Too much background!
Recently we were asked how to reduce the amount of penetrant background on a rough surface. For those familiar with castings and forgings, this is a common problem. Typical approaches to solving this problem are longer and more powerful rinses or longer emulsification times for those using the post emulsifiable method. If we take a microscope and look at the surface of a rough
material, we would see peaks and valleys across the surface. This is an ideal surface to entrap penetrant. In the January 2018 Penetrant Professor we demonstrated that penetrants can enter discontinuities with openings as small as 550nm, so entrapment on a rough casting is no challenge.
The customer wanted to know if a lower sensitivity penetrant would reduce the amount of background. This approach sounds logical, but would not entirely reduce the background as it may only reduce the brightness, along with that of any indications.
After further discussions with the customer the culprit for the high background was the type of developer being used.
In this job, the technicians were using a non-aqueous developer. Nonaqueous developers have a unique way of developing penetrant indications as the solvent mixes with the penetrant causing it to expand out of a crack and spread, making the indication much larger than they actually are. On a rough surface, this dilution of penetrant by the solvent will not only expand the crack indications, but will thin and spread the surface entrapped penetrant creating an excessive amount of background bleed out.
Other factors effecting the bleed out are heavy and wet developer spray applications. Proper spray technique is important when using nonaqueous developer.
A dry powder developer on the other hand does not have the solvent diluting effect. Dry powder developers are attracted to and cling to areas that are wet, mainly flaw openings. The lack of solvent and the lower attraction to the relatively dry surface will generally result in reduced background compared to nonaqueous developer.
Aqueous developers also can produce lower backgrounds with PE penetrants, but with WW penetrants a general blue white haze can result.
On very porous surfaces such as sintered metal or as found in additive manufacturing (3D printed metal) surfaces penetrant inspection is difficult because of the high background. Inspection without developer or inspecting the water wet surface before drying will show larger cracks with very little background. Neither approach is recognized by most process specifications such as
ASTM E-1417 or AMS-2647
Rough surfaces are a challenge for penetrant inspection, but selection of the right materials and process technique can greatly improve flaw detectability.
Non-aqueous developer (left), dry powder developer (right)