US 3811961 A
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United States Patent O 3,811,961 BORIDIZED STEEL-BONDED CARBIDES Martin Weinstein, San Antonio, Tex., Subhash C. Singhal, Monroeville, Pa., and John L. Ellis, White Plains, N.Y., assignors to Chromalloy American Corporation, San Antonio, Tex.
No Drawing. Filed Mar. 9, 1972, Ser. No. 233,282
Int. Cl. B231! 3/00 US. Cl. 148-34 2 Claims ABSTRACT OF THE DISCLOSURE A surface-hardened cemented carbide shape is provided consisting essentially of primary grains of a refractory metal carbide distributed through a metal matrix, the metal matrix adjacent the surface between the carbide grains having a boridized layer characterized metallographically by the presence of an iron-group metal boride.
, It is known to produce sintered composite metal articles of manufacture from a powdered mixture of refractory metal carbide and metal powder by compressing the powder mixture in a mold to produce a desired shape and sintering the shape at an elevated temperature under nonoxidizing conditions, whereby to cause the compressed particles to sinter together into a dense structure.
. In the case of refractory carbide, hard metals comprising in large part of carbides of titanium, tungsten, and/or other refractory carbides cemented together by liquid phase sintering in the-matrix metal, the favorable properties are due in large part to the rather high intrinsic hardness of the carbides combined with the strengthening effects of the bonding metal. By the term primary carbide grains is meant the grains or particles of the refractory carbide added directly to the composition during the formation thereof, and which still retain substantially their identity in the final composition as compared to secondary carbide grains which form by reaction during heat treatment.
In recent years, a machinable refractory carbide body comprising primary titanium carbide grains dispersed through a heat-treatable steel matrix has been developed which utilizes the intrinsic hardening effect of the primary carbide combined with the hardenability of the steel matrix. The machinable carbide body has one advantage over conventional cemented carbides in that the matrix of a composition containing about 50% by volume TiC and the balance substantially steel can be softened by annealing so as to lower the gross hardness of the composition to say 40 R such that the body can be machined to a desired shape and then hardened to upwards of 72 R by quenching the alloy from an elevated temperature similarly as is done in certain of the alloy tool steels.
In producing refractory carbide compositions of the foregoing type with heat treatable or non-heat treatable metal matrices, refractory carbide powder, such as titanium carbide, is mixed with finely divided steel-forming ingredients followed by compacting the mixture into the desired shape in a mold, and then subjecting the resulting compact to liquid phase sintering by heating it to a temperature above the lowest melting phase of the matrix metal but below the melting point of the refrac- 3,811,961 Patented May 21, 1974 tory carbide. We find liquid phase sintering advantageous for our purposes in that dense products substantially free from porosity are obtainable. The fore-going type of composite metal has found use in a wide variety of applications such as forming dies, die nibs, wear resisting parts, size gages, machine parts, and the like.
In utilizing such refractory carbide compositions in the fabrication of a wide variety of tools or machine parts, it is sometimes desirable to modify the metallurgical characteristics of the matrix metal in order to meet the particular requirements stipulated by a specific end use intended for the tool. For example, in the case Where a composition comprising 45% by volume of TiC dispersed through a stainless steel matrix making up 55% by volume of the composition is employed as a part in a pump involving sliding friction at an elevated temperature, a preferential wearing is apt to occur in the matrix metal between the carbide grains. This composition has an average hardness in the neighborhood of about 45 R Since stainless steel of the 18/8 variety is substantially non-heat treatable, the intrinsic hardness of the steel is substantially below that of the actual hardness of the carbide. Therefore, when a machine part of the foregoing material is subjected to sliding friction at an elevated temperature, the soft matrix metal is preferentially worn away from around the carbide grains. While the primary carbide grains themselves provide resistance to wearing, they only do so provided they are securely embedded and anchored in the matrix metal. However, as the matrix metal is selectively worn or eroded away from around the grains, the grains tend to loosen and fall out. Such preferential wearing has its disadvantages in that it may result in a surface notch effect which lowers the resistance of the metal to impact. Thus, it would be desirable where the matrix is concern to provide high retained hardness at the surface of the machine part to avoid the aforementioned difiiculties.
In the case of a heat treatable composition, for example, a composition comprising about 50% by volume of TiC and the balance 50% by volume of a low chromium low molybdenum steel, it may be desirable to temper the foregoing composition from a hardness of about 70 R to a hardness of about 50 to 55 R,,, let us say in the case of a draw die. However, it would be important to insure a hard wear-resisting surface at the throat or orifice of the die, particularly between the hard primary carbide grains embedded in the matrix. Thus, it would be desirable to provide a hardened surface between the carbide grains to anchor said grains in place so as to obviate any preferential wearing away of the matrix and inhibit dislodgement of carbide particles.
The hardened surface should be one having as low a. coeflicient of friction as possible, while providing optimum hardness in addition to anchoring the primary grains of refractory metal carbides.
However, the surface hardening process should be one capable of hardening not only a steel surface but other alloys of the iron-group matrix metals, such as nickelbase and cobalt-base matrix metals. For example, it would be desirable to provide a tungsten carbide-cobalt composition in which the cobalt matrix metal is surface hardened to improve its resistance to wear in uses involving sliding friciton.
OBJECTS OF THE INVENTION It is thus the object of the invention to provide a cemented refractory carbide composition in which the surface of the matrix metal is boridized to provide a hardened low friction surface.
Another object is to provide a steel-bonded carbide in which the surface of the matrix surrounding the carbide grains is boridized to improve its resistance to erosion,
while providing a hard low friction surface particularly STATEMENT OF THE INVENTION In its broad aspects, the invention provides a surface hardened cemented refractory metal carbide article of manufacture consisting essentially of about 20% to 80% by volume of primary grains of a hard refractory metal carbide distributed through a metal matrix comprised substantially of an iron-group metal, e.g. a matrix alloy based on at least one iron-group metal, the surfacehardened cemented carbide being characterized by a microstructure adjacent the surface thereof consisting essentially of primary grains of said refractory metal carbide dispersed through a boridized layer of said metal matrix, the boridized layer being comprised of an irongroup metal boride, such as iron boride, cobalt boride and nickel boride.
The primary grains of refractory metal carbides include those selected from the group consisting of carbides of chromium, tungsten, molybdenum, titanium, zirconium, hafnium, niobium, tantalum and vanadium. While the amount of carbide in the composition may range from about 20% to 80% by volume, a range of about 30%.
to 70% by volume may be preferred.
The invention is particularly applicable to cemented steel-bonded carbides of the type dislosed in US. Pats. No. 2,828,202, No. 3,053,706, No. 3,183,127, No. 3,369,- 891, No. 3,369,892 and No. 3,416,976.
Examples of matrix steel compositions which can be boridized are SAE 1010 to SAE 1080 steels and such other steels as: 0.8% Cr, 0.2% M0, 0.3% C and iron substantially the balance; 5% Cr, 1.4% M0, 1.4% W, 0.45% V, 0.35% C and iron substantially the balance; 8% Mo, 4% Cr, 2% V, 0.8% C and iron substantially the balance; 18% W, 4% Cr, 1% V, 0.75% C and iron substantially the balance.
A preferred steel matrix composition for boridizing is one containing by weight about 1% to 6% Cr, about 0.3% to 6% M0, about 0.3% to 0.8% C and the balance substantially iron.
Another preferred steel matrix composition for boridizing is one containing by weight about 6 or 7% to 12% Cr, about 0.6 to 1.2% C, about 0.5% to 5% Mo, up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% cobalt and the balance substantially iron.
A further steel matrix composition is a low carbon heat treatable composition containing by weight about to 30% Ni, 0.2% to 9% Ti, and up to 5% Al, the sum of the Ti and Al not exceeding about 9%, up to about 25% Co, up to about 10% Mo, and substantially the balance of the matrix at least about 50% iron; the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of the Al and Ti is less than 1.5% and the Co and Mo contents are each at least about 2%; and such that when the Ni content ranges from about 18% to 30% and the Mo content is less than 2%, the sum of the Al and Ti exceeds 1.5%. This matrix alloy forms soft martensite when quenched from a solution temperature of about 1400 F. to 2150 F. (760 C. to 1165 C.) and age hardens when heated at a temperature of about 500 F. to 1200 F. (260 C. to 650 C.) for about three hours.
Examples of other matrix metals other than steel are the iron-group metals nickel, cobalt and nickel-base and cobalt-base alloys. These metals can be boridized to form nickel boride and cobalt boride.
Cobalt is a well-known matrix or binder metal em ployed in the production of cemented tungsten carbide for use in cutting tools, drill bits, etc.
4 DETAILS OF THE INVENTION The cemented carbides mentioned hereinabove are produced by powder metallurgy. In the case of a steelbonded carbide, for example, a composition comprising primary grains of titanium carbide dispersed through a low chromium, low molybdenum steel composition, such a composition comprising 40% by weight of TiC and 60% by weight of steel may be produced as follows:
A l000-gram charge of TiC powder of about 5 to 7 microns in size is mixed with a 1500-gram charge of a steel-forming mixture calculated to form a matrix steel containing about 1.25% Cr, 2.5% M0, 0.4% C and the balance carbonyl iron powder of about 20 microns in size. The powdered ingredients have mixed therewith 1 gram of parafiin wax for each 100 grams of ingredients. The mixing is carried out in a steel mill for about 40 hours with the mill half full with steel balls, using hexane as the vehicle. 1
After completion of the milling, the mixture is removed and vacuum dried. A proportion of the mixed product is compressed in a die at 15 tons/sq. in. to form a die nib of the desired size. The die nib is then liquid phase sintered at a temperature of about 1435 C. (about 2615 F.) for about one-half hour at a vacuum corresponding to about 20 microns of mercury or better. Afterv completion of sintering, the assembly is cooled and then annealed by heating to 900 C. (about 1565 F.) for 2 hours followed by cooling at a rate of about 15 C./
hour to about 100 C. (212 F.) to produce an annealed microstructure containing spheroidite. Following annealing, the die nib is finished machined to the desired size. The nib is then cleaned up and boridized. After the nib has been boridized, it is then hardened by austenitizing at 1750 -F., followed by quenching in an oil bath.
In carrying out the boridizing step, a pack cementation process is used. A stainless steel retort is used in which the cover is sealed to the retort flange, the retort cover and the flange being machined to a smooth finish before each run to assure a gas-tight seal. A new nickel-plate A151 321 stainless steel O-ring is placed between the retort flange and the cover for each run.
A typical powder pack is a mixture by weight of about 5% boron and about 95% aluminum oxide, the mixture containing 0.75% by weight of a halide energizer (e.g. ammonium bifluoride and potassium fluoride). The boron powder used is amorphous, has a particle size of about 5 microns and is about 90% to 92% pure. The main impurities are 6.2% Mg, 0.1% Fe and 2.2% O. The
aluminum oxide had a particle size of about 325 mesh.
The pack may contain by weight from about 1% to 50% boron, a small but effective amount of a halide energizer (e.g. up to about 2%) and the balance an inert diluent such as alumina. Examples of the inert metals.
In producing the pack, the powders in appropriate quantities are mixed together and blended in a rotary blender containing stainless steel balls for 1 to 2 hours.
The samples to be boridized are thoroughly degreased in trichloroethylene vapors and then honed with '400 mesh alumina grit. The honed samples are then packed in the retort containing the boridizing pack. The retort is filled densely with the powder pack so as to leave no empty space at the top and the cover then sealed to the retort flange as described hereinabove.
The charged retort is heated in a furnace to 1550 F. (or the range of 1350 F. to 1700" F.) and held at temperature for about 10 to 35 hours, e.g. 24 hours. During this period, boron is deposited onto the surface of the sintered shape via boron halide vapors formed in the pack. The boron then diffuses into the surface and forms iron boride. At the end of the 24-hour period, the retort tive compositions as follows:
is removed from the furnace and cooled. The retort is then opened and the pack removed.
The surface-hardened samples are either cleaned in the ultrasonic cleaner or by liquid honing to remove any powder sticking to them.
' The compositions of the samples were-as follows:
TABLE 1 Vol. percent No. carbide Vol. percent matrix 1 45' TiC 55 Matrix contains by weight 2.87% Cr, 2.87% Mo 0.65% C and bal. Fe. p
2 45 T 55 Matrix contains by weight 10% Cr, 3.0% M0,
0.8% C, bal. Fe.
3 45 TiC 55 Mgtriixlcontains by weight Cr, 0.8% C and 4"..- 45 TiC 55 Matrix contains by Weight 18% Ni, 8.5% Co,
4.75% Mo, 1% Ti, bal. Fe.
5....-. 50 T10 50 Matrix contains by weight 18% Ni, 8.5% Co,
4.75% Mo, 1% Ti, bal. Fe.
6 55 TiC 45 Matrix contains by weight 18% Ni, 8.5% Co,
4.75% Mo, 1% Ti, bal. Fe.
7.-." 45 TiC 55 Matrix contains by Wei ht 14% Or, 6% Ni, 5% Co, 1.5% Ti, bal. Fe.
8.- 45 T10 55 Matrix contains by weight 71% Ni,-18% Cr, 8%
Fe, 2% Ti, 1% Al.
The samples were hardened according to theirrespec- No. 1 composition.Quenched from 1750 F., using a blast of inert gas, followed by tempering at 375 F. for 1 hour.
No. 2 composition.Quenched from 2000 F. using a blast of inert gas, followed by tempering two times-at 975 F., 1 hour each.
No. 3 composition.--Quenched from 1875 F. in an inert atmosphere and then tempered between 400 F. and
and aged at 900 F. for 3 hours and then air cooled.
No. 7 c0mposition.-Solution annealed at 1800 F. and hardened at 900 F. for 10 hours and then air cooled.
No. 8 composition.--Solution annealed by quenching in a non-oxidizing atmosphere from about 1950 F. 'followed by aging 8 hours at 1600 and then 4 hours at,
In the iron-base matrix, the eutectic FeFe B tends to form which melts at about 2100 F. and therefore the hardening heat treatment should preferably be confined below this temperature to preserve the hard coating.
perature of heat treatment should be well below 2000 F. p 3
The boride coating on the surface of each sample was identified by X-ray diffraction analysis and showed the presence of Fe B. and FeB. Approximately 1.5 to 2 mils thick coatings were obtained on all samples. The microhardness of the coating was measured by making Vickers :microhardness indentations using 25 g. load in the surface in the matrix region clear of any of the dispersed primary carbides. The hardness determinations are given in Table 2 as follows before and after boridizing:
TABLE 2 Surface microhardness (V HN) Before coating Alter coating pend upon the hardness to be achieved in the substrate.
Since generally the substrate may be heat treated after .boridizing, it is advisable to employ a boridized layer of up to about 2. mils in thickness (0.002 inch) in order to minimize cracking of the boridized layer due to quenchmg.
' On the other hand, in applications in which an annealed substrate is preferred, such as in thread gages and the like or complex shapes, the boridized layer may range above 2 mils (0.002 inch) and up to about 7 mils (0.007
'inch) in thickness. Thus, it may be desirable to employ an annealed titanium carbide steel substrate in the case of drills and broaches, with the boridized surface confer- .ring improvedwear resistance and the annealed substrate providing improved vibration damping characteristics. As illustrative of additional embodiments of the inven- "tion, the following example is given with respect to the production of a thread ga-ge.
A rod is. first produced by compacting a titanium carbide steel mixture comprising about 45% by volume of TiC and the balance (about 55% by volume) of a steel matrix having a composition consisting essentially of about 3% Cr, 3% Mo, 0.65% C and the balance iron.
The compacted rod is liquid phase sintered in vacuum at a temperature of about 1435 C. (about 2615 F.) for about one-halfhour. After completion of the sintering, the rod is furnace cooled to room temperature to provide an annealed hardness of about 47 R The annealed rod is finished machined to the desired diameter and the threads are: precision out along one length of the rod while the opposite length is knurled. Thereafter, the threaded gage is boridized as describedherein, the threads of which are thereafter cleaned and lapped to the final The boridized thread gage is used without any subsequent heat treatment to avoid volumetric size change and distortion normally caused by high temperature oil 'quench. The surface hardness'is generally infexcess of 1500VHN..
Boridized drills and broaches are similarly produced from the titanium carbide steel compositions oftheaforementioned type. i
As stated hereinbefore, an advantage of the boridized 'lay er'is that it confers low friction propertiesto the surface ofthe sintered refractory "carbidematerial, While at the same. time, conferring improved wear resistance to the metal. matrix surrounding the refractory carbide grains- Where the matrixisheattreatable, particularly a steel matrix, the coated sintered refractory carbide material can be hardened andtemperd by the usual heat treatment employed for the matrix in'the uncoated con- 'dition, provided sufficient care is takento avoidoxidizing or otherwise chemically changing the coating. Thus, where the hardness is achieved by oil-quenching, the steelbonded carbide from a relatively high temperature, eg.
from 1750 F., it is preferred to effect the cooling with a blast of inert 'gas, such as argon.
Examples of other compositions which may be boridized in accordance with the invention are as follows:
TABLE 3 Percent v No. carbide Percent vol. matrix 9....- 20 TiC 80 Byi 1%71, Cr, 3% M0, 0.5% C and balance essena y e.
10.... 30 V 70 By wt. 6% Fe, 15% Cr, 2.5% A1, 2.5% Ti, balance essentially Ni.
11...- 45 W0 55 Matrix substantially Co.
12.... 50 T10 50 By wt. 8% Cr, 3% M0, 1% V, 0.9% 0, balance essentially Fe.
13..-- 60 NbC 40 By wt. Cr, 1.4% M0, 1.4% W, 0.45 V, 0.35% C and balance essentially Fe.
14.... 70 TiC 30 By wt. 10% Cr, 2% Mo, 2% W, 1% G and balance essentially Fe.
-... 55 T10 45 By wt. 60% Ni, 15% Co, Cr and 5% Fe.
16-.-. ZrC 75 By wt. 80% Ni20% Or.
The matrix metals employed in sintering and bonding the refractory carbide preferably contain at least 50% by weight of at least one of the iron group metals iron, nickel and cobalt, the refractory metal carbide ranging from about 20% to 80% by volume with the matrix metal making up substantially the balance.
Preferably, the invention is applicable to steel-bonded carbides and, in particular, to titanium carbide tool steel compositions comprising primary grains of refractory carbide, such as TiC, of about 20% to 80% by volume dispersed through the following steel matrices making up substantially the balance:
(A) The matrix containing by weight about 1% to 6% Cr, about 0.3% to 6% Mo, about 0.3 to 0.8% C and the balance essentially iron;
(B) The matrix containing by weight about 6% to 12% Cr, about 0.5% to 5% Mo, about 0.6 to 1.2% C, up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% Co and the balance essentially iron; and
(C) The matrix comprising a high nickel alloy containing by weight about 10% to Ni, about 0.2 to 9% Ti, up to about 5% Al, the sum of Ti and Al content not exceeding about 9%, less than about 0.15% C, up to about 25% Co, up to about 10% Mo, substantially the balance of the matrix being at least about 50% iron, the metals making up the matrix composition being proportioned such that when the nickel content ranges fromabout 10% to 22% and the sum of Al and Ti is less than about 1.5 the molybdenum and cobalt contents are each at least about 2%, and such that when the nickel content ranges from about 18% to 30% and the molybdenum content is less than 2%, the sum of Al and Ti exceeds 1.5%.
The foregoing matrix steels are characterized in the heat treated state by the presence of martensite. Thus, as regards matrix steel (A), the refractory carbide tool steel produced therefrom is heat treated by heating it to above the austenitizing temperature, e.g. to 1750 F., and then quenching it to form hard martensite. Where the refractory carbide tool steel is boridized, it is preferred that the quenching be achieved by a blast of inert gas to avoid spalling of the boridized layer. Following the quench, the steel may be tempered by heating over the temperature range of about 250 F. and 550 F. for up to about 5 hours.
In the case of a refractory carbide tool steel using steel (B) as the matrix, the matrix is similarly heat treated by quenching from above the austenitizing temperature in the range of about 1700 to 2050" F., e.g. from 1750 F. However, the tempering is conducted at a higher temperature, e.g. from about 900 F. to 1050 R, such as 1000 F., for about 1 or 2 hours, wherein secondary hardening effects are obtained due to the formation of secondary carbides.
In the case of steel matrix (C), refractory carbide steel com ositions produced therefrom by subjecting the steel to a solution treatment by cooling it (e.g. air cooling) from a solution temperature of about 1400 F. to 1950 F. (760 C. to 1100 C.) to produce a microstructure in the matrix characterized by the presence of soft martensite. Thereafter, the matrix surrounding the carbide grains is age-hardened by heating the refractory carbide steel at a temperature of about 500 F. to 1200 F. (260 C. to 650 C.) for about three hours. A typical agehardening temperature is 900 F. (483 C.).
One of the advantages of the invention is that the matrix can be maintained in a relatively soft condition to provide toughness while providing a very hard surface by virtue of the lboridized layer.
The boride coatings enable the production of refractory carbide articles such as molds, dies, sand blasting nozzles, rolls, etc. Such articles in use usually contact hard particulate matter which normally selectively wears away the matrix surrounding the primary carbide grains. One of the advantages of the steel matrix is that it can be annealed to enable the machining of sintered refractory carbide stock which can then be coated. The dimensional changes during boridizing are negligible as the coating is formed substantially by the inward diffusion of boron.
Sand blasting nozzles were produced from an annealed titanium carbide tool steel and boridized in accordance with the invention. The composition comprised 45 vol. percent TiC and the balance a matrix steel containing 2.87% Cr, 2.87% Mo, 0.6 5% C and the remainder iron. Following boridizing, the nozzles were heat treated to achieve a substrate hardness of 68 to 70 R The life of the nozzles was markedly improved. The rate of wear and erosion was substantially less than that experienced on nozzles made of ceramic, hardened tool steels, boron carbide and the aforementioned titanium carbide tool steel in the hardened but unboridized state.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.
What is claimed is:
1. A surface-hardened cemented carbide article of manufacture consisting essentially of about 20% to by volume of primary grains of a hard refractory carbide selected from the group consisting of primary carbides of chromium, tungsten, molybdenum, titanium, zirconium, hafnium, niobium, tantalum and vanadium distributed through a steel matrix selected from the group consisting by weight of (A) about 1% to 6% Cr, about 0.3% to 6% Mo, about 0.3% to 0.8% C and the balance essentially iron; (B) about 6% to 12% Cr, about 0.5% to 5% Mo, about 0.6% to 1.2% C, up to about 5% W, up to about 2% V, up to about 3% Ni, up to about 5% Co and the balance essentially iron; and (C) a high nickel alloy steel containing about 10% to 30% Ni, about 0.2 to 9% Ti, up to about 5% Al, the sum of the Ti and Al content not exceeding about 9%, up to about 25 Co, up to about 10% Mo, substantially the balance of the matrix being at least about 50% iron, the metals making up the matrix composition being proportioned such that when the nickel content ranges from about 10% to 22% and the sum of Al and Ti is less than about 1.5%, the molybdenum and cobalt contents are each at least about 2%, and such that when the nickel content ranges from about 19% to 30% and the molybdenum content is less than 2%, the sum of Al and Ti exceeds 1.5 said surface-hardened cemented carbide being characterized by a microstructure at least adjacent the surface thereof comprising primary grains of said refractory metal carbide anchored and dispersed through a boridized layer of said steel matrix; said matrix of said compositions (A), (B) and (C) being in the hardened state and being characterized metallographically 10 at least at said surface by the presence of martensite and FOREIGN PATENTS Iron borlde- 25-21 1950 Japan 148-6 2. The surface-hardened cemented carbide article of claim 1, wherein the refractory carbide is substantially WALTER SATTERFIELD p Examiner titanium carbide and wherein the amount of titanium 5 carbide ranges from about 30% to 75% by volume. 11.5. C1. X.R.
References Cited 148-6, 6.3, 31.5; 29182.7, 182.8 UNITED STATES PATENTS 3,712,798 1/1913 Van Thyne et al 1486.3
3,244,482 4/1966 Culbertson et al 423-297 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: Dated May 21 1.9211
Inventor s Martin Weinstein et a1 Itis certified th at error appears in the-above-identifiedpatent;-
and that said Letters Patent are hereby corrected as shown below:
Column 8, dzlaim 1, line 67, "19%" should read 18% Signed and sealed this 24th day of September 1974.
McCOY M. Attesting Officer GIBSON JR. C. MARSHALL D'ANN Commissioner of Patents USCOMM-DC B0376-P69 w u.s. GOVERNMENT PRINTING ornc: 190s o3ss-ss4.
FORM PO-1050 (10-69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 1 1 (1 Mm Dated May 21, 191
lnventofls) Martin Weinstein et a1 It is certified that error appears in theabove-identifiedpatent; Q
and that said Letters Patent are hereby corrected as shown below:
Column 8, (claim 1, line 67, "19%" should read 18% Signed and sealed this 24th day of September 1974.
McCOY M. GIBSON JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents USCOMM-DC 00376-P69 w u.s. GOVERNMENT PRINTING orrlc: 1 Isis o-Ju-au,
M PO-l 050 (10-69)