A method for heat treating and surface hardening a workpiece made of an alloy steel material and a workpiece made of an alloy steel material

Abstract

This invention relates to a method for heat treatment and surface tempering of a workpiece (1) made of alloy steel, wherein the workpiece is subjected to hardening in a vacuum furnace at a hardening temperature (T). H The workpiece (1) is preferably subjected to initial hardening (I.) under a vacuum heat treatment, followed immediately by gas nitriding (II.) of the edge layer (3) by bringing the workpiece (1) to the nitriding temperature (T) in a nitrogen atmosphere. N ), wherein the tempering (III.) coupled thereto is integrated into the gas nitriding (II.), for which the nitriding temperature (T) is set. N The selection is made so that the hardening stress in the workpiece (1) is reduced at the same time.

Description

Technical Field

[0001] The present invention relates to a method for heat treatment and surface tempering of workpieces made of alloy steel, and workpieces produced by the manufacturing method, wherein the steel material has a chromium content of at least 2% by mass or is made of high alloy steel.

[0002] The application of this invention extends primarily to high-load components made of the aforementioned steel materials, which are further hardened and have a nitrided edge layer to meet particularly high fatigue strength, corrosion, and / or wear requirements, as seen in, for example, automotive components subjected to high stress, particularly for nozzle bodies, valves, valve plates, valve bodies, throttle plates, valve carriers, pistons, guide plates, and the like. For this purpose, chromium steels of particular interest are used, especially X40 and X90 steels, such as X40CrMov5-1 or X90CrW18. X40 steel can be used to manufacture, for example, nozzle bodies or valve components for high-pressure applications in the automotive technology field. In contrast, X90 steel forms more chromium carbides and tungsten carbides and is primarily suitable for valve seat components or piston components subjected to even higher stresses. Background Technology

[0003] A method for heat treatment and surface tempering of workpieces made of high-alloy steel for the aforementioned applications is disclosed in DE 10 2016 221 891 A1. The heat treatment disclosed herein follows immediately after initial soft machining of the workpiece from a blank, which can be performed by initial forming, shaping, and / or cutting. After subsequent hardening of the workpiece, tempering is performed to reduce stress and achieve the desired hardness; this tempering can be performed, for example, in a vacuum-sealed, oxygen-deficient tempering furnace. To obtain a workpiece surface that is as wear-resistant as possible, gas nitriding is preferably performed at 480°C to 600°C after surface activation for surface tempering, during which nitrogen diffuses into the workpiece surface to form a hard edge layer. Finally, the workpiece is hard machined to obtain a finished component and its associated final dimensions and surface quality, for example by grinding, hard turning, or similar methods.

[0004] Specifically, in the known manufacturing method described above for heat treatment and surface tempering of a workpiece made of high-alloy steel, the workpiece is heated to a first temperature in a vacuum environment, wherein the first temperature is kept constant during a first holding phase, and the workpiece is subsequently further heated to a second temperature higher than the first temperature. The second temperature is kept constant during a second holding phase, and the workpiece is quenched, for example, after the end of the second holding phase, to perform, for example, martensitic hardening. To activate the workpiece surface, in the first processing step, a process gas or process gas mixture that releases hydrogen is circulated around the workpiece surface during the first holding phase. In the immediately following second processing step, which also occurs during the first holding phase, a process gas or process gas mixture that releases nitrogen is additionally circulated around the workpiece to form a nitride-containing layer or nitride crystals, thereby optimizing the subsequent gas nitriding process.

[0005] The aforementioned activation step, due to the cleaned and activated surface, facilitates the decomposition of ammonia and the diffusion of atomic nitrogen into the workpiece surface during the second processing step.

[0006] Therefore, nitriding of high-alloy steel, preferably gas nitriding, is carried out in several separate steps. Here, hardening is performed first, followed by tempering, and then activation of the workpiece surface is performed as an intermediate step before gas nitriding. Typically, several hours to weeks pass between the tempering (i.e., hardening and tempering) and the activation and gas nitriding. During this time, a thin, naturally occurring oxide layer forms on the workpiece surface, typically a few nanometers thick, and is already stably constructed in normal air or even at room temperature. This thin oxide layer consists of alloying elements with oxygen affinity, such as Cr, Mo, V, Si, Al, and Fe, or other oxidizable alloying elements of the steel. Therefore, these alloying elements are no longer dissolved in the steel's crystal lattice and, as a thin oxide layer, affect the diffusion of atomic nitrogen at temperatures between 400°C and 600°C. This thin oxide layer, formed due to storage or tempering, often or even completely inhibits the diffusion of atomic nitrogen. The consequence is the formation of an inhomogeneous compound layer and a diffusion layer with undesirable electrical, magnetic, and chemical properties.

[0007] For these reasons, according to known prior art, such interfering thin oxide layers are removed before gas nitriding, and specifically preferably chemically by acid pickling, electrically by applying voltage to decompose the oxide layer, by adding a weak acid or compound directly in a chamber furnace at elevated temperatures, mechanically by surface finishing (such as brushing, grinding, honing), or by a so-called sputtering process with or without temperature support.

[0008] The objective of this invention is to further improve a method for surface tempering of a material made of alloy steel by gas nitriding to produce a wear-resistant edge layer, wherein the improvement is that the wear-resistant edge layer is reliably and reproducibly produced in a simplified manner with minimal manufacturing time. Summary of the Invention

[0009] This task is accomplished by the manufacturing method according to claim 1. For the specific workpiece thus created, see claim 11. The respective dependent claims provide advantageous extensions of the invention.

[0010] This invention includes a methodological teaching that, in order to heat treat and surface temper a workpiece made of alloy steel, the workpiece is vacuum heat-treated by preferably initial hardening in a vacuum furnace at a hardening temperature depending on the material, followed immediately by, i.e., without an intermediate step of surface activation, gas nitriding of the edge layer of the surface, in such a way that the workpiece is brought to a nitriding temperature depending on the material in a nitrogen atmosphere, wherein a coupled tempering is integrated into the gas nitriding, in such a way that the nitriding temperature is adjusted to a range within which the hardening stress in the workpiece can be reduced and the microstructure tempered throughout the nitriding duration.

[0011] Tempering, which is integrated into and coupled with the typically long gas nitriding period, can be tuned by temperature variations within the range of gas nitriding temperatures, as will be explained in more detail below.

[0012] In other words, in the solution according to the invention, compared to the prior art, the intermediate tempering step is not performed after hardening and before gas nitriding. This means that gas nitriding is performed directly after quenching to complete hardening. Here, the invention utilizes the advantage of the highly reactive component surface produced by the vacuum process of hardening, making it virtually possible to omit the separate oxidation performed by tempering. Tempering can be integrated into gas nitriding because the former acts on the component core, which is only affected by temperature, while gas nitriding only affects the component surface to produce a wear-resistant and corrosion-resistant surface. Therefore, these two heat treatment steps can be performed simultaneously in a functionally integrated sense.

[0013] A particular advantage of the solution according to the invention is that it eliminates the need for any type of component surface activation after tempering and before gas nitriding. This avoids the formation of conventional oxide layers. The solution according to the invention utilizes the fact that the technically clean workpiece surface, formed during hardening and typically remaining for a sufficiently short period even after workpiece quenching, is available for subsequent gas nitriding.

[0014] Therefore, actual gas nitriding for surface tempering is performed immediately after, or directly after, a short, defined dwell time before significant oxide layer formation occurs. During gas nitriding, the workpiece is also tempered at the nitriding temperature to reduce internal component stresses generated during hardening. Thus, the pre-treatment of the workpiece, which is usually performed before gas nitriding, can be completely omitted and integrated into a subsequent gas nitriding process, preferably performed within a nitriding temperature range of 400°C to 600°C. Experiments have shown that both tempering and gas nitriding can be performed within this temperature range. Below this temperature range, although a slight stress reduction still occurs in the component, the chemical reaction for gas nitriding is no longer effectively initiated, while above this temperature range, an undesirable braunit microstructure is produced, leading to unacceptably degraded fatigue strength.

[0015] The technically clean workpiece surface after hardening allows surface chemical reactions (e.g., the catalytic decomposition of ammonia into atomic nitrogen and hydrogen, and the decomposition of ammonia into nitrogen and hydrogen in the furnace atmosphere) to proceed more uniformly, undisturbed, and with reduced complexity, thereby avoiding impurities. Furthermore, in the gas nitriding according to the invention, supplying atomic nitrogen to the technically clean workpiece surface is simpler, which further increases the speed of the nitriding process. Interfering localized chemical reactions that require the reduction, transformation, or decomposition of oxides still present on the surface do not occur.

[0016] Alternatively, additional pre-oxidation can be performed at a temperature of 350°C to 450°C prior to gas nitriding to improve nitrogen absorption during actual gas nitriding if needed. This is because it creates an expanded surface area, which provides a higher absorption capacity for nitrogen. Pre-oxidation is preferably performed over a period of 5 to 60 minutes. Experiments have shown that oxidation within this temperature range provides sufficiently rapid results during gas nitriding without affecting the latter. Conversely, oxidation above this temperature range will produce iron oxides that will have a passivating effect. On the other hand, oxidation below this temperature range will take too long to produce iron oxides that allow nitrogen diffusion for gas nitriding.

[0017] If the workpiece requires specific functional characteristics in the core and edge regions (such as a certain core toughness and hardened layer, i.e., the edge thickness of the layer constructed during nitriding), these can be achieved through a tempering process integrated according to the invention within the nitriding process, preferably through the duration and / or nitriding temperature T. N These characteristics can be adjusted. Here, this can be done at the start and / or during and / or at the end of gas nitriding by adjusting the temperature relative to the normal nitriding temperature T. N Compared to the increased nitriding temperature T that produces a tempering effect within the desired time period. NTempering is performed to set defined functional workpiece characteristics in the core and edge regions of the workpiece (1) for a shorter tempering duration compared to the entire nitriding duration. This flexibility in achieving these overlaps is possible because, in principle, gas nitriding lasts much longer than tempering. Therefore, for example, a tempering effect can be achieved by maintaining a tempering temperature of 550°C for 90 minutes. N Together they are integrated into the normal nitriding temperature T of 450°C, which is maintained for 180 minutes under other conditions. N Nitrogen permeation of the gas.

[0018] If necessary, additional tempering can be performed after gas nitriding is completed.

[0019] Alternatively, multiple tempering processes can be performed at different tempering temperatures for different durations to achieve the desired hardness in combination with a reduction in retained austenite. Thus, for example, X40 steel can be treated with a first tempering at 500°C for 90 minutes to reduce retained austenite, followed by a second tempering at 600°C for 90 minutes to adjust the hardness.

[0020] Since the intermediate step of activating the workpiece surface is completely eliminated according to the present invention, it is unnecessary to compensate for the disadvantages associated with this step. These disadvantages include localized surface changes (such as pitting), various rinsing, cleaning, and drying steps for removing residues after acid activation, and similar issues. The generation of acid-activated residues with the same passivating effect due to incomplete cleaning, or cleaning residues with a passivating effect, are also eliminated. This is particularly significant in small, deep grooves in the workpiece that cannot be thoroughly cleaned by liquid due to capillary action.

[0021] In contrast, in the solution according to the invention, cleaning of the workpiece surface in preparation for gas nitriding during hardening, particularly during heating, during isothermalization, and during austenitization, is preferably carried out in multiple vacuuming steps at pressures less than 1 mbar and in longer vacuum phases experienced at different temperatures and for different times. The parameters are determined based on the experience of those skilled in the art, within ranges determined according to typical material values.

[0022] According to a further improvement to the invention, it is proposed that, in order to support optional cleaning of the workpiece surface, hydrogen, nitrogen, acetylene, carbon dioxide, or similar reducing gases be added, which may flow into the vacuum furnace at different temperatures and times during the aforementioned hardening process.

[0023] According to another improvement of the invention, the targeted inflow of nitrogen, for example, during hardening leads to nitriding of the workpiece surface to form various nitrides of alloying elements, particularly Cr, Fe, Mo, Mn, and the like, on and within the workpiece edge regions. Advantageously, these forming nitrides act as nitride nuclei during subsequent gas nitriding, actively supporting further nitrogen absorption or enabling more uniform, rapid, and simpler catalytic decomposition of ammonia into hydrogen and nitrogen. This nucleation during hardening also stabilizes the technically clean workpiece surface at room temperature by combining the alloying elements with nitrogen. The nucleated workpiece surface further directly minimizes air ingress, thereby minimizing oxide formation and repassivation of the workpiece surface. If hydrogen is optionally added, it acts as a reducing agent for existing oxides.

[0024] According to another improvement to the invention, the hardening process ending with quenching is performed by gas quenching, preferably high-pressure gas quenching, until the workpiece reaches approximately room temperature. During high-pressure gas quenching, cooling gas is flowed into the vacuum furnace previously used for hardening, and this cooling gas flows around the workpiece, thereby causing a desired rapid temperature reduction.

[0025] Experimental studies have shown that after hardening in a vacuum furnace, the workpiece surface contains only a very low concentration of oxygen. This is further facilitated by the fact that heating during hardening is preferably performed under a nitrogen atmosphere at a pressure range of 100 to 3000 mbar and / or in a vacuum at a pressure ≤ 1 mbar. Further resistance to oxide formation can be achieved by additionally introducing a gas with oxygen affinity at different temperatures, preferably in the range of 600°C to 1300°C, for different durations. The solution according to the invention enables the targeted preparation of oxide-poor or even oxide-free workpiece surfaces so that gas nitriding can be performed directly and immediately after activation of the workpiece surface. Tempering of the hardened workpiece is effectively compensated for during gas nitriding. Conversely, if tempering is performed in a conventional manner before gas nitriding, the desired oxide-poor or even oxide-free workpiece surface cannot be achieved. It is thus understood that it is advantageous to remove oxidizing tempering from the heat treatment and surface tempering process chain, as activation can only be completely eliminated in this way. Attached Figure Description

[0026] Further improvements to the invention are shown in more detail below, together with a description of preferred embodiments of the invention with reference to the accompanying drawings. The drawings show: Figure 1 A flowchart of a method for surface tempering of a workpiece; Figure 2 A schematic diagram of a workpiece surface-tempered using this method, and Figure 3 A graphical representation of the method flow along the timeline. Detailed Implementation

[0027] according to Figure 1 By having a hardening temperature T ranging from 800°C to 1300°C in different stages at pressures below 1 mbar. H Initial hardening (I) is then performed to achieve surface tempering of the workpiece made of alloy steel (here, X40CrMov5-1). For example, for X40 steel, the hardening temperature T... H It is 1020℃; for X90 steel, for example, the hardening temperature T H The temperature is 1050°C. Hardening I is performed under vacuum in a conventional vacuum furnace. At least temporarily, Hardening I is performed under a nitrogen atmosphere to form surface nitride nuclei. Thorough cleaning of the workpiece surface is achieved by adding, at least in stages, a reducing gas, which may consist of nitrogen and a reducing gas consisting of hydrogen, acetylene, carbon, or the like. In this embodiment, Hardening I is quenched under high pressure gas to room temperature T. R And so it ends.

[0028] Immediately following, gas nitriding II is introduced, which is done by bringing the workpiece to a nitriding temperature T in a vacuum furnace under a nitrogen atmosphere, within the range of 400°C to 600°C. n At least one tempering (III) is performed during gas nitriding II, and to some extent, it is integrated into gas nitriding II. This is achieved by setting the nitriding temperature T... n The nitriding temperature T is chosen to simultaneously reduce hardening stress in the workpiece, thus producing a sufficient tempering effect. For X40 steel, the nitriding temperature T... n In principle, the nitriding temperature Tn is 400-600℃ for these steels, with the best results for high edge hardness typically achieved at 550-590℃, and ideally at 570℃. For X90 steel, the nitriding temperature Tn is also 400-600℃, with the best results for high edge hardness typically achieved at 500-540℃, and ideally at 520℃.

[0029] In this embodiment, an additional subsequent tempering IV can be performed after gas nitriding II to improve stress reduction in the workpiece.

[0030] according to Figure 2 The manufacturing method produces a hardened and surface-tempered workpiece 1. The workpiece 1, exemplified here, is made of X40 steel, which, in addition to the dominant alloying element chromium, also contains molybdenum as another alloying element in this embodiment, and contains 1.2 to 1.75% by mass according to the alloy composition given above.

[0031] As a result of the aforementioned gas nitriding, in this embodiment, the workpiece 1 has a wear-resistant edge layer 3 on the entire outer surface 2.

[0032] according to Figure 3 In the hardening stage I, the workpiece first undergoes hardening at room temperature T. R The starting point is set at a hardening temperature T that varies from 800°C to 1300°C, depending on the material. H The hardening temperature, appropriately matched to the material and charge weight, is maintained for 20 to 320 minutes within a time period t1. This is followed by quenching to room temperature T. R This is immediately followed by gas nitriding II with integrated tempering III, which is carried out at a nitriding temperature T of 550°C. n The downward-looking functional requirements and the materials used are determined within a time period t2, which lasts from 1 to 120 hours. Finally, in this embodiment, tempering after gas nitriding II is carried out by a subsequent isolated tempering IV within a time period t3 (here, 180 minutes).

[0033] The invention is not limited to the preferred embodiments described above. Instead, modifications are conceivable and are included within the scope of the appended claims. Thus, for example, it may be possible to have only a portion of the workpiece surface tempered according to the invention, while the remaining areas are, for example, shielded. During gas nitriding, pre-oxidation performed at a temperature range of 350°C to 450°C can also be additionally incorporated into the actual nitriding. Furthermore, hardening can be performed in multiple stages, and tempering integrated into gas nitriding can be continued separately and isolated after gas nitriding is completed (i.e., upon reaching the desired edge layer thickness), or omitted entirely.

Claims

1. A method for heat treating and surface tempering a workpiece (1) made of alloy steel, said workpiece being heat-treated and surface-tempered in a vacuum furnace at a hardening temperature (T). H The initial hardening (I.) performed under a vacuum is preferably heat-treated by vacuum, followed immediately by gas nitriding (II.) of the edge layer (3), in which the workpiece (1) is brought to the nitriding temperature (T) in a nitrogen atmosphere. N ),in, The coupled tempering (III.) is integrated into the gas nitriding (II.) for this purpose, and the nitriding temperature (T) is set accordingly. N The selection is made so that the hardening stress in the workpiece (1) is reduced at the same time.

2. The method according to claim 1, characterized in that, The tempering (III.) is performed at the beginning and / or during and / or at the end of the gas nitriding (II.) by adjusting to the normal nitriding temperature (T). N Compared to the increased nitriding temperature (T) that produces a tempering effect within the desired time period. N The process is performed to set defined functional workpiece properties in the core and edge regions of the workpiece (1) for a shorter tempering duration than the entire nitriding duration, the functional workpiece properties including the core toughness and edge thickness of the hardened layer.

3. The method according to claim 1, characterized in that, Integrated nitriding temperature (T) N The temperature range is between 400°C and 600°C, depending on the steel material.

4. The method according to claim 1, characterized in that, An additional subsequent tempering (IV) is performed after the gas nitriding (II).

5. The method according to claim 1, characterized in that, An additional pre-oxidation at a temperature between 350°C and 450°C is performed prior to the gas nitriding (II.) in order to improve nitrogen absorption during the gas nitriding (II.) by means of pre-oxidation.

6. The method according to claim 1, characterized in that, The hardening (I.) is performed by quenching to room temperature (T). R And so it ends.

7. The method according to claim 6, characterized in that, The quenching is performed as gas quenching, preferably high-pressure gas quenching.

8. The method according to claim 1, characterized in that, During the hardening (I.), multiple vacuuming stages and / or multiple vacuum stages with pressures less than 1 mbar are traversed at different temperatures and for different durations.

9. The method according to claim 1, characterized in that, Prior to the gas nitriding (III.), the cleaning of the surface (2) of the workpiece (1) is supported by the phased addition of a reducing gas selected from the group consisting of hydrogen, nitrogen, acetylene and carbon dioxide.

10. The method according to claim 1, characterized in that, The hardening (I.) of the workpiece (1) is performed at least temporarily in a nitrogen atmosphere to form a surface nitride nucleus.

11. The method according to claim 1, characterized in that, Multiple tempering processes (III.) are performed at different tempering temperatures and for different times in order to adjust the desired hardness in combination with the desired reduction of residual austenite.

12. A workpiece (1) made of alloy steel material containing more than 2% by mass of chromium up to high alloy steel, wherein the surface (2) of the workpiece is subjected to gas nitriding at least in part according to any one of the preceding claims in order to produce a wear-resistant edge layer (3).

13. The workpiece (1) according to claim 12, characterized in that, In addition to chromium, the dominant alloying element, the alloy steel contains at least one other alloying element selected from the group consisting of aluminum, calcium, cobalt, copper, magnesium, manganese, molybdenum, nickel, silicon, titanium, zinc, and mixtures thereof, which constitute more than 1% by mass of the alloy.

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