Metallurgical Testing

Non-Destructive Testing

This is carried out on components rather than on test pieces, they are designed to indicate flaws occurring due or after manufacture. They give no indication of the mechanical properties of the material.

Surface flaws may be detected by visual means aided by dye penetrant or magnetic crack detection. Internal flaws may be detected by X-ray or ultrasonic testing.

In addition to this there are special equipment able to exam machine finish.

Liquid Penetrant Methods

The surface is first cleaned using an volatile cleaner and degreaser. A fluorescent dye is then applied and a certain time allowed for it to enter any flaws under capillary action. Using the cleaning spray, the surface is then wiped clean. An ultra violet light is shone on the surface, any flaws showing up as the dye fluoresce.

The more commonly used Dye penetrant method is similar in application. The surface is cleaned an d the low viscosity penetrant sprayed on. After a set time the surface is again cleaned. A developer is then used which coats the surface in a fine white chalky dust he dye seeps out and stains the developer typically a red colour.

Both these methods are based loosely on the old paraffin and chalk method.

Magnetic crack detection

A component is place between two poles of a magnet The lines of magnetism concentrate around flaws. Magnetic particles are then applied, in a light oil or dry sprayed, onto the surface where they indicate the lines of magnetism and any anomalies. This method of testing a has a few limitations. Firstly it cannot be used on materials which cannot be magnetised such as austenitic steel and non-ferrous metals. Secondly it would not detect a crack which ran parallel to the lines of magnetism.