How to Interpret Radiography Results

Radiographic testing (RT) is a non-destructive examination method that uses X-rays or gamma rays to produce an image of a pipe weld on film (conventional RT) or a digital detector (digital RT). Interpreting radiography results requires understanding how weld defects appear as density variations on the radiographic image, and comparing those indications against the acceptance criteria of the applicable code.

How to Read a Radiographic Film

1. Verify film identification: Check the lead markers identifying the weld joint number, pipe spool, date, and RT technician.
2. Check image quality: Verify the Image Quality Indicator (IQI) is visible and meets the required sensitivity (typically 2% of wall thickness per ASME V, Article 2).
3. Verify film density: Measure density using a densitometer. Acceptable range: 1.8 to 4.0 per ASME V.
4. Scan the weld systematically: View on a properly illuminated viewer. Scan the entire weld length including the heat-affected zone (HAZ).
5. Identify indications: Note density variations (dark or light spots, lines) differing from the surrounding weld and base metal.
6. Measure indication size: Determine the length, width, and location of each indication.
7. Apply acceptance criteria: Compare against the applicable code (e.g., ASME B31.3 Table 341.3.2).

Common Weld Defect Indications on Radiographs

Defect Type	Appearance on Film	Shape	Cause
Porosity (gas pores)	Round dark spots	Circular, scattered or clustered	Gas entrapment during welding
Slag inclusion	Irregular dark spots	Elongated, irregular edges	Trapped slag between weld passes
Lack of fusion (LOF)	Linear dark line along fusion line	Straight or slightly curved	Insufficient heat input at sidewall
Lack of penetration (LOP)	Dark line at weld root centerline	Straight, uniform	Root gap too narrow, low amperage
Crack (longitudinal)	Fine dark line, often branching	Jagged, sharp edges	Thermal stress, hydrogen embrittlement
Crack (transverse)	Fine dark line across weld	Perpendicular to weld axis	Shrinkage stress, high restraint
Undercut	Dark line at weld toe	Follows weld edge	Excessive heat input, poor technique
Burn-through	Very dark localized area at root	Circular or oval	Excessive heat at root pass
Tungsten inclusion	Bright (light) spot	Small, dense, well-defined	Tungsten electrode contamination (GTAW)
Concavity (root suck-back)	Slight darkening at root	Uniform along root	Excessive purge gas, high root gap

ASME B31.3 Acceptance Criteria (Summary)

Indication Type	Acceptance Limit (ASME B31.3, Table 341.3.2)
Slag inclusions or cavities	Max. individual length: lesser of 1/3 wall thickness or 2.4 mm; max. cumulative in 12t length: wall thickness
Porosity	Max. individual pore: 1/4 wall thickness or 1.6 mm (whichever is less); scattered porosity per charts
Lack of fusion or lack of penetration	Not acceptable
Cracks	Not acceptable (any length)
Undercut	Max. depth: 1 mm or 12.5% of wall thickness (whichever is less)
Root concavity	Acceptable if total wall thickness (including concavity) exceeds minimum required

Common Mistake

Do not interpret radiography results under insufficient lighting. A film viewer (illuminator) must produce uniform illumination of at least 1000 cd/m2 for standard density films. Viewing films against ambient light or on a low-intensity viewer leads to missed defects, particularly fine cracks and small slag inclusions.

Key Points

• Interpretation must be performed by a certified Level II or III RT technician per ASNT SNT-TC-1A or ISO 9712.
• RT is most effective for volumetric defects (porosity, slag). For crack detection, ultrasonic testing (UT) or TOFD is more sensitive.
• All RT reports must be retained as part of weld quality documentation, reviewed during piping inspections.

Read the full guide to piping inspections