US2860972A - Molybdenum-cobalt-nickel alloy - Google Patents

Description

United States atent O finghouse Electric Corporation, East Pittsburgh, Pa., a

corporation of Pennsylvania N Drawing. Application July 2,1956

. Serial No. 505,045

'2 Claims. C(Cl. 75-176) This invention relates to alloys and, more particularly, to ,an alloy of molybdenum, cobalt'and nickel.

Heretofore, high-strength alloys for tool steels and similar applications have comprised tungsten carbides and high-speedtool steels, among others. While the tungsten carbides are very suitable from the standpoint of hardness, they are.difficult to fabricate and sharpen, necessitating grinding withdiamondbits because of their extreme hardness. High-speed tool steels are considerably softer than tungsten carbides and may be easily ground with conventional aluminum oxide or silicon carbide grinding .wheels but their relatively low "hardness and other physical properties do not enable these materials tostand up very "well when cutting very hard materials. M

It is the general object-of this :inventionto avoid and overcome the foregoing "and other diffi'culties' of and objections to,prior.tart practices by the provision of a molybdenum-cobalt-nickel alloy which combines some of the desirable propertiesof high-speed tool steel with some oflthe.desirahlmproperties of tungstenca-rbide.

The aforesaidobjectoftthe invention, and other objects which .will becomeapparent as the description .proc'eeds, are achieved by providing permissible constituent ranges and optimum composition for an alloy of molybdenum, cobalt and nickel.

Although the uses .of .the alloyherei-n described are broadly applicable to :any type of tool or equipment where hardness, high wear resistance and strength are important, such-as drawingand extrusion dies, mandrels, gauges, guide, rings drills, forming tools, ,circular saws, reamers, counter .bores, .-etc., the alloy is particularly adapted for useas. a toolbit and hence its'properties as ap plie tlfto this use willfbe described.

"Basically the alloy consists of a .ternary alloy of molybdenum, cobalt and nickel wherein the cobalt is present in amounts from 2.5% to 6% by weight of the alloy and the nickel is present in amounts from 0.1% to 0.7% by weightof the alloy, with the balance being molybdenum. Thus .the .molybde'num nray be present -'in amounts of from 93.3% to 97.4% by weight of the alloy. The preferredcomposi't-ion-forthe alloy is abount 96.5% :by .weight molybdenum, about 3% by. Weight cobalt and about 0.5% by weight nickel.

A binary alloy of molybdenum and cobalt within the ranges as specified will be quite hard and wear resistant, but will display a tendency for sudden failure under stress. That is, when the alloy fails under stress it will develop very large fractures with the possibility of flying fragments, which is of course undesirable. It has been found that the addition of nickel, within the ranges as specified, eliminates this tendency toward sudden failure and the ternary alloy will wear or at most tend to chip 01f in small flakes rather than display the sudden-fracture type of failure. If less than 0.1% by weight of nickel is used, this sudden-failure defects of the molybdenum-cobalt binary alloy is not substantially eliminated 7 2,860,972 Patented Nov. 18, 8

ice

andif more than 0.7% by weightof nickel is utilized in the ternary alloy, the alloy hardness is decreased.

Regarding the cobalt component of the ternary alloy, if less than 2.5% by weight of-cobalt is utilizedfthe alloy is not sufiiciently hard and if more than '6%'=by weighttof cobalt is utilized, the alloy has some "tendency to become brittle, thereby limiting its usefulness as a tool bit, for example. I

In fabricating the molybdenum cobalt -nickel galley, molybdenum dioxide may be admixed with cobalt and nickel acetatein such proportions that the metallic content of the components of the mixture correspondin g -to the desired percentages of eachcomp'ound in the final alloy. These materials are preferably thoroughly =a'dmixed by means of awater slurry, after whichthe water is'dried off leaving a very 'even mixture-of molybdenum dioxide and the finely-divided cobalt and nickel acetates. This dried material is then reduced to finely-divided metallic form by reduction in a hydrogenatmosphere at a temperature of 1100 C. 'foryabout 6 hours. The reductiontemperature may vary from 1000"C. to -1 2"0'0 C., for example, the lower the temperature the longer-the period of reduction.

The reduced metal thus comprises a very even admixture of the elements of the ternary alloy in amounts corresponding to the final composition of the-alloy. The finely-divided powders comprising this admixture are then compressed into a green ingot of 'the desired configuration. This green ingot is sintered undernonoxidizing conditions at a temperature otirom 1550" C. to 1660 C. forfrom 1 to 3 'hours. As a specific example, the sintering temperature maybe l600 Cfifor atperiod of 2 hours in a hydrogen atmosphere. 'Th'e'preierred atmosphere for sintering isa hydrogen atmosphere although any other non-oxidizing conditions would be suitable.

After sintering, the ingot is age-hardened 'by maintaining it under non-oxidizing conditionsat a temperatureof from 650C. to 750 C. for a period b'firdm 4 to 24 hours, although the preferred temperature'for age-hardening is about 700 C. for a period of from 8 10.12 hours. The-preferred atmosphere for age hardening is hydrogen.

If molybdenum dioxide is "not available, the trioxide maybe reduced to dioxide byprefiring at 'a temperature of about 550 C. to 650 C. for af-period of about '6 hours, for example. If the molybdenum triox'ide h is utilized and is first reduced to the dioxide,-i't ispreferab'le toadd'the other constituents of the alloy after the first preliminary reduction to the dioxide, although the'all'oy constituents may be added directly tothe trioxide if desired, "before the firstreduction tothe dioxide. Also,""it is desirable, although not mandatory, to screen thefin'elydivided metallic material through a 40 mesh screen (see A. S. T. .M.:Designation: Ell-'39) after "reduction-t0 the rinetallic state and before formation of the green ingot. u i it In the foregoing method, cobalt and nickel acetate have been given. as specific examples. These materials are not meant to be limiting and any water-soluble salts of cobalt and nickel which can be readily decomposed to a residue of cobalt and nickel powders respectively under high-temperature reducing conditions may be utilized, and cobalt nitrate and nickel nitrate would be as acceptable as the acetates, if it is desired to use these compounds in forming the slurry.

It is possible to mix finely-divided molybdenum, cobalt and nickel directly inthe prescribed proportions, prior to forming the green ingot, thus eliminating any'reducing steps. However, the finely-divided metallic powders normally cannot be mixed as thoroughly as with the water-slurry method. Thus the preferred method of forming the metallic powder. admixture, prior to forming the green ingot, is the aforementioned water slurry technique.

After the alloy has been age-hardened, it is very hard andcannot be machined as a practical matter, but must be ground to the desired configuration. Since grinding is a fairly slow and expensive process as compared to machining, it may be desirable to prefire the green ingot at atemperatnre of from 800 C. to 1000 C. for a period of 1 to 2 hours, for example. In this stage, the green ingot has sufficient strength to enable it to be machined readily, but is not sufliciently hard as to make machining difficult. After this prefire, the machine ingot may be sintered and age hardened under'conditions'as hereinbefore specified.

-Thephysical and mechanical properties of the molybdenum-cobalt-nickel alloy is specified are excellent with respect to the requirements of high-speed tool bits. For purposes of comparison, the physical and mechanical properties of thisalloy can be compared to a tungsten carbide-tool bit having a composition of 94% tungsten carbide and 6% cobalt and a high speed tool steel having as composition 18% tungsten, 4% chromium, 1% vanadium, 0.7% carbon and the remainer iron. These property comparisons will be examined in the following paragraphs.

With respect to hardness, the molybdenum-cobaltnickel alloys display a Rockwell hardness of 60-62 which is comparable to the Rockwell hardness of high-speed tool steel. This is somewhat less than the hardness of tungsten carbide which displays a Rockwell hardness of from 76 to 80. This relatively low hardness as compared to tungsten carbide has both advantages and disadvantages. For example, the relatively low hardness means that the tools can be easily ground with conventional grinding wheels whereas the tungsten carbide must be ground with diamond wheels, although the relatively low hardness with respect to tungsten carbide would seem to indicate that the ternary alloy would not stand up as well when cutting very hard materials. However, another factor enters into this, for the hardness at high temperatures, which is most important in high-speed cutting tools, is apparently very good for the ternary alloy as compared to tungsten carbide which softens rapidly above 900 C. and high speed tool steels which soften rapidly above 700 C. The results of the actual machining tests conducted would seem to indicate'that the molybdenum-cobalt-nickel alloy has excellent hightemperature characteristics. I a

The modulus of elasticity for the ternary alloy was found to be about 41.5 10 p. s. i. while the modulus for the high speed tool steel was 31.5 X10 and the modulus for tungsten carbide was measured as 88x10. It should be noted that a high modulus is desirable in a cutting tool to minimize the elastic deflection of the tool and thereby obtain machining accuracy.

The thermal expansion and thermal conductivity for the ternary alloy were found to be about 5.08 10 (per C.) and 0.12 (cal. cm. sec. C.- respectively, compared to x10 and 0.19 for tungsten carbide.- This p the ternary alloy in order to machine a 2" diameter bar is to be compared with a thermal coeflicient of 1123x10 and a thermal conductivity of 0.058 for the high speed tool steel. It should be noted that a low thermal expansion and a high thermal conductivity are very dew; sirable in a tool material, since the former permits more accurate machining and since the latter permits heat conduction from the tool tip and thereby helps keep down tool-tip temperatures. p In actual cutting tests, the high-speed tool steel, and carbide-tipped tool were mounted in a turret along with of austenitic stainless steel, with the dimensions of all of the tips being the same. Under a fairly light load all I of the bits held up equally well and there was so marked evidence of wear or other type 'o'f failure. In order to test the individual tools to failure, a second series of machining tests was made in which the feed and depth of the cut was held constant andqthe cutting speed i n= creased until failure of the bit occurred or until the lathe stalled. In this test, the high-speed tool steel bit failed 1 at relatively low cutting speeds. The tungsten carbide tipped bit and the molybdenum-cobalt-nickel alloy bit' g- Withstood the highest cutting speeds obtainable with the a lathe used for the test and the lathe stalled before the bits failed. However, the tungsten .carbide bit showed evidence of rapid wear at the highest cutting speed and the molybdenum-cobalt-nickel alloybit did not show any evidence of wear, indicating excellent high temperature. hardness. While this foregoing test is of a qualitative nature, it indicates the excellent characteristics of the molybdenum-cobalt-nickel alloy as a cutting tool and this is borne out by the measured physical and mechanical properties of this alloy. It will be recognized that the objects of the invention f l have been achieved by providing a molybdenum-cobalti nickel alloy having high hardness, high wear resistance and strength.

-While this alloy appearsparticularly desirable for use H in high speed tool bit applications it should be mentioned again that it should also be useful in many other types of applications where its hardness, strength and wear resistance make its'use desirable.

While in accordance with the patent statutes, one bestknown embodiment of the invention has been illustrated and described in detail, it is to be particularly understood. that the invention is not'liinited thereto or thereby.

I claim: 1. -A metallic alloy containing from 93.3% to 97.4% by weight of molybdenum, from 2.5% to 6% by weight of cobalt, and from 0.1% to 0.7% by weight of nickel.

2. A-metallic alloy containing about 96.5% by weight of molybdenum, about 3% by weight of cobalt, and about 0.5% byweightof nickel.

FOREIGN PATENTS Canada Ian. 3, 1950 ,5 t.

Claims (1)

1. A METALLIC ALLOY CONTAINING FROM 93.3% TO 97.4% BY WEIGTH OF MOLYBDENUM, FROM 2.5% TO 6% BY WEIGHT OF CABALT, AND FROM 0.1% TO 0.7% BY WEIGHT OF NICKEL.