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 (t.sub.U) to its material-specific pre-oxidation temperature (T.sub.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.sub.A) and maintaining it for at least one hour; (D) cooling the component to its material-specific nitriding temperature (t.sub.N) for gas nitriding the component; (E) further cooling the component to the ambient temperature (t.sub.U).

Claims

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.sub.U) to its material-specific pre-oxidation temperature (T.sub.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.sub.A) and maintaining the material-specific tempering temperature (T.sub.A) for a holding period; (D) cooling the component to its material-specific nitriding temperature (T.sub.N) for gas nitriding the component; (E) further cooling the component to the ambient temperature (T.sub.U).

2. The method according to claim 1, wherein the pre-oxidation temperature (T.sub.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.sub.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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Further measures for improving the invention are described in greater detail hereinafter, together with the description of a preferred exemplary embodiments of the invention, with reference to the figures. Shown are:

[0022] FIG. 1 a flow chart of the method according to the invention for the thermal treatment of chromium steels, and

[0023] FIG. 2 a graph of the temperature-time curve of the thermal treatment according to FIG. 1.

DETAILED DESCRIPTION

[0024] According to FIG. 1, the thermal treatment according to the invention of a component consisting of chromium steel, which has already been hardened, initially includes heating it under a nitrogen atmosphere from an ambient temperature T.sub.U to a material-specific pre-oxidation temperature T.sub.V of approximately 350 C. in step A. The heating as well as the performance of the following method steps is carried out in a conventional chamber furnace, which is equipped with a gas exchange device.

[0025] After heating and pre-oxidizing, a tempering frequency is started in step B for tempering the heated component while adding a nitride process gas, which is ammonia in this exemplary embodiment. Once a critical process temperature is reached, the ammonia is removed again according to intermediate step B1. Further heating is carried out in step B1 up to the material-specific tempering temperature T.sub.A of above 550 C. In this exemplary embodiment, this tempering temperature is maintained for about 3 hours.

[0026] After tempering in step C, nitriding is initiated by cooling the component to its material-specific nitriding temperature in step D, so that gas nitriding occurs. Then, in step E, the component is further cooled to the ambient temperature T.sub.U.

[0027] FIG. 2 illustrates a temperature-time curve of the process flow described above, whereby the component is heated in step A, starting from the ambient temperature T.sub.U for any heating time t.sub.A under a nitrogen atmosphere. Then, in step B, after pre-oxidation, a tempering step is started with the addition of the process gas ammonia, wherein the pre-oxidation temperature T.sub.V resulting from the heating initially remains at approximately 400. Upon reaching a critical process temperature T.sub.K of approximately 500 C., in intermediate step B1, the process gas ammonia is removed again and instead the cracked gas is introduced. Further heating is carried out in step C to the tempering temperature T.sub.A, which is maintained under a nitrogen atmosphere up to the time t.sub.K for approximately 3 hours until the tempering is completed. Subsequently, in step D, cooling to nitriding temperature T.sub.N for gas nitriding takes place again in the presence of ammonia gas and cracked gas. After completion of the gas nitriding, a further cooling of the component to ambient temperature T.sub.U is carried out from the time t.sub.N.

[0028] The invention is not limited to the preferred exemplary embodiment described above. Rather, it is also conceivable to make modifications to this, which are included in the scope of protection of the following claims. For example, it is also possible to omit the intermediate step B1. The specific temperatures, in particular the pre-oxidation temperature T.sub.V, the tempering temperature T.sub.A and the nitriding temperature T.sub.N, are based on the specific composition of the thermally treated chromium steel, i.e. in particular its alloying elements.