Examination method
Radiographic testing (RT)
Penetrating radiation passes through the weld onto film or a digital detector, and differences in how much is absorbed produce a permanent image of the joint's internal condition.
How it works
A source of X-rays or gamma rays is placed on one side of the joint and a film or detector on the other. Radiation passing through the material is absorbed according to thickness and density, so more radiation reaches the detector wherever there is less material in the path, such as at a void or an inclusion of lower density.
The result is a shadow image in which cavities, slag, and missing metal appear darker than sound weld, while denser material such as a tungsten fragment appears lighter. Image quality indicators placed on the part demonstrate the sensitivity actually achieved, giving objective evidence that the technique could resolve flaws of a given size.
Detection depends strongly on flaw orientation. A void reduces material in the beam path from any direction, so porosity and slag show clearly. A tight planar crack removes almost no material unless the beam lies nearly along its plane, which is why radiography is excellent for volumetric flaws and unreliable for cracks and sidewall fusion problems.
Strengths
- Produces a permanent, reviewable record of the weld's internal condition
- Excellent at volumetric flaws such as porosity, slag, and incomplete penetration
- Images the full weld width in a single exposure rather than point-by-point scanning
- Interpretation is largely independent of the person who took the exposure, since the record can be reviewed later
Limitations
- Poor at detecting tight planar flaws unless the beam happens to align with the crack plane
- Radiation hazard requires exclusion zones, monitoring, licensing, and often night-shift working
- Gives no depth information, so a flaw's position through the thickness is unknown from one exposure
- Access to both sides is normally needed, and thickness limits apply for a given source strength
Key variables
- Source type and energy
- X-ray tubes give adjustable energy and better contrast on thin sections; gamma sources are portable and penetrate heavy material.
- Source to film distance
- Greater distance sharpens the image by reducing geometric unsharpness, at the cost of longer exposure time.
- Exposure time and film speed
- Together set image density; too little exposure gives a pale image with no usable contrast for interpretation.
- Image quality indicator
- A calibrated step or wire set placed on the part that demonstrates what size of detail the technique could actually resolve.
- Beam orientation
- Decides whether planar flaws are seen at all, which is why the method is chosen for volumetric flaws and paired with UT for cracks.
What an inspector watches for
- Check image quality indicator visibility, since it is the objective proof the technique achieved the required sensitivity
- Verify identification and location markers so the image can be tied back to a specific length of a specific weld
- Assess film density and contrast before interpreting, because an underexposed or fogged image hides indications rather than showing none
- Confirm radiation safety controls, boundaries, and dosimetry are in place, as this is the one method that endangers everyone nearby