Working Principle of Weld Magnetic Particle Examination
Magnetic Particle testing is most common non-destructive weld inspection technique used for the examination of surface flaws/openings in ferromagnetic materials. Basic magnetic principles are used to make the indication of surface cracks and other subsurface discontinuities. The test part is made magnetized with the help of some magnetizing system and then some magnetic particles are applied on the test surface. The behavior of those magnetic particles in the presence of magnetic field, provides the obvious manifestation of surface discontinuities. Below is the demonstration for How the Magnetic Particle Examination Works in evaluating the Weld components.
Also See Comparison between Magnetic Particle and Dye Penetrant Examination.
Magnetization
Magnetization of material can be done by different techniques, normally the Yoke Technique is most common in weld and weld repair. Magnetic field is generated in the test part by applying strong magnetic flux to it. Say if the test material is clean and free from any kind of flaw then the magnetic field lines will pass straight through it without any interruption. If there is any breaking in surface, the magnetic field lines gets interrupted and consequently cause the leakage of field which is often referred to as “Leakage Field”. These leakage fields are responsible to attract any magnetic material in surrounding.
Pre-Cleaning
As no strict conditions are required for cleaning the test material before magnetic particle inspection. Less level of cleaning is needed to remove dust, organic pollutants, oil and paint etc. Normally less level of surface pollutants don’t create troublesome effects during inspection.
Introducing the Magnetic Particles
Different types of magnetic particles are used, depending on the certain application and magnetizing technique. Fluorescent and colored magnetic particles are available in market. Pigmented magnetic particles make color contrast for indication while fluorescent particles are viewed by mean of some black light (ultraviolet).
After applying on surface, excess particles are removed with the help of low pressure air jet because in excess they can’t help proper examination. The leakage fields if present due to discontinuities, tends to attract these particles. The accumulation of particles make a clear investigation of cracks, openings or breaking if present any.
Manipulation
The technique is easy to perform however there are some critical points that need to be focused. Leakage fields are only formed if the magnetic field lines are at ninety degrees to discontinuities. Say if the surface breaking is parallel to these magnetic lines, the particles will not get entrap thus giving no indication. For this, equipment is applied on test weld part in two directions provided one direction is perpendicular to other. The Inspector must make sure that test weld is examined in both directions. See the pre-requisites of visual inspection.
Indication and Evaluation
The accumulation of particle over the surface flaws, direct the indication to the inspector. The weld discontinuities are evaluated (giving the size and shape).
Post Cleaning
After examination, it is sometimes necessary to demagnetize the test weld components and the magnetic particles are removed from the surface.
Last comments
The evaluation of weld discontinuities using magnetic particle technique, requires suitable light intensities. Codes/standards codifies the minimum amount of light which must be fulfilled before the examination to start. The test must be carried out in perpendicular directions in order to provide adequate magnetic flux to discontinuities. Make sure the equipment is kept in good condition and must be calibrated according to standards.
