WO1999005340A1

WO1999005340A1 - Case hardening of steels

Info

Publication number: WO1999005340A1
Application number: PCT/GB1998/002120
Authority: WO
Inventors: Jeffrey Kinder; Andrew Dood; Andrew Higgins; Hiroshi Narai
Original assignee: Nsk-Rhp European Technology Co. Limited
Priority date: 1997-07-21
Filing date: 1998-07-17
Publication date: 1999-02-04
Also published as: AU8351898A

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 HARDENING OF STEELS

TECHNICAL FIELD

The present invention relates to case hardening of steels

More particularly, the invention relates to nitrogen surface hardening of stainless

steel vvorkpieces. 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

Λ.s 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

DISCLOSURE OF THE INVENTION

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 to 1200T in hardening stainless steel aπicles 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

BRIEF DESCRIPTION OF THE DRAWINGS

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

DESCRIPTION OF BEST MODES OF CARRYING OUT 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 N':. N₂ and H₂. NF . 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 1200°C.

Using temperatures in this range means that nitrogen will diffuse through the

austenite phase. The austenite phase has a face centred cubic (fee.) crystal structure.

Nitrogen occupies octahedral interstitial sites in the lattice. As these octahedral sites are

larger in a f e e. 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 450°C. and

b) a Pyrowear 675 steel sample, carburised, and tempered in an inert

atmosphere at 465^ϋC

The pitting potentials of each treated sample at a current density of 100A are shown in Table 1

Table

ARTICLE PITTING POTENTIAL /mV

Nitrided Pyrowear 675

Carburised Pvrowear 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 lOOO 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

1 1 A stainless steel workpiece or article with characteristics substantially as

represented in the accompanying drawings

12 An article according to claim 10 or 1 1, 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% chromium.