Concurrent flow of activating gas in low temperature carburization

A process for surface hardening a workpiece made from an iron, nickel and/or chromium based alloy by gas carburization in which an unsaturated hydrocarbon is contacted with the workpiece inside a carburization reactor under a pressure of about 3.5 to 100 torr and at an elevated carburization temperature to cause carbon to diffuse into the workpiece surfaces thereby forming a hardened primary surface layer which is essentially free of carbide precipitates as well as a thermal oxide film, the process further comprising increasing the rate at which carburization occurs by feeding a carbon-free, halogen-containing activating compound to the carburization reactor simultaneously with feeding the unsaturated hydrocarbon to the carburization reactor, wherein the unsaturated hydrocarbon is acetylene, the concentration of the of the unsaturated hydrocarbon inside the carburization reactor is about 8 to 35 vol. % and the concentration of the carbon-free, halogen-containing activating compound inside the carburization reactor is about 0.5 to 3 vol. %.