In this video Magnaflux’s Brian Batteiger goes through the common steps of magnetic particle inspection with dry, visible magnetic particles on ferrous parts as outlined in ASTM E1444: Standard Practice for Magnetic Particle Testing. The specific sequence of steps may vary depending on the method of particle application and magnetization technique.
- Surface preparation: It is important for the test surface to be clean, dry and free from any dirt, grease, scale, paint or other materials which might interfere with the inspection, Brian is using SKC-S solvent cleaner/remover.
- Magnetization set-up: The test part or area must be set-up to have the magnetizing current applied to it. This will be different for various applications, in the video Brian demonstrated how to test a weld using an electromagnetic yoke.
- Magnetization method: There are different types of magnetic current, ways to apply current to a part and magnetic field direction, check the part procedure for the correct magnetization method. Here Brian uses an Y-1 AC yoke.
- Amperage: Make sure to consult the part procedure for the correct amperage which will vary depending on the part, type of metal and type of defect needing to be found.
- Magnetic field strength: Check the magnetic field strength with a Pie Gauge, Hall Effect Meter, QQI, known defect or Magnetic Flux Indicator Strip.
Apply magnetic current: For dry, continuous method the magnetic current is started before magnetic powder is applied and the magnetic current is terminated only after magentic particle application is complete.
Apply magnetic particles: Brian demonstrates dry magnetic particle application, continuous method by apply the mag particles to the part while applying the current and gently removes the excess magnetic powder while the current is still being applied.
Inspection: Evaluate the part under appropriate UV lighting conditions, inspect the part for any indications and take the appropriate steps to interpret and document any findings.
Additional magnetization: Depending on the part and the process, additional magnetization may be required to find indications in different orientations on the part or test surface. In this case, repeat steps 2-5 for the additional magnetization.
- Demagnetization: Using the appropriate magnetic current, field strength and direction, demagnetize the part after the final inspection is complete and use a Field Meter to check for any residual magnetism.
- Postcleaning: Parts are cleaned after inspection to remove any remaining magnetic particles.
Dry-Method Magnetic Particle Advantages
- Effective at low temps
- Effective at high temps
- Easy removal, not as messy as wet-method
- Good particle mobility with AC and HWDC
- Equipment may be less expensive
Dry-Method Magnetic Powder Disadvantages
- Not as sensitive as wet-method for very fine and shallow cracks
- Difficult to use in overhead inspections
- Hard to tell when/if complete coverage is achieved
- Not good for high-volume
- Difficult to adapt to any type of automatic system
Dry-Method Mag Particle Uses
- Detecting large, course discontinuities
- Locating surface and subsurface discontinuities
- Large objects or for field inspections
- With portable equipment
- Finding relatively deep flaws with HW rectified current
- Weld testing
Just like all nondestructive testing methods, the results of a mag particle inspection are only reliable if the testing is performed accurately and consistently. This is meant to provide a general overview of a typical MT / MPI process; all NDT testing must be performed in accordance with the prescribed specifications or test procedures.