What Is Lack of Fusion? Weld Defect
Lack of fusion (LOF) is a weld defect in which the weld metal fails to fuse completely with the base metal or with the previous weld pass. It creates a planar discontinuity (a flat, crack-like interface) within the joint that significantly reduces strength and is particularly dangerous under fatigue or cyclic loading. Most piping and pressure vessel codes treat lack of fusion as a rejectable defect regardless of size.
| Type | Location | Description |
|---|---|---|
| Sidewall LOF | Between weld metal and groove face (bevel) | Most common type; weld metal sits against the base metal without bonding |
| Inter-run LOF | Between successive weld passes | Occurs when a new pass does not melt into the surface of the previous pass |
| Root LOF | At the root of the joint | Weld metal does not fuse with the root face or backing |
Key Causes and Remedies
| Cause | Remedy |
|---|---|
| Insufficient heat input | Increase amperage or reduce travel speed |
| Improper electrode angle | Direct the arc into the groove face, not just the center of the joint |
| Excessive travel speed | Slow down to allow adequate melting of the sidewall |
| Narrow groove angle | Widen the bevel (e.g., from 30 deg to 37.5 deg per side) for electrode access |
| Contaminated surfaces | Remove oxide, scale, and slag between passes by grinding or wire brushing |
| Wrong welding process | Use a process with better penetration (e.g., GMAW spray vs short-circuit for thick sections) |
| Magnetic arc blow | Use AC current or demagnetize components before welding |
Detection Methods
Lack of fusion is a planar defect oriented parallel to the sound beam in conventional UT, which makes it detectable but angle-dependent. Radiographic testing (RT) can detect LOF when the X-ray beam is aligned with the defect plane, but it may be missed if the beam is perpendicular to the flat interface. Phased-array UT (PAUT) and time-of-flight diffraction (TOFD) are the most reliable methods for detecting and sizing LOF.
Acceptance Criteria
Lack of fusion is generally not acceptable under any piping or pressure vessel code:
- ASME B31.3: LOF is a rejectable indication
- ASME Section VIII: Not acceptable per UW-51
- AWS D1.1: Zero tolerance in CJP welds
- API 1104: LOF exceeding defined length limits is rejectable
- EN ISO 5817: Not permitted at Level B or C
Any detected LOF must be removed by gouging or grinding and re-welded per an approved repair procedure, followed by re-inspection.
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