Transformation Temperature

(also critical point; in Russian, Chernov point), any one of several specific critical temperatures at which a change in the phase and structure of steel occurs during heating and cooling. The most important are the two discovered by D. K. Chernov in 1868, which he called points a and b; the determination of their values and relative positions on the temperature scale marked the beginning of the scientific heat treatment of steel and was of enormous practical value.

According to Chernov the significance of point a (dark cherry temperature color) is that when steel is heated to a temperature below point a, it cannot be hardened no matter how rapidly it is cooled. For point b Chernov gave the following evaluation: when steel is heated to a temperature below point b, its structure does not change and a fracture retains its earlier form. According to Chernov, point a is the temperature above which steel must be heated in order to be hardened, and point b is the temperature above which heating tends to refine a coarse-grain structure. Later, in 1878, Chernov proposed point d (he defined point c as the melting point of steel), with a value of approximately 200°C, as the temperature to which steel must be rapidly cooled in order to achieve full hardening.

In modern representations, point a (now designated as A₁) corresponds to the eutectoid temperature. Point b is usually identified with the temperature, now designated as A₃, at which the dissolution of ferrite in austenite is completed when steel is heated. According to Chernov, steel heated above point b acquires after cooling a fine-grain structure, which thus determines the value of the point for the heat treatment of steel. Since in a number of cases heating above A₃ is not followed by the refinement of a coarse crystalline structure, the unqualified identification of point b with A₃ is apparently incorrect. Point d is now known as the temperature of martensitic transformation when steel is hardened, usually designated by M_s.