Composite Nitrided Materials

Abstract

Graded nitrided articles, surface modified in alloy composition 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.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 16,595 filed March 4, 1970, now U.S. Pat. No. 3,674,547, which is in turn a continuation-in-part of application, Ser. No. 755,658, entitled "WEAR RESISTANT MATERIALS" filed Aug. 27, 1968 now U.S. Pat. No. 3,549,427.

Description

BACKGROUND OF THE INVENTION

In our parent application, Ser. No. 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 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 offer 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 may be employed to effectuate 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 U.S. Pat. No. 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. No. 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 determined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the optical pyrometer 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.

The 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., U.S. Air Force Report IR-718-7 (II) (1967); molybdenum alloys (U.S. Pat. No. 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 U.S. Pat. No. 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 surface 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 conjuction 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 100 to 750 surface feet per minute (SFM) on AISI 4340 steel having a hardness of around Rockwell C, (Rc), 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.degree. back rake and a 15.degree. 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 4,340 steel at speeds of both 100 and 750 SFM.

At 750 SFM 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 "SFM" 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 instance 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 SFM and 2 cu. in. removal, there is a uniform wearland of less than 0.025 in. Furthermore, we would note that although chipping is seen in some compositions upon testing at 750 SFM the chipping propensity is aggravated at lower speeds and better assessed at 100 SFM. 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 titanum 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 25 percent of the nitrogen weight pickup is replaced by oxygen and/or 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-40Ti-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 3,375.degree.F for a time of 2 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 2,950.degree.F for 2 hours and showed a microhardness grading and structure similar to the nitrided material in bulk form. The microhardness at a depth of 1/2, 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-10 Mo was also produced on molybdenum by fusing in argon. This was subsequently nitrided at 2,250.degree.F for one-half hour resulting in a nitrogen weight pickup of 1.6 mg per sq. cm. The microhardness at a depth of one-third mil from the surface was 1,680 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 hardening 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 0.1 to 1 mg per sq. cm.

Another useful 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-10Mo vacuum treated at a pressure of 5 .times. 10.sup.-.sup.5 Torr at 2,850.degree.F for 4 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 2,950.degree.F for 6 hours showed even greater titanium loss. A Cb-45Ti-20W alloy vacuum treated at 2,850.degree.F for 4 hours lost 33 mg for a 3/8 .times. 3/8 .times. 1/8 inch specimen weighing 1.9 gram, and a similar size sample of Cb-50Ti-20W vacuum treated at 2,950.degree.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 significant amounts of porosity. Ta-40Ti-10Mo, initially 2.4 g., vacuum treated at 2,950.degree.F for 6 hours lost 54 mg for a 3/8 .times. 3/8 .times. 1/8 sample. Upon nitriding at 3,250.degree.F for 2 hours, this material cut at both 750 and 100 SFM. 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-40 Ti-10Mo vacuum treated at 2,850.degree.F for 4 hours prior to nitriding at 3,250.degree.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 effect on the microstructural morphology. Such morphology effects due to annealing, which result in greater regularity of structure may produce improvements for certain uses, but the compositional effect due to treatment in vacuo is of value by itself.

Since our nitrided materials present as a homogeneous material or as a coated article are in a thermodynamically metastable condition, those skilled in the art will realize that a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction structure and resulting properties 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 nitrided 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.

TABLE I __________________________________________________________________________ Nitriding Argon Microhardness (DPN) at Alloy Treatment Treatment Depth (mils) __________________________________________________________________________ Compositon .degree.F Hrs .degree.F Hrs 0.5 1 2 4 8 __________________________________________________________________________ Cb-17Ti-20W 3450 2 none 2570 2090 1890 1140 906 do do do 3450 1 1220 1017 1040 857 -- do do do 3450 1* 2190 1420 1250 835 765 do do do 3250 2 3060 2600 2570 2160 985 Ta-20Ti-10Mo 3550 2 none 2060 -- 1675 1480 1110 do do do 3250 1 1690 -- 1175 1250 946 do do do 3250 4 1790 -- 1160 996 1060 __________________________________________________________________________ *Argon -- 0.1 percent nitrogen atmosphere.

The alloy Cb-17Ti-20W, nitrided at 3,450.degree.F for 2 hours shows substantial softening when subsequently annealed in argon for 2 hours at this same temperature. If the annealing is carried out in an atmosphere of A-0.1%N.sub.2 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 3,250.degree.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 SFM, we have found regulation of the nitrogen content of the atmosphere to be a useful parameter. It should be noted that the A-0.1%N.sub. 2 atmosphere will harden unnitrided or moderately nitrided alloys but results in softening when used with the highly nitrided alloys in the examples above. A 3/8 .times. 3/8 .times. 1/8 inch specimen of Cb-30Ti-20W reacted in nitrogen at 3,250.degree.F for 2 hours, cuts well at 750 SFM. When subsequently treated in A-0.1%N.sub.2 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 Cb-20V-40Ti-10Mo passed our cutting test criteria at 750 and 100 SFM, improvement was achieved by nitriding at 3,250.degree.F for 2 hours followed by annealing in argon at 3,250.degree.F for 1 hour. Also, good combined performance at 750 and 100 SFM was shown for Cb-30Ti-20W nitrided at 3,550.degree.F for 2 hours and annealed at 3,550.degree.F for 1 hour. Annealing at 3,250.degree.F for 1 hour did not produce any significant improvement and annealing for 4 hours at 3,550.degree.F resulted in failure in cutting at 750 SFM. 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 hardness slightly below the surface and grades inwardly therefrom. Such materials can be effective cutting tools or abrasion resistant articles.

We have also nitrided materials directly in an environment sufficiently low in nitrogen potential that the effect is noted. Nitriding in flowing A-0.1%N.sub.2 produces reduced nitrogen pick-up compared to 100 percent nitrogen. Another method involves sealing the furnace with a measured amount of nitrogen and allowing the nitrogen content 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.sub.2 balance argon and ending with 0.03%N.sub.2. A specimen treated in this manner cut well at both 750 and 100 SFM. The alloy Cb-80Ti-10Mo falling outside our invention, was nitrided in A-0.1%N.sub.2 for 2 hours at 3,050.degree.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 SFM. 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 reduced 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 2,950.degree.F for 3 hours in vacuo by holding in a pack of fine titanium "sponge" which causes diffusion of titanium into the surface. This treatment resulted in a 6 mil titanized layer which upon nitriding for 2 hours at 3,250.degree.F yielded a graded reaction zone similar to Cb-Ti-Mo materials. This contrasts with the 4 mil continuous nitride layer formed on Cb-10Mo without the prior titanizing treatment which exhibits cracking of the continuous nitride layer.

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:

Ratio A = Cb/(Cb+Ta+V)

(that is, the concentration of columbium to total columbium, tantalum and vanadium). Similarly,

Ratio B = Ta/(Cb+Ta+V)

Ratio C = V/(Cb+Ta+V)

Ratio D = Mo/(Mo+W)

Ratio E = W/(Mo+W)

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 per cent of such metals must be equal to or less than the total of

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)]

[10 (Ratio E) + 25 (Ratio D) ] + 15 (Ratio C)

Furthermore, when there is more than 1 metal of the group columbium, tantalum and vanadium present the maximum amount of titanium permitted in the alloy system is equal to or less than the amount determined by the formula

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 + 0.66 (Ratio C):1

Additionally, when both tungsten and molybdenum are present the maximum amount thereof is determined by the formula

60 (Ratio A + Ratio C) (Ratio D) +

50 (Ratio B) (Ratio D) + 80 (Ratio E)

We would further note that when columbium alone is used of Group A metals and both molybdenum and tungsten are present the minimum amount of columbium required is determined by the formula

10 (Ratio E) + 20 (Ratio D)

The minimum titanium is 1 percent and the minimum tungsten and/or molybdenum is 2 percent.

Up to 3 percent of the titanium content may be replaced by zirconium.

Ta-10W was also titanized with the following procedure: vacuum pack titanized 2,950.degree.F -- 6 hours; annealed in argon 2,950.degree.F -- 2 hours plus 3,050.degree.F -- 2 hours; nitrided 2,850.degree.F -- 2 hours. Such treatment produced a 6 mil titanized diffusion zone. Structural grading is shown in the microhardness traverse data presented below: Microhardness (DPN) at depth (mils) 0.5 1. 2 4 8 ______________________________________ 1520 1100 955 666 215

A strip specimen 72 mils thick was prepared using the same titanizing and nitriding procedures and was subsequently bent 45.degree.. Cracking of the hard nitrided case occurred 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 an 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 2,950.degree.F for 3 hours yields a thinner diffusion zone than that observed for the titanizing of Cb-15Mo. The depth of the diffusion zone was about 11/2 mil with molybdenum and less with tungsten. After nitriding at 3,250.degree.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 temperture 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 although 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-20W was nitrided at 3,250.degree.F for 2 hours and subsequently boronized at 2,650.degree.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 90 percent 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-20 W alloy nitrided at 3250.degree.F for 2 hours exhibits a microhardness of 2680 DPN at a distance of one-third mil from the surface. After the subsequent boronizing treatment, the hardness was 4,550 DPN at the same depth. This duplex treated material passes our test at 750 and 100 SFM but the chipping propensity is increased. Up to 25 percent 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 effects 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 effectively 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.

The excellent cutting properties and wear resistance of the nitrided materials can be effectively employed with the other useful properties of the alloys and nitrided materials 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 corrosion-abrasion 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.

Claims

1. An article consisting essentially of: a metal or alloy substrate member; a graded nitrided ternary or higher alloyed surface zone on said substrate member, which surface zone consists of at least one metal selected from each of 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 pickup of said surface zone is at least 0.1 milligram per square centimeter of surface area and in said zone;

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;

d. 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;

f. 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) + 15 (Ratio C);

k. when more than one metal of the group columbium, tantalum and vanadium are present with molybdenum, 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)] + 15 (Ratio C);

l. 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

85 (Ratio 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;

n. 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;

o. 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 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 + 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

Ratio A = Cb/(Cb + Ta + V)

Ratio B = Ta/(Cb + Ta + V)

Ratio C = V/(Cb + Ta + V)

Ratio D = Mo/(Mo + W)

2. The article as defined in claim 1 wherein in said surface zone up to 3

3. The article as defined in claim 1 wherein the surface zone thereof is

4. The article as defined in claim 1 wherein the graded, nitrided portion of the surface zone thereof is depleted in titanium content to the desired

5. The article as defined in claim 1 wherein said surface zone comprises a

6. The article as defined in claim 1 wherein up to 25 percent of the nitrogen weight pick-up is replaced by a material selected from the group consisting of oxygen and boron and mixtures thereof.