SURFACE TREATMENT OF ROLLING ELEMENT BEARING STEEL

Abstract: Rolling-element bearing components made from high quality steel are subjected to ion nitriding in a plasma as a final step in the manufacturing process to produce a layer with a hardness around 800-1200 HV with a depth no more than 150 mu m. By controlling the conditions no friable surface layer is created and no further steps need to be performed on the component.

Description: SURFACE TREATMENT OF ROLLING ELEMENT BEARING STEEL FIELD OF THE INVENTION The present invention relates in general to a process of manufacturing a rolling element bearing component and more particularly to a final surface treatment of the bearing component. BACKGROUND It is well known to subject steels to thermo-chemical treatments to create a hardened surface layer or case. Such treatments can involve several heating cycles to achieve the desired properties and post-machining is often necessary to compensate for dimensional changes and unwanted surface features. It is also known to subject steel to ion nitriding but in the case of conventionally nitrided bearing components additional steps are needed after nitriding to remove the friable compound layer which forms on the surface. SUMMARY OF THE INVENTION The present invention relates in general to a process applied to rolling element bearing components made in pre-hardened steel in the fully machined condition. Rolling element bearing components for use in aircraft engines in particular are made from M50, M50 NiL RBD and Pyrowear 675 steels. It has now been found that such components can exhibit superior properties if, as a" final step in the manufacturing process, the component is subjected to a plasma (ion) nitriding treatment designed to increase the hardness of the outer surface layer up to 150μm in depth producing a surface hardness between 800 to 1200 HV0.3 and no other treatment is carried out on the component after nitriding. The nitriding treatment can involve exposing the component to a plasma established by subjecting a gaseous atmosphere, principally composed of N 2 and preferably N 2 and H 2 , to electrical energy and maintaining the exposure at a temperature in the range of 375°C to 592°C. By careful control of the plasma (ion) nitriding conditions the formation of a compound layer is prevented and the distortion minimised thereby negating the need for subsequent machining. DESCRIPTION OF PREFERRED EMBODIMENT Bearing components made from M50, M50 NiL, RBD and Pyrowear 675 steels were subjected to the normal final machining operations, typically grinding and honing and then placed in a furnace containing a gaseous atmosphere composed of N 2 and H 2 in a ratio 2% N 2 and 98% H 2 at a pressure of 1 to 10 more preferably 1 to 3 torr. The gaseous atmosphere is converted into a plasma by supplying electrical energy. The electrical energy can be a c. or d.c. or pulses of d.c. and additional heating energy can be supplied to the furnace to ensure the temperature of the plasma is in the range of 375°C to 592°C and preferably 460°C for 1 to 2 hours. The ion nitriding process parameters (temperature, duration, pressure and atmosphere) can "be adjusted according to the properties of the nitrided layer required. After removal from the furnace the components each had a hardened surface layer up to 500μm in depth but a preferred range of between 5μm to lOOμm. The maximum hardness of the surface layer was tested and found to be 1200 HV0.3 but more typically between 850 to 1000 HV0.3. Subsequent trials of the bearings components showed they exhibited improved wear resistance (particularly resistance to scuffing), resistance to debris damage, and rolling contact fatigue resistance. The improvement in rolling contact fatigue resistance ranged from over 4 times for M50 NiL to 12 times for M50. If the depth of the surface layer is greater than lOOμm the components are prone to change in dimensions during plasma nitriding. If the hardness is less than 800 HV0.3 wear resistance shows no significant improvement. The dimensions of the components do not suffer as a result of plasma nitriding and subsequent exposure to service conditions. The following tables provides an example of properties of bearing components made from M50 NiL when subjected to the aforementioned plasma nitriding treatment. Depth of hardened Surface Hardness Dimensional change Improvement in surface layer HV0.3 (significant x rolling contact (μm) insignificant -) fatigue resistance Untreated 700-750 - xl 25 850-900 - x3 900-960 - x4 940-1000 - x4 1000-1150 - x5 1100-1250 X x5 1100-1250 X x4

Claims: Claims 1. A process of manufacturing a rolling-element bearing component from pre- hardened steel, particularly M50, M50 NiL, RBD and Pyrowear 675, wherein as a final step in the manufacturing process the component is subjected to a plasma (ion) nitriding treatment to create a hardened outer surface layer up to a depth of 150μm with a hardness of 800 to 1200 HV0.3 and the outer surface layer is not subjected to any further treatment. 2. A process according to claim 1, wherein the treatment involves exposing the component to plasma created by subjecting a gaseous atmosphere principally composed of N 2 and preferably N 2 and H 2 to electrical energy and maintaining the exposure at a temperature in the range of 375°C to 592°C. 3. A process according to claim 2, wherein the temperature is maintained by supplying additional energy. 4. A process according to claim 2 or 3, wherein the gaseous atmosphere is 2% N 2 and 98% H 2 at a pressure of 1 to 10 and more preferably 1-3 torr. 5. A process according to claim 4, wherein the component is exposed to the plasma for 1 to 2 hours. 6. A rolling-element component, such as an inner or outer bearing ring, made by the process of any one of the preceding claims.

Inventors: DODD ANDREW, KINDER JEFFERY, DODD, ANDREW, KINDER, JEFFERY
Applicants: DODD ANDREW, KINDER JEFFERY, RHP BEARINGS LTD, DODD, ANDREW, KINDER, JEFFERY, NSK-RHP EUROPEAN TECHNOLOGY CO. LIMITED
Application year: 1997
Main classification: C23C8
IPC codes: C23C8, F16C33