Method for heat treating chrome steels

Abstract

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

Description

[0001] Description

[0002] Title:

[0003] Process for heat treating chromium steels

[0004] The present invention relates to a method for heat treating chromium steels, in particular high-alloy chromium-x steels, by gas nitriding and to a motor vehicle component resulting in particular from such a manufacturing method.

[0005] The field of application of the present invention extends primarily to automotive engineering and toolmaking. Automotive components subject to wear and vibration, made of high-alloy chromium steel, i.e., a steel material with chromium as the main alloying component, are usually tempered, activated, and nitrided after the mandatory martensitic hardening to achieve the desired component properties. Such components are typically made of specially high-alloy chromium steels, such as X40CrMov5-l, X50CrMov5-l, and X90CrW18, and are used in the automotive engineering sector of primary interest here, for example, as components for valves, throttles, or piston-cylinder assemblies in high-pressure applications.

[0006] State of the art

[0007] According to the technical data sheet "Heat Treatment of Steel - Nitriding and Nitrocarburizing" (Publisher: Steel Information Center Düsseldorf, 2005 edition, ISSN 0175-2006), the classic gas nitriding of low- and high-alloy steels of the type in question here is carried out in several steps. It begins with hardening, followed by several tempering sequences, usually at different temperatures and times, and then activation of the component surface as a separate process prior to the actual nitriding. Several hours, days, or even weeks can elapse between the quenching and tempering process and the subsequent activation and (gas) nitriding process.

[0008] Low- and high-alloy steels always exhibit a naturally thin oxide layer of a few nanometers, which forms stably in normal air and / or with varying water vapor concentrations, even at room temperature. This oxide layer consists of the oxygen-affine alloying elements chromium, molybdenum, vanadium, silicon, aluminum, and iron, as well as other oxidizable alloying elements of the steel material.

[0009] These alloying elements are therefore no longer dissolved in the crystal lattice and, as a thin oxide layer, negatively or completely impair the diffusion of atomic nitrogen at the subsequent nitriding temperatures in the range between 400 and 600°C. The result is inhomogeneous compound layers and diffusion layers with undesirable, different mechanical, electrical, magnetic, and chemical functional properties. These thin oxide layers are usually removed before the actual nitriding process. These layers are chemically removed by pickling with an acid, electrically by applying an electrical voltage to break down the oxide layer, or mechanically by surface treatment such as brushing, grinding, honing, or similar. This intermediate processing step between tempering and subsequent nitriding requires a correspondingly high manufacturing and time expenditure.In addition, there is no guarantee that all surface areas have been treated accordingly.

[0010] During the overall heat treatment process, the core hardness of the component is adjusted according to functional requirements through at least one tempering step via the tempering temperature. During a metallurgical tempering step, the component is typically heated in a targeted manner to influence its properties, including to relieve workpiece stresses. The object of the present invention is to create a heat treatment process for hardened chromium steels consisting of quenching and tempering and gas nitriding, in which the usual tempering and activation steps are eliminated and the core hardness of the component can be variably adjusted.

[0011] Disclosure of the invention

[0012] This object is achieved by a heat treatment method according to claim 1. The following dependent claims describe advantageous developments of the invention. The subordinate claim 10 specifies the product-by-process result of claim 1 for specific automotive components.

[0013] The invention includes the process engineering teaching that heat treatment of chromium steels, in particular high-alloy chromium-x steels, by gas nitriding comprises the following process steps:

[0014] (A) heating a previously hardened component made of chromium steel under a nitrogen atmosphere from an ambient temperature Tu to its material-specific pre-oxidation temperature Tv to carry out a pre-oxidation step;

[0015] (B) carrying out at least one tempering step by adding a nitriding process gas to the nitrogen atmosphere around the component;

[0016] (C) further heating the component to its material-specific tempering temperature TA and maintaining it at this temperature for preferably at least one hour;

[0017] (D) cooling the component to its material-specific nitriding temperature TN for gas nitriding of the component; and

[0018] (E) further cooling of the component to the ambient temperature Tu. The advantage of the inventive solution lies in the fact that the functionally important core hardness of the material can now be freely adjusted independently of the nitriding temperature. This makes the inventive method usable for a broader range of applications. The inventive solution enables the core hardness to be adjusted independently of the required nitriding temperature. When nitriding between 480 - 600 °C, the core hardness only results in a range between 620 HV1 - 460 HV1, depending on the nitriding temperature. An independent selection of the core hardness of, for example, 200 - 440 HV1 is not possible, since nitriding above a material-specific limit leads to a functionally poor microstructure.

[0019] The pre-oxidation temperature of step (A) is preferably selected in the range between 300 and 450°C, preferably in the range between 350 and 450°C. The range depends on various material properties of the previously hardened steel component and, to this extent, on its alloying elements. Tests have shown that the aforementioned preferred range with a higher lower limit for the pre-oxidation temperature offers greater energy efficiency, since the energy input for subsequent further heating after the tempering step is lower. In addition, the pre-oxidation temperature is selected according to the diffusion-permeable oxides to be formed. During heating, the nitrogen atmosphere prevents surface oxidation.

[0020] Preferably, the annealing of step (B) should begin immediately after heating, i.e., once the pre-oxidation temperature has been reached, but no later than when the component temperature reaches between 350 and 450°C, with the temperature distribution in the furnace as homogeneous as possible. The latter control parameter can be used to simplify the control of the temperature profile. Pre-oxidation can be carried out using synthetic air, atmosphere, or oxygen-containing gases. The start of the annealing step is preferably achieved by adding a nitriding process gas after evacuation. A suitable nitriding process gas for this purpose is an oxygen-affine process gas, for example, ammonia gas, an ammonia gas-fission gas mixture, or the like. Fission gas is understood to be a gas mixture consisting of the gases nitrogen and hydrogen.During this tempering step, it is irrelevant whether the temperature is kept constant or changed. However, above a critical process temperature of 450 to 550°C, the nitriding process gas can be removed, for example, via a vacuum process step as an intermediate step B1 or via a nitrogen purge process.

[0021] Following this tempering step, the component is further heated to the material-specific tempering temperature TA, preferably under nitrogen or, depending on the tempering temperature, under ammonia or cracked gas. A holding time of at least one hour, preferably 2 to 4 hours, and most preferably 3 + / - 0.5 hours, is selected. The length of the holding time, in turn, depends on the grade of chromium steel to be heat-treated. Process gases or pure nitrogen are still supplied.

[0022] The component can preferably be heated in a conventional chamber furnace with a suitable gas exchange device. After the tempering step in step C, the component is cooled to the nitriding temperature TN for gas nitriding. After passing through a material-specific cooling temperature, the nitrogen can be replaced with the nitriding process gas if necessary. Once the nitriding temperature is reached, the component is then gas nitrided in the conventional manner.

[0023] The heat treatment process described above can be used to produce automotive components made of chromium steel, preferably a high-alloy chromium-x steel, which are particularly exposed to wear and vibration. These can be, for example, automotive components such as nozzle bodies, valve pieces, valve plates, valve bodies, throttle plates, valve supports, or pistons, which are primarily used in the high-pressure area of ​​a motor vehicle's fuel supply. Description of the Figure

[0024] Further measures improving the invention are described in more detail below, together with the description of a preferred embodiment of the invention, with reference to the figures. It shows:

[0025] Fig. 1 is a flow chart of the process according to the invention for the heat treatment of chromium steels, and

[0026] Fig. 2 is a graphical representation of the temperature-time curve of the heat treatment according to Fig. 1.

[0027] According to Fig. 1, the heat treatment according to the invention of an already hardened component made of chromium steel initially includes, in a step A, heating under a nitrogen atmosphere from ambient temperature Tu to a material-specific pre-oxidation temperature Tv of approximately 350°C. The heating and the implementation of the subsequent process steps are carried out in a conventional chamber furnace equipped with a gas exchange device.

[0028] After heating and pre-oxidation, in step B, a tempering frequency is started to temper the heated component by adding a nitriding process gas, which in this embodiment is ammonia. Once a critical process temperature is reached, the ammonia is removed again according to intermediate step B1. Further heating in step B1 is carried out up to the material-specific tempering temperature TA of above 550°C. In this embodiment, this tempering temperature is maintained for approximately 3 hours.

[0029] 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 begins. Subsequently, in step E, the component is further cooled to the ambient temperature Tu.

[0030] Fig. 2 illustrates a temperature-time curve of the process sequence described above, in which in step A of heating, starting from the ambient temperature Tu, the component is heated under a nitrogen atmosphere within an arbitrary heating time tA. Subsequently, in step B, after pre-oxidation, an annealing step is started with the addition of the process gas ammonia, wherein the pre-oxidation temperature Tv of approximately 400°C resulting from the heating is initially maintained. Once a critical process temperature Tk of approximately 500°C is reached, the process gas ammonia is removed again in the intermediate step B1 and cracking gas is introduced instead. Further heating takes place in step C to annealing temperature TA, which is maintained under a nitrogen atmosphere until time t« for approximately 3 hours, until annealing is completed.Subsequently, in step D, the component is cooled again to the nitriding temperature TN for gas nitriding in the presence of ammonia gas and fission gas. After completion of gas nitriding, the component is further cooled to ambient temperature Tu in step E, starting at time IN.

[0031] The invention is not limited to the preferred embodiment described above. Rather, modifications thereof are also conceivable, which are also encompassed by the scope 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 Tv, the tempering temperature TA, and the nitriding temperature TN, depend on the specific composition of the chromium steel to be heat-treated, i.e., in particular, its alloying elements.

Claims

Claims 1. A process for heat treating chromium steels, in particular high-alloy chromium-x steels, by gas nitriding, comprising the following process steps: (A) heating a previously hardened component made of chromium steel under a nitrogen atmosphere from an ambient temperature (Tu) to its material-specific pre-oxidation temperature (Tv) to carry out a pre-oxidation step; (B) carrying out at least one tempering step by adding a nitriding process gas to the nitrogen atmosphere around the component; (C) Further heating of the component to its material-specific tempering temperature (TA) and maintaining it for a holding time; (D) Cooling the component to its material-specific nitriding temperature (TN) for gas nitriding of the component; (E) Further cooling of the component to the ambient temperature (Tu).

2. Process according to claim 1, characterized in that the pre-oxidation temperature (Tv) of step (A) is set in the range between 300°C and 450°C, preferably in the range between 350°C and 450°C.

3. Method according to claim 1, characterized in that the tempering step in step (B) which starts immediately after the heating-up begins when the pre-oxidation temperature is reached, but at the latest when a component temperature between 350°C and 450°C is reached.

4. The method according to claim 1, characterized in that the nitriding process gas is selected from a group of oxygen-affine process gases, comprising: ammonia gas, ammonia gas-fission gas mixture.

5. The method according to claim 1, characterized in that in step (B) the nitriding process gas is removed or replaced by pure nitrogen in an intermediate step (B1) upon reaching a critical process temperature of 450°C to 550°C.

6. The method according to claim 1, characterized in that the further heating of the component in step (C) is carried out to a tempering temperature between 550°C and 700°C.

7. The method according to claim 1, characterized in that the holding time of the tempering temperature in step (C) is at least 1 hour, preferably between 2 and 4 hours, most preferably 3 + / - 0.5 hours.

8. Method according to claim 1, characterized in that the component contains a mass fraction of at least 2% chromium as alloying metal.

9. Method according to claim 1, characterized in that the heating of the component is carried out in a chamber furnace with a gas exchange device.

10. Motor vehicle component made of a chromium steel, in particular a high-alloy chromium-x steel, which is tempered by a manufacturing process according to one of the preceding claims.

11. Component according to claim 10, characterized in that it is selected from a group of wear- and vibration-loaded motor vehicle components, comprising nozzle body, valve piece, valve plate, valve body, throttle plate, valve carrier, piston.

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