METHOD FOR THERMALLY TREATING CHROMIUM STEELS

Description

BACKGROUND

The present invention relates to a method for thermally treating chromium steels, in particular high-alloy chromium-x-steels, by gas nitriding, as well as a motor vehicle component, resulting particularly from such a manufacturing process.

The field of application of the present invention primarily extends to automotive technology as well as tool construction. Motor vehicle components made of high-alloy chromium steel, i.e. a steel material with the main alloy component chromium, are usually tempered, activated and nitrated to achieve the desired component properties after the obligatory martensitic hardening. Typically, such components are specifically made of high-alloyed chromium steels, such as X40CrMov5-1, X50CrMov5-1, and X90CrW18, and are used in the area of automotive technology, which is of primary interest here, for example as components of valves, chokes, or piston-cylinder arrangements in high pressure applications.

According to the technical fact sheet “Thermal treatment of steel—nitriding and carbonitriding” (publisher: Stahl-Informations-Zentrum Dusseldorf, edition 2005, ISSN 0175-2006), classical gas nitriding of low-alloyed and high-alloyed steels of the type discussed herein takes place in several steps. Hardening is first carried out, then several tempering sequences at mostly different temperatures and times, followed by activating the component surface as a separate process before the actual nitriding. Several hours, days or even weeks may pass between the thermal treatment process, consisting of hardening and tempering, and the subsequent activating and (gas) nitriding process.

Low-alloyed and high-alloyed steels always have a natural thin oxide layer of a few nanometers, which forms stably in normal air and/or different water vapor contents even at room temperature. This oxide layer is composed of the oxygen affine alloying elements chromium, molybdenum, vanadium, silicon, aluminum and iron as well as other oxidation-capable alloying elements of the steel material.

These alloying elements are thus no longer dissolved in the crystal lattice and, as a thin oxide layer, negatively affect or completely impair diffusing of atomic nitrogen at the subsequent nitriding temperatures in the range of 400 to 600° C. Inhomogeneous bonding layers as well as diffusion layers with undesirable different mechanical, electrical, magnetic and chemical functional properties are the result. Typically, these thin oxide layers are removed chemically via pickling with an acid, electrically via applying an electric voltage to energize the oxide layer, or mechanically via surface processing by, for example, brushing, grinding or honing, or the like, prior to the actual nitriding process. This intermediate processing step between at the thermal treatment and subsequent nitriding requires a correspondingly high amount of manufacturing effort and time. In addition, it is not ensured that all surface areas have been processed accordingly.

In the overall thermal treatment process, the core hardness of the component is adjusted according to the functional requirements by means of at least one tempering step above the tempering temperature. As part of a metallurgical tempering step, the component is usually heated in a targeted manner to affect its properties, and also to reduce workpiece stresses.

SUMMARY

It is the object of the present invention to create a thermal treatment process consisting of tempering and gas nitriding of hardened chromium steels, in which the usual tempering and activation is eliminated and the core hardness of the component can be variably adjusted.

The object is achieved with a method for thermal treatment according to the disclosure.

The invention includes the procedural instruction that a method for thermally treating chromium steels, in particular high-alloy chromium-x-steels, by gas nitriding, comprises the following process steps:

• • (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 it for at least one hour;
  • (D) cooling the component to its material-specific nitriding temperature T_N for gas nitriding the component; and
  • (E) further cooling of the component to the ambient temperature T_U.

The advantage of the solution according to the invention is in particular that the functionally important core hardness of the material can now be freely adjusted independently of the nitriding temperature. The method according to the invention can thereby be made use of for a broader application. The solution according to the invention allows the core hardness to be adjusted independently of the required nitriding temperature. This is because, when nitriding between 480-600° C., the core hardness only results in between 620 HV1-460 HV1 according to the nitriding temperature. An independent choice of the core hardness of 200-440 HV1, for example, is not possible because nitriding above a material-specific limit results in a functionally poor structure.

Preferably, the pre-oxidation temperature of step (A) is selected in the range between 300 to 450° C., preferably in the range between 350 to 450° C. The range breadth results from different material properties of the previously hardened steel component and is dependent on its alloy elements in this respect. Attempts have shown that the above-mentioned preferential range with a higher lower range limit for the pre-oxidation temperature provides higher energy efficiency because the energy input for subsequent further heating is lower after the tempering step. In addition, the pre-oxidation temperature is used according to the diffusion permeable oxides to be formed. During heating, the nitrogen atmosphere prevents surface oxidation.

Preferably, the tempering in step (B) should be started immediately following the heating, i.e. upon reaching the pre-oxidation temperature, but from a component temperature of between 350 to 450° C. at the latest, with a temperature distribution as homogeneous as possible in the oven. The latter control parameter may be used to simplify control of the temperature profile. Pre-oxidation may be performed via synthetic air, atmosphere, or oxygen-containing gases. The start of the tempering step is preferably caused by a nitride process gas being supplied after evacuation. An oxygen-affine process gas, for example ammonia gas, ammonia cracked gas mixture, or the like, is suitable as a nitride process gas. A cracked gas is understood to be a mixture of gases consisting of nitrogen and hydrogen. During this tempering step, it is irrelevant whether the temperature is kept constant or the temperature changes. However, from a critical process temperature of 450 to 550° C., the nitride process gas can be removed via, for example, a vacuum process step as intermediate step B1 or via a purging process with nitrogen.

Following this tempering step, the component is further heated to the material-specific starting temperature T_A under preferably nitrogen or as a function of the tempering temperature, also under ammonia or cracked gas, wherein a holding time of at least one hour, preferably 2 to 4 hours, quite preferably 3+/−0.5 hours, is selected. The duration of the holding time is, in turn, dependent on the type of the chromium steel to be thermally treated. Process gases are continued to be supplied or pure nitrogen is supplied.

The heating of the component may preferably be carried out in a conventional chamber furnace with a corresponding gas exchange device. After the tempering step in step C, the component is cooled to nitriding temperature T_N for gas nitriding. After passing through a material-specific cooling temperature, the nitrogen may be replaced by the nitride process gas. After reaching the nitriding temperature, the gas nitriding of the component takes place in a conventional manner.

According to the thermal treatment method described above, motor vehicle components can in particular be made from a chromium steel, preferably a high-alloyed chrome-x-steel, which are particularly subject to wear and vibration. These may be, for example, automotive components, such as nozzle bodies, valve pieces, valve plates, valve bodies, throttle plates, valve carriers, or pistons, which are primarily used in the high-pressure area of the fuel supply of a motor vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

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

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

DETAILED DESCRIPTION

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_U to a material-specific pre-oxidation temperature T_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.

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_A of above 550° C. In this exemplary embodiment, this tempering temperature is maintained for about 3 hours.

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_U.

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_U for any heating time t_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_V resulting from the heating initially remains at approximately 400°. Upon reaching a critical process temperature T_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_A, which is maintained under a nitrogen atmosphere up to the time t_K for approximately 3 hours until the tempering is completed. Subsequently, in step D, cooling to nitriding temperature T_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_U is carried out from the time t_N.

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_V, the tempering temperature T_A and the nitriding temperature T_N, are based on the specific composition of the thermally treated chromium steel, i.e. in particular its alloying elements.