Pearlite stabilization

By adding alloy elements, or possibly by varying the basic composition, setting of a completely pearlitic basic structure (see Pearlite) in the cast condition.

As the formation of ferrite requires a carbon diffusion and this reduces with falling temperature, alloy elements such as copper, tin, nickel, manganese or chromium promote the transformation of pearlite and delay the formation of ferrite.

This pearlite which is formed at lower temperatures is finer and consequently stronger, harder and better resistant to wear. A similar pearlite refinement and with it a solidification effect can be achieved through hot shake-out and accelerated cooling. However, residual stresses here are considerably and unavoidably larger.

The influence of the alloy elements on the eutectoid reaction can be seen in the TTT diagram for continuous cooling. Most elements shift ferrite and pearlite transformations to the right and to the bottom (Fig. 1).

Molybdenum, however, behaves ambivalently. It strongly delays the pearlitic transformation but, at the same time, has a small effect on the delay of the formation of free ferrite. As a result, adding molybdenum delays the pearlite reaction but also leaves more time for carbon diffusion of the formation of free ferrite. As a consequence, the addition of molybdenum can cause ferrite formation in unalloyed iron (warning, carbide formation!). For this reason, the addition of molybdenum is always combined with pearlite formers, if a fully pearlitic structure is desired.

Additional references:
Perlite peak
Pearlite point
Pearlitic influence factor
Pearlite period

  • Figure 1: Influence of selected alloy elements on the austenite decomposition during continuous cooling