Weld Inspection: A Guide

Weld inspection is a process in which welds are examined for quality, strength, safety, and a number of other important factors.

In a weld inspection, trained specialists make a series of quality assurance checks before, during and after the welding process, ensuring that the fabrication is as safe and secure as possible. 

Whether made of metal, thermoplastics or another material entirely, welds are used in many industries and environments across the world, and proper inspection and care is vital to keeping both workers and civilians safe.  



What Do Welds Do?

Welding is a fabrication process that uses extreme heat to fuse together metal and other materials into a joint. 

In a weld, a base or parent metal is melted down and combined with a filler material in what is known as the weld pool. This pool is then cooled and attached to a second piece to create a joint that is stronger and sturdier than the original material on its own. 

Welders and weld inspection experts are needed across a number of industries, including: 

  • Aerospace

  • Amusement Parks

  • Automobiles

  • Construction

  • Infrastructure 

  • Military

  • Manufacturing

  • Railroads 

  • Ship & Boat Building

Why Is A Weld Inspection Important?

Weld inspection takes place before, during, and after the welding process, emphasizing safety at every stage. When performed correctly, weld inspection should catch any and all weld discontinuities before a piece even leaves the shop or factory. 

Before and during the weld, proper weld inspection consists of keeping workers safe and ensuring regulatory compliance. After the weld is complete, weld inspection keeps civilians protected by making sure no product with a faulty weld enters public use. 

Annual inspection of welds that experience high stress is also an aspect of weld inspection, though such procedures are usually part of a separate, comprehensive safety protocol. 

Welding is an occupation with a number of inherent dangers that weld inspectors help workers avoid. Both the heat and light of various welding energy sources can be dangerous, causing burns or eye damage if proper precautions are not taken. Welders typically wear personal protective equipment like masks, goggles, gloves or fireproof jackets to minimize their exposure to these hazards.

Other welding risks include the inhalation of dangerous gasses and working in unusual environments like factories, underwater, or even outer space. 

By ensuring that workers are following the strict safety procedures laid out by their regulatory bodies or workplace, and by not letting any faulty products enter the public sphere, weld inspectors aim to prevent safety issues before they ever occur. 

Because they are required to understand a variety of processes including welding symbols, welding design, safety codes, and weld testing, most weld inspectors are formally qualified by the American Welding Society or another similar certifying board. 

The 3 Stages of Weld Inspection

As noted above, proper weld inspection techniques should be present in all three stages of the welding process: before the weld, during the weld and after the weld. 

1. Pre-Weld Inspection

This kind of weld inspection refers to any task that needs to take place before the actual welding begins. This includes making sure that all safety measures are observed, reading and understanding documents that explain the job at hand, gathering all needed materials, and preparing all tools or energy sources. 

2. During-Weld Inspection

Weld inspection tasks that take place during the weld include keeping track of heat input, cleaning between individual weld passes, monitoring welding technique and joint position, and looking for visual clues that indicate problems. 

3. After-Weld Inspection

After-weld inspection takes place after the weld has cooled and fully fused. Tasks include checking for adequate weld size, looking for discontinuities or flaws, ensuring that the product meets task specifications, and testing the weld strength through various means. 

There are a number of weld inspection techniques that can be deployed at every step of the welding process. Many of these are explored in detail further below, with their own specific use case and application listed. 

Types of Welds

There are many different kinds of joint types in welding. Below, we’ll explore the five most popular. Then, we’ll look at some of the most common ways those welds can fail, and why they need to be inspected in the first place. 

Butt Joint Welding

A butt joint is the most common type weld in large structures and piping systems. In a butt weld, two pieces of metal on the same plane are joined on their side. Butt joints are easy to prepare and can be customized in a number of different ways.

weld-inspection-flyability-2Butt joint weld


Lap Joint Welding

A modification of the butt joint, a lap joint is formed when two metallic pieces are placed in an overlapping pattern on top of each other. Lap welds are useful for fusing two pieces of differing thickness together, though they are typically only used for sheet metal and materials of a similar size.

weld-inspection-flyability-3Lap joint weld | Source


Tee Joint Welding

Intuitively named, a tee weld occurs when two pieces of metal are fused at a 90 degree angle, with one piece intersecting the other in the exact middle. Tee joint welds can also be formed when a pipe or tube is welded to a base plate.



Corner Joint Welding

Similar to tee welds, corner joint welds fuse two metal pieces together at a 90 degree angle. However, instead of intersecting in the middle and creating a joint in the shape of the letter T, a corner joint weld forms an L shape.

These welds are most commonly used in the fabrication of boxes, frames, shipping containers, and other applications in the sheet metal industry. 

weld-inspection-flyability-5Corner joint weld


Edge Joint Welding

When the edges of two pieces are parallel or nearly parallel, an edge joint is likely to be the best option.

Edge joints are not as strong as other types of welds like butt joints, and are typically not used if the joint is expected to experience high levels of stress. However, edge welds are very useful inside of structures for supplemental support. 

weld-inspection-flyability-6Edge weld | Source

While these five joint types are some of the most common kinds of welds, they are far from the only ones used.

Each of these joint types has a long list of variations and customizations (fillet weld, plug weld, surfacing weld, etc.) that come with their own set of benefits and applications.

Understanding the different kinds of weld types and knowing which joint should be used where is one of the central responsibilities of a weld inspector. Weld inspection is as much about prevention as it is correction, and a careful understanding of weld types is vital to a successful inspection. 

Common Faulty Welds

In this section we’ll look at some of the most common types of faulty welds. 

Some of these failures are beginner mistakes that would be visible even to an untrained eye. Others are more difficult to notice and may not present themselves until after the initial weld has been completed.

This is why proper weld inspection techniques must be observed both during and after the welding process. 

After we identify these common faulty welds, we’ll explore the different methods of locating these failures through weld inspection. 


Also known as melt-through, this faulty weld occurs when the entire base metal of the weld is melted, creating a hole that burns all the way through a welded seam.

Burn-through is a common beginner’s mistake, and in many professional settings a weld with burn-through would be immediately rejected by a manufacturer or buyer.

If the part is still accepted, it should be regularly inspected, as any major discontinuity can greatly reduce joint strength.  


Welds are exposed to corrosion of all kinds throughout their lifetime, and much of it is unavoidable. Because of this, weld inspection that determines how corroded a particular weld may be is vital.

Environmental forces, material degradation, excessive stress, microstructure discontinuities, and more can each lead to different types of corrosion and weld failure. 



Incomplete Penetration

When the base metal and the filler metal fail to fuse together at the root of the joint, it is a weld failure known as incomplete penetration. Frequently, this faulty weld occurs when a joint design is not suitable for the particular welding process that is used.

An incorrect angle, a root opening that is too big or too small, or a high rate of travel can also lead to incomplete penetration.

Lack of Fusion

When the weld metal flows over the base metal instead of fusing with it, the resulting weld failure is called, unsurprisingly, lack of fusion. This common weld failure is also known as overlap. Typically, lack of fusion occurs due to low temperatures, dirty plates, or improper fluxing.

Lamellar Tearing

When high stress causes cracks or breaks to appear at the base of a weld line, it is a defect known as lamellar tearing. This most often occurs due to unintended stress, incorrect material use or improper weld orientation. Lamellar tearing most often occurs in butt joints and certain types of T welding. 


When a pocket of the weld contains a gas instead of solid material, it is a weld fault known as porosity. These gasses can come from chemical reactions in the weld materials during welding or cooling. Overheating is the most common cause of porosity.  



Slag Inclusions

Slag is the hardened layer of nonmetallic material sometimes left on the top of the weld, and is most common in flux-cored welding, shielded metal arc welding, and submerged arc welding. When these metallic oxides are not removed, they can slip into the weld and create porous discontinuity. 


When the base metal is burned away at the toe of the weld, it is known as undercutting. Like many of the other faulty welds mentioned above, undercutting is most often a result of an arc that is too long or an energy source that is too hot.


These are just a few of the faults and discontinuities that a weld inspector will be looking for during a weld and after it is completed. Let’s now look at the various types of weld inspection techniques that can be used to quickly and accurately identify many of these faults. 

Types of Weld Inspection 

Most weld inspection techniques can be divided into two categories: non-destructive weld inspection and destructive weld inspection. 

As the names imply, non-destructive weld inspection involves visually examining the weld or inspecting its interior without breaking any pieces of the fabrication. Destructive weld inspection destroys the weld in an attempt to ascertain certain types of important information.

Both non-destructive and destructive weld inspections are usually performed after the weld is complete. 

Non-Destructive Weld Inspections

These kinds of weld inspections are performed in an attempt to find any discontinuities without altering or destroying the finished piece. They vary widely in application, price, and operator knowledge. In some cases, they may be performed by the welder themself, and not a separate inspector. 

Visual Inspection

Visual inspections are the easiest, quickest, and least expensive type of weld inspection, and they are widely used. In a visual weld inspection, welding technicians will look for any noticeable discontinuities like burn-through or cracks.

Visual inspections should also be performed regularly on all gear and materials. There are of course some disadvantages to visual weld inspections. Human eyesight is fallible, and visual weld inspections cannot identify any internal discontinues like porosity or slag inclusions. 

Liquid Penetrant Surface Crack Inspection 

Also called dye penetrant testing, this type of weld inspection is a type of weld inspection designed to identify hairline or microscopic cracks on the surface of the weld that may be invisible to the naked eye. 

In a liquid penetrant weld inspection, a brightly colored or fluorescent dye is spread across the entirety of the weld surface and allowed to dry. Small imperfections can then be seen due to the dye’s sharp contrast with the welding material. 

Like visual inspection, surface crack detection only identifies flaws on the surface, and cannot identify internal weld failure. 

weld-inspection-flyability-10Liquid penetrant testing

Magnetic Particle Weld Inspection 

In this inspection method, a magnetic field is established around the weld and magnetic probes are applied to its surface. Accumulations of particles to these magnet ends may indicate certain kinds of weld discontinuities.

If no particles accumulate to the magnet ends or poles, there are likely no cracks in the weld. 

Magnetic particle weld inspection can only be used on certain kinds of materials in certain circumstances, but it is one of the most effective and accurate kinds of non-destructive weld inspection available. 

weld-inspection-flyability-11Magnetic particle testing

Radiographic Weld Inspection

Radiographic weld inspection is one of the ways to detect discontinuities and weld faults within the part itself, not just on its surface. In radiographic weld inspection, an X-ray is used to display the weld part’s internal structure onto a piece of photographic film. This process is not dissimilar to how medical X-rays on the human body are gathered. 

Variations in shape and color on the film can indicate different types of weld discontinuities, including slag inclusion, porosity, and cracks. Just as with medical radiographs, there are health and safety risks included in this process, and only trained professionals should perform this type of weld inspection.

weld-inspection-flyability-12Radiographic weld inspection

Ultrasonic Weld Inspection

In ultrasonic testing, a high powered mechanical frequency is beamed into a weld piece and, when this beam strikes a discontinuity within the piece, it is reflected back to a receiver known as a transducer.

The reflected signal can be measured in a number of different ways that can be interpreted to help determine the size, shape, and nature of the weld failure. 

This non-destructive method is incredibly effective at determining the exact location of internal discontinuities, but it requires a high level of operator knowledge and, often, expensive equipment. 

weld-inspection-flyability-13Ultrasonic weld inspection

[Related read: NDT (Non-Destructive Testing): A Complete Guide]

Destructive Weld Inspections

Destructive weld testing involves the breaking off of a welded piece in an attempt to inspect and analyze the piece’s mechanical and internal physical traits. 

Destructive weld inspection can be performed for a number of reasons, including weld failure analysis, research inspection, sampling inspection, or certification training, though the latter is most common. 

Consider an amateur welder about to work on her first automobile. A destructive weld inspection could be performed on a piece of scrap metal that is similar in thickness to the car, so that the technician can perfect her practice and ensure the final weld is safe, secure and strong.

Macro etch Weld Inspection

In this method, small samples from the welded joint are removed and then carefully polished. These pieces are then etched at their center with a mild acid mixture that varies depending on the parent metal that was used.

The acid etch sample can then provide a visual snapshot of the weld’s internal structure, including any internal fusion failure or cracking. It is also called a cut and etch inspection.

weld-inspection-flyability-14Macro etch destructive testing | Source

Transverse Tension Weld Inspection

This is a method of weld inspection intended to help determine the tension per unit in a cross-sectional area of a welded piece, or, simply, the breaking point of certain welds made out of different metals. 

In a transverse tension weld inspection, a specimen is pushed to failure. The maximum load required to do so is then divided by the cross sectional area. Transverse tension weld inspection is a great way to determine limits and minimum requirements when working with new or different kinds of materials. 

Guided Bend Test Weld Inspection

This destructive weld inspection method involves bending a weld part to a predetermined bend radius. Guided bend tests are used to evaluate the ductility and structural integrity of certain kinds of welded joints. 

There are many different kinds of bend tests, but they are most often used in training and qualification tests. By pushing the welds to a stress limit they almost certainly would be exposed to in the field, weld inspectors can make sure welders are prepared to practice their craft on the “real” thing. 

Weld Inspection Checklist

Below is an example of a checklist that an inspector or experienced welder may use when performing a weld inspection. 

While the specific steps of a weld inspection will vary based on the job and the type of welding materials used, the important thing to note here is how there are important checks and regulations at all three stages of the welding process: before, during and after the weld is complete. 

Pre-Weld Inspection Checklist

  • Review all blueprints and drawings to see if any changes or special considerations should be taken into account. 

  • Ensure that all materials (base, consumable, and filler) are those which have been specified in the schematics.  

  • Check the physical condition of all materials that are to be welded. 

  • Ensure that the workspace, tools and welding materials are clean. 

  • Calibrate all tools and energy sources as needed. 

  • Check that the welder is qualified and certified for this job. 

  • Preheat energy source. 

During-Weld Inspection Checklist

  • Ensure that safety precautions and parameters are followed at all times.

  • Monitor heat, voltage, technique, temperature, and more, if applicable. 

  • Check individual weld passes.

  • Look for any visual clues that could indicate a problem. 

After-Weld Inspection Checklist

  • Visually check for discontinuities. 

  • Perform other types of after-weld inspection testing, if applicable. 

  • Check weld length and size. 

  • Ensure that any post-heat treatment is performed, if applicable. 

Weld Inspection Equipment

Welders and weld inspectors use a variety of equipment to ensure safety across all steps of the welding process. Depending on the location of the weld and the inspection, different tools may be needed.

For example, a weld inspector in a factory may only require his eyes and protective equipment, while a weld inspector working underwater may require a whole host of other weld inspection gear. 

Some common weld inspection equipment includes: 

  • Borescope

  • Bridge Cam

  • Codes, Weld Signs

  • Flashlight

  • High-low Gauge, Pit Gauge, Other Gauges

  • Log Book

  • Mirror

  • Owner Specifications and Drawing

  • Personal Protective Equipment

  • Radiograph Machine

Drones as Weld Inspection Equipment

One final piece of weld inspection equipment that should not be overlooked is the drone, or uncrewed aerial vehicle. Ideally, the bulk of weld inspection will occur before the welded piece even leaves the shop.

However, as part of annual inspections and other safety regulations, different welds in hard to reach places may have to be examined up close. 

Drones, which are growing more and more accessible, may be part of many weld inspectors' solutions in the coming years.

With the ability to get into hard to reach places like oil tanks, hangars, mines and more, drones will be key to keeping workers and inspectors safe during weld inspection. A drone outfitted with ultrasonic or LiDAR payload would only increase accuracy and safety.

weld-inspection-flyability-15The Elios 3 drone inspecting water pipes

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