Patent ID: 8317939
Filing Date: 2012-11-27
Classification: C23C

Abstract:
1. A method of rapid gas carburizing, using a carburizing atmosphere composed of an admixture of a hydrocarbon-based gas as a carburizing gas and a diluting gas, comprising: a. first step in which a steel treatment object is heated in the carburizing atmosphere, the carburizing atmosphere having a constant, predetermined carbon potential set to a constant value by setting the flow rate of the carburizing atmosphere to a constant value and setting the concentration of the carburizing gas to a constant value, the first step continuing until a surface carbon concentration of the treatment object reaches a final target value that is not higher than a solid solubility limit at a carburizing temperature that is not higher than the peritectic point, at which steel transforms from δ iron and liquid phase to γ iron, and is not less than the eutectic point, at which steel transforms from liquid phase to γ iron and cementite, the solid solubility limit existing on a solidus line, JE, of γ iron in an iron-carbon phase equilibrium diagram; and a second step in which the gas carburization at a carburizing temperature not higher than the peritectic point and not less than the eutectic point is advanced after the first step so that a carburizing depth of the treatment object increases while the surface carbon concentration of the treatment object is kept at the final target value by reducing a carbon potential of the carburizing gas over time at a rate-of-reduction, the rate-of-reduction of the carbon potential being decreased at a rate that decreases over time.