GB2118207A

GB2118207A - Method of making a part for a rolling element bearing

Info

Publication number: GB2118207A
Application number: GB08209553A
Authority: GB
Inventors: Geoffrey William Meetham
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-03-31
Filing date: 1982-03-31
Publication date: 1983-10-26

Abstract

A method of making a part for a bearing in which a Boron containing iron alloy is produced as a metallic glass by rapid solidification, and pulverised. The pulverised material is compacted to form a consolidated material which is then hot formed to produce a fine-grained crystalline bearing part. The part may have good hot strength and wear resistance in its bearing application.

Description

SPECIFICATION Method of making a part for a rolling element bearing This invention relates to a method of making a part for a rolling element bearing, and the part made thereby.

It is known to produce metallic glasses by solidifying molten metal at an extremely rapid rate (108 degrees/second), and that the metallic glass may be pulverised and compacted to produce a fine grained material. We have determined that by using particular constituents, the material thus produced may have properties which are very advantageously used for the parts of rolling element bearings.

According to the present invention, a method of making a part for a bearing comprises melting an iron based alloy having 712% Boron and other alloying elements, solidifying the molten metal in such a way as to form a metallic glass, pulverising the metallic glass, compacting the pulverised material, and forming the compacted material into the shape of the desired bearing part.

The molten metal may be solidified by melt spinning or melt extraction.

The alloy may comprise 18% chromium, 9% Boron, 2% Molybdenum and 1% Silicon, the balance comprising Iron and impurities.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Fig. 1 illustrates the melt spinning process comprising the first stage of the method of the invention, and Fig. 2 shows the compaction of a metal powder resulting from the pulverising of the product of the Fig. 1 apparatus.

In Fig. 1 there is shown a crucible 10 containing a volume of molten metal 11 which is heated by an induction heating coil 12 to keep it molten. The metal may comprise one of a number of iron alloys with high boron content, but the particular material used in this instance comprises 18% chromium, 9% Boron, 2% Molybdenum, and 1% Silicon, the remainder comprising Iron and impurities.

A nozzle 1 3 in the bottom of the crucible 10 allows a very thin stream of the molten metal to issue from the crucible, and a copper wheel 14 is positioned so that this thin stream falls on its broad rim 1 5. The wheel 14 is rapidly rotated about its axis, by means not shown, in the direction of the arrow 1 6. As the metal falls upon the copper it solidifies very rapidly, and the rotation of the wheel causes the solidified metal to be moved out of the path of the descending metal stream very rapidly.

The result of this continuous process is the formation of a thin ribbon of rapidly solidified metal, and if the parameters of alloy constituents and solidification rate are correctly chosen this will be in the form of a metallic glass. With the alloy used in this case a cooling rate of some 106 degrees K/second is required. The process can clearly be allowed to continue until the supply of molten metal is exhausted or until a sufficient quantity of the metallic glass ribbon is produced.

The next step in the process is to pulverise the metallic glass ribbon. This may be done using one of several mechanical processes.

The powder thus produced is loaded into a steel can 1 8 and the can is then heated and extruded through an extrusion nozzle 1 9 by a piston 20.

This extrusion causes the powder to be compacted into a solid material, which may then have the can 18 stripped from it and is in a form which is amenable to forming by conventional means to produce the necessary bearing parts.

During this process the alloy is heated to a temperature above its transition temperature, and devitrification occurs with the production of a microcrystalline material of extremely fine grain size (0.1--0.2 microns diameter). Extrusion conditions are chosen so that this fine grain size is maintained in the finished product. Most amorphous metals become brittle when heated above their transition temperatures and in order to achieve the required level of ductility it is necessary to select alloy compositions which are sufficiently close to, but not of, eutectic alloy compositions.

As an alternative, it would be possible to use the hot isostatic pressing process to form the powder into a solid metal part. In this process the powder is again encased in a can, but is then compacted by an inert gas such as argon at high pressure and temperature. This process could have the advantage of producing the solidified part in a shape nearer to that eventually required; for example a ring could be made which will require very little machining to produce a bearing race.

However the powder is consolidated, the end product will have a microstructure containing a significant volume fraction of borides dispersed in a very fine discrete manner throughout the microcrystalline structure. This structure is very stable and its retention of hardness at high temperature and rear resistance are both very good, this combination making for a material particularly useful in forming bearing components with improved fatigue strength.

It will be appreciated that alternatives to the steps referred to above could be devised, for instance melt extraction using a rotating wheel contacting the surface of a pool of molten metal, and other methods of giving the desired rapid solidification could be used.

1. A method of making a part for a bearing comprising melting an iron based alloy having 712% Boron and other alloying elements, solidifying the molten metal in such a way as to form a metallic glass, pulverising the metallic glass, compacting the pulverised material, and forming the compacted material into the shape of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Method of making a part for a rolling element bearing This invention relates to a method of making a part for a rolling element bearing, and the part made thereby. It is known to produce metallic glasses by solidifying molten metal at an extremely rapid rate (108 degrees/second), and that the metallic glass may be pulverised and compacted to produce a fine grained material. We have determined that by using particular constituents, the material thus produced may have properties which are very advantageously used for the parts of rolling element bearings. According to the present invention, a method of making a part for a bearing comprises melting an iron based alloy having 712% Boron and other alloying elements, solidifying the molten metal in such a way as to form a metallic glass, pulverising the metallic glass, compacting the pulverised material, and forming the compacted material into the shape of the desired bearing part. The molten metal may be solidified by melt spinning or melt extraction. The alloy may comprise 18% chromium, 9% Boron, 2% Molybdenum and 1% Silicon, the balance comprising Iron and impurities. The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which: Fig. 1 illustrates the melt spinning process comprising the first stage of the method of the invention, and Fig. 2 shows the compaction of a metal powder resulting from the pulverising of the product of the Fig. 1 apparatus. In Fig. 1 there is shown a crucible 10 containing a volume of molten metal 11 which is heated by an induction heating coil 12 to keep it molten. The metal may comprise one of a number of iron alloys with high boron content, but the particular material used in this instance comprises 18% chromium, 9% Boron, 2% Molybdenum, and 1% Silicon, the remainder comprising Iron and impurities. A nozzle 1 3 in the bottom of the crucible 10 allows a very thin stream of the molten metal to issue from the crucible, and a copper wheel 14 is positioned so that this thin stream falls on its broad rim 1 5. The wheel 14 is rapidly rotated about its axis, by means not shown, in the direction of the arrow 1 6. As the metal falls upon the copper it solidifies very rapidly, and the rotation of the wheel causes the solidified metal to be moved out of the path of the descending metal stream very rapidly. The result of this continuous process is the formation of a thin ribbon of rapidly solidified metal, and if the parameters of alloy constituents and solidification rate are correctly chosen this will be in the form of a metallic glass. With the alloy used in this case a cooling rate of some 106 degrees K/second is required. The process can clearly be allowed to continue until the supply of molten metal is exhausted or until a sufficient quantity of the metallic glass ribbon is produced. The next step in the process is to pulverise the metallic glass ribbon. This may be done using one of several mechanical processes. The powder thus produced is loaded into a steel can 1 8 and the can is then heated and extruded through an extrusion nozzle 1 9 by a piston 20. This extrusion causes the powder to be compacted into a solid material, which may then have the can 18 stripped from it and is in a form which is amenable to forming by conventional means to produce the necessary bearing parts. During this process the alloy is heated to a temperature above its transition temperature, and devitrification occurs with the production of a microcrystalline material of extremely fine grain size (0.1--0.2 microns diameter). Extrusion conditions are chosen so that this fine grain size is maintained in the finished product. Most amorphous metals become brittle when heated above their transition temperatures and in order to achieve the required level of ductility it is necessary to select alloy compositions which are sufficiently close to, but not of, eutectic alloy compositions. As an alternative, it would be possible to use the hot isostatic pressing process to form the powder into a solid metal part. In this process the powder is again encased in a can, but is then compacted by an inert gas such as argon at high pressure and temperature. This process could have the advantage of producing the solidified part in a shape nearer to that eventually required; for example a ring could be made which will require very little machining to produce a bearing race. However the powder is consolidated, the end product will have a microstructure containing a significant volume fraction of borides dispersed in a very fine discrete manner throughout the microcrystalline structure. This structure is very stable and its retention of hardness at high temperature and rear resistance are both very good, this combination making for a material particularly useful in forming bearing components with improved fatigue strength. It will be appreciated that alternatives to the steps referred to above could be devised, for instance melt extraction using a rotating wheel contacting the surface of a pool of molten metal, and other methods of giving the desired rapid solidification could be used. CLAIMS

1. A method of making a part for a bearing comprising melting an iron based alloy having 712% Boron and other alloying elements, solidifying the molten metal in such a way as to form a metallic glass, pulverising the metallic glass, compacting the pulverised material, and forming the compacted material into the shape of the desired bearing parts.

2. A method as claimed in claim 1 and in which said metallic glass is devitrified after it has been pulverised.

3. A method as claimed in claim 2 and in which devitrification is caused by a heat treatment.

4. A method as claimed in claim 1 and in which solidification of the molten metal is performed using a melt spinning process.

5. A method as claimed in claim 1 and in which said iron based alloy comprises 18% chromium.

9% boron, 2% molybdenum, and 1% silicon, balance iron and impurities.

6. A method substantially as hereinbefore particularly described with reference to the accompanying drawings.

7. A part for a bearing made by the method of any one of the preceding claims.