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MAGNETIC PARTICLE EXAMINATION (M.P.I.)

MAGNETIC PARTICLE EXAMINATION (M.P.I.)

INTRODUCTION
This method is used for detecting cracks and other discontinuities open to surface or sub surface in ferromagnetic materials. Fine magnetic particles are applied to the surface of a part which has been suitably magnetized. The particles are attracted to regions of magnetic non-uniformity associated with defects and discontinuities, thus producing indication which are observed visually.

PRINCIPLE
When a piece of metal is placed in a magnetic field and the lines of magnetic flux get intersected by a discontinuity such as a crack or slag inclusion in a casting, magnetic poles are induced on either side of the discontinuity. The discontinuity causes an abrupt change in the path of magnetic flux flowing through the casting normal to the discontinuity, resulting a local flux leakage field and interfering with the magnetic lines of force. This local flux disturbance can be detected by its affect upon magnetic particles that collect on the region of discontinuity and pile up and bridge over the discontinuity.
A surface crack is indicated by a line of fine particles following the crack outline, and a subsurface defect by a fuzzy collection of magnetic particles on the surface near the discontinuity. Maximum sensitivity of indication is obtained when the discontinuity lies in the direction nominal to the applied magnetic field and when the strength of the magnetic field is just enough to saturate the section being inspected.



MAGNETIC PARTICLES INSPECTION MEDIUM
1. Dry or wet magnetic particles shall be used as the inspection med ium. Dry powder is applied in the form of spray or colour. It is better for locating near surface defects. Moreover all the powder can be recovered after the test. In wet method, the powder is suspended in low viscosity non-corrosive fluid such as kerosene and is sprayed over the casting; alternatively casting may be immersed in the liquid for the purpose. The material shall have a high permeability and low retentivity. It shall be of a colour that will provide adequate contrast with the background surface being tested.
2. The inspection medium shall consist of finely divided ferromagnetic particles, which may be suspended in a suitable liquid medium or used in dry powder form.
3. The size and shape of the particles and their magnetic properties, both individually and collectively, are important.
4. The magnetic powder of iron or black magnetic iron oxide base and having elongated individual particles is used for the purpose. Metallic iron particles are coated to prevent oxidation and sticking.

MAGNETIC MEDIUM TO BE USED
Colour contrast consumables(Wet)
Colour contrast consumables(Dry)

SURFACE PREPARATION
1. Defects may be satisfactorily revealed on shot blasted or otherwise cleaned forged surfaces, or on surfaces having small amounts of heat treating scale without any special surface preparation.
2. The surface to be examined and all adjacent areas within at least I" shall be clean and free of grease, oils, rust, paint, contamination's or other substances to which particles may adhere.
3. Rough surfaces may hamper the mobility of magnetic powders due to mechanical trapping, which in tum produces false indications. Such areas should be surface ground.

MAGNETIZATION METHOD
The two general types of magnetization with regard to direction are :
1. Longitudinal : When a piece is magnetized longitudinally, the magnetic flux lines are usually parallel to the axis of the piece. A longitudinally magnetized piece always has definite poles readily detectable by compass or magnetometer.
2. Circular :Circular magnetization is obtained by passing a current directly through the piece or induced through a conductor or conductors threaded through an opening in the piece.

POLE SPACING
The pole spacing shall be limited to 2 to 8 inches.

INSPECTION AREA
Inspection area is limited to a maximum distance of Y.of the pole spacing on either side of the line joining the two poles.

INTERPRETATIONS AND EVALUATION OF INDICATIONS
The indication is the visual particle build up resulting from leakage fields in the magnetic field. All observation of indications shall be done when the magnetizing current is on. It can be interpreted as
  • Appearance of the indication. 
  • Direction and shape of the indication. 
  • Type of material from which the part is made. 
  • Past experience with similar parts based on destructive tests such as fracturing, chipping, grinding etc. 
The indications may be as subsurface defects such as inclusions, cracks in under bead of welds etc. Non-relevant or false indications such as flow lines, edge of weld etc.




ADVANTAGES
1. MPT is a relatively simple and easy technique.
2. It is free from any restriction as to size, shape, composition and heat treatment of a ferromagnetic specimen.
3. Defects such as cracks, laps, inclusions, hot-tears etc can be easily revealed by MPT.

LIMITATIONS
1. The electromagnetic yoke is heavy in weight.
2. It is not practicable for large castings.
3. The indications are to be observed minutely.
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