Views: 222 Author: Loretta Publish Time: 2025-12-24 Origin: Site
Content Menu
● External vs Internal Welding Defects
● Common Types of Welding Defects (with Causes and Prevention)
>> 1. Weld Cracks (Hot and Cold)
>> 3. Undercut
>> 4. Porosity (Gas Pores, Wormholes, Surface Porosity)
>> 5. Spatter
>> 9. Incomplete Fusion (Lack of Fusion)
>> 15. Whiskers
● Quick Reference Table for Key Welding Defects
● Modern NDT Methods to Detect Hidden Welding Defects
● Weld Discontinuity vs Weld Defect in Practice
● Partner for Reliable Welded Components
● Frequently Asked Questions About Welding Defects
>> 1) What is the most dangerous welding defect?
>> 2) How can manufacturers reduce welding defects in production?
>> 3) Which NDT method is best for detecting internal porosity?
>> 4) Can small welding defects be accepted without repair?
>> 5) How often should welded structures be inspected in service?
Welding defects are flaws in a weld that reduce strength, safety, or appearance, and understanding them is critical for reliable metal fabrication and OEM production. This guide explains the most common types of welding defects, their causes, how to prevent them, and how professionals inspect welds using modern NDT methods.
Welding defects are irregularities in a weld that exceed acceptable limits defined by widely used welding standards such as ISO classifications and quality level specifications. When a weld imperfection affects structural performance, safety, or function, it is treated as a defect and must be repaired or rejected.
- Weld defect: An imperfection that makes the weld unacceptable for its intended use under specified standards or project requirements.
- Weld discontinuity: An observable irregularity that may still be acceptable if it remains within allowed limits for size, location, and service conditions.
Clear acceptance levels for imperfections are usually defined in international welding standards and project specifications.
Weld defects are often grouped by where they occur in the joint and how they are detected.
- External defects: Visible on the surface and typically found by visual inspection or simple surface testing, such as undercut, overlap, surface porosity, surface cracks, and spatter.
- Internal defects: Hidden within the weld metal or heat affected zone and detected with volumetric testing, such as incomplete fusion, slag inclusions, or internal porosity.
In structural welding, pressure vessels, and precision fabricated parts, both categories can lead to serious failures if not properly controlled.
The following sections describe the most frequent weld defects, their typical causes, and practical prevention measures used in modern welding production and fabrication.
Weld cracks are planar fractures in the weld or base metal and are among the most critical defects because they concentrate stress and can propagate quickly under load.
Typical causes:
- Poor ductility or contamination in base or filler metal.
- High restraint and high residual stress during cooling.
- High sulfur or carbon content in base steel.
- Lack of preheat and rapid cooling, especially in thick or high strength steels.
- Excess hydrogen in the weld from moisture or unsuitable shielding gas.
Prevention:
- Use low hydrogen consumables with compatible base materials and keep them dry.
- Apply adequate preheat and control heat input to slow down cooling.
- Optimize joint design to reduce restraint and stress concentration.
- Avoid hydrogen bearing shielding gases for susceptible steels.
Crater cracks form at the end of a weld bead when the molten pool is not properly filled before extinguishing the arc.
Typical causes:
- Abrupt arc stop without crater fill.
- Insufficient weld metal at the termination area.
- Incorrect torch or electrode manipulation at the end of the weld.
Prevention:
- Use crater fill techniques or current down slope functions at the bead end.
- Maintain correct travel speed and torch angle during weld termination.
- Apply short weave or circular motions to fully fill the crater before stopping.
Undercut is a groove along the weld toe where base metal is melted away but not filled with weld metal, reducing effective cross section and fatigue strength.
Typical causes:
- Excessive voltage or travel speed.
- Incorrect electrode angle or too large electrode diameter.
- Inadequate filler metal or poor technique near the weld toe.
Prevention:
- Reduce arc voltage and travel speed to stabilize the pool at the toe.
- Keep the electrode angle in the recommended range for the chosen process.
- Use appropriate filler size and position the arc correctly at the joint edges.
Porosity occurs when gas is trapped in the weld metal as it solidifies, creating spherical or elongated cavities that reduce strength and can cause leaks or fatigue issues.
Typical causes:
- Contaminated surfaces, such as oil, rust, paint, or moisture.
- Damp or corroded electrodes and contaminated shielding gas.
- Excessive gas flow or turbulence around the arc.
- Incorrect shielding gas type or poor gas coverage.
Prevention:
- Clean base metal surfaces thoroughly and store consumables in dry conditions.
- Use high purity shielding gas and correct flow rate for the welding process.
- Adjust welding speed so gas can escape before the pool solidifies.
- Apply preheat when necessary to drive off moisture.
Spatter consists of small metal droplets ejected from the arc that adhere around the weld bead, impacting appearance and increasing cleanup time.
Typical causes:
- Incorrect polarity, low voltage, or excessive amperage.
- Long arc length and unstable arc conditions.
- Unsuitable shielding gas or poorly cleaned base metal.
Prevention:
- Set proper current, voltage, and polarity for the process and joint.
- Maintain a short, stable arc length during welding.
- Use recommended shielding gas mixtures and clean the workpiece thoroughly.
Overlap occurs when molten weld metal flows over the base metal without fusing, forming a lip that can act as a severe stress raiser.
Typical causes:
- Excessive weld size or high heat input.
- Slow travel speed causing the pool to overflow.
- Incorrect electrode or torch angle.
Prevention:
- Reduce weld size and heat input to appropriate levels.
- Increase travel speed to keep the pool under control.
- Maintain correct torch or electrode angle and position.
Lamellar tearing appears as step like cracks in rolled steel plates, usually parallel to the surface and associated with through thickness inclusions.
Typical causes:
- High through thickness restraint at T or corner joints.
- Plate with low through thickness ductility and non metallic inclusions aligned in rolling direction.
Prevention:
- Use steels with improved through thickness properties.
- Modify joint design to reduce through thickness stress.
- Plan welding sequence to limit restraint in critical areas.
Slag inclusions are non metallic particles trapped in the weld metal or between passes, reducing toughness and effective cross section.
Typical causes:
- Inadequate slag removal between passes.
- Low current or improper travel speed.
- Poor electrode angle and tight joint geometry that traps slag.
Prevention:
- Clean each weld pass thoroughly before depositing the next pass.
- Adjust current and travel speed within recommended ranges.
- Modify electrode angle and joint design to allow slag escape.
Incomplete fusion occurs when weld metal fails to fuse with base metal or earlier weld beads, leaving unbonded interfaces that reduce joint strength.
Typical causes:
- Low heat input or excessive travel speed.
- Incorrect electrode diameter for thickness.
- Poor joint preparation, contamination, or inadequate cleaning between passes.
Prevention:
- Increase heat input and adjust travel speed to ensure full sidewall fusion.
- Select suitable electrode or wire size and bevel angle.
- Clean joint faces and remove defects before welding subsequent passes.
Incomplete penetration occurs when the weld does not reach the root, leaving an unwelded section at the joint bottom.
Typical causes:
- Inadequate root opening or overly thick root face.
- Low current or excessively fast travel speed.
- Poor alignment and incorrect torch or electrode positioning.
Prevention:
- Prepare correct joint geometry, including root gap, bevel, and backing when necessary.
- Use adequate welding current and moderate travel speed to reach the root.
- Ensure accurate alignment and maintain correct torch position along the joint.
Distortion is the permanent change in shape of welded parts caused by uneven heating and cooling.
Typical causes:
- High heat input and unbalanced weld placement.
- Thin sections with insufficient fixturing or clamping.
- Incorrect welding sequence along the structure.
Prevention:
- Use balanced welding sequences and back step techniques.
- Apply proper fixturing, clamping, and tack welds.
- Control heat input and use intermittent welds where design allows.
Burn through occurs when too much heat melts through the base material, creating a hole or excessively wide root opening.
Typical causes:
- Excessive current and slow travel speed.
- Large root gap or thin material without backing or chill bars.
- Incorrect wire size or bevel angle for the material thickness.
Prevention:
- Reduce current and optimize travel speed for material thickness.
- Improve fit up and use backing bars or chill bars on thin materials.
- Choose suitable wire size and joint configuration.
Mechanical damage includes gouges, dents, and stray arc strikes caused by improper handling of tools or aggressive grinding.
Typical causes:
- Excessive force during chipping and grinding operations.
- Careless use of electrode holder or arc strikes outside the joint area.
Prevention:
- Train personnel on correct mechanical finishing techniques.
- Restrict grinding and chipping to specified locations only.
- Avoid accidental arc striking on the base metal.
Excess reinforcement is too much weld metal at the root or face, which can create stress concentration, interfere with fit, or violate design rules.
Typical causes:
- Excessive filler metal feed rate.
- Inconsistent travel speed or poor bead control.
Prevention:
- Control filler addition and maintain steady travel speed.
- Follow specified bead size and weld profile in the procedure.
- Adjust voltage and amperage for flatter and more controlled beads.
Whiskers are short wire stubs protruding from the root side, mainly in MIG or wire feeding processes, which can cause flow disturbances in piping.
Typical causes:
- Excessive wire feed speed.
- High travel speed and poor torch positioning.
Prevention:
- Match wire feed speed to welding current and travel speed.
- Place the wire correctly at the leading edge of the molten pool.
Misalignment is the offset between joined parts, seen as steps across the weld that reduce fatigue performance and may violate tolerance requirements.
Typical causes:
- Poor fixturing, clamping, or joint fit up.
- Excessive distortion during welding.
- Inaccurate or insufficient tack welds.
Prevention:
- Use accurate fixtures, jigs, and tack welds.
- Control distortion with balanced sequences and correct heat input.
- Inspect and adjust alignment before and during welding.
Defect type | Main risk | Typical cause | Core prevention |
Cracks | Sudden brittle failure | High stress, hydrogen, rapid cooling | Preheat, low hydrogen consumables, reduce restraint |
Porosity | Reduced strength, leaks | Contamination, poor shielding | Clean surfaces, correct gas, correct flow |
Undercut | Fatigue cracking | High voltage, fast travel | Lower voltage, slower travel, correct angle |
Lack of fusion | Loss of load bearing area | Low heat input, high travel speed | Increase heat, adjust speed, prepare joint |
Incomplete penetration | Weak root, premature failure | Poor prep, low current | Correct gap and bevel, higher current |
Slag inclusion | Lower toughness, crack origin | Poor cleaning, low current | Clean passes, adjust current and electrode angle |
Modern production relies heavily on non destructive testing (NDT) to detect internal defects without damaging parts.
- Visual Testing (VT): First line of inspection for surface defects, weld contour, and general workmanship.
- Dye Penetrant Testing (PT): Highlights surface breaking cracks and porosity on non ferromagnetic materials.
- Magnetic Particle Testing (MT): Reveals surface and near surface cracks in ferromagnetic materials using magnetic fields and indicators.
These methods are cost effective and widely used for day to day weld inspections.
- Ultrasonic Testing (UT): Uses high frequency sound waves to detect internal cracks, lack of fusion, and inclusions, allowing depth and size evaluation.
- Radiographic Testing (RT): Uses X rays or gamma rays to produce images that show internal porosity, slag inclusions, and incomplete penetration.
Advanced techniques such as phased array ultrasonic testing and digital radiography enhance detection and documentation for critical applications.
In actual projects, quality engineers distinguish between tolerable discontinuities and rejectable defects using the applicable codes and service conditions.
- A discontinuity is acceptable if it remains within defined limits for size, length, and location in the relevant standards.
- The same indication may be acceptable in non critical components but treated as a defect in highly stressed or safety critical parts such as pressure equipment or structural members.
Clearly defined acceptance criteria in welding procedure specifications and inspection plans reduce ambiguity and rework.
If your business relies on high precision metal parts, plastic components, silicone products, or stamped metal assemblies, consistent weld quality is essential to overall performance and brand reputation. To reduce welding defects, stabilize your supply chain, and scale production with confidence, consider partnering with an OEM manufacturer that combines strict process control, qualified welding procedures, and comprehensive inspection capability. Contact our team today to discuss your drawings, technical requirements, and target volumes, and explore how a dedicated Chinese OEM partner can support your next project from prototype to full scale production.
Cracks are often considered the most dangerous welding defect because they can propagate rapidly under load and cause sudden failure, even when other weld parameters appear acceptable.
Manufacturers usually reduce welding defects by qualifying robust welding procedures, training and certifying welders, preparing joints carefully, and applying systematic inspection and non destructive testing at critical stages.
Radiographic testing is often preferred for detecting internal porosity patterns because it shows the shape and distribution of pores, while ultrasonic testing is useful for thicker sections and complex geometries.
Small indications can be accepted without repair if they comply with the acceptance criteria defined in the applicable standard or project specification, taking into account size, location, and service conditions.
Inspection frequency depends on industry codes, loading conditions, environment, and risk level, but critical welded structures often require scheduled non destructive testing during service to monitor fatigue sensitive or highly stressed areas.
