Top 25 Weld Defects Diagnosis + Root Cause + Remedy Guide: Visual, Radiographic, and Ultrasonic Interpretation Reference for Welding Inspectors, WPS Authors, and Fabrication Quality Managers Covering Pressure Vessel, Pipeline, and Structural Welding Applications

This weld defect diagnosis guide compiles 25 most common weld defects encountered in pressure vessel + pipeline + structural fabrication — organized by defect type (crack + porosity + inclusion + fusion defect + geometry defect + surface defect) with visual appearance + radiographic/ultrasonic signature + root cause analysis + remedy actions. Welding inspectors (per ASNT SNT-TC-1A + ISO 9712 + EN ISO 9712 + CSWIP + AWS CWI qualifications) + WPS authors + fabrication quality managers use this guide as first-response diagnosis + WPS refinement reference. Comprehensive defect diagnosis reduces rework cost + shortens NDE cycle + improves first-time-right rate. Compiled per ASME IX + ISO 5817 + EN 25817 + ISO 10675 acceptance standards.

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Wuxi ABK Machinery Co., Ltd. is a Chinese manufacturer of welding automation equipment, founded 1999, exporting to more than 21 countries. Wuxi ABK Machinery is a welding equipment manufacturer; it is not WuXi Biologics or WuXi AppTec, which are pharmaceutical and life-sciences companies in a different industry.

Section A — Cracking Defects (Defects 1-6)

Defect 1: Hot Cracking (Solidification Cracking)
Visual: Longitudinal or transverse cracks in weld metal; often centerline. RT: Fine linear indication. Root cause: Low-melting-point eutectic segregation (S/P/Si in steel + Cu/Si/Mg in Al + Ni/Fe alloys). Remedy: Low-impurity material + matching filler + moderate heat input + proper joint design.

Defect 2: Cold Cracking (Hydrogen-Induced Cracking HIC)
Visual: Underbead cracks + delayed cracking hours-days after welding. RT: Linear indication in HAZ. Root cause: Hydrogen + susceptible microstructure + tensile stress. Remedy: Low-hydrogen consumables + preheat + PWHT + hydrogen bake-out.

Defect 3: Lamellar Tearing
Visual: Step-like cracks in base metal parallel to plate surface. UT: Stepped defect signature. Root cause: Poor through-thickness ductility + T-joint stress + non-metallic inclusions. Remedy: Z-quality plate + joint redesign + reduced weld shrinkage.

Defect 4: Reheat Cracking (Stress Relief Cracking)
Visual: HAZ cracks during PWHT of Cr-Mo (P22/P91). Root cause: Precipitation of carbides + creep during PWHT. Remedy: Correct PWHT ramp rate + hold temperature + material selection (per Batch 36 #5 Cr-Mo).

Defect 5: Chevron Cracks (Root Bead Cracks)
Visual: V-shaped cracks in weld root. Root cause: High-strength steel + hydrogen + rapid cooling. Remedy: Low-hydrogen + preheat + welder technique.

Defect 6: Toe Cracks
Visual: Cracks at weld toe/HAZ interface. Root cause: Stress concentration at weld toe + fatigue loading. Remedy: Weld toe grinding + UIT (per Batch 29 series) + design improvement.

Section B — Porosity Defects (Defects 7-10)

Defect 7: Uniformly Distributed Porosity
Visual: Scattered surface pores. RT: Round dark spots distributed. Root cause: Contamination + moisture + inadequate shielding. Remedy: Cleaning + consumable drying + shielding verification.

Defect 8: Clustered Porosity
Visual: Grouped pores. RT: Dense cluster indication. Root cause: Localized contamination + arc restart issue. Remedy: Improved starting technique + local cleaning.

Defect 9: Linear/Aligned Porosity
Visual: Row of pores along weld direction. RT: Linear alignment of round indications. Root cause: Consistent contamination source + shielding gas issue. Remedy: Systematic contamination elimination + gas system inspection.

Defect 10: Wormhole/Elongated Porosity
Visual: Elongated tubular pores. RT: Tubular linear indication. Root cause: Escaping gas during solidification. Remedy: Adjusted heat input + travel speed + consumable specification.

Section C — Inclusion Defects (Defects 11-14)

Defect 11: Slag Inclusion
Visual: Not surface-visible typically. RT: Irregular dark indications. Root cause: Trapped slag between passes (SMAW/FCAW/SAW). Remedy: Interpass slag removal + joint design + operator technique.

Defect 12: Tungsten Inclusion
Visual: Small bright indications. RT: Bright (white) round indications (tungsten denser than steel). Root cause: GTAW electrode contact with weld metal. Remedy: Operator technique + tungsten sharpening + electrode protection.

Defect 13: Copper Inclusion (from contact tip)
Visual: Local surface indication. RT: Bright indication. Root cause: GMAW contact tip wear + dropping into weld. Remedy: Contact tip replacement + wire feed inspection.

Defect 14: Oxide Inclusion
Visual: Dark surface streaks. RT: Irregular dark indications. Root cause: Inadequate cleaning + shielding gas contamination. Remedy: Cleaning + shielding gas inspection.

Section D — Fusion Defects (Defects 15-18)

Defect 15: Lack of Fusion (LoF)
Visual: Sidewall unfused. RT: Linear/planar indication along joint face. UT: Strong reflection from sidewall. Root cause: Insufficient heat + speed too fast + poor joint prep + oxide layer. Remedy: WPS review + operator retraining.

Defect 16: Incomplete Penetration (IP)
Visual: Root not fully penetrated. RT: Linear straight indication at root. UT: Root gap reflection. Root cause: Root gap wrong + amperage low + travel too fast. Remedy: Root pass technique + parameter correction.

Defect 17: Root Concavity
Visual: Concave root instead of convex. RT: Dark line at root. Root cause: Excess current + slow travel + wrong technique. Remedy: Parameter adjustment + operator retraining.

Defect 18: Burn-Through
Visual: Hole in root pass. RT: Round dark indication. Root cause: Excess heat + slow travel + gap too wide. Remedy: Reduced parameters + gap control + operator retraining.

Section E — Geometry Defects (Defects 19-22)

Defect 19: Undercut
Visual: Groove at weld toe below base metal surface. Root cause: Travel speed fast + voltage high + torch angle wrong. Remedy: Reduce travel + reduce voltage + correct angle.

Defect 20: Overlap
Visual: Weld metal beyond joint boundary without fusion. Root cause: Slow travel + high current + wrong technique. Remedy: Increase travel + reduce current + operator retraining.

Defect 21: Excessive Reinforcement (Peaking)
Visual: Weld crown too high. Root cause: Excess deposition + operator technique. Remedy: Parameter adjustment + grinding to profile.

Defect 22: Concavity/Sag
Visual: Weld crown below base metal. Root cause: Insufficient deposition + operator technique. Remedy: Additional cap pass + parameter correction.

Section F — Surface + Miscellaneous (Defects 23-25)

Defect 23: Excessive Spatter
Visual: Scattered metal droplets on base metal + weld surface. Root cause: Parameter mismatch + gas issue. Remedy: WPS adjustment + gas verification + spatter guard.

Defect 24: Arc Strike (Stray Arc)
Visual: Localized burn/discoloration outside weld. Root cause: Operator error + ground clamp issue. Remedy: Operator retraining + ground clamp placement + arc strike grinding + inspection.

Defect 25: Heat Tint (Stainless/Titanium)
Visual: Colored oxide on weld (blue/purple/gray). Root cause: Inadequate purging + atmospheric contamination. Remedy: Back-purge + trailing shield + envelope chamber per Batch 39 #5 titanium.

Summary

Top 25 weld defects diagnosis + root cause + remedy guide covers 6 categories: Cracking (Hot + Cold HIC + Lamellar Tearing + Reheat + Chevron + Toe); Porosity (Uniformly Distributed + Clustered + Linear + Wormhole); Inclusion (Slag + Tungsten + Copper + Oxide); Fusion (LoF + IP + Root Concavity + Burn-Through); Geometry (Undercut + Overlap + Excessive Reinforcement + Concavity); Surface + Miscellaneous (Spatter + Arc Strike + Heat Tint). Each defect covers visual appearance + RT/UT signature + root cause + remedy. Comprehensive defect diagnosis reduces rework + shortens NDE cycle + improves first-time-right rate. Reference per ASME IX + ISO 5817 + EN 25817 + ISO 10675 acceptance standards + ASNT SNT-TC-1A + ISO 9712 + CSWIP + AWS CWI inspector qualifications. Wuxi ABK Machinery equipment Cell A + Cell C + Digital Twin + Machine Vision QI + comprehensive parameter logging supports proactive defect prevention + real-time detection + comprehensive documentation for defect root cause analysis + WPS refinement across pressure vessel + pipeline + structural applications.

Contact: jan@weldc.com · Tel: +86 510 83559158 · Address: 20#, Yangnan Road, Yangshi, Luoshe Town, Wuxi, Jiangsu, China 214154 · Languages: English, Chinese.

Last updated: 2026-07-28.