Visual Inspection

Magnetic particle inspection (MT)

Magnetic particle inspection (MT) is used for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt and some of their alloys. MT is performed by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles. If there’s is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage flux is displayed by covering the surface with very fie iron particles applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually even when the crack is very narrow.

MT is highly portable, generally inexpensive, and does not need a stringent pre-cleaning operation. It is also one of the best options for detecting fine, shallow surface cracks. MT is fast, easy, and will work through thin coatings.

Magnetic particle testing (MT) is very sensitive test method. It can detect tight in-service fatigue cracks in rotating parts or creep cracks on steam piping. 

Magnetic Particle Inspection (MT) cannot be used for non-ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels.  

Dye penetrant inspection (PT)

Dye penetrant inspection (PT) is a widely applied and low-cost inspection method used to locate surface breaking flaws. To be detected, the flaw must reach the surface to be tested. Dye penetrant (PT) can be applied to both ferrous and non-ferrous materials and all non-porous materials.

PT is used to detect cracks, surface porosity, lack of penetration in welds and defects resulting from in-service conditions, such as fatigue cracks of components or welds, in castings, forgings and weldments. The dye penetrant in drawn into the surface-breaking crack by capillary action and excess surface penetrant is then removed. A developer is then applied to the surface to draw out the penetrant in the crack and produce a surface indication. Cracks as narrow as 150 nanometers can be detected.

This method of NDT is useful for testing on-site as it is portable, and when applying certain techniques such as visible colour contrast there is no need for additional resources such as electricity or water.

Penetrants are classified by their sensitivity levels and, when selecting a penetrant, factors such as environment under which the test is to be performed, surface finish and the size of defects are considered.