Patent Publication Number: US-2011052442-A1

Title: Bearing component

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of metallurgy and to a bearing component such as a rolling element or ring formed from a bearing steel comprising lower bainite as the predominant phase. 
     BACKGROUND 
     Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (balls or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling fatigue, wear and creep. 
     Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished components so as to improve, for example, wear resistance and fatigue resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue performance. 
     An alternative to case-hardening is through-hardening. Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example. For through-hardened bearing steel components, two heat-treating methods are available: martensite hardening or austempering. Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed. 
     The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be low-temperature tempered to stabilize the microstructure. 
     The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels, instead of martensite through-hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance. 
     Numerous conventional heat-treatments are known for achieving martensite through-hardening and bainite through-hardening. 
     WO 01/79568 describes a method for the production of a part for a rolling bearing. 
     SUMMARY 
     The present invention provides a bearing component formed from a steel composition comprising:
     (a) from 0.5-1.2 wt. % carbon,   (b) from 0.15-2 wt. % silicon,   (c) from 0.25-2 wt. % manganese,   (d) from 0.85-3 wt. % chromium,   (e) optionally one or more of the following elements
       from 0-5 wt. % cobalt,   from 0-2 wt. % aluminium,   from 0-0.6 wt. % molybdenum,   from 0-0.5 wt. % nickel,   from 0-0.2 wt. % vanadium,   from 0-0.1 wt. % sulphur,   from 0-0.1 wt. % phosphorous, and   
       (f) the balance iron, together with unavoidable impurities.   

     The bearing component is formed from the alloy as herein described and preferably comprises lower bainite as the main phase (typically at least 60% bainite, more typically at least 80% bainite) or as essentially the only phase (i.e. &gt;95% bainite). Bainite is preferably obtained by carrying out the transformation at a relatively low temperature, typically less than 350° C., more typically from 110 to 325° C. One result of the low transformation temperature is that the plates of bainite are very fine. In particular, the material preferably has a microstructure comprising plates of bainite of less than 100 nm, typically from 10 to 50 nm, more typically from 20 to 40 nm. The plates of bainite are typically interspersed with retained austenite. The bainite typically forms at least 60% of the microstructure, more typically at least 80%. 
     The steel is preferably essentially carbide-free. Typically, the microstructure will comprises less than 5% carbides, more typically less than 3%. 
     The steel typically has an ultimate tensile strength of 2500 MPa, a hardness at 600-670 HV, and toughness in excess of 30-40 MPam 1/2 . The microstructure and resulting mechanical properties lead to improved rolling contact fatigue performance in the bearing component. 
    
    
     The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. 
     The steel composition preferably comprises 0.7-1.1 wt. % carbon, more preferably from 0.75 to 1.05 wt. % carbon. In combination with the other alloying elements, this results in the desired fine (lower) bainite microstructure. Carbon acts to lower the bainite transformation temperature. 
     The steel composition preferably comprises 0.25-2 wt. % silicon, more preferably from 0.25-1 wt. % silicon, still more preferably from 0.4-1 wt. % silicon. In combination with the other alloying elements, this results in the desired fine carbide-free microstructure (or essentially carbide-free). Silicon helps to suppress the precipitation of cementite. However, too high a silicon content may result in undesirable surface oxides and a poor surface finish. For this reason, the maximum silicon content is 2 wt. %, more preferably 1 wt. %. 
     The steel composition preferably comprises 0.25-1.9 wt. % manganese, more preferably from 0.25-1.8 wt. % manganese, still more preferably from 0.25-1.7 wt. % manganese. Manganese acts to increase the stability of austenite relative to ferrite. 
     The steel composition preferably comprises 0.95 to 2.05 wt. % chromium, more preferably from 0.95-1.5 wt. % chromium, still more preferably from 0.95-1.4 wt. % chromium, still more preferably 0.95-1.3 wt. % chromium. Chromium acts to increase hardenability and reduce the bainite start temperature. 
     While cobalt and aluminum are optional elements, it is preferable for one or both elements to be present. Accordingly, in a preferred embodiment, the steel composition comprises one or both of:
         from 0.1-5 wt. % cobalt, and/or   from 0.1-2 wt. % aluminium.       

     More preferably, the steel composition comprises one or both of:
         from 1-4 wt. % cobalt, and/or   from 0.5-2 wt. % aluminium.       

     More preferably, the steel composition comprises one or both of:
         from 1.8-4 wt. % cobalt, and/or   from 1-2 wt. % aluminium.       

     More preferably, the steel composition comprises one or both of:
         from 2-4 wt. % cobalt, and/or   from 1.2-2 wt. % aluminium.       

     Aluminium has been found to improve the intrinsic toughness of the bearing component, possibly due to it suppressing carbide formation. 
     Cobalt has been found to improve the corrosion resistance of the bearing component. This is very important for bearing components for wind turbines or marine pods, for example. Such bearings may become contaminated by sea water, which can drastically reduce the service life of the bearing. 
     If present, the alloy preferably also comprises from 0.05-0.5 wt. % molybdenum. Molybdenum acts to avoid austenite grain boundary embrittlement owing to impurities such as, for example, phosphorus. Molybdenum also acts to increase hardenability and reduce the bainite start temperature 
     It will be appreciated that the steel for use in the bearing component according to the present invention may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt. % of the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than 0.3 wt. % of the composition, more preferably not more than 0.1 wt. % of the composition. The phosphorous and sulphur contents are preferably kept to a minimum. 
     The alloys according to the present invention may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements which are mandatory other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence. 
     The bearing component according to the present invention is formed from a steel that transforms to bainite at a temperature of typically 110 to 350° C., more typically 115 to 250° C. The transformation time for complete bainite formation is typically from 3 hours to 80 days, more typically from 6 hours to 60 days. The transformation time depends on the transformation temperature: the time is longer for lower temperatures. The amount of bainite that is formed depends on the transformation temperature: more bainite is formed at lower temperatures. The amount of retained austenite increases for higher transformation temperatures. 
     The process for the manufacture of the steel for the bearing component avoids rapid cooling so that residual stresses can be avoided in large component pieces. 
     In order to accelerate transformation, the addition of cobalt and/or aluminium to the steel composition has been found to be beneficial. 
     Some examples of suitable bainitic steel compositions for use in the present invention include (the balance being Fe):
     0.79 wt. % carbon,   1.59 wt. % silicon,   1.94 wt. % manganese,   1.33 wt. % chromium,   0.3 wt. % molybdenum,   0.11 wt. % vanadium.   0.98 wt. % carbon,   1.46 wt. % silicon,   1.89 wt. % manganese,   1.26 wt. % chromium,   0.26 wt. % molybdenum,   0.09 wt. % vanadium.   0.83 wt. % carbon,   1.57 wt. % silicon,   1.98 wt. % manganese,   1.02 wt. % chromium,   0.24 wt. % molybdenum,   1.54 wt. % cobalt.   0.78 wt. % carbon,   1.49 wt. % silicon,   1.95 wt. % manganese,   0.97 wt. % chromium,   0.24 wt. % molybdenum,   1.60 wt. % cobalt,   0.99 wt. % aluminium.   

     If desired, various mechanical properties can be improved by carrying out any of the conventional post-bainite transformation steps. For example, in some cases, the yield strength can be improved by carrying out a post-bainite transformation deformation step followed by tempering. 
     The bearing component may be part of a rolling element bearing, for example the bearing inner or outer ring, or the ball or roller element. The bearing component could also be part of a linear bearing such as ball and roller screws. 
     The present invention also provides a bearing comprising a bearing component as herein described.