When cooled slowly below 670 degrees, martensite yields a heterogeneous mixture of pearlite and ferrite (or cementite, if the original mixture contained between 0.8 per cent. and two per cent. of carbon).

Moderate reheating or annealing changes this structure largely into troostite, which is a partly transformed martensite, possessing much of the hardness of martensite, but with

When heated to 670 degrees, it becomes homogeneous, an amount of carbon up to two per cent. dissolves in the iron, and hard steel or martensite is formed.

The engineers are running programs that show how adding tiny amounts of alloys to ferrite and martensite—crystalline structures of iron—during the steelmaking process changes the steel's strength, as well as its ability to be shaped into different parts.

This toughness is the chief characteristic of the next material in the transformation series, sorbite, which is merely martensite wholly transformed into

In the ordinary practice of hardening steels, the quenching is not so drastic, and the transformation of austenite back to ferrite and cementite is more or less completely effected, giving rise to certain transitory forms which are known as "martensite," "troostite," "sorbite," and finally, pearlite.

When cooled rapidly, however, as in the tempering of steel, martensite remains a homogeneous solid solution, or hard steel.