Rapid Nitriding Through Nitriding Potential Control

Abstract

A disclosure is directed to a method for rapidly nitriding steel, the method including: placing the steel in a furnace having an atmosphere comprising partially dissociated ammonia gas; heating the steel to a highest temperature in a range of 400 to 600° C. while holding a nitriding potential below 15 atm −1/2 during heat-up from 400° C. up to the highest temperature; and holding the steel at the highest temperature while continuing to maintain the nitriding potential below 15 atm −1/2 , where a total time taken for the heating and holding the steel in the range of 400 to 600° C. during the nitriding is 15 hours or less, and where a composition of the steel comprises at least one of the group consisting of Al, Cr, Mo, V, and Ti.

We claim: 
     
         1 . A method for rapid nitriding of steel, comprising:
 placing the steel in a furnace having an atmosphere comprising partially dissociated ammonia gas;   heating the steel to a highest temperature in a range of 400 to 600° C. while holding a nitriding potential below 15 atm −1/2  during heat-up from 400° C. up to the highest temperature; and   holding the steel at the highest temperature while continuing to maintain the nitriding potential below 15 atm −1/2 ;   wherein a total time taken for the heating and holding the steel in the range of 400 to 600° C. during the nitriding is 15 hours or less, and wherein the steel comprises at least one of the group consisting of Al, Cr, Mo, V, and Ti.   
     
     
         2 . The method according to  claim 1 , further comprising:
 heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the nitriding potential below 15 atm −1/2  during the heat-up from 400° C. up to the highest temperature; and   subsequently holding the steel for 5 hours or less at or below the highest temperature in the range of 400 to 600° C. and at a nitriding potential in a range of from 0.5 to 10 atm −1/2  followed by the remainder of the time up to 15 hrs in total using a nitriding potential of 2 atm −1/2  or less.   
     
     
         3 . The method according to  claim 2 , further comprising:
 heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the nitriding potential below 15 atm −1/2  during heat-up from 400° C. up to the highest temperature and   subsequently holding the steel for 3 hours or less at or below the highest temperature in the range of 400 to 600° C. and at a nitriding potential in a range of from 0.5 to 10 atm −1/2  followed by the remainder of the time up to 15 hrs in total using a nitriding potential of 2 atm −1/2  or less.   
     
     
         4 . The method according to  claim 1 , further comprising:
 determining a Nitriding Intensity, NI, for the nitriding, wherein
   NI= a   (αTn-725)   ×c [(βK n ) d   −b βK n ]
 
   
       wherein: NI is dimensionless; Kn is a nitriding potential; a is a temperature impact constant; T n  is a temperature in Kelvin; c is a nitriding impact multiplier; d is a nitriding impact constant; b is a nitrogen potential constant; α is a unit conversion factor, 1/K°; and β is a unit conversion factor, atm 1/2 , wherein the temperature impact constant, the nitriding impact multiplier, the nitriding impact constant and the nitrogen potential constant are a function of an alloy composition of the steel. 
     
     
         5 . The method according to  claim 4 , further comprising:
 adjusting the temperature impact constant, the nitriding impact multiplier, the nitriding impact constant and the nitrogen potential constant so that, for the steel under consideration, the Nitride Intensity of a value of 1 corresponds to a minimum detectable limit of nitriding and a Nitride Intensity of 100 corresponds to a level at which a white layer forms within 15 minutes of the nitriding.   
     
     
         6 . The method according to  claim 4 , wherein a is 1.0105; b is 0.05; c is 3; and d is 0.92. 
     
     
         7 . The method according to  claim 5 , further comprising:
 maintaining a maximum NI value at 100 or less during the full cycle of the nitriding.   
     
     
         8 . The method according to  claim 5 , further comprising:
 maintaining the NI at 50 or less at a temperature (T n ) in the range of 400 to 600° C. during the full cycle of the nitriding once the highest temperature is reached.   
     
     
         9 . The method according to  claim 5 , further comprising:
 heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the NI in a range of from 1 to 100 during heat-up from 400° C. up to the highest temperature; and   subsequently holding the steel at or below the highest temperature in the range of 400 to 600° C. for 5 hours or less and at a NI in a range of from 1 to 50 followed by the remainder of the time up to 15 hrs in total using a NI in a range of from 1 to 20.   
     
     
         10 . The method according to  claim 5 , further comprising:
 heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the NI in a range of from 1 to 100 during heat-up from 400° C. up to the highest temperature; and   subsequently holding the steel at or below the highest temperature in the range of 400 to 600° C. for 3 hours or less and at a NI in a range of from 1 to 50 followed by the remainder of the time up to 15 hrs in total using a NI in a range of from 1 to 20.   
     
     
         11 . The method according  claim 1 , further comprising:
 increasing the nitriding potential to a highest nitriding potential and maintaining the steel at the highest nitriding potential for five hours or less and subsequently decreasing the nitriding potential during the full cycle of the nitriding.   
     
     
         12 . The method according  claim 11 , further comprising:
 increasing the nitriding potential to a highest nitriding potential and maintaining the steel at the highest nitriding potential for three hours or less and subsequently decreasing the nitriding potential during the full cycle of the nitriding.   
     
     
         13 . The method according  claim 11 , wherein the highest nitriding potential is in a range of from 6 atm −1/2  to 10 atm −1/2 . 
     
     
         14 . A nitrided steel obtained by the method according to  claim 1 , wherein the nitrided steel has an alloy composition comprising: by weight,
 C: from 0.1 to 2.2%;   Mn: from 0 to 1.2%;   Al: from 0 to 1.5%;   Cr: from 0 to 5.5%;   Mo: from 0.15 to 1.8%;   Si: from 0 to 1.8%;   V: from 0 to 1.2%; and   Iron and acceptable trace elements: remaining balance.   
     
     
         15 . The nitrided steel according  claim 14 , wherein the alloy composition further comprises:
 C: from 0.1 to 0.43%;   Mn: from 0.75 to 1.2%;   Al: from 0.08 to 0.13%;   Cr: from 0.8 to 1.2%;   Mo: from 0.15 to 0.25%;   Si: from 0.15 to 0.35%;   V: from 0.05 to 0.1%; and   Iron and acceptable trace elements: remaining balance.   
     
     
         16 . The nitrided steel according to  claim 14 , wherein the nitrided steel maintains a hardness value of 57 HRC or higher in a region from a surface to a thickness of 75 μm or more of the steel. 
     
     
         17 . The nitrided steel according to  claim 14 , wherein the nitrided steel maintains a hardness value that is 35 HRC or higher in a region from a surface to a thickness of 250 μm or more for steels with core hardness of 32 HRC or lower. 
     
     
         18 . The nitrided steel according to  claim 14 , wherein a white layer on a surface of the nitrided steel has a thickness of 2 μm or less. 
     
     
         19 . The nitrided steel according to  claim 14 , wherein a white layer on a surface of the nitrided steel has a thickness of 1 μm or less. 
     
     
         20 . The nitrided steel according to  claim 14 , wherein a TGP (thin γ′phase) layer on a surface of the nitrided steel has a thickness of 5 μm or less.