US2757108A - Surface hardening of molybdenumcobalt alloys - Google Patents

Description

July 31, 1956 A. G. CATERSON SURFACE HARDENING OF MOLYBDENUM-COBALT ALLOYS 2 Sheets-Sheet 1 Filed Sept. 30, 1953 $1 0/70 0F Co 61917 IN VEN TOR. 4. 6. 67 725250 July 31, 1956 A. G. CATERSON SURFACE HARDENING OF MOLYBDENUM-COBALT ALLOYS Filed Sept. 30, 1955 2 Sheets-Sheet 2 INVENTOR. f7. 6.675/66'0/8 SURFACE HARDENING F MOLYBDENUM- COBALT ALLOYS Application September '30, 1953, Serial No. 383,409

6 Claims. 01. 148-32 This invention relates to the production ofarticles made from molybdenum-cobalt alloys, the surfaces of which have been hardened by the inclusion of molybdenum carbide therein, making said articles suitable for certain applications, particularly for high-temperature use.

The principal object of my invention, generally considered, is to produce molybdenum alloyed with a small proportion of cobalt, and case-hardened by the inclusion of molybdenum carbide the proportion of which gradually decreases'inward from the surface.

, Another object of my invention is to surface harden molybdenum by gas carburizing, with the production of agood bond between the surface-hardened case and the relatively-soft core, thereby adapting articles made therefrom for such specific uses as hot piercing tools, hot shears, and other devices designed to operate in the Zrange of 600 to 1000" C.

A further object of my invention is to case-harden molybdenum which has been alloyed with .05% to .5 cobalt, by treating in a carburizing atmosphere Within the temperature range of 1400 to 1600 C. for from 1 to 4 hours, to produce from .01 to .25 of molybdenum carbide'as a second phase in the surface-adjacent portions thereof.

,Other objects and advantages of the invention will become apparent as the description proceeds.

"Referring to the drawings:

Figure '1 is a vertical sectional view of a receptacle for mixing molybdenum oxide with a solution of a salt of cobalt;

1 Figure 2 is a similar view of the same apparatus after steam has been admitted to evaporate the mixture to dryness;

Figure 3 is a vertical sectional view of an electric furnace in which the mixture is to be reduced to metal powder Figure 4 is a vertical sectional view of a receptacle in which the metal powder is mixed with a solution of a lubricant;

Figure 5 is an elevational view, partly in vertical section, of a furnace which may be employed for sintering the pressed powder to coherent metal;

Figure 6 is a photo-micrograph with a magnification of 250, after etching in an electrolyte of concentrated sulphuric acid with 50 volt direct current applied to an open circuit, followed by polishing, showing the center of an ingot or slug, of molybdenum-cobalt alloy embodyilllghmy invention, in-which the carburization is very s ig t;

Figure 7 is a view corresponding to Fig. 6, but showing the transition zone where the carburization is somewhat greater; and

Figure 8 is a view also corresponding to Fig. .6, but showing the relatively-heavy surface carburization.

.Molybdenurn is'one of the few metals which retains its strength at high temperatures. On the other hand, molybdenum and its malleable alloys are relatively soft. Even such alloys in a highly cold-worked condition are United States Patent 0 Patented July 3l, 1956 2 softer than 300 VPN at 600" C. Alloys of molybdenum which show very high hardnesses are britfle and unworkable.

For applications where high hardness is desirable, it is preferable to have a soft ductile core which has been surface hardened to a very high degree. Alloys of molybdenum, with only small proportions of other materials such as .05 to .2% cobalt, have the following desirable properties:

1. Good hot strength 2. Readily malleable 3. Good hot resistance to impact.

Previous attempts to surface-harden pure molybdenum, or such alloys of molybdenum as above mentioned, have been relatively unsuccessful. Certain experimenters surface hardened molybdenum strip by nitriding in raw ammonia. However, molybdenum nitride is unstable at temperatures of 900 and higher. Molybdenum has been successfully carburized, but in all previous cases the hard, brittle phase did not form a good bondwith the soft ductile core. Experience has been that the case would crack away from the core in machining operations.

In accordance with my invention, 1 provide a method of surface-hardening molybdenum, whereby a good bond is formed between the hardened case or surface-portions and the core, thereby adapting such material for use'in the manufacture of such specific applications as hot piercing tools, hot shears, and other tools designed to operate in the range of 600 to 1000 C.

Although molybdenum containing from .05 to .5 Of cobalt may be manufactured in any approved manner, yet it is essential that the cobalt portion be uniformly diffused therethrough. For this reason, a good method of making an alloy suitable for my purpose will now be disclosed.

Referring to the drawing in detail, like parts being designated by like reference characters, in Fig. 1 there is shown a receptacle 11 in which is placed a desired quantity of molybdenum oxide, preferably the dioxide. To this is added, preferably during agitation as by means of a stirrer 15, a desired quantity 13 of a solution of a cobalt salt, which upon fin'ng breaks down to cobalt oxide. This salt is preferably cobalt acetate, but the chloride 'or other soluble compound which breaks down to the oxide upon heating, without evolving undesirable material, may be employed. The concentration and amount of the solution is such that the metals molybdenum and cobalt are present in the ratio desired in the alloy ture or slurry of the components, is evaporated to dry ness as by the admission of steam into the hollow interior of the receptacle While the stirrer 15 is operated. The dried mixtureis then placed in suitable refractory boats or receptacles 16 and introduced into a furnace 17. The furnace 17 preferably comprises an alumina or other refractory tube 18, surrounded by nichrome or other suitable refractory metal wire 19, for heating to the desired extent in a dry hydrogen or other suitable reducing atmosphere, on an appropriate time-temperature schedule to get a metal powder satisfactory for pressing.

Reduction of the metal oxides is desirably done in a furnace consisting of a 1%" diameter Inconel tube wound with nichrome wire to get three separate heat zones each about 24" long. Hydrogen is sent into the tube at one end of a flow of 40 to 50 cubic feet per hour.

Iron boats about 1" x 1" in section and about 29" are filled with about 250 grams of the oxide mixture long and,

ing atmosphere.

pushed through the furnace at a rate of about 10" every half hour. The first heat zones are held at from 600 C. to 650 C., the second at from 700 to 750 C., and the third atfrom 800 C. to 350- C.

It is, however, to: be understood that, the reductioncan be carried out satisfactorily under different conditions; of furnace size, temperature, hydrogen flow rate,,weight-and thickness ofcharge,,. and speed'of travel through -the. furnace. The molybdenum dioxide, preferably employed in. this. process,-v may be produced from-either the tnoxlde or ammonium molybdatc ina known manner.

After complete reduction, the metal powder is-removed from the furnace and may be impregnated with lubricating material, to facilitate the compacting of theparticles' on pressing, as in a receptacle 21, while being stirred by rod 22. The lubricant maybe steari'c acid, paraiiin, diglycolstearate, or diglycol laurate, preferably from 62% to 6% by weight, dissolved in sufiicient vehicle, such: as carbon tetrachloride, to completely wet the. powder so. that the lubr-icant' is entirely mixed therewith. The. lubrieatingsolution is represented in Fig. 4' by the reference character 23..

The powder or lubricated mixture isthen-pressed to a suitable form in a conventional press under a pressure. of preferably between about 10 and 50 tons per square inch. After removal from the press, the slugs or articles are heated to first slowly drive off the lubricant; if used, as by introducing them through the-entrancewdoor- 26: into the pre-heating portion Z'Zof a sinteringfurnac'e' zb,illus trated' in Fig. 7, after which they are pushed aiong; into the sintering'portion 29 o said furnace and fired. at temperatures above 1500 (1., say about 1650 (-3., for from about 2 /2 to about 5 -hours, preferably in a wet hydrogen or other wet reducing. atmosphere, inaccordance' with the process described and claimed from the Hall et al'.v Patent No. 2,431,690, dated. December 2, 1947.

The finished articles,v when sintered to approximately maximum density, are then pushed along. to the cooling. portion 31 of said furnace and, after reaching a s'afetemperature, removed as upon opening the exit door 32,, in a condition suitable for working, such as forging. or. roll' ing, if desired. The product, that is,; in.- either sintered, forged orrolled condition and alloyed with from'.05%- to .5% cobalt, may then be formed to the desired shape, as by rough machining.

It is then' placed- -inan Alundum' refractory furnace, such as the. sintering furnace of' Fig. 5, where it is main.-

the diffusion of carbon in tungsten, as has been shownby the use of cobalt in the manufacture of tungsten carbide. Carburized molydenurn-cobalt has been found to have the following properties:

1. There is excellent bonding between the case and core.

2. A hardness gradient is established from 800 VPNao in the most severely carburized areas at the surface, to lSOV-PNso in the-core.

3. The hardness at 600 C. ranges from 600 VPlfI to 260: VPN in the areawhere, a second phase. is. visible; dependingon the amount ofsecond. phase in-the-area of the impression.

4. Pieces of the product embodyingmy inventlonmay be ground with conventional grinding equipment, or quenched in water-from. a temperature.of'900 C., without the case portion cracking away from the core.

Figs. 6, 7 and 8 are referred to as showing the variation in carburization from very slight at the center encore, as viewed in Fig. 6; through a medium carburization'as seen in Fig. 7 between the center and the surface, to a heavy carburized surface. zone, as viewed in Fig. 8.

Before carbur-izing,,the metal consists of a single" phase, molybdenum. In spite of the fact that oxygen, nitrogen, traces of carbon, iron, other metallic impurities, and cobalt are present-,theseare allxpresent in solution. These are present as either substitutional atoms, as in the case of cobalt, where an atom of cobalt takes the placeof a molybdenum. atom in the. lattice, or in interstitual solid solution, as in the case of oxygen or carbon, where: the foreign. atoms fit between the molybdenum atoms. Now as. more carbon is added, the molybdenum lattice can no longer tolerate itin solid solution, so that we now have a mixture of molybdenum and molybdenum carbide. Referring to Figures 6, 7, and 8,.in Figure 6 we have practically a-single phase, molybdenum, the'black specks indicating voids. In Figures17 and 8, we havetwo phases, the:continuous.phase, which. is molybdenum with dissolved impurities, a little. carbon and cobalt, and a second, di'stained at temperatures in the range of- 1400" to 1600! C.

for L to 4 hours. During. this treatment, 'asuitable carburizing atmosphere ismaintained inthe furnace, that is, onecontaining usable carbon, suchascracked naturalv gas, introduced into the furnace as by means of an"Endo-g as generator, rawnatural gas, methane on propane. After such treatment, it will be found that the article is casehardened to a depth of from .01"-to .25".

This case. is formed of extremely hard molybdenum carbide in the alloy, present as. a second phase. It find there is no abrupt transition but a gradual lessening ofi the amount of the second phase carbide with. distance from the exposed surface. The specific differences in the product of this invention, as compared with molybdenum carburized. in accordance with previous attempts; are: as-

follows: i

1. The carbon is obtained from acarbonaceous reduc 2. Cobalt has been added to the molybdenum-which: is to be carbur-ized. Cobalt is. observed to: promote formation of a good bond between the molybdenum'carbide' of the second phase-and the core. The-precise mechanism by which the cobalt acts is not known. 1 However, it mayresult' in the formation of acomplex-molybdenumcobalt carbide, or the cobalt may act toincrease'the rate of diffusion of the carbon into the molybdenum.

The above is contrary to the'general effect of" an alloying agent, but seems probable because" cobalt promotes continuous phase with a molybdenumv carbide compound. This molybdenum carbide is the discrete, outlined white particles.

The amount of carbon. which molybdenum will dissolve is not definitely known. The best estimate is probably to be found in the work of Manning and. Few of Battelle Memorial Institute, published: in the 1952 Transactions of the A. I. M. E. However, it seemsprobable' that molybdenum must hold considerably more carbon in solution at the carburizing, temperatures than it doesat room temperature. Then as the-rnetal cools, thecarbon, which was in solution, separates out as a molybdenum carbide.

It is essential, forthc wear resistance, that the outside of the piece be-a mixture of two phases, one a soft matrix phase, and the other, a hard,finely-divided, discontinuous phase.

It is virtually impossible to make a chemical analysis of a case-hardened piece, since the amount of carbon varies from the surface in toward the center. If we were able to make such an analysis, by some such technique as shaving off layers, we would find that the. distribution of carbon would conform to a sine function or a Bessel function, falling 01f sharply from the surface.

In Figure 8., /3. of the surface is. considered to be molybdenum and A3. molybdenum carbide. Fromthis the atomic percent may be calculated, and therefore the weight percent of carbon present.

Although preferred: embodiments have been disclosed, it will be: understood: that modifications may be made within the spirit and scope of my invention.

claim? 1. The method of case-hardening molybdenum-cobalt alloys containing notmore than about 0.5% cobalt. comprising'hold-ing within the temperature range of 1400 to 1600 C. in a carburizing atmosphere to cause the former tion of'molybdenum carbide at the surface for a depth of about 0.25 inch and extending further inwardly but gradually decreasing in proportion toward the interior.

2. The method of case-hardening molybdenum containing from .05 to .5 cobalt alloyed therewith, comprising holding within the temperature range of 1400 to 1600 C. in a carburizing atmosphere, to cause the formation of molybdenum carbide at the surface for a depth of from 0.01 inch to 0.25 inch and gradually decreasing in proportion toward the interior.

3. The method of surface-hardening molybdenum when alloyed with small proportions of cobalt ranging from 0.05 to 0.5%, comprising forming a piece of such molybdenum-cobalt alloy to the shape desired, placing said piece in a furnace and maintaining it in said furnace at a temperature in the range of 1400 to 1600 C. for from 1 to 4 hours in a carburizing atmosphere, to form a case from .01" to .25" thick hardened by a proportion of second phase molybdenum carbide, with no abrupt transition but a gradual lessening of the amount of carbide with distance from the exposed surface.

20 4.=The method of producing case-hardened molybdenum comprising adding enough of a cobalt solution to molybdenum dioxide to produce a slurry with the ingredients in the ratio of 99.95 to 99.5% molybdenum and -.0s% to 5% cobalt, drying the mixture, reducing it to 6 metal, sintering to coherent form, working, and holding within the temperature range of 1400 C. to 1600 C. in a carburizing atmosphere to cause the formation of molybdenum carbide at the surface for a depth of from 0.01 inch to 0.25 inch and extending further inwardly but gradually decreasing in concentration.

5. An article of molybdenum and from about 0.05% to 0.5% cobalt, the skin of which is hardened by the inclusion of molybdenum carbide for a depth of from about 0.01 inch to 0.25 inch.

6. An article of molybdenum containing between .05 and .5% of cobalt alloyed therewith, the surface of which is hardened by the inclusion of a proportion of molybdenum carbide for a depth of from 0.01 inch to 0.25 inch and further decreasing from the surface toward the interior with no abrupt transition.

References Cited in the file of this patent UNITED STATES PATENTS 1,512,191 Baumhauer Oct. 21, 1924 2,032,694 Gertler Mar. 3, 1936 FOREIGN PATENTS 449,847 Great Britain June 24. 1936

Claims (2)

1. THE METHOD OF CASE-HARDENING MOLYBDENUM-COBALT ALLOYS CONTAINING NOT MORE THAN ABOUT 0.5% COBALT COMPRISING HOLDING WITHIN THE TEMPERATURE RANGE OF 1400* TO 1600* C. IN A CARBURIZING ATMOSPHERE TO CAUSE THE FORMATION OF MOLYBDENUM CARBIDE AT THE SURFACE FOR A DEPTH OF ABOUT 0.25 INCH AND EXTENDING FURTHER INWARDLY BUT GRAD-

5. AN ARTICLE OF MOLYBDENUM AND FROM ABOUT 0.05% TO 0.5% COBALT, THE SKIN OF WHICH IS HARDENED BY THE INCLUSION OF MOLYBDENUM CARBIDE FOR A DEPTH OF FROM ABOUT 0.01 INCH TO 0.25 INCH.

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