FATIGUE-RESISTANT BEARING STEEL

Abstract

A steel alloy for a bearing, the alloy having a composition that provides: from 0.8 to 1.0 wt. % carbon, from 0.1 to 0.5 wt. % silicon, from 0.2 to 0.9 wt. % manganese, from 2.0 to 3.3 wt. % chromium, from 0 to 0.4 wt. % molybdenum, from 0 to 0.2 wt. % cobalt, from 0 to 0.2 wt. % iridium, from 0 to 0.2 wt. % rhenium, from 0 to 0.2 wt. % vanadium, from 0 to 0.1 wt. % niobium, from 0 to 0.5 wt. % tungsten, from 0 to 0.2 wt. % nickel, from 0 to 0.4 wt. % copper, from 0 to 0.05 wt. % aluminum, from 0 to 150 ppm nitrogen, and the balance iron, together with any unavoidable impurities.

Claims

1. A steel alloy for a bearing, the alloy having a composition comprising: from 0.8 to 1.0 wt. % carbon, from 0.1 to 0.5 wt. % silicon, from 0.2 to 0.9 wt. % manganese, from 2.0 to 3.3 wt. % chromium, from 0 to 0.4 wt. % molybdenum, from 0 to 0.2 wt. % cobalt, from 0 to 0.2 wt. % iridium, from 0 to 0.2 wt. % rhenium, from 0 to 0.2 wt. % vanadium, from 0 to 0.1 wt. % niobium, from 0 to 0.5 wt. % tungsten, from 0 to 0.2 wt. % nickel, from 0 to 0.4 wt. % copper, from 0 to 0.05 wt. % aluminum, from 0 to 150 ppm nitrogen, and the balance iron, together with any unavoidable impurities.

2. The steel alloy of claim 1, comprising from 0.8 to 0.9 wt. % carbon.

3. The steel alloy of claim 1, comprising from 0.1 to 0.45 wt. % silicon

4. The steel alloy of claim 1, comprising from 0.35 to 0.8 wt. % manganese.

5. The steel alloy of claim 1, comprising from 2.3 to 3.3 wt. % chromium.

6. The steel alloy of claim 1, comprising from 0.1 to 0.4 wt. % molybdenum.

7. The steel alloy of claim 1, comprising no more than 0.1 wt. % molybdenum.

8. The steel alloy of claim 1, comprising one or more of: 0.05 to 0.2 wt. % cobalt, 0.05 to 0.2 wt. % iridium, 0.05 to 0.2 wt. % rhenium, 0.05 to 0.2 wt. % vanadium, 0.03 to 0.1 wt. % niobium, and/or 0.05 to 0.5 wt. % tungsten.

9. The steel alloy of claim 1, comprising from 0.05 to 0.2 wt. % nickel.

10. The steel alloy of claim 1, comprising from 0.05 to 0.4 wt. % copper.

11. The steel alloy of claim 1, comprising from 0.005 to 0.05 wt. % aluminium.

12. The steel alloy of claim 1, comprising: from 0.85 to 0.95 wt. % carbon, from 0.15 to 0.3 wt. % silicon, from 0.5 to 0.8 wt. % manganese, from 2.5 to 2.9 wt. % chromium, from 0.3 to 0.4 wt. % molybdenum, from 0.2 to 0.35 wt. % copper, from 0 to 0.2 wt. % cobalt, from 0 to 0.2 wt. % iridium, from 0 to 0.2 wt. % rhenium, from 0 to 0.2 wt. % vanadium, from 0 to 0.2 wt. % tungsten, from 0 to 0.1 wt. % nickel, from 0 to 0.05 wt. % aluminium, from 0 to 150 ppm nitrogen, and the balance iron, together with any unavoidable impurities.

13. The steel alloy composition of claim 1, further comprising a microstructure including at least one of (i) martensite, bainitic ferrite, (ii) carbides and carbonitrides, and (iii) optionally retained austenite.

14. A bearing component, comprising: a steel alloy having; from 0.8 to 1.0 wt. % carbon, from 0.1 to 0.5 wt. % silicon, from 0.2 to 0.9 wt. % manganese, from 2.0 to 3.3 wt. % chromium, from 0 to 0.4 wt. % molybdenum, from 0 to 0.2 wt. % cobalt, from 0 to 0.2 wt. % iridium, from 0 to 0.2 wt. % rhenium, from 0 to 0.2 wt. % vanadium, from 0 to 0.1 wt. % niobium, from 0 to 0.5 wt. % tungsten, from 0 to 0.2 wt. % nickel, from 0 to 0.4 wt. % copper, from 0 to 0.05 wt. % aluminum, from 0 to 150 ppm nitrogen, and the balance iron, together with any unavoidable impurities, and one of a rolling element, an inner ring or an outer ring for a bearing.

15. The bearing according to claim 14, further comprising a bearing component.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0096] The invention will now be described further, by way of example, with reference to the accompanying non-limiting drawings, in which:

[0097] FIGS. 1a and 1b show a micrograph of a top surface and a cross-section through a wear scar produced on a test coupon made from Steel A, being an example in accordance with the invention.

[0098] FIGS. 2a and 2b show a micrograph of a top surface and a cross-section through a wear scar produced on a steel coupon made from Steel B, being a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

Examples

[0099] A steel with the chemical composition: (wt. %) 0.84C-0.24Si-0.51Mn-2.92Cr-0.28Mo was used in the present work (Steel A). Chemical analysis of a sample made from Steel A revealed the presence of further elements: (wt. %) 0.003P-0.001S-0.01Ni-0.018Cu-0.029Al-0.004As-0.001Sn, as well as trace amounts of Ti, Pb, Ca, Sb and O. The balance is made of iron together with any unavoidable impurities. Steel A is suitable for use in the production of large-size bearing rings and has high hardenability. The expected Ideal Critical Diameter for the composition is 160.3 mm (see C. F. Jatczak: Hardenability in high carbon steels. Metallurgical Transactions Volume 4:2267-2277, 1973).

[0100] As a reference, a known steel with an equivalent level of hardenability was used, having the following composition: (wt. %) 0.96C-0.52Si-0.93Mn-1.86Cr-0.57Mo (Steel B). Chemical analysis of a sample made from Steel B revealed the presence of further elements: (wt. %) 0.003P-0.001S-0.01Ni-0.017Cu-0.029Al-0.003As-0.002Sn, as well as trace amounts of Ti, Pb, Ca, Sb and O. The balance is made of iron together with any unavoidable impurities. The expected Ideal Critical Diameter for the composition is 163.9 mm.

[0101] Steel A and Steel B were prepared in an identical manner. Each composition was vacuum induction melted and cast into ingots of 100 kg each, having a thickness of approx. 80 mm. The ingots were homogenised and then annealed, to soften the material, after which blocks were sectioned from the ingots of Steel A and Steel B. The blocks were then hot rolled to produce plates with a thickness of approx. 20 mm. The plates were heat-treated in an identical manner, using conventional processes such as described above, comprising steps of:

[0102] normalising;

[0103] spheroidising-annealing;

[0104] martensitic hardening;

[0105] tempering.

[0106] Test coupons of each steel were soft-machined from the plates, after the step of spheroidising-annealing. After hardening and tempering, the test coupons were ground and polished and hardness was measured. The measured hardness for Steel A was 61.3 HRC; the measured hardness for Steel B was 62.4 HRC.

[0107] The test coupons were subjected to a fretting wear test, in which a hardened steel ball with a diameter of 12.7 mm was pressed against the test coupon surface, without lubrication, whilst oscillating with tangential micro-displacements of 30 μm. The maximum contact pressure between the ball and the test coupon surface (given by the Hertzian distribution) was fixed at 2 GPa. The test was conducted for 15×103 cycles, at an oscillation frequency of 20 Hz and in ambient conditions, without any deliberate heating or cooling.

[0108] Due to the slightly lower hardness of Steel A, the coupons made from this material experienced a slightly higher tangential friction force of 110 N during the fretting wear test compared with the 106 N experienced by the coupons made of Steel B.

[0109] After the test, the resulting wear scars were lightly polished and acid-etched using a 1.5% Nital solution, to reveal white etching areas. A micrograph of the wear scar produced on the top surface of coupons of Steel A and Steel B is shown in FIG. 1a and FIG. 2a respectively. The direction of fretting motion is indicated by the arrows in each figure. Furthermore, each coupon was sectioned along the wear scar in a direction essentially perpendicular to the fretting motion. A micrograph of the sectioned coupons of Steel A and Steel B is shown in FIG. 1b and FIG. 2b respectively.

[0110] The fretting test simulates rolling contact fatigue (RCF), which is one of the failure modes in bearings. Frequently, this failure mode is accompanied by the formation of white etching matter in damaged zones, driven by microstructural changes or decay of the steel. The RCF damage is typically associated with the initiation of surface or subsurface cracks which propagate in fatigue and eventually lead to flaking of the material from the raceway. White etching areas are generally localised along subsurface fatigue cracks.

[0111] Looking at FIG. 1a and FIG. 2a, it can be seen that the wear scar on both test coupons provides crack formation and white etching matter. However, the white etching matter on the coupon made from Steel A is superficial only. As may be seen from the cut section shown in FIG. 1b, white etching matter is not present in the subsurface of the wear scar.

[0112] This is in contrast with the wear scar produced on the test coupon made of Steel B. As may be seen from FIG. 2b, white etching matter is present in the subsurface. Furthermore, loss of material has occurred, possibly due to flaking. The wear scar on the test coupon made from Steel A exhibits barely any material loss.

[0113] It may therefore by concluded that the steel in accordance with invention has improved fatigue resistance and is less susceptible to white etching damage.

[0114] 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.