WO2000077265A1

WO2000077265A1 - Wear-and fracture-resistant steel

Info

Publication number: WO2000077265A1
Application number: PCT/NL2000/000414
Authority: WO
Inventors: John Michael Beswick; Aidan Michael Kerrigan; Jan Ture Slycke
Original assignee: SKF Engineering and Research Centre BV
Current assignee: SKF Engineering and Research Centre BV
Priority date: 1999-06-15
Filing date: 2000-06-15
Publication date: 2000-12-21
Anticipated expiration: 2001-12-15
Also published as: NL1012329C2; AU5714400A

Abstract

Roller bearing steel and method for the production of a part for a rolling structure. The ball bearing steel contains 0.37 - 0.46 % by wt. C, 0.1 - 0.3 % by wt. Si, 0.7 - 1.3 % by wt. Cr, 0.60 - 1.2 % by wt. Mn, 0.15 - 0.30 % by wt. Mo and also Fe and usual impurities. After carburising/carbonitriding a further heat treatment (re-hardening) takes place after the initial quenching, which further heat treatment is followed by quenching once again and tempering, martensite with a dispersion of hard carbides being formed at the surface.

Description

Wear-^:and fracture-resistant steel

The present invention relates to a method for the production of a part for a transmission, comprising the provision of a steel containing C, Si, Cr, Mn, Mo, and also Fe and customary impurities, carburising/carbonitriding, quenching, followed by further heating (re-hardening), quenching again and tempering.

A steel comprising above elements is known from US 5019182A. It is proposed to use such a steel for the production of gears in gearboxes of cars. After heat treatment the steel is subjected to shot peening, i.e. the surface structure is compressed by a bombardment of foreign particles. After that further mechanical treatment is not possible because the effect of the surface bombartment would be nullified.

More particular this steel comprises .l-.4% by wt. C, .06-.05% by wt. Si, .3-1.0 by % wt. Mn, .9-1.2 by % wt. Cr, .6-.5 by % wt. Mo and remaing Fe.

Subject invention aims to provide a part of a toroidal IVT transmission. The two most important parts are the disk and roller. An example thereof is given in the British patent specification 2286023. Therein at least two opposed disks are provided between which at least two power rollers are provided. By tilting the angle of the power rollers the transmission ratio can be changed. The requirements for a INT are different from those for roller bearings in that friction is required in order to transmit torque from the engine to the wheels. In a lubricated contact, such as in a INT, friction can only be generated if there is slip.

The prior art discloses low-alloy steels for the production of parts for rolling bearings which also have a low carbon percentage. 0.2 % by wt. C may be mentioned as an example. The core of parts for ball bearings produced from such a material is relatively tough because of the low carbon content. In order to make the surface sufficiently wear-resistant, carbon and/or nitrogen are introduced by carburising/carbonitriding. Rehardening is carried out after a treatment of this type.

Although parts for rolling bearings can be produced relatively inexpensively in this way, the low resistance to wear constitutes a limitation to the use thereof. This plays a role, for example, in papermaking installations and other constructions where oil- or grease-filled casings and the like are used and wear particles pass into the oil or grease, which particles are not filtered off, whilst the oil is not changed or is not changed sufficiently frequently. In such cases wear particles of this type will inevitably come between the various moving parts of a rolling bearing and attack the surface thereof. On the one hand this gives an increase in the noise generated by the bearing and on the other hand this will lead to failure in the longer term.

Another possibility described in the prior art for eliminating this problem in an inexpensive manner is the use of a steel containing more carbon. The hardness will increase as a result of this. One example of this is a ball bearing steel that contains approximately 0.6 % by wt. carbon. Although the core of rolling bearing parts is stronger with this steel, it is more susceptible to cracking. This applies in particular bearing rings mounted on shaft with high interference fit, where the tensile hoop stress can result in through cracking failure.

A ball bearing steel containing 0.2 - 0.6 % by wt. C, 0.3 - 2.0 % by wt. Si, 0.5 - 2.5 % by wt. Cr, 0.3 - 1.7 % by wt. Mn and other customary elements is described in US Patent 5,085,733. It is claimed that a tempering treatment can be carried out immediately after carbonitriding. The indicated range of contents is particularly broad so that it is not possible clearly to indicate which of the contents specified in this patent leads to a type of steel that, on the one hand, has adequate wear resistance and, on the other hand, does not display any fracturing or cracking, and, finally, can be produced relatively inexpensively. Because of the quantity of silicon used, such a type of steel will be appreciably more expensive than the types of steel described above which are generally known in the prior art.

In a INT the contact stress between the disc and the power rollers is much higher than in typically found rolling bearings. Fatigue cracking and fracture due to repeatative bending stresses have been reported as well as the combined rolling contact fatigue strength of the traction surfaces and improved fatigue fracture strength requirements have to be set.

A wear-and fracture-resistant ball bearing steel, as described above, is known from GB-A-2,272,909. The nitrogen to carbon ratio (N/C) in this specification is 0,8 to 2,0 i.e. the C/N ratio is 0,5 - 1,25. This means that a relatively low carbon content is combined with a relatively high nitrogen content. Because of that nitrides, retained austenite and carbonitrides will result. However, because of the low carbon percentage it is difficult to control the hardness of the surface of a ball bearing steel according to this British patent specification.

The invention aims to provide steel for an IVT component. This aim is realised with a method as described above in that the steel comprises .37-.46% by wt. C, .01-.35% by wt. Si, .7-1.3% by wt. Cr, .60-1.2% by wt. Mn, .15- .30% by wt. Mo and remaining Fe and usual impurities, wherein said part of a toroidal IVT is subjected after retreatment to a polishing treatment. More particular at the surface of said part .95-1.25% by wt. Carbon is present. If carbonitriding is used, the nitrogen content will be .05-0.25 % by wt.

The steel described above is generally available as structural steel. For example, it is known under the SAE designations 4140 and 4140H. Such types of steel are used for a wide variety of structural components in general. Surprisingly, it has been found that if these types of steel are used for rolling structure parts on the one hand the resistance to wear is adequate and on the other hand there is no risk of cracking or fracturing at the hoop stress levels typically found in the application.

A considerable increased structural strength is obsvered. In this context it is important that the structure described below is produced. The surface of types of ball bearing steel obtained in this way will have a Rockwell hardness HRc of at least 58 and the hardness of the core is between HRc 45 and 60, whilst there is no or a negligible amount of retained austenite present in the core. With the method according to the invention the surface will comprise a considerable quantity of fine carbides. With carbonitriding sufficient nitrogen is present to obtain tempering resistance for use at elevated temperature. The first quenching step can comprise quenching in oil at 50-80 °C or quenching in salt at 300-400°C, the second quenching step will be in oil at 50-80°C . Compared with the prior art, the hardness both of the surface and of the core are greater than is the case with generally known ball bearing steels having a low carbon content. The resistance to wear is improved as a result. That is to say, if the lubricant used is contaminated, for example by metallic particles or other particles originating from the environment, this will not lead to failure of the bearing in the short term. Moreover, as a result of these measures the loading capacity of the bearing is increased, as a result of which the latter can be of smaller construction. The fatigue resistance on rolling contact also increases, so that the component life of the bearing can be extended.

On the other hand, the disadvantage that through cracking occurs, described above in the prior art, is not found. It is assumed that the reason for this lies in a residual stress at the surface in the sense of compression combined with the toughness of the core. After all, at approximately 0.4 % by wt., the proposed carbon percentage is clearly lower than in the case of the types of steel described above which have a high carbon percentage (0.6 % by wt.). On the other hand, it will be understood that the costs of such a bearing are relatively low in view of the price of the steel used for its production and the treatment carried out.

In contrast to the British patent specification 2,272,909 the carbon content in the subject application is relatively high whilst the nitrogen content is relatively low. This result in a distribution of fine carbides, a martensite strengthed primarily with carbon but with additional nitrogen for temper restistance and a controlled quantitiy of retained austenite.

The invention also relates to a ball bearing steel containing 0.37 - 0.46 % by wt. C, 0.1 - 0.29 % by wt. Si, 0.7 - 1.3 % by wt. Cr, 0.60 - 1.2 % by wt. Mn, 0.15 - 0.30 % by wt. Mo and also Fe and usual impurities, wherein the microstructure of the surface comprises martensite with a dispersion of hard carbides having a diameter of between 0.2 and 2.0 μm and a proportion of retained austenite, which will be inversely related to the tempering temperature .

The costs of the type of steel now proposed and the method for the production thereof are appreciably lower than the costs of types of ball bearing steel which are also carburised but are alloyed with nickel. Because of the higher initial percentage of carbon, the carbonitriding or carburising time will be appreciably (approximately 50 %) shorter than in the case of types of steel containing a lower percentage of carbon (0.2 % by wt.). This comparison is made for the same diffusion depth, which in practice is approximately 3.0 mm. The combination of composition of the ball bearing steel now proposed and the associated method for the production thereof is found to constitute an advantage with regard to both the mechanical properties and the cost price.

The ball bearing steel described above has numerous applications. This steel can be used, for example, in the drying section of papermaking installations where bearings are subjected to high loads and contamination of the lubricant occurs, whereas when types of ball bearing steel containing a high percentage of carbon are used fracture and cracking may be encountered.

According to an advantageous embodiment of the invention, the further heating is carried out at at least 800°C for at least 0.4 hours. In practice said further heating step can take up to 1,5 hours. The actual time is dependant on the temperature used and on the desired final structure. At a low temperature (830 °C) this time will be longer than at a somewhat higher temperature, such as 850 °C. The various aspects must be matched to the tempering treatment that follows quenching. The tempering treatment takes place between 160 °C and 260 °C and takes 1,5-6 hours. The temperature for the further heating and re-hardening can be raised to 870 °C. Of course, the various operations must be carried out such that the carbide dimensions are smaller than 2.0 μm, but in practice it has been found that the value of 0.2 μm mentioned above is easily achievable. As a consequence of these very fine carbides, the surface has a sufficiently high hardness and fatigue resistance whilst adequate hardness is guaranteed. The actual tempering temperature and tempering time applied will be related to the application temperature and the application dimensional stability requirements.

Preferably carburising/carbonitriding is carried out at approximately 950°C. This is higher than suggested in the prior art, for example British patent specification

2,272,909. Because of the higher temperature the diffusion rate is increased shortening the term for this process.

By way of illustration the composition of a number of types of steel with which the invention can be employed is given in the table below. It will be understood that further types of steel are conceivable which fall within the scope of the appended claims and yield equally good properties.

Table

In the enclosed figure 1 the range both for the nitrogen content and the carbon content is^~shown as function from the surface from the related part.

Another field where the steel according to the invention can be used are INT steels. Below an example of such a IVT is described referring to fig. 2 which shows schematically a IVT. Such a INT is generally referred to by 1. This comprises an output disc 3 which is driven by the power rollers 4. The input disc 2 is driven by the engine. In between the input and output discs, three power rollers 4 are provided. Through tilting in the direction of arrow 5, the transmission ratio between the input disc 2 and the output disc 3 can be altered. The power rollers are subjected to considerable loads near their engagement surface with the raceways 7. The input and output discs are loaded in order to develop a friction force between the power rollers and the discs. By applying a load, the slip will be set such that the required force (torque) is deliverd. Area 8 is regarded as a critical area with respect to structural fatigue loads.

Claims

I . Method for the production of a part for a rolling structure, comprising the provision of a steel containing C, Si, Cr, Mn, Mo, and also Fe and customary impurities, carburising/carbonitriding, quenching, followed by further heating (re- hardening), quenching again and tempering, characterised in that, said part comprises a component of a toroidal INT and in that said steel contains 0.37 - 0.46 % by wt. C, 0.1 - 0.35 % by wt. Si, 0.7 - 1.3 % by wt. Cr, 0.60 - 1.2 % by wt. Mn, 0.15 - 0.30 % by wt. Mo and also Fe and customary impurities, wherein after said heat treatment said part is subjected to a polishing treatment.

2. Method according to Claim 1, wherein carburising/carbonitriding is effected such that at the surface of said part 0.95-1.25% by wt. carbon is present.

3. Method according to Claim 1 or 2, wherein carbonitriding is effected such that at the surface of said part at least 1 mm 0,05 - 0,25 % by weight Ν is present.

4. Method according to one of the preceding claims, wherein the further heating is carried out at at least 800°C for at least 0.5 hours.

5. Method according to one of the preceding claims, wherein tempering is carried out between 160°C -260°C for at least 1,5-6 hours.

6. Method according to one of the preceding claims, wherein the carburising/ carbonitriding is carried out at approximately 950 °C for at most 24 hours.

7. Method according to one of the preceding claims wherein said rolling structue comprises a rolling bearing and wherein 0.10 - 0.29 % by wt Si is present.

8. Method according to one of the claims 1-6, wherein said rolling structure comprises a CNT/TNT component.

9. Steel for a component of an INT containing 0.37 - 0.46 % by wt. C, 0.1 - 0.29 % by wt. Si, 0.7 - 1.3 % by wt. Cr, 0.60 - 1.2 % by wt. Mn, 0.15 - 0.30 % by wt. Mo and also Fe and usual impurities, wherein the microstructure of the surface comprises martensite with a dispersion of hard carbides having a diameter of between 0.2 and 2.0 μm wherein said surface is polished.

10. Steel according to claim 8, wherein said rolling structure comprises a rolling bearing comprising 0.10 - 0.29 % by wt Si.

I I . Steel for a rolling structure according to claim 10, wherein said rolling structure comprises a INT disc component comprising 10-35 % by wt Si.