9.1 INTRODUCTION

Martensite in steels can be a very strong and in its virgin condition rather brittle. It is then necessary to modify its mechanical properties by heat treatment in the range 150 700 C. This process is called tempering, in which the microstructure approaches equilibrium under the influence of thermal activation.

The tendency of the microstructure to temper depends on how far it deviates from equilibrium. The data in Table 9.1 show the components of the excess free energy of martensite in a typical low-alloy steel of chemical composition Fe-0.2C-1.5Mn wt%. The reference state is the equilibrium mixture of ferrite, graphite and cementite, with a zero stored energy. Graphite precipitates incredibly slowly in steels and is almost never observed during tempering - not surprising given that it only increases the free energy by a small amount (70 J mol ^?1). Preventing the substitutional solute, manganese, from partitioning between the ferrite and the austenite adds a substantial amount of energy but the greatest stored energy increase comes from the trapping of carbon in supersaturated ferrite. Martensite is in Table 9.1 distinguished from supersaturated ferrite by including the strain and interfacial energies due to its mechanism of transformation.

The general trend during the tempering of martensite therefore begins with...