US3173785A - Colored metal and method of making it - Google Patents

Description

United States Patent 3,173,785 COLORED METAL AND METHOD OF MAKlNG IT Samuel J. Manganello, Penn Hills Township, Allegheny County, Pa, assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Filed July 1'7, 1963, Ser. No. 295,812

4 Claims. (Cl. 75--201) This invention relates to a through-colored sintered metal product and a method of making it. More particularly, it relates to a cohesive, sintered mass of intimately dispersed minute particles of a metal and a colored inorganic compound.

Colored metal products are widely used in industry. Usually, they are colored with paints, enamels, chemicals, plastics and other surface-treating agents. These products are susceptible to scratching, chipping, abrasion, heat and wear, resulting in the destruction or marring of the colored surface or exposure of the uncolored metal.

An object of the invention is to provide a throughcolored sintered metal product that is not susceptible to the above-described defects and may be used in the general applications for sintered metal products.

A further object of the invention is to provide a method of making such a product comprising admixing a major portion of a metal powder and a minor portion of a colored inorganic compound powder that is chemically and physically stable under the sintering conditions, compacting the admixture and sintering it in a substantially inert atmosphere.

in carrying out my method, 1 preferably admix, in percent by volume, between about 60 and 80% metal powder, between about 20 and 40% of a colored inorganic compound powder, and between about 0 and of a suitable binding and lubricating agent. Preferably, the admixture particles should all pass through a standard 100 mesh screen. The metal powders may be pure or prealloyed, such as those used in conventional powder-metallurgy techniques. It is preferred to use metals that can tolerate large amounts of non-metallic inclusions, for example, such face-centered-cubic pure metals as nickel, copper, aluminum and the prealloyed austenitic stainless steels and bronzes. However, body-centered-cubic metals such as iron and plain carbon steels may also be used for products that have static or low-stress applications, such as decorative and ornamental sections.

The colored inorganic powders usable in my invention include oxides, nitrides, carbides, mixtures thereof and other refractory materials which retain color and are stable in admixture with the above-described base metals at sintering temperatures. These powders may be single or mixed compounds or mixtures such as the fri-t mixtures or other commercial, proprietary mixtures used as coloring agents for vitreous enamels and ceramics.

I have found that an important guide to the chemical and physical stability of a coloring agent with respect to a base metal, within the preferred range of sintering temperatures, may be determined from tables or curves plotting the standard free energies of formation of the coloring materials against temperature. Thus, for example, where a metal-oxide coloring powder is used with an ironcontaining base metal, such as iron, one of the austenitic stainless steels, or a carbon steel, it is only necessary to find the iron plus oxygen line in a chart showing the standard free energies of formation of the oxides per gram mole of oxygen. These charts for carbides, oxides, phosphides, silicides, nitrides and sulphides are given in various references, such as Thermochemistry for Steelmaking by J. F. Elliott and M. Gleiser, volume 1, published 1960, by Addison-Wesley Publishing Co., Inc, Reading, Mass.

Thus, within the preferred sintering temperature range between about 1500 and 2200 F., a colored metal oxide in intimate contact with an iron-containing base metal will be reduced to its metallic state *as the iron in the base metal is oxidized, if the colored metal oxide has a less negative free energy of formation than iron oxide, i.e., a free-energy of formation curve above that for iron oxide in a chart of oxides. Conversely, a colored metal oxide having a free energy of formation substantially more negative, i.e., a free energy of formation curve below that for iron oxide, will probably be stable and retain its color during sintering. Thus, I have found that, when admixed with an austenitic stainless steel and sintered at about 1850 F., red tungsten oxide and blue cobalt oxide having curves above that for iron oxide do not retain their color, rather becoming drab gray. However, green chromium oxide and brown cerium oxide, having curves below iron oxide, do retain their colors and produce a through-colored product.

Accordingly, other stable oxides for use with an ironcontaining base metal comprise the white aluminum and magnesium oxides, black titanium oxide, green manganese oxide and yellow titanium and vanadium oxides. As with iron, some of the other metals occur in more than one oxide form, often exhibiting dilferent colors, as well as in mixtures of oxides. However, white powders, being so similar in color to silver-colored, basemetal powders, are not as effective as coloring agents for iron-containing base metals as in the darker-colored powders. Other coloring materials, such as carbides or nitrides, can be similarly investigated from other charts. Thus, golden tantalum carbide, black boron carbide, yellow-brown titanium or zirconium nitrides, dark-blue tantalum nitride, and greenish-yellow magnesium nitride, would be coloring materials that may be successfully used with iron-containing base metals. In like manner, other base metals and prospective coloring agents may be investigated in their ranges of sin-tering temperatures. Thus, coloring agents chosen from the hereinabove mentioned oxides, carbides, nitrides and mixtures thereof may be admixed vn'th other base metals, compacted and sintered at the appropriate temperature, for example, with nickel powder at about 1850 F., with Monel metal powder (a nickel-copper alloy) at about 1750 F., with copper powder at about about 1350 F. and with aluminum powder at about 950 F.

Proprietary coloring agents are often mixtures, for example, of oxides. Since their compositions are usually not disclosed, they would require testing to determine their chemical and physical stability in intimate contact with base metals, under sintering conditions. The coloring agents should be stable to all constituents of a base metal, for example, to chromium as well as iron, where a stainless steel is the base metal.

The addition of nonmetallic binders and lubricants to the above admixture often serves to improve the compressing characteristics of the compacting step by reducing the interparticle friction and the friction between particles and die Walls. Between about 0 and 10% of such an agent or agents may be added, for example, zinc stearate, polyethylene glycol or talcum powder.

The admixture may be compacted at ambient or elevated temperatures and before or during the sintering step. After sintering, it may be cold rolled or otherwise worked. The compacting means may include pressing in a die or mold, roll forming, explosive or pneumaticmechanical compacting, or a slip-casting process, where a liquid dispersion of the metal powders is cast and subsequently sintered.

The compacted admixture may be sintered in a temperatureand atmosphere-controlled furnace. The sintering temperatures should not be high enough to cause the coloring agent or base metal to change their chemical or physical forms. The atmosphere should prevent oxidation of the base metal, reduction of the coloring agent and should not enhance interaction between the mixed powders. The temperature range may vary between about 1500" and 2200 F. for iron-containing base metals, but will vary according to the type of base metal and will usually be lower than when sintering only the base metal. For example, the range may be between about l500 and 2000 F. for iron or carbon-steel mixtures and between about 1800" and 2200 F. for the austenitic stainless steels. For relatively thin products, a sintering period of no more than about 20 minutes is preferred.

The sintering atmosphere should be free of such components as oxygen, sulfur dioxide or hydrogen sulphide. Preferably the atmosphere should be inert. Such an atmosphere may comprise argon, nitrogen or helium. The dew point of the atmosphere may range between about 60 and 60 F. Up to about hydrogen may be tolerated in the atmosphere to prevent possible oxidation of the base metal without reducing the coloring agent, if it is an oxide. Where hydrogen is present in the atmosphere, it is preferred to operate on the high (wet) side of the dew-point range. The sintered product is preferably cooled in the same inert atmosphere, usually for no more than about one hour.

A complete understanding of the invention may be obtained from the following typical examples of process showing how the products are made.

EXAMPLE 1 Substantially equal spoonfuls of powders were measured out and admixed in percent by volume to produce a mixture of about 70% of 310 stainless steel, 25% green chromium oxide and 5% Zinc stearate.

ders all passed through a standard 100 mesh screen. A portion of the mixture was poured into a 1% diameter by 3 high cylindrical mold to a height of about 1". At room temperature, a pressure of tons per square inch was applied to the mixture with a tight-fitting ram, the resulting disc being about A thick. The disc was sintered in a temperature-gradient furnace for 16 minutes at about 1850 F. in an argon atmosphere having a dew point between about 5 and 5 F., and then cooled in another part of the furnace for 7 minutes in the same atmosphere. The disc exhibited a through-colored, cohesive mass of intimately dispersed minute particles of silvery stainless steel and green chromium oxide. The coloring appeared speckled; that is, particles of green oxide were interspersed between bonded, silvery, metal particles. Upon fracturing, cutting or grinding the disc, the color appeared uniform throughout the cross section.

EXAMPLES 27 Example 1 was repeated as Examples 2 to 7 inclusive, substantially similar conditions being maintained, except for the indicated changes. The conditions and the results obtained from the seven examples are summarized in Table I. In Example 3, the frit was Solaramic frit, a product of Ferro Corp., Cleveland, Ohio. This is a hightemperature, vitreous-enamel frit believed to comprise a silicate or boro-silicate and is not in itself a coloring agent. The Example 3 coloring material comprised green chromium oxide and about 10% of the Solaramic frit. In Examples 6 and 7, the P105 coloring agent was a proprietary product of Ferro Corporation sold as a coloring agent for ceramics. The product is believed to comprise a mixture of calcined oxides of iron, chromium and zinc and has a medium-brown color. This coloring agent The powpowder was used as received from the manufacturer.

Table I Example 1 Example 2 70% 310 stainless steeL 25% chromium oxide. 5% zinc stearate 40 70% 310 stainless steel. 25% cerium oxide. zinc stearatc.

7. Silver and brown.

Example 3 Example 4 Volume Percent Base MetaL. Volume Percent Coloring Agent Volume Percent Binder Compacting Pressure, Tons/sq. in Sintcring Temperature, F Sintering Time, minutes Sintcring Atmosphere Atmosphere Dew Point, F Cooling Time, Minutes Final Color of Product, Particles of..-

65% 310 stainless steel..- 30% chromium oxide and frit. 2% talcum powder Nitrogen and 23% hydrogen. 0 to 20 15 Silver and green 70% electrolytic iron. 25% chromium oxide.

5% talcum powder.

5. Nitrogen.

Silver and green.

Example 5 Example 6 Example 7 Volume Percent Base Metal.... 316 stainless .et e

Volume Percent Coloring 30% chromium 30% P105 Agent. ox e.

Volume Percent Binder 5% zinc stcaratc Compacting Pressure, Tons/sq. 30

lIl. Sintering Temperature, F Sintering Time, minutes G Sintering Atmosphere Atmosphere Dew Point, F. Cooling Time, Minutes Final Color of Product, Particles of.

Silver and green.

65% 316 stainless 310 stainless 5% zinc stearatm. 5% zinc steer-ate. 3O 40.

60 Silver and gray Silver and brown.

The foregoing examples illustrate a preferred method and the products obtained with a variety of base metals, coloring agents and conditions. In the Example 3 product, the Solaramic frit did not enhance the color of the product, when compared with products made without its use. In contrasting Examples 5 and 6, the mediumbrown proprietary coloring agent P105 turned gray, hence was not stable at a sintering temperature of 2150 F., Whereas the green chromium oxide was stable. Example 7 indicated the P105 material to be stable at an 1850 F. sintering temperature, since it retained its brown color.

The surface of the colored-metal product may be buffed or abraded to bring out more of the base-metal color and thus render the product more speckled in appearance. Such treatment removes or fixes any excess colored material on the surface that may tend to rub off during subsequent use. The strength and ductility of my products will be less than that exhibited by products of the sintered metal alone. For this reason they are most useful for static or low-stress applications. Their mechanical and physical properties may be varied by such changes as the proportions, ingredients and pauticle size in the mixture, the density of the compact, sintering technique and subsequent work done thereon.

It will be evident from the foregoing that my invention provides novel through-colored sintered products and a method of making them.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as Well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

1. A method of making a sintered ornamental product comprising mixing in percent by volume between about 60% and 80% of a metal powder, between about 20% and 40% of colored-compound powder, said compound being chosen from the group consisting of metal oxides, carbides, nitrides and mixtures thereof having a more negative free energy of formation at the sintering temperature than said metal powder and less than about 10% or" a binder, compacting said admixture, sintering said admixture at a temperature between about 1500" and 2200 F. in an atmosphere comprised principally of inert gases chosen from the group consisting of argon, nitrogen, hciium and containing not more than about 5% hydrogen, maintaining the dew point of said atmosphere between about -20 F. and 20 F., and then cooling the sintered product in said atmosphere to produce a sintered product wherein colored specks of said compound are uniformly dispersed throughout the metallic, silver-colored matrix and contrast in color therewith.

2. A method as defined in claim 1 characterized by maintaining the dew point of said atmosphere between about 0 F. and 20 F.

3. A method as defined in claim 1 characterized by said admixture containing about 70% metal powder, about 25% colored-compound powder and about 5% of said binder.

4. A method as defined in claim 1 characterized by said admixture containing between about and stainless steel, between about 25% and 30% oxide powder chosen from the group consisting of cerium oxide and chromium oxide and about 5% of said binder.

References Cited in the file of this patent UNITED STATES PATENTS 2,479,914 Drugmand et a1 Aug. 23, 1949 3,012,951 Storcheim Dec. 12, 1961 3,047,383 Slayter July 31, 1962 3,061,756 Henderson Oct. 30, 1962 3,087,234 Alexander et al. Apr. 30, 1963

Claims (1)

1. A METHOD OF MAKING A SINTERED ORNAMENTAL PRODUCT COMPRISING MIXING IN PERCENT BY VOLUME BETWEEN ABOUT 60% AND 80% OF A METAL POWDER, BETWEEN ABOUT 20% AND 40% OF COLORED-COMPOUND POWDER, SAID COMPOUND BEING CHOSEN FROM THE GROUP CONSISTING OF METAL OXIDES, CARBIDES, NITRIDES AND MIXTURES THEREOF HAVING A MORE NEGATIVE FREE ENERGY OF FORMATION AT THE SINTERING TEMPERATURE THAN SAID METAL POWDER AND LESS THAN ABOUT 10% OF A BINDER, COMPACTING SAID ADMIXTURE, SINTERING SAID ADMIXTURE AT A TEMPERATURE BETWEEN ABOUT 1500* AND 2200*F. IN AN ATMOSPHERE COMPRISED PRINCIPALLY OF INERT GASES CHOSEN FROM THE GROUP CONSISTING OF ARGON, NITROGEN, HELIUM CONTAINING NOT MORE THAN ABOUT 5% HYDROGEN MAINTAINING THE DEW POINT OF SAID ATMOSPHERE BETWEEN ABOUT -20*F. AND 20*F., AND THEN COOLING THE SINTERED PRODUCT IN SAID ATMOSPHERE TO PRODUCE A SINTERED PRODUCT WHEREIN COLORED SPECKS OF SAID COMPOUND ARE UNIFORMLY DISPERSED THROUGHOUT THE METALLIC, SILVER-COLORED MATRIX AND CONTRAST IN COLOR THEREWITH.