vessel 2 made of stainless or fireproof steel the vessel 2 contains activated boron carbide powder 5 thereafter the vessel is placed in an oven which consists of a base 4 made of refractory bricks and of a cover 1 the cover 1 is made of fireproof ceramic fibre material into which electric resistor units 6 have been embedded the temperature of the resistor units 6 is regulated by means of a separate control unit the temperature is raised by means of the resistor units to a temperature from about 800 u00b0 c to about 1050 u00b0 c at which temperature the diffusion of the boron into the steel takes place the particle size of the boronizing powder is in the range of from about 10 microns to about 1500 microns depending on the groove size on the bar to be boronized on the base material and on the required surface quality the reaction time varies from 10 minutes up to 30 hours depending on the desired thickness of the hard layer the type and the alloying of the base material also affect the reaction time the boronizing powder is available commercially e g under the name ekabor which is a registered trademark of messrs elektroschmelzwerk kempten gmbh the boronizing powder consists of activated boron carbide b 4 c whose boron concentration is from about 40 to about 70 in.