No. 36b. Steel with 2 Per Cent Carbon
No. 109. White Cast Iron with 3 Per Cent Carbon
(Magnification 60 Diameters.)
Such are illustrated in photomicrographs Nos. 24a, 36b and 109 which contain 1.25%, 1.98% and 3.00% of carbon respectively. While steels with the typical white, free ferrite areas are so soft that a needle-point will plow furrows across them, those with over 1.25% of carbon have such excess of free cementite that they are very hard to scratch and too brittle to use except for special purposes.
Austenite (White) and Martensite (Dark) Magnified 1,000 Times Their Actual Size
So during ordinary cooling from the molten alloy or the slower cooling of the steel during the annealing process, the martensitic structure breaks down at the recalescent temperature into pearlite and ferrite (soft iron) if the carbon content of the steel is lower than about .90%, or pearlite and the other and very hard constituent, “cementite,” if the steel has more than .90% of carbon. If the carbon content happens to be just .90%, or thereabouts, there is exactly sufficient pearlite to make up the total area of the field shown under the microscope.
Another constituent which is of great interest scientifically, though not at all commercially, is “austenite.” By quenching very high carbon steels from a very high temperature very suddenly and completely, we can fasten the “austenite” structure, which exists only at temperatures higher than martensite, i.e., austenite is our gamma iron with the carbon of the alloy in solid solution, perhaps as iron carbide, while martensite is thought to be the beta iron solid solution, perhaps with some gamma iron mixed with it.
While ordinary quenching fastens structures pretty well, it is not usually quick enough to prevent the austenite from sliding along down into martensite. However, carbon discourages such slipping, so, with high carbon to act as a brake, we can fasten some of it by chilling very suddenly and completely from a very high temperature. Steels with 1.5% of carbon and temperatures of 2000° F., or over, are usually necessary to accomplish it.