The present invention is directed to a process for tempering steel comprising carburizing said steel in a vacuum furnace in the presence of a hydrocarbon carburizing gas in combination with hydrogen wherein said carburizing gas/hydrogen combination is administered to the vacuum furnace by cyclically reducing the pressure in the furnace followed by the pulsed addition of the hydrocarbon carburizing gas consisting of an acetylene/hydrogen mixture is in a ratio of from about 1:1 to about 1:10 to replenish the air removed in the pressure reduction step.