Magnetic Particle Testing
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.
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.
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
- Prod
- Coil
Electromagnetic Yokes (Figure – 4)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.
.
Watch a video now
المراجعات
لا توجد مراجعات بعد.