The objective of the present work is to establish the heat treating parameters necessary to impose to a pair of high chromium white cast irons a hardness figure in the range from 35 to 40 HRC, acceptable to guarantee a reasonable machinability of the material. Samples of two high chromium white cast irons have been isothermally annealed under different conditions. The evolution of the phase transformations during the annealing process has been evaluated by dilatometry, supported by optical and scanning electron microscopy, x-ray microanalysis (EDS) and hardness evaluation. The results from this work demonstrate that the recommended procedure for the soft annealing treatment of the high chromium white cast irons consists of a destabilizing step in the austenitic domain followed by an isothermal decomposition of the austenite in the ferritic domain.