Magnetic particle testing magnetizes a ferromagnetic part, usually with a hand-held yoke, and applies dry or wet iron particles, often fluorescent. Cracks and other surface-breaking flaws disturb the field and gather particles into visible lines. It is fast and sensitive for toe cracks, arc strikes and back-gouged roots on carbon steel, but it does not work on austenitic stainless, aluminum or other nonmagnetic metals.
Run MT on the back gouge before you weld the second side, I want to see clean metal.
Run the yoke in two directions roughly at right angles; a crack lying parallel to the field lines will not show.
Also heard as
- mag particle
- MPI
- magnetic particle inspection
- mag
Do not mix it up with
- PT: MT works only on magnetic metals but can find slightly subsurface flaws, while PT works on any nonporous metal but only finds flaws open to the surface.
Related terms
PT
Penetrant testing: revealing surface-breaking cracks by drawing colored or fluorescent dye out of them.
Hydrogen cracking
Delayed cracking in steel welds caused by hydrogen, hard microstructure and stress acting together.
Arc strike
A spot where the arc touched base metal outside the weld, leaving a small melted or hardened scar.
VT
Visual testing: inspecting welds by eye with gauges, lights and magnifiers, the first and most-used NDE method.