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Inspecting Welds, Part 2 More on defects and how to find them Ron Alexander & Scott Helzer

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n July’s discussion of welding defects and discontinuities we looked at cracks and incomplete fusion and what they mean to the integrity of a weld joint. However, there are other common problems you will need to spot as you inspect your welds. It is important that you examine your work carefully for any defective weld. Depending on the nature of the joint, the metals involved, and the kinds of stresses the joint will experience, a defect may or may not be serious.

Inclusions An inclusion is a foreign metallic or nonmetallic solid material trapped in a weld. As the name implies, slag inclusions are regions within the weld cross section or at the weld surface where the molten flux used to protect the molten metal is trapped within the solidified metal. This represents a portion of the weld’s cross section where the metal is not fused to itself, which can result in a weak weld that impairs the component’s serviceability. Although slag inclusions are normally contained 94

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totally within the weld cross section, they can sometimes be observed at the surface of the weld as well. Other inclusions can result if the tungsten electrode in a gas tungsten arc welding (GTAW) torch touches the molten weld puddle. The arc can extinguish, and the molten metal can solidify around the electrode’s tip. Removing the electrode will likely break off the tip, leaving it embedded in the weld unless removed by grinding. Tungsten inclusions can also result if the current being used for GTAW is in excess of that recommended for a particular electrode diameter. In such a case, the electrode may begin to decompose, leaving pieces deposited in the weld metal. Decomposition can also occur if the welder does not properly grind the point on the tungsten electrode. If the grinding marks are oriented such that they form rings around the electrode instead of being aligned with its axis, they could form stress risers that would cause

the tip of the electrode to break. Other reasons tungsten inclusions might occur include contact of filler metal with the hot tip of the electrode, electrode tip contamination from spatter, extension of electrodes beyond their normal distances from the collet, inadequate tightening of the collet, inadequate shielding gas flow rates or excessive wind drafts resulting in oxidation of the electrode tip, use of improper shielding gas, and defects such as splits or cracks in the electrode. Creating inclusion-free welds is not that difficult, provided you take care to avoid introducing any of these potential defects into the welding process.

Porosity Porosity results when the solidified weld metal contains gas pockets or voids. Because most such voids are spherical, porosity is normally considered to be the least detrimental discontinuity. However, if the weld must form some pressure boundary to contain a gas or liquid—such as a fuel tank or other pressurized struc-

ture—porosity might be more damaging because the voids may provide a leak path. Describing the shape and location defines porosity as uniformly scattered porosity, cluster porosity, linear porosity, and piping porosity. A single cavity is also referred to as porosity pore. Wormhole porosity is where the individual gas pockets are elongated rather than spherical. Such a surface condition can occur when gases are trapped between the molten metal and solidified slag. Porosity is normally caused by contaminants or moisture in the weld zone, which decompose and form gases due to the welding heat. The contamination or moisture can come from the electrode, the base metal, the shielding gas, or the surrounding atmosphere. Variations in welding technique can also cause porosity, however. Therefore, porosity is generally a signal that some aspect of the welding

operation is out of control. It is then time to investigate further to determine what factors are responsible for the presence of this weld discontinuity.

Undercut/Underfill Undercut is a surface discontinuity that occurs in the base metal directly adjacent to the weld. It occurs when the base metal has been melted away during the welding operation and insufficient filler metal was deposited to adequately fill the resulting depression. The result is a linear groove in the base metal that may have a relatively sharp configuration. Because it is a surface condition, undercut is particularly detrimental in structures fatigue loading. It is interesting to note that for groove welds, the undercut may occur at either the face or root surface of the weld. Undercut is normally the result of improper technique. More specifi-

cally, there may not be sufficient filler metal deposited to adequately fill depressions caused by the melting of the base metal adjacent to the weld. This is generally the result of excessive weld travel speed. Undercut could also result when the welding heat is too high, causing excessive melting of the base metal, or when electrode manipulation is incorrect. Like undercut, underfill is a surface discontinuity that results in a loss of material cross section. However, underfill occurs in the weld metal of a groove weld, whereas undercut is found in the base metal adjacent to the weld. In simple terms, underfill results when there is not enough filler metal deposited to adequately fill the weld joint. When discovered, it usually means the welder has not finished making the weld, or has not understood the welding requirements. Like undercut, underfill can occur

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aircraft building at both the face and root surfaces of the weld. Underfill at the weld root of pipe welds is sometimes referred to as internal concavity or the slang term “suckback.” It can be caused by excessive heating and melting of the root pass during deposition of the second pass.

Overlap Another surface discontinuity that can result from improper welding techniques is overlap. Overlap is the protrusion of weld metal beyond the weld toe or weld root. It appears as though the weld metal has overflowed the joint and is lying on the adjacent base metal surface. As was the case for both undercut and underfill, overlap can occur at either the weld face or weld root of groove welds. Overlap is significant because it can result in a sharp notch at the surface of the weldment. Furthermore, if the amount of overlap is great enough, it can hide a crack that may propagate from the stress riser. Overlap is normally due to improper welder technique. If the welding travel speed is too slow, more filler metal will be deposited

Cracking in a fillet weld originating from a crater crack, which is a type of hot crack that results from poor welder technique. Crater cracks can start from a shallow region in the weld pass. than is required to sufficiently fill the joint. The excess metal spills over and lies on the base metal surface without fusing. Filler metals that become very fluid when molten are more prone to this type of discontinuity because they can run. Therefore, they may only be used in positions in which gravity will tend to hold the molten metal in the joint. Overlap and undercut often occur when welding in the horizontal position.

Convexity Convexity is a discontinuity that applies only to fillet welds. It results

when the amount of weld metal buildup on the face of the fillet weld exceeds what would be considered flush. By definition it is the maximum distance from the face of a convex fillet weld perpendicular to a line joining the weld toes. Convexity generally happens when welding travel speeds are too slow, when too little heat is used, or when the electrode manipulation is incorrect. In those cases, excess filler metal is deposited that does not properly wet the base metal surfaces. Convexity is not damaging within certain limits. In fact, a slight

Key Terms & Definitions Arc Strike—a discontinuity resulting from an arc consisting of any localized remelted metal, heataffected metal, or change in the surface profile. Convexity—the maximum distance from the face of a convex fillet weld perpendicular to a line joining the weld toes. Defect—a discontinuity that exceeds the permissible limits and requires repair or replacement. Discontinuity—any irregularity in the normal pattern of a material or interruption of the uniform nature of an item. Inclusion—entrapped foreign solid material, such as slag, flux, tungsten, or oxide. Incomplete Fusion—a discontinuity where fusion did not occur between weld metal and fusion faces or adjoining weld beads. Incomplete Joint Penetration—a joint root condition in a groove weld where weld metal does not extend through the joint thickness. Intergranular—conditions that occur at or fol-

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low the metal’s grain boundaries. An intergranular crack would initiate and propagate along a metal’s grain boundaries. Overlap—in fusion welding, the protrusion of weld metal beyond the weld toe or weld root. Porosity—cavity-type discontinuities formed by gas trapped during solidification of a weld bead. Safe Ending—(or stop drilling) drilling a small hole at each end of a crack to keep it from growing. Stress Risers—conditions such as notches, cracks, or geometry that increase the applied stress. Transgranular—conditions that cross or pass through the metal’s grains. A transgranular crack runs across the grains, as opposed to an intergranular crack, which runs along the grain boundaries. Undercut—a groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

amount of convexity is desirable to ensure that concavity is not present. Concavity can reduce the size and strength of a fillet weld. However, when the amount of convexity is excessive, this discontinuity becomes a significant flaw. The real problem created by the excess convexity is that the resulting fillet weld profile can have sharp notches present at the weld toes. Those notches can produce stress risers that weaken the structure, especially when it is loaded in fatigue. Therefore excessive convexity should be avoided during welding, or at least be corrected by depositing additional weld metal at the weld toes to provide a smoother transition between the weld and base metals.

Arc Strikes The presence of an arc strike can be a real problem, especially on the

low-alloy, high-strength steels. Arc strikes result when the arc is initiated on the base metal surface away from the weld joint, either intentionally or accidentally. When this occurs, there is a localized area of the base metal surface that is melted and then rapidly cooled due to the massive heat sink created by the surrounding metal. On certain materials, especially high-strength steels, this can produce a localized heataffected zone that may contain martensite. If this hard brittle microstructure is produced, the tendency for cracking can be great. Numerous failures of structures and pressure vessels can be traced back to the presence of a welding arc strike that provided a spot where cracks could begin, resulting in a catastrophic failure. Arc strikes are normally caused

by improper welding techniques. However, an arc strike can also appear if the work clamp is improperly connected to the work. Welders should be aware of the potential damage caused by arc strikes. Because of the potential damage they represent, arc strikes are never acceptable. Welding discontinuities exist in a number of different forms, including cracks, incomplete fusion, incomplete joint penetration, inclusions, porosity, undercut, underfill, overlap, convexity, weld reinforcement, arc strikes, spatter, laminations, lamellar tears, seams/laps, and dimensional types. By knowing how these discontinuities can form, the aircraft builder can be successful at spotting these problems and preventing failures in the structure when it’s completed.

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