Weld Quality Testing Guide

Weld quality testing ensures welded structures' safety, reliability, and longevity. Improperly welded structures can result in catastrophic failures, leading to injuries, property damage, and even loss of life.

Weld quality testing helps to identify defects such as cracks, porosity, incomplete penetration, and misalignments that could compromise the integrity of the weld. By proactively addressing these defects, weld quality testing is more than a safety measure, but can also prevent expensive repairs.

Why Is Weld Quality Testing Important?

The ability to be able to test welds can have a direct impact on both costs and safety in a welding project. For example, if a weld in a structural steel building fails because it does not meet the acceptance criteria in the designated codes, there could be a risk of serious injury or loss of life. Likewise, if a pipe or tank carrying a hazardous chemical fails, there could be a severe environmental impact, not to mention health hazards to anyone who comes in contact with the dangerous chemicals.

Common Weld Faults

To better understand some common weld flaws, it's important to be familiar with three key terms and their definitions.

  • Discontinuity: a discontinuity is defined as an irregularity or a lack of continuous cohesion or uniformity. A discontinuity may or may not be a defect depending on code allowances.
  • Defect: a defect is a discontinuity that exceeds the allowable limitations described in the code and can affect the usability of the end product, and will require repair.
  • Indication: this term is typically used in volumetric weld examination, i.e., X-ray and ultrasonic testing, and refers to an anomaly found in the examination process. Like discontinuity, an indication may or may not constitute a defect requiring repair.  

Incomplete Penetration

Incomplete penetration is a weld flaw that occurs when the weld fails to penetrate through the entire thickness of the joined parts, leaving some of the original weld joint unwelded. Although some codes may allow for a small amount of incomplete penetration, it's generally not recommended as it can compromise the integrity of the weld.

Lack of Fusion

When the weld fails to penetrate the base metal or other weld passes, the resulting defect is called incomplete fusion. In most commonly used welding codes, there is absolutely no allowance for lack of fusion or incomplete fusion, and will almost always require repair when found.


Undercutting is when the base metal has been gouged out by the welding process and not filled back in. This may be caused by a wide array of factors, such as excessive heat, travel speed being too fast, poor joint preparation, and poor welding technique. Most codes give some minor allowance for this discontinuity, though it is typically limited to 1/32” to 1/16”.

Slag Inclusions

A slag inclusion occurs when the preceding weld pass has not been adequately cleaned, and the next weld pass is not hot enough to burn out any residual slag left by the cleaning process. Welding codes vary on how much slag inclusion, if any, will be allowed.


Porosity is a discontinuity that is characterized by fairly symmetrical holes or a “Swiss cheese” type of appearance and can be caused by an absence of proper shielding of the arc during the welding process or moisture or other contaminants like dirt, grease, oils, or paints in the base metal. Welding with electrodes that have been improperly stored and have come into contact with moisture can also cause porosity in the weld. Porosity can be found on the surface or even under the surface of the weld. The structural steel welding code does allow some porosity of limited size and frequency, but pipe and pressure vessel codes tend to be much more stringent in the acceptance limits for porosity.


Burn-through is a common welding flaw that occurs when the second pass is applied over the initial pass and generates enough heat to partially or completely melt through the first pass. This can happen due to excessive grinding of the root pass or when the second pass is made with too much heat or too slow a travel speed, resulting in excessive heat input. The need for repair depends on the depth and severity of the burn-through.

Underfill or Undersized Welds

If the weld profile in the final pass in a weldment falls below the base metal and/ or the weld is thinner than the base metal, this could be considered underfill. If a drawing calls for a weld to be ⅜” and the inspector measures that weld at ¼,” this would be considered an undersized weld. 

Poor Weld Profile “Overlap” or “Cold Lap”

If a fillet weld, for example, falls far more onto the bottom flat portion of the joint and very little onto the vertical surface of the joint, then this dynamic would be considered a poor weld profile issue.

“Overlap” can occur when excessive molten weld metal does not adequately fuse into the base metal and is considered an incomplete fusion defect.

“Cold lap” is typically found in MIG welding and occurs when weld metal has been deposited on the surface but has not adequately fused into the base metal.


Cracks in welding can occur in the base metal, weld metal, or both and are often the result of stress in the weld joint or lack of pre-heat in the base metal during welding. Cracks are among the most severe defects that can occur in a welding operation and will always require repair, which, in many cases, it will require cutting out the part with the crack in it and replacing it. In addition, some applications, such as heavy equipment repair work, may have a repair procedure for repairing cracks. 


Weld Quality Testing Equipment

Weld testing equipment is essential for evaluating the quality and properties of welds in various applications. It helps identify and measure weld discontinuities, verify weld quality, and even assess the mechanical properties of the welded material. 

Common types of weld testing equipment include ultrasonic testers, magnetic particle testers (both portable and stationary), liquid penetrant testers, and radiographic or "X-ray" testing equipment.

Weld Quality Inspection Checklist

In order to ensure that the welder is making welds that will meet applicable code standards and be free from defects, there are a number of things that the welder must keep in mind, and it helps to have a checklist for specific issues that the welder must monitor.

Pre-Weld Inspection Checklist

Prior to starting the welding operations, the welder should check the following;

  • Is the material clean and free of rust, dirt, and other potential contaminants?
  • Are the electrodes correct and compatible with the material to be welded?
  • Is the welding machine capable of making the weld? (Constant current or constant voltage required)
  • Is the polarity set correctly? (AC or DC, electrode positive or negative.)
  • Is pre-heat of the materials required?

During-Weld Inspection Checklist

Once you have started welding, it is vital to consider several factors to achieve the desired weld quality. The following considerations should be kept in mind by the welder throughout the welding process;

  • Is the proper interpass temperature being maintained? Too much or too little heat could prove detrimental to the completed weld.
  • Are you cleaning adequately between passes? Take care not to trap slag that could result in a weld defect and the need to repair it.

Post-Weld Inspection Checklist

Once the weld is complete, the welder must check the weld to ensure that the weld is fully finished and ready for visual inspection. Things to look for include;

  • Is the weld adequate in size? Not underfilled but not excessive in reinforcement either.
  • Has the completed weld been cleaned, and is now free of slag?
  • Are there any indications of undercut, incomplete fusion, or porosity that should be corrected prior to the final inspection?

The quality-focused welder should always remember that you are your best quality control! You should be the first to inspect your completed weld, and if you have any doubts about any potential defects, it's better to address them proactively. By submitting your work with full confidence that it's free from any issues, you can ensure a job well done.

Types of Weld Quality Testing

There are two basic types of weld quality testing; destructive and non-destructive. In the general sense, completed products and work pieces will be tested with a non-destructive test method.

Non-destructive Testing

Visual Inspection

The first and most basic weld inspection method is a visual inspection. A trained and competent inspector will examine the weld visually and look for any indications that exceed the visual acceptance criteria in the applicable code. The advantages of visual inspection are;

  • Minimal cost in time and equipment. Usually a flashlight, and maybe a mirror, or in some cases, a magnifying glass, are all that is needed to perform a visual inspection.
  • Does not require an interruption in production. 
  • Minimal training is required. Welders, supervisors, and inspectors can be trained relatively quickly and at a minimal cost to perform a visual inspection.

There are some obvious limitations to visual inspection, however. This inspection is limited to surface discontinuities only. Visual inspection is only possible on the final cap pass and maybe the root pass, if accessible. There is no way to inspect anything between passes in a visual inspection.

Liquid Penetrant Surface Crack Examination

Liquid penetrant testing is a popular method to enhance visual inspection. First, the part is cleaned, then a dye penetrant is sprayed or brushed onto the weld. Once the dye has been allowed to dry, the excess is removed with a cleaner and a cloth. A developer is then applied. After 10 minutes minimum, the inspector can evaluate the part.

The dye that may have penetrated a discontinuity will now show up as a bright red contrasted against the white developing agent and can be evaluated as being either a “relevant indication,” meaning the discontinuity exceeds allowable limits and must be addressed, or as a “non-relevant indication,” meaning that the discontinuity falls within established acceptable limitations and can therefore be disregarded. 

Dye-penetrant testing is a quick and cost-effective method for weld examination but is limited to surface discontinuities only.

Magnetic Particle Weld Examination

Magnetic particle examination, or mag-particle as it is commonly known, works by establishing a magnetic field within the part to be inspected. Then, fine iron filings, usually colored for greater visibility, are applied to the weld within the magnetic field. Given the nature of the magnetic field, the iron filings will be drawn to a break in the field and line up along the defect. Mag particle is an excellent way to detect cracks but is limited to surface or very near surface discontinuities.

Radiographic Weld Examination

Radiography, commonly referred to as "X-ray," is a highly effective method for examining the quality of a weld throughout its entire cross-section. This method is particularly useful for pipe welds as it allows inspectors to evaluate every aspect of the weld, from the root pass to the final cap. A skilled radiography film interpreter can identify discontinuities such as incomplete penetration or fusion at the root or between weld passes and porosity and trapped slag inclusions.

The potential drawbacks to radiography are;

  • The immediate area around the part to be X-rayed must be barricaded for a distance of at least 25 feet, sometimes as much as 75 feet, depending on the strength of the source.
  • The necessity to handle hazardous materials.
  • Extensive training is necessary to perform radiography examinations.
  • Extensive and costly regulatory concerns.
  • Expensive equipment.

Ultrasonic Weld Examination

Ultrasonic examination uses acoustic “sound waves” through the use of a transducer and a couplant applied to the surface of the part to be examined. Therefore, any discontinuity will produce a spike on the display screen of an ultrasonic monitor. The examiner then must assess if the decibels generated by the discontinuity exceed allowable levels established by the applicable code and must be addressed or if they fall within acceptable limits.

Destructive Testing

Destructive testing is most commonly used to test materials, qualify welding procedures, and to determine a welder's ability to make a weld with a particular process, i.e., a weld test.

Bend Testing

Bend tests are performed by cutting a strip of material, usually about 1” x 8” long, and placing the test specimen in a bending apparatus. Usually, a block and dye with predetermined dimensions are used to bend the submitted test specimen. The test is rejected if a crack or some other discontinuity is found in excess of allowable limits.

Nick Break Testing

A nick-break test is very similar to the bend test. It involves making a small cut or "nick" perpendicular to the weld, then bending or pulling the specimen so the weld cross-section can be visually inspected. 

Macroetch Testing

Macroetch testing is often used to test fillet welds, as fillet welds can be difficult to bend test. The test specimen is cut in a saw or a cutting wheel on a grinder. The weld cross-section is then cleaned, usually with a Tiger disc or sanding disc. An acid is then applied to the cross-section of the weld and visually inspected for any discontinuities.

Transverse Tension Testing

Tension or tensile testing is commonly used in qualifying welding procedures. The weld specimen is put in a tension testing apparatus and pulled apart. The test is successful if the material fails in the base metal and not the weld. When testing materials or qualifying welding procedures, it is necessary to use a gauge and determine the actual load required to pull the test specimens apart to ensure the material and the welding process can produce welds of the necessary strength. 

Weld Quality Testing FAQ

What are the five methods to test the quality of welding?

The five most commonly used methods for examining weld quality are:

  • Visual inspection
  • Liquid penetrant examination
  • Mag particle examination
  • Radiography
  • Ultrasonic examination

What is the most commonly used method of weld inspection?

The most common method for inspecting welds is the visual inspection method. All welding codes and generally accepted quality standards require a visual inspection at a minimum for all completed welds. Furthermore, visual weld inspection requires no special equipment, just good eyesight and maybe a mirror and a flashlight. 

What are the two methods to test the quality of a weld?

Destructive testing and non-destructive testing. 

Destructive testing, as its name suggests, involves some form of cutting apart a completed weldment and subjecting it to further analysis as in the form of bend testing, a commonly used procedure for qualifying welders and welding procedures.

Non-destructive weld testing is used to examine welds in a finished part or product. Examinations such as penetrant testing, mag-particle testing, ultrasonic and X-ray examination leave no lasting effect on a finished product.

Who determines the inspection requirements and applicable codes for a welding project?

The owners of any given project usually determine the inspection requirements but only after carefully considering the regulatory requirements of the jurisdiction in which the project is to take place and the engineers who will design the connections involved.

Photo: Image courtesy Ries Bosch.

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