Magnetic Particle Testing
Magnetic Particle Inspection (MPI) is a non-destructive testing method that can be used to detect surface and subsurface flaws in ferromagnetic materials. MPI is often used to find the job’s fitness for use & conformity. Since it is a Quick & relatively easy technique, it is widely used in various sectors including aerospace, automotive, petrochemical, structural steel and power generation in order to inspect a variety of products and equipment such as engine, suspension and braking system components, castings, forgings and weld joints.
One of the main advantages of using MPI is it gives instant indications of defects and discontinuities.
The Technique works by inducing a magnetic field in the component tested. If a surface or near-surface flaw is present, the magnetic flux is distorted and ‘leaks’ around the flaw. By dusting the surface of the tested item with fine magnetic particles such as ferrous iron filings, (applied either dry or suspended in liquid), the particles will be attracted to the area of the flux leakage creating a visible indication of the defect.
Our MPI Methods include:
⦁ Yoke Method (portable)
⦁ Bench type (Direct method, Head shot, Central Conductor)
⦁ Prod Method
⦁ Coil/Cable wrap method
Aurora’s experienced & skilled Engineers are flawlessly equipped in performing Magnetic particle Inspection either on on-site at Customer’s premises or at Aurora’s accredited laboratories for Testing.
A yoke has an electric coil in the unit creating a longitudinal magnetic field that transfers through the legs to the examined part. The yoke technique is easy to use with minimal training. It can be used indoors, outdoors, inside vessels and tanks, and in all positions. Yoke method is the MPI Technique widely used for on-site inspections.
BENCH TYPE (Direct Method/Head Shot/Central Conductor):
A magnetic bench generally consists of a generator of longitudinal magnetic field by solenoid or heads, of a transverse field generator by passing current in the part, a lighting device and a system of spraying. A wide range of options is also proposed to optimize the operation of our equipment in your production environment. We have separate attachment coils which are used to induce transverse magnetic field in the component in order to achieve 100% quality.
Two hand-held electrodes which are pressed against the surface of a part to make contact for passing magnetizing current through the metal. The current passing between the two contacts creates a field suitable for finding defects with magnetic particles. The field produced is Circular Magnetic Field.
If a part is too large to put onto a bench testing unit, then local magnetization can be done. One method of such local magnetization is by use of prods. These are usually copper posts on the ends of the conductors from the magnetic particle machine. The hand held prods are held against the test piece as shown in Figure 4-4 and a circular field results with the current applied.
Discontinuities parallel to the line between the prods are made visible by the magnetic particles. This has long been a useful technique in MPI. It is readily portable and therefore convenient for field use. The sensitivity to subsurface defects is greater with this method than any other especially when using half wave rectified current.
COIL/CABLE WRAP METHOD:
A longitudinal magnetic field will be induced by way of the coil or cable wrap. The black light shall be employed during this inspection process. Lighting requirements shall be met prior to the start of the inspection. Determine if a residual field exists in the parts to be inspected. If so determine the polarity of the existing magnetic field, if any, in each end of the test piece using the Pocket Gauss Meter. The coil shall be placed so as to enforce the existing field.
Parts longer than the diameter of the coil shall be examined in sections. The inspection area shall be limited to half of the coil diameter up to 9" on each side of the coil. When using Cable Wraps; three wraps will usually be sufficient for inspection. Wet fluorescent magnetic particles shall be applied either by spraying or flowing over the areas to be inspected just as the magnetizing field is induced. The particle application will then be diverted with the current remaining on and the inspection performed with the current remaining on. A Castrol strip or Pie gauge will be used to confirm that the coil or cable wrap has been able to produce the indications required. The black light will be used in a darkened area during the entire time of the inspection process. When inspecting threads the last engaged thread of roots of pins and boxes should receive particular attention. A mirror shall be used to inspect thread roots and surfaces in the box connections as well as the Pin IDs. Unless the pipe is vertical, each length shall be rolled to allow 360° examination and to allow areas under solution “puddles” to be inspected. For inspection of Slip/ Upset areas, on drill pipe and the like, DC coils must be able to produce transverse, and three-dimensional flaws. The inspected area includes the first 36” from the pin tool joint shoulder and the first 48” from the box shoulder. If slip cuts are found beyond the first 48” then 6” on either side of the slip cuts shall be inspected. If this method is applied to HWDP, the area also includes the first 36” of tube on either side of the center upset. When inspecting smaller parts, in relation to the size of the coil diameter, parts may be turned inside the coil to achieve bi-directional inspection requirements, however, care shall be taken and the Castrol strip and/or Pie-gauge check shall be performed in both directions.