US2826805A

US2826805A - Sintered stainless steel metal alloy

Info

Publication number: US2826805A
Application number: US403922A
Authority: US
Inventors: Robert L Probst; Brasse Gordon J Le
Original assignee: Federal Mogul LLC
Current assignee: Federal Mogul LLC
Priority date: 1954-01-13
Filing date: 1954-01-13
Publication date: 1958-03-18
Anticipated expiration: 1975-03-18

Description

SINTERED STAINLESC; STEEL METAL ALLOY Application January 13, 1954 Serial No. 403,922

Claims. or. 29-152 No Drawing.

This invention relates to permeable, sintered, stainless steel articles and to a method of making same.

There is a need in the art for metal articles having a desired uniform permeability, high resistance to corrosion, and which retain their strength at relatively high temperatures. Metal filters are typical examples of such articles. The property of uniform permeability can best be obtained by a loose sintering process; and, in order to effect a proper sintering of metal powder without destroying this property in the finished article, it is necessary to heat the metal to a partially liquid condition and .to hold it at this temperature until the individual metal particles are properly fused together.

Austenitic stainless steel is a metal pro-eminently suited for this purpose, but to our knowledge no one. has been able heretofore to make articles having uniform permeability characteristic from this metal by commercially-practicable procedures. Difficulty invariably is encountered because of the narrow solidus-liquidus range characteristic of this metal. For example, one powdered austenitic stainless steel admirably suited for use in the manufacture of metal filters and the like when tested produced weak, open specimens when heated to 2615" F. and formed a hard, dense specimen due to almost complete melting of the metal when heated to 2630 F. Manifestly, this range is too narrow for adequate furnace control, particularly in a commercial operation.

Carbon, sometimes used to broaden the solidus-liquidus range of steel alloys, has not been deemed practicable forthepurpose at hand because it is generally recognized in this art that carbon causes undesirable phase variations in the steel and in general destroys essential properties-of the metal. The instant invention is concerned primarily with the use of carbon .in the production of uniformly permeable articles of austenitic stainless steel so as to broaden the solidus-liquidus rangeofthe metal sufficiently to assure proper furnace control without losing desirable properties inherent in the metal which are essential to the.

article .and which are normally destroyed by the presence of carbon.

Specifically, it has been discovered that in the manufacture 10f sintered articles from austenitic stainless steel, carbon can be incorporated initially as an alloying element in the metal powder, if confined to certain critical limits, to broaden the liquidus-solidus range so as to permit proper sintering of the particles under conditions affording adequate furnace control in a commercial operation; and then, either simultaneously with the sintering operation or after the sintering operation has been completed, this carbon can be removed by decarburization due to the open permeable structure of the sintered article without adversely affecting the desired properties of the metal to any significant or appreciable extent. After the decarburization step the finished article is, for all practical purposes, the same in carbon content as though it had been made initially from metal particles free or substantially free of carbon. The discovery and appreciation of these critical factors makes it possible for the first, time,

2,826,805 Patented Mar. 18, 1958 2 sosfar as we know, to make uniformly permeable articles of high strength and resistancetocorrosion using austenitic steel powder by commercially practical procedures, and this makes available to the art articles .of .this character having properties not heretofore available.

Austenitic stainless steel alloys ito which this invention pertains are generally definedin therart as iron-chromiumnickel steels which contain not less :than approximately seven percent nickel and not less than approximately seventeen percent chromium. It .is generally recognized that some variation .inthe proportions given are permissible without removing the alloy from the category of austenitic stainless steel. .Tests and experience indicate that the instant invention isnoperative in the case of all metal alloys identifiable in,.the metallurgical art as an austenitic stainless steel. Also, the austenitic stainless steel used in this invention may have .various minor amounts of other elements conventionally .used .to affect the properties of, the metal. Typical examples of such other metals are molybdenum, titanium, .columbium, and silicon.

,The shape of the individual particles comprising the austenitic stainless steel powder used in the manufacture of sintered articles under this invention is of some significance. Commercially usable articleshaving a fair degree of uniformness of permeability can be made using metal particles of irregular shape. However, a marked and for some purposes exeedingly importantimprovement in permeability characteristics and particularly in the uniformness of permeability of the sintered article is achieved by using a metal powder in which the individual particles are spherical in form, It is not possible, of course, to produce a metal powderin which eachparticle is a theoretically perfect sphere, but austenitic stainless steel powders have been produced in which all or substantially all the particles are approximately spherical in form utilizing techniques disclosed in United States Patents Nos. 2,460,992 and 2,460,993. Experience has shown that permeable sintered articles such as filters and the like made from powder producedaccording to the teachings of these patents are unexpectedly and markedly superior to similar but commercially usable articles made from other commercially available austenitic stainless steel powder.

As suggested, the amount of carbon in the steel must be confined within relatively critical limits. Specifically we have found that carbon should be added in amounts greater than about 0.5% but not more than about 1.25% This carbon of course is added to and uniformly mixed in metal as an alloying elementbefore the metal is atomized to produce the powder so that the carbon is incorporated'in each individual metal particle used in the sintering process to produce the finished article. The primary purpose of the carbon is to 'broaden the solidus liquidus range of the metal sufiiciently to permit proper furnace control in the manufacture of the article and to produce sintered articles having uniform permeability characteristics by commercially practical .methods. The carbon also serves in a secondary capacity to reduce the melting point of the metal. This latter consideration is of some practical importance inasmuch as austenitic stainlesssteels melt at exceedingly high temperatures in the order of 26002700 F. and this temperature is destructive of vessels used to confine the metal during heating. Any significant drop in the melting temperature of the metal therefore adds a corresponding amount to the life of the heating vessel. Silicon also may be added as an alloying element in the steel to further reduce the melting temperature. Of course any amount of carbon in the steel broadens the solidus-liquidus range to some extent but we have, foundv that when added in amounts 1 less than 0.5% the carbon does not broaden the solidusliquidus range sufliciently to permit adequate furnace control under commercially practical conditions. On the other hand, if carbon is added in amounts greaterthan about 1.25% it produces undesirable phase variations in the steel which adversely affect properties desired in the sintered article made from the powdered metal. For instance, excess amounts of carbon adversely affect the uniform permeability characteristics of the article, and in the case of a filter, for example, this characteristic is of prime importance. Best results are obtained in any particular instance by employing carbon in an amount sufficient to increase the solidus-liquidus range of the metal to a point which just affords adequate furnace control.

The following are typical examples of austenitic staiuless steel powders useful in the practice of this invention:

Example I Percent Chromium 18.0 Nickel "8.0 Manganese 1.5 Silicon 5.0 Carbon 0.6 \Iron 66.9 Example II c t Chromium 18.0 Nickel 8.0 Manganese 1.5 Carbon 0.6 Iron 71.9 Example 111 Pe cen Chromium 18.0 Nickel 8.0 Manganese 1.5 Silicon 2.5 Carbon 0.5 Iron 69.5

I Example IV Percent Chromium 18.00 Nickel 11.00 Molybdenum g 2.50 Manganese 0.25 Silicon i 5.00 Carbon q 0.60 Iron 62.65

Example V i 7 Percent Chromium 18.0 Nickel 8.0 Manganese 1.5 Silicon 5.0 Carbon 1.0 Iron 66.5

Example VI i Percent Chromium 20.0 Nickel 8.0 Manganese 0.5 Silicon 5.0 Carbon 1.0 Iron 65.5

The procedure for making sintered metal articles from powdered alloys embodying the instant invention -is relatively simple. The powdered metal is confined in any suitable manner as on' a supporting sheet or in a mold which negatively reproduces the desired article and heated in a furnace to a temperature sufiicient to partially liquefy the metal particles. Regardless of how 'it is confined, the

powder is loose in the sense that there is no compacb ing of the powder or application of pressure to the powder before or during the heating step which results in sintering of the particles. The alloy identified in example I above reaches the desired partially liquid state at about 2325 F., and the amount of carbon in this alloy broadens the solidus-liquidus range thereof sufliciently to afford good furnace control and to permit a sintered article having excellent uniformity of permeability to be produced commercially.

After the sintering operation has been completed, it is necessary to remove a large proportion and in some instances substantially all the carbon in order to produce a satisfactory end product. This is accomplished according to the present invention by a decarburizing procedure. In some instances where the metal powder is exposed during the sintering step, decarburization may be carried on simultaneously with the sintering operation. However, in'situations Where the metal powder is not exposed but is substantially entirely confined as in a mold or the like during the sintering operation, it is necessary to carry out' the decarburization as a separate step after the sintering operation has beencompleted and the article has been removed from the mold.

The preferred decarburizing procedure is to heat the sintered article in an atmosphere of hydrogen for a length of time sufficient to remove the carbon. If the article is heated during the idecarburization step to the sintering temperature, carbon is, of course, removed progressively therefrom as the temperature is raised; and as the carbon is removed, the melting point of the metal increases. Thus, in some instances, the sintered article can be heated to a temperature equal to or slightly in excess of the sintering tempreature without adversely affecting the permeability characteristics thereof to any appreciable or significant extent. The length of time required to remove all the carbon varies considerably, depending on the size and shape of the article, but thirty minutes to two hours usually is sufiicient. The open, porous structure of the sintered article permits the hydrogen to permeate through all portions thereof and to remove carbon substantially completely from the interior as well as the exterior portions. All but negligible amounts of carbon can be removed in this way, and in some instances final carbon contents in the order of 0.01% have been achieved.

Where conditions are such that decarburization can be carried out simultaneously with the sintering operation the powder is placed initially in the furnace and brought quickly to the sintering temperature in an atmosphere of hydrogen. Of course sintering occurs to produce the desired article when the temperature enters the liquidussolidus range of the metal. As the temperature is brought to this point, some carbon of course is removed, but as this part of the operation is performed relatively quickly, not enough carbon is removed to narrow the liquidussolids range to any appreciable extent. As soon as sintering .is accomplished the article is held at a temperature that will not adversely affect the sintered article but which will produce the desired decarburization.

In those situations where decarburization cannot be carried on simultaneously with the sintering operation as where the metal powder is confined in a mold during sintering so that the hydrogen gas does not have access thereto, it is necessary to remove the sintered article from the mold and then place it in a furnace where it is exposed to hydrogen in order to eifect decarburization. This of course can be done by batch or continuous methods according tothe exigencies of the particular situation.

' Having thusdescribed the invention, we claim:

1. The method of making uniformly permeable articles of austenitic stainless steel comprising the steps of loose sintering austenitic'stainless steel powder containing carbon prealloyed therewith in amounts from about 0.5% to about 1.25%, and decarburizing the'sintered article thusformed by exposing the same to a decarburizing atmosphere. i

2. The method of making uniformly permeable metal articles having high resistance to corrosion and which retain their strength at relatively high temperatures comprising the steps of sintering austenitic stainless steel powder containing from about 0.5% to about 1.25% carbon, and simultaneously decarburizing the same by exposing said powder during the sintering operation to a decarburizing atmosphere.

3. The method of making uniformly permeable metal articles of austenitic stainless steel comprising the step of heating loosely confined austenitic stainless steel powder containing from about 0.5% to about 1.25% carbon in an atmosphere of hydrogen to a temperature at which said alloy is in a partially liquid state so as to sinter said powder and simultaneouly decarburize the same.

4. The method of making uniformly permeable metal articles having high resistance to corrosion and which retain their strength at relatively high temperatures comprising the steps of loose sintering austenitic stainless steel powder containing from about 0.5% to about 1.25% carbon, and then exposing the sintered mass of powder to a decarburizing atmosphere for a length of time sufllcient to remove substantially all of said carbon.

5. The method of making uniformly permeable articles of austenitic stainless steel com-prising the steps of loosely confining austenitic stainless steel powder containing from about0.5% to about 1.25% carbon and subjecting the same to a temperature within the solidus-liquidus range of the steel for a length of time suflicient to sinter the same, and then exposing the sintered article to an atmosphere of hydrogen for a sutficient length of time to decarburize the same.

6. The method of making uniformly permeable metal articles of austenitic stainless steel comprising the steps of loose sintering austenitic stainless steel powder of a type wherein the individual particles of the powder are essentially spherical in form and which contains from about 0.5% to about 1.25% carbon, and decarburizing the sintered article to remove substantially all the carbon from the sintered powder mass by exposing the sintered article to a decarburizing atmosphere.

7. The method of making uniformly permeable metal articles having high resistance to corrosion and which retain their strength at relatively high temperatures com prising the steps of sintering austenitic stainless steel powder containing from about 0.5% to about 1.25 carbon, and decarburizing said powder by subjecting said powder to a decarburizing atmosphere.

8. A new article of manufacture comprising a metal. article composed of a sintered mass of austenitic stainless steel powder, said article being the product of claim 7 and containing negligible amounts of carbon approaching 0.01% and being characterized by high resistance to corrosion and oxidation, being capable of retaining a relatively high strength at high temperatures, and being essentially readily uniformly permeable throughout the entire mass thereof.

9. The method of making uniformly permeable metal articles of austenitic stainless steel comprising the steps of sintering austenitic stainless steel powder of a type wherein the individual particles of the powder are essentially spherical in form and which contains from about 0.5% to about 1.25% carbon, and decarburizing said powder to remove substantially all the carbon therefrom, by subjecting said powder to a decarburizing atmosphere.

10. Austenitic stainless steel powder for making unifornily permeable metal articles containing not less than approximately seven percent nickel and not less than approximately seventeen percent chromium and carbon pro-alloyed therewith in amounts from about 0.5% to about 1.25%, said powder being characterized by its broadened solidus-liquidus range and by having substantially all of its individual particles in spherical form.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Kinzel et al.: The Alloys of Iron and Chromium, vol. II, 1st ed., McGraw-Hill Book Co. (1940), pp. 368, 390.

Seymour: Powdered metal filters, The Mining Magazine (London), vol. 77 (Oct. 1947), p. 207.

Goetzel: Treatise on Powder Metallurgy, vol. II, Inter- 5 science Publishers, Inc. (1950), pp. 389-399, 538,

'UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,826,805

March l8, 191 Robert L Probst (-21, 8-1.

'Column 1, line 31 characteristics for "in metal" head in the metal column 4, lines 54, and 55,

for "to narrow the liquidus-solids range" .read to narrow the liquidus-solidus range Signed and sealed this 27th day of May 1958.

(SEAL) Attest:

KARL mm ROBERT C. WATSON Attesting Officer Conlnissioner of Patents UNITED STATES PATEN'I 7 ICE CERTIFICATE OF CORRECTION I Patent No. 2,826,805 March 18, 1958 Robert L. Probst et al. I It is hereby certifiedthat error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

' Column 1, line 31, for "permeability characteristic" rea'd permeability characteristics column 2, line 51, for "in metal" read in the I metal --Y; column 4, lines 54, and 55, for to narrow the liquidus solids range" read to narrow the liquidus -solidus range A Signed and sealed this 27th day of May 1958.

Attest:

KARL mm ROBERT c. WATSON Attesting Officer Comnissioner of-Patents

Claims

1. THE METHOD OF MAKING UNIFORMLY PERMEABLE ARTICLES OF AUSTENITIC STAINLESS STELL COMPRISING THE STEPS OF LOOSE SINTERING AUSTENITIC STAINLESS STEEL POWDER CONTAINING CARBON PREALLOYED THEREWITH IN AMOUNTS FROM ABOUT 0.5% TO ABOUT 1.25%, AND DECARBURIZING THE SINTERED ARTICLE THUS FORMED BY EXPOSING THE SAME TO A DECARBURIZING ATMOSPHERE.

10. AUSTENITIC STAINLESS STEEL POWDER FOR MAKING UNIFORMLY PERMEABLE METAL ARTICLES CONTAINING NOT LESS THAN APPROXIMATELY SEVEN PERCENT NICKLE AND NOT LESS THAN APPROXIMATELY SEVENTEEN PERCENT CHROMIUM AND CARBON PRE-ALLOYED THEREWITH IN AMOUNTS FROM ABOUT 0.5% TO ABOUT 1.25%, SAID POWDER BEING CHARACTERIZED BY ITS BROADENED SOLIDUS-LIQUIDUS RANGE AND BY HAVING SUBSTANTIALLY ALL OF ITS INDIVIDUAL PARTICLES IN SPHERICAL FORM