Heat treatment method for chrome steel and automotive parts made of chrome steel

Abstract

The present invention relates to a method for heat treating chromium steels, particularly high alloy chromium x steels, by gas nitriding, comprising the following steps: (A) heating a pre-hardened part made of chromium steel from ambient temperature (tU) to a material-specific pre-oxidation temperature (tV) in a nitrogen atmosphere to perform a pre-oxidation step; (B) performing at least one tempering step by adding a nitriding process gas to the nitrogen atmosphere surrounding the part; (C) further heating the part to a material-specific tempering temperature (tA) and holding that temperature for at least one hour; (D) cooling the part to a material-specific nitriding temperature (tN) to gas nitride the part; (E) further cooling the part to the ambient temperature (tU).

Description

[Technical Field]

[0001] The present invention relates to a method for the heat treatment of chromium steels, in particular high alloy chromium x steels, by gas nitriding, and in particular to automotive parts resulting from such a manufacturing method.

[0002] The application field of this invention is primarily in automotive technology and toolmaking. Automotive parts made of high-alloy chromium steels, i.e., steels with chromium as the main alloying element, that are subject to wear and vibration loads are usually tempered, activated, and nitrided after the required martensite hardening to achieve the desired component properties. Typically, such parts are made of specially highly alloyed chromium steels, such as X40CrMov5-1, X50CrMov5-1, and X90CrW18, and are used in the automotive technology field, which is the primary focus here, for example, as components for valves, throttles, or piston-cylinder systems in high-pressure applications. [Background technology]

[0003] According to Non-Patent Document 1, traditional gas nitriding of low-alloy and high-alloy steels of the type of interest here is carried out in several steps: hardening is first carried out, then a series of tempering steps, usually several times at different temperatures and times, followed by activation of the component surface as a separate process before the actual nitriding. Hours, days or weeks may pass between the hardening and tempering thermal refining process and the subsequent activation and (gas) nitriding process.

[0004] Low-alloy and high-alloy steels always have a natural oxide film of a few nanometers thick that forms stably even at room temperature in normal air and / or with different water vapor contents. This oxide film consists of the oxygen-compatible alloying elements chromium, molybdenum, vanadium, silicon, aluminum, and iron, as well as other oxidizable steel alloying elements.

[0005] These alloying elements are no longer dissolved in the crystal lattice, but rather form thin oxide films that impede or completely prevent the diffusion of atomic nitrogen at the subsequent nitriding temperatures in the range of 400 to 600°C. The result is an inhomogeneous compound layer and diffusion layer with various undesirable mechanical, electrical, magnetic, and chemical functional properties. These thin oxide films are usually removed chemically by pickling with an acid, electrically by applying a voltage that breaks down the oxide film, or mechanically by surface treatment, such as brushing, grinding, or honing, before the actual nitriding process. This intermediate treatment between tempering and subsequent nitriding requires correspondingly high production costs and is time-consuming. Furthermore, there is no guarantee that all surface areas have been properly processed.

[0006] In the overall process of heat treatment, the core hardness of the component is adjusted via the tempering temperature in at least one tempering step according to the functional requirements. Within the framework of metallurgical tempering steps, the component properties are usually influenced and Stress in the workpiece To release the heat, the part is heated appropriately. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Technical report "Heat Treatment of Steels - Nitriding and Carbonitriding" (Technisches Merkblatt,,Warmebehandlung von Stahl - Nitrieren und Nitrocarburieren), Publisher: Stahl-Informations-Zentrum, Düsseldorf, 2005, ISSN 0175-2006 Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide a method for the heat treatment of hardened chromium steels by means of tempering and gas nitriding, which allows the core hardness of the part to be variably adjusted, without the usual tempering and activation steps. [Means for solving the problem]

[0009] This problem is solved by a heat treatment method according to claim 1. The following dependent claims present preferred developments of the invention. Parallel independent claim 10 defines the product-by-process of claim 1 as a special automotive part.

[0010] The present invention encompasses the process technology teaching that the heat treatment of chromium steels, particularly high alloy chromium x steels, by gas nitriding comprises the following steps: (A) A pre-hardened part made of chromium steel is heated to ambient temperature T in a nitrogen atmosphere to carry out a pre-oxidation step. U to the material-specific pre-oxidation temperature T V heating until (B) performing at least one tempering step by adding a nitriding process gas to a nitrogen atmosphere surrounding said component; (C) The part is tempered at a tempering temperature T A and holding that temperature preferably for at least 1 hour; (D) To gas-nitride the part, the part is heated to a material-specific nitriding temperature T N cooling to 100°C; and (E) The part is heated to the ambient temperature T U Further cooling process. [Effects of the Invention]

[0011] The advantage of the solution according to the invention is that, in particular, the core hardness of functionally important materials can be freely adjusted independently of the nitriding temperature. This allows the method according to the invention to be used in a wider range of applications. The solution according to the invention makes it possible to adjust the core hardness independently of the required nitriding temperature, since nitriding between 480 and 600 °C only results in core hardnesses of 620 HV1 to 460 HV1, depending on the nitriding temperature. It is not possible to freely select a core hardness between, for example, 200 and 440 HV1, since nitriding above the material-specific limit would result in a functionally impaired structure.

[0012] Preferably, the pre-oxidation temperature in step (A) is selected from the range of 300 to 450°C, preferably from the range of 350 to 450°C. The width of this range results from the various material properties of the pre-hardened steel component, which in turn depends on its alloying elements. Experiments have shown that the above preferred pre-oxidation temperature range, which has a higher lower limit, provides higher energy efficiency by reducing the energy input for further heating following the tempering step. In addition, the pre-oxidation temperature is used depending on the diffuse permeable oxide to be formed. During heating, a nitrogen atmosphere prevents surface oxidation.

[0013] Preferably, the tempering step (B) is initiated immediately after the heating, in a furnace with as uniform a temperature distribution as possible, once the pre-oxidation temperature has been reached, but at the latest when the part temperature is between 350 and 450 °C. The latter control parameter can be used to simplify the temperature change control. Pre-oxidation can be carried out using synthetic air, atmospheric air, or an oxygen-containing gas. The tempering step is preferably initiated by supplying a nitriding process gas after evacuation. Suitable nitriding process gases are oxygen-compatible process gases, such as ammonia gas or ammonia-decomposition gas mixtures. Decomposition gas is understood to be a gas mixture consisting of nitrogen and hydrogen gases. During this tempering step, it is not important whether the temperature is kept constant or varied. However, once the critical process temperature of 450 to 550 °C has been reached, the nitriding process gas can be removed, for example, by a vacuum treatment step as an intermediate step B1 or by purging with nitrogen.

[0014] This tempering step is followed by tempering at a material-specific tempering temperature T, preferably under nitrogen. A The part is then further heated to a temperature of 1000 K or under ammonia or decomposition gas, depending on the tempering temperature, for a holding time of at least 1 hour, preferably 2 to 4 hours, and particularly preferably 3±0.5 hours. The holding time also depends on the type of chromium steel being heat-treated. At this time, process gas or pure nitrogen is also supplied.

[0015] The heating of the part may preferably be carried out in a conventional chamber furnace with a corresponding gas exchange device. After the tempering step in step C, the part is heated to a nitriding temperature T N The part is cooled to a temperature above 1000 K. After the material-specific cooling temperature has been passed, the nitrogen may optionally be replaced with a nitriding process gas. After the nitriding temperature has been reached, the part is gas-nitrided in a conventional manner.

[0016] The heat treatment method described above makes it possible to produce automotive parts, in particular made of chromium steel, preferably high-alloy chromium x steel, which are particularly subject to wear and vibration loads, such as nozzle bodies, valve pieces, valve plates, valve bodies, throttle plates, valve supports or pistons, which are primarily used in the high-pressure range of automotive fuel supplies.

[0017] Further improvements of the invention are shown in more detail in the following description of preferred embodiments of the invention and in the accompanying drawings. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a process diagram of a heat treatment method for chromium steel according to the present invention. [Figure 2] 2 is a graph showing the change in temperature over time in the heat treatment shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0019] According to FIG. 1, the heat treatment according to the invention of a hardened part made of chromium steel is carried out in step A by first heating the hardened part at ambient temperature T U to a material-specific pre-oxidation temperature T of approximately 350°C V This heating and subsequent steps are carried out in a conventional chamber furnace equipped with a gas exchange device.

[0020] After heating and pre-oxidation, in step B, a tempering cycle is initiated to temper the heated part by adding a nitriding process gas, which in this example is ammonia. After reaching the critical process temperature, an intermediate step B1 removes the ammonia again. Further heating in step B1 is performed to a material-specific tempering temperature T above 550°C. A In this example, the tempering temperature is maintained for approximately 3 hours.

[0021] After tempering in step C, nitriding occurs in step D by cooling the part to the material-specific nitriding temperature, and gas nitriding begins. U Further cooling of the part occurs until

[0022] FIG. 2 illustrates the temperature variation over time of the process flow. U Any heating time starting from t A In the heating step A, the part is heated in a nitrogen atmosphere. Then, in the step B after the pre-oxidation, the tempering process is started by adding ammonia as a process gas. The pre-oxidation temperature T of about 400°C resulting from the heating is V The critical process temperature T of about 500°C remains K After the tempering temperature T A Further heating is performed to a temperature of t K The temperature is maintained for about 3 hours until the tempering is completed. Subsequently, in step D, the nitriding temperature is increased to T N After the gas nitriding is completed, cooling is again carried out in the presence of ammonia gas and decomposition gas until the temperature reaches the ambient temperature T U Further cooling of the part occurs until

[0023] The invention is not limited to the above-described advantageous embodiment, but rather variations thereof are possible, which are within the scope of the following claims. Thus, for example, it is possible to omit the intermediate step B1. The specific temperatures, in particular the pre-oxidation temperature T V , tempering temperature T A and nitriding temperature T N is adjusted to the specific composition of the chromium steel to be heat treated, in particular its alloying elements.

Claims

1. A method for heat treatment of chromium steels, in particular high alloy chromium x steels, by gas nitriding, comprising the steps of: (A) Pre-hardened parts made of chromium steel are heated to ambient temperature (T) in a nitrogen atmosphere to carry out a pre-oxidation step. Ｕ ) to the pre-oxidation temperature (T Ｖ ) heating the mixture to a temperature of 100°C; (B) performing at least one tempering step by adding a nitriding process gas to a nitrogen atmosphere surrounding the component; (C) The part is tempered at a tempering temperature (T Ａ ) and holding at that temperature for a holding period; (D) To gas-nitride the part, the part is heated to a material-specific nitriding temperature (T Ｎ ) cooling to 50°C; (E) subjecting the part to the ambient temperature (T Ｕ ) and further cooling.

2. The preliminary oxidation temperature (T Ｖ 2. The method according to claim 1, characterized in that the temperature is adjusted in the range of 300 to 450°C, preferably in the range of 350 to 450°C.

3. 2. The method of claim 1, wherein the tempering step of step (B), which begins immediately following the heating, begins after the pre-oxidation temperature is reached, but at the latest when the part temperature is between 350°C and 450°C.

4. 2. The method of claim 1, wherein the nitriding process gas is selected from the group of oxygen-compatible process gases including ammonia gas, ammonia gas-decomposition gas mixtures.

5. 2. The method of claim 1, wherein in step (B) a critical process temperature of 450°C to 550°C is reached, and then the nitriding process gas is removed or replaced with pure nitrogen in an intermediate step (B1).

6. 2. The method of claim 1, further comprising heating the part in step (C) to a tempering temperature of from 550°C to 700°C.

7. 2. The method according to claim 1, characterized in that the holding time at the tempering temperature in step (C) is at least 1 hour, preferably 2 to 4 hours, particularly preferably 3±0.5 hours.

8. 2. The method of claim 1, wherein the component comprises at least 2% mass fraction of chromium as an alloy metal.

9. 2. The method of claim 1, wherein the heating of the part is carried out in a chamber furnace having a gas exchange device.

10. 10. An automotive part made of chromium steel, in particular high alloy chromium x steel, which has been tempered by the method according to any one of claims 1 to 9.

11. 11. The part according to claim 10, characterized in that the part is selected from the group of automotive parts subjected to wear and vibration loads, including nozzle bodies, valve pieces, valve plates, valve bodies, throttle plates, valve supports, and pistons.

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