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 good mechanical properties, such as a 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 1 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 oil decomposition or electric current breaking through the layer of oil, resulting in the decomposition of oil 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 100Cr6 has the following composition: 0.974 wt% carbon, 0.282 wt% silicon, 0.276 wt% manganese, 0.056 wt% molybdenum, 1.384 wt% chromium, 0.184 wt% nickel, 0.042 wt% aluminium, 0.21 wt% copper, 0.01 wt% phosphorus and 0.017 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, 100Cr6 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 steel alloy having a composition comprising: from 0.6 to 1 .2 wt% carbon

from 0.1 to 0.8 wt% manganese

from 0.5 to 2.5 wt% chromium

from 2.5 to 3.5 wt% vanadium optionally one or more of from 0 to 1 . .0 wt% silicon

from 0 to 2 wt% molybdenum

from 0 to 0. .5 wt% copper

from 0 to 3. .5 wt% nickel

from 0 to 0. .1 wt% aluminium

from 0 to 0. .05 wt% phosphorus

from 0 to 0. .05 wt% sulphur

from 0 to 0. .1 wt% titanium

from 0 to 0. .1 wt% niobium

from 0 to 0. .1 wt% tantalum

from 0 to 0. .1 wt% tungsten

from 0 to 0. .1 wt% boron

from 0 to 0. .1 wt% nitrogen

from 0 to 0. .1 wt% oxygen

from 0 to 0. .1 wt% calcium

from 0 to 0. .1 wt% cobalt 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 0.6 to 1 .2 wt% carbon. Preferably, the steel alloy composition comprises from 0.8 to 1 .2 wt % carbon, more preferably from 0.9 to 1.1 wt% carbon, even more preferably from 0.95 to 1.05 wt% carbon. In one example, the alloy comprises about 0.99 wt% carbon. The presence of carbon in the specified amount may serve to increase the hardness of the steel alloy. 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 comprises from 0.1 to 0.8 wt% manganese, more typically from 0.1 to 0.6 wt% manganese. Preferably, the alloy comprises from 0.2 to 0.5 wt% manganese, more preferably from 0.2 to 0.4 wt% manganese. In one example, the alloy comprises about 0.28 wt% manganese. The manganese, in combination with the other alloying elements, may increases hardness and may contribute to the steel's strength. Manganese may also have a beneficial effect on surface quality. The steel alloy comprises from 0.5 to 2.5 wt% chromium. Preferably, the alloy comprises from 1.0 to 2.0 wt% chromium, more preferably from 1.2 to 1.6 wt% chromium. In one example, the alloy comprises about 1 .42 wt% chromium. The presence of chromium in the specified amount may provide an improved corrosion resistance property to the steel alloy. The chromium may lead to a hard oxide on the metal surface to inhibit corrosion. Chromium may also have a beneficial effect on hardenability.

The steel alloy comprises from 2.5 to 3.5 wt% vanadium. Preferably, the alloy comprises from 2.5 to 3 wt% vanadium, more preferably from 2.6 to 2.9 wt% vanadium, still more preferably from 2.7 to 2.8 wt% vanadium. In one example, the alloy comprises about 2.75 wt% vanadium. In combination with the other alloying elements, vanadium in the specified amounts can provide the steel alloy with increased resistance to hydrogen embrittlement. Vanadium may also act to increase the hardness of the alloy and preferably also the yield strength and/or tensile strength. Vanadium can form carbides, such as, for example, V ₄ C ₃ . Such carbides, which are preferably nanometre-scaled, may act as hydrogen traps. The presence of such carbides is believed to provide the steel alloy with increased resistance to hydrogen embrittlement. The presence of vanadium in the range of 2.5 to 3.5 wt% may make carbide formation (for example V ₄ C ₃ ) thermodynamically possible at about 600°C, and may also be beneficial for delaying grain growth during austenitisation.

Vanadium levels lower that 2.5 wt% can result in the high temperature formation of cementite. In contrast, vanadium levels of 2.5 wt% or higher can avoid the formation of cementite at high temperature. This is advantageous since cementite may then be formed at a lower tempering temperature resulting in improved mechanical properties (primarily high hardness). Vanadium levels lower than 2.5 wt% may result in the formation of detrimental carbides such as, for example, M ₆ C and M ₂ 3C ₆ . Vanadium levels higher than 3.5 wt% may result in the formation of other undesirable phases.

The steel alloy may optionally comprise up to 0.5 wt% copper, for example from 0.1 to 0.5 wt% copper. Preferably, the alloy comprises from 0.2 to 0.5 wt% copper, still more preferably from 0.2 to 0.4 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 may optionally comprise up to 1.0 wt.% silicon, more typically up to 0.5 wt% silicon, for example from 0.1 to 0.5 wt% silicon. Preferably, the alloy comprises from 0.1 to 0.4 wt% silicon, more preferably from 0.2 to 0.3 wt% silicon. Silicon may be added during the steel making process as a deoxidizer. Silicon may also act to increase strength and hardness.

The steel alloy may optionally comprise up to 2 wt% molybdenum, for example from 0.01 to 2 wt% molybdenum. Preferably, the alloy comprises from 0.01 to 0.3 wt% molybdenum, more preferably from 0.01 to 0.2 wt% molybdenum, even more preferably from 0.05 to 0.1 wt% molybdenum. In one example, the alloy comprises about 0.093 wt% molybdenum. In combination with the other alloying elements (particularly the vanadium and the carbon), molybdenum in the specified amounts may improve the 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. In addition, Molybdenum may improve grain boundary cohesion.

The steel alloy may optionally comprise up to 3.5 wt% nickel, typically up to 1 wt% nickel, more typically up to 0.1 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 may act to increase hardenability and impact strength.

The steel alloy may optionally comprise up to 0.1 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 may also act to control grain size in the alloy. The steel alloy may optionally comprise up to 0.1 wt% of one or more of titanium, niobium, tantalum, tungsten, boron, nitrogen, calcium and cobalt.

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 0.05 wt%. Typically the phosphorus content will be about 0.004 wt%. If sulphur is present, the content should generally not exceed 0.05 wt%. Typically the sulphur content will be about 0.003 wt%. If oxygen is present, the content should generally not exceed 0.1 wt%. Preferably, the oxygen content does not exceed 15 ppm.

It will be appreciated that the steel alloy may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt.% of the composition. Preferably, the alloy 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.

A most preferred steel alloy according to the present invention comprises: about 0.994 wt% carbon

about 0.282 wt% manganese

about 1 .42 wt% chromium about 0.247 wt% copper

about 2.75 wt% vanadium

about 0.272 wt % silicon

about 0.093 wt % molybdenum

about 0.01 wt% nickel

about 0.003 wt% aluminium

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 martensite, optionally cementite, and carbides comprising vanadium and carbon. If the alloy undergoes a tempering heat- treatment, then cementite is present in the final microstructure.

The carbides may consist of vanadium and carbon, for example V ₄ C ₃ , 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 carbo-oxy-nitrides and also mixed metal carbides, carbo-nitrides and carbo-oxy-nitrides.

The microstructure typically comprises at least 70 vol. % martensite, more typically at least 75 vol. %. Preferably, the microstructure comprises from 1 to 5 vol. % carbides (comprising vanadium and carbon) and from 5 to 20 vol. % cementite, the remainder being martensite. Most preferably, the microstructure comprises about 2 vol. % carbides (comprising vanadium and carbon), about 10 vol. % cementite, and the remainder being martensite.

Within the martensite matrix the carbide precipitates comprising vanadium and carbon may act as hydrogen traps. The presence of cementite precipitates may impart strength. The cementite precipitates are typically nanometre-sized, preferably having a mean diameter of from 10 to 500 nm. The carbide precipitates comprising vanadium and carbon 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 are particularly effective as hydrogen traps.

The structure of the steel alloy described herein can be determined by conventional microstructural characterisation techniques such as, for example, optical microscopy, TEM, SEM, AP-FIM, TDA and X-ray diffraction, including combinations of two or more of these techniques.

In a further aspect, the present invention provides an engine component or an armour component comprising a steel alloy as defined herein. The material may also be used in marine and aerospace applications, for example gears and shafts.

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

In a further aspect, the present invention provides a method of heat-treating a steel alloy comprising:

(i) providing a steel alloy composition as herein described;

(ii) heating the composition at a temperature of from 780 to 950°C to at least partially austenitise the composition;

(iii) further heating the at least partially austenitised composition to a temperature of from 1200 to 1400°C;

(iv) ageing the alloy at a temperature of from 540 to 660°C; and

(v) optionally carrying out a tempering heat-treatment following the ageing step (iv). In step (ii), the composition is at least partially austenitised, preferably completely austenitised. This is achieved by heating the alloy composition to a temperature of from 780 to 950°C, preferably from 820 to 900°C, more preferably from 840 to 880°C, and most preferably about 860°C. The composition may be maintained in this temperature regime for up to 30 minutes, preferably from 5 to 20 minutes, even more preferably for about 15 minutes. However, longer heating times are also possible. Step (ii) may result in the formation of vanadium carbides. Step (ii) may result in only minimal formation of cementite.

Further heating of the at least partially austenitised composition in step (iii) is carried out at a temperature of from 1200 to 1400°C, preferably from 1300 to 1380°C, more preferably from 1340 to 1360°C, even more preferably at about 1350 °C. Step (iii) may result in the dissolution of any coarse vanadium carbides formed on austenitisation. The composition may be maintained in this elevated temperature regime for up to 10 minutes, preferably from 30 seconds to 5 minutes, even more preferably for about 1 minute.

Between steps (iii) and (iv), the composition may optionally be quenched, preferably to a temperature lower than 200°C, more preferably to a temperature lower than 150 °C. The quenching may occur using helium quenching gas, and may occur at a cooling rate of 10 °C/minute or more, preferably 25 °C/minute or more.

In step (iv), the alloy is aged at a temperature of from 540 to 660°C, preferably from 560 to 640°C, more preferably from 580 to 620°C, and most preferably about 600°C. The alloy may be aged for up to 120 minutes, preferably from 30 to 90 minutes, even more preferably for about 60 minutes. However, longer heating times are also possible.

Following step (iv), the composition may optionally undergo a quench, preferably to a temperature lower than 200°C, more preferably to a temperature lower than 150 °C. The quenching may occur using helium quenching gas, and may occur at a cooling rate of 10 °C/minute or more, preferably 25 °C/minute or more.

The heat-treatment method according to the present invention may further comprise carrying out an optional spheroidising treatment prior to the austenetising step (ii). This may increase the machinability of the alloy composition. A tempering heat-treatment (v) may be optionally carried out following the ageing step (iv). The optional tempering heat-treatment (v) may be carried out at a temperature of from 150 °C to 300 °C, preferably from 190 °C to 260 °C, more preferably from 200 to 230 °C, even more preferably about 215 °C. The optional tempering heat-treatment (v) may be carried out for at least 30 minutes, preferably from 40 minutes to 240 minutes, more preferably from 90 minutes to 150 minutes, even more preferably for about 120 minutes. Longer heating times may also be employed. The optional heat-treatment (v) results in the formation of fine cementite particles, which can impart the steel with high hardness.

One or more of the heat-treating steps may be carried out via induction heating. The use of induction heating improves efficiency. Figure

The present invention will now be described further, by way of example, with reference to the following figure: Figure 1 shows a possible heat treatment schedule according to the present invention.

In the exemplary heat treatment depicted in Figure 1 , a steel alloy composition as described herein is heated to approximately 860°C and maintained at that temperature for about 15 minutes in order to at least partially austenise the composition. Vanadium carbides are also formed during this step. The composition is then further heated to approximately 1350°C and held at that temperature for about one minute in order to dissolve any coarse vanadium carbides formed during the austenisation before being quenched. Aging is then carried out at a temperature of approximately 600 °C for about 60 minutes, after which the composition is again quenched. In the final step, the composition is tempered at a temperature of approximately 215 °C for about 120 minutes, resulting in the formation of fine cementite particles.

The foregoing detailed 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.