Nitrided Nickel-Base Article and lon-Nitriding Method Field of the Invention The present invention relates generally to the ion nitriding art, and is more particularly concerned with a new ion nitriding method and with case-hardened nickel-base alloy articles of unigue hardness and wearability and resistance to corrosion and oxidation.

Cross Reference This invention is related to my invention disclosed and claimed in allowed patent application Serial No. 07/796,635 filed November 11, 1991 and entitled Steel Article and Method, from which a patent is soon to be issued covering stainless steel articles of unique hardness and wearability produced by removing surface portion to a depth of about 0.001 inch and ion nitriding it to depth of about 0.003 inch.

Background of the Invention There has long been general recognition that there were important advantages to be gained if nickel-base alloys could be case-hardened like some other alloys. All attempts in that direction involving established nitriding practice have, however, been unsuccessful. Thus hardness and wear resistance were not substantially enhanced and oxidation and corrosion resistance were not improved.

Summary of the Invention In accordance with the present invention, based upon the discoveries and new concepts set out below, case-

hardened nickel-base alloy articles of unique hardness and wear resistance and resistance to corrosion and oxidation can be consistently produced. The method for producing these novel articles is likewise new in the art in respect to the steps and the series of steps resulting in the novel mechanical and corrosion resistance properties. Thus, as the method of this invention is carried out to full extent, the end product nickel-base alloy article is unigue in respect both to mechanical properties and corrosion resistance properties and is gualified well above nickel- base alloy articles known heretofore for purposes and applications involving jet engines.

A major discovery of mine is that it is possible under certain circumstances to case harden nickel-base alloys, not by nitriding as previously attempted to no avail, but by ion nitriding in a special manner.

Another major discovery underlying this invention is that the condition of the surface of a nickel-base alloy article is critically important. Specifically, I have found that the surface must be such that ion nitriding bombardment results in uniform penetration of the article to a depth of the order of at least 0.0004 inch.

Additionally, I have found that such a receptive surface can consistently be provided by precision machining, that is, light stress machining the article just prior to the ion nitriding step. Particularly good results are obtained when such machining is done to a depth of between about 0.004 to 0.006 inch and following a heat

treatment step to relieve residual cold work stresses in the article.

Briefly described, the method of this invention comprises steps of providing a nickel-base alloy article of approximate desired size and shape and precision machining it to ultimate desired size and shape, then subjecting the resulting cold worked article to ion bombardment and electrical glow discharge under an atmosphere of hydrogen and nitrogen at pressure between about one and seven Torr and elevated temperature until the article is nitrided substantially uniform to depth between about 0.0004 and 0.0015 inch.

A novel article this invention is the case-hardened nickel-base alloy product having an unique combination of superior hardness and wearability and resistance to corrosion oxidation because of a nitride surface portion composed of a primary phase of uniform depth from about 0.0004 to 0.0015 inch.

Brief Description of the Drawings Those skilled in the art will gain a further and better understanding of this invention from the drawings accompanying and forming a part of this specification, in which

Figure 1 is a flow diagram of the process of this invention; and

Figure 2 is an enlarged fragmentary view of an article of this invention showing the ion nitrided surface region.

Detailed Description of the Invention In presently preferred practice of this invention, a nickel-base alloy article of approximate desired size and shape is precision machined to ultimate desired size and shape. This light stress machining operation is followed by an ion nitriding step, an optional intervening heat treating operation being omitted in the best present practice. As indicated above, because of the condition of the lightly machined surface of article 10, penetration of the ion bombardment indicted at 13 is substantially deeper than that achieved in accordance with prior practice involving the ion nitriding of more heavily cold worked nickel-base alloy bodies. As a consequence, ion nitrided article 10 has hardness and wear resistance properties much superior to those of the best of the prior art practice and also has resistance to corrosion and oxidation and conseguently fully satisfies jet engine manufacturer reguirements.

In regard to the materials aspect of this invention, it will be understood that new results and advantages of the invention can be consistently gained and obtained with nickel-base alloys generally which are subject to case hardening treatment of the ion nitriding type conducted in the manner of the practice described herein and variations thereof not presently preferred but within the scope of the present invention and the appended claims. As indicated above, however, the nickel-base alloys of choice are those used generally in providing jet engine components, those

being the products facing special hardness, wear resistance and corrosion and oxidation resistance requirements. Such alloys include widely known and used In-713, In-718, In-738 of International Nickel Company and Rene 80 and Rene 95 of General Electric Company and a number and variety of other nickel-base alloys of generally similar compositions, properties and uses.

In regard to the ion nitriding process of this invention, in the initial stage it will be carried out at a temperature in the range of 800 to 1200° F in a hydrogen- nitrogen atmosphere in proportion of one to ten parts of hydrogen to one part of nitrogen. The pressure of the hydrogen-nitrogen atmosphere will be about 1-7 Torr and the electric potential will be in the range of 400 to 600 volts DC. The depth of ion penetration will be from 0 0004 - 0.0015 inch and the time reguired depends upon the prevailing conditions, but will be long enough to provide the reguired ion penetration depth.

The following illustrative, but not limiting, example of the preferred practice of this invention as it has actually been carried out in production operations will serve to further instruct those skilled in the art concerning the details of the several steps of this new process and the unigue characteristics of the resulting products.

Example Bar stock of Inconel™ 718 nickel-base alloy product of International Nickel Company was cut and rough machined

to provide a jet engine shaft bushing of approximate size and shape of the desired ultimate article, this alloy being of the following nominal composition:

Elements %

Cr 19.00

Fe 18.50

Cb 5.13

Ta

Mo 3.05

Ti 0.90

Si 0.18

N 0.04

C 0.04

Cu 0.15

Mn 0.18

S 0.008

Ni Balance

After annealing to relieve residual cold work stress, the article was subjected to precision machining to bring it to ultimate desired size and shape within specified tolerances, this operation involving only light work much diminished from that of the rough machining step and resulting in removal of metal to depth of about 0.005 inch. The precision-machined article was then subjected to depassivation treatment by heating for one hour from initial temperature of 300°F to final temperature of 975- 1000°F under a 5-7 Torr atmosphere of pure hydrogen. Voltage applied throughout the period was in the range of

400-500DC and current was 15-25 amperes. Immediately thereafter the resulting depassivated article was subjected in the same treatment vessel to ion nitriding for 36 hours during which a 75% hydrogen and 25% nitrogen atmosphere of one Torr was maintained. The temperature in the ion nitriding chamber throughout the operation was 975-1000°F while electrical glow discharge and ion bombardment continued under DC potential of 400 to 600 volts and current from 15-25 amperes. At the end of this 37-hour period, the article was removed from the chamber and cooled to room temperature and examined and tested. Ion nitriding depth was found to be uniformly approx. 0.0004 inch. Hardness and wear resistance proved to be surprisingly improved over the same alloy articles in non-ion nitrided condition. Rockwell hardness in fact, was measured at 65- 75, an increase of fully ten percent. Corrosion and oxidation resistance 100-hours tests were then carried out at 1200° F in the air, in salt fog at room temperature, and at 1200° F in salt fog. In no instance did visually detectable corrosion or oxidation occur. Thus, instead of being diminished by the superficial conditioning treatment, the corrosion and oxidation resistance properties were actually markedly enhanced.

Having thus described this invention so that others skilled in the art can understand and practice the same, I state that while I desire to secure by Letters Patent as set forth in what is claimed.