Carburising iron@ workpieces in gas phase process - with carbon mon:oxide to hydrogen@ ratio increased during specific phases of process to optimise carbon transfer to iron@ surface

摘要

Carburising iron workpieces uses a carburising atmos. of gaseous H2 and Co at 800-1050 deg. C in which during a certain phase of the process the Co to H2 ratio is raised to above 1:2 by drawing off some of the atmos. and passing it through a gas sepn. plant from where the Co is recycled back to the carburising furnace. This may be enhanced by supplying fresh Co from a special fixed source or the special source can be used alone. The gas sepn. plant is a membrane gas sepn. plant. During certain phases of the process, 20-70% of the atmos. feed is drawn off. This gas is cooled, filtered, compressed and then fed to the gas separator with the Co amt. fed back to the furnace as the main gas feed is reduced. At the start of the process the atmos. is essentially N2-free Co from the special fixed source is fed into the furnace in a short phase at the start of the process (i.e. 10-36% of the total process time) N2 is added to the atmosphere during a later stage of the process. ADVANTAGE - The Co to H2 ratio is economically raised above 1:2 thus approaching the optimum 1:1 ratio required for good carbon transfer into the iron surfacee ga