Patent ID: 8128761

Claim:
A carbonitriding method for carbonitriding a workpiece formed of steel that contains at least 0.8 mass % of carbon by heating in an atmosphere including ammonia, carbon monoxide, carbon dioxide, and hydrogen, the method comprising: an atmosphere control step of controlling an atmosphere in a heat treatment furnace, and a heating pattern control step of controlling a temperature history applied to said workpiece in said heat treatment furnace, wherein said atmosphere control step comprises an undecomposed NH 3 partial pressure control step of controlling a partial pressure of undecomposed ammonia in said heat treatment furnace, a CO/CO 2 partial pressure control step of controlling the partial pressure of at least one of carbon monoxide and carbon dioxide in said heat treatment furnace, and an H 2 partial pressure control step of controlling the partial pressure of hydrogen in said heat treatment furnace, said undecomposed NH 3 partial pressure control step, said CO/CO 2 partial pressure control step and said H 2 partial pressure control step are carried out such that the partial pressure of hydrogen in said heat treatment furnace is at least 0.1 atmospheric pressure and not more than 0.3 atmospheric pressure, and γ defined by a following equation (1) is at least 2.0 and not more than 6.0, in said atmosphere control step, wherein γ = a c * C NH 3 ( 1 ) where a c * = ( Pco ) 2 K × Pco 2 ( 2 ) P CO : partial pressure of carbon monoxide (atm); P CO 2 : partial pressure of carbon dioxide (atm); K: equilibrium constant at <C>+CO 2 2CO; C NH 3 : undecomposed ammonia concentration (volume %).