Method for thermally treating chromium steels

Abstract

The invention relates to a method for thermally treating chromium steels, in particular high-alloy chromium-x-steels, by gas nitriding, comprising the following process steps: (A) heating a previously hardened component consisting of a chromium steel under a nitrogen atmosphere from an ambient temperature (tU) to its material-specific pre-oxidation temperature (TV) for performing a pre-oxidation step;(B) performing at least one tempering step by adding a nitride process gas to the nitrogen atmosphere around the component;(C) further heating the component to its material-specific tempering temperature (tA) and maintaining it for at least one hour;(D) cooling the component to its material-specific nitriding temperature (tN) for gas nitriding the component;(E) further cooling the component to the ambient temperature (tU).

1 . A method for thermally treating chromium steel by gas nitriding, the method comprising:
 (A) heating a previously hardened component consisting of a chromium steel under a nitrogen atmosphere from an ambient temperature (T U ) to its material-specific pre-oxidation temperature (T V ) for performing a pre-oxidation step;   (B) performing at least one tempering step by adding a nitride process gas to the nitrogen atmosphere around the component;   (C) further heating the component to its material-specific tempering temperature (T A ) and maintaining the material-specific tempering temperature (T A ) for a holding period;   (D) cooling the component to its material-specific nitriding temperature (T N ) for gas nitriding the component;   (E) further cooling the component to the ambient temperature (T U ).   
     
     
         2 . The method according to  claim 1 ,
 wherein the pre-oxidation temperature (T V ) of the step (A) is adjusted in a range between 300° C. to 450° C.   
     
     
         3 . The method according to  claim 1 ,
 wherein the tempering step started directly following the heating in step (B) begins upon reaching the material-specific pre-oxidation temperature, but no later than at a component temperature of between 350° C. to 450° C.   
     
     
         4 . The method according to  claim 1 ,
 wherein the nitride process gas is selected from a group of oxygen affine process gases comprising: ammonia gas, ammonia cracked gas mixture.   
     
     
         5 . The method according to  claim 1 ,
 wherein, in step (B), the nitride process gas is removed upon reaching a critical process temperature of 450° C. to 550° C. in an intermediate step (B1) or is replaced with pure nitrogen.   
     
     
         6 . The method according to  claim 1 ,
 wherein further heating of the component is carried out in step (C) to a tempering temperature of between 550° C. to 700° C.   
     
     
         7 . The method according to  claim 1 ,
 wherein the holding period of the tempering temperature in step (C) is at least 1 hour.   
     
     
         8 . The method according to  claim 1 ,
 wherein the component contains a mass fraction of at least 2% chromium as an alloy metal.   
     
     
         9 . The method according to  claim 1 ,
 wherein heating of the component is carried out in a chamber furnace with a gas exchange device.   
     
     
         10 . A motor vehicle component made of a chromium steel, which is thermally treated according to a manufacturing method according to  claim 1 . 
     
     
         11 . The component according to  claim 10 ,
 wherein the component is selected from a group of automotive components subject to wear and vibration comprising nozzle bodies, valve pieces, valve plates, valve bodies, throttle plates, valve supports, pistons.   
     
     
         12 . The method according to  claim 1 , wherein the chromium steels are high-alloy chromium-x-steels. 
     
     
         13 . The method according to  claim 2 , wherein the pre-oxidation temperature (T V ) of the step (A) is adjusted in a range between 350° C. to 450° C. 
     
     
         14 . The method according to  claim 7 , wherein the holding period of the tempering temperature in step (C) is between 2 to 4 hours. 
     
     
         15 . The method according to  claim 14 , wherein the holding period of the tempering temperature in step (C) is 3.0+/−0.5 hours. 
     
     
         16 . The component according to  claim 10 , wherein the chromium steel is a high-alloy chromium-x-steel.