This is a complete guide for everyone involved in weld quality control, covering topics like the basics of weld inspection, the principles and proper use of visual inspection and non-destructive testing (MT/PT/RT/UT), typical welding defects & their judgment criteria, and automated inline inspection using 3D profile sensors.
Weld Inspection Guidebook
Inquiries About Weld Inspection
What is Welding Inspection?
This section provides a cross-comparison of various welding inspection methods based on detection targets, accuracy, speed, cost, and suitability for automation.
Definition and Purpose of Welding Inspection
Welding inspection is a series of processes to confirm whether the quality of a weld meets design requirements and standard criteria. Welding is considered a 'special process,' and because its quality cannot be fully judged by visual appearance alone after completion, quality is assured by combining multiple inspection methods for each process.
There are three main purposes of welding inspection. First, to ensure the strength, durability, and safety of the structure. Second, to enable early detection of product defects and minimize rework costs. Third, to ensure traceability and accumulate quality records.
Weld inspection is fundamental to quality in all manufacturing industries, including automotive, structural steel, pressure vessels, shipbuilding, plant piping, and electronic components. In recent years, a shortage of inspectors and more advanced quality requirements have led to the increased use of 3D profile sensors and cameras in the automation of weld inspection.
Four Times for Weld Inspection
Weld inspection is performed in four stages: before welding, during fabrication, after completion, and during operation. The items to be inspected and the applicable inspection methods differ at each stage.
- 1 Pre-weld Inspection – This includes confirming the base material, measuring the groove shape, dimensions, and angle, and checking the surface for rust, oil, and dirt. It also includes confirming the suitability of welding materials (rods and wires) and the preheat temperature.
- 2 In-process Weld Inspection – Check the penetration condition at the first layer (root pass). After removing slag, inspect each pass for its shape, cracks, lack of fusion, and slag inclusion. Management of the interpass temperature is also important.
- 3 Post-weld Inspection – Check the bead shape, undercut, pits, and reinforcement shape through visual inspection. As necessary, detect internal defects with RT and UT. Leg length and throat thickness are measured through dimensional measurement.
- 4 In-service Inspection – Periodically inspect for fatigue cracking, stress corrosion cracking, and creep damage during the use of the structure. This is conducted as a statutory inspection for plants, bridges, pressure vessels, and more.
Types and Features of Weld Inspection
Weld inspection is broadly divided into "visual inspection (surface)" and "nondestructive testing (internal)." This section explains the principles, detection targets, advantages, and notes for each method.
Visual Testing (VT) – Surface
This is the most basic method of weld inspection. An inspector uses the naked eye, a magnifying glass, or an inspection gauge to evaluate the appearance of the weld bead (shape, dimensions, and surface defects). It is the starting point for all weld inspections and is performed at all stages.
- Benefits: No special equipment required, low cost, wide range of applications
- Note: Dependent on inspector's skill, lacks quantitative evaluation, internal defects cannot be detected
3D Automated Visual Inspection – Surface
A 3D profile sensor (such as the LJ-X8000 Series or LJ-S8000 Series) quickly captures the cross-sectional shape of the weld bead and automatically measures leg length, throat thickness, reinforcement height, bead width, etc. It performs OK/NG judgment based on numerical criteria, eliminating the subjectivity of visual inspection. In combination with a robot arm, 100% inline inspection is possible.
- Benefits: Quantitative judgment, 100% inspection, traceability, high speed and high accuracy
- Note: Internal defects cannot be detected (requires combined use with UT/RT)
Magnetic Particle Testing (MT) – Surface
This method involves magnetizing a ferromagnetic material (such as steel) and detecting cracks on and just below the surface by attracting magnetic particles to them. Using fluorescent magnetic particles allows for the visualization of fine cracks in the dark, providing high detection sensitivity.
- Benefits: High sensitivity to cracks on and just below the surface, real-time judgment
- Note: Applicable only to ferromagnetic materials, cannot determine the depth of cracks, demagnetization is required
Penetrant Testing (PT) – Surface
A highly penetrative colored (or fluorescent) liquid is applied to the surface, which seeps into cracks through capillary action. After removing the excess liquid, the penetrant is drawn to the surface, visualizing surface cracks and pinholes. It is applicable regardless of the material.
- Benefits: Applicable to almost all materials, simple equipment
- Note: Only for surface-opening defects; drying and cleaning are time-consuming
Radiographic Testing (RT) – Internal
X-rays or gamma rays are irradiated onto the weld, and the difference in transmission is recorded on film (or a digital detector) to detect internal defects. It has high detection capability for volumetric defects such as blowholes and slag inclusions, and offers excellent recordability.
- Benefits: High sensitivity to internal volumetric defects, objective due to film recording
- Note: Requires radiation safety management, poor detection of planar defects (cracks), and time-consuming imaging/developing process.
Ultrasonic Testing (UT) – Internal
Ultrasonic pulses are directed into the weld, and echoes (reflected waves) from internal defects or the bottom surface are detected to estimate the presence, location, and size of defects. It has high detection capability for planar defects such as cracks and lack of fusion, and is highly safe as it does not use radiation.
- Benefits: High sensitivity to planar defects, safe due to no radiation use, and applicable to thick plates.
- Disadvantages: Poor detection of spherical defects, dependent on the skill of the inspector, and inferior recordability compared to RT.
Typical Defects to be Detected in Weld Inspection
This section outlines the typical defects that can occur in welds, their causes, and effective inspection methods for their detection.
Undercut – Surface
A groove-like depression melted into the base metal at the toe of the weld bead. Caused by excessive welding current or speed. It becomes a source of stress concentration and reduces fatigue strength. Depth and length can be quantitatively measured with a 3D profiler.
Overlap – Surface
A condition where the weld metal covers the base metal at the toe of the bead without fusing. Occurs due to insufficient heat input or excessively slow welding speed. Detectable from the angle of the bead toe with a 3D profiler.
Improper Reinforcement – Surface
A condition where the reinforcement is excessive (stress concentration) or insufficient (insufficient strength). The allowable range for reinforcement height is specified in standards like JASS 6. The height can be automatically measured for OK/NG judgment using a profiler.
Insufficient Leg Length – Surface
A condition in fillet welding where the leg length is below the design value. A direct cause of insufficient joint strength. Both leg lengths can be automatically measured for tolerance management using a profiler.
Pits/Blowholes – Surface/Internal
Spherical cavities formed by gas trapped in the weld metal during solidification. Those open to the surface can be detected with a 3D profiler. Internal ones are detected by RT or UT.
Weld Cracking – Surface/Internal
Includes hot cracking during solidification and cold cracking after cooling. One of the most dangerous defects. Cracks open to the surface are detected by 3D profiler or MT, while UT is effective for internal cracks.
Lack of Fusion – Internal
A state where the weld metal and base material, or between passes, are not sufficiently fused. UT is the most effective detection method. Detection is difficult with RT depending on the direction.
Lack of fusion – Internal
Incomplete penetration at the root. Detected by UT/RT. The main causes are insufficient welding current and poor groove preparation.
Slag inclusion – Internal
A state where slag remains within the weld metal. High detection rate with RT. Caused by insufficient inter-layer cleaning in multi-layer welding.
Comparing Weld Inspection Methods
This section provides a cross-comparison of various weld inspection methods based on detection targets, accuracy, speed, cost, and suitability for automation.
Detection of Surface Defects
3D Automated Visual Inspection (Profiler)
Excellent
Visual Inspection (VT)
Excellent
Magnetic Particle Testing (MT)
Excellent
Penetrant Testing (PT)
Excellent
Radiographic Testing (RT)
Limited
Ultrasonic Testing (UT)
Limited
Detection of Internal Defects
3D Automated Visual Inspection (Profiler)
Not available
Visual Inspection (VT)
Not available
Magnetic Particle Testing (MT)
Not available
Penetrant Testing (PT)
Not available
Radiographic Testing (RT)
Excellent
Ultrasonic Testing (UT)
Excellent
Quantitative Dimensional Measurement(Leg length, reinforcement, etc.)
3D Automated Visual Inspection (Profiler)
Excellent - Automated measurement
Visual Inspection (VT)
Limited - Gauge
Magnetic Particle Testing (MT)
Not available
Penetrant Testing (PT)
Not available
Radiographic Testing (RT)
Not available
Ultrasonic Testing (UT)
Not available
Inspection Speed
3D Automated Visual Inspection (Profiler)
Excellent - Fast
Visual Inspection (VT)
Limited
Magnetic Particle Testing (MT)
Limited
Penetrant Testing (PT)
Not available - Slow
Radiographic Testing (RT)
Not available - Slow
Ultrasonic Testing (UT)
Good
Inline Suitability
3D Automated Visual Inspection (Profiler)
Excellent
Visual Inspection (VT)
Not available
Magnetic Particle Testing (MT)
Not available
Penetrant Testing (PT)
Not available
Radiographic Testing (RT)
Not available
Ultrasonic Testing (UT)
Limited
100% Inspection
3D Automated Visual Inspection (Profiler)
Excellent - 100%
Visual Inspection (VT)
Not available - Sample Inspection
Magnetic Particle Testing (MT)
Not available - Sample Inspection
Penetrant Testing (PT)
Not available - Sample Inspection
Radiographic Testing (RT)
Not available - Sample Inspection
Ultrasonic Testing (UT)
Limited
Inspector Dependency
3D Automated Visual Inspection (Profiler)
Low (Automated)
Visual Inspection (VT)
High
Magnetic Particle Testing (MT)
Medium
Penetrant Testing (PT)
Medium
Radiographic Testing (RT)
Medium
Ultrasonic Testing (UT)
High
Traceability
3D Automated Visual Inspection (Profiler)
Excellent - Digital
Visual Inspection (VT)
Limited - Difficult to record
Magnetic Particle Testing (MT)
Limited
Penetrant Testing (PT)
Limited
Radiographic Testing (RT)
Excellent - Film
Ultrasonic Testing (UT)
Good
Safety Management Burden
3D Automated Visual Inspection (Profiler)
Not required
Visual Inspection (VT)
Not required
Magnetic Particle Testing (MT)
Low
Penetrant Testing (PT)
Low
Radiographic Testing (RT)
High (Radiation)
Ultrasonic Testing (UT)
Not required
Robot Mounting
3D Automated Visual Inspection (Profiler)
Excellent - Compact and lightweight
Visual Inspection (VT)
Not available
Magnetic Particle Testing (MT)
Not available
Penetrant Testing (PT)
Not available
Radiographic Testing (RT)
Not available
Ultrasonic Testing (UT)
Limited
Automating visual inspections offers the best return on investment
Automating Weld Inspections with 3D Profilers
From visual weld inspections to inline automated inspections with 3D profiler. This guide explains how to solve the challenges of inspector shortages, quality variations, and the need for 100% inspection all at once.
Why is the automation of weld inspections necessary now?
Due to the aging and retirement of skilled inspectors, the number of personnel capable of performing visual weld inspections is decreasing year by year. Meanwhile, with the rise of electric vehicles and increased automotive component complexity, the number of welding points and the stringency of quality standards are both increasing. This has led to a rapid increase in factories facing the structural problem of 'not enough people, but more inspections.'
Automated weld inspection with 3D profilers fundamentally solves this problem. The sensor captures the cross-sectional shape of the weld bead with 3200 points per profile, automatically measuring values such as leg length, throat thickness, reinforcement height, bead width, and undercut depth. This enables quantitative and highly reproducible inspections that do not depend on the inspector's experience.
Inline Automated Inspection Procedure
- Install the sensor on a robot or conveyor – Mount the LJ-X8000 Series or LJ-S8000 Series on the arm of a welding robot or on a conveyor. Scan the bead inline immediately after welding.
- Acquire 3D cross-sectional profiles at high speed – Capture the cross-sectional shape of the weld bead on a moving workpiece in real time. The data is detailed and dense, with up to 3200 points per line.
- Automatically measure leg length, throat thickness, reinforcement, etc. – Automatically calculate dimensional values for each inspection item from the acquired profile data. Perform OK/NG judgment by comparing against set tolerances.
- 100% Recording and Traceability – All inspection results are digitally recorded. The system provides end-to-end support, from automatic ejection of NG workpieces and feedback control for welding conditions to the accumulation of quality statistics.
The Optimal 3D Profilers for Weld Inspection
KEYENCE's 3D profilers have a proven track record in automating the visual inspection of weld beads. You can select the optimal series according to the inspection target and production line.
Most Adopted for Weld Inspection – 2D/3D laser profiler LJ-X8000 Series
Acquires weld bead cross-sections with ultra-high-definition data of 3200 points per profile. It simultaneously and automatically measures undercuts, reinforcement, leg length, and bead width to perform OK/NG judgments. Supports 100% inline weld inspection on high-speed conveyor lines or mounted on robot arms.
Thanks to its proprietary multi-slit optical system, it can acquire stable profiles even on glossy metal surfaces or rough surfaces with adhered spatter. It is designed to be robust against the unique reflection conditions of weld beads.
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No Scanning Mechanism Required, Easy to Install – 3D laser snapshot sensor LJ-S8000 Series
This series inherits the technology of the LJ-X8000 Series while pursuing ease of installation by eliminating the need for motion. It is ideal for retrofitting in limited spaces or mounting on small robots. It provides sufficient accuracy and profile speed for automatic measurement of weld bead leg length, throat thickness, and reinforcement height.
The built-in measurement tools allow for easy setup of dimension measurement programs from bead cross-sections.
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Case Studies of Automated Weld Inspection
Automated weld inspection using 3D profilers has been introduced in a wide range of industries, including automotive, construction machinery, metals, and electrical equipment.
Automotive Parts – For Welding Robots
Equipped with the LJ-X8000 Series, achieve 100% bead inspection immediately after welding.
Construction Machinery
Inline quantitative management of weld bead shapes on large components.
Electronic Components
Space-saving automation of laser weld bead visual inspection with the LJ-S8000 Series.
Frequently Asked Questions about Weld Inspection
Weld inspection is broadly classified into 'visual inspection' and 'non-destructive testing.' Visual inspection includes visual testing (VT) and 3D automated visual inspection. Non-destructive testing includes magnetic particle testing (MT), penetrant testing (PT), radiographic testing (RT), and ultrasonic testing (UT). In recent years, the adoption of inline automated visual inspection using 3D profilers has been rapidly advancing.
It is performed at four main times: before welding (checking base material and groove), during welding (first layer inspection, post-slag removal inspection), after welding is complete (visual inspection, non-destructive testing, dimensional measurement), and after the structure is in operation (in-service inspection). The inspection items and applicable methods differ at each stage.
Install a 3D profiler (such as the LJ-X8000 Series or LJ-S8000 Series) on a robot arm or conveyor to acquire the cross-sectional shape of the weld bead inline. It automatically measures leg length, throat thickness, reinforcement height, bead width, undercut depth, etc., and performs OK/NG judgment by comparing them with specified values.
Visual inspection depends on the experience and skill of the inspector, which can easily lead to variations in judgment and missed defects, and makes quantitative recording difficult. Automated inspection with a 3D profiler performs quantitative judgments based on numerical data of the cross-sectional shape, eliminating variations among inspectors and enabling 100% inspection and digital traceability.
Automated visual inspection with a 3D profiler can detect surface defects such as undercut, overlap, insufficient/excessive reinforcement, insufficient leg length, pits (surface), weld cracks (surface), bead meandering, and spatter. Detecting internal defects like lack of penetration or incomplete fusion requires combined use with UT and RT.
Please feel free to contact us for test measurements on actual workpieces, proposals for inspection configurations, and more.
- 2D/3D Laser Profiler LJ-X8000 series
2D/3D Laser Profiler
LJ-X8000 series
- 3D Laser Snapshot Sensor LJ-S8000 series
3D Laser Snapshot Sensor
LJ-S8000 series
