Cold-Shift Penetrant Results on Aluminium Forgings
On the night shift the shop is at 9 degrees C and the penetrant in the tank measures 8 degrees C. The written procedure states a qualified temperature range of 10 to 50 degrees C. You inspect a batch of 40 machined 7075-T6 forgings using a water-washable fluorescent penetrant with a 15 minute dwell, a 30 second coarse rinse, forced-air dry, and a thin non-aqueous developer with 30 minutes development. On forging 17 a faint dot appears on a machined fillet at 20 minutes and by 30 minutes it has spread into a line about 6 mm long and under 0.1 mm wide. The forging is fully recleaned and re-inspected the next morning at 22 degrees C; a 1 mm dot appears and grows no further. The fillet has a 1.5 mm radius and shows no visible discontinuity.
1. Was the first inspection valid? State what must happen to the batch, not just to forging 17.
Read the worked answer
No - the first inspection was outside the qualified temperature range and is not valid evidence either way.
Why the temperature matters, physically:
- Penetrant viscosity rises as temperature falls, so capillary entry into tight, shallow defects is slower and the 15 minute dwell no longer represents the qualified dwell.
- Wash and rinse behaviour changes at low temperature: water-washable penetrants can be harder to rinse cleanly from a rough or filleted surface, leaving a background film that later bleeds into a line.
- Developer drying and draw-out kinetics slow down, so a 30 minute development is no longer equivalent.
The defect-detection capability is therefore unproven, not merely degraded. Every forging inspected in that window - all 40, not only 17 - must be re-inspected after bringing the part and the penetrant to at least 10 degrees C and holding them there, with the dwell adjusted per the procedure if the procedure allows it. Record the temperature excursion as a nonconformance; the 22 degrees C re-inspection of forging 17 is the only valid data you currently hold for the batch.
2. Explain the difference between the two results on forging 17 and describe the test you would run to settle it.
Read the worked answer
The two results differ in shape and in growth behaviour, and that difference is diagnostic:
- Cold inspection: a dot that grew into a 6 mm line during development. A line that keeps extending usually means a reservoir of penetrant is draining along a surface channel or a lap, or the rinse left a film on the fillet that the developer drew out.
- Warm inspection: a 1 mm dot that stopped growing. That is the signature of a small, closed reservoir - a pore, a forging lap or a machining tear - bleeding out a finite volume and then exhausting.
A genuine tight crack refills from a long thin volume and typically reproduces as a line of similar length each time. It did not.
Decisive test: local material removal in controlled steps (about 0.05 mm per step by blending or light grinding), re-inspecting after each step, until the source is exposed or the indication disappears. Record the depth at which it disappears; a defect that vanishes within the machining allowance is generally dispositioned as a surface irregularity, while one that persists needs engineering evaluation. Confirm the location with magnification first so you grind in the right place.
3. If the 6 mm line had been produced under qualified conditions, how would you classify and measure it?
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Classification: the indication is 6 mm long and under 0.1 mm wide. Because the length is far greater than three times the width, it is a linear indication, not a rounded one. The shape category drives which acceptance clause applies, so this is the first call to make.
Measurements to take (on the qualified result only):
- Length along the indication, end to end, at the surface - here about 6 mm. Do not measure the halo.
- Width of the tight core, not the bleed-out.
- Orientation relative to the forging grain flow and to the principal stress direction, since a linear indication transverse to grain flow in a forging is treated far more severely than one parallel to it.
- Location referenced to a datum on the drawing, so an evaluation can be made against the permitted indication size for that zone.
Caution: acceptance criteria apply to the indication as produced under the qualified process. The low-temperature result is not a valid basis for classifying or rejecting anything - using it would reject a part on an artefact.