Fig. 1 The Fe-C equilibrium diagram up to 6.67 wt% C. Solid lines indicate Fe-Fe3C diagram; dashed lines indicate iron-graphite diagram.

Source: Ref 1

The Fe-C diagram in Fig. 1 is of experimental origin. The knowledge of the thermodynamic principles and modern

thermodynamic data now permits very accurate calculations of this diagram (Ref 4). This is particularly useful when

phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to

develop.

If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the

eutectoid composition are changed. Classical diagrams introduced by Bain (Ref 5) show the variation of A1 and the

eutectoid carbon content with increasing amount of a selected number of alloying elements (Fig. 2). It suffices here to

mention that (1) all important alloying elements decrease the eutectoid carbon content, (2) the austenite-stabilizing

elements manganese and nickel decrease A1, and (3) the ferrite-stabilizing elements chromium, silicon, molybdenum, and

tungsten increase A1. These classifications relate directly to the synergisms in quench hardening as described in the

articles "Quantitative Prediction of Transformation Hardening in Steels" and "Quenching of Steel"in this Volume.