Magnetic Particle Theory and Processing Steps
Magnetic Particle Testing is performed by inducing a magnetic field in ferromagnetic material and covering the surface with magnetic particles. Surface and near-surface discontinuities will distort the magnetic field and create a leakage field. The magnetic particles will concentrate near the leakage field, thus indicating their presence by Visible Light or Black Light.
The apparent simplicity of the magnetic particle process is misleading. It is often incorrectly assumed that the process steps are simply magnetizing, applying the particles and inspection. This misunderstanding results in the assignment of personnel with little or marginal training who are not aware of the importance of their particular tasks and the effects they have on the overall process. The magnetic particle method requires a diligent awareness of each processing step from initial application to process completion.
Magnetic Particle MaterialsFluorescent particles are used in the wet method to aid visibility, which requires the use of a black light and fluoresce at 365 nm in the longwave region. The particles are suspended in water or oil. To achieve the required test sensitivity, the particle concentration in the bath must be correct and a water based material must have the correct chemistry. Magnetic Particles must possess two important characteristics:
Magnetic particles containing these characteristics will give maximum response in a leakage field, but will not remain magnetized when the magnetic field is removed.
The shape of the magnetic particle should be smooth and must have a high degree of mobility and still have substantial attractive power.
There are two types of magnetic particles: Iron Oxides (Iron and Oxygen combination) and “Pure” Iron. It is important to have an understanding of the type discontinuity that needs to be detected. Either particle is functional, but chosen dependent upon the size discontinuity.
Iron Oxides are finer, lighter particles and ideal for very tight, critical discontinuities. The Iron Oxide will stay in suspension with simple agitation. The Iron Oxide shape can be controlled by the oxidation/reduction process. The Iron Oxide will remain in the discontinuity more effectively than the “Pure” Iron.
The “Pure” Iron particles are larger in particle size and can be used to detect medium to large discontinuities. The geometry and size of these particles will render a much brighter particle than the Iron Oxide particle. The Iron particles will not stay in suspension as easy as the Iron Oxides and requires a more aggressive, constant agitation. These Straight Iron particles will migrate quicker to the discontinuity than the Iron Oxide. Every application and discontinuity must be evaluated in the selection of the optimal particle size and type.
Magnetic Particle Testing has many facets to consider when applying to a component. Once a specific component or part is determined, the testing sensitivity must be defined. It is very important to monitor the inspection system to verify that the sensitivity of the component has not changed. In order to ensure magnetic particle testing results obtained are reproducible, it is critical to maintain consistent control over the magnetic particles in the system. This relates to controlling the concentration of the inspection bath and if in a water bath system, factors such as corrosion inhibition, wetting and shelf life properties must be considered as well.
ApplicationsIn the magnetic particle testing method there are many variables to be considered-before talking about applications-and understood when choosing a magnetic particle type. The primary factor is the characteristics of the component part to be inspected. The following characteristics must be considered:
Having a good working knowledge of the part and possible discontinuities will provide a better solution for the magnetic particle choice, specifically in regards to complex parts, although either particle type may readily capture the discontinuity, the inspector needs to consider the overall inspection area. With Flaw to Background interference always a concern, it is important to keep the background minimized. There are several factors that affect the background such as particle concentration, contamination, lighting, and particle types–iron oxide vs. pure iron.
We recently have addressed a request to evaluate the best particle type for threaded products. As stated above, both particle types may readily capture the discontinuity; however, their backgrounds will remain uniquely different. Due to the finer sizes of the oxides and slightly slower migration rates, the oxides “deposit” a more noticeable background. The thread size and the coarseness of the thread part surface can provide ample area for the particles to “lag” behind and generate a background. The iron particles with their slightly larger size and density may “move” over the threaded area in such a manner that the background appears less significant, and these iron particles will migrate quicker to a discontinuity than the iron oxide particle.
We evaluated a small pure iron particle and found that it generated less background and thus, a sharper contrast for the discontinuity. Our evaluation also included a standard iron oxide particle at a concentration of 0.4ml, a standard iron oxide particle at a concentration of 0.15ml and a pure iron particle at a concentration of 0.15ml. The best results were using a low concentration of pure iron particles.
Light SourcingWhen the fluorescent inspection method was developed the only practical sources of high intensity fluorescence-exciting light were bulbs that created a discharge in mercury vapor. Mercury has intense spectral emission lines at 254 and 365 nm in the longwave ultraviolet region. The manufacturers of fluorescent magnetic particles successfully produced materials that fluoresced with adequate intensity when excited by these lights.
Since the only concern was that the magnetic particles fluoresce under 365nm excitation, their response to other wavelengths was unimportant and not controlled for. It turns out that the same particles can indeed be excited by other wavelengths, both in the ultraviolet and the visible. There are two basic ways to determine whether another wavelength will make magnetic particles fluoresce: 1) try a light and see if it works; and 2) measure the excitation spectrum for the particle. The excitation spectrum is a measurement of the relative ability of different wavelengths of light to make something fluoresce. Both methods demonstrate that a wide range of wavelengths, in both the ultraviolet and the blue, can be used to make particles fluoresce, sometimes with even greater brightness than when excited with an equivalent intensity of 365nm ultraviolet. The exact relationship can be determined by measurement, enabling one to specify the intensity of an alternative wavelength needed to excite the brightness that would you would expect from a conventional ultraviolet light that produces 1000 W/cm2.
Intense blue light can now be easily produced through the use of high intensity light-emitting diodes (LEDs), and LED-based blue lights are now available on the market. The blue light reflected from the inspection area would interfere with and potentially mask the fluorescence, so when using these lights it is imperative that the inspector wears yellow filter glasses that completely block the reflected blue and still transmit the fluorescence with high efficiency. A scientific study on the use of blue light in combination with yellow filter glasses for magnetic particle inspection has been conducted with successful results. The use of alternative excitation wavelengths, such as blue, for magnetic particle inspection is now permitted by ASME under the guidelines in Mandatory Appendix IV to Article 7 of the Surface Inspection code.