This subject was discussed in our 2nd anniversary issue of the Penetrant Professor in 1995. Funny how time flies but the questions remain the same.
Every penetrant manufacturer is asked about what temperature their penetrant should be used at. In the case of Met-L-Chek penetrants, the label on the aerosol can gives the recommended use range which reflects the range specified in ASTM E-165 and ASTM E-1417; 40˚F-125˚F(4˚C-52˚C). In ASTM E-1417 it states that penetrant dwell times should be increased to 20 minutes rather than the normal recommended minimum of 10 minutes(5 minutes in ASTM E-165), for temperatures between 40˚F-50˚F (4˚C-52˚C). Most users take this as a guide and some think it means penetrants do not work below 40˚F(4˚C). There are two factors which are involved in deciding what to do if the temperature is not in the recommended range. The two factors are the parts to be tested, and the penetrant materials. Each is important.
This is where it is really important to understand the factors, because low temperatures pose special problems for penetrant inspection. Penetrants, like any liquid become more viscous as the temperature drops. Increased viscosity results in slower penetration. Many in-house specifications recognize this and state that the penetration time should be doubled or even lengthened to half an hour. Some simply state that it should be increased, but by how much should it be increased? This depends on the assessment of the factors, perhaps some tests of the actual conditions, as called for in ASTM E-165 ¶10.2, and then a decision by the level III, if the specification being used does not have specific guide lines.
Can penetrants be formulated specifically for low temperatures? Sure they can and there are some on the market. As expected, these penetrants may have very low viscosity at room temperature, so that when they are cold their viscosity is about what one would expect for a conventional penetrant at room temperature. Does this do the job? Not always, because it neglects
the other factor – the part. The part just like the penetrant, is subject to physical laws. At low temperature, the coefficient of expansion of the metal part will cause it to shrink. When the part shrinks, any flaws or cracks will tighten up, become smaller, more difficult to penetrate, and harder to locate. This phenomenon is well known to mechanical engineers, mechanics, and others who work with metals. So, even though you might be using a “low temperature” penetrant, you may have trouble because you forgot that the metal, and the cracks, have shrunk. Because of this, they will be harder to locate as the indications will be smaller than expected. If one had been using a visible dye penetrant a better choice might be a fluorescent penetrant, or if a fluorescent had been used, a higher sensitivity fluorescent penetrant would make the smaller cracks more detectable. Then use plenty of penetration time. Without actual testing of a part with a known defect under the low temperature conditions with the intended use penetrant, it is not possible to say how long a penetration time should be used. Doubling or tripling the penetration time is a nice rule of thumb but is no substitute for actual application verification.
With high temperature inspection, the part is hot and the cracks have enlarged and are easier to locate. Within reason, there is little problem with the penetrants at higher temperatures because any penetrant becomes less viscous, which is a good thing. But, if the temperature is too high, it may cause some of the ingredients to evaporate and this may adversely affect how the penetrant
performs. At still higher temperatures the dye in the penetrant may decompose and render the penetrant useless for flaw detection. Fluorescent penetrants will begin to lose their fluorescence at
around 200˚F(93˚C), one of the reasons dryer temperatures are not to exceed 160˚F(71˚C) per ASTM E-1417¶6.6.2.
For test temperatures 150˚F(65˚C) to roughly 350˚F(176˚C) special high temperature penetrants are available, relying on visible dyes. The properties of this type of penetrant must be such that there is a minimum of evaporation of the constituents and no degradation of the dye. Before the penetrant is used, it should be applied to a small area and observed over five to ten minutes for any indication of dye fading or color change. If this occurs then the part temperature is too high for the penetrant material. If the color holds the inspection can proceed. Keeping penetration time to a minimum will help against over exposure and dye decay.
Met-L-Chek standard pentrants are designed to work well between 40˚F(4˚C) and 150˚F (65˚C). When working at very low temperature we would recommend the use of visible red penetrant VP-31A because of its low initial viscosity, and availablity in spray cans. Increased dwell time must be determined. We would recommend FP-95A(M) level 3 fluorescent penetrant for highlighting
cracks if fluorescent penetrant is the choice. Again low viscosity, high fluorescent brightness, and its availablity in 16 oz. spray cans. For high temperature applications, 150˚F(65˚C) – 50˚F(176˚C), VP- 302 visible penetrant has been proven to work well.