High Temperature Limitations

Figure 2-11. Approximate Drying Times for Two Types of Nonaqueous Developers at Various Temperatures

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 2.4.5.4.2 High Temperature Limitations. Sensitivity is improved slightly when test part temperatures are 120^oF (49^oC) to 150^oF (65.5^oC). 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 120^oF (49^oC), 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 120^oF (49^oC), 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 120^oF (49^oC), unless special high temperature penetrants are used. In general if a part is too hot to handle, it is too hot for penetrant testing. 2.4.5.5 Penetrant Dwell. 2.4.5.5.1 Definition. 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. 2.4.5.5.2 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 item. a. Width and depth of the void. b. Type of penetrant. c. Part material and form. d. Type of discontinuity. e. Penetrant viscosity. f. Cleanliness of the void. 2.4.5.5.2.1 Void Size. 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. 2.4.5.5.2.2 Penetrant Sensitivity. 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. 2.4.5.5.2.3 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. 2.4.5.5.2.4 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). Change 2 2-41

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