GB2328953A

GB2328953A - A process for hardening high alloy steels

Info

Publication number: GB2328953A
Application number: GB9719033A
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: 1997-09-08
Filing date: 1997-09-08
Publication date: 1999-03-10
Anticipated expiration: 2017-09-08
Also published as: GB2328953B; GB9719033D0

Abstract

The method comprises the steps of preoxidising a steel article followed by two stage carburising and carbonitriding at a temperature between 700‹C and 1000‹C and then hardening (e.g. by oil quench). At least one diffusion treatment may be made after carburising and carbonitriding but before hardening (e.g. two 1 ¢ hour treatments at 1100‹C). The method may be applied to a stainless steel article containing 4-13 wt % cobalt, the article being a bearing element or gear tooth.

Description

IMPROVED STEEL HARDENING PROCESS The present invention relates to case hardening of steel. More particularly the invention relates to hardening high alloy steel, particularly carburisable stainless steels, by carbonitriding.

It is known in the art of hardening steels to subject steels, including some stainless steels, to iurising treatments. Such treatments are austenitic thermo-chemical processes whereby the steel is initially treated by exposing it to a carbon-containing atmosphere within a certain temperature range. This temperature range is typically 800-900"C, although both higher and lower temperatures may be employed. During the treatment the carbon diffuses into the steel which is then quenched to transform the austenite phase to martensite to provide sufficient case hardness.

It is also known to harden plain carbon and low-alloy steels by a process known as carbonitriding. By carbonitriding it is meant an austenitic thermo-chemical treatment as detailed above wherein a small amount of nitrogen is introduced to the carbon atmosphere and is taken up by the steel simultaneously with the carbon. Carbonitriding produces steels with greater hardness and resistance to wear than carburising. Steel case hardened by carbonitriding is also less prone to uneven case hardness which gives rise to so-called "soft spots" upon quenching, than steel hardened by carburising.

The improved behaviour exhibited by plain carbon and low alloy steels when carbonitrided rather than carburised has led to demands for stainless and other high alloy steels with similarly enhanced properties, particularly in the aerospace industry where hardened stainless steels are extensively used for bearing and gear applications.

These stainless steels are presently carburised as part of their hardening process. The length of time a steel workpiece must spend in the carburising furnace to acquire sufficient hardenability and sufficient case depth for such applications makes carburising an expensive exercise, as the temperature inside the fumace must be maintained at 800" - 900"C throughout the cycle.

According to the invention there is provided a method of producing a hardened high alloy steel, such as Carpenter Technology's Pyrowear 675 steel, for gear and bearing applications said method comprising: i) preoxidising the steel article, ii) subjecting the steel article to a two-stage carburising and carbonitriding treatment at a temperature between 700"C and 1000 C, and iii) hardening the steel article.

The invention also provides a method of hardening stainless and other high alloy steels, particularly carburisable stainless steels, and more particularly carburisable stainless steels containing at least 4 wt.% cobalt, which comprises a carbonitriding step thereby imparting to the steel the advantages of improved hardenability and wear resistance, and a uniform, hard surface layer upon quenching.

A filrther advantage of the present invention is that the process delivers a steel of comparable hardness to a carburised steel in a significantly shorter time. This further advantage is surprising and unexpected in view of the fact that nitrogen, with its greater bulk, has a lower rate ofdiffision through iron than does carbon.

This reduction in the duration of the heat treatment also makes the process of the instant invention more energy efficient than previously known processes.

According to another aspect of the invention there is provided a hardened steel article produced in accordance with the above method.

The invention will now be further described with reference to the accompanying drawings in which.

Figure 1 shows the hardness profile of a hardened steel, Pyrowear 675, produced in accordance with the invention against that of the same steel given a standard carburising treatment, and Figure 2 shows the pitting corrosion potentials of the hardened steel produced in accordance with the invention against those of the carburised steel.

A stainless steel containing at least 4 wt% cobalt, preferably 4-13 wt% cobalt is pretreated to promote its susceptibility to active carbon and placed in a conventional carbonitriding furnace and exposed to an atmosphere of gas selected from the group consisting of CO2, CO, CO2 and H2, CO and H2, C, lower alkanes excluding C, or a combination of these. Preferably the gas comprises CO2 or CH4.

The furnace is heated to a temperature of between 700"C-1000"C. Preferably the temperature is about 900"C.

The steel is carburised for up to 50 hours. Oxygen free nitrogen containing gas, conveniently ammonia, is introduced into the furnace towards the end of the heat treating cycle. Both the length of the cycle and the length of exposure to nitrogen containing gas will be dependent on the depth of case and hardenability required.

The exact process parameters for a given case depth and hardness can be selected by the skilled without undue experimentation but a typical treatment cycle suitable for treating Pyrowear 675 is given below.

TYPICAL TREATMENT CYCLE i. pre-oxidise the steel for 1 hour at 900"C; ii. carburise for 20 hours at 900"C; iii. add 4% by volume ammonia and carbonitride for 4 hours, iv. subject to two difflisions treatments each of 11/2 hours at 1100 C; v. harden from 1050 C by quenching in oil followed by sub zero treatment for 30 minutes at -70 C, and vi. temper twice for 2 hours at 465"C in an inert atmosphere.

The carbon potential is typically maintained at 0.95% during steps ii) and iii).

The steel treated in accordance with the instant invention can be given a case of comparable depth and hardness to that of a carburised steel in a significantly shorter period of time.

Figure 1 shows the hardness profile of Pyrowear 675 steel treated in accordance with the invention including a 24 hour carbonitriding stage conducted at 9000C.

It can be seen that from its surface to a case depth of 2mm the steel piece exhibits a similar hardness to a hardened steel piece subjected to a process involving a 48 hour carburising treatment at a higher temperature viz. 925"C.

Steel articles made in accordance with the invention will of course have the advantage of a superior corrosion resistance to carburised steel, as is evidenced by Figure 2.

That steel of good hardness, good resistance to wear, and uniform case for use in, for example, bearing and gear applications and particularly bearing and gear applications in the aircraft manufacture industry may be produced in a shorter time not only allows steel producers to make cost savings, but also to meet increasing demand for such steel in an energy efficient manner Those persons skilled in the art will have no difficulty in making adjustments and alterations to the described process without departing from the scope of the invention

Claims

Claims 1. A method of producing a hardened high alloy steel article for use in bearing and gear applications the method comprising the steps of i) preoxiding the steel article, ii) subjecting the steel article to a two-stage carburising and carbonitriding treatment at a temperature between 700"C and 1000"C, and iii) hardening the steel article.
2. A method according to claim 1, wherein the steel article is subjected to at least one diffusion treatment between steps ii) and iii)
3. A method according to claim 1 or claim 2, wherein the article is hardened by quenching in oil.
4. A method according to claims 1, 2 or 3, wherein the high alloy steel is a stainless steel containing at least 4wt% cobalt.
5. A method according to claim 4, wherein the stainless steel contains ;13 wt% cobalt.
6. A method of producing a hardened high alloy steel article substantially as hereinbefore described and with properties according to any one or more Figures of the accompanying drawings.
7. A high alloy steel article produced in accordance with the method of any one of the preceding claims.
8. A high alloy steel article according to claim 7, wherein the article is a bearing element.
9. A high alloy steel article according to claim 7, wherein the article is a gear tooth.
10. A high alloy steel article substantially as hereinbefore described with proportions according to one or more of the Figures of the accompanying drawings.