When a steel piece 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 growing particles produce a visible beam of sparks, which may be short or long, continuous of 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 patterns. 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 patterns. The frequency of explosion and the shape of the ray pattern produced depend upon the composition of the steel. One distinguishes between barbed, flower-shaped lobar, drop-shaped and lance like patterns.
Hardened samples generally yield a lighter and livelier spark pattern than similar samples in annealed of tempered states. The contact pressure greatly influences the length and density of the ray pattern and the frequency of explosions.
A ray pattern of moving-glowing particles is very difficult to reproduce in the form of a picture which is even somewhat constant. The temperature shown here can therefore only illustrate broad distinctions in shape and color which actually change and transpose into each other in reality. The pictures and descriptions can function only as guidelines on how to evaluate a sparking test. 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
The spark patterns of ordinary structural steels and unalloyed tool steels are determined by the carbon content. As the carbon content increases, the frequency of C explosion increases accompanied by lateral branching of the ray pattern.
The sparking pattern of alloyed tool steels is altered by changes the in alloying components. Manganese alloyed tool steels produce bright bulges in the main beam. Molydbenum alloyed tool steels produce ray encs with lance tips chromium-tungsten alloyed tool steels produce discontinuous beam with tongue shaped end sparks, high speed steels produce broken rays of sparks, practically without C explosions.
The test is best carried out by grinding a comparison steel of known composition along with.