GB2550380B

GB2550380B - Roller Element

Info

Publication number: GB2550380B
Application number: GB1608699.3A
Authority: GB
Inventors: Andrew Stewart David
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2016-05-18
Filing date: 2016-05-18
Publication date: 2019-06-12
Anticipated expiration: 2036-05-18
Also published as: GB2550380A; GB201608699D0

Description

ROLLER ELEMENT

TECHNICAL FIELD

The present disclosure concerns a roller element, a bearing, a nuclear power plant, and/or a method of manufacturing a roller element and/or a bearing.

BACKGROUND

Roller element bearings (or rolling bearings) are used in multiple locations on a nuclear power plant, in particular in locations within the primary circuit and/or steam generation portion of a nuclear power plant. A roller element bearing includes roller elements (which may also be referred to as rollers) between two races. The roller elements roll between the races. There are many different types of roller elements that can be used in bearings of a nuclear power plant, including ball bearings, cylindrical rollers, spherical rollers, tapered rollers, gear bearings or needle rollers. In some bearing types, cages are provided to locate the roller elements between the races.

To ensure durability and load bearing capability of the bearing, the roller elements need to be harder than the races (and cage when used) so as to encourage the races (and as applicable the cage) to wear preferentially to the roller elements. In a nuclear power plant, the roller elements generally have a hardness of approximately 600 Vickers, and the races often have a hardness of around 550 Vickers. When roller elements are used in a nuclear power plant, they often also need to be corrosion resistant.

To achieve the required properties the roller element bearings are conventionally made from a cobalt base alloy. The cobalt base alloy may include elements such as chromium, tungsten, carbon, nickel, iron and silicon. However, cobalt has a long half-life and as such when cobalt based alloys are used in a nuclear power plant there is a risk that cobalt-60 will form. Cobalt-60 is a radioactive isotope that emits gamma radiation, and as such the formation of this isotope is undesirable, as it requires additional safety considerations and shielding.

Alloys have been developed to remove the need to use cobalt based alloys. However, none of these alloys have been used for roller element bearings because they do not have the required hardness, along with other properties such as corrosion resistance, to be suitable for use in a roller element bearing.

SUMMARY

According to an aspect there is provided a roller element for a roller element bearing as claimed in claim 1.

The roller element may have a hardness greater than or equal to 550 Vickers, e.g. greater than or equal to 600 Vickers.

According to an aspect there is provided a roller element bearing comprising the roller element according to the previous aspect.

The roller element bearing may comprise two races and/or a cage. The rolling-element bearing may comprise a plurality of roller elements. The plurality of roller elements may be positioned between the two races. The roller elements may be located in the cage. The races may be made from a material having a hardness greater than or equal to 550 Vickers. The cage may be made from a material having a hardness greater than or equal to 500 Vickers. The hardness of the roller element bearing will be greater than the hardness of the races.

When a cage is provided, the hardness of the roller element will be greater than the hardness of the cage.

The races and/or the cages may comprise 1.7 to 2.5 % carbon, 8.5 to 9.5 % nickel, 5 to 6 % silicon, and 19 to 22 % chromium, optionally, 0.5 % manganese; optionally, 0.2 % cobalt; and optionally, up to 0.5% nitrogen; wherein the alloy further comprises: 8 to 14 % niobium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; 8 to 14 % molybdenum and optionally, 0.3 to 0.5 % titanium; 8 to 14 % of one or more of tantalum, tungsten, zirconium, and/or vanadium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; or additional chromium, such that the total chromium by weight is 28 to 33 % and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum, the balance being iron and impurities.

For example, the race of the bearing may be manufactured from an iron based alloy comprising: 1.7 to 2 % carbon, 8.5 to 9 % nickel, 5 to 5.5 % silicon, and 19 to 20.5 % chromium, wherein the alloy further comprises: 8 to 12 % niobium; 8 to 12 % molybdenum; 8 to 12 % of one or more of tantalum, tungsten, zirconium, and/or vanadium; 8 to 12 % titanium; or additional chromium, such that the total chromium by weight is 28 to 30 %.

According to an aspect there is provided a nuclear power plant comprising the roller element bearing and/or the roller element as described in the previous paragraphs of the summary.

According to an aspect there is provided a method of manufacturing a roller element for a roller element bearing as claimed in claim 6.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect within the scope of the claims. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein within the scope of the claims.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the Figures, in which:

Figure 1 is a schematic of a nuclear power plant; and Figure 2 is a schematic end view of a roller-bearing.

DETAILED DESCRIPTION

Referring to Figure 1, a nuclear power plant is indicated generally at 10. The plant includes a nuclear reactor 12, a primary circuit 14, a heat exchanger 16, a secondary circuit 18 and a turbine 20. The primary fluid in the primary circuit is heated by the nuclear reactor. The primary fluid then flows to the heat exchanger, where it heats secondary fluid in the secondary circuit. The heated secondary fluid is then used to drive the turbine 20. In the present example, only a primary and secondary circuit is provided, but it will be understood that additional circuits may be provided, for example, the primary fluid from the reactor may be used to heat an intermediary fluid which is then used to heat the secondary fluid in the heat exchanger.

Roller element bearings are used at various locations throughout the nuclear power plant, as is understood by the person skilled in the art. The present disclosure is concerned in particular with the roller elements that are provided in the region of the primary circuit, including the steam generator.

Referring now to Figure 2, a roller element bearing is indicated generally at 22. The roller element bearing includes a plurality of roller elements 24 (only one labelled in Figure 2), which in this example are spherical in shape, but in alternative embodiments may be any suitable shape, for example cylindrical with or without a taper. The roller elements are provided between an outer ring component 26 and an inner ring component 28. The outer ring component defines an outer race 30 and the inner ring component defines an inner race 32 that the roller elements contact and roll along. In alternative embodiments, the inner and/or outer race may be formed integrally with components of the nuclear power plant and be defined by said components instead of providing the outer and/or inner ring. In further alternative examples, a roller bearing may include a cage to locate the roller elements.

The races (and cage where applicable) can be made from a variety of materials. Generally the material of the races (and cage where applicable) will have a hardness of approximately 550 Vickers.

The roller elements 24 are made from an iron based alloy. The iron based alloy includes carbon, nickel, silicon and chromium. The alloy may include impurities such as cobalt and magnesium, but the cobalt is minimised to approximately 0.2 percent by weight. The following describes five example alloy compositions. Each composition includes at least 19% chromium (Cr), 1.7 to 2.5 % carbon (C), 8.5 to 9.5% nickel (Ni), and 5 to 6 % silicon (Si). In each composition there may be 0.2 % cobalt (Co), and 0.5% manganese (Mn). As the alloy is an iron based alloy, iron (Fe) forms the remaining balance (with some possible impurities) of the elements of each alloy.

In addition to the elements mentioned above, each example alloy includes niobium (Nb) (not claimed), molybdenum (Mo), titanium (Ti), additional chromium, tantalum (Ta), tungsten (W), zirconium (Zr), and/or vanadium.

Composition 1

In the first example (not claimed), the alloy of the roller element has the following composition, all percentages are by weight:

Base of Fe

1.7 to 2.5% C 8.5 to 9.5 % Ni 5 to 6 % Si 19 to 22 % Cr 8 to 14 % Nb 0.3 to 0.5 % Ti 0.3 % Mo 0.2 % Co 0.5 % Mn

Composition 2

In the second example, the alloy of the roller element has the following composition, all percentages are by weight:

Base of Fe

1.7 to 2.5% C 8.5 to 9.5 % Ni 5 to 6 % Si 19 to 22 % Cr 0.3 to 0.5 % Ti 8.0 to 14.0 % Mo 0.2 % Co 0.5 % Mn

Composition 3

In the third example, the alloy of the roller element has the following composition, all percentages are by weight:

Base of Fe

1.7 to 2.5% C 8.5 to 9.5 % Ni 5 to 6 % Si 19 to 22 % Cr 0.3 % Mo

8 to 14 %Ta, W, Zr and/or V 0.2 % Co 0.5 % Mn

Composition 4

In the fourth example, the alloy of the roller element has the following composition, all percentages are by weight:

Base of Fe

1.7 to 2.5% C 8.5 to 9.5 % Ni 5 to 6 % Si 19 to 22 % Cr 8 to 14.0 % Ti 0.2 % Co 0.5 % Mn

Composition 5

In the fifth example, the alloy of the roller element has the following composition, all percentages are by weight:

Base of Fe

1.7 to 2.5% C 8.5 to 9.5 % Ni 5 to 6 % Si 28 to 33 % Cr 0.3 to 0.5 % Ti 0.2 % Co 0.5 % Mn

In further alternative examples the alloy may comprise less than or equal to 0.5% nitrogen, e.g. any one of the compositions 1 to 5 may comprise approximately 0.5% nitrogen.

The above described alloys may also be used to manufacture the race and/or the cage of the bearing. In such examples the elements listed may be selected towards the lower end of the provide range for the race and/or cage and towards the upper end of the range for the roller element.

In each of the above examples, the roller elements are manufactured by providing a powder having the above composition. The powder is then consolidated in a hot isostatic press, commonly referred to as HIPPING. The roller elements may be cast using the powder and then HIPPING may be used to consolidate the material of the roller element to remove/reduce the porosity of the material. Use of HIPPING can result in much finer carbides which are better for wear and also will aid sphere shape definition of a roller element.

In further alternative examples, the roller elements may be formed using a can to form a near net shape of the roller element using the HIPPING process. In yet further examples, the roller element may be formed using additive layer manufacturing followed by HIPPING.

Once formed, the roller elements may be ground, and polished to achieve the desired final size, shape and surface finish.

The roller elements formed using the described method have a solid body formed of one of the described alloys, e.g. a monolithic, bulk body of one of the described alloys. The roller elements do not require coating.

Roller elements described in the examples provided have been found to have a hardness equivalent to that of the conventional cobalt base alloys used conventionally for roller elements. That is, the roller elements have been found to have a hardness greater than or equal to 560 Vickers, and often greater than or equal to 600 Vickers. As such, the described roller elements have the required hardness to meet the functionality requirements of roller elements, without including high levels of cobalt which can undesirably form cobalt-60.

In exemplary alternative embodiments, a coating may be provided on the roller element and/or the race and/or as applicable the cage. The coating may be a solid lubricant such as silver, or a ceramic, e.g. CrN-Ag. The ceramic coating may be applied using plasma vapour deposition.

In the present disclosure, where ranges are provided the ranges are inclusive of the upper and lower limits provided.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the scope of the claims. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and subcombinations of one or more features described herein and within the scope of the claims.

Claims

1. A roller element for a roller element bearing for use in a nuclear power plant formed from an iron based alloy comprising by weight: 1.7 to 2.5 % carbon; 8.5 to 9.5 % nickel; 5 to 6 % silicon; 19 to 22 % chromium; optionally, 0.5 % manganese; optionally, 0.2 % cobalt; and optionally, up to 0.5% nitrogen; wherein the alloy further comprises: 8 to 14 % molybdenum and optionally, 0.3 to 0.5 % titanium; or 8 to 14 % of one or more of tantalum, tungsten, zirconium, and/or vanadium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; or 8 to 14 % titanium and optionally, 0.3% molybdenum; or additional chromium, such that the total chromium by weight is 28 to 33 % and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum, the balance being iron and impurities.

2. A roller element bearing comprising the roller element according to claim 1.

3. The roller element bearing according to claim 2, wherein the race of the bearing is manufactured from an iron based alloy comprising: 1.7 to 2.5 % carbon, 8.5 to 9.5 % nickel, 5 to 6 % silicon, and 19 to 22 % chromium, optionally, 0.5 % manganese; optionally, 0.2 % cobalt; and optionally, up to 0.5% nitrogen; wherein the alloy further comprises: 8 to 14 % niobium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; or 8 to 14 % molybdenum and optionally, 0.3 to 0.5 % titanium; or 8 to 14 % of one or more of tantalum, tungsten, zirconium, and/or vanadium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; or additional chromium, such that the total chromium by weight is 28 to 33 % and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum, the balance being iron and impurities.

4. The roller element bearing according to claim 2, wherein the race and/or the cage of the bearing is manufactured from an alloy comprising: 1.7 to 2.5 % carbon; 8.5 to 9.5 % nickel; 5 to 6 % silicon; 19 to 22 % chromium; 7.7 to 13.5 % titanium; the balance being iron and impurities.

5. A nuclear power plant comprising the roller element bearing according to any of claims 2 to 4 and/or the roller element according to claim 1.

6. A method of manufacturing a roller element for a roller element bearing comprising: providing an iron based powder having the composition by weight of: 1.7 to 2.5 % carbon; 8.5 to 9.5 % nickel; 5 to 6 % silicon; 19 to 22 % chromium; optionally, 0.5 % manganese; optionally, 0.2 % cobalt; and optionally, up to 0.5% nitrogen; wherein the alloy further comprises: 8 to 14 % molybdenum and optionally, 0.3 to 0.5 % titanium; or 8 to 14 % of one or more of tantalum, tungsten, zirconium, and/or vanadium and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum; or 8 to 14 % titanium and optionally, 0.3% molybdenum; or additional chromium, such that the total chromium by weight is 28 to 33 % and optionally, 0.3 to 0.5 % titanium and optionally, 0.3% molybdenum, the balance being iron and impurities; hot isostatic pressing said powder to form a consolidated alloy material; and forming and/or machining said alloy material to form one or more roller elements.