A weld defect is a defect that affects the integrity of a weldment. There are a variety of types of weld defects. They are classified according to ISO 6520, while the permissible limit values are specified according to ISO 5817 and ISO 10042.
According to the American Society of Mechanical Engineers (ASME), the causes of welding defects can be broken down as follows: 45% bad process conditions, 32% operator errors, 12% incorrect technology, 10% bad weld conditions and 5% bad weld seam.
Hydrogen embrittlement is an inclusion of hydrogen inside the metal. This can lead to corrosion starting from the inside, which occurs immediately after welding. Therefore, different thermal methods are often used for prevention. These guide the trapped hydrogen out of the metal.
The stress state created by welding can be roughly calculated by the following forms:
Cracks at ignition points
Cracks at ignition points occur when the welding arc is ignited but the point does not weld. This is because the material is heated above its upper temperature limit and then quenched. This creates martensite , which is brittle and leads to a higher probability of micro-cracks. Normally the arc is ignited above the weld seam so that these types of cracks do not occur, but if the arc is ignited outside the seam it will be necessary to weld over again to prevent cracking. If this is not possible, the weld seam can be subsequently heated using an acetylene-oxygen torch and then slowly cooled down again.
Residual stresses can reduce the integrity of the base material and lead to catastrophic failures due to cold cracking, such as the hulls of some Liberty freighters from World War II . Cold cracking occurs in steel and occurs when martensite is formed when the weld material cools. Hydrogen can accumulate in the crystalline structure of martensite, which increases the internal stress of the material in the heat-affected zone. If this internal stress exceeds a critical level, cold cracking occurs. In order to reduce the welding distortion and the residual stress, the heat supplied should not be too great and the welding sequence should not be from one end to the other, but rather in sections.
Cold cracking occurs when these circumstances coincide:
- vulnerable microstructure (like martensite)
- Hydrogen in the welding material or the atmosphere
- mechanical stresses (thermal or residual stress)
If any of these conditions are eliminated, cold cracking will no longer occur.
End crater crack
End crater cracks occur when a crater is not filled before the weld arc breaks. This causes the outer edges of the crater to cool faster than the crater, which creates enough stress to cause a crack. Longitudinal breaks, transverse cracks or radial cracks can occur.
Cracks on the melt line
The following section is an excerpt from research on the damage behavior of welded joints with defects in the melt line .
Two models for defects in welded joints, with notches and cracks on the melt line, were applied to the influence of the heterogeneity of the mechanical properties and geometric heterogeneity, such as the adaptation to the intensity, the elongation at break of the base metal, speed of work hardening and the width of the welding line to analyze the fatigue distribution and the relationship between damage parameters and breaking stress parameters. In addition, the expansion behavior of cracks in the welded joints was examined in relation to the entire damage zone.
The mechanical properties of the base metal have a major impact on the distribution of damage in the welded joints. Under the conditions of equal loads, the damage value increased with a decrease in the elongation at break and an increased cold deformation exponent and intensity adjustment.
For poorly matched welds, the damage zone of the welds near the groove is shifted from the weld material side to the base material side when the elongation at break decreases and the cold deformation exponent increases. For over coordinated welded joints, the damage zone is shifted from the base material side to the welding material side when the elongation at break increases and the cold deformation exponent decreases.
On the other hand, changing the weld line width has little influence on the damage distribution in welded joints. For the welded joints with cracks on the welding line, when they are under load and the crack extent reaches a certain level, the maximum damage value before the crack increases as long as the cold deformation exponent and the intensity coordination increase and decreases as long as the elongation at break increases.
The introduction of an error principle on the welded structure should take into account the mechanical property of heterogeneity. The numerical calculation of the crack expansion behavior, which is based on the entire damage zone, shows that the crack expansion path parallel to the weld line under the condition of plane stress but under the condition of plane load the angle between the crack extension and the weld line is approximately 45 ° . The numerical calculation and analysis indicate that micro-damage parameters and macro-fracture parameters can both describe stress-strain behavior and that their relationship to one another is based on the heterogeneity of the mechanical properties. The investigation of micro-damage parameters and macro-breakage parameters is undoubtedly a supplement to the assessment of the safety and the expected life span of welded joints, which is of great importance in theory.
The hat crack takes its name from the shape of the cross section of the weld, as the weld material bulges on the surface. The crack starts at the weld line and expands through the weld material. These cracks are usually created by too much tension or too slow a speed.
Hot cracking, also known as freezing cracking, can occur in all metals and occurs in the fusion zone of the seam. To avoid this type of cracking, excess material should be limited and appropriate filler material used. Other causes include excessive welding current, poor joint design that does not dissipate heat, contaminants (such as sulfate and phosphorus ), burns, excessive speed, or welding arcs that are too long.
Undercuts can lead to cracks in the heat affected zone (HAZ). These are cracks that form a short distance from the melting line. These cracks occur in low-alloy steel and high-alloy steel . The exact causes of these cracks have not yet been fully explored, but it is known that dissolved hydrogen must be present. Another factor affecting this type of crack is residual stress resulting from the unequal contraction of the base metal and weld metal, retention of the base metal, stresses from the formation of martensite, and stresses from the precipitation of hydrogen from the metal.
Longitudinal cracks run along the weld bead. There are three forms: "check cracks", "root cracks" and "full centerline cracks". Check cracks are visible on the surface and partly go into the weld. They are usually caused by large shrinkage stresses , especially in final stages, or in hot cracking mechanisms. Root cracks start at the root and partially expand into the weld. They are the most common type of longitudinal crack due to the small size of the first weld bead. If this type of crack is not addressed, it will continue in further welding operations, creating full cracks (cracks from the root to the surface).
Cracks when reheating
Cracks appear during reheating in high-strength, low-alloy steels, especially in chrome , molybdenum and vanadium steel during reheating. This phenomenon has been observed in austenitic stainless steels. It is caused by poor creep ductility in the heat affected zone. Existing flaws or notches increase the crack formation. To prevent it, a two-layer welding technique can be used first with a low temperature unit and then with rapidly increasing temperatures, grinding or shot peening of the weld seam transitions to improve the heat-affected zone.
Cracks at the root and weld seam
A root crack is a crack caused by a short bead at the root (at the joint preparation) at the beginning of welding, caused by low welding current at the beginning of welding and by insufficient filler material for welding. The main reason for these cracks is hydrogen embrittlement . This type of error can be eliminated by using a strong welding current at the beginning and suitable filler material. Seam breaks are caused by moisture in the welding area and can be quickly discovered as part of the surface crack. These errors can be avoided by preheating and forming an appropriate connection.
Transverse cracks run perpendicular to the welding direction. They are generally caused by the longitudinal shrinkage stress that occurs in weld metals with low ductility . Crater cracks occur in craters when the welding arc is terminated too early. Crater cracks are mostly superficial, hot cracks usually form single or star-shaped cracks. These cracks usually start in a crater funnel and extend longitudinally into the crater. However, they can expand into longitudinal cracks in the remainder of the weld.
Welding methods that melt the metal on the sides of the joints often result in shrinkage as the metal cools. The shrinkage then leads to internal stress and distortion . Warpage can be a major problem as the end product will not be in the desired shape. To alleviate some types of warpage, the workpieces can be offset so that they are in the correct shape after welding. The following pictures show different types of weld distortion:
Gas inclusions can cause a variety of defects, such as “porosity” and “ blowholes ”. The basic cause of gas entrapment is the entrapment of gas in the hardened weld. Gas formation occurs due to the following causes: high sulfur content in the workpiece or the electrode , high humidity on the electrode or the workpiece, welding arc that is too short, or incorrect welding current or incorrect electrical polarity .
There are two types of inclusions: “linear inclusions” and “round inclusions”. Inclusions can occur either in isolation or cumulatively. Linear inclusions occur when slag or flux appears in the weld. Slag is created from the use of flux, which is why such defects usually occur in welding processes that use flux, such as gas metal arc welding, cored wire welding, and submerged arc welding, but gas arc welding can also occur. This error usually occurs in welds that require multiple welds and there is poor overlap between the operations. The poor overlap means that the slag from the previous process cannot melt out and can rise to the tip of the new weld bead. It is also possible that the previous process left an undercut or an uneven surface. To prevent the slag from becoming trapped, it should be sanded, brushed off with a wire brush or chipped off between operations.
Defects in connection and root penetration defects
A binding defect is the poor adhesion of the weld bead to the base material; the root penetration defect describes a weld bead that does not start at the root of the weld joint. Root penetration defects form a channel and crevices at the root of the weld joint, which can lead to major problems as corrosive substances can accumulate here. These types of errors can occur when welding processes are not performed properly; The causes can include the application of the welding current, welding arc length, electrode angle or electrode operation. The errors can vary and can be classified as critical or non-critical. Porosity in the weld is usually acceptable to some extent. Inclusions of slag, nicks and cracks are mostly unacceptable. Some porosities, cracks and slag inclusions are visible and do not need to be further inspected to order removal. Small defects can be detected by liquid penetrant testing (dye check). Inclusions of slag and cracks just below the surface can be detected by magnetic inspection. Deeper faults can be detected by radiographic or ultrasonic testing techniques.
Deck cracks are a form of weld failure that occurs in rolled steel plates that have been welded together due to shrinkage forces perpendicular to the ends of the plates. Since the 1970s, changes in manufacturing techniques for reducing sulfur have greatly contributed to minimizing the occurrence of this problem.
Terrace breaks are mostly caused by sulfur inclusions in the material. Another cause can be an excess of hydrogen in the alloy. This error can be reduced by an upper limit of 0.005% sulfur in the alloy. Adding rare elements , zirconium or calcium to the alloy to control the configuration of sulfur inclusions in the metal lattice can also alleviate the problem.
Undercuts occur when the weld reduces the cross-sectional thickness of the base material, which reduces the integrity of the weld and the workpiece. One reason for this error can be excessive welding current, which melts the corners of the joint and runs into the weld; which leaves a channel-like impression along the length of the weld. Another reason is poor technique that doesn't put enough filler material on the edges of the weld. Other reasons are an electrode angle that is too small, a moistened electrode, a welding arc that is too long or too low a welding current speed.
- Howard B. Cary, Scott C. Helzer: Modern Welding Technology. Pearson Education, Upper Saddle River NJ 2005, ISBN 0-13-113029-3 .
- Baldev Raj, R. Jayakumar, M. Thavasimuthu: Practical non-destructive testing. 2nd Edition. Woodhead Publishing, 2002, ISBN 1-85573-600-4 .
- Hoobasar Rampaul: Pipe welding procedures. 2nd Edition. Industrial Press, 2003, ISBN 0-8311-3141-1 .
- Preto Moreno: Welding Defects. 1st edition. Aracne, 2013, ISBN 978-88-548-5854-1 .
- Klas Weman: Welding processes handbook. CRC Press, New York 2003, ISBN 0-8493-1773-8 .
- Understanding Hydrogen Failures ( Memento from January 9, 2015 in the Internet Archive )
- Radiograph Interpretation - Welds
- BS EN ISO 6520-1: Welding and allied processes - Classification of geometric imperfections in metallic materials - Part 1: Fusion welding (2007)
- BS EN ISO 5817: Welding - Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections (2007)
- BS EN ISO 10042: Welding. Arc-welded joints in aluminum and its alloys. Quality levels for imperfections (2005)
- Clifford Matthews: ASME engineer's data book . ASME Press, 2001, ISBN 0-7918-0155-1 , pp. 211 .
- hydrogen brittleness. Retrieved October 28, 2019 .
- Magnitude of stresses generated. Archived from the original on December 6, 2009 ; Retrieved March 16, 2000 .
- Rampaul: Pipe welding procedures. Pp. 207-208.
- Cold cracking. Retrieved May 1, 2015 .
- Cary, Helzer: Modern Welding Technology. Pp. 404-405.
- Raj, Jayakumar, Thavasimuthu: Practical non-destructive testing. P. 128.
- Factors promoting hot cracking. Archived from the original on December 6, 2009 ; Retrieved December 6, 2009 .
- Raj, Jayakumar, Thavasimuthu: Practical non-destructive testing. P. 126.
- Rampaul: Pipe welding procedures. P. 208.
- reheat cracking. Archived from the original on December 7, 2009 ; Retrieved December 6, 2009 . or reheat cracking. Archived from the original on December 7, 2009 ; Retrieved December 6, 2009 .
- Weman: Welding processes handbook. Pp. 7-8.
- Welding Faults and Defects. Archived from the original on December 6, 2009 ; Retrieved March 16, 2000 .
- Defects / imperfections in welds - slag inclusions. Archived from the original on May 16, 2010 ; Retrieved December 5, 2009 .
- Welding Faults and Defects. Archived from the original on December 5, 2009 ; Retrieved December 5, 2009 .
- Rampaul: Pipe welding procedures. P. 216.
- Welding Faults and Defects. Archived from the original on December 4, 2009 ; Retrieved December 3, 2009 .
- Understanding Hydrogen Failures. Archived from the original on January 9, 2015 ; Retrieved December 3, 2009 .
- Vladimir B. Ginzburg, Robert Ballas: Flat rolling fundamentals . CRC Press, 2000, ISBN 0-8247-8894-X , pp. 142 .
- Magnitude of stresses generated. Retrieved December 6, 2009 .
- Rampaul: Pipe welding procedures. Pp. 211-212.