Abstract
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.
Highlights
Austenitic stainless steel is used in a variety of industrial applications due to excellent corrosion resistance and mechanical properties, as well as especially low-temperature toughness.it has poor wear resistance and hardness when employed in applications requiring high mechanical properties
Direct low-temperature vacuum carburization was conducted to investigate the effects of the C2 H2 and H2 gases mixture ratios on the formation of the carburization layer, which is well known as expanded austenite layer
The expanded austenite layer clearly separated from the substrate can be observed in Figure 1a,b, cross-sectional images of samples carburized at the conditions of A-1 and A-4, observed in Figure 1a,b, cross-sectional images of samples carburized at the conditions of A-1 and Arespectively
Summary
Austenitic stainless steel is used in a variety of industrial applications due to excellent corrosion resistance and mechanical properties, as well as especially low-temperature toughness. It has poor wear resistance and hardness when employed in applications requiring high mechanical properties. Austenitic stainless steel has a limitation in that it requires a low temperature in the surface hardening treatment as a high temperature would reduce its corrosion resistance. Low-temperature carburization was proposed and commercialized to enhance surface mechanical properties such as hardness and wear resistance, while maintaining the inherent corrosion resistance of austenitic stainless steel [2,3,4,5,6,7]. Carbon atoms penetrate into the austenite matrix to form a supersaturated solid solution [8]
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