WO2008025344A1

WO2008025344A1 - Process for producing a highly case-hardenable roller bearing component

Info

Publication number: WO2008025344A1
Application number: PCT/DE2007/001533
Authority: WO
Inventors: Christian SCHULTE-NÖLLE; Werner Trojahn; Tomas Bauer
Original assignee: Schaeffler Kg
Priority date: 2006-08-31
Filing date: 2007-08-28
Publication date: 2008-03-06
Also published as: DE102006040814A1

Abstract

A process for producing a highly case-hardenable roller bearing component which can be controlled in terms of process engineering and can be realized with a justifiable outlay provides for the roller bearing component to be produced from a high-alloy, corrosion-resistant steel having high proportions of carbide-forming alloying elements, in particular chromium, and to be subjected to a temperature in the range from at least 700°C to not more than 1000°C in a process chamber into which methane is introduced to carburize the roller bearing component and in which a plasma is generated in the vicinity of the surface of the roller bearing component by the methane being dissociated with liberation of pure carbon.

Description

Name of the invention

Method for producing a highly case-hardenable

rolling bearing component

description

The invention is in the field of rolling bearing technology for particularly critical and highly stressed applications, such. in turbines or engine construction and relates to a method for producing a highly hardenable rolling bearing component.

Rolling bearings or their components, in particular bearing rings and rolling elements, are exposed to extremely high operational stresses, in particular high operating temperatures, depending on the application and site of use. Nevertheless, a wear-free, reliable storage is expected from them, even with high rolling and sliding stress between the rolling elements and the associated raceways.

Depending on the area of application, various materials, in particular case hardening steels, are known for the production of roller bearings. In case hardening by the so-called carburizing - so the targeted and metered introduction of carbon atoms by diffusion into the surface layer of the workpiece - the necessary for the respective case steel and produces the best possible carbon gradient. By subsequent clamping of the workpiece material, ie the respective atomic lattice, by the embedded carbon atoms ultimately the desired hardness is generated.

In the case of workpieces made of low-alloyed steels, it is (still) possible to carburize them in a furnace atmosphere of an evacuatable furnace enriched with propane as carbon donors, by generating such high temperatures that the propane is split after the propane pyrolysis known per se with release of atomic carbon.

However, propane has the disadvantage that it is split thermally in the low pressure range already at temperatures of about 400 ⁰ C, which would lead to uneven carburizing in high-alloy corrosion-resistant steels and also carries the risk of carbon fouling in the gas inlet nozzles. On the other hand, if the temperature is lowered, very long treatment or diffusion times would have to be accepted, although still no fully satisfactory results would be achieved.

As a result, high-alloyed steels are far more problematic than low-alloyed steels in terms of the carburising process. Metallurgical cause is u.a. the comparatively low rate of diffusion of carbon in high-alloy steels, which is due to the high proportion of dissolved elements in the steel. In addition, especially chromium, which may be present in high-alloy steels to a considerable extent, precipitating carbides to immediately connect to the diffusing carbon, so that it is virtually trapped and is no longer available for the desired carburizing.

Against this background, an object of the present invention is to provide a manufacturing technology controllable and implementable with reasonable effort method for producing a highly hardenable rolling bearing component. This object is achieved by a method for producing a highly hardenable rolling bearing component, wherein the rolling bearing component of a high alloy, corrosion-resistant steel with high levels of carbide-forming alloying elements, in particular chromium, is prepared and for carburizing in a process chamber at a temperature between at least 700 ⁰ C. and is exposed to at most 1000 ° C, wherein in the process space for carburizing the rolling bearing component methane is introduced and in the process space, a plasma near the surface of the carburizing rolling bearing component is generated by the first methane is split to release pure carbon.

An essential aspect of the invention is that with this actually a splitting of the methane to release the desired carbon atoms only by applying a suitable voltage by means of a so-called plasma circuit. The rolling bearing component can thus be exposed to a relatively high temperature of up to about 1000 ⁰ C, because methane is still very stable up to these temperatures and no thermal decomposition occurs. The actual carburization by providing the carbon atoms in the immediate vicinity of the surface of the rolling bearing component is advantageously carried out only by the high voltage gradient or a very high field strength, which leads there selectively to a splitting of the methane.

In lengthy and detailed investigations, it has surprisingly been found that so rolling bearing components made of high-alloy, heat-resistant commercial case-hardened steels, such as M50NiL or sold under the trade name Pyrowear 675 by Carpenter Technology Corporation (USA) steel, manufactured with reasonable equipment and very much be carburized homogeneous. Namely, such case hardening steels have the following, hitherto curing technology as extremely problematic properties seen on:

a) they have a high content of carbide-forming alloying elements, b) they have low carbon solubility and c) they have a low diffusion coefficient.

It is also particularly problematical for such case steels (and thus particularly preferred is the use of the method according to the invention) if carburization is to take place in a large, densely packed batch.

Surprisingly, excellent results can also be achieved under these aggravating conditions with the invention and rolling bearing components can be produced in the best quality.

A preferred embodiment of the method according to the invention provides that the steel contains a total of at least 12% chromium and / or molybdenum.

A preferred application of the method according to the invention consists in the production of case-hardenable rolling bearing components for bearings in engines or turbines.

Further features, advantages and aspects of the invention will become apparent or supplementary from the following description of the invention with reference to the drawings. Showing:

FIG. 1 hardness profiles with respect to the edge thickness in the case of samples which have been carburized by means of propane on the one hand and by means of methane on the other hand, 2 shows two samples of a rolling bearing component in each case in the cut, which were carburized according to a conventional method using propane on the one hand and methane on the other hand as Kohlenstoffträ- ger.

FIG. 1 shows the Vickers hardness (HV) in each case in the Y direction and the examination depth per one sample in the X direction. The first sample was carburized with propane at a process temperature of 940 ⁰ C as a carbon donor and without activation of a suitable plasma generating on the sample surface tension. The second sample was also carburized at a process temperature of 94O ⁰ C by means of propane as a carbon donor, but with the inclusion of a suitable, plasma-generating voltage at the sample surface. The hardness represents a measure of the carbon absorption or the penetration depth of the carbon, ie for the carburization depth.

It can be seen in FIG. 1 that the propane decomposes even without the plasma-generating voltage and the carbon released in the process only acquires a relatively small penetration depth. The lower penetration depth of the carbon, also referred to as the diffusion depth, is not directly related to the activation of the plasma. Rather, the depth of penetration of the carbon depends, inter alia, on the position of the particular sample within the batch or more precisely on the distance of this sample to the gas inlet nozzles. The _* reason for this is that propane decomposes in a relatively short time after it has left the Begasungsdüsen. As a result, the respective samples located near the nozzles are heavily carburized, but more remote samples are less carburized. A uniform carburizing of the entire batch (with the steel and the temperature mentioned) is not or only slightly possible. Methane, on the other hand, does not decompose immediately after it leaves the fumigation nozzles. It can distribute evenly in the oven. Only when a voltage is applied does methane dissociate directly at a cathodically connected, electrically conductive surface, such as the surface of a sample or the furnace frame.

The penetration depth of the carbon, i. The depth of diffusion in each sample thus depends essentially on the total amount of reactive carbon present. When carburizing with propane, there is a large supply of carbon, even without a plasma switch, in the immediate vicinity of the gassing nozzles. When carburizing with methane, there is a large supply of carbon in or near the plasma (so-called plasma chamber), which in this case is independent of the proximity to the gassing nozzles.

As a result, there is no significant influence on whether a plasma-generating voltage was switched on, because the propane already decomposes thermally at the process temperature (after the propane pyrolysis described above).

In contrast, the right side of Figure 1 shows in the same representation, namely in the Y direction, the Vickers hardness (HV) and in the X direction, the depth of investigation of the sample, the hardness profile of each sample, with methane as carbon donors at a process temperature of 940 ° C and carburized at a process pressure of about 10 mbar. Here it can be clearly seen in the first sample that, without the connection of the plasma-generating voltage, there is only a relatively small carburisation over the sample depth. This shows that hardly any splitting of the methane took place without plasma-generating voltage.

By using the plasma-generating voltage, the second sample has undergone considerable carburisation and thus a clear hardening even in deeper-lying boundary layers or regions. In other words, only with the plasma-generating voltage is a carbon release accurate takes place where a carbon supply for carburization is desired. Due to the high process temperature of 940 ^° C., for example, this carbon could diffuse sufficiently deep into the sample.

FIG. 2 shows sections of the workpiece samples corresponding to FIG. The sample carburized according to a conventional method using propane clearly indicates so-called overcooling (left part of FIG. 2), whereas carburization (FIG very metered carburization can be seen.

Claims

claims

1. A method for producing a highly hardenable rolling bearing component, wherein the rolling bearing component

- is manufactured from high-alloy, corrosion-resistant steel with high levels of carbide-forming alloying elements, in particular chromium,

- is exposed in a process room to a temperature between at least 700 ° C and at most 1000 ⁰ C,

- Is introduced into the process space for carburizing the rolling bearing component methane, and

- In the process space, a plasma near the surface of the rolling bearing component is generated by the first of the methane is split to release pure carbon.

2. The method of claim 1, wherein the steel contains at least 12% chromium and / or molybdenum.

3. Use of a case-hardenable rolling bearing component prepared according to claim 1 or 2 for bearings in engines or turbines.