Bearing steel

Abstract

A steel alloy for a bearing, the alloy having a composition comprising: (a) from 0.5 to 0.9 wt. % carbon, (b) from 1.2 to 1.8 wt. % silicon, (c) from 1.1 to 1.7 wt. % manganese, (d) from 0.7 to 1.3 wt. % chromium, (e) from 0.05 to 0.6 wt. % molybdenum, and optionally any of: (f1) from 0 to 0.25 wt. % nickel, (f2) from 0 to 0.02 wt. % vanadium, (f3) from 0 to 0.05 wt. % aluminium, (f4) from 0 to 0.3 wt. % copper, (f5) from 0 to 0.5 wt. % cobalt, (f6) from 0 to 0.1 wt. % niobium, (f7) from 0 to 0.1 wt. % tantalum, (f7) from 0 to 150 ppm nitrogen, (f8) from 0 to 50 ppm calcium, and (f9) the balance iron, together with any unavoidable impurities, wherein the steel alloy has a microstructure comprising bainitic ferrite and retained austenite, and a hardness (Vickers) of at least 650 HV.

Claims

1. A method of heat treating a bearing component composed of a steel alloy composition that comprises: 0.6 to 0.7 wt. % carbon, 1.3 to 1.7 wt. % silicon, 1.2 to 1.6 wt. % manganese, 0.8 to 1.2 wt. % chromium, 0.15 to 0.4 wt. % molybdenum, 0.05 to 0.25 wt. % nickel, 0.003 to 0.01 wt. % vanadium, 0.005 to 0.05 wt. % aluminium, 0.05 to 0.3 wt. % copper, 0 to 0.5 wt. % cobalt, 0 to 0.1 wt. % niobium, 0 to 0.1 wt. % tantalum, 0 to 150 ppm nitrogen, and 0 to 50 ppm calcium, the balance being iron and 0.3 wt. % or less of unavoidable impurities, the method comprising: (i) heating the steel alloy composition at a temperature of 865-900 C. for 50-100 minutes to at least partially austenitise the composition; (ii) quenching the steel alloy composition to a temperature of 190-210 C. and holding the steel alloy composition at the temperature of 190-210 C. for 12-36 hours; (iii) isothermally heating the steel alloy composition at a temperature of 200-280 C. until the steel alloy composition has a microstructure that comprises 5 to 10 vol.% retained austenite and at least 80 vol. % bainitic-ferrite and has a Vickers hardness of at least 650 HV; and (iv) subjecting the bearing component having 5 to 10 vol.% retained austenite and at least 80 vol. % bainitic-ferrite to a surface finishing technique.

2. The method of claim 1, wherein the at least 80 vol. % bainitic-ferrite is at least 80 vol. % lower bainite.

3. The method of claim 2, wherein step (i) is performed at 865-880 C. for 50-100 minutes.

4. The method of claim 3, wherein step (ii) is performed for 12-16 hours.

5. The method of claim 4, wherein the steel alloy composition consists of: 0. 65 to 0.7 wt. % carbon, 1.4 to 1.6 wt. % silicon, 1.3 to 1.5 wt. % manganese, 0.9 to 1.1 wt. % chromium, 0.2 to 0.3 wt. % molybdenum, 0.08 to 0.2 wt. % nickel, 0.005 to 0.007 wt. % vanadium, 0.01 to 0.03 wt. % aluminium, 0.1 to 0.2 wt. % copper, 0 to 0.1 wt. % cobalt, 0 to 0.1 wt. % niobium, 0 to 0.1 wt. % tantalum, 0 to 150 ppm nitrogen, and 0 to 50 ppm calcium, the balance being iron and 0.1 wt. % or less unavoidable impurities.

6. The method of claim 1, wherein step (i) is performed at 865-880 C. for 50-100 minutes.

7. The method of claim 1, wherein step (ii) is performed for 12-16 hours.

8. The method of claim 1, wherein the steel alloy composition consists of: 0.65 to 0.7 wt. % carbon, 1.4 to 1.6 wt. % silicon, 1.3 to 1.5 wt. % manganese, 0.9 to 1.1 wt. % chromium, 0.08 to 0.2 wt. % nickel, 0.01 to 0.03 wt. % aluminium, 0.1 to 0.2 wt. % copper, 0 to 0.1 wt. % cobalt, 0 to 0.1 wt. % niobium, 0 to 0.1 wt. % tantalum, 0 to 150 ppm nitrogen, and 0 to 50 ppm calcium, the balance being iron and 0.1 wt. % or less unavoidable impurities.

9. The method of claim 1, wherein the steel alloy composition consists, in wt. %, of 0.67C, 1.53Si, 1.42Mn, 1Cr, 0.12Ni, 0.25Mo, 0.13Cu, 0.006V, and 0.028Al, the balance being iron and unavoidable impurities.

10. The method of claim 9, wherein the at least 80 vol. % bainitic-ferrite is at least 80 vol. % lower bainite.

11. The method of claim 10, wherein step (i) is performed at 865-880 C. for 50-100 minutes.

12. The method of claim 11, wherein step (ii) is performed for 12-16 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described further, by way of example, with reference to the following non-limiting Figures, in which:

(2) FIG. 1 is a plot of change in length against temperature for dilatometer measurements (austenitising temperature 860 C./20 minutes).

(3) FIG. 2 is a plot of change in length against temperature for dilatometer measurements (austenitising temperature 870 C./50 minutes).

(4) FIG. 3 is a plot of change in length against time for dilatometer measurements (austenitising temperature 870 C./50 minutes).

(5) FIG. 4 are electron micrographs showing a fine bainitic microstructure with a small amount (approx. 8 vol. %) of retained austenite.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(6) The present invention will now be described further with reference to the following non-limiting examples.

(7) A steel with the chemical composition 0.67C-1.53Si-1.42Mn-1Cr-0.12Ni-0.25Mo-0.13Cu-0.006V-0.028Al, in wt. %, was used in the present work. The balance is made of iron together with any unavoidable impurities.

(8) After a full-annealing heat treatment to soften the structure for improved machinability, the steel was austenitised at 870 C., in a dilatometer, and soaked at this temperature for 50 minutes. Thereafter, the specimen was gas-quenched, using nitrogen gas, to a temperature of 200 C., and held at this temperature for 72 hours until the bainite transformation has ceased. Finally, the specimen was allowed to cool to room temperature.

(9) Austenitising at 870 C. for 50 min was found to be important to ensure that the martensite start temperature of the austenite matrix, the MS temperature, is depressed sufficiently below the intended bainite transformation temperature. Initially, holding a similar specimen at 860 C. for 20 minutes resulted in the experimentally measured MS temperature of about 200 C., upon quenching (see FIG. 1). In contrast, as shown in FIG. 2, for the sample austenitised at 870 C., no dilatation (circled), which signifies the martensite transformation, was observed in the measured dilatometer curve prior to the bainite transformation. The bainite transformation stage may be seen more clearly in FIG. 3.

(10) FIG. 4 shows a typically fine bainitic structure that was obtained according to the specified heat treatment on the alloy of this example. X-ray measurements showed the presence of only approximately 8 vol. % retained austenite.

(11) The very fine bainitic structure results in very high toughness and hardness. The low level of retained austenite in the bainitic-hardened structure results in improved dimensional stability.

(12) Hardness measurements performed after the heat treatments gave a hardness of 681 HV (average of 3 measurements). This is approximately 50 HV higher than previous heat-treated alloys.

(13) This difference in hardness equates to an increase of about 2 HRC (Rockwell Hardness). The alloy's hardness of at least 59 HRC exceeds the minimum required 58 HRC for bearing applications.

(14) 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.