US2827407A - Method of producing powdered steel products - Google Patents

Description

Eugene E. Carlson, Ann Arbor, Mich, and Fritz V. Eenel, Troy, N. Y., assignors to Federal-Mogul Corporation, Detroit, Mich, a corporation of Michigan Application June 15, 1954, Serial No. 436,323

3 Claims. (Cl. 148-465) The present invention relates generally to the art of powder metallurgy and more specifically to an improved article of stainless steel and to an improved method of making same from powdered metallic ingredients.

The molding of metal powders has been extensively employed in the production of complicated shapes of soft metals, particularly iron and low-carbon steels. The method usually employs a fine iron powder which is pressed or compacted under high pressure to cold weld the metal particles and then sintered at a high temperature sufficient to form a solid article. This method is quite satisfactory for soft iron and iron low in alloying constituents.

If it is desired to increase the density of the final articles, it has been suggested, as shown in Patent No. 2,411,073, to coin the once-sintered compact and then resinter to relieve the cold-working stresses.

it has sometimes been the practice to carburize and heat treat the soft iron to produce a carbon steel structure. For example, Patent No. 2,333,573 discloses a method of making low-carbon steels by powder molding, the method including the use of carbon-free electrolytic iron, compacting and then sintering in a carburizing atmosphere which would be in equilibrium with a carbon steel of the desired carbon content. In the latter method,

the sintering is continued until the carbon is homogeneously distributed through the compact. While the latter method is quite satisfactory for ordinary carbon steels, the presence of chromium, nickel and other alloytion of the chromium and carbon through the specimen.

Another procedure is disclosed in Patent No. 2,489,839, wherein there is disclosed the compacting of soft iron at low pressures, sintering, coining the resulting porous soft iron compact at higher pressures, carburizing it in a rich atmosphere above equilibrium to obtain a highcarbon case on the surface, and finally soaking or homogenizing in an equilibrium carburizing atmosphere (i. e. in equilibrium with the'final carbon content desired) to distribute the carbon throughout the specimen.

When it has been desired to produce articles from high alloy steels, particularly high chromium and high chromiurn-nick'el steels, i. e. the stainless steels, alloy powders having the desired composition have been employed. These pre-alloyed powders have been pressed and the resulting compacts sintered as described above. However, because of the poor compressibility of these powders, it has not been possible to produce compacts of high density, i. e. above 95% of the theoretical or of a forged article, even if the compacts are coined and resintered. High density inthese alloys is needed for good fatigue resistance. It has also not been possible to produce heat-treatable article from these alloy powders since the usual atmosphere used for sintering the compacts d'ecarburize them effectively. It has not been possible predictably to carburize stainless steel compacts by the usual cracked ammonia, combustion gas, or hydro gen-hydrocarbon gas mixtures. Cracked ammonia and combustion gases contain high proportions of nitrogen which strongly favors the formation of the austenite phase, the latter then interfering with carbon difiusion and favoring the production of an extremely hard high carbon case on the article. Combustion gases also contain carbon monoxide, a gas which is easily reduced by chromium. The result'is an article of poor properties due to the presence of unreduced chromium oxides in the piece. A principal disadvantage of the hydrogenhydrocarbon mixtures is that they are diflicult to use and control.

The art of powder molding, therefore, has long sought a process which would be capable of producing dense, heat-treatable articles of stainless steels having the fatigue resistance and other physical properties of a wrought article of the same composition.

It has been found, in accordance with this invention, that an article of stainless steel containing at least 11% and up to 25% chromium and up to 2.5% nickel can be formed having a density between 96 and 98% or more of theoretical and having the tensile strength, hardness, elongation, and fatigue resistance closely approaching that of a forged article of the same alloy by a process including the steps in the sequence of (1) mixing a pure, deoxidized, low-carbon powdered iron of high compressibility, powdered chromium, ferrochrome or other powdered form of chromium, powdered nickel or nickel alloy and other powdered alloying ingredients; (2) compacting the mixture by pressing at a pressure of at least 50 t. s. i. (ton/sq. in.), and preferably at 70 t. s. i. or more; (3) sintering the compact at a temperature between 2,000 and 2,500 F. and preferably between 2,300 and 2,450 F. in a reducing atmosphere having a dew point below -25F.; (4) coining the sintered compact at a pressure of at least 60 t. s. i., and preferably at 70 t. s. i. or more, to obtain a high density, and (5) carburizing the compact at a temperature between 2,000 and 2,300 F., and preferably at temperatures between 2,100 and 2,300 F., in a reducing atmosphere having a low dew point and in the presence of solid carbon, the atmosphere employing dry hydrogen to react with the solid carbon and form a carburizing gas. Also, according to this invention, when a stainless steel article having a relatively thick cross section is desired (i. e. thicker than M3), the carburized compact is subsequently soaked or homogenized at a temperature between about 2,100 and 2,300 P. in an atmosphere-free molten bath. Although the carburizing step of this invention does not produce a hard, high-carbon case, the articles of thicker cross section do not have the uniform carbon distribution so much to be desired in an article for rigorous service. The homogenizing step is carried out at a high temperature to speed the diffusional process.

The above-outlined process produces a molded stainless steel compact having little dimensional change (i. e. can be made to extremely close tolerances), having a density of at least 96 to 98% of theoretical, the strength, elongationyhardness, and fatigue resistance approaching those of a forged article of the same alloy, and with good reproducibility. This process can be employed to mold complicated air foil shapes such as rotor and stator blades for compressors in gas turbines, jet engines, and the like, which articles have heretofore been made only by expensive, time-consuming, forging, machining, finishing and polishing operations. The above process is believed to be the first to produce a satisfactory powder molded heat-treatable article of a high temperature, corrosion-resistant stainless steel having good fatigue resistance.

The significance of the above sequence of steps, and the conditions employed in'each step, isnot fully understood but is believed to be critical for the production of satisfactory powder-molded articles. of. stainless steel. For example, the use ofan iron powder of high compressibility rather than an alloy. powder makes possible a, higher density. The pro-alloyed powders do not readily weld together on cold pressing and the compacts formed from them have relatively low density. In contrast, the use of a very pure, soft iron produces a compact of higherv density in the initial pressing step, and permits a still further reduction in porosity during the coining step; The iron powder must be low in carbon, i. e. below 0.02% carbon, andlowlin oxygen,i. e. below 0.1% oxygen, in'. order toprevcnt. preferential reaction of chromium and other alloying metals with oxygen and carbon to form oxides and carbides which are not readily reduced or dissolved, thereby creating. a carbideor oxide-coated skeleton in the compactwhichresists. compacting during the coining operation.

The atmosphere in the sintering stepis more highly. critical in the method of this invention due to .the presence of pure chromium or high chrome ferrochrome particles. with the chromium and other metals at such high temperatures and the oxide coatingthus formed would interfere with sintering of the particles and with diffusion of the various alloying metals. For this. purpose, the dew point must be below -.2S E. and preferably is in the range of -35 to -45 F. or lower. Also, the presence of a reducing atmosphere tends still further to reduce the carbon content of the compact and facilitates the subsequent repressing or coining step so asztoobtain a-higher density. The use of" the higher sintering temperatures of 2,000 to 2,500.F; is believed necessary to obtain proper diffusion of the high melting chromium, nickel and other alloying ingredients throughout the iron phase. Also, the high temperatures reduce the length of the sintering cycle and thus prevent theformation of an excessively large-grained structure.

At any given temperature, there is aminimum sintering period that is required for optimum properties.

Any oxygen or moisture would readily react usually is non-hardenable due to its extremely low carbon content of 0.01 to 0.04%. Suflicient carbon must, therefore, be introduced to obtain a hardenable composition. Should an atmosphere be employed which has too high a dew point, the carbide diffusion and structure will be improper and the properties of the final compact will be inferior. If the carburizing step is carried out at too low a temperature, the carbon will not readily diffuse into the interior of the article, but will form a high car'- bon case containing massive chromium carbides on its surface. Such a hard case is unusually stable, since it is very difiicult to decompose the carbide during any subsequent homogenizing step and. to produce a uniform heat-treatable structure throughout the article.

The carburizing step is carried out using dry hydrogen as a carrier for the carbon. This is carried out according to the invention by placing the campact near but out of actual contact with carbon, graphite, and other solid forms of carbon. At the temperatures employed, the hydrogen reacts with thesolid carbon and forms a carburizing gas, the latter transferring the carbon to the sample, probably in the form of methane or some cracked, intermediate, reactive product thereof. The resulting atmosphere is not oxidizing in nature, in contrast to anatmosphere which contains carbon monoxide. The carburizing effect of this atmosphere is very uniform and is easily controlled, the carburization penetrating to advantage resides in extending'the time beyond this period, in. fact, increased grain. growth. obtained. during protracted sintering is generally disadvantageous. For

example, when a compact. is. sintered. at. 2,350? F.', its

ultimate tensile strength continues toincrease as the sintering time is increased up.to aperiodof'two hours and no significant. increase occurs beyond two hours. This would indicate that sintering of. the particles and diffusion of the ingredients. of the alloy is essentially complete in two hours. Of course, with. temperatures less than 2,350 B, longer sinteringv times will be required while with higher temperatures, slightly shorter periods will sufice.

The use of a coining stepemploying pressures of 60, 70 and up to 100 t. s. i. or more pressure effects a significant increase in density'inthe compactbut only when the latter has been properly. compacted and sintered. During this coining operation, the carbon content of the compact must be-very low (below 0.1%)

and preferably below 0.05%, in order to obtain any substantial increase in density. during the coining operation. In the sintering .operationof this invention, the

already low carbon content is markedly reduced. The sintering and coining steps canbe repeated'if desired, 'although one such cycle usually produces adensity of 96 to 98% or more of theoretical.

The carburizing step is especiallyimportant. inorder to produce a heat-treatable stainless steel alloy. The product of the compacting, sintering andcoining steps a greater depth without case formation. The use of temperatures above 2,000 F. also shortens the carburizing time and thereby reduces the tendency to form a casehardened skin. As a result, the carbon so introduced is more uniformly distributed throughout the specimen and is in a form more easily homogenized.

Whilea total carbon content of 0.08 to 0.15% is generally desirable for stainless steels of about 11-13% chromium, the higher chrome steels require increased amounts to be heat-treatable. At about 25% chromium up to 1% carbon may be required for heat-treatability.

The amount of carbon, per unit of compact surface area, introduced during the carburization step can be most accurately controlled by controlling the time and temperature of carburizing. This is a much more positive and simpler means of control than in the usual gas carburizing method in which the ratio of a carburizi-ng gas such as methane, and carrier gas such as hydrogen, is largely determinative of the amount of carburization. The carbon content introduced in representative times and temperatures is shown in Figs. 1 and 2 of-the accompanying-drawings. From Fig.- 1, which is a plot of time of carburization, as abscissae, and-lbs. carbon/ sq. in. las ordinates, it can readily be seen that for a 'givendesired carbon-content,- a time of carburization can-be determined'by reading horizontally.- The latter figure is useful whereit ispossible to operate at any of the three temperatures shown on the graph. Figure 2, however, is a plot of lbs. (carbon/ sq. in.) x" versus temperature. Using the latter curve, it is possible to determine the temperature to be used to obtain the desired carbon content during atime of one-half to one and one-half hours. A family of such curves can be, constructed from experimental data which willv enable the ready determination of both time andtemperatures.

The effect ofmoisture during carburizing is most pronounced, althoughnot to quite the same extent as during sintering, satisfactory carburizing being difiicult to obtain with an atmosphere having. a dew point. above -.l5 F. Better a-nd'more consistent results are obtained byrnaintaining thedew point in the carburizing furnace at least between F. and F. or even lower. Thislow dew point in the sinteringand carburizing steps is. easily obtained by passing commercial forms of pure hydrogen through-an oxidizing catalyst such as palladium to cause anyoxygen to react'with the hydrogen and then over activated alumina or otherchemical drying agents to remove the moisture thus formed. .Whenheated to the high carburizing temperatures employed, the dew point will then be very low. If the carburizing furnace is constructed with an impervious iron or other metal muffle, the mufile can be put under a slight pressure with hydrogen and the exit gases ignited and burned oif at the furnace exit. This will prevent backing up of moisture into the muflle.

The time required for carburizing will depend both on the amount of carbon desired to be introduced, on the temperature employed, and on the ratio of surface area to weight. As shown in the drawings, with temperatures between 2050 F. and 2250 F., carburizing for one-half to one and one-half hours will usually sufilce to introduce 0.01 to 0.2% carbon. For the usual application with this type of high chromium alloy, the latter amount of carbon will be sufiicient. As indicated in the drawings, the carburization may be carried further to obtain any desired carbon content.

The most convenient method of carrying out carburization is to pack the alloy compact in alu-ndum or other inert refractory material and place the whole'in a graphite or carbon boat or other vessel. The loaded vessel then is placed in the furnace and heated.

The homogenizing step is carried out in a molten bath such as any of the molten heat-treating salts, such as barium chloride, borax, and the like, molten glass and others. The exclusion of all atmosphere during this step prevents both oxidation and reduction, ensures efiicient heat transfer, and eliminates the difficulties attendant on controlling the atmosphere. The use of homogenizing or soaking temperatures above 2100 F. permits a sufficiently short cycle to be practical and prevent substantial grain growth. At these temperatures, the carbon freely diffuses through the compact. The time required for this step will depend on the time and temperature and also on the size and shape of the compact and on the carbon content introduced during carburizing. However, practical commercial operations at 2100-2300" F. can be accomplished in as little as one hour to as much as eight or twenty-four hours. The use of homogenizing periods of two to ten hours usually is suflicient.

The powdered metal mixture utilized in this invention should contain at least 11% and not more than chromium and may contain up to 2.5% nickel to obtain the desired heat-treatable, fatigue-resistant stainless steel composition. Small amounts of silicon (1% max.), sulfur, phosphorous, manganese, molybdenum, tungsten, titanium, columbium, zirconium, cobalt, and other alloying ingredients may be present, if desired, up to a total of not more than 5% by weight. The chromium may be added in the pure state as ferrochrome or chromiumnickel alloy powder. The use of ferrochrome and other chromium alloys is much preferred, not only because of lower cost, but also because the intensely reactive chromium is diluted and less apt to form carbides, oxides etc. during the initial sintering step. Ferrochrome containing from 67 to 72% chromium, 1% silicon (1.5% max), 1% carbon (max), sulfur up to 0.3%, phosphorous up up to 0.04% (max), and the balance iron is a commercial material found entirely satisfactory. Similarly, the nickel may be added as a pure metal or as a high nickel alloy of iron or chromium. The use of iron alloys of these more reactive constituents, and especially of chromium, reduces the formation of a surface oxide coating.

The iron powder employed generally should contain less than 0.02% carbon and be very low in occluded, absorbed or oxide-form of oxygen, i. e. below 0.1%. In any case, the iron powder must be soft and have a high compressibility. A suitable material of this type is electrolytic iron and any of the very pure commercial forms of iron powder. Any of these forms of iron may readilybe deoxidized by heating in hydrogen for one hour at'about 1900 F., and if caking occurs, reground to a suitable mesh size. Regrinding and classification does v 6 not reintroduce much oxygen. The iron preferably is finely ground and composed of uniform particles, any powder having a sieve analysis between to 325 mesh or smaller being satisfactory. Generally, the finer iron powders seem to facilitate difiusion of alloying ingredients. The size of the-other ingredient powders should be comparable.

The metal powders may be mixed in any convenient manner to obtain a thorough blending of the ingredients. The powdery mixture is then ready for pressing.

The invention will now be described with reference to certain specific examples which are intended to be illustrative only.

EXAMPLE 1 An iron powder known as Plast-Iron powder, a trade name, and having the analysis given below was mixed with 325 mesh ferrochrome powder (analysis below) in proportions to yield 12.5% chromium on the total mix.

Iron powder Analysis: Percent Iron 99.17 0 (hydrogen loss) 0.750 max. Carbon 0.020 Manganese 0.003 Phosphorous 0.005 Silicon 0.007 Sulfur 0.005 All others 0.037

Sieve analysis Mesh: Percent Meta1 Powder Association standard sieve analysis.

Ferrochrome Chromium 67-72% Silicon 1.50 max.

Carbon 0.75-1.00 Phosphorous 0.04 max. Sulphur 0.03 max.

Balance iron.

Before mixing with the ferrochrome the iron was heated in dry hydrogen at 1900 F. for one hour and reground in a Mikro-Pulverizer. Mixing of the two powders was continued for one hour. The resulting mixutre was placed cold in metal tensile specimen molds and compacted at- 70 tons per square inch. Sintering of the tensile specimens was carried out in a General Electric box-type high temperature electric furnace. A low-carbon steel muffle extended through the furnace to project on either side.

The ends of the muflle were closed in and provided with small burner-like jet openings. A current of commercially-pure hydrogen, treated as described, and having a dew point below 25 F., was introduced under a slight pressure and the jets on both ends of the mufiie ignited, and the muffle brought up to temperature. The compacts were then placed inside the muffie and heated for two hours at 2350 F., cooled and coined at 70 tons per square inch.

At this point, the compacts were found to contain only 0.03% carbon as against a minimum of about 0.08% required for this 12.5% chromium iron alloy. The density of the compacts were found to be about 7.50 or 97% of theoretical (7.75). The tensile bars were found to have a tensile strength of 47,500 lbs./ square inch as compared to a literature value of 50,000 lbs/sq. in. for a similar very low carbon wrought alloy.

When compacts of the above general composition were sintered in hydrogen of a higher dew point and higher 1 oxygen content, the specimens showed the presence of an, oxide coating on some of theferrochrome particles. These compacts, .afterc'ompacting', sintering, coining and resiri'tering, failed to reach 95% of theoretical density and showed some evidence of inadequate chromium difr fusion. This same effect is not observed with soft iron nor to the same degree with lower chromiumalloys containing less than 11% chromium. It appears, therefore,

that the furnace atmosphere is highly critical during the sintering of high chromium alloys.

'Several of the compacts prepared above were carburized to increase their carbon content to the range of 0.08 to 0.15%. The compacts were packed in alundum in closed graphite boats so that no metal came indirect contact with the boat. The boats were heated for thirty minutes at 2150 F. in the mufile described above, with the hydrogen atmosphere having a dew point below F;, then austenized for fifteenminutes at 1800 F. and quenched in oil. The resulting heat-treated specimens developed the following properties when'drawn at vari- It is readily apparent that the specimens prepared as above, by compacting, sintering, coining and carburizing produce a heat-treatable stainless steel article having a high density. Fatigue-resistance of these specimens was very good. As shown above, the physical properties closely approach those of a wrought high-chrome steel.

EXAMPLE 2 The addition of small amounts of nickel has quite a surprisingly large effect on the tensile strength of a 12.5% chromium alloy, when made by the powder molding method of this invention. A 12.5% chromium mixture such as was utilized in Example 1 was modified by the addition to separate portions thereof of, respectively, 0.5%; 1%; 1.5%; 2% and 2.5% nickel powder. The resulting mixtures were compacted at 70 't. s. i., "sintered for two hours at 2350 F; in a hydrogen atmosphere at 25 to 45 F. dew point, coined at 70 t. s. i., and resintered for two hours at 2350 F. The tensile strength of the as-resintered specimen containing 2% nickel was about 94,500 lbs./ sq. in., a value double that of 4748,000

for; a specimen prepared in the same manner but with y no nickel- The density of these compacts was about 97% of theoretical.

When the compacts containing 2% nickel are packed in alundum in a graphite boat and heated for to 60 minutes at 2350 F. in a low dew point hydrogen atmosphere, a heat-treatable product is obtained. After proper heat-treatment in a low dew point atmosphere, tensile strengths of up to 160,000 lbs./ sq. in. and increased hardness are, obtained. Homogenization of similarspecimens, but having a thickness of A inch or more is readily accomplished by heating for up to 8 or 10 hours at temperatures of 2250 to 2350 F. After the homogenized specimens are heat-treated, they have a uniformly finegrained structure.

EXAMPLE 3 A cylindrical specimen of a composition similar 'tothose' of Example 1 (containing 12.5 chromium and no nickel) is made by compacting a mixture of soft iron and ferrochrome. The diameter of the specimen is 1 inch and the thickness /s inch. The mixture is first compacted at 70 t. s. i., sintered for three hours at 2350 F. in a hydrogen atmosphere having a dew point between 'and Rand finally coinedat 70 t. s. i. The resulting'co-mpact, in the as-sintered condition, has a-carbon content of about point below -25 F. for the prescribed time.

0.04% and adensity of: about 7.50 or between-96 and 98% oftheoretical. The weight and surface area of the compact is then determined and the weight of carbon per unit areaneeded'to produce a final carbon content of 0.15%. is calculated. The value thus determined isused to determine from Fig.1 the time of carburization required at 2250 F. According to Fig. 1, only about 45 minutes is required for this purpose. The sample is then packed in alundum in a graphite boat and placed in the furnace, described above, having a hydrogen atmosphere and a dew The resulting carburized compact is then immersed in a molten bath of. barium chloride for three hours at 2100 F. for homogenization. Thecarburized, homogenized cylinder is found to be heat-treatable to yield a uniform finegruined structure.

The stainless steel powder molding method of this invention is highly useful and has a number of outstanding advantages. In the first place, the method is the first powder molding method to produce a heat-treatable stainless steel article having physical properties approaching those of a forged articleof the same or similar composition. Secondly, there is no need to control the carbon content during the preliminary compacting and sintering steps since the carbon is introduced after the article is compacted, sintered and coined. With high chromium steels, precise control of carbon content is a most difficult operation in the presence of a reducing atmosphere. Thirdly, the low carbon content produces a higher density product than previously known alloy powder molding methods. The obtaining of such high densities is believed due, in part, to the use of a highly compressible iron powder, to the lowcarbon content, and the use of a'low dew point reducing atmosphere during sintering. Fourthly, the method of this invention is believed to be the first powder method to be successfully applied to high alloy steels and-especially to-high chrome, stainless alloys. The production of articles ofcomplicated shapes by powder molding leads to large economies by elimination of expensive forging, machining, finishing and plating steps. Fifthly, the meth od produces a fatigue-resistant article. of such a material is not possible with known powder molding methods as applied to stainless or high alloy steels because of excessive porosity in the compacts. Other advantages reside in the method including the production of high strength, hardness, high finish and others.

The: heat-treatable, fatigue-resistant products made by the method of this invention require little further treatment for use in difiicult applications. Usually, further machining, finishing, polishing or plating is not required. If an exceptionally high finish is required, a simple electropolishing treatment usually is sufiicient without plating or other treatments' The products can be heat-treated according to the usual techniques employed on the forged or wrought forms of the same alloy. Compressor blades for gas turbines, steam turbines, jet engines and the like, when made by this method of this invention, approach the properties of the best wrought and precision ground blades now in use.-

What is claimed is:

1. A method of;making ,a heat-treatable article of stainess steel which comprises compacting a mixture of between 11 and 25% of a powdered form of chromium and up to 2.5 of apowdered form of nickel, and balance low carbon, deoxidized; highly-compressible iron powder at a pressureabove 50 tons per, square inch, sintering' the resulting compact at a temperature-above 2000 F. in a hydrogen atmosphere; having a dew point, below 25 F. to-decrease thecarbon content of the compact to'below 0.1%, coining, the resulting;sinteredcompact at a pres.-

sure above 60 tons per square inch, and carburizing saidv sintered and coined compact at a temperatureabove2000. Frin ahydrogen atmosphere having a dewpoint b'elow:

IS* F; and in the presence of, but out of contact with;

The production 2300 and 2450 F. in a hydrogen atmosphere having a 10 dew point below 25 F, to decrease the carbon content of the compact to below 0.1%, coining the resulting sin-' tered compact at a pressure above 60 tons per square inch, carburizing said siutered and coined compact at a temperature between 2100 and 2300" F. in a hydrogen atmosphere having a dew point below -15 F. and in the presence of solid carbon until the carbon content is in the range of 0.08 to 1.0%, and homogenizing said carbu- 10 rized compact in a molten atmosphere-free bath at a temperature between 2100 and 2300 F.

3. As a new article of manufacture, a product which is a heat-treatable powder molded stainless steel article containing between 11% and 25 chromium, up to 2.5 nickel, carbon in the range of 0.08% to 1% and balance iron, which has a density of at least 96% of theoretical and which is prepared in accordance with the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,228,600 Hardy Jan. 14, 1941 2,397,533 Chevigny Apr. 2, 1946 2,489,838 Webb Nov. 29, 1949 2,489,839 Witney Nov. 29, 1949 2,495,823 Rice Jan. 31, 1950 2,527,521 Bloom Oct. 31, 1950

Claims (1)

1. A METHOD OF MAKING A HEAT-TREATABLE ARTICLE OF STAINLESS STEEL WHICH COMPRISES COMPACTING A MIXTURE OF BETWEEN 11 AND 25% OF A POWDERED FORM OF CHROMIUM AND UP TO 2.5% OF A POWDERED FORM OF NICKEL, AND BALANCE LOW CARBON, DEOXIDIZED, HIGHTLY-COMPRESSIBLE IRON POWDER AT A PRESSURE ABOVE 50 TONS PER SQUARE INCH, SINTERING THE RESULTING COMPACT AT A TEMPERATURE ABOVE 2000*F. IN A HYDROGEN ATMOSPHERE HAVING A DEW POINT BELOW -25* F. TO DECREASE THE CARBON CONTENT OF THE COMPAFT TO BELOW 0.1%, COINING THE RESULTING SINTERED COMPACT AT A PRESSURE ABOVE 60 TONS PER SQUARE INCH, AND CARBURIZING SAID SINTERED AND CINED COMPACT AT A TEMPERATURE ABOVE 2000* F. IN A HYDROGEN ATMOSPHERE HAVING A DEW POINT BELOW -15*F. AND IN THE PRESENCE OF, BUT OUT OF CONTACT WITH, SOLID CARBON UNTIL SAID COMPACT CONTAINS CARBON IN THE RANGE OF 0.08 TO 1.0%.

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