The six major defects in welding—along with their causes, hazards, and preventive measures—are all here.

 

　　I. Surface Defects—Surface defects (also known as appearance defects) are imperfections that can be detected on the surface of a workpiece without the aid of instruments. Common surface defects include undercut, weld bumps, depressions, and welding distortion; sometimes, they also include surface porosity and surface cracks, as well as incomplete root penetration in single-sided welds.

　　A. Edge biting

　　

 

　　It refers to a depression or groove formed along the weld toe in the base metal. This defect arises from the gap left behind when the arc melts the base metal at the weld edge without sufficient replenishment by the filler metal.

　　The primary causes of undercutting are excessively high arc heat—resulting from too high a current and too slow electrode travel speed—as well as improper electrode-to-workpiece angle, unreasonable weaving motion, excessive arc length, and improper welding sequence. In DC welding, magnetic arc blow is also a contributing factor to undercutting. Certain welding positions (vertical, horizontal, and overhead) can exacerbate undercutting. Undercutting reduces the effective cross-sectional area of the base metal, thereby decreasing the structural load-carrying capacity. Moreover, it can lead to stress concentration, which may eventually develop into crack initiation sites.

　　Prevention of undercutting: Correcting operational posture, selecting appropriate welding parameters, and adopting proper electrode movement techniques can all help eliminate undercutting. When welding fillet joints, using AC welding instead of DC welding can also effectively prevent undercutting.

　　B. Weld Bead

　　

 

　　A weld bead is a metal buildup that forms when liquid metal from the weld pool flows onto the base metal that has not been sufficiently heated and remains unmelted, or overflows from the root of the weld. After cooling, this buildup does not fuse with the base metal. Excessive welding parameters, rapid electrode melting, poor electrode quality (such as eccentricity), unstable characteristics of the welding power source, and improper working posture can all easily lead to the formation of weld beads. Weld beads are particularly prone to forming in horizontal, vertical-up, and overhead positions.

　　Weld beads are often accompanied by lack-of-fusion and slag-inclusion defects, which can easily lead to cracking. At the same time, weld beads alter the actual dimensions of the weld seam, causing stress concentrations. Inside pipes, weld beads reduce the inner diameter, potentially leading to blockages in fluid flow.

　　Measures to prevent weld bumps: Position the weld seam in the flat welding position, select appropriate welding parameters, use non-eccentric electrodes, and perform operations properly.

　　C. Pit

　　

 

　　A pit refers to a localized area on the surface or backside of a weld that is lower than the base metal.

　　Pits are often caused by the welding rod (or wire) not being briefly paused at the arc termination point (such pits are referred to as arc pits). During vertical-up and horizontal welding, concave defects frequently occur at the root of the weld bead on the back side. Pits reduce the effective cross-sectional area of the weld, and arc pits often contain arc-pit cracks and arc-pit shrinkage cavities.

　　Measures to prevent crater formation: Select a welding machine equipped with a current-decay system; whenever possible, weld in the flat position; use appropriate welding parameters; and when ending the arc, allow the electrode to briefly remain within the molten pool or perform a circular oscillating motion to fully fill the arc crater.

　　D. Underfilled

　　

 

　　Underfill refers to continuous or intermittent grooves on the weld surface. Insufficient filler metal is the root cause of underfill. A specification that is too lenient, electrodes that are too thin, and improper electrode movement can all lead to underfill.

　　Lack of fusion also weakens the weld, making it prone to stress concentration. Meanwhile, due to the excessively weak specifications, the cooling rate increases, which can easily lead to porosity and cracks.

　　Measures to prevent insufficient weld penetration: Increase the welding current and add a cover pass weld.

　　E. Burn-through

　　

 

　　Burn-through refers to a defect in the welding process where the penetration depth exceeds the thickness of the workpiece, causing molten metal to flow out from the back side of the weld and form a through-hole.

　　Excessive welding current, too slow a welding speed, and prolonged arc dwell time at the weld seam can all lead to burn-through defects. If the workpiece gap is too large or the root face is too small, burn-through is also likely to occur.

　　Burn-through is a defect that is not permitted in boiler and pressure vessel products. It completely destroys the weld seam, causing the joint to lose its bonding strength and load-bearing capacity.

　　Preventive and control measures: Select a lower current and adopt an appropriate welding speed; reduce the assembly gap; add backing plates or flux pads on the reverse side of the weld; and use pulse welding—all of which can effectively prevent burn-through.

　　F. Other Surface Defects

　　(1) Poor forming refers to the weld seam’s external geometric dimensions failing to meet the requirements. This includes excessive weld height, an uneven surface, excessively wide weld seams, and an abrupt or unsmooth transition from the weld seam into the base metal.

　　(2) Misalignment refers to the displacement of two workpieces along their thickness direction by a certain amount. It can be regarded both as a weld surface defect and as an assembly/formation defect.

　　(3) Collapse: During single-sided welding, excessive heat input leads to over-melting of the metal, causing the molten metal to collapse toward the back side of the weld. After solidification, the back side of the weld appears raised, while the front side sinks down.

　　(4) Surface porosity and arc crater shrinkage.

　　(5) Various types of welding deformation, such as angular distortion, warping, and wave distortion, all fall under the category of welding defects. Angular distortion also qualifies as an assembly and forming defect.

　　II. Porosity and Slag Inclusions

　　A. Stomata

　　

 

　　A porosity defect refers to a cavity formed in the weld seam when gases within the molten pool fail to escape before the metal solidifies and remain trapped inside the weld. These gases may be absorbed from the surrounding environment by the molten pool, or they may be generated through chemical reactions during the welding metallurgical process.

　　(1) Classification of stomata

　　According to their shape, pores can be classified as spherical pores or worm-like pores. In terms of quantity, they can be divided into single pores and clustered pores. Clustered pores, in turn, can be further categorized into uniformly distributed pores, densely packed pores, and chain-like distributed pores. Based on the gas composition within the pores, they can be classified as hydrogen pores, nitrogen pores, carbon dioxide pores, carbon monoxide pores, oxygen pores, and so forth. Among fusion welding pores, hydrogen pores and carbon monoxide pores are most common.

　　(2) Mechanism of stomatal formation

　　The solubility of gases in solid metals at room temperature is only a few dozen to a few hundred times lower than that in liquid metals at high temperatures. During the solidification process, a large amount of gas must escape from the molten metal pool. When the solidification rate exceeds the rate at which gas can escape, porosity forms.

　　(3) The main causes of porosity

　　If the base metal or filler metal surfaces are contaminated with rust, oil, or other impurities, and if the welding electrodes and fluxes are not properly dried, the number of porosity defects will increase. This is because rust, oil, and moisture contained in the electrode coating and flux decompose into gases at high temperatures, thereby increasing the gas content in the molten metal. Additionally, if the welding heat input is too low, the weld pool cools rapidly, which hinders the escape of gases. Insufficient deoxidation of the weld metal can also lead to oxygen porosity.

　　(4) The Harm Caused by Stomata

　　Porosity reduces the effective cross-sectional area of the weld, making the weld porous and thereby decreasing the strength and ductility of the joint, and may also lead to leakage. Porosity is also a factor that causes stress concentration. Hydrogen porosity can further promote the formation of cold cracks.

　　(5) Measures to Prevent Porosity

　　a. Remove oil, rust, moisture, and debris from the welding wire, the work bevel, and the surfaces nearby.

　　b. Use alkaline electrodes and fluxes, and dry them thoroughly.

　　c. Use reverse polarity DC and employ a short arc for welding.

　　d. Preheat before welding to slow down the cooling rate.

　　e. Weld using slightly stronger specifications.

　　B. Slag Inclusion

　　

 

　　Slag inclusion refers to the phenomenon in which residual slag remains in the weld after welding.

　　(1) Classification of Slag Inclusions

　　a. Metal inclusions: Refers to metal particles such as tungsten and copper remaining in the weld seam; these are commonly referred to as tungsten inclusions or copper inclusions.

　　b. Non-metallic slag inclusions: Refers to residues such as unfused flux coating or welding flux, sulfides, oxides, and nitrides remaining within the weld seam. This indicates incomplete metallurgical reactions and poor slag removal performance.

　　(2) Distribution and shape of slag inclusions

　　There are single-point inclusions, strip-like inclusions, chain-like inclusions, and dense inclusions.

　　(3) Causes of slag inclusion

　　a. The bevel dimensions are unreasonable; b. There is contamination in the bevel; c. During multi-layer welding, slag removal between layers is incomplete; d. The welding heat input is too low; e. The weld pool cools too rapidly, causing the liquid metal to solidify too quickly; f. The chemical composition of the electrode coating and flux is inappropriate, with a melting point that is too high; g. In tungsten inert gas (TIG) welding, the power supply polarity is incorrect, the current density is too high, and the tungsten electrode melts and falls into the weld pool; h. During manual welding, improper electrode oscillation hinders the upward flotation of slag. Corresponding measures can be taken based on the above-mentioned causes to prevent the formation of slag inclusions.

　　(4) Hazards of slag inclusion

　　The hazards of spotted slag inclusions are similar to those of porosity. Slag inclusions with sharp corners can cause stress concentration at their tips, and these tips may even develop into crack sources, posing a significant hazard.

　　III. Cracks

　　

 

　　A crack is a gap formed when the atomic bonds in a weld are disrupted, creating a new interface.

　　A. Classification of Cracks

　　According to crack size, they are classified into three categories:

　　(1) Macrocracks

　　Cracks visible to the naked eye.

　　(2) Microcracks

　　It can only be detected under a microscope.

　　(3) Ultramicroscopic cracks

　　They can only be detected under high-magnification microscopy and generally refer to intergranular cracks and intragranular cracks.

　　From the perspective of generation temperature, cracks are divided into two categories:

　　(1) Hot cracking

　　Cracks that originate near the Ac3 line generally appear immediately after welding is completed; they are also known as solidification cracks. These intergranular cracks primarily occur along grain boundaries, and the crack surfaces exhibit an oxidized coloration, losing their metallic luster.

　　(2) Cold Cracks: These are cracks that occur after welding, when the temperature has cooled below the martensite transformation temperature M3. They typically appear some time after welding—hours, days, or even longer—and are therefore also referred to as delayed cracks.

　　According to the cause of crack formation, cracks can also be classified as:

　　(1) Reheat cracking

　　Cracks that occur when the joint is cooled and then reheated to 500–700℃. Reheat cracks typically form in the coarse-grained zone of the weld heat-affected zone in precipitation-hardened materials (such as metals containing Cr, Mo, V, Ti, and Nb). These cracks generally propagate from the fusion line toward the coarse-grained zone of the heat-affected zone and exhibit intergranular cracking characteristics.

　　(2) Layered tearing

　　This is mainly due to the fact that during the rolling process, impurities such as sulfides (MnS) and silicates become entrapped within the steel, resulting in anisotropy. Under the influence of welding stresses or external restraining stresses, these impurities along the rolling direction tend to crack.

　　(3) Stress Corrosion Cracking

　　Cracks that arise under the combined action of stress and a corrosive medium. Aside from factors such as residual stress or constraint stress, stress-corrosion cracks are primarily related to the microstructure and morphology of the weld seam.

　　B. The hazards of cracks

　　Especially cold cracks can cause catastrophic damage. Among pressure vessel accidents worldwide, with the very rare exception of those caused by improper design or inappropriate material selection, the vast majority are due to brittle failure triggered by cracks.

　　C. Hot Cracking (Crystallization Cracking)

　　(1) Formation mechanism of solidification cracks

　　Hot cracks occur during the final stages of solidification of weld metal, with the sensitive temperature range lying roughly in the high-temperature region near the solidus line. The most common type of hot crack is the crystallization crack. The underlying cause of these cracks is that, during the solidification process of weld metal, compositional segregation leads to the enrichment of low-melting-point eutectics formed by impurities at grain boundaries, creating what is known as a "liquid film." In the specific sensitive temperature range—also referred to as the brittle temperature range—the strength of this liquid film is extremely low. As the weld metal contracts during solidification, it experiences tensile stresses, ultimately resulting in cracking and the formation of hot cracks.

　　The most common type of solidification crack occurs along the length of the weld center, forming a longitudinal crack. Sometimes, it also appears between two columnar crystals within the weld, resulting in a transverse crack. Arc-crater cracks represent another form of commonly encountered hot cracks.

　　Thermal cracks always propagate along grain boundaries and typically occur in gas-welded joints of materials such as carbon steel with high impurity content, low-alloy steel, and austenitic stainless steel.

　　(2) Factors affecting crystallization cracks

　　The effects of alloying elements and impurities: An increase in carbon content as well as in impurity elements such as sulfur and phosphorus will widen the sensitive temperature range, thereby increasing the likelihood of solidification cracks.

　　b. The effect of cooling rate: As the cooling rate increases, first, compositional segregation during crystallization is exacerbated; second, the temperature range for crystallization widens. Both of these factors increase the likelihood of the occurrence of solidification cracks.

　　c. The effects of solidification stress and constraint stress: Within the brittle temperature range, the strength of the metal is extremely low, and welding stresses further subject this portion of the metal to tensile stress. When the tensile stress reaches a certain level, solidification cracks will occur.

　　(3) Measures to Prevent Crystallization Cracks

　　a. Reduce the content of harmful elements such as sulfur and phosphorus, and use materials with lower carbon content for welding. b. Add certain alloying elements to reduce columnar crystals and segregation. For example, elements like aluminum, vanadium, iron, and zirconium can refine the grain structure. c. Use welds with shallower penetration depth to improve heat dissipation, allowing low-melting-point substances to float to the surface of the weld and avoid being trapped within the weld bead. d. Select appropriate welding parameters and employ preheating and post-heating to slow down the cooling rate. e. Adopt a reasonable assembly sequence to minimize welding stresses.

　　D. Reheat Cracking

　　(1) Characteristics of reheat cracking

　　a. Reheat cracks originate in the overheated coarse-grained zone of the weld heat-affected zone. They develop during subsequent heating processes, such as post-weld heat treatment.

　　b. Temperature at which reheat cracks occur: For carbon steel and alloy steel, it’s 550–650℃; for austenitic stainless steel, it’s approximately 300℃.

　　c. Reheat cracks are intergranular cracks (intergranular cracking).

　　d. Most likely to occur in steels strengthened by precipitation.

　　e. Related to residual stresses from welding.

　　(2) Mechanism of the formation of reheat cracks

　　a. There are several explanations for the formation mechanism of reheat cracking. Among them, the model cracking theory offers the following explanation: Under the influence of high-temperature thermal cycling, strengthening phases—such as carbides (e.g., iron carbide, cementite, chromium carbide, and zirconium carbide)—deposit in the dislocation zones within the grains in the near-weld zone. As a result, the strength enhancement within the grains becomes significantly higher than that at the grain boundaries. Particularly when these strengthening phases are uniformly dispersed throughout the grains, they impede local adjustments within the grains and also hinder the overall deformation of the grains. Consequently, the plastic deformation resulting from stress relaxation is primarily borne by the metal at the grain boundaries. This leads to stress concentration at the grain boundaries, ultimately causing cracks—hence the term "model cracking."

　　(3) Prevention of Reheat Cracking

　　a. Pay attention to the strengthening effect of metallurgical elements and their impact on reheat cracking. b. Perform reasonable preheating or apply post-weld heat treatment to control the cooling rate. c. Reduce residual stresses and avoid stress concentrations. d. During tempering, try to avoid the temperature range where reheat cracking is most sensitive, or shorten the dwell time within that temperature range.

　　E. Cold Cracks

　　(1) Characteristics of cold cracks

　　a. They originate at lower temperatures and appear some time after welding, which is why they are also referred to as delayed cracks. b. They mainly occur in the heat-affected zone, though they can also appear in the weld zone. c. Cold cracks may exhibit intergranular cracking, transgranular cracking, or a combination of both. d. The failure of components caused by cold cracks is typically brittle fracture.

　　(2) Mechanism of cold crack formation

　　a. The hardened microstructure (martensite) reduces the metal’s plastic reserve. b. Residual stresses in the joint cause tensile stress in the weld. c. There is a certain hydrogen content within the joint.

　　Hydrogen content and tensile stress are two key factors that contribute to the formation of cold cracks (here referring to hydrogen-induced cracks). Generally, the arrangement of atoms within a metal is not perfectly ordered; rather, it contains numerous microscopic defects. Under the action of tensile stress, hydrogen diffuses and accumulates in regions of high stress—specifically, at defect sites. When the hydrogen concentration reaches a certain threshold, it disrupts the atomic bonds within the metal, giving rise to microscopic cracks. As the stress persists and hydrogen continues to accumulate, these microscopic cracks gradually expand, eventually evolving into macroscopic cracks and ultimately leading to fracture. There exists a critical hydrogen content and a critical stress value that determine whether cold cracks will form. If the hydrogen concentration within the joint remains below the critical hydrogen content or if the applied stress falls below the critical stress level, no cold cracks will occur (meaning the delay time becomes infinitely long). Among all types of cracks, cold cracks pose the greatest hazard.

　　(3) Measures to Prevent Cold Cracks

　　a. Use low-hydrogen alkaline electrodes, dry them thoroughly, and store them at 100–150°C, taking out and using them as needed.

　　b. Increase the preheating temperature, implement post-weld heat treatment measures, and ensure that the interpass temperature is no lower than the preheating temperature. Select appropriate welding parameters to prevent the formation of hard martensitic structures in the weld seam.

　　c. Select a reasonable welding sequence to minimize welding deformation and welding stress.

　　d. Perform post-weld hydrogen removal heat treatment promptly.

　　IV. Lack of Penetration

　　

 

　　Lack of fusion refers to the phenomenon in which the base metal remains unmelted and the weld metal fails to penetrate into the root of the joint.

　　A. Causes of incomplete fusion

　　(1) The welding current is low, resulting in shallow penetration.

　　(2) The bevel and gap dimensions are unreasonable, and the root face is too large.

　　(3) Influence of magnetic deflection.

　　(4) The electrode is excessively eccentric.

　　(5) Poor cleaning of interlayer and weld root.

　　B. Hazards of Incomplete Weld Penetration

　　One of the hazards of incomplete fusion is that it reduces the effective cross-sectional area of the weld, thereby decreasing the joint’s strength. Secondly, the damage caused by stress concentrations resulting from incomplete fusion is far more serious than the mere reduction in strength. Incomplete fusion significantly lowers the fatigue strength of the weld. Incomplete fusion can serve as a crack initiation site and is a major cause of weld failure. The damage caused by stress concentrations due to incomplete fusion is far more severe than the damage caused by reduced strength. Incomplete fusion significantly lowers the fatigue strength of the weld. Incomplete fusion can serve as a crack initiation site and is a major cause of weld failure.

　　C. Prevention of incomplete fusion

　　Using a higher welding current is a fundamental method for preventing incomplete penetration. Additionally, when welding fillet welds, switching from direct current to alternating current can help prevent magnetic blow. Properly designing the groove geometry, thoroughly cleaning the joint surfaces, and employing techniques such as short-arc welding can also effectively prevent the occurrence of incomplete penetration.

　　V. Lack of Fusion

　　

 

　　Lack of fusion refers to a defect in which the weld metal has not been fully melted and fused with the base metal or between weld metals themselves. Depending on their location, lack of fusion can be classified into three types: groove lack of fusion, interpass lack of fusion, and root lack of fusion.

　　A. Causes of Lack-of-Fusion Defects: (1) Insufficient welding current; (2) Excessive welding speed; (3) Incorrect electrode angle; (4) Arc blow phenomenon; (5) Welding performed in a downhill position, where the base metal is already covered by molten metal before it has time to melt; (6) Presence of contaminants or oxides on the surface of the base metal, which interfere with the fusion bonding between the weld metal and the base metal.

　　B. Hazards of Lack of Fusion: Lack of fusion is an area-type defect. Both groove lack of fusion and root lack of fusion significantly reduce the effective cross-sectional area available to carry loads, and they also cause severe stress concentrations. Their harmfulness ranks second only to that of cracks.

　　C. Prevention of Lack of Fusion: Use a larger welding current, perform welding operations correctly, and ensure that the bevel areas are kept clean.

　　6. Other Defects (1) Weld Seam Chemical or Microstructural Composition Does Not Meet Requirements: Improper matching between welding materials and base metals, or elemental burn-off during the welding process, can easily cause changes in the chemical composition of the weld metal or result in a microstructure that fails to meet the specified requirements. This may lead to a deterioration in the mechanical properties of the weld seam and also affect the corrosion resistance of the joint.

　　(2) Overheating and Burnout: If welding parameters are improperly selected or if the heat-affected zone remains at high temperatures for an extended period, the grain size will coarsen, resulting in an overheated microstructure. If the temperature rises further and the exposure time is prolonged, the grain boundaries may oxidize or even partially melt, leading to a burnout microstructure. Overheating can be eliminated through heat treatment, whereas burnout is an irreversible defect.

　　(3) White spots: White spots resembling fish eyes that appear on the fracture surface of weld metal are known as F-white spots. These white spots are caused by hydrogen accumulation and pose a significant hazard.