Magnetic particle inspection(MPI),also called magnetic particle testing(MT),works by magnetizing the specimen using a magnet or by producing a magnetic field by inducing an electric current in the specimen.If the specimen has a discontinuity,the magnetic field or flux flowing through the specimen is interrupted and flux leakage occurs.Finely milled iron particles coated with a dye pigment are applied to the specimen.These are attracted to the flux leakage fields and cluster to form an indication directly over the discontinuity.The indication is visually detected using proper lighting conditions,which could be traditional lighting or black light.
Magnetic particle testing for large diameter elbow welds
Material selection for magnetic particle inspection:
1, magnetic powder media
The function of magnetic powder is as a display medium,and its types include:
- Black magnetic powder – the composition is ferroferric oxide (Fe3O4),in the form of a black powder, suitable for workpieces with a light or bright background.
- Red magnetic powder – the composition is ferric oxide (Fe2O3),which is iron red powder,suitable for workpieces with dark background.
- Fluorescent magnetic powder – a fluorescent substance is coated on the ferroferric oxide particles, which emits yellow-green fluorescence under ultraviolet irradiation, and is suitable for dark-dark workpieces, especially due to the color-sensitive characteristics of the human eye. Magnetic powder inspection using fluorescent magnetic powder as magnetic medium has higher sensitivity than other magnetic powders.
- White magnetic powder – coated with white matter on the ferroferric oxide particles,suitable for darker workpieces.
In order to facilitate the use of on-site inspection, there are many kinds of commercial magnetic media,in addition to black,red and white magnetic powder,fluorescent magnetic powder,spherical magnetic powder (hollow, color, for dry powder method),and pre-configured magnetic paste.,concentrated magnetic suspensions, magnetic suspension spray cans, etc., as well as surface brighteners (contrast enhancers) provided to improve the testability of dark or rough surface workpieces.
In order to ensure the reliability of the magnetic powder inspection results,the magnetic powder (including magnetic properties,particle size,shape) and the concentration, uniformity,and suspension of the magnetic suspension must be verified before they can be used,and also need to be periodically used during use.Verification,in addition to observing the white illuminance of the environment at the time of evaluation,or the ultraviolet intensity of the ultraviolet lamp used in the fluorescent magnetic particle inspection,etc.,is also a verification item in order to ensure the quality of the inspection.
large diameter elbow after magnetic particle testing
2, sensitivity test piece
The sensitivity test piece can also be used to check the comprehensive performance of the flaw detection equipment, the magnetic powder and the magnetic suspension,and check the distribution law of the magnetic field at each point on the surface of the workpiece to be inspected.The sensitivity test piece is used by adhering the grooved side of the type A standard test piece to the surface to be cleaned of the test piece by tape paper,and spraying the magnetic suspension on the test piece while magnetizing the workpiece and the test piece, and Observe the magnetic marks on the test piece.When the magnetizing current is appropriate,a clear grooved magnetic mark appears on the surface of the groove where the test piece is not engraved.
When the sensitivity of the flaw detection is magnetized on a complex workpiece,the magnetic field intensity distribution on the surface of the workpiece is very uneven,and the magnitude of the magnetizing current is difficult to estimate.It is often difficult to achieve the ideal magnetization effect by merely selecting the magnetizing current by experience.Therefore,sensitivity test strips are generally used to select the best magnetization specification.
Haihao Group is one of the biggest steel pipes,butt welding and forged pipe fittings,and flanges manufacturers in China,our quality inspection department can carry out the NDT inspection as per clients requirement.If you have any questions,please feel free to contact us,our email address is:sales@haihaogroup.com
