The effect of the acetylene and hydrogen gases mixture ratios in direct low-temperature vacuum carburization was investigated. The gas ratio is an important parameter for producing free radicals in carburization. The free radicals can remove the natural oxide film by strong reaction of the hydrocarbons, and then thermodynamic activity can be increased. When the gas ratio was below one, carbon-supersaturated expanded austenite layers were formed on the surface of the AISI 316L stainless steel, which had a maximum carbon solubility up to 11.5 at% at 743 K. On the other hand, when the gas ratio was above one, the carbon concentration of the layers was low even if the process time was increased enough to reach the maximum carbon solubility. As a result, the carbon concentration underneath the surface was determined to be highly dependent on the gas mixture ratio of acetylene and hydrogen. In conclusion, it is necessary to restrict the ratio of acetylene and hydrogen gases in the total mixture of gases to form an expanded austenite layer with high carbon concentration in direct low-temperature vacuum carburization.

The effect of the acetylene and hydrogen gases mixture ratios in direct low-temperature vacuum carburization was investigated. The gas ratio is an important parameter for producing the free radicals in the carburization. The free radicals can remove the natural oxide film by the strong reaction of the hydrocarbons, and then thermodynamically activity can be increased. When the gas ratio was below 1, the supersaturation expanded austenite layers were formed on the surface of the AISI 316L stainless steel, which had the maximum carbon solubility up to 11.5 at.% at 743 K, were formed. On the other hand, when the gas ratio was above 1, the carbon concentration of them remained low even if the process time was enough increased to reach the maximum carbon solubility. As a result, the carbon concentration underneath the surface was determined to be highly dependent on the gas mixture ratio of acetylene and hydrogen. In conclusion, it is necessary to restrict the ratio of acetylene and hydrogen gases to total mixture gases to form the expanded austenite layer with the high carbon concentration in the direct low-temperature vacuum carburization.