A great advantage of this form of test is that it can be carried out on the steel at any point, e.g.. as a billet, an ingot. a bar, a forging, or often a finished piece. The test is carried out on the steel as it stands, and the elaborate drilling of separate samples with the possibility of confusion is eliminated. At the present time, also, the test has great utility, because it enables pieces of undesired metal in a batch of different composition to be picked out quickly and cheaply, and set aside for scrap or salvage, whereas to have to analyse them chemically would constitute a prohibitive charge.
The principle on which the test is based is this: The effect of bringing a piece of steel into contact with the face or cutting edge of a grinding wheel is to force or wrench off tiny fragments of the steel. The wheel runs at a high speed, and the friction is so great that the temperature of these fragments is raised to such a height that they become white hot. This makes them brilliantly visible against a dark background, and their passage through the air as they are flung off has an almost comet-like trajectory, which is termed a " carrier line."
The basis of the test is that different metals give off sparks or particles of incandescent character each having a different trajectory and form. For example, wrought or ingot iron will give off a little bundle of individual lines called a " spark picture." A O.2 per cent. carbon steel will give a line of brighter colour and will throw off a series of fine branches from this line known as " forks," or " primary bursts." These are due to the presence of carbon. It will thus be seen that wrought iron can readily be distinguished from carbon steel by means of the spank given off.
Raising the Temperature. The effect of raising the temperature of a metallic particle to white heat and hurling it through the air at great velocity is to cause any carbon existing in the fragment to combine with oxygen in the atmosphere to form carbon dioxide. The change from solid carbon to gaseous carbon dioxide results in an increase of volume. This increase of volume is withstood to the best of its ability by the particle, and the result is the setting up of an internal stress that ultimately leads to the complete disruption of the particle thus causing the fork or burst responsible for the branching out of the line. This, at all events, is the theory. The greater the percentage of carbon in the steel, the more marked is the branching effect, and this has proved fairly conclusively that carbon is the element causing these forks or bursts.
Fig. 1 shows cast iron,
which possesses a dull red, non-explosive spark that thickens towards the
end. Fig, 2 shows wrought iron, whose spark is brighter, as indicated,
and has a luminous extremity. If any traces of carbon are found in the
iron, the extremity may reveal a burst or fork.