What Is Dye Penetrant Testing?
What Is Dye Penetrant Testing?
Dye Penetrant Testing (PT or DPI) is a type of non destructive testing using a dye penetrant to inspect for surface breaking faults in non-porous materials such as metals, plastics and ceramics. This form of testing is often applied to non-ferrous materials, as magnetic particle inspection is more often applied to ferrous materials for its reliability and ability to detect a small amount of subsurface discontinuities. In dye pen weld inspections, a liquid penetrant is applied to a surface and left to stand for a period of time, before being cleaned away, where a developer is applied to the surface to draw the penetrant out of any flaws in the material, which will make the flaw show up in an obvious way. It is often used to detect surface discontinuities and defects in forging, casting and welding such as surface porosity, hairline cracks and fractures, fatigue damage and leaks.
The History Of Dye Pen NDT
One of the first known uses of NDT penetration testing was in the 1900’s where a method known as oil and whiting was used in the railroad industry to detect cracks and defects on trains, tracks, and engines. This method used the application of an oil solvent mixed for cleaning, which was then followed by the application of a chalk coating (whiting), which absorbed oil from the cracks allowing them to be detected. By the 1940’s, the technique had developed so that a dye (often fluorescent) was used. Over time, the importance of temperature and the amount of time the material was left to soak was discovered, and the process was gradually refined over many decades until any manufacturers could guarantee a fast, low cost and accurate result by visiting a Dye Pen NDT Technician.
How Does It Work?
The first step in any dye pen weld inspection is to clean the surface thoroughly beforehand to remove paint, dirt, oil, and any impurities on the surface of the material that is to be inspected. This is important as these impurities can show up as if they were cracks and give skewed results if not adequately removed. Modern day cleaning methods include solvents, vapour degreasing, and alkaline cleaning steps. The penetrant is then applied to the surface. The penetrant used is normally a brightly coloured dye with high wetting capabilities, often fluorescent dye is used. This is then allowed to soak into the material for an amount of time, normally between 5 minutes and half an hour, which depends on the size of the flaws being tested for. Larger and more obvious flaws need less soaking time, whereas testing for smaller, harder to detect flaws require much longer. Once the penetrant has been left to settle for an appropriate amount of time, the excess penetrant is removed from the surface. Emulsifiers are a common method used to do this, as they chemically react with the oily penetrant to make it removable by applying water, though solvent removers are also often used, though the Dye Pen NDT Technician must be careful not to spray solvent removers directly onto the surface as this could also remove the penetrant from the flaws needed to be inspected. A white developer is then applied to the surface, normally in the form of a dry powder or non-aqueous wet developer. This developer draws the penetrant out of the defects and onto the surface to give a visual indication of the flaw. This is commonly known as a bleed-out. Areas that bleed out can show the location, orientation, severity and even the possible type of the discontinuity. Development time varies, but is normally between 10 to 30 minutes. An inspector will then use a bright, intense light source (sometimes ultraviolet if fluorescent penetrant has been used) to inspect the flaws and to see if the material has reached the stand it needed to reach to be considered usable.
Advantages & Disadvantages Of Dye Pen Weld Inspection
Dye Penetration Testing has two major advantages over other forms of NDT Weld Inspection, namely its comparative speed and relatively low cost. A Dye Pen NDT Technician is able to carry out dye penetrant testing both onsite and offsite with relative ease. However, some disadvantages are that it can only detect surface flaws, and needs to be carried out on a smooth, flat surface to fully be effective and not come up with false indications.
Some common errors that may impede the quality of dye penetrant test results include: Test surfaces are not cleaned adequately, the contact time between the penetrant and the test surface is too short, or the excess penetrant is removed carelessly (i.e. from flaws as well as from the test surface). For these reasons, it is important that personnel carrying out liquid penetrant inspections are properly trained and qualified (for example, in accordance with the British Institute of Non Destructive Testing’s PCN certification scheme or equivalent schemes such as those operated by CSWIP and ASNT) and work for reputable NDT companies in the UK.
At SureCheck, you can be confident in our diligent staff, high quality equipment and years of experience to conduct the job properly and efficiently. Get in touch for a quote or to find out more.