GB2328448A

GB2328448A - Case hardening of steels containing cobalt and chromium

Info

Publication number: GB2328448A
Application number: GB9726071A
Authority: GB
Inventors: Jeffrey Kinder; Andrew Dodd; Andrew Higgins; Hiroshi Narai
Original assignee: NSK European Technology Co Ltd; RHP Bearings Ltd
Current assignee: NSK European Technology Co Ltd; RHP Bearings Ltd
Priority date: 1997-07-21
Filing date: 1997-12-09
Publication date: 1999-02-24
Also published as: GB9715345D0; GB9726071D0

Abstract

A method of hardening a workpiece made from stainless steel containing 4 to 10% by weight of cobalt and 9-17% by weight of chromium by subjecting the workpiece to an austenitic nitriding treatment with gaseous atmosphere at a temperature of 1000‹C to 1200‹C and a pressure of 1kPA to 200kPA for a relatively long time. Thereafter the workpiece is quenched and then exposed to a low temperature typically -65‹C or lower before a final tempering treatment.

Description

Case Hanlenine of Steels The present invention relates to case hardening of steels.

More particularly, the invention relates to nitrogen surface hardening of stainless steel workpieces, i.e., nitriding stainless steel. Stainless steel is a high alloy steel containing a minimum of 9% chromium and which may contain other materials for example, cobalt, nickel or molybdenum.

It is known in the art to subject steel to a nitriding treatment to improve the hardness of the steel. However, such treatment normally only generates a very shallow hardened layer.

As an alternative, certain stainless steels may be hardened by carburising wherein the stainless steel to be hardened is subjected to active carbon at high temperatures and then quenched to precipitate a phase change in the carbon-containing layer. A drawback of carburising stainless steel is that the carbon binds to the chromium as discrete chromium carbides, reducing the matrix chromium content to levels below that necessary for effective corrosion resistance. Consequently, as steel users demand steel with ever better performance manufacturers find it increasingly difficult to supply the requirements of certain industries for hard, corrosion resistant stainless steel.

It is an object of this invention to provide stainless steel with good resistance to corrosion, high surface hardness and good core toughness for bearing and gear applications in, for example, the aircraft industry.

In one aspect the invention provides the use of austenitic nitriding treatment in a gaseous atmosphere containing active nitrogen at a pressure of less than 200kPa at a temperature in the range 1000 C to 1200"C in hardening stainless steel articles with a significant amount of cobalt and a minimum of 9% chromium.

By austenitic nitriding treatment it is meant an austenitic thermochemical treatment, by the addition of the interstitial nitrogen into the austenite phase by diffusion and subsequent transformation of the austenite into martensite to produce a high surface hardness.

In another aspect the invention provides a method of hardening a workpiece made from stainless steel containing cobalt and 9 - 17 % by weight of chromium, said method comprising; subjecting the workpiece to an austenitic nitriding treatment, at a temperature greater than 700 "C, a preferred temperature range being 1000"C - 1200 C; and thereafter exposing the workpiece to a low temperature below 0 C.

The invention also provides stainless steel workpieces or articles, hardened in accordance with the invention.

The invention will now be further described with reference to the accompanying drawings, in which: Figure 1 shows the hardness profile of stainless steel hardened in accordance with a preferred method of the present invention, and Figure 2 shows the pitting corrosion potentials of a carburised steel workpiece, and one nitrided in accordance with the method of the invention.

A stainless steel article or workpiece which is to be hardened is first pre-treated non - chemically (e.g. by grinding) to provide a surface suitable for accepting active nitrogen and then placed in a furnace capable of sustaining a positive pressure of nitrogen at temperatures up to 1200"C. Inside the furnace the workpiece is exposed to a suitable gaseous atmosphere.

Preferably, the atmosphere comprises oxygen free N2. The gas can be selected from the group consisting of N2, N2 and H2, NH3, or a mixture of these.

The pressure of the nitriding atmosphere is set at a value of between 1 kPa and 200 kPa.

The nitrogen atmosphere is then subjected to elevated temperatures preferably of about 1000 C to 12000C.

Using temperatures in this range means that nitrogen will diffuse through the austenite phase. The austenite phase has a face centred cubic (f.c.c.) crystal structure.

Nitrogen occupies octahedral interstitial sites in the lattice. As these octahedral sites are larger in a f.c.c. lattice arrangement than in a body centred cubic lattice arrangement, i.e.

the crystal structure of the ferrite phase, nitrogen diffuses better at the temperatures employed in the invention than in the lower, ferrite phase, temperatures previously used when nitriding. Thus a greater case depth of nitrided steel per unit time is available by using the method of the present invention.

Typically the workpiece is nitrided in this manner for up to 50 hours but as with all other process parameters the skilled technician will select the conditions suitable to achieve the desired case depth and hardness.

Following the austenitic nitriding step the workpiece is preferably quenched, for example in oil, and is exposed to a sub-zero temperature. After the sub-zero treatment the workpiece is allowed to reach room temperature prior to tempering in a pressurised nitrogen atmosphere at a temperature of typically 450"C. The sub - zero temperature treatment involves exposing the workpiece to a temperature below 0 C, typically - 65"C or lower, for a period of time, the duration of which is determined by the size of the workpiece. During the sub - zero treatment the stability of the retained austenite phase in the workpiece is reduced, so that it is more readily transformed to the martensite phase.

To obtain a more complete transformation the operation may be repeated. The sub - zero temperature treatment can be carried out in various ways, for example, the workpiece can be exposed to gaseous or liquid nitrogen.

Suitable stainless steels to which the method may be applied include those containing significant amounts of cobalt and chromium, i.e. 4-10% Co and 9 - 17% Cr, such as Carpenter Technology's Pyrowear 675. This material exhibits good hardness when subjected to the above nitriding process, as can be seen in Fig. 1.

Fig. 1 shows the hardness/depth profile of a sample piece of Pyrowear 675 after being treated in according with the invention over a 24 hour period. The hardness was tested using the Vickers method with a 0.3 kgf (2.9N) loading. The nitrided steel articles of the invention show surprising resistance to corrosion as demonstrated in Fig. 2 which shows the pitting corrosion potentials of a) a Pyrowear 675 steel sample, treated in accordance with the invention and tempered in a pressurised nitrogen atmosphere at 4500C, and b) a Pyrowear 675 steel sample, carburised, and tempered in an inert atmosphere at 465"C The pitting potentials of each treated sample at a current density of 100A are shown in Table 1.

Table 1

ARTICLE PITTING POTENTIAL /mV Nitrided Pyrowear 675 30 Carburised Pyrowear 675 -350 Clearly the present invention is capable of satisfying the hitherto irreconcilable advantages of good hardness and corrosion resistance in a stainless steel.

Those skilled in the art will be able to envisage further embodiments of the present invention and practice the same without departing from the scope of the invention.

Claims

1. A method of hardening a workpiece made from stainless steel containing cobalt and 9 - 17 % by weight of chromium, said method comprising; subjecting the workpiece to an austenitic nitriding treatment, at a temperature greater than 700"C; and thereafter exposing the workpiece to low temperature below 0 C.

2. A method according to claim 1, wherein the nitriding temperature is within the range 1000"C to 1200"C.

3. A method according to claim 1 or 2, wherein the nitriding treatment involves exposing the workpiece to a gaseous mixture containing active nitrogen at a pressure of up to 200 kPa.

4. A method according to claim 3, wherein the nitriding pressure is within the range of 1 kPa to 200 kPa.

5. A method according to any one of the preceding claims, wherein gaseous or liquid nitrogen is used to expose the workpiece to the low temperature.

6. A method according to any one of the preceding claims, wherein the workpiece is quenched following the austenitic nitriding step.

7. A method according to any one of the preceding claims, wherein the workpiece is tempered in a pressurised nitrogen atmosphere following the sub - zero temperature treatment.

8. A method according to any one of the preceding claims wherein the stainless steel to be hardened comprises 4 to 10 wt% cobalt.

9. A method substantially as hereinbefore described.

10. A stainless steel workpiece or article hardened in accordance with the method of any one or more of the preceding claims.

11. A stainless steel workpiece or article with characteristics substantially as represented in the accompanying drawings.

12. An article according to claim 10 or 11, wherein the article is a gear tooth.

13. An article according to claim 10 or 11, wherein the article is a rolling bearing element.

14. The use of austenitic nitriding treatment in a gaseous atmosphere containing active nitrogen at a pressure of less than 200kPa and at a temperature in the range 1000"C to 1200"C in hardening stainless steel articles with a significant amount of cobalt and a minimum of 9 /O chromium.

Amendments to the claims have been filed as follows 1. A method of hardening a workpiece made from stainless steel containing cobalt and 9 - 17 % by weight of chromium, said method comprising, subjecting the workpiece to an austenitic nitriding treatment, at a temperature greater than 700 C; and thereafter exposing the workpiece to low temperature below 0 C.

2. A method according to claim l, wherein the nitriding temperature is within the range 1000"C to 1200"C 3. A method according to claim 1 or 2, wherein the nitriding treatment involves exposing the workpiece to a gaseous mixture containing active nitrogen at a pressure of up to 200 kPa.

9. A method according to claim 1 and substantially as hereinbefore described.