Magnetic Particle Testing (MT)
Description
This NDT method involves inducing a magnetic field in a ferromagnetic material and dusting the surface with iron particles, either dry or suspended in liquid. Surface and near-surface flaws disrupt the flow of the magnetic field, causing some of it to leak out at the surface. Iron particles are attracted to and accumulate at the sites of magnetic flux leakages, indicating defects on the material's surface.
Application
Magnetic particle testing (MT) detects impurities on or just beneath the surface of a workpiece by creating a magnetic field with a permanent magnet or an electromagnet. An electromagnet requires an electric current. Both methods generate magnetic flux lines that change in the presence of impurities. Unfortunately, this is not detectable by the human eye. Therefore, NDT technicians introduce colored magnetic particles in either powder or liquid form. If defects are present, the particles will be drawn to them, enabling the technician to inspect them.It is widely used in weld inspections, forgings, and castings in the oil and gas industry.
step by step Process
- Surface Preparation: Clean the surface to remove contaminants.
- Magnetization: Apply a magnetic field using permanent magnets, electromagnetic yokes, prods, coil wraps, clamps, or horizontal bench units.
- Particle Application: Spread dry or wet magnetic particles over the surface.
- Inspection: Observe particle patterns under white or UV light to locate defects. All interpretations and discontinuities found are evaluated, sized, and compared to the applicable code, standard or customer specifications to determine if they meet the requirements to be deemed acceptable or rejectable.
- Demagnetization: Remove residual magnetism after testing.