Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
  • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S75/00—Specialized metallurgical processes, compositions for use therein, consolidated metal powder compositions, and loose metal particulate mixtures
  • Y10S75/95—Consolidated metal powder compositions of >95% theoretical density, e.g. wrought

Definitions

• Powdered austenitic chromium-nickel stainless steel is blended with a powdered metal ferrite stabilizer, such as molybdenum, and the resulting blend is sintered to produce a new steel, namely, an unwrought austenitic-ferritic chromium-nickel alloy having, as sintered, desirably high tensile and yield strengths and other desirable properties.
• This invention relates to powder metallurgy of stainless steel. In another aspect, it relates to the manufacture of sintered stainless steel articles from powdered metals. In another object, it relates to a novel mixture of powdered stainless steel and powdered metal ferrite stabilizers. In a further aspect, it relates to a shaped article of a novel chromium-nickel steel alloy and a method for its preparation using principles of powder metallurgy.
• Austenitic chromium-nickel stainless steels are enjoying an increasing wide-spread industrial application as engineering alloys because of their resistance to corrosion and desirable mechanical properties.
• these steels are not significantly hardenable by or responsive to heat treatment; phase transformation is suppressed by the nickel constituent in these steels and austenite (the gamma form of iron) is substantially retained on cooling from the gamma region.
• the mechanical properties of these austenitic steels cannot be controlled or varied by the usual type of heat treatment, such as quenching and tempering. Changes in mechanical properties, such as strength, are brought about only by expensive, timeconsuming cold working (rolling) and annealing, resulting in the so-called wrought austenitic stainless steels. (See Forming of Austenitic Chromium-Nickel Stainless Steels 2nd Ed. (1954), published by The International Nickel Co., Inc., New York, N.Y.).
• Powder metallurgy broadly is not a new type of metallurgical process but it is receiving increasing application in the manufacture of metallic articles, extending as it does the design limits of liquid metallurgy. An excellent description of this metallurgical process is found in Review of the Powder Metallurgy Process, July, 1966, published by the U.S. Army Production Equipment Agency, Manufacturing Technology Division, Rock Island Arsenal, Ill.
• Powder metallurgy has been used to make metal articles approaching the physical properties, such as density and strength, of cast or wrought alloys of similar composition.
• powder metallurgy has been applied to stainless steel see Progress in Powder Metallurgy," Vol. 16, pp. 120l29 (1960, Capital City Press, Montpelier, Va.).
• useful stainless steel articles have been made by the powder metallurgy technique generally high-compacting pressures and prolonged sintering at elevated temperatures have been found necessary in order to produce high density articles.
• Generally stainless steel articles commercially produced by powder metallurgy procedures have densities of 80 90 percent of theoretical density and interconnected porosity.
• 2,792,302 discloses making sintered articles from 18-8 stainless steel using 10 to 15 weight percent of a binder which can contain a relatively small amount of molybdenum disulfide as a die lubricant, which, upon subsequently being reduced during the sintering operation, has an insignificant effect, if any, on the properties of the sintered article.
• MOTT molybdenum disulfide
• 3,223,523 discloses a powder metallurgy technique in which stainless steel powder (A151 302) is blended with an aqueous solution of a salt of molybdenum, copper, or nickel, such as ammonium molyb' denate, which salt is reduced to form a metallic coating on the stainless steel powder, the amount of metallic coating being sufficient to improve the green strength of the powder compact and apparently less than that which would increase the strength of the resulting sintered article or change the finished properties thereof.
• a salt of molybdenum, copper, or nickel such as ammonium molyb' denate
• this invention provides a new alloy or steel, characterized as an unwrought chromium-nickel steel alloy, by sintering a mixture of powdered austenitic chromium-nickel stainless steel and powdered metal ferrite stabilizer, such as molybdenum, to form during sintering an austenitic-ferritic structure.
• This new alloy has a number of desirable properties and can be made in accordance with this invention with densities ranging from those which are relatively low to those which approach theoretical density, with strengths equaling or surpassing that of cast or wrought and annealed stainless steels of substantially the same elemental composition.
• the low density products of this invention have particular utility as filter elements and the highly dense articles can be used, for example, in fabricating complex shapes that ordinarily would be cast, forged, or machined.
• the stainless steel powders used in this invention are commonly known in the art as austenitic chromium-nickel stainless steels, these alloys generally containing 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weight percent nickel, 0.03 to 0.25 weight percent (max.) carbon, and occasionally some other elements added to develop certain specific properties, such as 1.75 to 4.00 weight percent molybdenum or small amounts of titanium, tantalum, and niobium to minimize formation of chromium carbides, especially in welding. Standard types of these steels have been assigned numbers and specifications by the American lron and Steel Institute.
• stainless steels of the A181 300 series types 301, 302, 304, and 305 generally referred to as l8-8f" stainless steel, and the workhorse type 316 generally referred to as l 8-8-Mo.” All of these AlSl stainless steels of the 300 series are applicable in the practice of this invention. However, AISI 303 and 304 are preferred over grades such as AIS! 316 and 317 because a greater amount of the ferrite stabilizer can be used without producing sintered articles with reduced ductility.
• Powdered AISl stainless steels of the 300 series are commercially available in various grades or sizes and can be prepared by the atomization of molten metal.
• the powdered stainless steel used in this invention will have a mesh size of -50.
• I prefer to use -325 mesh and in making less dense or porous articles I prefer to use S0+325 mesh, such as I200+325, -+200, 50+100, or blends therof, suitably selected to produce the desired micronic rating or bubble point, and. to that end, small amounts, e.g.. 12() Weight percent, of 325 mesh can be blended with the coarser powder, i.e., the 50+325 mesh.
• the stainless steel powder is used in its alloyed form, sometimes referred to as being a prealloy"; however, it is within the scope of this invention to use blends of the powdered individual metal elements in the same proportions found in the prealloyed steels since the amounts of the elemental metal constituents in the sintered articles will be equal to those found in prealloyed stainless steel.
• the metal ferrite stabilizers used in this invention are a known class of materials, most of them having body centered, cubic crystal form. They are distinguished from the austenite stabilizers, such as nickel and cobalt, which do not produce the desired results, such as high density, when used in a similar fashion in the practice of this invention, even when used in fine particle size (1.2-3 microns) at a level of 6 weight percent.
• the ferrite stabilizers used in this invention include molybdenum, titanium, vanadium, tungsten, chromium, zirconium, silicon, tantalum, and niobium.
• the particular stabilizer to be used and the amount thereof can be determined by those skilled in the art in possession of this disclosure by simple routine tests involving correlating various levels of stabilizer with the mechanical properties of the corresponding sintered articles.
• the amount of stabilizer used will amount to l to 11 weight percent, based on the total weight of the blend of powdered stainless steel and powdered ferrite stabilizer.
• low amounts do not impart the desired increase in strength and fast sintering rate, and high amounts will result in brittleness of the sintered article.
• amounts of l to 3 weight percent may be sufficient to achieve the desired results.
• these stabilizers are preferably used in amounts of 3 to 9 weight percent, and molybdenum is preferably used in amounts of to 7 weight percent.
• the mesh of the powdered ferrite stabilizer can vary from relatively coarse to relatively fine, but fine mesh of 325 is preferred because of the greater distribution of the resulting ferrite in the grain boundaries.
• the size of the ferrite stabilizer powder is preferably expressed in terms of the Fisher Standard Subsieve Series. Generally, powdered ferrite stabilizer having Fisher Numbers in the range 0.5 to 44 microns will be applicable, though that in the range of 2 to 10 microns is preferred. it is also within the scope of this invention to use reducible oxides, hdyrides, and, less desirably, salts of such ferrite stabil' izers, since such precursors will be reduced during sintering to the metal. Such salts include the nitrates, sulfates acetates, halides such as chlorides and bromides, and the like, as well as ammonium molybdenate.
• the blended mixture of the austenitic stainless steel powder with the powdered ferrite stabilizer can be deposited in the form of a loose powder on a suitable substrate or in a suitable mold, as in the case of slip casting, and sintered to form a rigid sintered article.
• the blended powdered mixture can be compacted or pressed to form a shaped article which is then sintered.
• binders of the nature disclosed in U.S. Pat. Nos. 2,593,943, 2,709,651, and 2,902,363 can be employed, such as methylcellulose.
• Various solvents can be used in conjunction with these binders, such as water, as well as various plasticizers, such as glycerin.
• Useful die lubricants which can also be used include stearic acid, and zinc, calcium, and lithium stearates.
• the resulting shaped articles can be dried or slowly heated prior to sintering, or even partially sintered, e.g., at l050-l 200 C., in a reducing atmosphere, in order to volatize, burnoff, and/or decompose the organic material, taking suitable precautions to minimize any carbon from being left in the sintered article.
• the blending of powdered stainless steel, powdered ferrite stabilizers, and binders and other various adjuncts where used can be carried out in a conventional manner in various types of commercially available mixers, tumblers, blenders, rotating drums, and the like, care being taken to insure that the blend is homogeneous and the components well dispersed.
• a binder is used, the blend will be in the nature ofa plastic mass, dough, or clay, and can be shaped and dried, for example, on a rolling mill or by means of extrusion, injection molding, etc. The shaped article can then be compacted under pressure, if desired, before sintering.
• Compacting of the blended powdered mixture can also be carried out in a conventional manner, using either hot or cold pressing, such as die pressing, isostatic pressing, etc., the compacting pressures range from 4,000 to 200,000 p.s.i.
• hot or cold pressing such as die pressing, isostatic pressing, etc.
• the high compacting pressures normally used in compacting powdered stainless steel will not be required in the practice of this invention in order to obtain desirably high strength and density in the sintered article, and thus the longevity of die parts, etc., will be far greater, with the attendant cost savings.
• the desired objects of this invention can be readily achieved with low compacting pressures in the range of 20,000 to 50,000 p.s.i.
• the sintering step of this invention will be generally carried out at sufficiently high temperatures and have sufficient duration to achieve at least during the sintering step the austeniteferrite structure and the desired increased tensile strength in the sintered article.
• the sintering temperature will be below that at which any melting of the metal powders occur.
• Sintering temperatures useful in the practice of this invention will be generally in the range of l200 to [400 C, and preferably from 1250" to 1350 C., this latter preferred temperature range being the range where the ferrite phase is readily formed.
• the duration of sintering will vary and can be from 10 minutes to 2 or 3 hours or longer.
• the sintering temperature will be sufficiently high and of sufficient duration to cause the formation of two-phase austenite-ferrite microstructure.
• the sintering operation is carried out in a conventional reducing atmosphere or under vacuum or in an inert gas such as argon.
• the reducing atmospheres particularly useful include hydrogen and anhydrous or cracked ammonia, the dew points of these gases being 40 F. or lower.
• the sintering furnaces which can be used include the conventional resistance or induction heated gastight shell or muffle furnace of the pusher, hump, or batch types.
• the sintered articles are preferably rapidly cooled through the region where the ferrite phase is partially unstable, so as to minimize rejection of ferrite formers and maintain the ferrite formed during the sintering operation.
• the rate of cooling necessary to retain the ferrite formed during sintering can be determined empirically by simple routine cooling tests by those skilled in the art. Means for effecting the rapid cooling necessary to preserve the ferrite phase, or at least 50 to volume percent of that formed during sintering, are available in the art, such cooling being carried out by quenching sintered articles from their sintering temperatures in cooled furnace gas, other gas such as argon or nitrogen, air, water, oil, or the like.
• Slow cooling, such as furnace cooling, of the sintered articles can be employed but generally is not preferred since this favors the formation of sigma and related undesired phases, which impart brittleness and other generally undesirable properties to the sintered article.
• slow cooling can be used in those instances wherein the presence of these normally deleterious phases is desired or is of no consequence.
• the microstructure of the preferred sintered articles of this invention is substantially two-phased: austenite and ferrite.
• Other phases namely sigma and/or related phases, may be present if the sintered article is slowly cooled as described above.
• the austenite-ferrite two-phase structures can be heated further at higher sintering temperatures very near the melting point of the structure to form structures which are substantially all ferritic, or, by appropriate heat treatment, the twophase structures can be transformed into substantially all austenitic structure.
• These essentially single-phase structures will revert to the two-phase austenite-ferrite structure if reheated to temperatures, e.g., l250-l 350 C., favoring their coexistence.
• inrorderto produce sintered articles having desired properties such as strength and density
• these will exhibit reduced ductility and toughness and reduced corrosion resistance to salt solution, i.e., sea water.
• the ferrite phase will comprise 4 to 80 volume percent, preferably to'60 volume percent, and the balance will be substantially austenite.
• the presence of the ferrite phase results in a faster rate of sintering due to the increased diffusion rate of this phase and it imparts magnetism to the sintered article.
• the grains of austenite and ferrite are randomly distributed and the grain size of these phases in the sintered article is relatively fine, e.g., 5-8 according to ASTM El9-33, and is in contrast to the relatively coarse grain of prior art sintered stainless steel caused by the costly long high sintering temperatures necessary to obtain dense articles.
• X-ray studies of quenched sintered articles of this invention show, for example, face centered, cubic (FCC) diffraction lines, attributed to austenite, with a relative intensity of about 100, and body centered cubic (BCC) diffraction lines, attributed to ferrite, with a relative intensity of 70.
• FCC face centered, cubic
• BCC body centered cubic
• Rapidly cooled specimens when viewed under an optical microscope show microstructure characterized as grains of austenite dispersed in an essentially continuous matrix that was ferrite during sintering but has transformed at least partially to a very fine mixture of austenite and ferrite during cooling from the sintering temperature.
• a relatively large amount, e.g. 9 percent, of the molybdenum stabilizer is used, the grains of austenite will be needlelike or lenticular, and where a relatively small amount of molybdenum (e.g., 3 percent) is used, the grains of austenite will be irregular equiaxed in shape.
• Low-temperature sintering e.g., 1,250C.
• high sintering temperatures e.g., l,350 C.
• the amount of austenite decreases and the grains of austenite appear lenticular or needlelike.
• the, tensile strength of the sintered article will be significantly greater than that obtained by sintering powdered stainless steel of the A181 300 series in the absence of ferrite stabilizers.
• Dense sintered articles of this invention made with -325 mesh austenitic stainless steel powder (and ferrite stabilizer) will have as sintered ASTM E8-66 tensile strengths as high as 55,00080,000 p.s.i., and even as high as 110,000 p.s.i., these values being as much as 25 to 200 percent greater than those obtained by sintering stainless steel powder without ferrite stabilizer addition.
• the sintered articles of this invention also have very high yield strengths, a property of considerable importance to structural designers.
• Yield strength is usually defined as the stress required to impart a permanent deformation of 0.2 percent in the article.
• Dense sintered articles of this invention prepared from -325 mesh 'austenitic stainless steel powder (and ferrite stabilizer) will have ASTM E8-66 yield strengths as sintered as high as 25,000 to 80,000 p.s.i., which values are 50-400 percent higher than that of as sintered stainless steel articles of similar composition produced by powder metallurgy (without ferrite stabilizer). These high yield strengths even substantially exceed that obtained by annealed wrought stainless steel of similar composition,
• the apparent density of the dense sintered articles will also be significantly greater (e.g., 5-25 percent greater) and generally will be in a range of to 95 percent of the theoretical density (as sintered), as determined by mercury porousimetry described by the American Instrument Co. in its Bulletin 2300 1960).
• the as sintered tensile strength and absolute micronic rating values of the sintered articles can be multiplied to obtain a product value which is useful as a parameter for evaluating the mechanical properties of the articles without reference to the particle size of the stainless steel powder used in preparing them.
• a tensile strength of 17,475 psi. multiplied by an absolute 'niicronic rating of 14 microns gives a product value or parameter of 244,650.
• Theparameter values of the porous articles of this invention will be as high as 200,000 to 500,000 and as much as to 450 percent higher than that of porous articles made of sintered stainless steel powder without ferrite stabilizer addition.
• the desirably high strengths and/or densities of the articles of this invention can be obtained in the prior art onlyby repeatedly cold working and annealing stainless steel of the casttype obtained by liquid metallurgy, or by prior art powder metallurgy techniques involving significantly greater compacting or pressing pressures and long high temperatures sintering and subjection of the sintered article to subsequent repeated cold working and annealing.
• the novel alloy ofthis invention can be used in manufacturing articles of either a relatively low density or porous nature, which would be particularly suitable where the sintered articles are used as filter elements, or relatively dense articles having densities approaching theoretical densities. Such high densities are particularly suitable in the fabrication of such articles as die pressedor injection molded parts, such as a cam, valve housing, etc., seamless tubing for heat exchangers and immersion heaters, corrugated recuperative or regenerative heat exchangers (made without welding or brazing).
• the dense'shapecl articles can also be used for architectural applications such as window casings and decorative railing supports, burner grids of corrugated or foamed structure, acoustic materials made as a foamed structure, catalyst carrier and catalyst support structures, dinnerware, etc.
• Dense sintered articles of this invention can be made highly impervious, for example, by injection molding, such articles being advantageously employed in applications where leakage or corrosion would present problems if relatively porous sintered stainless steel were used. It is also within the scope of this invention to subject the sintered articles to finishing operations which result in even denser articles or better mechanical properties,-such operations including, for example, coining and resintering. However, the as sintered articles in most cases will have the properties desired and further processing will be unnecessary though useful in some cases to achieve final dimensional tolerances.
• EXAMPLE 1 Two batches of powdered metal were made using 325 mesh prealloyed powdered stainless steel of the AIS] 316L type, one batch being made in accordance with this invention using 3 weight percent powdered molybdenum with a Fisher Number of 3.27 microns.
• 100 g. of the powdered material were mixed with 3 g. of methyl cellulose (4,000 c.p.s.) in a twin shell blender for 1 hr. and then for about 20 min. in a sigma blade mixer with 9.5 cc. of l6.6 weight percent solution of glycerin in distilled water until a stiff clay or plastic mass was produced.
• the wet clay was then rolled to a 0.060 inch thick sheet on a rubber mill with a roll speed ratio of 1.4:1.
• the sheets were cut into specimens linch X2 inches, placed in a vacuum drying oven and dried at ll5 F.
• the specimens were then pressed at 20,000 p.s.i. and sintered in a vacuum furnace by heating from room temperature to 350 C. in about 3 hrs. and then heating to l325 C. and holding for 8 hrs., the samples being suspended in l00 mesh alumina during sintering and furnace cooled (734 C./min. over l3006 00 C.). in table I below, the in table of these two runs are shown.
• Metallographic mounts of the sintered specimens were etched with ferric chloride.
• light-appearing grains were considered austenite and the relatively darker matrix was considered as mixed grains of austenite (light) and ferrite (dark) which was considered all ferrite at sintering temperature.
• EXAMPLE 2 In this example, a series of six runs was made in which compacted mixtures of stainless steel of A151 303 type and varying amounts of molybdenum (Fisher Number 3.27 microns) were sintered and furnace cooled. The compacting sintering, and
• EXAMPLE 4 in this example, a series of runs was made in which compacts of 325 mesh AlSl 304L or MS] 316L stainless steels dry blended with varying amounts ofmolybdenum (Fisher No. 4.2 microns) were pressed at 20,000 p.s.i. and sintered at various temperatures for various periods of time in dry hydrogen and rapidly cooled (ll0 C./min.) and the properties of the green compacts and sintered articles determined and compared. For purpose of comparison, other runs were made in which stainless steel compacts made without molybdenum addition were prepared and sintered under these varying conditions. Results are summarized in table lV,
• the powdered materials were blended in a twin shell blender for about 30 min. 'Those sintered articles whose tensile strengths were determined were made from compacts prepared by pressing the blended powders in a Haller DL-lOOl die in accordance with ASTM 158-66 and MPIF 1063, these specimens having been pressed at 20 tsi before sintering. All sintering was performed at sintering temperatures of l300 C for 1.25 hrs. (except in Runs 3, l2, and I3, where sintering was at [350 C. for 2 hrs). in a palladiumsilver purified hydrogen atmosphere using induction heating. The sintered articles were rapidly cooled in the furnace. The
• the sizes of the ferrite stabilizers used are shown in table V TABLE V Powder Size Mo 2-4 microns Cr 3 microns W 0.9 microns TiH, 6-9 microns ZrH, 2-8 microns Co 1.2 microns Ni 3-5 microns VH, 325 mesh" Si 325 mesh bilizers in the practice of this invention as well as combinations thereof, and also show that Ni and Co, by comparison, are inferior.
• EXAMPLE 6 Following the procedure of example 5, a compact was made and sintered from a blend of elemental metals used in amounts matching the composition ofAlSl 3041.. In one run, the blend contained 6 weight percent of powdered molybdenum and the compact prepared from this blend had a green density of 70 percent that of the theoretical density, the sintered density of this compact being 87.4 percent of theoretical. The compact made from the blend without molybdenum addition had a comparable green density of 70.9 percent of theoretical but by contrast the sintered density of this compact was only 81.5 percent of theoretical. The sintered article made with molybdenum had an 0.2 percent yield of 47.1 KSl, an ultimate strength of 56.1 KS1, and an elongation of 2.5 percent. The sintered article made without molybdenum had an 0.2 percent In place of methylcellulose, other binders can be used, such as polypropylene, polystyrene, stearin mixed with vegetable oil, etc.
• EXAMPLE 8 Six batches of various commercial grades of stainless steel powder were prepared, some of which were blended with powdered molybdenum (Fisher Number 4.2 microns).
• Parameter is the product of ultimate strength multiplied by absolute mieromie rating.
• EXAMPLE 9 Two 3,000 g. batches of A181 3 l 6L +200 mesh) were prepared in a manner like example 8, one of these batches containing 5 weight percent molybdenum (Fisher Number 4.2 microns). The batches of clay were extruded to form 4-foot long tubes with an outside diameter of 0.504 inches and an inside diameter of 0.314 inches. The green tubes were sintered in dry hydrogen for 3 hrs. at 1,150" C. to bumoff the binder and partially sinter the tubes. The tubes were then isostatically pressed and resintered at l,350 C. for 2 hrs. in dry hydrogen. The tubes were furnace cooled to form porous tube useful as filter elements. The tubes were machined to form tensile specimens and the ultimate tensile strengths were determined. These runs and results are summarized in table IX.
• lclaim l.
• stainless steel of said ferrite phase further comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, tungsten, chromium, zirconium, silicon, tantalum, and mixtures thereof.
• Alloy according to claim 1 having as sintered" an ultimate tensile strength of 55,000 to I 10,000 p.s.i., 0.2 percent offset yield strength of 25,000 to 80,000 psi.
• a relatively dense article made of the alloy of claim 6 and having an apparent density of to 99 percent of the theoretical density.
• Alloy according to claim 1 comprising 16.0 to 26.0 weight percent chromium. 6.0 to 22.0 weight percent nickel, and up to 0.03 to 0.25 weight percent carbon, based on the weight of said alloy.

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• 1968
  • 1968-07-10 US US743588A patent/US3620690A/en not_active Expired - Lifetime
• 1969
  • 1969-06-30 NL NL6909986A patent/NL6909986A/xx unknown
  • 1969-07-09 FR FR6923280A patent/FR2012653A1/fr not_active Withdrawn
  • 1969-07-09 DE DE19691935676 patent/DE1935676A1/de active Pending
  • 1969-07-09 GB GB34578/69A patent/GB1281734A/en not_active Expired

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