METHOD OF HEAT TREATING A STEEL COMPONENT

Abstract

The present invention provides a method of heat treating a steel component including the sequential steps of: (a) carbonitriding the steel component; (b) quenching the steel component; (c) optionally tempering the steel component; and (d) ferritically nitrocarburizing the steel component.

Claims

1. A method of heat treating a steel component comprising the sequential steps of: a) carbonitriding the steel component; b) quenching the steel component; c) optionally tempering the steel component; and d) ferritically nitrocarburizing the steel component.

2. The method of claim 1, wherein carbonitriding the steel component in step (a) comprises carbonitriding the steel component for 5 to 30 hours.

3. The method of claim 1, wherein quenching the steel component in step (b) comprises quenching the steel component in a salt or oil bath.

4. The method of claim 1, wherein tempering the steel component in step (c) comprises tempering the steel component at a temperature of about 150 to about 620° C.

5. The method of claim 1, wherein ferritically nitrocarburizing the steel component in step (d) comprises ferritically nitrocarburizing the steel component at a temperature of about 580° C. or less.

6. The method of claim 1, wherein the method further comprises either: a step of furnace cooling (e.1) the steel component after step (d); or a step of further quenching (e.2) the steel component after step (d) and then subsequently tempering the steel component.

7. The method of claim 1, wherein the method further comprises a step (x) of machining the steel component between steps (c) and (d) and/or a step (y) of tumbling the steel component after step (d), step (e.1) or step (e.2).

8. The method of claim 1, wherein the steel component has a carbon content of 0.10 to 1.20 wt. %, based on the total weight of the steel component, preferably wherein the steel component comprises a 100Cr6 steel or a 100CrMo7-3 steel.

9. The method of claim 1, wherein the steel component comprises or constitutes a rolling element or roller, or a steel component for an application in which the steel component is subjected to alternating Hertzian stresses.

10. A heat-treated steel component treated by the method of heat treating a steel component having the sequential steps of: a) carbonitriding the steel component; b) quenching the steel component; c) optionally tempering the steel component; and d) ferritically nitrocarburizing the steel component, preferably wherein the heat-treated steel component comprises or constitutes a rolling element or roller, or an inner or outer ring of a bearing.

11. The method of claim 2, wherein quenching the steel component in step (b) comprises quenching the steel component in a salt or oil bath.

12. The method of claim 2, wherein tempering the steel component in step (c) comprises tempering the steel component at a temperature of about 150 to about 620° C.

13. The method of claim 2, wherein ferritically nitrocarburizing the steel component in step (d) comprises ferritically nitrocarburizing the steel component at a temperature of about 580° C. or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] At least one of the embodiments of the present invention is accurately represented by this application's drawings which are relied on to illustrate such embodiment(s) to scale and the drawings are relied on to illustrate the relative size, proportions, and positioning of the individual components of the present invention accurately relative to each other and relative to the overall embodiment(s). Those of ordinary skill in the art will appreciate from this disclosure that the present invention is not limited to the scaled drawings and that the illustrated proportions, scale, and relative positioning can be varied without departing from the scope of the present invention as set forth in the broadest descriptions set forth in any portion of the originally filed specification and/or drawings. The invention will now be described in relation to the following non-limiting drawings in which:

[0039] FIG. 1 shows a method according to an embodiment of the invention;

[0040] FIG. 2 shows a method according to an embodiment of the invention; and

[0041] FIG. 3 shows a steel component according to an embodiment of the invention.

[0042] FIG. 4 is a plot showing the change (in micrometres) of different parts of a steel component due to nitrocarburizing treatment.

[0043] It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] Those of ordinary skill in the art will appreciate from this disclosure that when a range is provided such as (for example) an angle/distance/number/weight/volume/spacing being between one (1 of the appropriate unit) and ten (10 of the appropriate units) that specific support is provided by the specification to identify any number within the range as being disclosed for use with a preferred embodiment. For example, the recitation of a percentage of copper between one percent (1%) and twenty percent (20%) provides specific support for a preferred embodiment having two point three percent (2.3%) copper even if not separately listed herein and thus provides support for claiming a preferred embodiment having two point three percent (2.3%) copper. By way of an additional example, a recitation in the claims and/or in portions of an element moving along an arcuate path by at least twenty (20°) degrees, provides specific literal support for any angle greater than twenty (20°) degrees, such as twenty-three (23°) degrees, thirty (30°) degrees, thirty-three-point five (33.5°) degrees, forty-five (45°) degrees, fifty-two (52°) degrees, or the like and thus provides support for claiming a preferred embodiment with the element moving along the arcuate path thirty-three-point five (33.5°) degrees. FIG. 1 shows a heat treatment cycle according to an embodiment of the present invention, on a plot of temperature (T) vs. time (t). Temperature (T) being on the y-axis and time (t) being on the x-axis. The method comprises subjecting a steel component to a CN process as described herein (step (a)), followed by a quenching process as described herein (step (b)), followed by an optional tempering process as described herein (step (c)), followed by a FN process as described herein (step (d)), optionally followed by a cooling step (step (e.1) or (e.2), preferably (e.1)).

[0045] FIG. 2 shows a flow chart of a heat treatment cycle according to an embodiment of the present invention. The method comprises subjecting a steel component to a CN process as described herein (step (a)), followed by a quenching process as described herein (step (b)), followed by an optional tempering process as described herein (step (c)), optionally followed by a machining process (step (x)), then followed by a FN process as described herein (step (d)), optionally followed by a cooling step (step (e.1) or (e.2), preferably (e.1)) which is then optionally further followed by a further tumbling process (step (y)).

[0046] FIG. 3 shows an example of a steel component according to an embodiment of the invention, namely a rolling element bearing 34 that may range in size from 10 mm diameter to a few metres in diameter and have a load-carrying capacity from a few tens of grams to many thousands of tonnes. The bearing 34 according to the present invention may namely be of any size and have any load-carrying capacity. The bearing 34 has an inner ring 36 and an outer ring 38 and a set of rolling elements 40. The inner ring 36, the outer ring 38 and/or the rolling elements 40 of the rolling element bearing 34, and preferably at least part of the surface of all of the rolling contact parts of the rolling element bearing 40 may be subjected to a method according to the present invention.

[0047] The invention will now be described in relation to the following non-limiting examples.

[0048] FIG. 4 is a plot showing the change (in micrometres) of different parts of a steel component due to nitrocarburizing treatment, i.e. the amount of distortion of the steel component due to the heat treatment. The steel components were rings for bearings. The steel grade used for the steel components was ISO 683-17 steel designation 100CrMo7-3. Tests A, B and C involved a tempering step (c) being performed at a temperature of 160° C. Tests D, E and F involved a tempering step (c) being performed at a temperature of 200° C. Test G involved a tempering step (c) being performed at a temperature of 620° C. Tests A and D involved an FN step (d) being performed at 510° C. for 5.5 hours including an additional post-oxidation treatment. Tests B and E involved an FN step (d) being performed at 620° C. in N.sub.2 followed by a temperature of 580° C. and a standard Corr-i-Dur (registered trademark) process. Tests C, F and G involved an FN step (d) being performed at 580° C. including an additional standard Corr-i-Dur (registered trademark) process.

[0049] The plot shows the change in different measurements of different steel components after the nitrocarburizing process is performed. Measurements D1 and D2 are bore diameters, D3 is the shoulder diameter, and Measure 4 is the difference between the average of D2 and the average of D1 (i.e. (average D2)—(average D1)). The ring has a conical bore and therefore Measure 4 is the inclination of the bore. The numbers on the plot indicate each component tested.

[0050] Advantageously, the amount of distortion due to the nitrocarburizing processes is within tolerance. However, Tests A, C, D and F show even less distortion than Tests B and E, which involve a higher nitrocarburizing temperature of 620° C. Moreover, Test G, which involves a tempering step (c) at 620° C., shows very little distortion in the bore diameters, although larger shoulder diameter distortion.

[0051] 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.