Defects in Welding – What Is Piping: All about Piping Engineering

Welding defects can be defined as an unacceptable imperfection. Any unacceptable deviation with respect to set technical and design requirements in the welding process is termed as welding defects. Various parameters could cause the deviation from the ideal welding process as defined by codes and standards. Wrong welding, human behavior, wrong electrode, poor process condition, improper job preparation, unskilled welder, incorrect weld parameters, etc could be a few of such reasons. Defects in Welding can occur at any stage of the welding process and they can easily be detected in forms of geometric imperfections. Such weld defects can affect both the inside and outside of the metallic structure. Various codes and standards like BS EN ISO 6520-1, BS EN ISO 5817, BS EN ISO 10042 provides the acceptable limits for welding irregularities. When any discontinuity exceeds those acceptable code limits they are considered as welding defects. It is important that a welding defect is correctly identified so the cause can be established and actions are taken to prevent further occurrence. Types of Welding Defects / Welding Defect Types Depending on the location of the Welding defects, they are classified into two groups: External Welding Defects andInternal Welding Defects. External welding defects are found on the surface itself and can sometimes be recognized by naked eyes. Surface Cracks, Undercut, Porosity, Overlaps, Crater, Underfill, Spatters, Excessive penetration, Arc Strike, etc are examples of external welding defects. Internal welding defects exist in the material at some depth and are hidden from naked eyes. Incomplete penetration, Slag inclusion, Internal porosity, Internal crack, Incomplete fusion, Internal Blowholes, etc are examples of internal welding defects. In the welding process, both internal and external types of welding are quite common. In the next paragraphs, we will explore these welding defects in Detail. Welding Defects Crack Welding defects Crack is an imperfection produced by a local rupture in the solid state arising from the effect of cooling or stresses. Welding Cracks are more significant than other types of welding defects as their geometry produces a very large stress concentration at the crack tip making them prone to cause fracture. Welding cracks are normally found in the weld metal, parent metal or the Heat Affected one. Welding Defect cracks can be of various types like: Longitudinal.Transverse.Radiating.Crater (found only in the weld metal)Branching. Welding defect cracks can be of various shapes and sizes and may appear on the surface or at any depth or even at the root. The main reason for crack is the localized stress exceeds the UTS of that material. Depending on their nature, these welding cracks can be: Hot cracksCold cracks Lamellar tearing. Hot Cracks: Hot cracks normally occur during the solidification stage; means soon after welding. Depending on their location and mode of occurrence, hot cracks can be of two types: Solidification cracks: They occur in the weld metal during the solidification process.Liquation cracks: They occur in the coarse grain HAZ, as a result of elevated temperature heating of the material that causes liquation of the low melting point constituents on the grain boundaries. Fig. 1: Welding Defects-Solidification Hot Crack Solidification cracking (Fig. 1) normally occur when: Weld metal possesses high carbon or impurity content.The solidifying weld bead has a larger depth-to-width ratio (deep and narrow).Heat Flow disruption occurs. The cracks can be wide and open and possibly narrow. Solidification cracking occurs in compositions having a wide freezing temperature range. For steels, solidification cracks occur due to the presence of high carbon content and impurity elements like sulfur and phosphorus. During solidification, these elements segregate such that intergranular liquid films remain once the bulk of the weld has solidified. The thermal shrinkage of the cooling weld bead can cause these to rupture and form a crack. Cold Cracks: Cold cracks occur after the weld metal solidification in the grain coarsened region of the HAZ. They are also known as delayed cracking as they can normally develop after several days of welding. It lies parallel to the fusion boundary and its path is usually a combination of inter and transgranular cracking. The direction of the principal residual tensile stress can result in toe cracks cause the crack path to grow progressively away from the fusion boundary towards a region of lower sensitivity to hydrogen cracking. When this happens, the crack growth rate decreases and eventually arrests. Fig. 2: Welding Defects-Cold Crack The main causes of Cold cracks (Fig. 2) are Lack of preheating, Low temperature, high stresses, susceptible material structure, high hydrogen content, etc. Lamellar tearing: Lamellar tearing normally occurs in rolled steel plates. They are distinguished by the cracking feature having a terraced appearance. Lamellar Cracking occurs in joints where: A thermal contraction strain can occur in the through-thickness direction of the steel plate.The possibility of non-metallic inclusions as very thin platelets with their principal planes parallel to the plate surface is high. Fig. 3: Welding Defect-Lamellar tearing Welding Defects Undercut Undercuts are welding defects that present themselves as irregular narrow grooves on the parent metal. They are characterized by their depth, length, and sharpness. The undercut runs parallel to the weld metal and acts as a stress raiser during fatigue loading due to the weakened section. Welding defects Undercut is of three types; Continuous undercut, Intermediate undercut, and Inter-run undercut. Causes of Undercuts Probable causes of undercut welding defects are Melting of the top edge due to fast weld speed or high voltage.High arc voltage.Too large electrode; Incorrect electrode angle.Use of wrong filler metal.Incorrect shielding gas selection.Excessive weaving Fig. 4: Welding Defect-Undercut Remedies/Prevention of Welding undercut The following steps should be exercised to prevent the possibility of undercut welding defects. Decrease the travel speed, Reduce the power input.Use the right electrode size with correct positioning; between 30 to 45 degrees angle.Reduce the length of the arc and lower the voltage.Weld in flat positions.Use proper current with attention to thinner areas and edges.Use the correct gas mixture based on material type and thickness. Heat input must be controlled during weld repairs of undercut welding defects. Welding Defects Porosity Welding Defect Porosity is caused by entrapment of gas or air bubbles in the weld metal. These trapped gases collapse over time and weaken the weld section. They can be localized or uniformly distributed. Depending on the Porosity formations they are of various types like: Gas PorosityWorm HolesSurface Porosity Gas Porosity: Gas porosity is a small cavity of spherical shape generated in the weld metal due to the trapped gases. They are of various forms like Isolated; Uniformly distributed porosity, Surface pore, Localized Clustered porosity; Elongated cavity, or Linear porosity. Worm Holes: Sometimes, during solidification of the trapped gas can form tubular or elongated cavities which are known as Wormholes. They can appear singly or in groups over the weld surface. Progressive entrapment of gas between the solidifying metal crystals causes wormholes. Surface Porosity: Porosities that break the surface are known as surface porosities. They are similar to uniform porosities. Fig. 5: Welding Defects-Porosity Causes of Porosity Major causes of welding defect porosity are: Electrode is not well coated / Corroded ElectrodePresence of oil, grease, hydrocarbon, water, or rust on the weld surface.Use of incorrect shielding gas or improper shielding or air entrapment.Too high gas flow/ Too great arc voltageGas evolution due to improper surface treatment. Prevention or Remedies of Porosity: To reduce the formation of welding defect porosity the following remedial actions can be taken: Clean the weld surface and materials; Ensure that the prepared surface is free from oil, rust, or other contaminants.Use of dry, good quality electrodes.Optimize the welding process to allow gases to escape.Gas flow meter to be configured with the correct flow settings. Welding Defects Overlap Welding defect overlap occurs when the weld pool overflows on the welding surface of the parent metal. In such scenario, the molten metal does not fuse with the base metal. Causes of Overlaps: Large weld deposition in one go.Poor electrode manipulation; Wrong electrode coating.Using the electrode at the wrong angle.High current/Heat Input.Incorrect Weld positioning.Longer arc. Fig. 6: Welding Defects-Overlap Remedies of Overlap: Use correct welding technique to avoid wrong arc length.Position the electrode at the appropriate angle.Change to flat position.Use correct deposition during each run; Correct Electrode CoatingUse low welding current; Low heat input. Welding Defects Solid Inclusions Various solid particles may be included during the welding process. These welding defects are known as solid inclusions. They can be of various types as listed below: Slag inclusionOxide inclusionFlux inclusionMetallic inclusionTungstenCopperOther Metal Fig. 7: Welding Defects-Slag Inclusion Welding Defect Lack of Fusion Welding defect lack of fusion arises due to incomplete fusion between the weld metal with the parent metal. Lack of fusion is also known as Cold lapping or cold shuts. Lack of fusion is an internal welding defect, but it can occur on the external surface too. Lack of fusion can be categorized into three groups: Lack of sidewall fusionLack of inter-run fusion andLack of root fusion Fig. 8: Welding Defects-Lack of Fusion Causes for welding defect Incomplete or Lack of fusion: The following parameters can contribute to the lack of fusion welding defect: Low heat input/Low Arc currentWrong electrode diameter with respect to the material thickness.High travel speed.Large Weld Pool.Improper bead placement.Oxide or Scale in weld preparation.Large Root Face/Small root gap/Excessive root misalignment Remedies/Prevention of Lack Of Fusion:; To prevent or reduce the possibility of incomplete fusion the following steps can be followed: Reduce travel speed.Appropriate bead positioning.Maintaining Correct root gaps.Increase current or heat input.Inprove edge preparation. Welding Defect Lack of Penetration When the weld metal doesn’t completely penetrate the joint, It creates the weld defect known as lack of penetration or incomplete penetration. As the weld depth is not sufficient, this zone will be highly stressed and can fail easily. They are of two types; Incomplete penetration and Incomplete root Penetration. Fig. 9: Welding Defects-Lack of Penetration Causes behind Lack of Penetration welding defects: Excessive thick root face.Root gap too smallFast travel speedUse of vertically down weldingLow heat inputToo large electrode Prevention of welding defect Incomplete Penetration: Reduce Electrode SizeProper joint preparation i.e. providing a suitable root gap.Proper heat inputCorrect travel speedVertical up procedure Welding Defects Spatter Spatters as welding defects are small globular weld metal droplets expelled during welding process and stuck to the base metal surface. Reasons For Spatter: High Welding current can cause this defect.Damp Electrodes.The longer the arc the more chances of getting this defect.Magnetic Arc Blow.Incorrect polarity.Improper gas shields may also cause this defect. Fig. 10: Welding Defects-Spatter Remedies for Spatter: Reduce the arc length and welding currentUse dry electrodes.Using the right polarity and according to the conditions of the welding.Use of AC power.Increasing the plate angle and using proper gas shielding. However, Spatter does not affect the weld integrity and can be easily removed by brushes. Other Welding defects There are few other welding defects as mentioned below: Shrinkage cavityBlow-HolesWarpageExcessive penetrationExcess weld MetalBurn-ThroughLinear MisalignmentIrregular WidthRoot ConcavityStray ArcTorn SurfaceGrinding MarkUnder-flushingChipping Mark, etc Fig. 11 shows few of those welding defects. Fig. 11: Various Welding defects