Nitriding method and nitrided part production method

A nitriding method, comprising a gas nitriding step in which a low alloy steel is heated to a temperature ranging from 550 to 620° C. in a gas atmosphere containing NH3, H2, and N2, the gas nitriding step being performed for a total process time of A ranging from 1.5 to 10 hours,

• the gas nitriding step including the steps of:
• performing a high KN value process with a nitriding potential KNX determined by Formula (1) ranging from 0.15 to 1.50 and with an average value KNXave of the nitriding potential KNX, the average value KNXave ranging from 0.30 to 0.80, the high KN value process being performed for a process time of X in hours, and
• performing a low KN value process after the high KN value process, the low KN value process being performed with a nitriding potential KNY determined by Formula (1) ranging from 0.02 to 0.25 and with an average value KNYave of the nitriding potential KNY, the average value KNYave ranging from 0.03 to 0.20, the low KN value process being performed for a process time of Y in hours,
• wherein an average nitriding potential value KNave determined by Formula (2) ranges from 0.07 to 0.30,
• wherein a thickness of a compound layer, which is formed on a surface of the alloy steel by the gas nitriding step, is not more than 3 μm,
• KNi=(NH3 partial pressure)/[(H2 partial pressure)3/2] (1)
• KNave=(X×KNXave+Y×KNYave)/A (2)
• where i is X or Y.