An inspector’s primary concern is ensuring that all the welds they inspect are good sound welds and are acceptable to code. It’s essential for an inspector examining the weld to be able to spot a variety of weld discontinuities, including:
- Porosity
- Incomplete fusion
- Incomplete joint penetration
- Unacceptable weld profiles
- Cracking
The first step toward understanding weld discontinuities is to examine some welding terminology.
Discontinuity
Discontinuity is an interruption of the typical structure of a material. Including a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity could result from a defect but isn’t necessarily a defect. On the other hand, a defect is a discontinuity that by nature or accumulated effect renders a part or product unable to meet minimum applicable acceptance standards or specifications. A defect results in the rejection of the part or product.
Because we’re examining these phenomena outside the requirements of specific welding codes or standards and don’t discuss their limitations in terms of these documents, we’ll use the word discontinuities to describe them. So, the first type of weld discontinuity we’ll address is porosity.
Porosity
Porosity is a cavity-type discontinuity formed by gas entrapment during solidification. These trapped gasses in the molten weld may form bubbles or pockets as the weld solidifies.
The four main reasons for the presence of gasses because of porosity is:
- Dirty base material contaminated with hydrocarbons such as oil, grease, or paint.
- Moisture on the joint surface, electrode in water, hydrated oxides, or water leaks from poorly maintained cooling systems can introduce hydrogen into the welding process.
- An inadequate shielding gas flow rate causes insufficient or improper shielding. Gas contaminated from its source or delivery system, or wind or draft prevents the gas from protecting the molten weld metal.
- Incorrect welding conditions or techniques.
- Porosity is often identified by its shape and distribution within the weld. Such as uniformly or randomly scattered, cluster, or linear. These porosity distributions may have different acceptance levels within a welding standard.
The most practical methods for controlling or eliminating porosity are:
- Using clean base materials
- Properly storing uncontaminated welding consumables
- Adequately maintaining welding equipment
- Using proven welding procedures
- Welding in acceptable environmental conditions.
Incomplete Fusion and Incomplete Joint Penetration
These terms are sometimes misused. It’s essential to understand the difference between these two weld discontinuities.
Incomplete Fusion
Incomplete fusion is a weld discontinuity in which fusion doesn’t occur between the weld metal and fusion faces or adjoining weld beads. This absence of fusion can occur at any location within the weld joint and be present in fillet welds or groove welds.
Incomplete fusion may result when the temperature of the base material or previously deposited weld metal is not elevated to its melting point during the welding process. Incomplete fusion often is found on one leg of a fillet weld and is caused by an incorrect welding angle, which distributes heat nonuniformly between both sides of the joint.
It also may be caused by oxides or other foreign material on the surface of the base material.
Incomplete Joint Penetration
Incomplete joint penetration is a discontinuity in a groove weld in which the weld metal doesn’t extend through the joint thickness. Instead, it’s the filler or base metal failing to fill the weld’s root completely.
Some common causes of incomplete joint penetration are a bad groove weld design or a fit-up unsuitable for the welding conditions. Incomplete joint penetration can occur if the root face dimensions are too large, the root opening is too small, or the included angle of a V-groove weld is too narrow. These joint design problems restrict the weld’s ability to penetrate the joint’s thickness.
Incomplete joint penetration can be prevented. It needs the correct design and fit-up per welding procedure requirements.
Understanding these weld discontinuities will help welding inspectors identify them and, more important, prevent them from occurring in production. Using welding inspection as a preventive tool within the quality system is more efficient than only as an appraisal technique to sort bad welds from good welds.
Unacceptable Weld Profiles
The profile of a completed weld may have a considerable effect on the performance of that weld in service. Welding inspectors must identify discontinuities through visual inspection. As well as, evaluate their acceptance or rejection according to the applicable welding code or standard acceptance criteria.
An Unacceptable Weld Profile
An unacceptable weld profile scan causes a reduction in base material thickness, weld size, or stress concentrations on the weld or plate surface. These types of weld discontinuities often seriously detract from the overall performance of a welded component in service.
Some weld profile discontinuities are undercut, overlap, insufficient throat, and excessive convexity. An undercut is a groove melted into the base metal adjacent to the weld toe and left unfilled by weld metal.
The term undercut describes two specific conditions. The melting away of the base material at the sidewall of a groove weld at the edge of a bead, which produces a sharp recess in the sidewall where the next bead is to be deposited. This undercut can entrap inclusions within the cavity, which a subsequent weld bead may cover.
This condition can usually be corrected by grinding down the recess before depositing the next bead. If the undercut is slight, an experienced welder who knows how deep the arc will penetrate may not need to remove it. The undercut of the sidewall of a groove weld won’t affect the completed weld if the condition is corrected before the next bead is deposited.
The second condition is a reduction of the base metal’s thickness. This is at the line where the weld bead on the final layer of weld metal ties into the surface of the base metal. This position is known as the toe of the weld. This condition can occur on a fillet weld or a butt joint.
The amount of undercut permitted at the completed weld’s surface is usually specified within the welding code or standard. The maximum permissible undercut requirements for completed welds should be followed stringently. Excessive undercut can seriously affect the performance of a weld, particularly if it’s subjected to fatigue loading in service.
Both undercut types are usually caused by:
- An incorrect welding technique
- Incorrect electrode positioning
- Incorrect travel speed.
High currents and a long arc increase the probability of undercut.
Overlap
Overlap protrusion of weld metal beyond the weld toe or weld root. This condition occurs in fillet welds and butt joints. It produces notches at the toe of the weld that is undesirable because of their stress concentration under load. This discontinuity can be caused by incorrect welding techniques or insufficient current.
Insufficient Throat
Insuffucuent throat usually occurs in concave fillet weld and butt joint profiles. Excess concavity reduces throat thickness, which considerably reduces weld strength. This condition usually is caused by excessive welding current or arc lengths.
Excessive Convexity
Excessive convexity can produce a notch effect in the welded area. Consequently, the stress concentration is under load. For this reason, some codes and standards specify the maximum permissible convexity of a weld profile. Insufficient current or incorrect welding techniques typically cause this condition.
Cracking
Cracks in a weldment are probably the most dreaded weld discontinuities. Cracking is a complex subject because so many materials and applications are used in welding.
The base material’s crack sensitivity may be associated with its chemistry and susceptibility to the formation of elements that reduce its ductility. Excessive stresses in the weld joint, mainly if the material is in a crack-sensitive condition, can cause cracking.
The welding operation can produce stresses in and around the weld. This can introduce extreme localized heating, expansion, and contraction.
Cracking is often caused by stress concentration. This can be near discontinuities in welds and base metal and near mechanical notches in the weldment design. Hydrogen embrittlement, which causes a loss of elasticity and exists in weld metal because of hydrogen absorption, can contribute to crack formation in some materials.
Cracks are classified as one of two types: hot or cold.
Hot Cracks
Hot cracks develop at:
- Elevated temperatures
- Propagate between the grains of material
- Commonly form during the solidification of weld metal
Cold Cracks
Cold cracks develop after the solidification of the weld. This is due to stresses, and propagates both between grains and through grains. Cold steel cracks are sometimes called delayed cracks. They are often associated with hydrogen embrittlement.
Hot and cold cracks can be categorized further as base material cracks and weld metal cracks.
Base Material Cracks
The heat-affected zone (HAZ) cracking most often occurs with base material that can be hardened. High hardness and low ductility in a HAZ often result from a metallurgical response to thermal welding cycles. In ferritic steels, hardness increases, and elasticity decrease with an increase in carbon content and a faster cooling rate.
The HAZ hardness depends on the base material’s ability to be hardened. In turn, depends on the base material’s chemical composition. Carbon has a predominant effect on steel’s hardenability. Cast iron contains between 2 and 4.5 percent carbon, giving high alloy hardness and low ductility. Welding this material without seriously considering cooling rates and residual stress invariably will result in base material cracking.
Weld Metal Cracks
Weld metal cracks can be divided into three types:
- Transverse, which is perpendicular to the direction of the weld.
- Longitudinal travels in the same direction as the weld and is often confined to the weld’s center. This type of crack may be an extension of a crack initially initiated at the end of a weld.
- An abrupt weld termination can form a crater if a crater is left unfilled with weld metal. These cracks usually are star-shaped and initially extend only to the crater’s edge. However, they can propagate into longitudinal weld cracks.
Dealing With Cracks
Cracks are unacceptable discontinuities and are detrimental to weld performance. A crack, by its nature, is sharp at its extremities, so it acts as a stress concentration. The stress concentration effect of a crack is greater than that of most other discontinuities.
Cracks tend to propagate, contributing to weld failure under stress. Cracks aren’t permitted in weldments governed by most fabrication codes regardless of size. Instead, they must be removed by grinding or gouging, and the excavation filled with sound weld metal.
Successful welding procedures incorporate the necessary controls to overcome the tendency for crack formation. Such controls are preheating, interpass, consumable type and preparation, and post-weld heat treatment.
Welding inspectors are responsible for evaluating these procedural controls during inspections to ensure welding is performed to minimize the possibility of weld cracking.
Detecting and Evaluating Discontinuities
Weld discontinuities often are detected through visual inspection. However, some are detected with inspection methods, such as radiography, ultrasonic, liquid penetrant, and magnetic particle inspection.
The maximum acceptable limitations for these discontinuities depend on the performance requirements of the welded component. They are specified in the appropriate welding code, standard, or specification. Therefore, the welding inspector must often determine the extent of discontinuities and establish their acceptance or rejection based on the relevant acceptance criteria.