of a single thermal cycle by exposing the metal to diffusible carbon and boron with no intermediate cooling between the carburizing and boronizing temperatures by varying the conditions at the ferrous metal surface the carburized substrate 14 will be formed initially the intermediate and outer layers 16 and 18 are formed subsequently in the preferred embodiment the temperature at the ferrous metal surface is selected as the variable condition to selectively carburize and boronize the metal in one process performed under the teachings of the present invention a 4815 steel having a carbon content between 0 13 and 0 18 was treated a pack composition was employed to form the surface layer 12 this composition consisted of between 5 and 15 by weight amorphous boron between 75 and 85 by weight of a carburizing compound consisting of a blend of between 10 and 20 by weight of barium carbonate between 5 and 10 by weight calcium carbonate and the remainder a hardwood charcoal and or coke and about 10 by weight of an alkali earth fluoride i e kf with reference to fig2 the initial carburizing phase 20 was performed at a temperature of approximately 1500 u00b0 f for a period of approximately seventeen hours at this temperature the carbide layer 14 is formed rapidly the temperature is then increased over a period of approximately one hour to a final temperature of about 2000 u00b0 f at 2000 u00b0 f the boride phase 22 rapidly diffuses boron into the metal to form layers 16 and 18 the rate of temperature change.