
Insufficient fusion of the joint in the welding zone is called lack of penetration. The main reasons are the remains of rust, oxidation, scale, and other adverse effects. As a consequence of the decrease in concentration, the possibility of stresses that adversely affect the structure as a whole increases. In case of deviations from tolerances, areas with an unfinished seam are cleaned to the ground, the welding operation takes place again.
Deviation from the specified parameters of the shape of the weld surface, the geometric state of the joint, is caused by a violation of the shape.
There are various violations, each of which occurs due to certain conditions.
- Undercuts - along the edges of the weld, a defect is formed in the form of recesses with a longitudinal arrangement. It is often formed at an increased welding speed, as a result of which the weld pool hardens faster than expected. The increased distance of the arc contributes to the spread of the width of the seam through the metal, because. with such a scheme, the heat transfer of the arc remains at the same level, the power is not enough to melt the entire metal cavity.
- Excess weld material found on the inside of the weld - excess penetration. Linear displacement defects are a condition in which the parts to be joined are located at different levels, there is a difference in height between the joints. There is an angular type of marriage, in situations where the angle is asymmetric to the butt element.
- Surfacing - 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, increased welding current.
- Burn-through - a through hole formed as a result of leakage of the metal component of the weld pool. Marriage is formed as a result of the use of 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 seam edges formed from the side of the root of the joint. Other inconsistencies are equated to scuffs of the surface type, an accidental arc, metal splashes, and others.
Methods of detection and control
A quality seam has the appropriate marking indicators. At large enterprises, each specialist sets a certain stamp on the joined area. The following methods are used to detect defects:
- visual inspection;
- color flaw detection;
- ultrasonic method for determining defective areas;
- radiation;
- magnetic method.
After a defect is discovered, the further fate of the part is determined by an employee of the quality department, in most cases they are sent for revision. The influxes are removed with the help of an abrasive tool, by mechanical influences. Welding is used for flaw detection of large cracks, with a preliminary cleaned place of residual welding.
Let's proceed to a detailed consideration of defects according to the classification.
Group 1 defects - Cracks
Cracks (100; E)- a defect in the welded joint in the form of a gap in the weld and (or) areas adjacent to it
or
- discontinuity caused by local rupture of the seam, which may occur as a result of cooling or the action of loads ()
Cracks are unacceptable defects, as they are a stress concentrator and a source of destruction. These are the most dangerous defects in a welded joint, often leading to its destruction. They appear as a gap in the weld or in areas adjacent to it. At first, cracks form with a very small opening, but under the action of stresses, their propagation can be commensurate with the speed of sound, resulting in the destruction of the structure.
Most often, cracks appear when welding high-carbon and alloy steels as a result of rapid cooling of the weld pool. The probability of cracking increases with rigid fastening of the welded parts.
crack formation contributes to the increased content of carbon in the molten metal, as well as silicon, nickel and especially harmful impurities of sulfur, phosphorus and hydrogen.
The reasons for the formation of cracks are most often non-compliance with the technology and welding modes. This can manifest itself, for example, in the wrong location of the seams in the welded structure, which leads to a high stress concentration. Large stresses in welded structures can also arise if the specified order of applying welds is not observed.
Crack removal. Surface cracks in welded structures are eliminated in the following order: first, the ends of the crack are drilled so that it does not spread further along the seam, then the crack is removed mechanically or by gouging, after which the defect removal site is cleaned and welded.
Internal cracks (as well as other internal defects) are removed mechanically or by gouging, followed by welding of this area.
By origin, cracks are divided into:
- cold cracks
- hot cracks
cold cracks occur at temperatures below 300 ° C, that is, immediately after the seam has cooled down. In addition, cold cracks can also occur after a long period of time. The reason for the appearance of cold cracks are welding stresses that occur during phase transformations, leading to a decrease in the strength properties of the metal. The reason for the appearance of cold cracks can be dissolved atomic hydrogen, which did not have time to stand out during welding. The causes of hydrogen ingress can be undried seams or welding materials, violations of the protection of the weld pool. Cold fracture cracks have a clean, shiny crystalline appearance.
hot cracks appear in the process of metal crystallization at temperatures of 1100 - 1300 ° C due to a sharp decrease in plastic properties and the development of tensile deformations. Hot cracks appear at the grain boundaries of the crystal lattice. The appearance of hot cracks is facilitated by an increased content of carbon, silicon, hydrogen, nickel, sulfur and phosphorus in the weld metal. Hot cracks can occur both in the weld mass and in the heat-affected zone. Hot cracks can propagate both along and across the seam. They can be internal or surface. Hot cracks at the fracture have a yellowish-orange hue.
According to the size of the cracks are divided into:
- macroscopic
- microscopic
Macroscopic cracks or just cracks (100; E)- visible to the naked eye or through a magnifying glass of small (2-4x) magnification with visual control
Microscopic cracks or microcrack (1001)- a crack of microscopic dimensions, which is detected by physical methods at least at a fifty-fold increase
According to the location of the cracks are divided into:
- longitudinal
- transverse
Longitudinal crack (101; Ea)- crack of the welded joint, oriented along the axis of the weld
The longitudinal crack may be located :
In weld metal (1011)
On the edge of fusion (1012)
In the heat affected zone (1013)
In base metal (1014)
Transverse crack (102; Eb)- a crack oriented across the axis of the weld.
The transverse crack can be located:
In weld metal (1021)
In the heat affected zone (1023)
In base metal (1024)
Gas cavity (200; A)- a cavity of arbitrary shape, without corners, formed by gases trapped in molten metal
or
- a cavity of arbitrary shape, without corners, formed by gases trapped in molten metal
Gas cavities are formed in the weld pool in the form of gas bubbles (hydrogen, nitrogen, carbon monoxide, etc.) that solidify in the metal during metal crystallization during welding.
The difference between a gas cavity and a gas pore in the form i.e. the pore has an almost regular spherical shape, and the gas cavity has the shape as shown in the figure above.
Gas time (2011; Aa) - a discontinuity formed by gases trapped in molten metal. It is usually spherical
or
- gas cavity usually spherical ( - 97)
Uniformly distributed porosity (2012)- a group of gas pores distributed evenly in the weld metal. Should be distinguished from Pore Chain (2014)
Pore Cluster (2013)- a group of gas cavities (three or more), arranged in a heap with a distance between them of less than three maximum dimensions of the largest of the cavities
Pore Chain (2014)- 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 (2015; Ab)- discontinuity, elongated along the axis of the weld. The length of the discontinuity is at least twice the height.
Fistula (2016; Ab)- a tubular cavity in the weld metal caused by gas evolution. 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.
The fistula is formed during accidental short circuits of the tungsten electrode or a sharp break in the arc, as well as as a result of improper arc extinguishing during manual and automatic welding.
A possible reason for the development of a fistula is most often poor-quality surface preparation and filler wire for welding.
The defect is detected visually and is subject to overcooking.
It is possible to correct such a defect only after the complete removal of the weld metal in this area.
Surface pore (2017)- gas pore that breaks the continuity of the weld surface
Shrink shell (202; R)- cavity formed due to shrinkage during curing
Crater (2024; K)- shrinkage cavity at the end of the weld bead, not welded before or during subsequent passes
or
- a defect in the weld, which is formed in the form of depressions in places of a sharp separation of the arc at the end of welding. Shrinkage friability may appear in the depressions of the crater, often turning into cracks.
Craters usually appear as a result of incorrect actions of the welder. During automatic welding, a crater may appear in places where the welding seam breaks. Craters reduce the working section of the weld, that is, reduce its strength. In addition, shrinkage friability can occur in the craters, which contribute to the formation of cracks. Craters are cut down to the base metal, cleaned and welded.
Group 3 defects - Solid inclusions
Such inclusions weaken the weld cross section, reduce its strength and become stress concentration zones.
Places of seams with solid inclusions are cut down to healthy metal or removed by gouging and subsequently welded.
Solid inclusion (300)- solid foreign substances of metallic or non-metallic origin in the weld metal. Inclusions with at least one acute angle are called acute-angled inclusions.
Slag inclusion (301; Va)- slag trapped in the weld metal.
Depending on the conditions of formation, such inclusions can be:
- linear (3011)
- divided (3012
- other (3013)
The slag formed during the melting of the electrode coating or flux always floats to the surface of the weld pool. Slag can remain inside the metal only if the technique and technology of the process is violated (high welding speed, incorrect electrode tilt, poor cleaning of the previously made bead). Most often, slag inclusions remain in the seam as a result of slag leakage during root rolls and deep grooves. Submerged arc welding of girth welds is accompanied by slag inclusions due to non-compliance with the recommended electrode displacement (zenith).
When welding in shielding gases, slag inclusions are rare. Slag inclusions can be up to several tens of millimeters in size and are therefore very dangerous. They reduce the cross section of the weld and lead to stress concentration in it.
The section of the seam, where slag inclusions exceed the permissible norms, is subject to cutting and overcooking.
Flux inclusion (302; g)- flux trapped in the weld metal
Depending on the formation conditions, flux inclusions can be:
- linear (3021)
- disconnected (3022)
- other (3023)
Flux inclusions are formed due to flux that has not reacted with the molten weld metal and has not floated to the surface of the weld. The reason for the formation of flux inclusions is the use of flux with a large granulation, overestimation of the welding speed, accidental entry of flux granules into the weld pool.
Oxide inclusion (303; J)- metal oxide trapped in the weld metal during solidification.
Oxide inclusions are obtained as a result of the formation of sparingly soluble refractory films. Most often they occur due to significant surface contamination or violations of the protection of the weld pool. Also, oxide inclusions can occur in the weld metal due to their low solubility and too rapid cooling.
Being an interlayer in the weld mass, oxide inclusions sharply reduce the strength of the welded joint and can lead to its destruction under the load applied during operation.
Metal inclusion (304, N)- a foreign metal particle that has entered the weld metal
Distinguish metal inclusions from:
- tungsten (3041)
- copper (3042)
- other metal (3043)
Tungsten inclusions occur when the protection of the weld pool is violated when welding with a non-consumable tungsten electrode. In addition, tungsten inclusions occur during short circuits or high current density. Tungsten inclusions are especially common when welding aluminum and its alloys, in which tungsten is insoluble.
Characteristic signs of the formation of tungsten inclusions are crackling closures and a sharp arc flash. In this case, the molten end of the electrode is splashed and enters the molten metal in the form of small (or one large) inclusions. If at the moment of closing the weld metal was sufficiently hardened, the tungsten inclusion will remain on its surface. Most often, the electrode closes when a drop of filler metal is separated during welding of joints in various (inconvenient for welding) spatial positions of the seam. A piece of tungsten separated from the electrode is carried away by the molten filler metal into the weld.
Group 4 defects - Non-fusion and lack of fusion
Non-fusion (401)- lack of connection between the weld metal and the base metal or between the individual weld rolls.
Distinguish non-fusion:
- on the side (4011)
- between rollers (4012)
- at the root of the weld (4013)
Non-fusion is formed during arc welding due to the fact that the arc did not melt part of the joint edge and did not form a seam with its participation.
Most often, non-fusions are formed due to the wrong choice of the shape of the corner and groove, poorly cleaned edge surfaces, due to poor cleaning of the seam between the passes, chemical heterogeneity of the metal, incorrect welding modes (low current, high welding speed).
Lack of penetration (incomplete penetration) (402; D)- 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
or
local violation of fusion between the welded elements, between the weld metal and the base metal, or between the individual layers of the weld in multilayer welding.
Incomplete penetration (lack of penetration) in butt joints can occur in the middle of the section during bilateral welding or at the root of the weld during one-sided welding, both without a lining and on a forming lining, due to its uneven fit.
A characteristic feature of lack of fusion is its ends, which have the form of a crack, the dimensions of which, for example, for the AMg6 alloy, are commensurate with the intergranular distances. Lack of fusion may also be accompanied by the presence of pores and oxide inclusions.
In welded joints that are not sensitive to lack of penetration under static loading, the weakening of the weld cross section can be compensated by reinforcement or penetration. For example, reinforcing a weld at the joints of low-carbon steel pipes with annular lack of penetration along the entire length at the root of the weld under static loads fully compensates for the weakening of the section created by lack of penetration up to 20% of the pipe wall thickness. Welded joints that are not sensitive to lack of penetration under static loads can reduce static strength in sectional or multilayer welding at low temperatures (- 60 - 70 ° C). This is due to reheating, which creates local thermoplastic deformation and aging of the metal. In places of lack of penetration, the margin of plasticity decreases - embrittlement, which leads to a sharp decrease in strength.
As a result of lack of penetration, the weld cross section decreases and local stress concentration occurs, which ultimately reduces the strength of the welded joint. Under vibration loads, even small lack of penetration can reduce the strength of the joint by up to 40%. Large root gaps can reduce strength by up to 70%.
Lack of penetration at the root of the weld occurs when the current strength is insufficient or at an increased welding speed, lack of penetration of the weld edge - when the electrode is displaced from the axis of the joint, lack of penetration between the layers - with poor cleaning of the previous layers, a large amount of deposited metal. Also, the reason for the formation of lack of penetration is poor cleaning of the metal from scale, rust and contamination, a small gap during assembly, large blunting, a small bevel angle, insufficient welding current, high welding speed, and displacement of the electrode from the center of the joint.
Areas with lack of penetration have to be cut down to the base metal, cleaned and brewed again.
Group 5. Violation of the shape of the seam
Form violation (500)- deviation of the shape of the outer surfaces of the weld or the geometry of the joint from the specified value
Defects in the shape and size of welds reduce strength and worsen the appearance of the weld. The reasons for their occurrence in mechanized welding methods are voltage fluctuations in the network, wire slippage in the feed rollers, uneven welding speed due to backlash in the mechanism for moving the welding machine, incorrect electrode tilt angle, liquid metal flow into gaps, their unevenness along the length of the joint, etc. .P. Defects in the shape and size of the seams indirectly indicate the possibility of the formation of internal defects in the seam.
Undercut continuous long (5011; F)- longitudinal depression on the outer surface of the weld bead formed during welding
Undercut intermittent local (5012; F)- longitudinal recess in separate sections on the outer surface of the weld bead
Undercuts lead to a weakening of the cross section of the base metal and local stress concentration under the influence of working loads. In electric arc welding, undercuts occur at increased current and arc voltage, and in gas welding, due to the increased power of the welded flame.
Undercuts are often formed when welding horizontal seams on a vertical plane. In manual arc welding of fillet joints, the cause of undercuts is often an incorrect technique for making seams, in particular, an incorrect position of the electrode in relation to the axis of the seam, especially when working in cramped conditions. Sometimes undercuts are formed on the inner beads of the seams made by argon-arc welding. The reason for their formation may be poor assembly (mixing of edges), inaccurate electrode guidance along the cut.
This defect is detected visually and, in case of deviations above the established norm, it will be necessary to weld thin (thread) seams with electrodes of small diameter.
Shrink groove (5013)- undercut from the side of the root of a one-sided weld, caused by shrinkage along the fusion line
When welding the inner bead, a shrinkage groove is sometimes formed, located along the axis of the seam. It can be eliminated by reducing the volume of the weld pool. To do this, it is necessary to reduce dullness or change the welding mode, increase its speed or reduce the strength of the welding current.
Butt Weld Swell (502)- excess of deposited metal on the front side of the butt weld in excess of the specified value
Excess fillet weld swell (503)- excess of weld metal on the front side of the fillet weld (over the entire length or in the area) in excess of the specified value
During welding, due to incorrect welding modes, as well as for a number of other reasons (low welding speed, uncomfortable spatial position, single-pass welding into a narrow groove), when forming a weld, excess metal crystallizes in the center of the weld pool in the form of a bulge exceeding the allowable values. Excessive bulge in other words is called excess reinforcement of the seam.
Exceeding the convexity is removed mechanically - with a grinding tool.
Excess penetration (504)- excess of weld metal on the reverse side of the butt weld in excess of the specified value
Local excess of penetration (5041)- local excess penetration in excess of the set value
Excess penetration most often occurs due to poor preparation of the welding edges (unequal gap in the joint, different thickness of the metal along the length of the seam) and chemical inhomogeneity of the welded metal.
Incorrect weld profile (505)- angle α between the surface of the base metal and the plane tangent to the surface of the weld, less than the specified value
The reasons for the formation of an incorrect profile of the weld are identical to the reasons for the excess penetration.
Influx (506) (aka influx) Excess weld metal deposited on the surface of the base metal, but not fused with it
They can be local - in the form of individual frozen drops, and also have a significant length along the seam. The reasons for the formation of sagging are high welding current, too long an arc, incorrect electrode tilt, a large angle of inclination of the product when welding downhill, poor cleaning of the edges to be welded. When performing circumferential seams, sagging is formed when the electrode is insufficiently or excessively displaced from the zenith. In places of influx, lack of penetration, cracks, etc. can often be detected.
The influxes are removed mechanically, checking if there is a lack of penetration in these places.
Linear displacement (507)- displacement between two elements to be welded, in which their surfaces are parallel, but not at the required level
Angular offset (508)- displacement between two elements to be welded, in which their surfaces are located at an angle different from the required one
Natek (509)- weld metal that has settled due to gravity and does not have fusion with the surface to be joined.
Depending on the conditions, this may be:
- 5091 sagging with horizontal welding ;
- 5092 leakage in the lower or overhead welding position;
- 5093 leakage in the fillet weld;
- 5094 leakage in the lap joint
Most often, sagging is formed when horizontal welds are made on a vertical plane. The reasons for the formation of sagging and the methods for their elimination are the same as for sagging (sagging).
Burn (510)- outflow of the metal of the weld pool, as a result of which a through hole is formed in the weld
Burn-throughs are most often formed on thin-walled joints or joints with backing strips, rings, when welding is performed at an increased mode or with an increased gap between the edges. In places of burn-through, the metal oxidizes and becomes loose, fragile, loose. If possible, such areas are carefully cleaned until the complete removal of low-quality metal. In places inaccessible for stripping, where burns may appear, when welding the first layer, the reverse side of the seam should be blown with protective gas. Burn-through can occur when the shielding gas supply suddenly stops. When welding rotary annular joints, burns are caused by the incorrect location of the electrode relative to the zenith.
Burn-throughs are a characteristic defect in welding thin-walled products: shells of bellows compensators, pipes of flexible metal hoses, fittings with pipes. In the process of assembling these parts, it is especially important to comply with the requirements for precision machining of mating surfaces and assembly quality. The dimensions of the bath here are so small that the slightest violation in processing or assembly leads to a change in heat dissipation, and therefore to a sharp change in heating. As a result of excessive heating of the edges to be welded, the bath instantly breaks, each edge melts independently and a burn is formed.
Burns are corrected by cutting them out, stripping defective places and brewing.
Incomplete groove filling (511)- longitudinal continuous or intermittent groove on the surface of the weld due to insufficient filler metal during welding
Incomplete filling of the groove occurs when welding modes are incorrectly selected (strength of welding current, welding speed), as well as when the groove is incorrectly selected. This defect can be eliminated after stripping and welding the defective area.
Excessively asymmetric fillet weld (512)- excessive excess of the dimensions of one leg over the other
Excessive asymmetry of the fillet weld is typical when welding metals with different thermal conductivity and inconvenient spatial position of welding.
The following defects do not need explanations. the reasons for the uneven width of the seam, uneven surface, the concavity of the root of the seam is most often in incorrectly selected welding modes, an uncomfortable position during welding, and the wrong choice of cutting edges.
The reasons for the occurrence and methods for eliminating pores in the root of the weld are identical to gas pores, and everything about the renewal defect is clear from the definition.
Uneven seam width (513)- deviation of the width from the set value along the weld
Rough surface (514)- rough irregularity of the shape of the surface of the reinforcement of the seam along the length
Root concavity (515)- a shallow groove on the side of the root of a one-sided weld, formed due to shrinkage
Porosity at the root of the weld (516)- the presence of pores in the root of the weld due to the appearance of bubbles during the solidification of the metal
Resumption (517)- local unevenness of the surface at the place where welding is resumed
Group 6. Other defects
Other defects (600)- all defects that cannot be included in groups 1 to 5
Random arc (601)- local damage to the surface of the base metal adjacent to the weld, resulting from accidental arcing.
Random arc is special, dangerous for stainless steels. may cause corrosion. When welding hardenable steels, an accidental arc can cause cracking.
Metal splash (602)- drops of weld or filler metal formed during welding and adhering to the surface of the hardened metal of the weld or the heat-affected zone of the base metal.
Yes, yes, yes, metal spatter is also a defect (it is especially difficult for novice welders to believe in this). Spatter on the welded metal not only spoils the external (marketable) appearance of the weld, but is also a source of corrosion for stainless steels and a place for cracking for hardened steels.
Tungsten Spray (6021)- particles of tungsten ejected from the molten zone of the electrode onto the surface of the base metal or solidified weld metal
Surface scratches (603)- surface damage caused by the removal of a temporarily welded fixture
The above defects of the 6th group are quite easily correctable, you just need to remove these places by grinding to a “healthy” metal.
Metal thinning (606)- reduction of metal thickness to a value less than permissible during machining
If you overdid it with the removal of defects and suddenly found thinning of the metal, do not be too upset - just surfacing in this place with subsequent machining.
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 for testing 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 connection 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 50 times 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 non-uniformity 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 range;
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
What are weld defects? In fact, these are deviations from the requirements for the technical characteristics of the weld, and, accordingly, the entire structure. It is welding defects that reduce the strength of the seam and the reliability of welding joints. They can be divided into several types.
Types of weld defects:
- deviations from the size and shape of the seam;
- flaws in micro- and macrostructure;
- warping and deformation of structures.
Deviations from the size of the seam and its shape
Dimensional indicators of the weld are determined by state standards. And each type of welding has its own GOST. For example, when welding, where the melting method is involved, weld defects are determined by the uneven fullness of the welded groove, plus the difference in the width and height of the seam along its entire length. As for the shape, it is uneven, there are so-called saddles (hollows), bumps, its structure is scaly.
The reasons for the occurrence of manual welding are the poor quality of the electrodes, the low qualification of the welder, and the violation of the welding technology. The reasons for automatic welding are power surges, the angle of inclination of the electrode feed is incorrectly selected, the filler wire slips in the feed mechanism, and so on.
If we talk about pressure welding, then its defects in welds are deep-type dents, uneven distribution of points along the weld, displacement of workpieces relative to each other can occur.
Shape defects include burns, undercuts, sagging and uncertified craters.
influxes
Typically, such defects in welds are formed when welding workpieces lying in a horizontal plane. And the welding process itself is done from above. An influx is a solidified liquid metal in the form of tubercles, which are formed at the moment of contact of the hot molten metal of the electrode with the cold surface of the workpiece. The sags can be of different sizes: from small drops to large rows, stretching for a decent length of the weld.
The reasons for the appearance of influxes can be a large current supplied to the electrode, a long electric arc, the inclination of the workpiece, and an incorrectly chosen angle of the electrode. As a result, cracks in the weld, lack of penetration and other flaws.
Undercuts
This defect is a groove (recess) in the weld, which is often formed during welding near the metal of the workpiece. The reasons may be a high current and a long arc, which create overheating of the metal itself, as well as the welding filler. It is the state of high temperature that causes the edges of the two workpieces to melt. If corner joints are welded, then most often the causes of undercutting are an incorrectly installed electrode, especially when there has been a shift towards a vertically mounted workpiece. In this case, overheating occurs precisely on the vertical wall of the joint, and here an undercut is formed. But on the horizontal at this time an influx is formed, because the metal begins to flow down.
In gas welding, undercuts can occur for only one reason - increased burner power. It should be noted that undercuts are a rather serious defect in the weld. It leads to a weakening of the workpiece in thickness, and this is the first reason for the destruction of the joint, and, accordingly, the entire welded structure.
burns
The name itself already speaks for itself. Holes are formed along the edges at the welding site and in the metals being welded. Causes:
- large distance between workpieces;
- high current and powerful torch for fast welding;
- irregularly shaped edges, very pointed;
- long duration of the process in one place.
Most often, this type of defect is obtained when thin sheets of metal are welded together, or when multilayer welding is carried out and the first layer is applied.
craters
These are recesses in the weld. Typically, this defect is formed when the arc breaks. Therefore, its experienced welders are trying to melt immediately. This is the simplest elimination of welding defects. When welding is carried out automatically, the crater usually appears at the exit from the seam, that is, on the exit bar.
There is a subspecies of craters called shrinkage shell. It is formed under the influence of shrinkage of the metal in the seam. The thing is that the metal decreases in volume as it cools.
Macrostructural defects
These types of defects in welded joints can be detected by increasing the structure of the weld by 10 times. This type of flaws includes cracks, lack of penetration, gas pores, slag inclusions.
Pores form when the seam cools rapidly. At the same time, the gas-forming elements in his body do not have time to go outside. This happens when the edges of the workpieces are covered with rust, oil or paint stains, a flux with high humidity is used, the welding machine has been incorrectly set up for current or gas, a high carbon content in the metals being welded, and so on.
The pores can be large and small, they can be located in a heap or evenly along the seam, there are through pores, called fistulas. In general, their number and size depend on the time during which the bath is in a liquid state. The longer the weld pool is liquid, the smaller the pores, because the gases have time to leave the liquid metal.
Slag inclusions are, in fact, negligence on the part of the welder when welding. This means that he poorly prepared the two metals to be joined for welding. They were covered in dirt and rust. If this type of defects appeared during multilayer welding, it means that the welder did a poor job of removing slag from the previous layers.
These defects can be several microns or several millimeters in size, and their shape varies from a sphere to a thin line. Location - throughout the body of the seam.
Lack of penetration is a serious defect. It turns out that the metal of the workpiece did not melt with the metal of the electrode (electric welding) or filler wire (gas welding). The layer of deposited metal can not melt with each other. There are many reasons for failure:
- too much welding current was used;
- edge contamination;
- the electrode was incorrectly brought to the axis of the seam;
- very small gap between two workpieces;
- the edges are too pointed ends;
- forced break, during which the metals cool down;
- increased welding speed.
As for cracks, they can be divided depending on the temperature of their appearance. That is, cold or hot. Hot ones appear when the metal solidifies, and crystallization begins at a temperature of 1100-1300C. At the same time, shrinkage stresses appear inside the suture metal, layers of a semi-liquid type begin to form. They eventually become cracks. If the weld metal contains a lot of hydrogen, carbon or silicon, then this is also the cause of hot cracks.
Cold cracks are formed at a temperature of 100-300C. The reasons are all the same stresses that arise in the body of the deposited metal when it begins to cool. In addition, hydrogen (gas) remains inside the weld, which tends to escape. And that's extra stress. By the way, hot cracks on the front of the seam are not visible, they are considered internal. But the cold ones immediately appear on the outside of the seam, they are clearly visible to the naked eye. These are external defects in welds and joints.
There are two more types of cracks: temper and lamellar. The first are formed already when welding is completed and operations are performed for the next metal processing. The latter have a very interesting appearance technology. They are formed even at high temperatures, but their further development is already in the cooled metal. By the way, most often this type of defect is formed from microscopic cracks. Both options belong to the category - an external defect.
Microstructural defects
Microstructural defects include microscopic cracks and pores, inclusions of a non-metallic type (oxygen, nitride), coarse grain structure of the deposited metal with elements of overheating and burnout.
The most dangerous of all these defects is burnout. With it, large grains of the metal structure appear inside the seam in large numbers, which have minimal strength bonds between themselves. Hence the high fragility of the joint. The causes of burnout are the presence of oxygen in the welding zone, which means that the insulation of the bath was poor. The high temperature of the welding process can also be added here.
Permissible and non-permissible defects
It is clear that all defects in welded joints negatively affect the quality of the welded structure. But there are those in which the structure can be operated without problems, and there are those in which it is strictly forbidden to operate it.
Therefore, before determining whether a welded structure can or cannot be operated, it is necessary to take into account all the circumstances and factors influencing the choice.
- It is necessary to determine whether the design corresponds to all geometric and dimensional parameters strictly according to the project or drawing.
- Type of defect, its size and place in the connection.
- What mechanical stresses will the building or structure be subjected to. Will their welds hold up?
- The nature of the environment. Natural loads adversely affect the state of the weld.
- Functions assigned to the design. That is, one defect can withstand certain loads, while others are contraindicated for it.
To determine the admissibility of defects is possible only with special equipment. Therefore, it is recommended to use equipment that, in terms of the degree of defect verification, was higher than the nominal allowable value of the defect itself. For example, a 3 mm crack cannot be measured with an instrument that detects minimum cracks of 5 mm in length.
By the way, the admissibility is affected not only by the size and shape of defects, not the last word for their number and frequency of location.
Conclusion on the topic
Defects in welded joints affect the quality of the joint between the elements of the assembled structure, and hence the entire structure as a whole. Therefore, special attention is paid to the correction of welding defects. They cannot remove themselves. There are flaws that can be easily eliminated, there are flaws that can be eliminated, but not easy. Ways to eliminate them are known. And there are defects that cannot be corrected. So it is better to carry out the process correctly. Therefore, study the processes of the appearance of seams and the reasons for their formation.
