What it finds

  • Surface cracks from fatigue, grinding, quenching or welding.
  • Laps, seams and other surface flaws in forgings and rolled products.
  • Some flaws just under the surface, though sensitivity falls off quickly with depth.

How it works

A magnetic field runs through the part. Where a crack cuts across the field, some of it leaks out into the air and forms tiny poles on either side of the crack. Fine iron particles applied to the surface are drawn to that leakage and build up into a visible line.

The particles may be dry powder, or carried in water or oil. Visible particles are viewed in good white light. Fluorescent particles are viewed under ultraviolet light in a darkened area, and they stand out strongly. The field can come from a portable yoke, from current passed through the part on a bench unit, or from a coil.

Direction matters. A crack shows best when it lies across the magnetic field, and it may not show at all when it runs along it. So parts are normally magnetized in two directions, roughly at right angles. Afterward, many parts must be demagnetized and cleaned.

Where it is used

MT is used on steel welds, forgings, castings, shafts, gears, crane hooks, lifting gear and landing gear parts, both in production and during maintenance. See our guides to automotive, amusement rides and construction.

Limits

  • Ferromagnetic materials only. Carbon steels and many alloy steels work. Aluminum, copper, titanium and austenitic stainless steel do not.
  • Surface-focused. It will not find deep internal flaws.
  • Field direction. A flaw lying along the field can be missed if only one direction is tested.
  • Surface condition. Thick coatings, scale and rough welds reduce sensitivity or create false indications.
  • Part risks. Current contact points can burn the surface, so techniques that pass current through a part are controlled carefully, and some specifications restrict them.

Who is allowed to do this work is set by a certification scheme. See How to Become an NDT Level I, II or III.

Frequently asked questions

MT or PT on steel?

On magnetic steel, MT is usually preferred. It is often quicker, less sensitive to surface cleaning and can pick up some flaws just under the surface. PT is the usual choice for non-magnetic materials.

Does MT work on stainless steel?

It depends on the grade. Austenitic stainless steels are essentially non-magnetic, so MT does not work on them, while some martensitic and ferritic grades can be tested.

Why does a part need demagnetizing?

Leftover magnetism can attract metal debris, interfere with nearby instruments or affect later welding and machining. Procedures state when demagnetizing is needed and how to check it.

How NDT LLC can help

Our team of Level IIIs helps with MT procedures, process control and audit readiness, and we take on field NDT and inspection work. Contact us.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASNT, ASME, ASTM International, or any other organization named in this article; names are used only to identify the organizations and documents referred to.