US3677749A - Method of making high-density sintered chromium-bearing iron alloys - Google Patents

Abstract

CHROMIUM-BEARING IRON ARTICLES AND PARTICULARLY CHROMIUM-BEARING STEEL ARTICLES SUCH AS STAINLESS STEEL OBJECTS ARE PRODUCED BY COMPACTING, REDUCING, AND SINTERING MIXTURES OF METAL COMPOUND POWDERS AND FERROCHROMIUM POWDERS. AT LEAST 35 PERCENT, BY WEIGHT, OF THE POWDER IS OF A PARTICLE SIZE LESS THAN 10 MICRONS.

Description

Int. Cl. B22f 1/00 U.S. Cl. 75-211 15 Claims ABSTRACT OF THE DISCLOSURE Chromium-bearing iron articles and particularly chromium-bearing steel articles such as stainless steel objects are produced by compacting, reducing, and sintering mixtures of metal compound powders and ferrochromium powders. At least 35 percent, by weight, of the powder is of a particle size less than microns.

BACKGROUND In US. patent applications Ser. Nos. 872,481, filed Oct. 30, 1969, entitled Method of Making High Density Sintered Metal, and 827,846, filed May 26, 1969, entitled Slip Casting, there is revealed the surprising dis covery that dense metal objects free of cracks, internal defects, and surface irregularities may be obtained by compacting, reducing, and sintering very fine-grained metal compounds. Particular success has been demonstrated in the production of fine-gage wire and thin wall tubing by first extruding plasticized metal-powder compounds, reducing in a hydrogen-yielding atmosphere, and sintering the extruded and reduced product. Particular success has also been demonstrated in the production ofhigh density slip cast articles by reducing in a hydrogen-yielding atmosphere and sintering.

The metal compounds are preferably those easily reduced, i.e., characterized by having standard free energies of reaction with hydrogen to form elemental metals of less than about kilocalories per gram atom of hydrogen at the reaction temperature. We have had particular success in reducing fine metal oxide particles-particularly iron oxide particles.

A prerequisite for our success has been the use of powders having a particle size distribution wherein at least 35 percent, by weight, of the particles are less than 10 microns in diameter. However, preferably the mean particle size will be no greater than about 6 microns and at least 25 percent of the powder has a particle size no greater than about 2.5 microns. We have had particular success in utilizing iron oxide particles wherein all of the particles are less than one micron in diameter as measured by Coulter counter.

When extruding fine-gage wire mils diameter or less) or thin-wall tubing (20 mils wall gage or less), none of the particles should have a diameter that exceeds about one third of the extrusion die orifice.

In the aforementioned patent application the fact is disclosed that metal alloys such as stainless steels may be fabricated from blends of differing reducible metal compounds or powder mixtures of one or more reducible metal compounds or powder mixtures of one or more re- 3,677,749 Patented July 18, 1972 ducible metal compounds and one or more metals. The use of ferrochrome metal powders in combination with iron oxide in the production of chromium-bearing steel is illustrated.

THE INVENTION This invention relates to the discovery that surprising superior chromium-bearing steel products substantially equivalent to wrought steel are obtainable when using the process of the aforementioned patent application by utilizing ferrochromium as the source of chromium.

Although Cr O powders may be blended with Pe O and reduced just prior to sintering, the time required for complete reduction of the chromium oxide below the melting or sintering temperature of iron or nickel is excessive. We have been able to make a successful product in this manner in that we have been able to produce finegage stainless steel wire having a continuous chromiumbearing corrosion-resistant surface but showing unreduced Cr O' particles and reduced but nondiifused chromium in the core of the product. Although this wire exhibits surprisingly good mechanical properties for having unreduced Cr O and is a useful and marketable product, it is not the equivalent of conventional wrought stainless steel either in respect to long time corrosion resistance or mechanical properties.

Other chromium compound particles 'which may be substituted for the Cr O are CrO (chromic acid) and CrCl These materials are deliquescent, making it diflicult to maintain a consistent water content in the plasticized mass for proper compaction. However, of equal importance is the fact that some Cr O forms during the reduction of the Fe- O (or Fe O where compounds such as Cr0 or CrCl are present to provide a structure similar to that obtained when the source of chromium is Cr O \Additionally, where chromium metal compounds such as Cr O CrO Crcl etc., are reduced to particles of elemental chromium or Where elemental chromium metal powders are used as the chromium metal source such elemental metal fails to migrate or diffuse into the iron or iron and nickel matrix at a sufficiently rapid rate to effect a reasonably homogenous chromium-containing steel except perhaps at the surface of the compacted, reduced, and sintered part.

We have found that powdered ferrochromium, particularly the commercially available grades of ferrochromium commonly used in the steel industry for chromium additions to steel in conventional melting practices, are ideal for providing chromium in the method of the present invention.

The commercially available ferrochromium alloys consist essentially of beneficiated and upgraded iron-chromium mineral deposits having chrominum contents ranging from about 50 to percent, by weight, and carbon contents ranging from a trace to 8.00 percent, by Weight. The significance of these alloys is that the chromium is already in solution in an iron matrix or perhaps it is more accurate to say that from 30 to 50 percent, by weight, iron is in solution in a chromium matrix. In any event, we have found that such an alloy in powdered form mixed with iron oxide powder upon reduction and sintering forms a homogenous or essentially solutionalloyed chromium containing iron or steel.

Ferrochromium frequently contains impurities such as phosphorus, sulfur, silicon, manganese, etc. Such impurities may be present in proportional amounts that will not unduly contaminate the chromium-bearing iron or steel articles being produced. For example, when producing A.I.S.I. or S.A.E. Type 304 stainless steel, the sintered product should contain no more than about .045 percent maximum phosphorus, or .030 percent maximum sulfur and accordingly a powdered ferrochromium should be selected and added to appropriate iron and nickel oxide powders in amounts that will not only yield from about 18 to 20 percent, by weight, chromium but which at such concentrations will yield no more than about .045 percent maximum phosphorus and .030 percent maximum sulfur. Corresponding selection is readily used to limit the silicon, manganese, and carbon contents of the ultimate product.

The use of ferrochromium is a source of chromium metal for the alloying of dense metal products made in accordance with the method of our aforementioned patent applications has the added advantage of providing an easy means for supplying carbon to the ultimate product. Carbon, of course, is an essential addition to steel and to meet A.I.S.I. of S.A.E. specifications for Type 400 series stainless steel or chromium containing low alloy steels such as Types 4140, 4340, and 5140, it is necessary to effect the addition of carbon. Commercially available ferrochromium alloy may contain a wide range of carbon contents so that practically any carbon contents can be easily established while meeting any of the desired commercial chromium contents.

The use of high carbon content ferrochromium alloys (1.00 percent carbon or greater) over low carbon content ferrochromium alloys is preferred because the high carbon content compositions are more easily comminuted to a fine particle size by mechanical means (crushing, grinding, etc.) due to its greater hardness and brittle characteristics. The particle size of the ferrochromium will preferably be the equivalent of the iron oxide powder (at least 35 percent, by weight, under microns; preferably a mean particle size not greater than 6 microns and at least 25 percent, by weight, being under 2.5 microns; and optimumly all of the particles being under 1 micron).

A method of producing a low carbon content product while using a high carbon content ferrochromium consists of decarburizing high carbon containing articles during the sintering step by providing a high moisture to 40 F. dew point) environment. In this reaction oxygen from the moisture forms CO with the surface emerging carbon. This procedure is particularly advantageous in producing low carbon, thin-gage wire shapes or thin-Wall tubing since decarburization is effective throughout the cross-sectional area of the extruded, reduced, and sintered article.

It will, of course, be understood that other alloying additions may be made either by including metal compounds of alloying metals in the iron oxide-ferrochromium powdered mixture or by including other alloying metal powders. In either event, the total particle size distribution of the powdered mixture must be within the above-recited parameters (at least 35 percent, by Weight, under 10 microns; preferably a mean particle size of 6 and percent, by Weight, being under 2.5 microns; and optimumly all of the particles being below 1 micron in diameter). The advantages of the present invention relating to high density, good surface, and high mechanical properties are largely lost where excessive quantities of metal powders are employed. Further, the pyrophoric nature of fine metal powders make them hazardous to handle particularly in a mixture of oxide particles. Consequently, it is preferable that the total quantity of metal powder (including ferrochromium) does not exceed about 50 percent, by volume, of the mixture.

For example, we prefer to blend nickel oxide powder with iron oxide powder and ferrochromium powder when we produce the Type 300 series stainless steels (essential- 1y l8 Cr-18 Ni-balance Fe). Such a mixture may be blended with a plasticized or binder such as starch and water, extruded into fine-gage wire (or thin-wall tubing), reduced with a temperature range of from about 930 F. to 1200" F. in the presence of reducing gaseous environment (hydrogen, hydrogen yielding, or CO) and sintered within the temperature range of from 1830" F. to 2450 F. to form a dense smooth surfaced austenitic steel wire which will meet most specifications for wire made of these grades of steel. High reducing and sintering temperatures and long time treatments such as are necessary to effect reduction and diffusion of chromium compounds and chromium metal are unnecessary and the ultimate product exhibits a structure essentially free of unreduced metal compounds or undiifused metal particles.

The preferred means of making further alloying additions depends, of course, on the physical properties of the addition. For example, in producing Type 316 stainless steel we may use elemental molybdenum powder or molybdenum oxide powder (M00 whichever is most economical to make the molybdenum addition. Where additions of silicon are to be made these may be made by the addition of ferro silicon powder. Further, manganese may be provided through the use of ferromanganese.

Although chromium-bearing steel products made in accordance with the method of the present invention closely approximate conventional wrought chromium-bearing steel products, some improvement of density and corrosion resistance may be effected by subsequent working. For example, the density and corrosion resistance of austenitic grades of stainless steel wire made in accordance with the method of the present invention is measurably improved by subsequent drawing and annealing or sintering.

The use of hydrogen to provide the environment for reducing the metal compound powders to elemental metal is a preferred embodiment of the present invention; however, we have found that other reducing materials may be employed. For example, we have found that the aboverecited metal compounds and particularly iron oxide can be reduced by partially or wholly substituting carbon monoxide for the hydrogen reducing environment.

Any metal compound powders having particles of any general shape (i.e., spherical, oblong, needles, or rods, etc.) and originating from any source (i.e., ore deposits, ore concentrates, precipitates, etc.) may be employed for compaction, reducing, and sintering in accordance with the present invention. The sintered article derived will possess a substantially pore free structure, a smooth surface, and will exhibit densities generally in excess of percent of theoretically completely dense material. We have, however, discovered that metal oxide powders obtained by the process of spray drying a dissolved metal compound provides superior compacts (particularly extrusions) that reduce and sinter in a manner to provide objects of greater density and better surface and structural integrity than slips made of metal oxides from other sources.

Spray drying of solutions containing dissolved metal compounds to effect metal oxide powders is a well-known prior art procedure. For example, this method is utilized to regenerate hydrochloric acid pickling solutions that have been used in the iron and steel industry to remove mill scale and other forms of iron oxide from iron and steel products. The used aqueous pickling solution containing up to about 11 percent, by weight, free hydrochloric acid, and up to about 35 percent ferrous chloride is sprayed through a nozzle into a heated chamber (about 1000 P.) where the ferrous chloride is converted into iron oxide and hydrochloric acid, as follows:

One version of the process is described in the article Liquor Regeneration Slashes Cost of Steel Pickling by Joseph A. Buckley, Chemical Engineering, Jan. 2, 1967, pages 56-58.

Regardless of the exact parameters or specific apparatus used, oxides produced as above described and particularly spray-dried iron oxides are believed to consist of minute hollow spheroids. The spheroids themselves cannot be used to make satisfactory compacts for reducing and sintering in accordance with the method of the present invention and it is our theory that when fragmented the resultant powders produce a compact of superior characteristics for use in conjunction with the method of the pres ent invention.

We believe that the fragmented spray-dried particles tend to agglomerate making accurate particle size determinations difficult. However, Coulter Counter measurements indicate that after three hours of dry ball milling the powder is substantially all under one micron size (average diameter).

By the term fragmented as it relates to the hollow spheroidal particles obtained by the above-described spray drying technique, we mean the breaking up of the hollow spheroids into smaller particles. Such breaking up is most conveniently accomplished by mechanical means such as grinding. We have had particular success in ball milling such spheroidal particles for periods of from about 1 to hours; however, other grinding techniques may be employed.

When practicing the preferred embodiment of the present invention wherein spray-dried and fragmented metal oxides are utilized to produce the compact or extrusion, it will be preferred that the alloying compounds also be of the spray-dried-fragmented variety. Some advantage will be experienced in utilizing any amount of spray-dried and fragmented metal oxides in the compact regardless of how small the proportion of these metal compound fragments are in relation to the metal compound particles; however, such advantages (green and sintered densities and sintered structure) are not readily discernible where such fragments do not constitute at least about 10 percent, by volume, of the particles present.

Accurate particle size determinations of fine-grained powders are diflicult to obtain, particularly where the particle size distribution of such powders includes a fraction that is less than 10 microns in diameter. Such determinations are most difiicult where the particles are of nonuniform shape, For example, if the particles consist of crushed or ground spheroids as is speculated in regard to ball milled spray dried HCl pickle liquor oxides many of the particles are likely to be of a relatively elongated or semicircular shape (sections of a hollow spheroid) so that it is diificult to determine actual diameter. Elongated particles will not pass through a screen having a mesh that is designed to accommodate a relatively symmetrically shaped particle of equivalent mass. As a result particle size and particle size distribution measurements vary to a considerable degree for a given powder between the known methods and procedures for making such determinations. For the purposes of the present specification and claims we have used Coulter Counter Analysis to make particle size determinations. In this system the particles are suspended in an electrically conductive liquid and drawn through a small orifice. A current is caused to flow through the orifice by means of two immersed electrodes, one on each side of the orifice. As the particles flow through the orifice, the change of electrical resistance between the electrodes is measured to determine particle size. Thus, the measure is one based on particle mass and is not affected by shape.

For the purposes of the present specification and the claims, all particle size determinations and limitations are in terms of Coulter Counter measurements and shall include metal compound particles meeting such determinations irrespective of particle size determinations by other means.

Also, for the purposes of the present specification and claims the terms compacting or compaction" shall include slip casting.

EXAMPLES Fine-gage steel wire having analyses in substantial conformity with stainless steel A.I.S.I. and S.A.E. Types 430, 431, 304, and 316 were made in accordance with the method of the present invention. The iron oxide was the spray dried byproduct of the closed-cycle HCl pickling process. This oxide is Fe O and has a purity of 98.6 percent or better, the principal impurity after sintering being about 0.4 percent manganese.

The ferrochromium had the following analysis:

C4.2 percent, by weight Cr68.l percent, by weight Feessentially the balance When making Type 316 stainless steel, both powdered M00 and elemental molybdenum metal were employed.

Nickel for the Type 304, 316, and 431 grades was derived from reagent grade NiO and NiCl All of the starting materials were dry ball milled in a steel-lined ball mill with an 8-inch inside diameter. A nominal charge of 150 grams of powder per 1500 grams of steel balls was added to the mill. The starting powders were less than 37 microns in size. Speed of the mill was r.p.m. The milling time was from 16 to 64 hours.

To grams of the mixture of ball milled metal compounds and ferrochromium were added 13.6 grams of a cooked water-starch binder consisting of 15 grams of No. 34-1 Buffalo cornstarch and 100 ml. water, heated with stirring until gelled. Or, 100 grams of the metal compound-ferrochromium powder was mixed with 1.76 grams of dry pregelatinizied starch and 12 ml. of water was added to this mixture.

The plasticized mixtures were then extruded into 18 mil diameter filaments on a 75 ton hydraulic press at pressures of 2880' to 7200 p.s.i. The die had a /z-inich 1diameter cavity, an 18 mil orifice, and was l A-inches ong.

After drying in air at 300 F. for 30 minutes to remove excess water, the filaments were heated rapidly to 1100 F. in an Inconel tube furnace with a dry hydrogen atmosphere (-40 dew point) and held at F. for 15 minutes to reduce the Fe O and NiO to iron and nickel. The filaments were next heated slowly to 2200 F. as follows:

Holding Holding time at time at tempera- Temperatempera- Temperature ture ure ture F.) (min.) F.) (min.)

TABLE I Particle size Tensile properties distribution sintering time Sintered Composition Percent of Tensile Elongation Identi- Alloy Mean iameter, Theoretical strength, percent fieation type 25 p Max. [1 Hours F. mils Cr Ni Mo density K 5.1. in 1 in.

& Determined by Culter counter after ball milling mixture.

b Made with molybdenum metal powder.

Made with molybdenum trioxide.

Microexaminat-ion of specimens identified in Table I above as 1 showed a relatively porous structure; however, all of the other specimens showed very little or only slight porosity. All of the specimens were ductile and could be bent into a tight When specimens identified as 6 were drawn to 3.8 mils diameter and resintered at 2000 F. for one hour, the tensile strength was 104 K. s.i., the elongation was 41.4 (percent in 1 inch), the density was nearly 100 percent of theoretical, and the microstructure revealed substantially no porosity.

Corrosion data on Types 304 and 316 wire are given in Table II below. The Type 304 (No. 5) wire, both as sintered and after cold drawing and resintering had good corrosion resistance. The Type 316 specimens 8 also showed good corrosion resistance in the as-sintered condition. The Type 316 specimen in which M00 was used as the source of molybdenum showed poor corrosion resistance in the as-sintered condition; however, after drawing and resintering this specimen showed good corrosion resistance.

mix of iron oxide (spray dried by-product of the closed cycle HCl pickling process), nickel oxide (NiO) and ferrochromium powder (C 4.2 percent, Cr 68.1 percent, balance essentially Fe). The mixture was balanced to meet Type 304 specifications. The HCl iron oxide was calcined at approximately 850 F. for eight hours and reground to eliminate excess acid. All of the starting materials were dry ball milled for 48 hours in the steel-lined ball mill with an 8-inch inside diameter in the manner described above. The average particle size was less than one micron. The slip was mixed by tumbling in a ceramic ball mill. It was necessary to make hydrochloric acid additions to lower the pH and render the slip castable.

A drain casting was made at a pH of 6.45. After drying and separation from the mold the casting was reduced at 1150 F. in a hydrogen atmosphere (held at temperature for approximately /z-hour) and inserted at 220 F. for three hours.

TABLE TIL-CORROSION RESISTANCE OF sTAINLlfiilsg lTtljiiggs WIRE PRODUCED FROM OXIDE-FERROCHROMIUM Density Percent Corrosion Wire 0 rate, Sample Alloy diameter, Carbon theomils per identification type Condition nnls percent G./ce. retical year Remarks As sintered 12. 5 0. O8 7. 2 91 7 Drawn and resintered 9. 0 0. 08 7. 9 100 5 Conunerci 5-20 Rating in 70 percent boiling nitric acid.

As sintered 12. 6 19 Produced from Mo powder. do 11. 8 0. 31 7. 8 98 Produced from M00; powder.

Drawn and resintered 8. 5 0. 09 8. 0 100 11 Do. Commercial 6-20 Rating in 70 percent boiling nitric acid.

B Determined according to ASTM Designation A-262-56T percent boiling nitric acid).

b Average of five 48-hour test periods. Completely dissolved in first 48-hour evaluation. 4 Metals Handbook, Volume 1, page 75, ASM (1968).

The Wire specimens of Tables I and II above that were redrawn after sintering were drawn from about 12 mils (as-sintered diameter) to about 4 mils. This wire was drawn through a sequence of 14 diamond wire dies of the following sizes:

Type 430 stainless-steel wire could be drawn to 4-mils diameter without an intermediate anneal. Type 304 required intermediate anneals at 1 850 to 2050 F. after drawing to 9 and 5 mils, and Type 316 wire required intermediate anneals at 1900 to 2100 F. after drawing to 11, 9, and 5 mils.

After drawing, some wires were resintered at 2200 F. in dry, purified hydrogen to produce wire of nearly 100 percent density.

A Type 304 stainless steel slip was prepared from a The resultant sintered product exhibited a clean, smooth, crack-free surface and a density of 92.3 percent of theoretical density.

We claim:

1. In a process for making chromium bearing articles from agglomerates that consist essentially of a mixture of particulate metal compounds, particulate ferrochromium, and a plasticizer or binder by compacting said agglomerate into a shaped compact, exposing said compact to a reducing environment for a period of time disposed to elfect reduction of substantially all metal compounds reducible in said environment and exposing said compact to a temperature disposed to eifect sintering of the reduced metal particles and ferrochromium particles so as to increase the density of said compact wherein the improvement comprises:

-(a) having at least 35 percent, by weight, of the particles of said mixture being less than 10 microns in diameter; and

(b) reducing said compact at a temperature in the range from 930 F. to 1200 F.

2. The method of claim 1 wherein said reducing environment is provided by at least one reducing agent selected from the group of hydrogen and carbon monoxide.

3. The method of claim 2 wherein a major portion of said metal compounds consist of iron oxides and said chromium-bearing article consists of a steel article.

4. The method of claim 3 wherein the mean particle size of said mixture of particles does not exceed about 6 microns and at least 25 percent, by weight, of said particles are less than 2.5 microns.

5. The method of claim 4 wherein said agglomerate is compacted by being extruded through a die orifice to form an elongated-shaped article.

6. The method of claim 4 wherein said metal compounds consist essentially of iron oxide, said reduction temperature is from about 930 F. to 1200" F. and said sintering temperature is from about 1830 F. to 2350" F.

7. The method of claim 6 wherein said agglomerate is compacted by being extruded through a die orifice to form an elongated-shaped article.

8. The method of claim 7 wherein said exrusion is a filament having a diameter of 20 mils or less.

9. The method of claim 6 wherein said agglomerate is compacted by being slip cast.

10. The method of claim 4 wherein said metal compounds include a mixture of iron and nickel compounds and said compounds and ferrochromium are present in proportions to meet the composition ranges of chromium, nickel, and iron for Type 300 series stainless steels when sintered.

11. The method of claim 4 wherein said iron oxides and ferrochromium are present in proportions to meet the composition ranges of chromium and iron for Type 400 series stainless steels when sintered.

12. The method of claim 8 wherein said metal com- 10 pounds consist of materials which in combination with said ferrochromium are disposed to yield a composition upon sintering that will meet the essential composition limits of Types 300 and 400 stainless steels.

13. The method of claim 12 wherein said filaments are subjected to drawing disposed to reduce the cross-sectional dimensions thereof after sintering.

14. The method of claim 7 wherein the particles of said mixture of particles are substantially all under 1 micron diameter as determined by a Coulter counter.

15. The method of claim 5 wherein said elongated shape is tubing having a wall thickness not greater than about 20 mils.

References Cited UNITED STATES PATENTS 2,175,850 10/1939 Patterson et al -211 2,315,302 3 1943 Volterra 7521 1 2,686,118 8/ 1954 Cavanagh 75-211 FOREIGN PATENTS 557,950 6/ 1959 Canada 75--211 OTHER REFERENCES Hausner, Harry: New Methods for the Consolidation of Metal Powders, 1967; Plenum Press, pp. 3-4.

CARL D. Q UARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner U.S. Cl. X.R. 75-2100