Magnetic Particle Examination (MT Inspection) is a popular, inexpensive form of nondestructive examination of ferromagnetic material. The term ferromagnetic is defined in ASME Section 5 as “a term applied t materials that can be magnetized or strongly attracted by a magnetic field.” MT examination checks for surface discontinuities and can also reveal discontinuities slightly below the surface. To better understand MT inspection, we must know how it works, its history, the different kinds of MT inspection, and its advantages and disadvantages.
How MT Inspection Works When ferromagnetic material is defect-free, lines of magnetic flux will transfer through the material without any interruption.
But when a flaw is present, the flux will then leak out of the material which will then collect ferromagnetic particles, making the size and shape of the discontinuity easily visible.
In order for this to work, the discontinuity must be perpendicular to the materials magnetic flux. If it is parallel to the flow, there will be no leakage and therefore no indication of a flaw. In order to detect a flaw such as this, each area needs to be examined twice. This is done by making the second examination perpendicular to the first, which will allow all flaws in any direction to be detected.
History of MT Examination The earliest form of MT examination can be dated back to 1868, when magnetism was used to detect flaws in cannon barrels. The cannon barrels were magnetized and then a magnetic compass was moved up and down the length of the barrel. If there were flaws in the barrel, the compass needle would move due to the leakage of the magnetic flux. About 50 years later, a man named William Hoke found that grindings from hard steel parts that were held by a magnetic chuck formed patterns that followed the discontinuities on the surface of the parts he was making. He also discovered that when putting fine ferromagnetic powder on the parts, the powder would build up at the flaws and make them more visible. By the 1930’s, MT examination was beginning to make its mark and rapidly started to replace the PT examination method (liquid penetrant). The reason being was that it was faster and cleaner the PT method since all that was left behind was iron powder, which could easily be removed.
Different MT Examination Methods According to the ASME Boiler and Pressure Vessel Code, Section 7, Article 7, there are five different magnetization techniques:
1. Prod technique
2. Longitudinal magnetization technique
3. Circular magnetization technique
4. Yoke technique
5. Multidirectional magnetization technique
Each of these different methods can use one of two kinds of ferromagnetic media: dry particles or wet particles. These can come in fluorescent and non-fluorescent and can come in many different colors in order to make the discontinuities more visible.
The three most popular kinds of MT examination methods are longitudinal and circular magnetization, and the yoke…