Nitriding treatment method for steel component

Abstract

The invention is a nitriding treatment method for a steel component including at least three nitriding treatment steps. The first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed at a temperature within a range of 500° C. to 590° C. The first nitriding potential for the first nitriding treatment step is a value within a range of 0.10 to 1.00. The second nitriding potential for the second nitriding treatment step is higher than the first nitriding potential and is a value within a range of 0.30 to 10.00. The third nitriding potential for the third nitriding treatment step is lower than the second nitriding potential and is a value within a range of 0.26 to 0.60.

1 . A nitriding treatment method for a steel component, comprising at least three nitriding treatment steps, i.e.,
 a first nitriding treatment step in which a nitriding treatment is performed to a steel component under a nitriding gas atmosphere of a first nitriding potential,   a second nitriding treatment step in which another nitriding treatment is performed to the steel component under another nitriding gas atmosphere of a second nitriding potential higher than the first nitriding potential, after the first nitriding treatment step, and   a third nitriding treatment step in which a further another nitriding treatment is performed to the steel component under a further another nitriding gas atmosphere of a third nitriding potential lower than the second nitriding potential, after the second nitriding treatment step,   wherein   the first nitriding treatment step is performed at a temperature within a range of 500° C. to 590° C.,   the second nitriding treatment step is also performed at a temperature within a range of 500° C. to 590° C.,   the third nitriding treatment step is also performed at a temperature within a range of 500° C. to 590° C.,   the first nitriding potential is a value within a range of 0.10 to 1.00,   the second nitriding potential is a value within a range of 0.30 to 10.00,   the third nitriding potential is a value within a range of 0.26 to 0.60,   a nitride compound layer consisting of a γ′ phase, or an ε phase, or mixture of a γ′ phase and an ε phase, is generated during the second nitriding treatment step, and   a γ′ phase is deposited in the nitride compound layer during the third nitriding treatment step.   
     
     
         2 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant,   three types of gases, which are an NH 3  gas, an AX gas and an N 2  gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the three types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant.   
     
     
         3 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant,   three types of gases, which are an NH 3  gas, an AX gas and an N 2  gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the three types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant.   
     
     
         4 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a batch type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the three types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant.   
     
     
         5 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the three types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of each of the NH 3  gas and the AX gas while keeping a total introduction amount of the two types of gases constant.   
     
     
         6 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant.   
     
     
         7 . The nitriding treatment method according to  claim 1 ,
 the first nitriding treatment step, the second nitriding treatment step and the third nitriding treatment step are performed in sequence in a same thermal processing furnace, which is a one-chamber type of thermal processing furnace,   two types of gases, which are an NH 3  gas and an AX gas, are used in the first nitriding treatment step,   a nitriding potential during the first nitriding treatment step is controlled to be close to the first nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant,   three types of gases, which are an NH 3  gas, an AX gas and an N 2  gas, are used in the second nitriding treatment step,   a nitriding potential during the second nitriding treatment step is controlled to be close to the second nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant,   two types of gases, which are an NH 3  gas and an AX gas, are used in the third nitriding treatment step, and   a nitriding potential during the third nitriding treatment step is controlled to be close to the third nitriding potential by changing an introduction amount of one of the NH 3  gas and the AX gas while keeping an introduction amount of the other of the NH 3  gas and the AX gas constant.   
     
     
         8 . The nitriding treatment method according to  claim 1 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         9 . The nitriding treatment method according to  claim 2 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         10 . The nitriding treatment method according to  claim 3 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         11 . The nitriding treatment method according to  claim 4 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         12 . The nitriding treatment method according to  claim 5 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         13 . The nitriding treatment method according to  claim 6 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.   
     
     
         14 . The nitriding treatment method according to  claim 7 ,
 a time for which the third nitriding treatment step is performed is 60 minutes or longer.