Technical Field The present invention relates generally to the field of metallurgy and to an improved steel alloy and a method of heat-treating an alloy. The steel alloy exhibits resistance to hydrogen embrittlement and high hardness. The steel alloy may be used in a number of applications, including, for example, bearings. 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 contact fatigue, wear and creep.

Steelmaking companies have been active in lowering the hydrogen content during casting, since this element can have an adverse effect on the rolling contact fatigue life. The hydrogen concentration should typically not exceed 2 ppm. Even if the hydrogen content is very low in the as-produced steel, its amount is likely to increase during service, for example due to lubricant (oil or grease) decomposition or electric current breaking through the layer of lubricant, resulting in the decomposition of lubricant molecules into products including free hydrogen, making its ingress into the bulk possible.

Hydrogen embrittlement is likely to occur when the steel contains mobile hydrogen. For this reason it has been proposed to immobilise hydrogen in the alloy microstructure.

The steel known as 100CrMnMoSi8-4-6 (EN ISO 683-17:1999) has the following composition: 0.97 wt% carbon, 0.50 wt% silicon, 0.90 wt% manganese, 0.50 wt% molybdenum, 1 .90 wt% chromium, up to 0.05 wt% aluminium, up to 0.30 wt% copper, up to 0.25 wt% nickel, up to 0.025 wt% phosphorus and up to 0.015 wt% sulphur, the balance being iron (and any unavoidable impurities). This steel exhibits high hardness and is suitable for use in a bearing component. However, 100CrMnMoSi8-4-6 exhibits moderate-to-low resistance to hydrogen embrittlement.

It is an object of the present invention to address or at least mitigate some of the problems associated with prior art, or at least to provide a commercially useful alternative thereto. Summary

In a first aspect the present invention provides a bearing steel alloy comprising from 0.8 to 1 .2 wt% carbon

from 0.3 to 0.9 wt% silicon

from 0.7 to 1 .3 wt% manganese

from 0.3 to 0.8 wt% molybdenum

from 2 to 2.6 wt% chromium

from 0.15 to 0.55 wt% vanadium optionally one or more of from 0 to 0.25 wt% nickel

from 0 to 0.3 wt% copper

from 0 to 0.05 wt% aluminium

from 0 to 0.015 wt% nitrogen

from 0 to 0.025 wt% phosphorus

from 0 to 0.015 wt% sulphur

from 0 to 0.015 wt% oxygen and the balance iron, together with unavoidable impurities. 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 alloy according to the present invention comprises from 0.8 to 1 .2 wt% carbon. Preferably, the steel alloy composition comprises from 0.9 to 1 .1 wt% carbon, more preferably from 0.95 to 1 .05 wt% carbon. In one example, the alloy comprises about 0.97 wt% carbon. The presence of carbon in the specified amount may serve to increase the hardness of the steel alloy in the hardened condition. In addition, the presence of carbon together with vanadium may enable the formation of carbides comprising carbon and vanadium. As discussed below, the presence of such carbides may increase the alloy's resistance to hydrogen embrittlement.

The steel alloy according to the present invention comprises from 0.3 to 0.9 wt% silicon. Preferably, the steel alloy comprises from 0.45 to 0.8 wt% silicon, more preferably from 0.5 to 0.8 wt% silicon. In one example the alloy comprises about 0.5 wt% silicon. Silicon may also act to increase strength and hardness.

The steel alloy according to the present invention comprises from 0.7 to 1 .3 wt% manganese. Preferably, the alloy comprises from 0.75 to 1 .2 wt% manganese, more preferably from 0.8 to 1 .1 wt% manganese. In one example, the alloy comprises about 0.9 wt% manganese. The manganese, in combination with the other alloying elements, may increase hardness and hardenability, and may contribute to the steel's strength. Manganese may also have a beneficial effect on surface quality. Manganese contents above the recited ranges will cause the excessive formation of retained austenite which is undesirable for hardness and dimensional stability.

The steel alloy according to the present invention comprises from 0.3 to 0.8 wt% molybdenum. Preferably, the alloy comprises from 0.4 to 0.7 wt% molybdenum, more preferably from 0.5 to 0.6 wt% molybdenum. In one example, the alloy comprises about 0.5 wt% molybdenum. In combination with the other alloying elements (particularly the vanadium and the carbon), molybdenum in the specified amounts may improve hydrogen-trapping capacity of the steel alloy, possibly owing to more favourable coherency strains. This may provide the steel alloy with increased resistance to hydrogen embrittlement. Molybdenum may also act to increase the hardenability of the alloy. Molybdenum levels above the recited ranges may suppress the formation of bainitic ferrite, which is undesirable. The steel alloy according to the present invention comprises from 2 to 2.6 wt% chromium. Preferably, the alloy comprises from 2.1 to 2.5 wt% chromium, more preferably from 2.2 to 2.4 wt% chromium. In one example, the alloy comprises about 2.25 wt% chromium. The presence of chromium in the specified amount may provide an improved corrosion resistance property to the steel alloy. The chromium leads to a hard oxide on the metal surface to inhibit corrosion. Chromium may also have a beneficial effect on hardenability.

The steel alloy according to the present invention comprises from 0.15 to 0.55 wt% vanadium. Preferably, the alloy comprises from 0.16 to 0.3 wt% vanadium, more preferably from 0.17 to 0.23 wt% vanadium. In one example, the alloy comprises about 0.2 wt% vanadium. In combination with the other alloying elements, vanadium in the specified amounts has been found to form carbides, such as, for example, V C3. Such carbides, which are preferably nanometre-scaled, may act as hydrogen traps. The presence of such carbides may provide the steel alloy with increased resistance to hydrogen embrittlement. The presence of such carbides may also provide the steel alloy with increased strength and hardness. The presence of vanadium in the range of about 0.15 to about 0.55 wt% makes carbide formation (for example V ₄ C ₃ ) thermodynamically possible at about 600°C, and is also beneficial for retarding grain growth during austenitisation. The steel alloy according to the present invention may comprise up to 0.25 wt% nickel. Preferably, the alloy comprises from 0.005 to 0.05 wt% nickel, more preferably from 0.007 to 0.02 wt% nickel. In one example, the alloy comprises about 0.01 wt% nickel. Nickel, in concentrations close to the cited upper limit, may act to increase hardenability and impact toughness.

The steel alloy according to the present invention may comprise up to 0.3 wt% copper. Preferably, the alloy comprises from 0.01 to 0.28 wt% copper, still more preferably from 0.1 to 0.27 wt% copper. In one example, the alloy comprises about 0.25 wt% copper. The copper may act to provide improved corrosion resistance.

The steel alloy according to the present invention may comprise up to 0.05 wt% aluminium. Preferably, the steel alloy comprises from 0.001 to 0.01 wt% aluminium, more preferably from 0.002 to 0.005 wt% aluminium. In one example, the steel alloy comprises about 0.003 wt% aluminium. Aluminium may be used as a deoxidizer. Aluminium, with other alloying elements, particularly nitrogen, may also act to control grain size in the alloy.

Other elements that may be present include oxygen, phosphorus and sulphur. Preferably, the presence of these elements is kept to a minimum. If phosphorus is present, the content thereof should generally not exceed 0.025 wt%. Typically the phosphorus content will be about 0.004 wt%. If sulphur is present, the content should generally not exceed 0.015 wt%. Typically the sulphur content will be about 0.003 wt%. If oxygen is present, the content should generally not exceed 0.015 wt%. Preferably, the oxygen content does not exceed 10 ppm. More preferably, the oxygen content does not exceed 5 ppm.

It will be appreciated that the steel alloy 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 alloy according to the present invention contains 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. As noted above, the phosphorus, sulphur and oxygen contents are preferably kept to a minimum.

Preferably, the steel alloy according to the present invention has a ratio Mo/V by weight, based on the total weight of the alloy, of from 0.6 to 4, preferably from 0.8 to 3.5. This ratio may ensure a high hydrogen trap concentration without the formation of coarse carbides at higher ratios.

A most preferred steel alloy according to the present invention comprises: about 0.97 wt% carbon

about 0.5 wt % silicon

about 0.9 wt% manganese

about 0.5 wt % molybdenum

about 2.25 wt% chromium

about 0.2 wt% vanadium

about 0.01 wt% nickel

about 0.25 wt% copper

about 0.003 wt% aluminium

about 0.010 wt% nitrogen

about 0.004 wt% phosphorus

about 0.003 wt% sulphur and the balance iron, together with unavoidable impurities.

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 alloy typically has a microstructure comprising bainite (bainitic ferrite), retained austenite, and carbides some of which comprise mainly vanadium and carbon. The typical retained austenite content is up to 5 vol% (optionally present). The bainite will typically be lower bainite with content up to 95 vol%. Optionally, aluminium nitrides are present in very small volume fraction.

The carbides may consist of vanadium and carbon, for example V C3, or may include one or more additional alloying elements. Thus, the term carbide as used herein is meant to encompass also, for example, carbo-nitrides and also mixed metal carbides, carbo-nitrides.

Preferably, the microstructure comprises from 1 to 10 vol. % carbides (some of which comprising mainly vanadium and carbon), the remainder being bainitic ferrite and retained austenite. Most preferably, the microstructure comprises about 5 vol. % carbides (some of which comprising mainly vanadium and carbon), the remainder being bainitic ferrite and retained austenite.

Within the structure the carbide precipitates comprising vanadium and carbon may act as hydrogen traps.

The carbide precipitates comprising vanadium and carbon have a plate shape with average aspect ratio of 4.5 to 6.5 and are advantageously nanometre-sized and, preferably, have a mean diameter of from 1 to 50 nm, more preferably from 1 to 30 nm, even more preferably from 5 to 25 nm. Most preferably, the carbides have a mean diameter of about 10 nm. Carbides having such sizes may be particularly effective as hydrogen traps.

Preferably the microstructure of the alloy is substantially free of retained austenite for better dimensional stability, strength and hardness.

In a further aspect, the present invention provides a bearing component comprising a steel alloy as defined herein. The bearing component may be at least one of a rolling element (for example ball or cylinder), an inner ring, and/or an outer ring. Other example application can be for the manufacture of linear motion components such as screws and nuts.

In a further aspect, the present invention provides a bearing comprising a bearing component as described herein.

Figures

The present invention will now be described further, by way of example, with reference to the following figure:

Figure 1 shows the calculated TTT diagrams for 100CrMnMoSi8-4-6 (dashed line) and an alloy according to the present invention comprising about 0.97 wt% carbon, about 0.5 wt % silicon, about 0.9 wt% manganese, about 0.5 wt % molybdenum, about 2.25 wt% chromium, about 0.2 wt% vanadium, about 0.01 wt% nickel, about 0.25 wt% copper, about 0.003 wt% aluminium, about 0.004 wt% phosphorus, about 0.003 wt% sulphur and the balance iron, together with unavoidable impurities (solid line). Referring to Figure 1 , the TTT diagrams were calculated using the MUCG83 algorithm (http://www.msm.cam.ac.uk/map/steel/prgrams/mucg83.html). The diagrams suggest that the transformation behaviour and the hardenability of the bearing steels of the present invention are similar to those of 100CrMnMoSi8-4-6. A suitable heat treatment for the inventive alloy is as follows:

Prior to hardening, the bearing steel may be hot-worked (forged and/or hot-rolled), heat-treated and softened by means of applying a suitable spheroidise-annealing heat treatment.

During the first step of hardening of steel components manufactured from the inventive steel alloy, in addition to the austenite that forms during austenitisation within the temperature range 800°C to 920°C for 10 min to 60 min, carbides, and optionally nitrides and/or carbo-nitrides will also be present that help in refining the austenite grains. Refining the austenite grains leads to faster transformation into bainite (bainitic ferrite) upon quenching and holding at the desired transformation temperature. Thereby it is possible to reduce the overall hardening heat treatment time and associated cost.

The martensite-start temperature (M _s ) is expected to be lowered as the austenite grains become finer which enables the bainite transformation to take place at even lower temperatures permitting the formation of harder structures. Moreover, the finer hardened structure yields better strength, toughness and hardness. All improve the resistance to rolling contact fatigue.

Typical temperatures for bainite transformation are within the range 150°C to 310°C for holding times between 30 min and 48 h.

In greater detail, the bainite transformation stage comprises:

After the austenitisation stage and the subsequent quenching in a manner such that all the reconstructive transformation products are avoided during cooling, the present invention describes a bainitic transformation schedule as follows (ordered 1 through 4):

1 . Holding the bearing components isothermally, typically in a salt bath, at a temperature just above the martensite-start (M _s ) temperature for a time that is sufficient for partial or full bainitic transformation. Example temperature is 200°C ;

2. Optionally, a second step may be employed which comprises heating the novel steel components, from the first bath temperature, typically by transferring the charge to a second salt bath kept at a temperature below the bainite-start temperature (B _s ) and holding isothermally until the bainitic transformation has ceased;

3. The bainite transformed bearing components are then air-cooled and subsequently cleaned;

4. Optionally, the bearing components may then be frozen to sub-zero temperatures followed by tempering to further reduce the content of retained austenite.

The foregoing description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.