Carburizing Behavior of AISI 4115 Steel According to the Flow Rates of Acetylene in Vacuum Carburizing

The vacuum carburizing behavior of AISI 4115 steel according to acetylene flow rates (1, 5, 10, 20, 30, 40, and 50 sccm) and temperatures (900, 950, and 1000 ^oC) was quantitatively investigated through microstructure, carbon concentration and hardness analyses. As the acetylene flow rate increased, the penetration rate of carbon into the surface of the samples (surface-carbon flux) increased, resulting in an increase in the solute carbon concentration at the surface up to the solubility of carbon in austenite. At the same time, the amount of carbon diffusion into the interior of the specimen and overall carburizing depth increased. When the carbon concentration in the surface of the carburized specimen exceeded the solubility of carbon in the austenite above a certain flow rate, cementite precipitated in the austenite grain boundaries. Accordingly, the surface hardness decreased and the amount of carbon diffusion into the specimen became constant as the solute carbon concentration in the austenite matrix at the surface of the specimen became constant. As a result, the overall solute carbon concentration and the effective case depth no longer increased. Meanwhile, the tendency of vacuum carburizing behavior according to temperature was the same, but as temperature increased, the surface-carbon flux and rate of carbon diffusion into the interior of the sample increased, resulting in increased carburizing depth. The appropriate flow rates (10, 20, and 30 sccm) and the corresponding surface-carbon fluxes (62.3, 135.7, and 216.7 g/m²h) were derived according to carburizing temperatures (900, 950, and 1000 ^oC), respectively.