US3708355A - Castable carbonitride alloys - Google Patents

Abstract

A REFRACTORY METAL BONDED CARBONITRIDE ALLOY IS DISCLOSED FOR USE AS CUTTING TOOLS AND IN OTHER APPLICATIONS WHERE HARDNESS AND ABRASION RESISTANCE ARE REQUIRED. THE MICROSTRUCTURE OF THE ALLOYS CONSISTS OF FINE-GRAINED, MECHANICAL MIXTURES OF CARBONITRIDE AND REFRACTORY METAL ALLOYS, THE FINE-GRAINED STRUCTURE BEING OBTAINED THROUGH SOLIDIFICATION OF CUTECTIC OR NEAR-EUTECTIC METALS. IN THE PREFERRED EMBODIMENT THE HARD CARBONITRIDE PHASE HAS TITANIUM AS ITS BASE METAL, WHILE THE BINDER PHASE IS A TUNGSTEN-RICH METAL ALLOY.

Description

United States Patent Oflice 3,708,355 Patented Jan. 2, 1973 3,708,355 CASTABLE CARBONITRIDE ALLOYS Erwin Rudy, Beaverton, Oreg., assignor to Teledyne Industries, Inc., Los Angeles, Calif. No Drawing. Filed Nov. 3, 1970, Ser. No. 86,622 Int. Cl. C22c /00, 31/04 U.S. Cl. 14832 9 Claims ABSTRACT OF THE DISCLOSURE A refractory metal bonded carbonitride allo is disclosed for use as cutting tools and in other applications Where hardness and abrasion resistance are required. The microstructure of the alloys consist of fine-grained, mechanical mixtures of carbonitride and refractory metal alloys, the fine-grained structure being obtained through solidification of eutectic or near-eutectic melts. In the preferred embodiment the hard carbonitride phase has titanium as its base metal, while the binder phase is a tungsten-rich metal alloy.

The present invention relates to carbonitride alloys and more particularly to a new family of carbonitride alloys which, due to their physical characteristics, are particularly useful as machine tools.

Probably the most common material used as machine tools today is sintered carbide of one form or another. Commercial sintered carbide tooling material usually consists of a hard carbide alloy, usually tungsten and titanium carbide, dispersed in a matrix or hinder of an iron group metal, usually cobalt or nickel. The binder provides toughness to the brittle carbide and also serves as a sintering aid during fabrication. However, the iron group metals have relatively low melting temperatures and the loss of strength of the binder alloys based on these metals at relatively low temperatures can cause thermal deformation and thermal wear to become the predominate wear mechanism at high cutting speeds and can cause premature failure of the tools.

To improve on these shortcomings of the iron metal bonded carbides, it has been proposed that sintered monocarbide alloys bonded by refractory metal alloys be used as machine tools. However, due to low strength and poor mechanical shock resistance, these compositions are not competitive with conventional sintered carbides. In a more recent development, refractory metal bonded castable carbide alloys were developed which showed a noted improvement over the sintered carbides when used as machine tools. The absence of thermal deformation under high thermal loads and the good wear resistance of these alloys cause them to be a superior material for tooling purposes.

It has now been discovered, in accordance with the present invention, that an even superior tooling material can be provided by bonding carbonitride alloys with refractory metal alloys.

'It is accordingly an object of the present invention to provide an improved composition of material for use as machine tooling material.

It is another object of the present invention to provide an improved carbonitride alloy for use as a machine tooling material.

It is yet another object of the present invention to provide an improved composition of material comprising a carbonitride alloy bonded to a refractory metal alloy.

DESCRIPTION OF THE PREFERRED COMPOSITIONS OF THE INVENTION The alloys of the invention are based on group IV metal (titanium, zirconium and hafnium)-rich carbonitride alloys (relative group IV metal content in excess of 60 atomic percent) bonded by group VI metal (molybdenum and tungsten)-rich refractory metal alloys (relative group VI metal content in excess of atomic percent).

As is discussed in greater detail below, the desired microstructure of the composition, which consists of a fine-grained aggregate of carbonitride and metal phase, is.obtained through rapid solidification of eutectic or near-eutectic alloys formed between the metal and the carbonitride phase. The lamellar, eutectic-type structure consists of the carbonitride phase, which is responsible for the cutting action, and the metal alloy, which contributes toughness and strength to the composite. In the presently preferred embodiment of the invention a hypereutectic composition of the carbonitride-metal is provided in which grains of primary carbonitride are dispersed throughout the lamellar eutectic structure. The presence of the primary carbonitride phase significantly improves the performance of the composite when employed as a machine tool. In another typical embodiment, the carbonitride metal composite comprises a fine-grained lamellar structure consisting of a tungsten-rich metal binder alloy and a carbonitride phase having titanium as its base metal. The carbonitride phase is a solid solution between isomorphous monocarbides and mononitrides. The molar ratio of nitrogen to carbon in the carbonitride phase can be varied at will but as is discussed below, for reasons related to fabricability and performance of the alloys, is usually kept below 0.40.

The carbonitride alloys of the invention are made possible by the existence of pseudobinary eutectic equilibria between the metal and the mononitride phases in the boundary systems titanium-tungsten-nitrogen, zirconiumtungsten-nitrogen and hafnium-tungsten-nitrogen, and the existence of nearly isothermally solidifying, eutectic-type equilibria between the carbonitride and the metal solid solutions in the systems titanium-tungsten-carbon-nitrogen, zirconium-tungsten-carbon-nitrogen, and hafniumtungsten-carbon-nitrogen. The phase equilibria in the metal-rich regions of the nitride and carbonitride systems are such, that over a wide range of metal-exchanges and temperatures, group IV metal-rich carbonitride, or nitride, solid solutions are in equilibrium with tungsten-rich metal alloys. Within the range of alloy compositions found useful in machine tool applications, the products of the eutectic crystallization consist of tungsten-rich binder alloys (relative tungsten content in excess of 80 atomic percent) and a group IV metal-rich carbonitride alloy (relative group IV metal content in excess of 60 atomic percent).

The nitrogen dissociation pressures of only nitride-containing alloys located within the range where suitable binder alloys are formed, are too high at melting temperatures to be of practical use. However, the nitrogen partial pressures as wellas the eutectic temperatures are substantially lowered by alloying the nitrides with carbides. Thus, for instance, at a carbon to nitrogen molar ratio of 1:1 and a nitrogen pressure of 1 atmosphere, eutectic solidification occurs at approximately 2750 C. along the join metal+carbonitride, at which the metal phase consists of a 94 atomic percent tungsten, 6 atomic percent titanium-alloy, and the interstitial alloy of almost pure titanium carbonitride. By comparison, TIN in contact with a 10 atomic percent titaniumatomic percent tungsten metal alloy melts at 2970" C. and the mixture has at this temperature a decomposition pressure of approximately 6 atmospheres.

The behavior of the other group IV metal nitrides (ZrN, I-IFN) in systems with tungsten are generally similar to the titanium systems, except that the nitrogen decomposition pressures at equivalent temperatures and pressures are somewhat lower than in" the titanium-based alloys. However, the eutectic temperatures in these systems are higher than in the titanium systems (3070" C. at the join ZrN-tungsten and 3100 C. at the join HfN-tungsten) and remain practically unchanged by alloying with carbon; additionally, the nitride or carbonitride content of the eutectic structure is lower than in the titanium systems, which results in substantially higher wear rates of zirconium and hafnium-based alloys in application as machine tools. The phase equilibria in the corresponding systems of group IV metals with molybdenum, nitrogen, and carbon are generally similar to the tungsten systems, although the eutectic temperatures, and hence the nitrogen decomposition pressures, of the alloys in the melting range are substantially lower. The hard alloy content of the eutectics in the molybdenum-containing alloys and the strengths of molybdenum-rich alloy binders are too low, however, in order to be competitive with the tungstenbased alloys as machine tools.

Considering fabricability and performance, the most preferred embodiment of the carbonitride-metal composite in applications as a machine tool is therefore based on the titanium-tungsten-carbon-nitrogen system. As is discussed below, certain improvements can be achieved by alloying with other elements, notably by partial substitution of titanium by zirconium or hafnium, and by small additions of group V metals (vanadium, niobium and tantalum). Partial substitution of tungsten by molybdenum and rhenium is also possible.

The compositions of the alloys of the invention are conveniently expressed either inatomic percent of the constituent elements, for example as Ti W C N (u+v+w+r= 100), where u, v, w and r are respectively, the atomic percent of titanium, tungsten, carbon and nitrogen present in the alloy; or as relative mole fractions of metal and interstitial elements in the form (Ti W (N C whereby x and l-x are, respectively, the relative mole fractions of titanium and tungsten, and y and 1y are respectively, the relative mole fraction of nitrogen and carbon. The stoichiometry parameter 2 measures the combined number of moles of interstitial atoms ofcarbon and nitrogen per gram atom of metal titanium+tungsten. The latter method of defining the overall composition of the alloys is especially useful in defining concentration spaces of interstitial alloys of the type discussed here. The relationships between the two sets of concentration parameters, which are interchangeably used throughout in the description of the alloys of the invention, are as follows:

The preferred compositions (Ti W (C,,N of the base alloys of the invention extend between limits of x between 0.28 and 0.60, of y less than 0.50, and of 2 between 0.28 and 0.37. Alloys located outside the preferred range, but inside the boundaries defined by limits of x between 0.25 and 0.70, of y less than 0.50, and of 2 between 0.25 and 0.45, are less suitable for machine tools but are acceptable for some applications.

Within the area of the preferred compositions, carbonitride-metal composites with low interstitial element contents consist of eutectic only and exhibit somewhat higher wear rates than hypereutectic alloys which contain excess primary carbonitride. Castability, however, becomes poor if the carbon plus nitrogen content of the alloys exceeds about 28 atomic percent (z 0.39). Machine tools fabri- 4 cated from such alloys are also more prone to edge-chipping than tools fabricated from the tougher eutectic, or slightly hypereutectic, alloys. The optimum interstitial element contents of the carbonitride-metal composites when applied as machine tools are between 25 and 26.5 atomic percent (z=0.33 to 0.36).

For a given titanium to tungsten ratio and fixed total carbon plus nitrogen content, increased substitution of nitrogen by carbon increases the hardness of the composites, with overall performance reaching a maximum at nitrogen to carbon molar ratios (y/ 1y) between 0.03 and 0.20. It is especially noteworthy that the top cratering rate of tools made with the composition of the present invention is almost cut in half by substitution of only small amounts "(y=0.02) of nitrogen in carbide alloys, while improvements in flank wear range between 0 and 50%. At fixed total interstitial carbon plus nitrogen content and fixed molar ratios of nitrogen to carbon, top cratering resistance improves with increasing titanium to tungstenratios (x/l-x) in the alloys, while flank wear is only moderately aifected by the relative metal exchange. Edge stability in heavy roughing applications is optimum at titanium exchanges corresponding to x-0.35 (at 2:034), decreases slightly for alloys with values of x between 0.35 and 0.50, and drops sharply towards higher titanium contents.

Maximum attainable molar ratios of nitrogen to carbon in the alloys at casting temperatures are a function of the metal exchange as well as the nitrogen pressure in the gas phase. For a nitrogen pressure of 1 atmosphere, a combined interstitial content corresponding to z=0.33 to 0.35, values for these ratios are listed in Table 1 below. These ratios become smaller if the nitrogen pressure is reduced, and are larger than the values listed in Table 1 if the nitrogen pressure is increased over 1 atmosphere.

TABLE 1 Maximum molar ratios of nitrogen to carbon in titaniumtungsten-carbon-nltrogen melts. Data. based on one atmosphere nitrogen pressure, a combined nitrogen and carbon content of 25 atomic percent, and C. superheating of the melt Maximum molar ratio N:C in the melt y Molar ratio TizW in the alloy z METHOD OF MAKING THE PREFERRED COMPOSITIONS The alloys of the invention may be prepared by are (skull) melting, for example in a nonconsumable electrode (tungsten) are furnace, and casting of the melt in Water cooled molds, or molds made of refractory materials, preferably graphite. Aside from the necessity for a certain nitrogen pressure in the furnace atmosphere to maintain the desired nitrogen to carbon balance in the alloys, are melting under pure nitrogen caused less electrode yvear and resulted in more stable arcs than are melting under noble gases such as helium and argon. This also represents an economic advantage in fabricating the tools. For reasons not well understood at the present time, cracking of the cast parts during cooling also appeared substantially reduced for the alloys which were melted under nitrogen. Induction melting of the composites in a lower frequency induction furnace (1000 to 2000 Hz.), using graphite as container materials, and direct resistance melting is also possible.

Centrifugal casting of the melt is preferable to casting techniques employing stationary molds, because the former technique minimizes the problems associated with the formation of shrinkage pipes, allows higher casting speeds, and also permits the development of processes by which multiple dies can be used to cast parts closely to shape. Dense bodies can also be prepared by powdermetallurgical techniques, using premelted and then comminuted alloy stock. The performance of sintered alloys as machine tools, however, is inferior to the performance of the cast alloys.

It is important in Whatever manner of fabrication that is employed, that the eutectic or near-eutectic melts of the alloys of the invention be solidified under a high temperature gradient (preferably above 20 C./sec.) in order to assure the formation of the fine-grained structure necessary to obtain composites with optimum mechanical properties and performance as machine tools. It has been observed that solidification under higher temperature gradients causes a finer-grained structure and, conversely, solidification under lower temperature gradients causes a coarser-grained structure.

POSSIBLE MODIFICATIONS IN THE PREFERRED COMPOSITIONS The quaternary alloys of the base alloy system can be extensively modified by alloying additions of other metals or interstitials without changing their principal characteristics. A correlation between amount and type of alloying additions, and performance of the resulting composites as machine tools, indicates that noticeable improvements can be achieved by certain low level alloying additions to the base alloys, but that higher level (above atomic percent) alloying is generally unnecessary or even undesirable; addition of selected elements up to certain concentrations proved inert while others resulted, even at low concentrations, in a substantial drop of the cutting performance. The summary of the effect of alloying additions to titaniumtungsten-carbon-nitrogen-based alloys given below is based on the performance of the carbonitride-metal composites in cutting annealed 4340 steel.

(1) Molybdenum, substituted in amounts up to atomic percent for tungsten in the base alloy system, resulted in no noticeable change in performance, although castability appeared impaired at concentrations higher than 5 atomic percent. Exchanges of 50 atomic percent resulted in nose breakdown and chip welding under the chosen test conditions.

(2) 10 atomic percent chromium in replacement for tungsten resulted in poor castability due to preferential vaporization of chromium in the arc and also caused an increase in brittleness of the composites. The maximum tolerable limit without impairing performance was placed at 3 atomic percent.

(3) Rhenium in amounts up to atomic percent in replacement for tungsten improves the cracking resistance of the composites, but had no measurable effect on cutting performance.

(4) Zirconium and hafnium, substituted in amounts up to 5 atomic percent for titanium in the base alloys, improved tool life, while concentrations in excess of 15 atomic percent adversely effected tool performance.

(5) Low level additions (below 5 atomic percent) of group V metals (vanadium, niobium and tantalum) in exchange for titanium were essentially inert, whereas concentrations in excess of 10 atomic percent resulted in cracking of the casting and edge-chipping of the tools.

(6) Boron in amounts up to 3 atomic percent in replacement for carbon or nitrogen entered substitutionally into the carbonitride solid solution and had no adverse effect on the cutting performance.

(7) Oxygen in amounts up to 4 atomic percent in replacement for carbon or nitrogen results in reduced friction and welding tendency, at some sacrifice in strength and cracking resistance.

(8) Additions of l to 5 atomic percent of a number of elements, including iron, nickel, copper, manganese, and aluminum to the tool batches prior to melting did not produce any adverse effects on tool performance.

SPECIFIC EXAMPLES AND TESTS RESULTS Specific examples of carbonitride alloys in accordance with the present invention are given below. Machine tools constructed from these compositions were subjected to tests and compared to tools constructed from the best previously known machine tooling material when these known materials were subjected to the same tests. The results of these tests and the comparisons with the known materials are given in Table 2 below.

The tests were basically conventional straight turning of test bars on a conventional lathe. The test bars consisted of 4340 steel annealed to have a Rockwell hardness value of R equal to 22 to 23 in one of the tests described below and of R equal to 28 to 30 in the other of the tests described below. The surface of the test bars was removed to a depth of 0.050 inch prior to testing the materials. The tool materials were cast into standard A square negative rake disposable inserts. Two test conditions are described below. In the first condition, indicated as being test condition A, the annealed 4340 steel having a Rockwell hardness of 22 to 23 was cut at a speed of 530 surface feet per minute using a depth or cut of 0.125 inch and a feed rate of 0.020 inch per revolution. The other test condition, indicated as being test condition B, using the 4340 steel having a Rockwell hardness value of 28 to 30 and was cut at a speed of 410 surface feet per minute while maintaining a cutting depth of 0.075 inch and a feed rate of 0.020 inch per revolution. In each case, tool life was empirically established at 0.016" total flank wear.

The comparison tools selected for the test were tools of identical dimension made from the best tooling materials known in the prior art. The first tool, indicated as comparison tool 1 in Table 2 below, was constructed from the highest grade sintered carbide material available. This top grade sintered carbide material, known as grade C-50 in the trade, is sold by the General Electric Company under the t-radename Carboloy 370. The second comparison test tool in Table 2 below, indicated as comparison tool 2, was constructed from the recently developed cast carbide alloys mentioned above, evaluated under identical test conditions. This material had the composition Ti Zr W C using the convention described above. An attempt was also made to obtain comparable data on commercial C2 grade sintered carbide material, but tools constructed from this material under the chosen test conditions failed almost immediately after initiation of the cutting test.

Example I An alloy with the initial composition TI37W33C18N7 was arc melted in a graphite-skull under 1 atmosphere nitrogen pressure and then drop-cast into a graphite mold. Analysis of the cast tool blank indicated a nitrogen pickup of 2 atomic percent from the gas atmosphere. Metallographic examination revealed substantial amounts of primary carbonitride phase dispersed in a matrix of hivariantly solidified metal-l-carbonitride eutectic. The average lamellae spacing in the eutectic structure was less than 1 micron and the hardness was R =88.

Example II An alloy with an initial composition Ti W C N was prepared in the same manner as described in Example I, except that the nitrogen pressure in the furnace was kept at atmosphere. The nitrogen pickup from the gas atmosphere was negligible, and the microstructure showed moderate amounts of primary carbonitride in addition to the eutectic. The hardness of the tool was R =87.

Example III An alloy Ti Zr- W C N was are cast under atmosphere nitrogen and fabricated to shape in the same manner as described in Example I. Nitrogen pickup from the gas atmosphere during melting was between 0.5 and 1 atomic percent. The microstructure of the sample showed primary carbonitride phase in a matrix of a very finegrained eutectic with an average lamellae spacing less than 0.5 micron. The hardness of the tool was R =86.6.

Example IV A metal-carbide alloy with the initial composition Ti Zr W C was arc melted in a graphite skull and nitrided in the melted state by a 2 minute exposure to nitrogen at atmospheric pressure prior to casting. The nitrogen content of the alloy was atomic percent, resulting in an approximate overall composition of the cast part corresponding to Ti Zr W C N Example V An alloy Ti Zr W C- N was are cast under 1 atmosphere nitrogen pressure as described in Example I. Postexperiment examination showed a total interstitial (carbon plus nitrogen) element content of 26.5 atomic percent, indicating a nitrogen loss of 3.5 atomic percent during melting. Hardness of the ground tool insert was R =88.3.

TABLE 2 Test Tool life,

Tool composition condition W T Wu We minutes ExampleI A 3 2 0.80 0.38 19 D B 8 2 0.50 0.12 18 Example II. A 5 2 0.55 0. 38 22 Do B 11 2 0.70 0.13 9 Examplel A 4 2 0.45 0.55 28 Do--- B 11 3 1.0 0.18 8 Example B 13 3 1.20 0.16 6 ExampleV B 7 3 0.75 0. 25 15 Comparison tooll A 6 1 1.70 2.70 5.5 Do B 8 2 2.00 1.42 6 Comparison t0ol2 A 4 2 0.90 0.88 15 D0 B 7 2 1.00 0.40 11 NOTE.Wn=Break-i1l flank wear in mils.; TB=Break-in time in minutes.; Wu Unifonn flank wear rate in mils/minute.; W. =Top crater wear rate in mils/minute.

While the invention is thus disclosed and several specific embodiments described in detail, it is not intended that the invention be limited to these shown embodiments. Instead, many modifications will occur to those skilled in the art which fall within the spirit and scope of the invention. It is intended that the invention be limited only by the appended claims.

What is claimed is:

1. A carbonitride-metal alloy composition comprising a base alloy composition of a Group IV metal, a Group VI metal, carbon and nitrogen having a fine-grained lamellar microstructure derived from a eutectic solidification reaction, said lamellar microstructure consisting of a refractory, hard carbonitride phase and a metal phase, with the metal phase being rich in said Group VI metal and contributing toughness to the composite and said carbonitride phase having said Group IV metal as its base metal, said alloy having the composition Group IV metal) (Group VI metal) [N C in which said Group IV metal is comprised of at least 85 atomic percent titanium, said Group VI metal is comprised of at least 90 atomic percent tungsten, and in which the mole fraction x is variable between the limits 0.25 and 0.70, the mole fraction y is less than 0.50, and the stoicniometry parameter z, which measured the combined 8 number of moles of nitrogen and carbon per gram atom metal alloy, is variable between the limits 0.25 and 0.45.

2. A carbonitride-metal alloy composite according to claim 1 wherein there are primary carbonitride grains dispersed throughout the lamellar eutectic matrix.

3. A carbonitride-metal alloy composition according to claim 1, wherein said group VI metal comprises at least 90 atomic percent tungsten and up to 10 atomic percent molybdenum.

4. A carbonitride-metal alloy composition according to claim 1 wherein said group VI metal comprises at least 97 atomic percent tungsten and up to 3 atomic percent chromium.

5. A carbonitride-metal alloy composition according to claim 1, wherein said group VI metal comprises at least 90 atomic percent tungsten and up to 10 atomic percent rhenium.

6. A carbonitride-metal alloy composition according to claim 1, wherein said group IV metal comprises at least atomic percent titanium and up to 15 atomic percent selected from the group consisting of zirconium and hafnium.

7. A carbonitride-metal alloy composition comprising a base alloy composition of a group V metal, group IV metal, a group VI metal, carbon and nitrogen having a fine-grained lamellar microstructure derived from a eutectic solidification reaction, said lamellar microstructure consisting of a refractory, hard carbonitride phase and a metal phase, with the metal phase being rich in said group VI metal and contributing toughness to the composite and said carbonitride phase having said group IV metal as its base metal, said alloy having the composition [(group V metal),,(group IV metal) (group VI metal) [N C in which said group V metal is selected from the group consisting of vanadium, niobium and tantalum, said group IV metal is comprised of at least 85 atomic percent titanium, said group VI metal is comprised of at least atomic percent tungsten, and in which the mole fraction v is less than 0.05, the mole fraction x is variable between the limits 0.25 and 0.70, the mole fraction y is less than 0.50, and the stoichiometry parameter z, which measured the combined number of moles of nitrogen and carbon per gram atom metal alloy, is variable between the limits 0.25 and 0.45.

8. A borocarbonitride-metal alloy composition comprising a base alloy composition of a group IV metal, a group VI metal, boron, carbon and nitrogen having a fine-grained lamellar microstructure derived from a eutectic solidification reaction, said lamellar microstructure consisting of a refractory, hard borocarbonitride phase and a metal phase, with the metal phase being rich in said group VI metal and contributing toughness to the composite and said borocarbonitride phase having said group IV metal as its base metal, said alloy having the composition [(group IV metal) (group VI metal) [B N C in which said group IV metal is comprised of at least 85 atomic percent titanium, said group VI metal is comprised of at least 90 atomic percent tungsten, and in which the mole fraction u is less than 0.03, the mole fraction x is variable between the limits 0.25 and 0.70, the mole fraction y is less than 0.50, and the stoichiometry parameter z, which measured the combined number of moles of boron, nitrogen and carbon per gram atom metal alloy, is variable between the limits 0.25 and 0.45.

9. An oxycarbonitride-metal alloy composition comprisinga base alloy composition of a group IV metal, a group VI metal, oxygen, carbon and nitrogen having a fine-grained lamellar microstructure derived from a eutectic solidification reaction, said lamellar microstructure consisting of a refractory, hard oxycarbonitride phase and a metal phase, with the metal phase being rich in said group IV metal and contributing toughness to the composite and said oxycarbonitride phase having said group IV metal as its base metal, said alloy having the composition [(group IV metal) (group VI metal) [O N C in which said group IV metal is comprised of a least 85 atomic percent titanium, said group VI metal is comprised of at least 90 atomic percent tungsten, and in which the mole fraction t is less than 0.04, the mole fraction x is variable between the limits 0.25 and 0.70, the mole fraction y is less than 0.50, and the stoichiometry parameter 2, which measured the combined number of moles of oxygen, nitrogen and carbon per gram atom metal alloy, is variable between the limits 0.25 and 0.45.

References Cited UNITED STATES PATENTS 10 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.