US2866259A - Powder metallurgy compositions of molybdenum, iron and silicon, shaped objects thereof, and their preparation - Google Patents

Description

United States Patent POWDER METALLURGY COMPOSITIONS OF MO- LYBDENUM, IRON AND SILICON, SHAPED OB- JECTS THEREOF, AND THEIR PREPARATION Max F. Bechtold, Kennett Square, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application February 20, 1956 Serial No. 566,358

18 Claims. (Cl. 29.182.5)

This invention relates to powder metallurgy compositrons, to shaped objects of alloys having high heat and oxidation resistance and to a method for preparing such shaped objects. More particularly this invention relates to new molybdenum-iron-silicon powder metallurgy compositions, to shaped objects of molybdenum-iron-silicon alloys which are resistant to oxidation and high temperature and to methods for preparing such shaped objects.

There has been considerable effort expended in the search for metals or their alloys that are resistant to attack by acids and alkalies and that can be subjected to oxygen-containing atmospheres at high temperatures without deleterious elfects, such as loss of strength or oxidation. Although some metallic compounds have been made that have some of these advantageous properties, they are usually difiicult to obtain in the form of shaped objects and they are usually quite expensive. In addition to the need for strong metallic structures having high heat and oxidation resistance, there is need for compositions that are resistant to abrasion and have low density.

.Metallic compositions that are useful as electrical heating elements likewise are desirable goals, particularly Err/hen the product can be readily obtained in a shaped orm.

It is an object of this invention to provide novel powder metallurgy compositions. A further object is to provide shaped objects of an alloy resistant to oxidation and high temperatures. A still further object is to provide a novel process of readily preparing strong metallic objects having high heat and oxidation resistance. Another object is to provide shaped objects of metallic compositions useful as electrical heating elements. Still another object is to provide shaped objects of metallic compositions which are resistant to abrasion and have relatively low density. Other objects still appear hereinafter.

These and other objects of this invention are accomplished by providing a powder metallurgy composition of an intimate mixture of metal powders having a particle size of less than 75 microns, preferably less than 50 microns of which at least 75% by weight are less than 5 microns, the intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18- 65% molybdenum, -50% iron and 17-57% silicon.

Strong metallic objects resistant to oxidation and high temperatures are prepared by pressing this intimate mixture of metal powders into the form of the desired object, and heating the pressed mixture of metal powders rapidly to a temperature of at least 800 C. These shaped objects have a minimum dimension of at least 1 mm. and are composed of the specified alloy. It has now been found that shaped objects of superior properties are obtained by heating to at least 800 C. an intimate mixture of powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in chemical composition to 18-65% molybdenum, 15- 50% iron and 17-57% silicon. At least 90%, generally above 95% and preferably at least 97%, of .this intimate mixture of powders consists of molybdenum, iron and silicon in the specified ratios.

A particularly preferred composition for the production of the shaped objects is obtained by grinding a ferrosilicon in the presence of molybdenum powder, the particle size of at least (by weight) of the resulting powder not exceeding 5 microns; with the composition containing 25-55% molybdenum, 20-45% iron and 20-45% silicon. Such a composition is readily shaped by cold pressing the powder into the form of desired objects and converted to an alloy by heating to a temperature, e. g., 1100-1400" C., within a short period of time.

The products thus obtained are substantially free from the uncombined elemental form of the major components of the compositions of this invention. When compositions outside the above-defined ranges are used, either the final product contains uncombined elements or the product obtained is deficient in such desired properties as strength, hardness, resistance to high temperature, oxidation, or freedom from mechanical defects. The new alloys are non-magnetic.

The following examples illustrate the preparation and properties of the compositions and resulting shaped objects of this invention.

EXAMPLE I Ten grams of molybdenum powder (maximum size 325 mesh with primary particle size about 0.17 micron, 99.9% pure) and 20 grams of ferrosilicon alloy (maximum size 20 mesh) analyzing: Fe49.62%, Si-48.23%, Al-- 1.1% [Mn0.5%, Ca-0.3%, Cr0.l%, Ti0.l%, Ni0.08%, Mg0.05%, Cu-0.05%, Mo0.03% approximate by emission spectography] were charged into a porcelain ball mill 250 cc. in capacity containing 12 porcelain cylinders x and then the sealed mill was rotated for 18 /3 hours at R. P. M. At the end of this time the powder (about 33.3% molybdenum, 33% iron and 32.2% silicon) recovered from the mill was of reduced particle size, particularly the initially coarse ferrosilicon alloy had been reduced to all fines.

Approximately 0.4 g. of this powder was loaded into a cylindrical steel die mold (1.033 cm. in diameter), pressed at 10,000 lbs. force and the resulting pellet ejected. The pellet was then fired in air for one minute in an oxygen gas torch, during which time the pellet underwent a mild calorescence and attained a temperature in the neighborhood of 1100-1300 C. The resultant pellet was strong and hard. It was inert to boiling 6 N potassium hydroxide.

A larger pellet was prepared in a similar manner from the dry ground powders with firing for 2 minutes. This gained 0.8% in weight and 8% in thickness during firing. Further heat treatment for 88 hours in air at 700 C. in an electric mufile furnace resulted'in an increase of weight of an additional 4.8% but with only 0.8% increase in thickness. The heat treatment strengthened and did not warp the pellet, which was harder than glass.

.It was cleaved into two parts with hammer and chisel.

It had pronounced capillarity to water. One half was submitted to the following cycles of heating in air:

This strong object showed no signs of scaling. These results establish that porous objects of the alloy undergo a limited reaction with the atmosphere at lowternpera- 89V; hours.

tures, and that after this treatment they are quite stable EXAMPLE :II

Molybdenum powder (45 g.)andcoarse ferrosilicon (90 g of 50% silicon'as used in Example I) were charged into a one-quart'ball mill'containing 400g. of A" glass beads and the wholerotated 44% hours at 54 R. P. M. The glass beadswere removed'and replaced with 48%" X porcelain cylinders and the mill again rotated for A sample (g.) of the resultant powder (about 33.3% Mo, 33%Fe and 32.2% Si) wasmixed on apolyethylene sheet with approximately 1 g. of 2% NaOH aqueous solution, which was just suificient to give a plastic mass,.then was molded into a /2 of a splitcylinder mold (a section of longitudinally'split A1 tubing,

1 cm. in diameter) and the assembly was 'dried at room temperature, then'at 100 C. without cracking. This object was removed from the mold and fired in an oxygengas torch within 3 seconds (With calorescence) to yield a hard, strong, half cylinder 2.5 cm. long x 1.0 cm. in diameter. Smoking due to M00 volatilization was notably'absentand the object had a glazed surface.

7 EXAMPLE III Molybdenum (1414'g1) an'd .ferrosilicon (16.8.g. of

50% .Si as usedin Example I) were subjected .to' ball -milling under the same-conditions asforExample'I' (for 46'hours). The resultant fine pow'der(about 46.2% "Mo,

26.7% Fe and 26% Si) was pelleted and fired for 60 seconds (calorescence occurred) as in ExampleI with about a 2% dimensional change and less than 1% weight change. The resultant strong metallic object was as hard as glass and upon polishing had'a very bright metallic lustre.

EXAMPLE IV Molybdenum powder, 99.9% pure which passed a'325 mesh screen and in Whichthe primary particle size was about 0.8 micron (14.4 g.) was ball-milled 89 /2 hours with ferrosilicon (25.2 g. of 50% Si) as in Example I. The powder (about 36.4% Mo, 31.6% Fe and-30.7% Si) was pelleted andzsintered withwalo'rescence as in Example I. A hard,rstrong,.metallic.object was obtained (with very slight'warpage). The'follovving .Weightdimensional changes occurred; during sintering:

'Weight, g. Diameter, "Thickness,

Before firing .1. 0213 1 033 ..0.2932 After firing 1. 0211 0:29am. 2970 This cylindrical pellet was roughly polished "on both faces to a thickness of 0.2332-0.220 cm. The electrical resistance of this object through its thickness to point probes was less than 0.1 ohm. Next the object was placed between fiat stainless steel plates and'pressed at atotal force of 14,250 lbs. without visible damageto-thepellet; deep indentations were jm ade'infthe'fsteeljfplates. M f

'EXAfMPLE 'V' n Molybdenum powder (48 'g.) 'was'milled as in EX- ample Iwith ferrosilicon'(112-g. of 50% Si) irra-onequart ball mill containing50%""-x:%" -porcelain' cylinders. A' sample'of the resultant powder- '(abont 30% 4 Mo, 34.7%, Fe and 33.8% Si) waspelleted as in Example I, then fired in an oxygen-gas torch for 10 seconds, during which time calorescence occurred. One half of the strong pellet was crushed, then pulverized for X-ray diffraction, which showed :the absence of Mo and molybdenum oxides. The other half of the pellet was sub- "jected' to'heating in the air in'an electric muflie furnace. Although'about 10% "weightjgain andl3l% increase in "diameter *occurredinthe"porous structure at 800 C. after '16:hours, .it wasthereafter essentially stable at 1000-C.on further heatingfor about 114.5 hours. It was easily polishable to brig'ht' hard metal'sur'face after such-heat treatment.

Another sample of the abovemixed powder was hotpressed in a graphite mold at 1150 C. and 3750 lb./ sq. in. to yield ahomogeneoushardmetal object with bulkdensity of 5.46 g./ cc. and a Knoop Hardness Number of.1000 at 0.1 kg. load and 750 ati3 kg. load. Heating this object for 16'hours.at"800 C. resulted in an increase .in weight of only 0.4% 'while an additional heating of 321.hours at.1000 C..resulted ina .further increase of about 0.02% a A furtherpreparation ofthe fine mixture of powders was effected by grindingforraalo-nger time. A sample (12 g.)wasmixed with about 1.5g. of 2% Na'OH and then:dried.1 hour at'iroom temperature, 1 hour at 50 C. and lhonrrat 100 C. .The driedmasswas coarsely ground'in amortar and pestle, and'then pressed in pellet form as inExarnple I. Firingfor 15 seconds in the oxygen-gas torch =caused thefollowing weight and dimen- -sional changes:

"The fired object"'was hard, strong, metallic and not Warped. 'This'techni'que' gave arelatively high green "density and 'an almost'equivalent fired density without the use of hot-pressing.

EXAMPLE -VI Molybdenum powder (64 g.) and coarse "ferrosilicon (37.3 g. 'of='50% Si as inExample I) were'ball-milled as in Example V and a sample of the resultant'powder (about63.2% Mo, 18.3% Fe and 17.8% Si) was pelleted and fired as in Example I to give a hard strong object which was heated in an electric-muffle furnace in air for'2 /2 hours at1000 Cxan'd' l6 /z hours at 850 C. without appreciable weight change. Further heating for 24 hours at 1000 C. brought about a lossof 0.25% in "weight.

Heattreatment ofanotherisimilarly prepared pellet for 16 hours each at 800, 900,'-1000, 1100=and 1200 C. "showeda gradual loss in weight with formation-of oxide 'scale particularly-at 1100l200 "C. Objects fro-m compositions high in molybdenum are preferably used at temperatures of lessthan 1100l200 C.

-A' sample (8 g.) ofzthe above powder was: pasted-with 'Fabout 075g. of an aqueoussolution"(piH-:10) consistsilica: (30%'-SiO 70% I-I O) until-a soft butter'consisten- 'cy was obtained. "This paste-was then formed into an open demounta-ble wooden bar mold- (ca-W 6 x /3 x 1 /8) lined with cellulosic tape. 1 The molded object Was dried at C. until set in 'shape-,"then at C.- until it --was very hard. It-was then removed from the mold and surfaces trued by. grinding. It'was-next'eured at C.. 1 hour and finally heated in 'an oxygen-gas torch in air "for 30 -seconds,- which causedcalorescence. While still .atred heat, -it wasquenched in water. There resulted a -strongnon-crackedmetallic object of hardness greater "than 'glass. A- portion of this"rectangular bar waspolishedto' the average'dimjensions of 2i543fcm. x 01761 cm.

5: 0.543 with a bulk density of 4.36 g./cc. It had a specific electrical resistance of 320 x ohm-cm.

EXAMPLE VII Molybdenum (8 g.) as in Example IV was ball-milled for 40 hours with chunks of a ferrosilicon alloy (14.02 g. of 33.44% Si). A sample (10 g.) of the resultant fine powder (about 36.3% Mo, 42.4% Fe and 21.3% Si) was mixed with 1.5 g. of 5% aqueous NaOH, the paste dried at room temperature, then briefly pulverized in a mortar and pestle. Next, the powder was compacted in a bar mold at 25,000 lb./sq. in. for 2 minutes, removed and fired for 40 seconds in the oxygen-gas torch, during which faint calorescence occurred. A smooth unwarped bar of high impact strength was obtained. A portion of this bar, when heat treated 16 hours each at 800, 900, 1000,

' 1100 and 1200 C. gained a maximum of 1.32% and had a net final gain in weight of less than 0.15%.

EXAMPLE VIII Molybdenum powder as in Example I (96 g.) was ballmilled for 48 hours by the general procedure of Example V with a 200 mesh ferrosilicon alloy (200 g. of 45.84% Si.). A sample of the resultant fine powder (about 32.4% Mo, 36.6% Fe and 31% Si) was mixed with of its weight of 5% NaOH, the paste dried at room temperature, then briefly pulverized in a mortar and pestle. A pellet was pressed from this powder at 80,000 lb./sq. in., then fired in an oxygen-gas torch with slight calorescence. When heat treated as in Example VII, this strong metallic pellet showed a net weight gain of less than 3.2% with substantially constant weight at 1000-1200 C.

EXAMPLE IX Molybdenum powder (96 g., see Example I) was ballmilled for 48 hours as in Example V with 200 mesh ferrosilicon (122 g. of 63.54% Si.). A portion of the resultant fine powder (about 44% Mo, 20.4% Fe and 35.6% Si) was mixed with 15% by weight of 5% NaOH, the paste dried at room temperature, then pulverized briefly in a mortar and pestle. This powder was next pelleted at 80,000 lb./sq. in., then removed and fired in an oxygen-gas torch (with slight calorescence). The hard metallic object was then heat treated as in Example VII. It had a total net weight gain of approximately 6.1% and maintained constant weight in the range 1000- 1200 C.

EXAMPLE X Molybdenum powder (14.4 g.) was ball-milled as in Example I for 40 hours with a 20 mesh ferrosilicon (21 g. of 68.87% Si.). The resultant fine powder (about 40.7% Mo, 17.4% Fe and 40.9% Si) was mixed with 5% NaOH, dried, pulverized, and pressed in a bar mold as in Example VII and was then fired in an oxygen-gas torch for seconds. Calorescence ensued during the firing. A portion of the metallic bar, when heat-treated as in Example VII, gained a total of about 5.3% in weight and was substantially constant in the range i000- EXAMPLE XI A 58.6% Mo-ferromolybdenum alloy powder (20 g.) was ball-milled for 48 hours as in Example I with a ferrosilicon alloy (44 g. of 49.59% Si.) The resultant fine powder (about 18.3% Mo, 44.7% Fe and 34.1% Si) was mixed with 5% NaOH, dried, pulverized, bar molded and fired (50 seconds) as in Example VII, with slight calorescence. The resultant smooth metallic bar was then heat-treated as in Example VII. The total weight gained was about 5.5% and the weight was almost constant in the range l000-1200 C.

EXAMPLE XII Molybdenum powder (99.9% pure and as in Example IV) was dry ground in a quart porcelain ball mill containing 50% x /a. porcelain cylinders for 48 hours at 70 R. P. M. with a commercial ferrosilicon alloy (49.59% Si, 46.02% Fe, 1.01% Al, 3.38% of other metals) in the proportions indicated in Table I. Samples (20 g.) of each batch were mixed with 3 g. of 5% aqueous NaOH and the paste spread on a polyethylene sheet, then dried at room temperature. Next, the dried powder was crushed briefly in mortar and pestle, then about 10 g. of each was pressed in 1 /2" x /2" bar mold at 25,000 lb./sq. in. at room temperature for 2 minutes. The ejected green bars were then weighed and measured and mounted on edge on three equally spaced stainless steel knife edges in preparation for firing. They were next fired with an oxygen-natural gas torch starting at one end of the rectangular bar, proceeding down both sides of the bar with the torch as calorescence progressed through the bar. A period of from 25 seconds for the Mo-rich bars to 40 seconds for the Mo-poor bars was required to insure that they were uniformly heated as judged by the whiteness of heat attained.

After being weighed and measured the bars were next ground down on both sides to a thickness of about 0.4 cm. and each was cut into several smaller rectangular cross section bars (specimens) of approximately 0.6 x 0.4 x 1.2 cm. One set of specimens from each batch was subjected to the following measurements and tests, the results of which are recorded in Tables I and II: bulk density from weight and geometrical volume, weight and dimensional change upon being heated in air in an electric mufiie furnace 16 hours at 800, 900, 1000, 1100 and 1200 C. inclusive. These specimens were weighed after cooling from each heat. After the final heat treatment, they were measured, bulk density calculated, then sectioned, and a small section mounted for examination of cross section and for hardness measurements. The remainder of these heat-treated specimen; were analyzed. A set of specimens not heat treated was used for immersion density determination by liquid displacement. Porosity was calculated from these results in conjunction with the bulk density. A further set of specimens not heat treated was used for determination of specific electrical resistance. Another set, not heat treated, was used for initial hardness tests. The results of these tests are 1200 C. summarlzed 1n Tables I and II.

Table 1 Experiment A B C D E F MO 60% M0 50% Mo 40% M0 30% Mo 25% N10 Input, Composition 16% Fe 18.5% Fe 23% Fe 27.5% Fe 32.5% Fe 34.5% Fe I 17.5% S1 20% S1 25% S1 30% Si 35% S1 37.5% S1 Weight Change on Firing (percent) 0. 59 O. 56 0. 61 O. 76 +0.01 +0. 06 Av. Linear Expansion on Firin (percent) 0. 52 O. 08 1. 25 O. 91 +1. 13 +1. 78 Bulk Density (g./cc.) 4. 78 4. 57 4. 44 3. 92 3. 73 3. 33 Immersion Density (g./cc.) 6. 8885 6. 7268 7. 0451 6. 0204 5. 6982 5. 6511 r I 830.1 kg. 2450.1 kg. 3321 kg. 1820.5 kg. 1,0 201 kg. 781 kg. knoop Hardness 3511 kg 1691 kg 2900.1 1: 1760.1 kg. 3 0.5 kg. 660.1 kg. l 2400.5 1.. 1660.5 1.... 2560.5 1.... 1611 1... 344. k... 600.. 1.... Specific Resistance (X10 ohmcm.) 3 3. 3. 2. 3 4. 9 6. 0

=this metal on heat treatment.

under a load of 0.3 kg.

Table II Heat'Treatment Properties of Heat-Treated Specimens Experiment Total Wt. Gain of Fired Wt.) after 16 hours at: Av. Appar- Linear ent Expan- Density Hardness Analysis 800 0. 900 C. 1,0000. 1,100 O. 1,200 C. sion (g./cc.)

(percent) (percent) (percent) (percent) (percent) (percent) 6800.1 k Mo44.96% C +5. 47 +5. 29 +5. 7 +5. 18 +5. 20 +5. 38 V 3. 97 2300.5 kg Fe-21.40% 2341.0 k Si21.32% o.i k Mo-35.59% D +9. 82 +9. 90 +9. 62 +9.60 +9. 60 +2; 44 3.98 4011].; k Fe24.95% 4671.0 kg S'k-23.53% 6270.1 kg Mo-26.18% E +9.82 +9. 52 +9. 40 +9. 40 +9. 40 +0.70 4.01 5240.5 k Fe29.19% 4701.0 k Si28.32% 5520.1 kg M21.05% F +15.-35 +14; 95 +1484 +14. 86 +14. 90 +2. 47 3. 59 332 kg. Fe29.77% i 3111.0 kg. Si27.07%

These experiments show that compositions high in molybdenum (see A and B) show'considerable loss of Compositions having about 25-40% of each element (see D, E and P) underwent little change in the rather complicated X-ray diffraction patterns upon heat treatment. Compositions having rela- =tively high iron and silicon (see F) increased in hardness on heat treatment.

EXAMPLE XIII Molybdenum powder (as in Example IV, 192 g.) was at room temperature. This air-drie-d powder was then pressed as inExampleI to a pellet,'which was'then'dried at 50-92" "C. for one hour. 'fire'd in an oxygen-gas torch for -15 seco-nds to obtain The dried pellet was next a hard, crack-free, metallic composition. The results of hardness tests with a mircohardness tester using the Knoop diamond indenter on an average of 5-8 indentations on a polished surface of this pellet are Knoop Hardness No. o-f 986 under a load of 1 kg. and of 1027 A pellet similarly prepared (except that the air-dried powder was further dried one hour at 100 C. and'the green pellet was not further dried before firing) had the following analyses after 'firingz Mo, 42.58%, Fe, 26.44%, Si, 26.11%, Al (from the ferrosilicon used), 0.96% and 3.91% other metals and oxides and nitrides (by difference). The net weight loss during firing was only 0.035%.

EXAMPLE XIV Molybdenum powder (as in Example.IV,'37.5 g.) and 112.5 g. of ferrosilicon (75.42%.Si) were ball-milled for 40.5 hours. The resultant fine powder Mo, 18.4% Fe" and 56.6% Si) was pasted with dilute NaOH, dried,

pulverized, molded, fired as. in Examplev XII with'slight calorescence to give a smooth high impact bar. 'Upon heat treatment (as in Example XII), the bar hardened somewhat but did not become harder than file steel. The total net gain in weight was about 14.5% withagain of 3% occurring at l00O-1200 C.

The new compositions of this invention are powder metallurgy compositions and shaped objects having a minimum dimension of at least one millimeter and initially-containing in homogeneous form 18-65% molyb- Not more than,l0% and preferably less thanv 5% These other metals are usually present in minor amounts as impurities in. ferrosilicon alloys.

The alloys or compositions of this invention do not have present appreciable amounts of elemental crystalline forms of the molybdenum, iron or silicon. The latter are not stable to high temperature oxidation. However, upon prolonged heating at high temperatures, some oxidation of the new alloys can take place. As shown in the tables of Example XII, the major three elements which comprise -98% or more of 'the original composition, may amount to about 78-90% of the final product after prolonged heat treatment in air. In such products, oxides of iron and silicon can be present in addition to the complex of molybdenum, iron and silicon. For ease of understanding and simplicity, the compositions listed are those of the initial components, before they are subjected to prolonged heating in air.

Within the above-defined range, shaped compositions 'having an initial molybdenum content within 20-60% have superior properties and of these, those with 25-55% are preferred. Particularly preferred are those having less than 50% molybdenum. An alloy having the approximate formula MoFe Si i. e., about 30% molybdenum, 35% iron and 35% silicon, or approximately equal parts by weight of molybdenum, iron and silicon, exhibits particularly outstanding properties. When the initial composition is high in molybdenum, the conversion from powder to hard composition takes place readily, but the resulting composition has a tendency to expand, crack and lose weight when heated in air (through loss of a volatile oxide of molybdenum and oxidation of iron and silicon). Compositions low in molybdenum undergo slow conversion from powder to hard alloy.

Although the iron content can be as high as 50%, preferred compositions have less than 50% iron, suitably 15-45% and generally between 20 and 40%. The silicon content can vary from 17-57% with alloys generally containing 20-45% and preferably 25-40%.

In the above compositions, the percentages given are of the total of the three major components, i. e., the essential constituents, of the alloy. They may contain amounts not exceeding 10% and usuallyless than 5% with generally less than 3.0% of other elements, including aluminum, manganese, calcium, chromium, and titanium, in the final alloy. Such additional elements are generally present in commercial ferrosilicons. As determined by emission spectra these additional elements are usually present in the following ranges: Mg 0.005- 0.025%; Mn 0.05-2.5%; Cu 0.05-0.25%; Ca 0.05- 0.25%; Ni 0.1-l.0%; Ti ODS-0.25%; and Al up to about 5.7%. Of these, alloys having larger amounts of titanium also give useful products. The alloy should be substantially free from carbon.

The compositions of this invention. are obtained by heating or firing with an external heat source of at least 1800"C. a composition containing molybdenum, iron and silicon in the ratios desired in the final product. Of

such, the iron is generally pro-alloyed with silicon while molybdenum may be pre-alloyed with iron, but not with silicon to any appreciable degree. The presence of about or more of unalloyed iron or silicon in the starting materials causes bubbling during the process of conversion to hard compositions. For example, when a composition obtained by balLmilling parts each of finely ground molybdenum, iron and silicon was heated in an oxygen-gas flame, the pellet underwent vigorous reaction and was warped and bubbled. X-ray diffraction analysis showed no elements remained uncombined after the reaction. However, when the more tedious conventional methods of conversion to alloy are practiced, such as hotpressing in virtual absence of air or sintering in vacuum or in inert atmosphere, appreciable concentrations of unalloyed Fe and Si can be used without bubbling or cracking.

Preferably the starting materials for the composition of this invention consist in an intimate mixture of elemental molybdenum and ferrosilicon, both of particle size of less than five microns wherein the ratio of molybdenum, iron and silicon present in the mixture corresponds to that desired in the resultant composition. Generally the finely ground material, i. e., the majority of the particles by weight, should be in the range of 0.1 to 2.5 microns, preferably about one micron. Such a powder is readily obtained by milling or dry grinding molybdenum with ferrosilicon until a powder is obtained in which the particle size is within the above range. This procedure is particularly preferred since the molybdenum powder by such a process becomes intimately associated with the ferrosilicon particles. The dry powders thus obtained are particularly easy to shape and convert by heat to the strong alloys.

The desired particle size of the powdered metallic compositions is obtained by conventional milling and grinding procedures. The size can be determined by microscopic techniques. Molybdenum powder can be obtained from commercial sources with a primary particle size of about 0.170.8 microns. Commercial ferrosilicons are usually much coarser (e. g., 8200 mesh). After grinding or milling the two materials, the powders obtained according to the processes of the examples had a primary particle size of the majority of the particles by weight of 0.2 to 1 micron with some clusters of par ticles that approached 5 microns in size.

Although small amounts of larger particles can be present, the properties of the resultant alloy are not as desirable. For example, a composition having considerable amount of particles of 6 microns with some particles as large as 96 microns did not react to give a useful alloy. The amount of particles present of size above about 5 microns should be less than 25% with the largest size present not exceeding about 75 microns to avoid coarse grains in the final alloys.

The dry powders wherein molybdenum, iron and silicon are intimately mixed can be shaped in the form of the desired ultimate product by compacting or pressing the powder into the form desired. The powder can be moistened with water or water containing a small amount e. g. about 0.25 to 1% of lithium or sodium hydroxide to give a material that can be shaped by a trowel into a coherent object. A mold can also be employed for such a composition or for one containing more moisture. Such mixtures which contain water or other volatile wetting agents should be dried before heat treatment to a moisture content of less than 3% to prevent rupture of the object during firing.

The shaped objects must have a minimum dimension of about one millimeter since it is not only difficult to-obtain thin objects but, if obtainable, they have low fiexural strength and are more subject to oxidation during firing than are thicker objects.

The shaped form of the mixed powder is converted into the solid alloy composition by external heating to a temperature of at least 800 C., generally above 1000 C. and preferably between 1100-1400" C. The maximum temperature employed is that at which the resultant alloy loses its dimensional stability and is generally about 1450 C. The minimum and maximum temperature employed is dependent upon the exact composition of each alloy.

The conversion step of the mixture of powders to solid alloy generally involves a calorescence (i. e. an exothermic reaction characterized by an increase in heat as exhibited by an increase in luminosity) that is induced when a major portion of the shaped object is heated to a minimum temperature of at least 800 C. The internal temperature of the shaped object (measured by thermocouples) during conversion is at least 1000 C. and generally 1100-1400 C. When the spontaneous heat increase is low, the temperature of the externally applied heat must be higher.

When pellets from powders of the composition of Example XII (Experiment D) were placed in a furnace at 700750 C., the alloy was unsatisfactory (soft and brittle) although the pellet underwent a mild calorescence, its maximum internal temperature was below 1000 C.. When the furnace temperature was 800 C., within about 50 seconds, the pellet underwent a rapid calorescence (towhite heat) and in about 2 minutes cooled to the temperature of the furnace. Simple and practical means of effecting this conversion involves heating the shaped powder object with an oxygen-gas torch or placing the object. in a hot furnace. Heating by induction or by direct resistance techniques can likewise be employed. When the starting powders are selected from the preferred compositions, the conversion step proceeds readily with little dimensional change of the shaped object.

To obtain maximum properties in the resulting alloy, it is preferred that the heating be rapid, e. g., at most 10 minutes and preferably less than 3-5 minutes. Slow heating in the range of 350-800 C. brings about powdering, expansion and cracking of the shaped object before conversion.

The use of iron or silicon or ferrosilicons having more than 85% silicon is not desirable in the preferred process since the conversion reaction (calorescence) is quite vigorous, especially in the presence of alkali metal hydroxides, such as sodium hydroxide, and results in shaped objects that have rough wavy surfaces or are cracked, warped or have bubbles, with little fiexural strength. On the other hand, with ferrosilicons of a low silicon content, c. g., 30% or less, the conversion step is more difficult to achieve in a short period of time since the calorescence step may be substantially absent. Notwithstanding this disadvantage, such compositions merit consideration when finished objects having a smooth surface with little if any warping during heating are desired.

Optimum properties are achieved by the use of powdered molybdenum and ferrosilicon alloys, the latter having between 33 and silicon (i. e., corresponding to FeSi-FeSi Particularly useful are ferrosilicons having 4263% silicon (i. e., FeSi to FeSi and of these ferrosilicons having 4555% silicon, i. e., of the approximate empirical formula FeSi are preferred. The presence of molybdenum powder in the dry grinding of ferrosilicon results in a superior product having a rnicroporous, strong metallic structure with little change in weight or dimension during the conversion step. A further novel feature is that ferrosilicons are hard to ball-mill to small particles alone but when mixed with molybdenum, the size reduction of the ferrosilicon is facilitated.

As shown in the examples, the compositions of this invention undergo little change in dimensions or weight on heating or firing at temperatures of the order of 1000 C.

When finely ground molybdenum powder was pressed into a form and heated under the conditions employed for molybdenum-ferrosilicon compositions, the molybdenum powder gave a very brittle object with considerable scratch glass. operations, e. g., for high-speed lathes, can be prepared i 11 weight lossand decrease in dimensions due to-formatidn ol -volatile oxide. Finely ground ferrosilicon having "50% silicon did not become strong or-hard and underwent considerableweight gain and some increase in dimensions.

The new alloys of this invention are hard, resistant to heat and degradation by oxidation. Such properties are quite surprising in the alloys prepared by the process of. this invention, e. g., Example XII, whichhave-a porosity of 25-50% of pores of extremely small'siz'e. Heat treatment of the alloys further'strengthens and hardens the compositions. AlthQughsOme-Weight increase takes place upon heating at 800-1000 C.,-little further increase is noted at temperatures of 1100-1200 C. The preferred heat-treated alloycomposition undergoes less than from these compositions.

A particular advantage is in the ease of fabrication of the objects and this may favor their use in contrast to presently used harder cutting compositions. The conversion to hard product-takes place in air thereby avoiding the necessity for inert atmospheres. The products are easy to make in the form in which they are to be used, e. g., as shaped or molded articles of manufacture, such as structural components of high temperature furnaces and heat engines.

As many apparently'widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as fol- 1. A powder metallurgy composition of an intimate mixture of metal powders having a particle size of less than 75 microns, of which at least 75% by weight are less than microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, 15-50% iron and 17-57% silicon.

2. A powder metallurgy composition of an intimate mixture of metal powders having a particle size of less than 50 microns, of which at least 75% by weight are less than 5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 25-55% molybdenum, 2045% iron and 2045% silicon.

3. A powder metallurgy composition of an intimate mixture of molybdenum and ferrosilicon metal powders having a particle size of less than 5 microns with the majority of the particles by weight within the range of 0.1 to 2.5. microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, 15-50% iron and 17- 57% silicon.

4. A powder metallurgy composition of an intimate mixture of molybdenum and ferrosilicon metal powders having a particle size of less than 5 microns with the majority of the particles by weight within the range of 0.1 to 2.5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to about equalparts by weight of molybdenum, iron and silicon.

5. Apowder metallurgy composition of an intimate mixture of ferromolybdenumand ferrosilicon metal powd'ers having a particle size of, less than 5 microns 'withthe majority ofthe particles by weight within the range of 0.1m 2.5 microns,"said intimate mixture of metal powders consisting by'chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65 %"molybdenum, 15-50% ironand 17-57% silicon.

6. A sintered'shaped object having a minimum dimension of at least 1 mm. composed of an alloy consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 25-55% molybdenum, 2045% iron and 2045% silicon.

7. A hot-pressed shaped object having a minimum dimension of at least 1 mm. composed of an alloy consisting by chemical analysis essentially of molybdenum, iron and silicon and correspondingin composition to 25-55% molybdenum, 2045% iron and 2045% silicon.

ticle size of less than 75'microns, of which at least 75% by weight are less than 5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, 15-50% iron and 17-57% silicon, and heating said mixture of metal powders to atemperature of 800 C. to 1450 C. within a period of, at most ten minutes.

10. Process for preparing a strong molybdenum-ironsilicon alloy object resistant to oxidation and high temperatures as'set forth in claim 9 wherein said intimate mixture of metal powders is an intimate mixture of molybdenum and ferrosilicon metal powders.

11. Process for preparing a strong molybdenum-ironsilicon alloy object resistant to oxidation and high temperatures as set forth in claim 9 wherein said intimate mixture of metal powders is an intimate mixture of ferromolybdenum and ferrosilicon metal powders.

12. Process for preparing a strong molybdenum-ironsilicon alloy object resistant to oxidation and high temperatures which comprises pressing into the form of said object an intimate mixture of metal powders having a particle size of less than 5 microns with the majority of the particles by weight within the range of 0.1 to 2.5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 25-55% molybdenum, 20-45% iron and 2045% silicon, and heating said pressed mixture of metalpowders to a temperature of 1100 to 1400 C. within a period of less than 5 minutes.

13. Process for preparing a strong'molybdenum-ironsilicon alloy object resistantto oxidation-and high temperatures which comprises pressing into the form of said object an intimate mixture of metal powders having a particle size of less than 5 microns with the majority of with a ferrosilicon alloy to an intimate mixture of metal powders having a particle size of less than microns with the majority of the particles by weight within the range of 0.1 to 2.5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, 15-50% iron and 17-57% silicon, pressing said intimate mixture of metal powders into the form of said object, and heating said pressed mixture of metal powders to a temperature of 800 C. to 1450 C. within a period of at most minutes.

15. Process for preparing a strong molybdenum-ironsilicon alloy object resistant to oxidation and high temperatures which comprises grinding a ferromolybdenum alloy with a ferrosilicon alloy to an intimate mixture of metal powders having a particle size of less than 5 microns with the majority of the particles by weight within the range of 0.1 to 2.5 microns, said intimate mixture of metal powders consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, -50% iron and 14 17-57% silicon, pressing said intimate mixture of metal powders into the form of said object, and heating said pressed mixture of metal powders to a temperature of 800 C. to 1450 C. within a period of at most 10 minutes.

16. A powder metallurgy composition a set forth in claim 1 containing a small amount of sodium hydroxide.

' 17. A sintered shaped objecthaving a minimum dimension of at least 1 mm. composed of an alloy consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18-65% molybdenum, 15-50% iron and 1757% silicon.

18. A hot-pressed shaped object having a minimum dimension of at least 1 mm. composed of an alloy consisting by chemical analysis essentially of molybdenum, iron and silicon and corresponding in composition to 18- molybdenum, 15-50% iron and 17-57% silicon.

Austria Aug. 10, 1927 France June 23, 1954

Claims (1)

1. 6. A SINTERED SHAPED OBJECT HAVING A MINUMUM DIMENSION OF AT LEAST 1 MM. COMPOSED OF AN ALLOY CONSISTING BY CHEMICAL ANALYSIS ESSENTIALLY OF MOLYBDENUM, IRON AND SILICON AND CORRESPONDING IN COMPOSITION TO 25-55% MOLYBDENUM, 20-45% IRON AND 20-45% SILICON.