Magnetic Particle Testing (MT) (also known as magnetic particle inspection – MPI) is a non destructive test (NDT) method, used to detect surface or subsurface (near to surface) discontinuities. This NDT method can be used on metals which can be easily magnetized (ferromagnetic). Metals can be classified as ferromagnetic, paramagnetic, or diamagnetic.
- Ferromagnetic metals: Ferromagnetic metals are those, which are strongly attracted to a magnet and can be easily magnetized. Examples ferromagnetic metals are iron, nickel, and cobalt.
- Paramagnetic metals: Paramagnetic metals are those which are very weakly attracted by magnetic forces of attraction and cannot be magnetized such as austenitic stainless steel.
- Diamagnetic metals: Diamagnetic metals are those which are slightly repelled by a magnet and cannot be magnetized. Examples of diamagnetic metals are bismuth, gold, and antimony.
Only ferromagnetic metals can be effectively inspected by Magnetic Particle Testing
Principle of Magnetic Particle Test
This method uses the phenomenon of a magnet by virtue of which path of magnetic lines of force gets disrupted when a surface breaking is occurred in the path. Any magnet has two poles i.e. north pole at one end and south pole at the other. Magnetic lines of force (also known as magnetic flux) travel from north pole to the south pole, as shown in the below figure (Figure 1).
(Figure — 1)
If there is any distinct change in the continuity (such as surface discontinuity) of magnet, then the magnetic lines of force will get distorted, this phenomenon is known as flux leakage. During flux leakage, additional North and South poles will be created near discontinuity (Figure — 2), and the magnetic lines of force will redistribute themselves in the material by bending around the discontinuity.
(Figure — 2)
How Magnetic Particle Test works:
When fine Iron particle (ferromagnetic particle) is spread over a magnet, it gets accumulated at the poles. But in case of any discontinuity, flux leakage would occur and the Iron powder will accumulate at the discontinuity, due to the creation of additional North and South poles at the discontinuity, as shown in Fig – 3.
Figure — 3
The accumulation of iron particle at the discontinuity would produce a visible indication of that discontinuity.
Particles used for Magnetic particle testing is similar to ferromagnetic particle and is called as detection media. These particles may be applied in dry form or may be mixed with liquid and spread over the area where Magnetic particle test has to be performed. Liquid like kerosene or a similar petroleum distillate may be used. Water can also be used by using suitable additives such as wetting agents and antifoam liquids. To provide better contrast with the test objects and enhanced sensitivity, these particles are coated, there are two types of coating;
1. Color contrast coating
2. Fluorescent coating
Color contrast coating: Color contrast coatings are available in several colors such as red, blue, black and Gray etc. Color of particle are selected so as to provide a good contrast with test object.
Fluorescent coating: Fluorescent particles can be seen under a ‘Black light illumination’. These particles emit light when seen by a black light in a dark background. These particles provide excellent contrast in dark background.
How to temporarily magnetize the test object:
To carry out Magnetic Particle Testing we need to temporarily magnetize the test object. Magnetization should be temporary in nature. To magnetize the test piece, common instruments which are used are;
- Electromagnetic yoke
- Permanent Magnet
Electromagnetic Yokes are also called as AC yokes, it’s very portable and commonly used in Industries. Yokes are connected with AC power source (Battery pack version is also available). Many yokes come with adjusting legs, to facilitate wide range of area profiles. These yokes produce longitudinal magnetization. Hence, for complete inspection, re-positioning of yokes in at least two 900 opposing direction is required.
Permanent magnets (Horse shoe types) are also used to temporarily magnetize the work piece. But due to their strong fields, particles gets attracted to the legs more readily instead of the test surface. Hence, many a time it becomes difficult to inspect the test object. Prod and coil method is also used for temporarily magnetizing the test object. Prods require good contact with test object otherwise it can damage the test object, due to electrical arcing. Coil method produce longitudinal magnetization.
Flux Direction Indicators:
Before inspection, the yoke shall be properly checked by the flux direction indicators. The most common flux direction indicator used in the industries is ‘A Pie Gauge’. Other flux direction indicators are Burmah Castrol strips and Quantitative quality indicator (QQI).
A pie gauge is an octagonal flat plate which consists of eight low-carbon steel segments. The octagonal flat plate is copper coated from the back side to hide the joint lines. When particles are suspended, from back side, on the pie gauge (under the influence of magnetic lines of force), particles get accumulated at these joint lines revealing the eight segments.
How to perform the Magnetic Particle Test
To carry out Magnetic Particle Testing, we need to follow the following activities
- Temporarily magnetize the test object
- Suspend (spread) the magnetic particles (either dry or wet) on the test object
- Inspection of test object either by black light (in case of wet fluorescent particle) in a dark area or inspection in sufficient light (in case of dry particle).
Various techniques are available to carry out the first two steps (mentioned above), these are;
- Dry Continuous
- Dry Residual
- Wet continuous
- Wet Residual
The term ‘Continuous’ is used when the particles are applied while the current is still flowing. And ‘Residual’ is used when particles are applied after the current has ceased. Out of these four techniques Dry continuous and Wet continuous techniques are widely used in Industries. A brief description of these two techniques are given below;
Dry Continuous: In dry continuous technique, dry magnetic particles are used. These particles are applied while the magnetizing force is on. Yokes or prods can be used to generate the magnetizing force. This technique is useful for detecting subsurface discontinuities, due to the higher permeability compared to the wet particles.
Wet Continuous: Particles are mixed with liquid and suspended on the test area, magnetization of test area and suspension of wet particles shall be done simultaneously. Liquids such as kerosene or petroleum distillates can be used for this purpose, water can also be used as a liquid carrier for particles. Kerosene and petroleum distillates are costly, highly inflammable and possesses safety related issues. However, water is inexpensive, available in abundance and possesses no safety related issues, but it can initiate corrosion in the test object.