CASE-HARDENABLE STAINLESS STEEL ALLOY

Abstract

A steel alloy for a bearing, the alloy having a composition having from 0.04 to 0.1 wt. % carbon, from 10.5 to 13 wt. % chromium, from 1.5 to 3.75 wt. % molybdenum, from 0.3 to 1.2 wt. % vanadium, from 0.3 to 2.0 wt .% nickel, from 6 to 9 wt. % cobalt, from 0.05 to 0.4 wt. % silicon, from 0.2 to 0.8 wt. % manganese, from 0.02 to 0.06 wt. % niobium, from 0 to 2.5 wt. % copper, from 0 to 0.1 wt. % aluminium, from 0 to 250 ppm nitrogen, from 0 to 30 ppm boron, and the balance iron, together with any unavoidable impurities.

Claims

1. A steel alloy for a bearing, the alloy having a composition comprising: from 0.04 to 0.1 wt. % carbon, from 10.5 to 13 wt. % chromium, from 1.5 to 3.75 wt. % molybdenum, from 0.3 to 1.2 wt. % vanadium, from 0.3 to 2.0 wt. % nickel from 6 to 9 wt. % cobalt, from 0.05 to 0.4 wt. % silicon, from 0.2 to 0.8 wt. % manganese, from 0.02 to 0.06 wt. % niobium, from 0 to 2.5 wt. % copper, from 0 to 0.1 wt. % aluminium, from 0 to 250 ppm nitrogen, from 0 to 30 ppm boron, and the balance iron, together with any unavoidable impurities.

2. The steel alloy of claim 1, comprising from 0.05 to 0.09 wt. % carbon.

3. The steel alloy of claim 1, comprising from 10.7 to 12.7 wt. % chromium, more preferably from 11 to 12.5 wt. % chromium.

4. The steel alloy of claim 1, comprising from 1.65 to 3.6 wt. % molybdenum.

5. The steel alloy of claim 1, comprising from 0.4 to 1.1 wt. % vanadium.

6. The steel alloy of claim 1, comprising at least 0.65 wt. % vanadium.

7. The steel alloy of claim 1, comprising from 0.3 to 1.9 wt. % nickel.

8. The steel alloy of claim 1, comprising from 6.5 to 7.7 wt. % cobalt.

9. The steel alloy of claim 1, comprising from 0.05 to 0.3 wt. % silicon.

10. The steel alloy of claim 1, comprising from 0.3 to 0.7 wt. % manganese.

11. The steel alloy of claim 1, comprising from 0.02 to 0.04 wt. % niobium.

12. A bearing component made from the steel alloy of claim 1, wherein a surface of the bearing component is case-carburised and/or carbonitrided.

13. A bearing comprising a bearing component according to claim 12, the bearing component being formed by at least one of an inner ring, an outer ring, or a rolling element of the bearing.

14. The bearing of claim 13, wherein the bearing component is an inner ring and an outer ring, and the bearing further comprises rolling elements made of a ceramic material.

15. The steel alloy of claim 2, comprising from 0.06 to 0.08 wt. % carbon.

16. The steel alloy of claim 15, comprising 0.07 wt. % carbon.

17. The steel alloy of claim 3, comprising from 11 to 12.5 wt. % chromium.

18. The steel alloy of claim 5, comprising from 0.5 to 1.1 wt. % vanadium.

Description

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLES

[0079] The chemical composition of a number of non-limiting examples of stainless steel alloys according to the invention is given in Table 1.

TABLE-US-00001 TABLE 1 The chemical composition of five stainless steels according to the invention. All quantities are in wt. %. The balance is iron, together with any unavoidable impurities. Element Cr Ni PRE N C Cr Mo V Ni Co Si Mn Nb eq. eq. core Example 0.08 12.1 2.5 0.5 1.0 7.2 0.16 0.47 0.03 18.7 10.8 20.4 1* Example 2 0.07 11.0 3.5 0.5 1.0 7.25 0.15 0.45 0.04 19.1 10.6 22.6 Example 3 0.07 12.0 2.5 0.5 1.8 6.5 0.15 0.45 0.02 18.6 10.6 20.3 Example 4 0.05 11.0 3.5 0.5 0.5 8.5 0.15 0.45 0.03 19.1 10.9 22.6 Example 5 0.07 11.0 1.8 1.0 0.5 8.0 0.2 0.65 0.03 19.0 10.9 16.9 *In Example 1, the alloy further contains 0.026 wt. % Al, 0.02 wt. % N and <0.005 wt. % Cu.

[0080] The stainless steels according to the present invention are designed for the manufacture of bearing components, particularly bearing rings, which are subjected to case-hardening. The case hardening may be carried out via carburizing, carbonitriding or a combination of both for larger case depths, preferably at reduced pressure (less than the atmospheric pressure), and usually after a suitable preoxidation step. For example, clean bearing components may be heated in air at 875 to 1050 C. for 1 hour, followed by air cooling. Carburising may be conducted at a temperature in the range of 870 to 950 C. in a carbon-containing medium. Such carburising treatments are conventional in the art and ensure sufficient carbon-enrichment in the carburised case such that there is adequate Ms (of the austenite) between the core and the case. This, in turn, ensures the development of a beneficial compressive residual stress profile through the thickness of the bearing component's hardened case and towards the core.

[0081] After case-carburising, or carbonitriding, or the combination of both, the bearing components are typically heat-treated and tempered. After the first temper, the parts may be deep-frozen at near liquid nitrogen temperature then re-tempered. Again, such treatments are conventional in the art.

[0082] The heat treatment consists of austenitisation at, for example, about 1100 C., followed by an oil or gas quench. Tempering can be double or, if necessary, even triple-tempering or more, with sub-zero treatments in-between the temper steps.

[0083] FIGS. 1a and 1b respectively show a phase diagram for a steel alloy with a composition in accordance with Examples 4 and 5 from Table 1. As can be seen, the formation of the -ferrite phase is avoided during soaking at 1200 C. FIG. 2 shows a micrograph of a steel alloy with a composition in accordance with Example 1 from Table 1, whereby the alloy was hot-rolled and homogenised. As may be seen, the alloy exhibits a fine grain structure and only small amounts (i.e. <10%) of delta-ferrite (dark grey) are present after homogenisation at 1150 C. for 24 hours. The measured grain size of the delta-ferrite is an average 28 m, with a standard deviation of 16 m. The predominant phase is martensite.

[0084] As noted above, to avoid excessive austenite grain growth during case-carburising or heat treatment, a small amount of Nb from 0.02-0.06 wt. % is added. Unlike known compositions, the addition of niobium results in the precipitation of niobium-rich precipitates, which are effective in refining austenite grains during high temperature processes. The phase diagrams of FIGS. 1a and 1b show that Nb-rich precipitates are formed.

[0085] Furthermore, niobium, in conjunction with the addition of vanadium, facilitates the precipitation of vanadium-rich precipitates which serve the same purpose as niobium-rich precipitates. The phase diagram for the steel alloy of example 5 (FIG. 1b) shows that both Nb-rich and V-rich precipitates are formed. The presence of two different types of precipitates is expected to enhance the austenite grain refinement, thereby resulting in a stronger steel.

[0086] In comparison, FIG. 1 c shows a phase diagram for a steel alloy with a composition similar to Pyrowear 675 stainless, comprising: 13 wt. % Cr; 1.8 wt. % Mo, 0.8 wt. % V, 2.6 wt. % Ni, 5.4 wt. % Co, 0.4 wt. % Si and 0.65 wt. % Mn and Fe. The composition differs from standard Pyrowear 675 stainless by having a higher vanadium content (the standard composition comprises 0.6 wt. % vanadium). As may be seen, soaking at 1200 C. leads to the formation of some of the undesirable delta-ferrite phase. There is barely any formation of V-rich precipitates.

[0087] Steel alloy compositions of the invention also exhibit superior hardness. FIG. 3 shows a graph of the results from a Vickers hardness test according to ISO 6507-1 performed on Samples A and Samples B prepared from stainless steel alloys with a composition according to the invention and on reference samples made from Pyrowear 675. The composition of the samples was as follows:

TABLE-US-00002 C Si Mn Mo Ni V Co Nb Samples A 0.054 0.16 0.47 11.19 3.46 0.51 7.18 0.033 Samples B 0.050 0.21 0.68 11.45 1.82 1.01 8.06 0.034 Reference 0.070 0.40 0.65 13.0 2.6 0.6 5.4 Samples

[0088] Quantities in wt. %. The balance is iron, together with any unavoidable impurities.

[0089] The steel alloys used to prepare all samples were heat-treated in the same manner: [0090] Low-pressure carburization at a temperature of 890-980 C.; [0091] Austenitization at a temperature of 950-1150 C., followed by quenching; [0092] Tempering at a temperature of 450-550 C.; [0093] Sub-zero cooling to a temperature below 120 C.; [0094] Tempering two more times at a temperature of 450-550 C.

[0095] In the graph of FIG. 3, the upper line 301 represents the Vickers hardness measured for Samples A; the middle line 302 represents the Vickers hardness measured for Samples B and the lower line 303 represents the Vickers hardness measured for the reference samples. As may be seen, the samples made from stainless alloys of the invention have a higher case hardness than the references samples.

[0096] The stainless steel alloys of the invention may be produced by, for example, a double vacuum melting VIM-VAR process, by a VIM-ESR process, by a powder metallurgy (PM) process route, or by spray-forming. Furthermore, if deliberately high nitrogen in the substrate alloy composition is desired, the VIM or P-ESR processes may then be used.

[0097] In addition, the core alloy, by virtue of being low in carbon, may also be 3D printed. These are also conventional manufacturing techniques. The Al content is reduced to trace level and preferably kept to a minimum in the PM or the spray-formed alloy variant.

[0098] For the VIM-VAR variant, the Al concentration can be in the range of 0.01 to 0.03 wt. %. The N concentration can be in the range of 30 to 60 ppm. Both elements help in pinning austenite grain boundaries in the form of aluminium nitride precipitates, thus ensuring a finer-grained structure that is beneficial for demanding bearing applications.

[0099] The forging process of the steel articles is controlled such that the grain sizes are sufficiently fine for the subsequent carburising process not to result in the formation of excessively large grain boundary carbides. For example, the grain sizes may typically range from 30 to 65 m.

[0100] For exceptional resistance to rolling contact fatigue, the case-hardened and tempered bearing components may be followed by surface nitriding or boriding, for example, to further increase the surface hardness of the bearing components. This is particularly applicable to the surface hardness of bearing raceways. Thus, in a preferred embodiment, once a surface of the bearing component has been case-carburised, the surface may be subjected to a surface nitriding treatment to further improve the mechanical properties of the surface layer.

[0101] The steel alloy or bearing component may be subjected to a surface finishing technique. For example, burnishing, especially for raceways, followed by, if necessary, tempering and air-cooling. Afterwards, the steel alloy or bearing component may be finished by means of hard-turning and/or finishing operations such as, for example, grinding, lapping and honing.

[0102] The burnishing and tempering operations may cause the yield strength of the affected areas to increase with significant improvement in hardness, compressive residual stress and better resistance to rolling contact fatigue.

[0103] 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.