2 98 to 100 0 by weight a pure silica sand washed with acids 90 of which passes through a 2 mm screen has been placed in the chamber 2 after pieces to be treated have been placed in the bed of the chamber 3 the chamber is swept with an inert gas namely nitrogen or argon and the temperature is simultaneously raised the bf 3 gas diluted if appropriate is then passed through when the temperature reaches about 500 u00b0 to 950 u00b0 c the latter is chosen as the boronising temperature the duration of the passage of the activating gas varies from half to the whole of the residence time of the pieces at 950 u00b0 c the said residence time being about 5 hours simultaneously the temperature of the bed of silica 7 is raised to about 850 u00b0 c two steels containing 0 1 and 0 35 of carbon were tested with a weight proportion b 4 c sic of 20 80 these steels being respectively designated xc 10 and xc 35 in accordance with the afnor designation after cooling the pieces were examined in the laboratory it was found see the micrograph sections in fig5 and 6 that in both cases the pieces were covered with a 170 u03bcm monophase layer a of oriented fe 2 b crystals.