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Penetrant enters a discontinuity by capillary action. The driving pressure is 2 sigma cos(theta) / r - surface tension divided by the half-width of the opening - and the rate of filling is set by that pressure against the viscous drag of the liquid moving along a very narrow channel. Both terms work against speed: the tighter the crack, the higher the driving pressure but the far greater the drag, so tight discontinuities fill slowly. Dwell time is the time allowed for that filling to happen, and it is the single step most often cut short in practice.
The code tables key dwell to material and product form because those predict the geometry of the discontinuity. Castings and welds contain relatively open defects - gas porosity, shrinkage, cold shuts, lack of fusion - which fill quickly, so 5 min is enough. Wrought material - forgings, extrusions, plate, bar - contains laps and tight forging cracks that have been squeezed nearly closed by the working process, so 10 min is required. Service-induced cracking such as fatigue and stress corrosion is tighter still and routinely needs far longer than any table minimum.
Temperature enters through viscosity. Penetrant thickens as it cools and fills more slowly, which is why the standard qualified range is 40 to 125 °F (5 to 52 °C); outside it the technique must be qualified on a cracked comparator block against a standard-temperature run. In the cold end of that range, extending dwell beyond the table minimum is normal practice.
The penetrant must not be allowed to dry on the part. A dried film cannot be removed cleanly and leaves background that hides indications, so long dwells on hot or windy sites need the surface replenished rather than simply left.
Minimum penetrant dwell, ASME V Table T-672: Metals - castings and welds: 5 min Metals - wrought (forging, plate, bar, extrusion, tube): 10 min Carbide-tipped tools, plastic, glass, ceramic: 5 min Minimum developer dwell, all of the above: 10 min Dwell used = greater of the table minimum and the procedure minimum Standard temperature range: 5 to 52 °C (40 to 125 °F)
- Table values are minimums, not targets. Tight service-induced cracking - fatigue, stress corrosion, grinding cracks - commonly needs far longer, and the written procedure governs.
- Dwell starts when the penetrant is applied and ends when removal begins, including any drain time.
- The penetrant must not dry on the surface. Replenish on hot or windy sites rather than letting the film go off.
- Dwell in excess of the maximum permitted by the procedure is a non-conformance in its own right; over-dwelled penetrant is difficult to remove and raises background.
- Developer dwell is measured from when the developer has dried on the part, not from when it was sprayed.
- Below about 10 °C consider extending the dwell to allow for the increased viscosity, even within the qualified range.
Reference: ASME BPVC Section V (2023 Ed.), Article 6, Table T-672 - minimum dwell times; ASTM E165/E165M-18 Table 1 gives the same values. Standard temperature range 40 to 125 °F (5 to 52 °C) per ASME V Article 6 and ASTM E165. Note that ISO 3452-1 uses 10 to 50 °C instead.
