AT520315B1 - Process for producing a sintered component - Google Patents

Abstract

The invention relates to a method for producing a sintered component (2), comprising the steps of: providing an iron-based powder with chromium; Filling the powder in a powder press and pressing into a green compact; Sintering the green compact to the sintered component (2); Re-compaction of the sintered component (2); Hardening of the sintered component (2). The sintering is performed in a decarburizing atmosphere, and the surface densification sintered member (2) is moved along an axis (3) from a first die opening (6) toward a second die opening opposite the first die opening (6) along the axis (3) (13) of a die tool (1) is moved, wherein the sintered component (2) during this movement a plurality of die sections (7-11) of the die tool (1) and thereby a surface region of the sintered component (2) is compressed, including in the pressing direction an inner diameter (17) of the successive die sections (7-11) is smaller and the individual die sections (7-11) are arranged such that a subsequent die section (7-11) of the plurality of die sections (7-11) each directly to the corresponding, in Pressing direction preceding die section (7-11) connects.

Description

Description: The invention relates to a method for producing a sintered component, comprising the steps: providing an iron-based powder with chromium as an alloy element; Filling the powder into a powder press; Pressing the powder into a green compact; Removing the green body from the powder press; Sintering the green compact to the sintered component; Post-compression of the sintered component; Hardening the sintered component.

Sintered components, that is, components that are produced by a powder metallurgical process, have the disadvantage in comparison to components made of cast materials that the strength after sintering is not sufficient for many applications due to the porosity of the sintered component. Various methods for post-compression or surface compression of sintered components after sintering have therefore already been proposed in the prior art.

A common method is the rolling of rotationally symmetrical components, such as gears. As a representative, reference is made to WO 1992/005897 A1.

[0004] A variant of the method is compacting in a die tool, which is described in EP 2 066 468 A2. Methods similar to this are known from JP 10 085 995 A, AT 517 989 A1 and RU 2 156 179 C2.

Although these methods of post-compression give good results per se, the result of the post-compression of hard materials, e.g. chromium-containing iron powders, e.g. problems are described in DE 10 2005 027 055 A1, which deals with the rolling of sintered gears. To this end, the method according to DE 10 2005 027 055 A1 comprises the steps of: filling a sintered material into a press which has an internal geometry to form a preform, an oversize being formed at least in the region of a flank of the toothing; Pressing the sintered material so that a preform is formed; Pre-sintering the pressed preform; Surface rolling of at least a region of the flank of the toothing; Sintering the component, sinter hardening the component and finishing.

The object of the invention is to provide a method with which hard sintered materials can be produced with a relatively high surface density.

The object is achieved in the aforementioned method in that the sintering is carried out in a decarburizing atmosphere, and that the sintered component for surface compression along an axis from a first die opening in the direction of a second, the first die opening along the axis opposite Die opening of a die tool is moved, wherein the sintered component passes through several die sections of the die tool during this movement and thereby compresses a surface area of the sintered component, for which purpose an inner diameter of the successive die sections becomes smaller in the pressing direction and the individual die sections are arranged such that a subsequent die section of the plurality Die sections each directly adjoins the corresponding die section preceding in the pressing direction.

The advantage here is that the hardness of the component is reduced by the decarburizing sintering, so that the subsequent compression of the component can be carried out more easily and efficiently. After the sintered component is “clamped” on all sides during the recompaction, very high surface densities can also be achieved for iron materials containing chromium, since the material cannot evade, as is the case when rolling the gearwheels according to DE-A1, in which the Pressure is only applied radially to the teeth of the gear. In addition, the method is not limited to rotationally symmetrical sintered components.

For the better formability of the sintered components has proven to be advantageous in the course of evaluating the method if at least one gas from a group consisting of oxygen, carbon dioxide, water vapor is contained in the decarburizing atmosphere, the proportion of the gas between 0, 1 vol .-% and 10 vol .-% in the atmosphere.

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Patent Office [0010] According to another embodiment variant of the method, the C content in the sintering green body is preferably reduced by a maximum of 0.6% by weight. Surprisingly, it was found that this minimal reduction in the carbon content in the sintered component is sufficient for the improved formability when the surface compaction is carried out in the die tool.

According to a further embodiment of the method it can be provided that after the surface compaction in a to the last die section with a decreasing internal diameter, a relaxation of the sintered component in a relief section immediately adjacent to the last die section, which is a compared to that formed immediately before last die section, the die section with a decreasing inside diameter has a larger inside diameter, the sintered component being calibrated in the relief section, for which purpose the inside contour of this relief section corresponds to the target contour with the target dimension of the sintered component. The advantage here is that prior to this calibration or intermediate calibration, the sintered component is not reshaped from the relieved state, as a result of which the burr formation on the sintered component caused by the kneading effect during surface compaction can be reduced. In addition, the die tool is also subjected to less mechanical stress, since further compression of the sintered component from the relieved state requires higher forming forces after it has already been superficially compacted in the previous compression steps.

Preferably, a powder is used which has a chromium content between 0.1 wt .-% and 10 wt .-%. The method can be used to produce sintered components with correspondingly good strength properties, which means that the area of use of sintered components can be enlarged.

As stated above, the hardening of the sintered component takes place after the surface compression. The hardening is preferably carried out by carburizing and subsequent quenching or sinter hardening and subsequent quenching or induction hardening.

It can be provided that the carburizing is carried out by means of low-pressure carburizing. The advantage can thus be achieved that even with sintered components that are very narrow in the axial direction, hardness profiles can be set in a very targeted manner in comparison to other carburizing processes, such as carbonitriding. It is also possible with these sintered components to obtain a softer core.

The quenching can be carried out with gas according to a further embodiment of the method. By avoiding liquids for quenching, the storage of these liquids in the sintered component can be avoided, whereby a very clean sintered component is already available after the process.

For a better understanding of the invention, this will be explained in more detail with reference to the following figures.

[0017] It shows in a simplified, schematic representation:

[0018] FIG. 1 shows a section through a section of a die tool for surface compaction.

As an introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numerals or the same component names, and the disclosures contained in the entire description can be applied analogously to the same parts with the same reference numerals or the same component names. The location information selected in the description, e.g. above, below, to the side, etc., referring to the figure described and illustrated immediately, and if the position is changed, these are to be applied accordingly to the new position.

The production of metallic sintered components, such as gears, takes place according to a powder metallurgical process (sintering process). Such procedures are out

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Patent office is already well known in the prior art, so that a detailed discussion of the basic features of this method is unnecessary. To this end, it should only be carried out so much that the method essentially comprises the steps of providing a powder, filling the powder into a powder press, pressing the powder into a green compact, removing the green compact from the powder press, sintering the green compact to the sintered component in one or more stages, Recompaction of the sintered component and hardening of the sintered component includes. Therefore, only the essential steps of the method according to the invention are discussed in more detail below. For the remaining process steps, reference is made to the relevant prior art.

The sintered component is made of an iron-based powder with chromium as an alloy element.

According to a preferred embodiment variant, the sintered component or the powder has a proportion of chromium which is selected from a range of 0.1% by weight and 10% by weight.

The iron-based powder can, for example, have the following composition: Fe + 3% Cr + 0.5% Mo + 0.5% C or also Fe + 1.8% Cr + 2% Ni + 0.5% C.

In general, in addition to chromium, the iron-based powder can contain the following constituents in the stated proportions, the proportions of the iron-based powder adding up to 100% by weight:

Fe: 90% by weight to 99.9% by weight C: 0% by weight to 1% by weight Mo: 0% by weight to 2% by weight Ni: 0 wt.% To 5 wt.% Cu: 0 wt.% To 5 wt.% For the powder used, the pure elements or master alloys, optionally with master alloys, be used.

The powder is poured into the die of a powder press and pressed into the so-called green compact, preferably coaxially. The pressing pressure can be, for example, between 600 MPa and 1200 MPa.

After removal of the green compact from the powder press, it is sintered to form the sintered component. The sintering can take place in one stage, for example at a temperature between 900 ° C. and 1350 ° C., or in two stages, the temperature in the first stage between 800 ° C. and 1200 ° C. and in the second stage between 1100 ° C. and 1350 ° C. can be.

The sintering (before post-compression) is carried out in a decarburizing atmosphere. For this purpose, the sintering atmosphere can contain at least one gas from a group consisting of oxygen, carbon dioxide, water vapor and mixtures thereof. The proportion of the at least one gas in the decarburizing atmosphere can be between 0.1 vol.% And 10 vol.%. In the case of a mixture, the total proportion of the decarburizing gases can likewise be between 0.1% by volume and 10% by volume. The rest forms nitrogen and / or hydrogen.

According to an embodiment variant of the method, the proportion of the gas in the atmosphere is preferably between 0.1% by volume and 2% by volume.

In general, the carbon content in the sintering green compact can be reduced by 0.01% by weight to 0.8% by weight during the sintering in the decarburizing atmosphere. According to a preferred embodiment of the method, the carbon content is only reduced by a maximum of 0.6% by weight.

Furthermore, it can be provided according to another embodiment variant that the carbon portion only in a surface layer with a layer thickness between 10 microns and

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500 μm is reduced. This is achieved through targeted gas flow in the sintering furnace.

Subsequent to the sintering, the sintered component is post-compressed, at least the surface and the adjoining region being compressed. The effect of surface compaction is greatest directly at the contact surface with the compaction tool and decreases towards the inside of the sintered component. With the help of the process, surface layers of chrome-containing sintered components with a thickness of a few hundredths of a millimeter up to several tenths of a millimeter and above can be compressed.

A die tool 1 is used for the surface compaction, as shown in FIG. 1 in longitudinal section using a preferred exemplary embodiment.

A sintered component 2, which is produced in accordance with the aforementioned method steps, is moved along an axis 3 by the die tool 1 for surface compaction.

The die tool 1 comprises a main tool body 4, which has a first (upper) die opening 6 on a tool surface 5, from which a plurality of die sections 7 to 11 lead along the axis 3 into the interior of the main tool body 4. The first die section 7 adjoins the first die opening 6, whereas the last die section 11 is closer to a second die surface 12 opposite the first die surface 5 along the axis and a second die opening 13 formed therein.

The sintered component 2 is disc-shaped in the illustrated embodiment for reasons of clarity and has on a radial outer surface 14, i. the end face, a diameter 15, which corresponds to a raw diameter before the surface compression and after the surface compression corresponds to a smaller final diameter. The shape of the sintered component 2 shown is not to be understood as limiting.

The surface compaction of the sintered component 2 takes place by inserting it through the first die opening 6 into the first die section 7 and subsequently moving it into all further die sections 8 to 11, the outer surface 14 of the sintered component 2 at least in each die section 7 to 11 is pressed on sections of the outer surface 14 against wall surfaces 16 of the die sections 7 to 11.

The pressing effect is achieved in that an inner diameter 17 of the die sections 7 to 11, which is defined by the clear width between opposing or interacting sections of the pressing surface of a die section 7 to 11, is respectively smaller than the diameter 15 of the sintered component 2 before it is inserted into the respective die section 7 to 11. In general, the die sections 7 to 11 preferably have an inner contour that corresponds to the outer contour of the sintered component 2, but each die section 7 to 11 has a circumference or a cross-sectional area that is smaller than the circumference or the cross-sectional area of the sintered component 2 before it is inserted into the respective die section 7 to 11.

The die sections 7 to 11 following one another along the axis 3 go immediately (continuously), i.e. without intermediate sections, one into the other and have (from the first die section 7 to the last die section 11) (monotonously) decreasing inner diameters 17 or cross-sectional areas, i.e. that successive die sections 7 to 11 become smaller, but not larger. The movement of the sintered component 2 in the die tool 1 preferably takes place in a straight line in the pressing direction from the first die opening 6 to the last die section 11, after which the sintered component 2 is removed from the die tool 1 preferably after the direction of movement has been reversed through the first die opening 6 counter to the pressing direction.

The rectilinear movement in the direction of the axis 3 can also be overlaid by a rotary movement, as a result of which the sintered component 2 executes a screwing movement in the die tool 1.

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Patent Office The relative movement between the sintered component 2 and the die tool 1 required for carrying out the method can be carried out by moving the sintered component 2 and / or by moving the die tool 1, the sintered component 2 and the die tool 1 in each case using a suitable drive or fixed frame are connected. During the surface compaction and the subsequent calibration, the sintered component 2 is clamped between an upper punch 18 and a lower punch 19. For the downward movement, the upper punch 18 presses on the sintered component 2 from above, the lower punch 19 can be pulled down or it is also pressed down by the upper punch 18. For the preferred ejection of the sintered component 2 via the first die opening 6, the lower punch 19 is pressed upward and the upper punch 18 can optionally be pulled upward. Corresponding drives, not shown, can be provided for these movements of the upper punch 18 and the lower punch 19.

The transition from a die section 7 to 10 to the adjoining die section 8 to 11 can be designed as a chamfer 20, or can be provided with a rounding, wherein a convex rounding can follow a convex rounding in the pressing direction. This allows a smooth transition of the sintered component 2 from a die section 7 to 10 to the subsequent die section 8 to 11 without an unintentional material removal on the sintered component 2 due to a sheep-edged step or without the edges breaking out at the transitions of the die tool 1. As can be seen from FIG. 1, such a chamfer can also be formed on the first die opening 6. The bevels 20 or the respective curves are part of the respective die section 7 to 11, so they do not form any intermediate sections.

1 and 2 five die sections 7 to 11 are shown, the die tool 1 may generally have between three and eight or more than eight such die sections.

The last die section 11 shown in FIG. 1 is that die section of the die tool 1 which has the smallest inside diameter 17 or the smallest clear width. Immediately after this last die section 11 with the smallest inside diameter 17, a relief section 21 can be provided or formed in the die tool 1 according to an embodiment variant of the method or die tool 1. This relief section 21 has a larger inside diameter 22 in comparison to the last die section 11, which is formed directly in front of it, with a decreasing inside diameter 17. As a result, the sintered component 2 can relax in this relief section 21. Simultaneously with this relaxation, the sintered component 2 is also calibrated in the relief section 21. For this purpose, the relief section 21 has an inner contour that corresponds to the target contour with the target dimension of the sintered component 2. The inner contour of the relief section 21 is therefore equal to the outer contour of the finished sintered component 2 both in terms of the geometry and the geometric dimensions (viewed in cross section).

It should be stated at this point that calibrating a sintered component is understood to mean its processing for at least approximately producing the target dimensions of the component in a tool by pressing stress. By “at least approximate” it is meant that deviations from the nominal size are permissible within the usual tolerances.

The term nominal dimension in the sense of the invention is understood to mean a gauge block that the finished sintered component 2 is to have, possibly minus the enlargement of the sintered component 2 after relaxation, which is defined by the springback behavior of the sintered material due to the elastic springback. The proportion of springback behavior can be determined empirically. In other words, the nominal size plus any enlargement that may occur due to the elastic springback gives the final dimension.

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Patent Office [0052] Subsequent to the relief section 21, the die tool 1 preferably has a further section 23. This section 23 has an inside diameter 17 or a clear width which corresponds to the inside diameter 17 or the clear width of the last die section 11 with the smallest inside diameter 17. The section 23 serves to guide the lower punch 19 in the die tool 1.

The inside diameter 22 or the inside width of the relief section 21 corresponds to the outside diameter 15 (FIG. 1) or the inside width of the finished sintered component

2. This inner diameter 22 or this clear width of the relief section 21 is at least 0.02%, in particular between 0.02% and 0.1%, larger than the inner diameter 17 or the clear width of the last die section 11 with the smallest Inner diameter 17. However, the inner diameter 22 or the inside width of the relief section 21 is not greater than the inside diameter or the inside width of the first die opening 6. The purpose of this is to enable the sintered component 2 to at least approximately completely relax.

According to an embodiment variant of the method for surface compaction of the sintered component 2, it can be provided that the penultimate die section 10, viewed in cross section, is identical in cross section to the cross section of the relief section 21 and thus to the calibration cross section, both with regard to the geometry and the geometric dimensions.

After the surface compaction and, if necessary, calibration of the sintered component 2, the latter is hardened. In principle, any suitable hardening method known from the prior art can be used for this.

According to a preferred embodiment of the method, the hardening is carried out by carburizing and subsequent quenching or by sinter hardening and subsequent quenching or by induction hardening.

Carburizing increases the carbon content in the sintered component 2. The carburizing can in principle be carried out by various methods, all methods having in common that a gas or gas mixture is used as the carbon source. For example, methane, propane, acetylene, etc. can be used as the gas. The carburizing can, for example, be carried out after the surface compaction in a further sintering step. Carburizing can also be done by carbonitriding. However, carburizing is preferably carried out by a low-pressure carburizing process.

The carbon content of the sintered component 2 after carburizing is preferably between 0.1% by weight and 1.0% by weight.

In particular, the carburizing can be carried out to a depth of the sintered component 2, measured from its surface, which is selected from a range from 100 μm to 2000 μm; preferably from a range of 100 μm to 1000 μm. The preferred carbon content mentioned above relates to this carburization depth. Areas of the sintered component 2 lying underneath can consequently have a lower carbon content.

After the carburizing, the sintered component is quenched. Quenching can also be done by any suitable method known in the art, such as oil quenching. However, the quenching of the sintered component 2 is preferably carried out with a gas, for example with N ₂ , N ₂ / H ₂ or He. The quenching rate can be selected from a range of 1 ° C / s to 7 ° C / s.

It can also be provided that a sinter-hardenable powder is used to produce the green compact. It is understood to mean an iron or steel powder which has a proportion of at least one alloy element which delays the eutectoid conversion from austenite to ferrite and pearlite. For example, in addition to chromium, the powder may contain nickel and / or molybdenum. The share of this at least one

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Alloying element on the powder for producing the green compact can be between 0.4% by weight and 5% by weight.

After sinter hardening, the sintered component is also quenched.

According to another embodiment variant of the method, it can be provided that the sintered component is carburized again. This carburizing step preferably takes place simultaneously with the second sintering step if the sintering is carried out in two stages, as was described above. A carburizing gas, such as methane or propane, can be added to the sintering atmosphere for the carburizing (recarburizing).

The carburizing can also be carried out with another known carburizing method.

The carbonization of the carbon content of the sintered component can again be increased by 0.1% by weight to 1.0% by weight.

If necessary, mechanical post-processing can take place after hardening.

With the process route described, sintered components 2 can be produced with little distortion.

The exemplary embodiments describe possible design variants, combinations of the individual design variants with one another also being possible.

For the sake of order, it should finally be pointed out that, for a better understanding of the structure, the die tool 1 is not necessarily shown to scale.

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LIST OF REFERENCE NUMBERS

die tool

sintered component

axis

Tool body

tool surface

die opening

die portion

die portion

die portion

die portion

die portion

tool surface

die opening

outer surface

diameter

wall surfaces

Inner diameter

upper punch

lower punch

chamfer

relief section

Inner diameter

section

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Claims (8)

claims 1. A method for producing a sintered component (2) comprising the steps: - Providing an iron base powder with chrome; - Pouring the powder into a powder press, Pressing the powder into a green compact, - removing the green body from the powder press, - Sintering the green compact to the sintered component (2), - post-compression of the sintered component (2), - Hardening the sintered component (2), characterized in that the sintering is carried out in a decarburizing atmosphere, and that the sintered component (2) for surface compaction along an axis (3) from a first die opening (6) in the direction of a second one first die opening (6) along the axis (3) opposite die opening (13) of a die tool (1), the sintered component (2) during this movement passing through a plurality of die sections (7-11) of the die tool (1) and thereby a surface area of the sintered component (2) is compressed, for which purpose an inner diameter (17) of the successive die sections (7-11) becomes smaller in the pressing direction and the individual die sections (7-11) are arranged such that a subsequent die section (7-11) of the plurality Die sections (7-11) each directly adjoin the corresponding die section (7-11) preceding in the pressing direction. 2. The method according to claim 1, characterized in that the decarburizing atmosphere contains at least one gas from a group consisting of oxygen, carbon dioxide, hydrogen, the proportion of the gas between 0.1 vol.% And 10 vol.% in the atmosphere. 3. The method of claim 1 or 2, that the C content in the sintering green body is reduced by a maximum of 0.6 wt .-%. 4. The method according to any one of claims 1 to 3, characterized in that after the surface compression in a to the last die section (11) with a decreasing inner diameter (17) a relaxation of the sintered component (2) in a directly to the last die section (11 ) subsequent relief section (21), which has a larger inside diameter (22) than the last die section (11) formed directly before it, the die section (7-11) with a smaller inside diameter (17), the sintered component (2 ) is calibrated in the relief section (21), for which purpose the inner contour of this relief section (21) corresponds to the target contour with the target dimension of the sintered component (2). 5. The method according to any one of claims 1 to 4, characterized in that the chromium content of the powder is between 0.1 wt .-% and 10 wt .-%. 6. The method according to any one of claims 1 to 5, characterized in that the hardening is carried out by carburizing and subsequent quenching or sinter hardening and subsequent quenching or induction hardening. 7. The method according to claim 6, characterized in that the carburizing is carried out by means of low pressure carburizing. 8. The method according to claim 6 or 7, characterized in that the quenching is carried out with a gas.