Grinding Spark Test to Metal

When a steel place is held against a revolving grinding wheel, the abrasive grains tear off small particles from the steel which are heated to melting temperature. The accelerating, glowing particles produce a visible beam of sparks, which may be short or long, continuous or broken (main beam), depending upon the force with which the piece is pressed against the wheel and the composition of the steel. Several of these beams together make up the ray pattern. If the flying particles attain a temperature which is high enough. The iron and iron companion burn, particularly carbon, silicon and manganese combining with atmospheric oxygen. Because the oxidation products of carbon are gases CO and CO₂, the small glowing particle explodes at a certain distance from the grinding wheel, forming new, shorter rays, radiating outwards from the center of the explosion of the sparking pattern. The frequency of explosion and the shape of the ray patterns produced depend upon the composition of the steel. One distinguishes between barbed, flower shaped, lobar, drop shaped and lance like patterns. Steel for engine should be apply this test first.

Hardened samples generally yield a lighter and livelier spark pattern than samples in annealed of tempered states. The contact pressure greatly influences the length and density of the ray pattern and the frequency of explosion.

A ray pattern of moving glowing particles is very difficult to reproduce in the form of a picture which is even somewhat constant. Long practice is necessary to be able to recognize finer differences and to use the sparking test as a means of distinguishing between different materials.

Differences between ferrous materials:

1. The spark patterns of ordinary structural steels and unalloyed tools steels are determined by the carbon content. As the carbon content increase the frequency of the explosion increases, accompanying by lateral branching of the ray pattern.
2. The sparking pattern of alloyed tool steels is altered by changes the in alloying components. Manganese alloyed tool steels produced bright bulges in the main beam. Molybdenum alloyed tool steels produced ray ends with lance tips, chromium tungsten alloyed tool steels produce discontinuous beams with tongue shaped end sparks high speed steels produce broken rays of sparks practically without C explosions.