DE102008031107B4 - Steel element with claimable surface and method for its production - Google Patents

Abstract

Process for producing a steel component (12, 14) starting from a structural component with a sulfur content of less than 0.01% by weight sulfur and a carbon fraction between 0.8 and 1.1% by weight, wherein at least one surface area (13, 15) of the Carbon component is carbonitrided, then the component is quenched at least at the first critical cooling temperature and then stored at a low temperature below 0 ° C, characterized in that the green component between the carbonitriding and the subsequent treatment step is cooled to room temperature.

Description

Field of the invention

The invention is in the field of mechanical engineering and more specifically in the field of tribology, that is, the design of components with the aim to increase the stability, durability and durability of components against surface wear and to optimize friction parameters.

Specifically, the invention is concerned with steel components which are subject to increased surface loading, for example when used in rolling bearings.

In principle, such components can be made of highly resistant steel grades or similar alloys in order to optimize both friction coefficients and hardness and abrasion. However, this is often not possible because of the cost of materials or, for example, because the components can not or should not be made homogeneous for various reasons.

Alternatively, various types of surface designs and treatments have been developed that can specifically treat surfaces of steels and other alloys to alter the surface properties to the properties of the particular component core.

The JP 2006-322 017 A describes a rolling bearing with a rolling element of a steel with a mass ratio of 03 to 1.2% carbon, 0.5 to 2% Cr, at least 0.5% Mn, at least 0.85% of Mn and Si total, at most 12 ppm O. The production process includes carbonitriding, quenching, storing at minus degrees, tempering, the residual austenite content in the surface layer being 5 to 24% by volume.

The DE 101 47 631 A1 describes a spindle bearing in which at least the bearing raceway of the bearing contains the following: C: 0.6 to 1.3%; Si: 0.3 to 3.0%; Mn: 0.2 to 1.5%; P: maximum 0.03%; S: maximum 0.03%; Cr: 0.3 to 5.0%; Ni: 0.1 to 3.0%; Al: maximum 0.050%; Ti: maximum 0.003%; O: maximum 0.0015%; N: 0.015% at maximum, the remainder being Fe and unavoidable impurities, and the race is tempered after quenching or carbonitriding and its surface hardness after tempering is at least HRC 58.

The present invention has for its object to provide a steel component and a method for its production, which combine a high material hardness with a low tendency to cracking and high wear resistance to abrasion with the lowest possible production costs in the area of a particularly loaded surface.

The object is achieved according to the invention with the features of claim 1.

In this case, a shell component is carbonitrided in a basically known method, that is, exposed at elevated temperatures carbon and nitrogen, so that corresponding atoms can diffuse in the surface region.

This can be done, for example, in an ammonia-enriched hydrocarbon gas, advantageously at a temperature between 820 ° C and 930 ° C. The duration of such treatment may vary between two and twelve hours depending on the choice of temperature and composition of the gas as well as gas pressure.

Then the shell component can be stored even further at a corresponding temperature, for example, between 820 ° C and 880 ° C (hardening temperature) in order to achieve a substantial conversion of ferrite to austenite (austenitizing). This is followed by quenching, ie rapid cooling, for example to room temperature (20 ° C.), at least at a cooling rate which corresponds to the so-called first critical cooling rate, which is decisive for martensitic hardening.

Accordingly, in the quenching process, at least a portion of the austenite is converted to martensite, which has a desired high hardness at least in the surface region.

The component is then stored for a time at a low temperature that is at least below 0 ° C to reduce retained austenite.

Since the shell part has a carbon content of between 0.8% and 1.1% in parts by weight, a high hardness can be achieved with the correspondingly formed martensitic part if a corresponding deep cooling takes place. Due to the preceding carbonitriding, the carbon content is particularly high in the surface area, so that the achievable by the storage at low temperature outside hardness particularly high values (62 to 67 HRC), while below the surface layer has a hardness of at least 60 HRC and a Austenithaltsgehalt less than 10% is present.

In the surface area, the conversion of retained austenite into martensite is not as pronounced as that due to the increased carbon content Core area and must be supported by cold storage.

In particular, by limiting the sulfur content to less than 0.01 percent by weight results in the surface area a good toughness against stress-induced cracking.

Due to the overall reduced Austenitgehalt also results in a high dimensional stability of the steel component, at least up to 135 ° C.

The inventive method is further configured in that between the carbonitriding and the quenching, the greenware is stored at a curing temperature between 820 ° C and 880 ° C for at least 20 minutes.

The storage time is advantageously at least 20 minutes after the heating, wherein in this hardening process a substantial austenitization takes place. This increases the rate of martensite achievable by deterrence.

Additionally or alternatively, it can also be provided that between the carbonitriding and the quenching the shell component is stored at a temperature between 600 ° C. and 750 ° C. for at least one hour.

Additionally or alternatively, it is also possible that between the carbonitriding and quenching, the shell component is exposed to an annealing treatment at a temperature between 500 ° C and 750 ° C.

An advantageous embodiment of the invention provides that the shell component between the carbonitriding and the subsequent treatment step is cooled to room temperature.

Due to the intermediate cooling to room temperature can take place in the core of the component partially a Martensitisierung, since there the carbon content is relatively low and the corresponding martensite start temperature is higher than for the outdoor area in which the carbon content of the component is much higher.

After quenching, the actual reduction of the residual austenite content takes place by further cooling and storage of the component at a low temperature which is advantageously below 0 ° C., particularly advantageously below -40 ° C., in particular -100 ° C. and occasionally advantageously even below -150 ° C is.

The component can also be brought to its temperature by means of liquid nitrogen and stored there in order to achieve the greatest possible conversion of the retained austenite into martensite.

This storage can last for example at least one hour.

This ensures that the component is thoroughly cooled to the core and stored for a correspondingly long time at the cooling temperature.

After storage at low temperature, the component can be heated and tempered.

In this case, the tempering at different temperature levels is conceivable, wherein advantageously the first tempering stage is provided between 120 ° C and 200 ° C. On tempering at the first annealing stage, the martensite passes into a modification step called cubic martensite.

In addition, additional tempering stages between 200 ° C and 320 ° C are conceivable for reducing the retained austenite.

The invention relates, in addition to a method for producing a steel component, to a steel component which is produced by the described method and which has a residual austenite content in the region of the surface of between 15% and 40%.

In the core area, the steel component has a much lower austenite content than in the surface area, for example less than 10%. As a result, the surface area becomes extremely wear-resistant as a result of a tough residual austenite content, high residual compressive stresses and high hardness, the component also having an outstanding resistance to stress-induced cracking and toughness due to the low sulfur content. In the following the invention will be shown with reference to an embodiment in a drawing and described below.

It shows

1 the spindle of a machine tool;

2 a spindle bearing in the form of an angular contact ball bearing;

3 a schematic process representation of the method according to the invention.

The inventive method is particularly suitable for the production of elements for rolling bearings, in particular rolling bearings for high-speed spindles of machine tools or other fast-rotating and highly loaded components. With such elements are particularly high Requirements placed on the wear resistance of the steel in the rolling bearing. In the appropriate camps advantageously ceramic rolling elements (cylindrical rollers or balls) are used. Advantageously, lubrication with grease or oil takes place.

The 1 shows a spindle 1 a machine tool not shown in detail, which is rotatably mounted in a plurality of bearings and by means of a drive 2 is driven in rotation.

There are in detail two angular contact ball bearings 3 . 4 shown, the axial movement of the spindle in the direction of the arrow 5 prevent and the spindle in the arrow 5 Support opposite axial direction. In addition, there are two angular contact ball bearings 6 . 7 shown, which is an axial movement of the spindle 1 in the direction of the arrow 8th prevent it and the spindle with it in the arrow 8th Support opposite axial direction.

In addition, the respective angular contact bearings provide 3 . 4 . 6 . 7 for radial support and storage. The corresponding rolling elements (balls) are designed as ceramic body.

In each case the inner and / or outer ring 9 . 10 a bearing is at least in the region of its tread, which comes into contact with the rolling elements, particularly wear-resistant.

This is closer in connection with the 2 shown. In the 1 is in addition to the angular contact ball bearings nor a radial cylindrical roller bearings 11 illustrated, will not be discussed in more detail below.

The 2 shows enlarged a single angular contact ball bearing with an outer ring 12 who has a tread 13 has, as well as an inner ring 14 who has a tread 15 for a spherical rolling element 16 having.

The treads 13 . 15 are arranged asymmetrically such that an axial relative movement in the direction of the arrows 17 . 18 prevents the two bearing rings against each other and the bearing is supported accordingly.

In addition, in the figure schematically a bearing cage 19 as well as a multi-part end shield on each end face 20 . 21 shown. The bearing is typically oil lubricated and is suitable for high speeds and exceptionally long tool life.

In the 3 is a schematic diagram of the inventive method based on time and temperatures for different process steps shown.

The process starts at a temperature Temp 5, which can be between 820 ° C and 930 ° C. The crude steel component, which is homogeneous at the beginning of the process and between 0.8 and 1.1 weight percent carbon, 1.3 to 2 weight percent chromium, 0.2 to 1.5 weight percent manganese, 0.2 to 0.9 weight percent silicon and containing less than 0.01% by weight of sulfur and other common and unavoidable impurities and accompanying elements, is subjected to carbonitriding in an ammonia-enriched hydrocarbon gas at this temperature, with the carbon content in the area of the tread (rolled-over functional area) being 0.9% to 1, 4% and the nitrogen content is enriched to 0.05% to 0.5%. The increase of the stated carbon and nitrogen contents extends to a depth between 1% and 6% of the corresponding component diameter in the area of the overrun surface. The carbonitriding at the temperature Temp 5, for example, take place over two to twelve hours and you. End is indicated by the time t1 in the diagram. Thereafter, the temperature can be lowered to room temperature Temp 2, held there from t2 to t3 and then held back to a temperature Temp 4 between 820 ° C and 880 ° C until time t4. Instead of this hardening between t3 and t4 at a temperature between 820 ° C and 880 ° C there can also be a heat treatment between 500 ° C and 750 ° C or annealing between 500 ° C and 750 ° C.

The transition between the carbonitriding temperature Temp 5 and the hardening temperature Temp 4 can also be effected directly without intermediate cooling to room temperature, as indicated by the dotted line 22 is indicated.

At time t4, a quench is provided at a cooling rate exceeding the first critical cooling rate for martensitic transformation and typically up to room temperature Temp2. However, cooling may be provided to another lower temperature, also less than room temperature Temp 2 can be.

After quenching, the component can be further cooled until time t5 up to a temperature Temp 1, which can be significantly below room temperature, often advantageously below -40 ° C or -100 ° C, for example, also at the temperature of the liquid nitrogen. At this temperature Temp 1, the component is stored until time t6 over a period of time, which provides complete cooling and then a storage time of at least half an hour.

Then, until the time t7, the component is heated again to room temperature to be started later at the time t8. The toughening to increase toughness typically takes place at the first tempering temperature Temp 3 in the range between 120 ° C and 200 ° C.

Thereafter, the steel component is cooled again and ready for use.

The described treatment provides a steel component with exceptional wear resistance and resistance to stress cracking in the surface area and a high hardness between 62 and 67 HRC in the surface area.

Limiting the sulfur content prevents the excretion of larger sulphides which would otherwise destroy the fracture toughness achieved by the treatment.

LIST OF REFERENCE NUMBERS

1

spindle

2

drive

3, 4, 6, 7

Angular contact ball bearings

5

Direction of movement of the spindle

8th

prevented axial movement of the spindle

9, 14

inner ring

10, 12

outer ring

11

Cylindrical roller bearings

13

Tread of the outer ring

15

Tread of the inner ring

17, 18

prevented relative movement of the treads

19

bearing cage

20, 21

end shield

22

direct transition between carbonation and hardening temperature

Claims (10)

Method for producing a steel component ( 12 . 14 ) starting from a shell component having a sulfur content of less than 0.01% by weight of sulfur and a carbon fraction of between 0.8 and 1.1% by weight, in which at least one surface area ( 13 . 15 ) is carbonitrided, then the component is quenched at least at the first critical cooling temperature and then stored at a low temperature below 0 ° C, characterized in that the green component between the carbonitriding and the subsequent treatment step is cooled to room temperature. A method according to claim 1, characterized in that between carbonitriding and quenching, the greenware is stored at a hardening temperature between 820 ° C and 880 ° C for at least 20 minutes. A method according to claim 1 or 2, characterized in that between the carbonitriding and the quenching, the greenware is stored at a temperature between 600 ° C and 750 ° C for at least one hour. A method according to claim 1, 2 or 3, characterized in that between the carbonitriding and the quenching, the shell component is subjected to an annealing treatment at a temperature of between 500 ° C and 750 ° C. A method according to any one of claims 1 to 4, characterized in that the low temperature below -40 ° C, particularly below -100 ° C, particularly advantageously less than -150 ° C. Method according to one of claims 1 to 5, characterized in that the storage at low temperature for at least one hour lasts. Method according to one of claims 1 to 6, characterized in that the shell component is heated and tempered after storage at low temperature. Method according to one of claims 1 to 7, characterized in that the carbonitriding takes place in an ammonia-enriched hydrocarbon gas for at least two hours. Steel component, produced by the method according to one of the preceding claims, characterized by a Restaustinitgehalt between 15% and 40% at the surface. Steel component according to claim 9, characterized by an austinite content in the core which is lower than in the region of the surface.

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