US3783039A - Surface depleted nitrided materials - Google Patents

Abstract

GRADED NITRIDED ARTICLES, SURFACE MODIFIED IN ALLOY COMPOSITIONS WHEREIN THE SURFACE ZONE CONSISTS OF NITRIDED ALLOYS CONSISTING ESSENTIALLY OF (A) ONE OR MORE METALS OF THE GROUP COLUMBIUM, TATALUM, AND VANADIUM; (B) TITANIUM; AND (C) ONE OR BOTH METALS OF THE GROUP MOLYBDENUM AND TUNGSTEN, A MINOR PORTION OF THE NITROGEN MAY BE REPLACED BY OXYGEN OR BORON. NITRIDED MATERIALS PREPARED FROM HOMOGENEOUS ALLOYS ARE ALSO INCLUDED. THE MATERIALS ARE CHARACTERIZED BY EXCELLENT WEAR AND ABRASION RESISTANCE.

Description

United States Patent ABSTRACT OF THE DISCLOSURE Graded nitrided articles, surface modified in alloy compositions wherein the surface Zone consists of nitrided alloys consisting essentially of (A) one or more metals of the group columbium, tantalum, and vanadium; (B) titanium; and (C) one or both metals of the group molybdenum and tungsten. A minor portion of the nitrogen may be replaced by oxygen or boron. Nitrided materials prepared from homogeneous alloys are also included. The materials are characterized by excellent wear and abrasion resistance.

CROSS REFERENCE TO RELATED APPLICATION This application is a division of our pending application Ser. No. 16,595, filed Mar. 4, 1970, now US. Pat. 3,674,-

574 and which in'turn is a continuation-in-part of our 7 pending application, Ser. No. 755,658, entitled Wear Resistant Materials filed Aug. 27, 1968, now US. Pat. 3,549,427.

BACKGROUND OF THE INVENTION In our parent application, SerialNo. 755,658, referenced above, we have disclosed and claimed certain nitrided alloys consisting essentially of (a) at least one metal of the group columbium, tantalum and vanadium;

(b) titanium; and

(c) at least one metal of the group molybdenum and tungsten Y in certain percentages by weight and compositional relationships as are therein set forth. Such nitrided materials are characterized by, among others, excellent wear and abrasion resistance and oifer substantial utility as cutting tool materials.

In such parent application, we have noted that the desired alloys to be nitrided may be formed as free standing thin sections or clad or by various means formed as a coating upon different substrates. Similarly, in such parent application, we have noted that a variety of nitriding treatments maybe employed to eifectuate the desired results.

In the present application, we wish to elaborate upon the teachings of said parent application. The compositions hereof which are nitrided or otherwise treated are the same as the alloy compositions which are disclosed in our parent application.

Accordingly, our parent application, Ser. No. 755,658, now US. Pat. 3,549,427, in its entirety, is incorporated herein by reference We would note that a counterpart of such parent application has issued as Belgium Pat. 720,398. As will be evident, we herein provide additional features to said basic invention and certain improvements thereof.

In our parent application, the temperatures are presented uncorrected. In the present application, temperatures are corrected. We used a correction factor deterice mined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the optical pyro'meter mentioned in the parent case.

Furthermore, we would note that it is well known that titanium can be nitrided to form a hard surface layer thereon but such material shows a chipping propensity due to brittleness. In the practice of our invention, such brittleness is avoided by specific alloying as taught herein prior to nitriding. Additionally, the alloying elements present in typical commercially available titanium alloys do not produce the same improvement and nitrided commercial titanium alloys show chipping similar to nitrided titanium.

'Ihe nitriding of titanium-rich alloys, i.e. containing about 90 percent titanium has been studied previously (for example, see E. Mitchell and P. J. Brotherton, J. Institute of Metals, vol. 93 (1964), p. 381). Others have investigated the nitriding of hafnium-base alloys (F. Holtz et al., US. Air Force Report IR-718-7 (II) (1967); molybdenum alloys (US. Pat. 3,161,949); and

tungsten alloys (D. J. Iden and L. Himmel, Acta Met.,

vol. 17 (1969), p. 1483). The treatment of tantalum and certain unspecified tantalum base alloys with air or nitrogen or oxygen is disclosed in US. Pat. 2,170,844 and the nitriding of columbium is discussed in the paper by R. P. Elliott and S. Komjathy, AIME Metallurgical Society Conference, vol. 10, 1961, p. 367.

In the present application, we wish to clearly point out the significance of alloying surface treatments or coatings or claddings with the present materials and surface treatments wherein nitriding is employed as the major constituent along with relatively minor amounts of oxygen and/or boron.

It should be noted that the alloys of the present invention may be employed on another metal or alloy as a service coating or cladding and with the proper substrate selection, a highly ductile and/or essentially unreacted substrate can be obtained. For example, columbium or tantalum are much less reactive to nitrogen when used in conjunction with the alloys hereof and tungsten and molybdenum do not form stable nitrides at the nitriding temperatures employed. Spraying and/or fusing the desired alloy onto the surface are included in the various coating methods available. A variety of direct deposition methods may be employed or alternate layers could be deposited followed by a diffusion annealing treatment.

As set out in our parent application in determining Whether or not a material falls within the scope thereof, certain test criteria were used as are set forth therein. More particularly, following nitrided sample preparation lathe turning tests were run thereon at surface speeds from to 750 surface feet per minute (s.f.m.) on AISI 4340 steel having a hardness of around Rockwell C, (R,,), 43 to 45. A feed rate of 0.005 in./rev. and depth of cut of 0.050 in. were used. A standard negative rake tool holder was employed with a 5 back rake and a 15 side cutting edge angle. Tool wear was measured after removing a given amount of material.

The principal criterion in our parent application in determining whether the nitrided materials pass or fail and thus whether or not they are included or excluded from the scope thereof was the ability to cut 2 cubic inch metal removal of the 4340 steel at speeds of both 100 and 750 s.f.m.

At 750 s.f.m. our high performance, nitrided materials readily pass the initial test of 2 cu. in. metal removal in about 1 minute. (We would note that by s.f.m. is meant the linear rate at which the material being cut passes the cutter.)

In some aspects of the present invention, such test criteria of the parent application are inoperative. This is particularly true of the thin sections and surface zones considered herein. The nitrided alloys are the same but in some instances in thin sections the test criteria of the parent case are not met herein. However, the materials still offer substantial wear and abrasion resistant properties.

In evaluating tools and tool materials, failure is often assumed to occur When the wearland reaches 0.030 inch. With the materials of this invention, we selected a rather severe test-we indicate those which are good (i.e., pass the test), when at 750 s.f.m. and 2 cu. in. removal, there is a uniform wearland of less than 0.025 in. Furthermore, we would not that although chipping is seen in some compositions upon testing at 750 s.f.m. the chipping propensit'y is aggravated at lower speeds and better assessed at 100 s.f.m. The latter is one of the reasons for selecting both speeds.

Accordingly, a principal object of our invention is to provide certain novel articles wherein the surface zone thereof is a nitrided alloy consisting essentially of: (a) at least one metal of the group columbium, tantalum and vanadium; (b) titanium; and (c) at least one metal of the group molybdenum and tungsten.

' Another object of our invention is to provide said novel articles aforesaid wherein up to three percent of the titanium content is replaced by zirconium.

A further object of our invention is to provide such nitrided articles wherein the nitrogen pickup is at least 0.1 milligram per square centimeter of surface area.

Still a further object of our invention is to provide such nitrided articles wherein up to twenty-five percent of the nitrogen weight pickup is replaced by oxygen and/o boron.

These and other objects, features and advantages of our invention will become apparent to those skilled in this art from the following detailed disclosure thereof.

DESCRIPTION OF THE INVENTION An alloy of the composition Cb-20V-4OTi-10Mo was readily reduced to foil by rolling and coatings thereof were made on molybdenum by fusing this alloy in argon at a temperature of about 3375 F. for a time of two minutes. The coating wet the substrate well, did not flow excessively, and did not seriously react with the molybdenum. A specimen with a 22 mil coating was nitrided at 2950 F. for two hours and showed a microhard'ness grading and structure similar to the nitride material in bulk form. The microhardness at a depth of /2, 1, and 2 mils was 2190, 1600, and 1365 DPN, respectively. A coating of similar thickness was produced on tungsten by dipping tungsten stock into molten Cb-18Ti-18W alloy.

A 3 mil coating of Cb-20V-40Ti-10Mo was also produced on molybdenum by fusing in argon. This was subsequently nitrided at 225 F. for one-half hour resulting in a nitrogen weight pickup of 1.6 mg. per sq. cm. The microhardness at a depth of /s mil from the surface was 1680 DPN. The nitriding temperature is sufficiently low that such alloys may be coated on a variety of substrate materials including ferrous alloys and successfully nitrided to produce a hard surface.

Much thinner coatings are readily produced by similar or other procedures. As the reactive alloy coating becomes thinner, the amount of nitrogen pickup for surface hardem'ng is reduced since the nitriding is concentrated near the surface. Accordingly, in such thin sections the depth of hardening is reduced. In relatively thin coatings, the weight pickup of nitrogen may be 0.1 to 1 mg. per sq. cm. or less and in thicker coatings the pickup will be over 1 mg. per sq. cm. of surface area.

In our copending, referenced parent application, we have shown that for noncoated homogeneous alloy stock the amount of nitriding required for equivalent surface hardening is dependent upon sample thickness. As the thickness is decreased, the required nitriding temperature and weight pickup are reduced. We have observed a pronounced effect of specimen thickness, particularly at knife edges where the required nitrogen pickup is greatly reduced. Also, such coated or homogeneous materials may be used for a wide variety of applications requiring wear and abrasion resistance Where the requirement for surface hardness or depth of hardening may be less than that required for metal cutting. Accordingly, in thin sections of homogeneous alloy material, similar to thin coatings of the alloys, the weight pickup of nitrogen may be 0. 1to1mg.persq.cm. a

Anotheruseful method for utilizing our nitrided materials involves controlled evaporation of titanium from the surface of an alloy (detitanizing). By this procedure, an alloy, for example, with 'a titanium content greater than that determined by our compositional limitations, can be depleted in titanium content to bring the surface alloy content within our prescribed ranges prior to nitriding. We have heated various alloys containing-the required metals of our invention in vacuo at temperatures below the melting point of the alloy. Titanium evaporation occurred without any substantial change in geometry. Most importantly, this was accomplished without the occurrence of significant amounts of porosity. Electron microprobe analyses confirmed the significant changes ,in weight that had been observed. A specimen of Cb -45Ti- IOMo vacuum treated at a pressure of 5X10 torr' at 2850 F. for four hours showed a. decrease in titanium content and a corresponding increase in columbium and molybdenum content. The decrease in titanium content extended to a considerable depth and in the outer 2 mils the decrease was about 10 percent. Other vacuum treatments run at 2950? F. for six hours showed even greater titanium loss. A Cb-45Ti-20W alloy vacuum treated at 2850 F. for four hours lost 33 mg. for a x x ,4: inch specimen weighing 1.9 gram, and a similar, size sample of Cb-SOTi-ZOW vacuum treated at 2950" F. for 6 hours lost 60 mg.

Similar detitanizing effects were shown for Ta-Ti-Mo alloys wherein substantial weight losses of titanium were observed without geometry changes or the development of vsignificant amounts. of porosity. Ta-40Ti-10Mo, initially 2.4 g., vacuum treated .at 2950 F..for 6 hours .lost 54 mg. for a x x sample. Upon nitriding at 3250* F. for 2 hours,'this' material cut at'both 750 and s.f.m. All such 'vacuum treated materials show high surface hardness. It will, of course, be appreciated that such surface evaporation techniques can be applied to alloys that are already within our prescribed composition ranges to effect desirable structural and property changes.

The. cutting performance of such Cb-40Ti-10Mo vacuurn treated at 2850 F. for 4-hours prior to nitriding at 3250 F. was better than the same alloy when nitrided without prior detitanizing. It should be noted that-annealing per se, that is, annealing under conditions where significant evaporation does not occur, has an efiect on. the microstructural morphology. Such morphology effects due to annealing, which result in greater regularity of structure may produce improvements for certainuses, but the compositionel-effect due to treatment in vacuo is of value by itself.

Since our nitrided materials present as ahomogeneous material or as a coated article are in a thermodynamically metastable condition, those skilledin the art will realize that a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction structure and resultingproperties whether performed as part the over-all nitriding reaction or as separate treatments. Improvement in cutting properties has been noted by nitriding at lower temperatures for longer times and by nitriding at lower temperatures followed by nitriding at higher temperatures. However, the required weight pickup for cutting at 750 SFM is similar to the amount of-nitriding necessary with a simple-2-hour nitriding treatment. The treatments have included typical nitriding followed by aging at lower temperatures in argon or nitrogen. We have also nitride-d at higher temperatures (and longer times) that normally would produce some embrittlement and then subsequently annealed in inert gas or at various partial pressures of nitrogen as a tempering or drawing operation to improve toughness. This duplex treatment results in a greater reaction depth with the hardness-toughness relationship controlled by the tempering temperature and time.

Such treatments can be employed to modify the properties of our nitrided materials to produce various combinations of hardness and toughness. The required annealing treatment is dependent upon the material usage, alloy composition and degree of prior nitriding.

The influence of annealing under various conditions for a variety of nitrided materials may be seen from the data presented in Table I.

We have also nitrided materials directly in an environment sufliciently low in nitrogen potential that the effect is noted. Nitriding in flowing A0.l% N produces reduced nitrogen pick-up compared to 100% nitrogen. Another method involves sealing the furnace with a measured amount of nitrogen and allowing the nitrogen con tent to be reduced during treatment as a result of the specimens absorbing the available nitrogen. For example, Cb-30Ti-10Mo was reacted in an atmosphere starting with 0.45% N balance argon and ending with 0.03% N A specimen treated in this manner out well at both 750 and 100 s.f.m. The alloy Cb-8 0Ti-10Mo falling outside our invention, was nitrided in A0.l% N for 2 hours at 3050 F. Similar to treating in nitrogen, the result was a thick continuous 3 mil nitride surface layer and such material fails immediately in testing at 750 TABLE I Nitriding Argon All treatment treatment Microhardness (DPN) at depth ofcomposition F. Hrs. F. Hrs. 0.5 mil 1' mil 2 mils 4 mils 8 mils Cb-17Ii-20W 2 None 2, 570 2, 090 1, 890 1, 140 906 Cb-l7Ti-20W 2 3, 450 1 220 1, 017 1, 040 857 Cb-17Ti-20W 2 3, 450 l 2, 190 1, 420 1, 250 835 765 Cb-17Ti-20W 2 3, 250 2 3, 060 2, 600 2, 570 2, 160 985 Ta-20Ti-l0M0 2 None 2, 060 1, 675 1, 480 1, 110 Ta-2OTi-10M0 2 3, 250 1 1, 690 175 1, 250 946 Ta-20Tl-10Mo 3, 550 2 3, 250 4 1, 790 1, 160 996 1, 060

Argon=0.1 percent nitrogen atmosphere.

The alloy Cb-l7Ti-20W, nitrided at 3450" F. for 2 hours shows substantial softening when subsequently annealed in argon for 2 hours at this same temperature. If the anealing is carried out in an atmosphere of A--0.1% N it may be noted that only a moderate decrease in hardness occurs and the material grades uniformly in a manner similar to the nitrided condition. 'If annealed at 3250 F. for 2 hours in argon the material hardens significantly. The influence of annealing in argon on reducing the uniform hardness gradient for the nitrided Ta-20Ti-10Mo alloy may also be seen from the above data. We have found that nitrided alloys containing higher amounts of tungsten or molybdenum soften readily when annealed in argon. To control this softening, that is, avoiding the formation of a surface-layer that is too soft to cut the hardened steel at 750 s.f.m., we have found regulation of the nitrogen content of the atmosphere to be a useful parameter. It should be noted that the A0.l% N atmospherewill harden unnitrided or moderately nitrided alloys but results in softening when used with the highly nitrided alloys in the examples above. A x X A; inch specimen of Cb-Ti-20W reacted in nitrogen at 3250 F. for 2 hours, cuts well at 750 s.f.m. When subsequently treated in A0.l% N for 2 hours, this material continues to nitride as evidenced by a further 8 mg. pick-up.

A number of our materials have been nitrided and subsequently annealed. Although the nitrided alloy passed our cutting test criteria at 750 and 100 s.f.m., improvement was achieved by nitriding at 3250 F. for 2 hours followed by annealing in argon at 3250 F. for one hour. Also, good combined performance at-750 and 100 s.f.m. was shown for Cb-30Ti-20Wnitrided at 3550 F. for 2 hours and annealed at 3550 F. for 1 hour. Annealing at 3250 F. for one hour did not produce any significant improvement and annealing for 4 hours at 3550 F. resulted in failure in cutting at 750 s.f.m. Thus, one should use due care in annealing conditions.

In most of our materials, the hardness (and nitride content) grades and lessens as one moves from the surface inwardly. However, we would note that in some cases such grading extends from a plateau or from a peak hardnesss slightly below the surface and grades inwardly therefrom. Such materials can be effective cutting tools or abrasion resistant articles.

s.f.m. These various alternate nitriding treatments may be applied to the materials of our invention whether used as a homogeneous alloy or as a coated or surface modified material. In all of the nitriding treatments and particularly for those involving reducing nitrogen potential, the effect of the varying stabilities of the metal nitrides must be considered since this can also contribute to surface compositional effects.

Surface alloying techniques are also useful for the preparation of the alloys to be nitrided to produce the materials of our invention. Cb-10Mo was titanized at 2950 F. for 3 hours in vacuo by holding in a pack of fine titanium sponge which causes difiusion of titanium into the surface. This treatment resulted in a 6 mil titanized layer which upon nitriding for 2 hours at 3250 F. yielded a graded reaction zone similar to Cb-Ti-Mo materials. This contrasts with the 4 mil continuous nitride layer formed on Cb-lOMo without the prior titanizing treatinent which exhibits cracking of the continuous nitride ayer.

In the present invention, as in the invention disclosed and claimed in our copending parent application, when one wishes to determine whether or not the material is useful in the nitrided state for purposes hereof certain compositional ratios and formulae must be employed in some cases. Such formulae represent linear proportionate amounts based on weight percentages.

A modest mathematical statement is required. In the present disclosure and claims, the following ratios shall have the following meanings:

Ratlo A =m When, in the present alloy systems, more than 1 metal of the group columbium, tantalum and vanadium is present the maximum total content, in terms of weight percent of such metals must be equal to or less than the total of I 85(Ratio A)+88(Ratio B)+90(Ratio C) and the minimum content thereof when tungsten and/or molybdenum are present must be equal to or greater than the total of [(Ratio A)+(Ratio B)] [(Rati0 E)'+ (Ratio D)]+(Ratio 0) Furthermore, when there is more than 1 metal ofthe group columbium, tantalum and vanadium present ,the maximum amount of titanium permited in the alloy system is equal to or less than the amount determined by the formula I 45(Ratio A+Ratio C)+35(Ratio B) and the ratio of the content of such metals to the titanium must be greater than the ratio determined by Ratio A+Ratio B+O.66(Ratio c) :1

Additionally, when both tungsten and molybdenum are 1 1 present the maximum amount thereof is determined by the formula 60(Ratio A+Ratio C)(Ratio D)+50(Ratio B) (Ratio D)+80(Ratio B) We would further note that when columbium alone is used of Group A metals and both molybdenum and tungsten are present the minium amount of columbium required is determined by the formula 10(Ratio E)+20(Ratio D) Microhardness (DPN) At depth (mils) 0.5

A strip specimen 72 mils thick was prepared using the same titanizing and nitriding procedures and was subsequently bent 45. Cracking of the hard nitrided'case occured on the tension (outer) side. The adherency of the hard nitrided 6 mil zone was shown by the fact that none of it spalled from the Ta-10W substrate which was intact.

Another surface alloying procedure involved the combined titanizing and vanadizing of molybdenum or tungsten. This can be accomplished by vacuum pack treatment since titanium and vanadium have similar vapor pressures. Such treatment of molybdenum or tungsten at 2950 F. for 3 hours yields a thinner diffusion zone than that observed for the titanizing of Cb-lSMo. The depth of the diffusion zone was about 1 /2 mil with molybdenum and less with tungsten. After nitriding at 3250 F. for 2 hours the microhardness of the molybdenum sample was 1000, 605, and 190 DPN at 0.5, 1, and 2 mils, respectively.

Use of surface alloying or coating techniques can enhance the utility of powder processing of the alloys prior to nitriding in a number of ways. For example, a powder processed alloy of Cb-Mo could be formed and then titanized or a porous molybdenum or tungsten presintered compact could be infiltrated by coating methods, These and other techniques can (1) lower sintering temperatures, (2) enhance filling of pores, and (3) reduce shrinkage as compared to making a homogeneous powder part.

We have modified our nitrided material by combining nitriding with oxidizing or boronizing. However, the amount of reaction with such other hardening agents must be limited, a majority of the weight pick-up is due to nitriding, and these are essentially nitrided materials. The alloys may be preoxidized at a temperature where little reaction would occur with nitrogen alone and then subsequently nitrided. Also, the alloys may be reacted with a combined oxidizing and nitriding environment al though the relative oxidizing potential must be low since for example in air the alloys will preferentially oxidize rather than nitride. A sample. of Cb-30Ti-2OW was nitrided at 3250 F. for 2 hours and subsequently boronized at 2650 F. for 4 hours. The structural features of such a material are very similar to the alloy only nitrided; the hardness grades inwardly-and of the total weight pick-up over is due to nitriding. A smooth surface layer about 0.4 mil thick forms due to the boronizing treatment that is harder than the nitrided surface.

For comparison, the Cb-30Ti-20W alloy nitrided at 3250" F. for 2 hours exhibits a microhardness of 2680 DPN at a distance of 6 mil from the surface. After the subsequent boronizing treatment, the hardness was 4550 DPN at the same depth. This duplex treated material passes our test at 750 and SFM but the chipping propensity is increased. Up to 25% of the nitrogen pick-up by weight may be replaced by oxygen and/or boron.

Although the alloys receptive to nitriding can be produced by coating or surface alloying techniques, many uses involve the forming and machining of a homogeneous alloy or a coated article. One of the advantages in utility of these materials is our ability to form the metallic alloys by cold or hot working and/or to machine (or hone) to shape in the relatively soft condition prior to final nitriding. Only minimal distortion occurs during nitriding and replication of the starting shape and surface finish is excellent. The final surface is reproducible and is controlled by original surface condition, alloy composition, and nitriding treatment. For some applications, the utility would be enhanced by lapping, polishing, or other finishing operations after nitriding. The nitrided surface is quite hard but only a small amount of material removal is required to produce a highly finished surface.

One of the nitrided elfects that we have noticed is an accentuation of sharp edges. Similar to the established technology'for aluminum oxide ceramic insert tools, we have blunted sharp cutting edges prior to nitriding. This has been accomplished by simple tumbling prior to nitriding or by'finishing subsequent to nitriding. High speed cutting performance will not be degraded if such edge preparation is limited. The nitrided material can be used as a mechanically locked insert or it can be bonded or joined by brazing, for example, to a substrate.

.We have also observed the excellent corrosion resistance of both the alloys and the nitrided alloys in strong acids, and these materials could efiectively be employed for applications requiring both corrosion and abrasion resistance. Both the alloys and the nitrided alloys possess good structural strength. Thus, the materials can be employed for applications involving wear resistance and structural properties (hardness, strength, stiffness, toughness) at room and elevated temperatures. Other useful properties of the nitrided materials include good electrical and thermal conductivity, high melting temperature, and thermal shock resistance.

Theexcellent cutting properties and wear resistance of the nitrided materials can be elfectively employed with the other useful properties of the alloys and nitrided ma terials to produce a wide range of products. Some of these are: single point cutting tools, multiple point cutting tools (including rotary burrs, files, routers and saws),

drills, taps, punches, dies for extrusion, drawing, and other forming operations, armor, gun barrel liners, impeller of fan blades, EDP (Electrical Discharge Machining) electrodes, spinnerets, guides (thread, wire, and other), knives, razor blades, scrapers, slitters, shears, forming rolls, grinding media, pulverizing hammers and rolls, capstans, needles, gages (thread, plug, and ring), bearings and bushings, pivots, nozzles, cylinder liners, tire studs, pump parts, mechanical seals such as rotary seals and valve components, engine components, brake plates, screens, feed screws, sprockets and chains, specialized electrical contacts, fluid protection tubes, crucibles, molds and casting dies, and a variety of parts used in corrosionabrasion environments in the paper-making or petrochemical industries, for example.

It will be understood that various modifications and variations may be affected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. Graded, nitrided ternary or higher refractory alloy material consisting essentially of: at least one metal selected from each of the Groups A, B, and C wherein Group A consists of columbium, tantalum and vanadium; Group B is titanium and Group C consists of molybdenum and tungsten and wherein:

(a) the nitrogen pick-up ranges from 0.1 to less than 1.0 milligram per square centimeter of surface area, and the surface of the material is depleted in titanium to the desired composition;

(b) when only columbium and molybdenum are present with titanium the range for the columbium content is from about 20 percent to 85 percent;

(c) when ,only columbium and tungsten are present with titanium the range for the columbium content is from about 10 percent to 85 percent;

((1) when only columbium, molybdenum and tungsten are present with titanium the minimum amount of columbium required is determined by the formula 10(Ratio E)+20(Ratio D) and the maximum content of columbium is about 85 percent;

(e) when only tantalum and molybdenum are present with titanium the range for the tantalum content is from about 25 percent to 88 percent;

(if) when only tantalum and tungsten are present with titanium the range for the tantalum content is about 10 percent to 88 percent;

(g) when only tantalum, molybdenum and tungsten are present with titanium the minimum amount of tantalum required is determined by the formula 10(Ratio E) +25 (Ratio D) and the maximum content of tantalum is about 88 percent;

(h) when only vanadium and a metal selected from the group consisting of molybdenum and tungsten and combinations thereof are present with titanium the range for the vanadium content is about 15 percent to 90 percent;

(i) when more than one metal of the group columbium, tantalum and vanadium are present with only molybdenum and titanium the minimum total content of the metals columbium, tantalum and vanadium must be at least equal to the amount of 20(Ratio A)+25(Ratio B) +15(Ratio C) (j) when more than one metal of the group columbium, tantalum and vanadium are present with only tungsten and titanium, the minimum total content of the metals columbium, tantalum and vanadium must be at least equal to the amount of 10(Ratio A)+10(Ratio B)+l5(Ratio C) (k) when more than one metal of the group columbium, tantalum and vanadium are present with mo- 10 lybdenum, tungsten and titanium, the minimum total content of the metals columbium, tantalum and vanadium must be at least equal to the amount of [(Ratio A) (Ratio B)] [10(Ratio E) +25(Ratio D)]+l5(Ratio C) (I) when more than one metal of the group columbium, tantalum and vanadium are present the maximum total content thereof must be equal to or less than (Rati0 A)+88(Ratio B)+90(Ratio C) (m) when titanium is present with only columbium and a metal selected from the group molybdenum and tungsten and combinations thereof, the titanium content ranges from about 1 percent to 45 percent and the columbium to titanium ratio is greater than 1;

(u) when titanium is present with only tantalum and a metal selected from the group molybdenum and tungsten and combinations thereof, the titanium content ranges from about 1 percent to 35 percent and the tantalum to titanium ratio is greater than 1;

(0) when titanium is present only with vanadium and a metal selected from the group molybdenum and tungsten and combinations thereof, the titanium content ranges from about 1 percent to 45 percent and the vanadium to titanium ratio is greater than 0.66;

(p) when titanium is present with more than one metal of the group columbium, tantalum and vanadium and a metal selected from the group molybdenum and tungsten and combinations thereof, the maximum content of titanium must be equal to or less than 45 (Ratio A+Ratio C) +35(Ratio B) and the ratio of the content of the metals columbium, tantalum and vanadium to titanium must be equal to or greater than the ratio of (Ratio A) +(Ratio B) +0.66(Ratio C) :1

and the minimum titanium content is 1 percent;

(q) when only molybdenum, titanium and a metal selected from the group columbium and vanadium and combinations thereof are present, the range for molybdenum content is from about 2 percent to 60 percent;

(r) when only molybdenum, titanium and tantalum are present the range of the molybdenum content is from about 2 percent to 50 percent;

(s) when only tungsten, titanium and a metal selected from the group columbium, tantalum and vanadium and combinations thereof are present the range for tungsten content is from about 2 percent to 80 percent;

(t) when molybdenum, tungsten, titanium and a metal selected from the group columbium, tantalum, vanadium and combinations thereof are present the maximum total content of molybdenum and tungsten must be equal to or less than 60(Ratio A-l-Ratio C) (Ratio D)+ 50(Ratio B)(Ratio D)+80(Ratio E) and the minimum content of molybdenum and tungsten is 2 percent; (u) and wherein in the foregoing (v) said depleted titanium surface content being produced prior to nitriding by treating the refractory alloy material in a vacuum at a temperature below the melting point of the alloy for a time suflicient to produce the desired depletion in titanium content and depth of depletion.

2. The material as defined in claim 1 wherein up to 3% of the titanium content is replaced by zirconium.

References Cited UNITED STATES PATENTS 12 12/1964 Lenning et al. 2- 75-474 12/1964 Douglass et al. 14834 3/1965 Wlodek et al. 75-174 10/1969 Wood 148-31.5 7/1972 Hill et al 1483l.5

OTHER REFERENCES CHARLES N. LOVELL, Primary Examiner Berger et a1 75-177 15 48-2 .3

US. Cl. X.R.