High Temperature Limitations.
Sensitivity is improved slightly when test part temperatures are 120oF (49oC) to 150oF (65.5oC). The higher
temperature evaporates some of the liquid, which increases the dye concentration and improves the visibility
of indications. The elevated temperature also reduces viscosity, which speeds penetration. However, the
disadvantages of elevated temperatures outweigh the advantages. At temperatures of 120oF (49oC), the
volatile components of penetrants are rapidly evaporated. During penetrant dwell, the layer of penetrant is
very thin and with a part temperature of more than 120oF (49oC), the loss of volatile components will
drastically change the penetrants composition. Elevated temperatures also reduce visible dye color and
f luorescence (heat fade), making indications less visible. Penetrant inspection SHALL NOT be performed on
parts whose temperatures exceed 120oF (49oC), unless special high temperature penetrants are used. In
general if a part is too hot to handle, it is too hot for penetrant testing.
Penetrant dwell is the total length of time the penetrant is allowed to remain on the part before removal of
the penetrant. This includes immersion, soak and drain times. The purpose of dwell is to allow the penetrant
to seep into and fill any surface openings.
Factors Inf luencing Penetrant Dwell.
There are a number of interacting factors that inf luence the length of time required for penetrant to enter
and fill a surface void. Some of the factors are included in the list, which is followed by descriptions of each
Width and depth of the void.
Type of penetrant.
Part material and form.
Type of discontinuity.
Cleanliness of the void.
The dwell time required for a penetrant to enter and fill a surface void depends mainly on the width of the
surface opening and depth of the void. Penetrant enters and fills voids with wide openings more rapidly than
those with narrow openings. Very narrow or tight f laws, such as those associated with fatigue cracking, may
require 2 to 5 times the length of dwell time needed for a wider f law caused by over-stressing. The larger void
depth requires more time to fill because there is more volume of void.
The sensitivity level of penetrants is affected by the length of penetrant dwell time. The differences in dwell
times are due to the differences in surface tension, contact angle, and viscosity of the various penetrant types
and sensitivities. While viscosities between manufacturers of the same type and sensitivity level vary, the
combination of factors tends to stabilize dwell time for each type and sensitivity. This allows penetrants
within each of the sensitivity levels to have equivalent dwell times.
Part Material and Form.
The effect of part material (steel, magnesium, aluminum, etc.) and form (castings, forgings, welds, etc.) on
penetrant dwell relates to the type of f law typically found. For example, cold shuts in steel casting tend to
have tighter openings than cold shuts in magnesium castings. Therefore, the dwell time for cold shuts in
steel castings is longer than the dwell time in magnesium and aluminum castings. Discontinuities occurring
in forgings are tighter than in castings and require more dwell time.
Type of Discontinuity.
The various types of discontinuities differ in the width of the opening. Laps are tighter than porosity, and
fatigue cracks are tighter than either laps or porosity. The required length of penetrant dwell increases as
the discontinuity width decreases (surface opening becomes tighter or narrower).