US 3545705 A
Description (OCR text may contain errors)
United States Pat ent John B. Hodgson Inventor Ihmpstead, Quebec, Canada Appl. No. 641,088
Filed April 14, 1967 Continuation-impart of Ser. No. 548,568, May 9, 1 966, abandoned.
Patented Dec. 8, 1970 Assignee .IWI Limited Montreal, Quebec, Canada STAINLESS STEEL FOURDRINIER CLOTH 8 Claims, 4 Drawing Figs.
[1.8. CI. 245/8 Int. Cl. B2lf27/l8 Field ofSelrch 245/8, 10;
 References Cited UNITED STATES PATENTS 3,100,729 8/l963 Goller 148/12 3,311,511 3/1967 Goller 75/l28(A)UX Primary ExaminerRichard J. Herbst Attorney-Alan Swabey ABSTRACT: A woven fourdrinier belt for paper making machinery, the belt having interwoven warp and weft strands of corrosion resistant wire which possesses substantial tensile strength and a high resistance to fatigue. The wire is made of a drawn and annealed stainless steel alloy and the warp strands have a flattened cross section to facilitate weaving.
RELATED APPLICATION This application is a continuation-in-part of my copending Pat. application Ser. No. 548,568, filed May 9, 1966, now abandoned.
This invention relates to an improved fourdrinier wire cloth used for paperrnaking, and to a method for improving the service life of the cloth by the use of stainless steel strands which are drawn and annealed to provide improved characteristics.
The principal object of the invention is to provide means whereby the working life of fourdrinier wire cloth belts, used in papermakin'g machines, is prolonged.
Another object of the invention is to provide a corrosion resistant fourdrinier wire cloth belt having a high resistance to abrasion and flexural fatigue.
Another object of the invention is to provide a fourdrinier wire cloth belt in which the warp strands are drawn from a stainless steel alloy, the drawn strands having excellent weaving characteristics.
Another object of the invention is to prepare the stainless steel alloy warp strands in such size and shape to enable them to be effectively woven into a fourdrinier wire cloth belt.
A further object of the invention is to prolong the working life of stainless steel fourdrinier wire cloth belt seams.
With the foregoing more important objects and features in view and such other objects and features as may become apparent as this specification proceeds, the invention will be understood from the following description taken in conjunction with the accompanying drawings, wherein like characters of reference are used to designate like parts, and wherein:
FIG. 1 is a fragmentary plan view showing a portion of the fourdrinier wire cloth belt of the invention;
FIG. 2 is a cross-sectional view, taken substantially in the plane ofthe line 2-2 in FIG. 1;
FIG. 3 is a cross-sectional view, taken substantially in the plane of the line 3-3 in FIG. 1; and
FIG. 4 is an enlarged cross-sectional view of one of the flattened stainless steel alloy warp strands.
Conventional fourdrinier wire drainage screens are made in the form of endless belts of woven wire cloth, ranging in width up to 30 feet and in total loop length up to about 180 feet. The endless belts are formed by carefully brazing together, with gold or silver alloy, the projecting warp ends of a length of woven wire cloth. The weaves range in mesh count from about 40 per inch to about 100 per inch, depending upon the fineness of the paper which is to be made thereon. Normally, the warp strands are made from phosphor bronze containing 8 percent tin, and the weft strands are made from brass or bronze. The conventional weave is a semitwill, wherein the warp strands are woven under two weft strands and over one weft strand. Semitwill weaving is the recommended weave for high abrasion resistance, there being a high differential of wear resistance between the two sides of twill woven cloth. This is done because, in practice, the life of a fourdrinier screen is largely limited by the abrasion of its inside or bottom surface.
The main reasons for the choice of phosphor bronze for the machine direction warp strands of fourdrinier screens are:
( l It has a reasonably high tensile strength with good ductility;
(2) It is fairly corrosion resistant;
(3) It has a fairly good abrasion resistance; and
(4) It has an exceptionally good flexural fatigue resistance under conditions of constant bending.
Stainless steel has long been known to be superior to phosphor bronze in many of these respects, but it has also been known that it was lacking in flexural fatigue resistance under conditions of constant bending. lts poor performance in this respect is due in part to the high modulus of elasticity (Young's Modulus), 28 X l p.s.i.. which is common to all steels. in contrast, the elastic modulus of phosphor bronze is 16 X 10 psi. Thus, although stainless steel has a fatigue strength of 35,000 p.s.i. in the annealed condition, and
phosphor bronze has only a fatigue strength of 30,000 p.s.i.,
phosphor bronze wires of the same diameter as stainless steel will withstand many more bending reversals at a constant bending radius because the stresses in stainless steel produced by the bending are higher, in the ratio of 28: 16. This increase in stress far outweighs the slight advantage which stainless steel has in true fatigue strength.
It is also well known that the fatigue strength of stainless steel can be greatly increased by cold working. Correspondingly, the tensile strength and yield strengths of the stainless steel are increased, and the ductility is reduced. Thus, it is possible to overcome the deficit imposed by the unfavorable elastic modulus by adequately work hardening the stainless steel and then tempering it to the desired degree. However, when such work hardening and tempering is performed, it is found in practice that the ductility is reduced to a point where satisfactory weaving is not possible.
The present invention overcomes these difficulties with the use of stainless steel by: l
(1) Choosing a stainless steel alloy which has higher than normal fatigue strength while maintaining adequate ductility;
(2) Choosing the proper conditions of cold working and subsequently tempering (annealing), so that the optimum relationship between fatigue strength and ductility is maintained; and
(3) Choosing a cross-sectional area and a cross-sectional shape of the warp strands, so that the weaving of good stable cloth is facilitated and fatigue stresses due to bending are minimized.
Referring now to the accompanying drawings, FIGS. 1-3 illustrate the plan and sections of cloth woven in accordance with the teachings of this invention. The weave is semitwill wherein the warp strands 10 are woven under two weft strands 12 and over one weft strand 12, to provide a greater bearing surface on the underside 14 of the cloth, where wear due to abrasion is greatest. The warp strands 10 are flattened in the plane of the cloth as shown in FIG. 4 to facilitate weaving.
By this means a stainless steel cloth can be manufactured having superior flexural fatigue resistance to a phosphor bronze cloth, while at the same time retaining the superior tensile strength and corrosion resistance inherent in stainless steel. The three elements of the invention as set forth above will now be described in greater detail.
ALLOY The method herein outlined can be applied to a great variety of stainless steel alloys, one alloy which has been found superior is commonly known as 21-6-9 Stainless Steel, the same having the following composition and mechanical properties:
Chemical composition: Percent arbon 1 0. 08 Manganese 8 00-10. 00 Phosphorus 1 0. 06 Sulphur 1 0. 03 Silicon 1 1. 00 Chromium 19. 00-21. 50 Nickel 5. 507. 50 Nitrogen 0 150. 40
Typical mechanical properties at room temperature Sheet and Strip Annealed 0.2% Y.S., K s.i.-70. Elongation, percent; in 2"-45. Hardness, li -92.
The distinguishing property of this alloy is that it has a high ultimate tensile strength while maintaining good elongation to failure. It also has a high fatigue strength.
Another alloy to which this invention can apply is commonly known as MS] Type 317 Stainless Steel, which has the following composition and mechanical properties:
Chemical composition: Percent Carbon 1 0. 08 Molybdenum 3. -4. 00 Chromium 18. 00-20. 00 Nickel 11. 00-15. 00
Mechanical properties Sheet and Strip Appealed 0.2% Y.S., K s.i.40. Elongation, percent in 245. Hardness, R -85.
Other corrosion resistant alloys having good tensile strength and ductility may be also suitable, specifically, any stainless steel alloy having a tensile strength in the range of 1 10,000 to 170,000 pounds per square inch with an elongation in the range from 20 to 50 percent. Wire having these properties may be produced from stainless steel alloys of the 300 series as well as of the 200 series, for example, stainless steel types 201, 202, 302, 304, 316 L and 317 and, including in this group, the low carbon content grades of these alloys. The composition of stainless steel alloy 201 is as follows:
Chemical composition: Percent Carbon 1 0. Manganese 5 50-7. 50 Phosphorus 1 0. 60 Sulphur 0. 03 Silicon 1 1. 00 Chromium 16. 00-18. 00 Nickel 3. 50-5. 50 Nitrogen 1 0. 25
The chemical composition of the aforementioned 200 series and 300 series of stainless steel alloys may be compared as follows:
1n the low carbon grade of these alloys, the carbon content is not greater than 0.03 percent.
Alloys other than stainless steel which possess characteristics similar in tensile strength and elongation and have adequate resistance to corrosion and fatigue may also be utilized in the embodiment of this invention.
Many metals which have desirable properties for fourdrinier warp wire, but heretofore have been found to be too tough to be woven in the conventional form may, by this invention, be woven into cloth having exceptionally good physical properties imparted by the very strength which has made them difficult to weave in the conventional form.
The invention is primarily concerned with the selection ofa special stainless steel alloy for and the cross-sectional shape of the warp strands of a fourdrinier screen. However, while the weft strands would normally be of circular cross section and of the same material as the warp strands, it has been found that ordinary grades of stainless steel, having a softer temper, may be effectively woven as weft strands.
COLD WORKING AND TEMPERING It has long been known in the art of fourdrinier wire manufacture that a fine grain structure in bronze warp strandsmay be obtained by cold working the warp strands in the drawing process and then subsequently tempering them by annealing at a known temperature for a known period of time. ln this manner optimum properties of ductility and strength are achieved. Also, in the same art, different drawing and annealing procedures are used for different purposes. The same general method is used for the drawing and tempering of stainless steel, but it has been found that the number of drawing reductions to achieve a finished size, and the annealing temperatures and times are vastly different for the production of stainless steel wire having the required characteristics.
For the aforementioned 21-6-9 alloy, it has been found that drawing a fully annealed wire from 0.036 inches to about 0.008 inches in 16 reductions, followed by tempering at a temperature between 1,600 F. and l,900 F. for between 6 to 18 seconds, produces a wire having superior tensile and fatigue strength with adequate ductility, and which is imminently suitable for weaving into a wire cloth for use in fourdrinier wire belts.
The 317 alloy has been investigated in this context in US. Pat. No. 3,100,729 by George W. Goller, disclosing that an percent reduction by cold working, followed by tempering between l,200 F. and 1,750 F. for times between one tenth to 30 minutes, provided a stainless steel wire having an adequate combination of tensile strength and fatigue strength,
together with good ductility. Manifestly, specific drawing and annealing conditions should be selected for each of the different alloys under consideration.
CROSS SECTION AND SHAPE OF WARP STRANDS It has been found that despite the use of ductile stainless steels, their stress-strain properties, cold worked and annealed for adequate fatigue strength, limit their usefulness because of the difficulties encountered in the weaving thereof. However, it has also been found that alterations in the size and shape'of the warp strands overcome these difficulties. Thus, the crosssectional area of the warp strands is slightly reduced, as compared with the cross-sectional area of a conventional phosphor bronze warp strand of a similar mesh count, and at the same time the warp strand is flattened in the plane of the woven cloth. The flattening of the strand serves the dual purpose of facilitating the weaving of tough materials by reducing the strand overlapping bend radii, while at the same time reducing the stress to which the warp strands are subjected in operation on a paper machine.
The flattened cross-sectional shape of the warp strand 10 is shown in FIG. 4. The wire may be flattened by rolling, or it may be drawn through a flattening die. The dimension x is limited by clearance in the weaving reed, and the dimension y is selected according to the degree of flatness desired. Suitable ratios of x: y are 1.32:1 or 1.4: 1, but actual limitations on sizes and shapes are found by considerations of the cross-sectional area consistent with allowable reed clearance and weavability. A possible ratio range ofx:y is 1.3:] to 1.6: 1.
It is understood that the principle of flattening the warp strands to improve the flexural fatigue resistance of woven cloth is old in the art. However, the combination ofa superior stainless steel alloy, proper drawing and annealing conditions, and the flattening of the warp strands produces fourdrinier cloth of outstanding qualities which cannot be attained by any one of these features alone.
In order to discover optimum annealing conditions for the various alloys, round wire and flattened wire in sizes suitable for the manufacture of fourdrinier screens were drawn in the conventional manner and then annealed under wide ranges of temperature and annealing time. The strands were then tested for tensile strength, ductility and fatigue strength, and pertinent results of such physical tests are given in the following Table I. It will be observed that for any except the lowest annealing temperature, the tensile strength of the 21-6-9 alloy is superior to that of the 317 alloy. Where the 317 alloy has a superior tensile strength, its loss in ductility (low percent elongation) makes it unsuitable for weaving. On the other hand, the 21-6-9 alloy retains its ductility while maintaining higher tensile strength, a property which renders this alloy particularly suitable for weaving the cloth of this invention. Moreover, the fatigue resistance of the 21-6-9 alloy is generally superior to that of the 317 alloy. It was also found that, in the instance of these alloys, for reasonable weaveability the elongation should preferably be in the range from about percent to 50 percent.
It will be also noted from the following Table I that, while the tensile and elongation properties of these alloys remain practically unchanged over the annealing time range, the fatigue resistance is generally best at an annealing time of 8 to 10 seconds.
From these experiments it was also found that the improved characteristics of the alloys are available over a range of annealing conditions. For example, in the instance of wire of 317 or 21-6-9 alloy, the annealing temperatures may range from 1600 F. to 1900 F for annealing periods ranging from 6 to 18 seconds. However, although the optimum results were obtained by annealing at 1800 F. for 9 seconds, this is not restrictive since the time depends to some extent upon the size of wire to be annealed.
TAB LE I.ST RAND TESTS Legend:
' T=Tensile Strength-lbsJsq. in.
E=Elongationpercent. F=Fetlgueminutes on strand tester before failure.
Material: Type 317 alloy .008" dia., area .0000503 sq. in.
Time 18 see. 12 sec. '1 see F '1 E F '1 E F 4 5 sec 3 6 sec Material: Type 21-6-9 alloy .008" d1a., area .0000503 sq. in.
Time. 18 see. 12 sec. 9 sec.
6 sec. 4 5 sec 3 6 sec i4,6d6"46"ii7'llll.l .IIII'Q l 141,500 41 79 143,000 41 135,000 41 72 115,200 43 73 117,200 41 58 Material: Type 317 alloy (flattened) .0065 x .0086 dia., .0000506 sq. in.
Time. 18 sec. 12 sec. 11sec.
TABLE I.STRAND TESTS(ontinued 6 sec. 4:5 sec. 3.6 see Material: Type 21-6-9 alloy (flattened) .0065 x .0086 dia., .0000506 sq. in.
Time.. 18sec. 12sec. 9500.
6 sec 4.5 see. 3.6 sec.
Material: Type 201 alloy (flattened) .0058 x .0085 die.., .0000435 sq. in.
Time 9.0 sec.
The following Table II shows properties of conventional phosphor bronze wire cloth, conventional stainless steel wire cloth, and stainless steel wire cloth woven in accordance with the invention.
In Table I1, example 1 is one of the most commonly used fourdrinier wire cloths, having a bronze warp and a brass weft- It should be noted that its warp direction tensile strength is 200 lbs/inch of width, and its warp direction flexural fatigue.
strength is 1 1,600 reversals.
Example 2 is cloth woven from round strands of 317 stainless steel for both the warp and the weft. The warp strands have been annealed in such a way that excellent ductility is obtained. It can be seen that cloth having similar construction to the phosphor bronze cloth is obtained. A smaller weft size must be used to obtain the same mesh count. Superior tensile strength is achieved and adequate drainage is obtained, as measured by the percentage open area. It is to be noted, however, that the flexural fatigue of the cloth is much below that of the phosphor bronze cloth of example 1.
Example 3 is a 317 stainless steel cloth utilizing flattened warp strands having the dimensions shown. It is noteworthy here that some improvement of flexural fatigue strength is obtained, although the cloth is still inferior to phosphor bronze in this respect.
Example 4 was woven with 21-6-9 warp material, drawn and annealed according to the teachings of this invention except that the warp was not flattened. It is to be noted that a superior fatigue life to that of phosphor bronze cloth was obtained, but it was found that this cloth was not tight, but was sleazy (unstable perpendicular relation between warp and weft strands), despite the use of a much smaller diameter weft size than normal. Even using the small diameter weft, it was not possible to crimp the warp adequately, and the weft mesh count of 50 was the maximum obtainable. The resultant drainage (percent open area) is high and this cloth would not be satisfactory for fourdrinier operation because of its difficulty of manufacture, its sleaziness, and its high drainage.
Example was woven according to the full teachings of this invention. It is to be noted that the warp mesh count has been reduced to 64 from the normal 68, and that the cross-sectional seam failures have been the most common reason for failure of the stainless steel fourdrinier cloth. Measures to improve the fatigue strength can be taken, but a large departure from 50 percent of the life of the cloth can rarely be achieved.
Seams made in cloth manufactured according to the present invention have a life in excess of 10,000 reversals. They too follow the usual relationship between the fatigue strength of the seam and the cloth, but in this instance the cloth fatigue strength has been raised to such a high level as to overcome the problem of premature seam failure in stainless steel cloth. The same factors which improve the fatigue strength of the cloth also improve the fatigue strength of the seam, that is, the choice of alloy, the particular conditions for cold working and tempering, and the particular shaping of the cross section of area of the warp strands has been maintained similar to that of the p- TABLE II.CLOTH TESTS Tensile Flexural strength Percent fatigue Warp Warp size, Weft warp. dir., open life of Ex. No. material Mesh in. size in. lb./in. area cloth 68 x 52 .008 0007 200 22. 6 11, 600 68 x 52 .008 .0085 315 25. 4 5, 700 68 x 57 0065 x 0086 0076 275 23. 5 6, 600 68 x 50 008 008 300 27. 4 17, 160 64 x 52 .0065 x .0080 .000 355 23. 0 20, 500 68 X 54 0058 x 0085 0088 280 22. 2 14, 248 68 x 54 0058 x .0085 .0088 368 22. 2 42,478 68 x 54 0058 x 0085 0088 420 22. 2 30, 742
TESTING PROCEDURES the flat warp used in example 3. The resultant cloth is slightly reduced in tensile strength with respect to example 3, but is still far superior to the tensile strength of conventional phosphor bronze cloth. Its drainage is similar to that of conventional phosphor bronze cloth, and its outstanding property is its far superior warp direction fatigue strength.
Example 6 is a cloth woven from strands which have been annealed according to the teachings of the Goller US. Pat. No. 3,100,729 already mentioned, with the additional treatment that the warp strands have been flattened to improve weavability and to provide improved flexural fatigue strength. In this example the preferred alloy of the aforementioned patent was used, that is, alloy 316L, whereas in example 3, stainless steel alloy 317 was used, which was not the most preferred alloy.
Example 7 is comparable to example 5, except that the mesh has been increased to 68 and the smaller warp size has been used, identical to the warp size in example 6. The warp material has been given the preferred treatment of the present invention by annealing at 1850 F. for 9 seconds. It should be noted that the resultant warp direction tensile strength is greatly increased over example 6 and the flexural fatigue life is more than doubled. Example 8 is identical to example 7 except that alloy 201 was used instead of alloy 2145-9. It should be noted that a further improvement in tensile strength has been obtained. The flexural fatigue life of the cloth is greatly increased over example 6, and the cloth would be perfectly adequate for use in paper machines. This example further illustrates how the teachings of the invention permit the use of very high strength stainless steel alloys in the manufacture of fourdrinier wire cloth for use in paper machines.
SEAMS Fourdrinier wire cloth has always been more susceptible to failure by fatigue at the seam than in the body of the cloth and, characteristically, failure occurs in the warp strands immediately adjacent to the seam. For example, the flexural fatigue strength ofa scam in phosphor bronze cloth rarely exceeds 5,000 reversals, that is, about one half the life of the body of the cloth. With conventional stainless steel having a fatigue life of 5,000 reversals, the seam will not withstand much more than about 2,500 reversals, and conseouentlv.
The following procedures were used to determine the characteristics of the different wire cloths:
Tensile Strength Tensile strength and percentage elongation of strands were measured on a standard tensile tester with a 5 inch gauge length.
The tensile strength of the cloth was determined by pulling a one-inch strip of the cloth, out parallel to the warp direction, in a conventional tensile testing machine.
Flexural Fatigue Life The flexural fatigue life of cloth was measured by the number of reversals to failure which occur when two rollers, each one inch in diameter, are rolled backwards and forwards over a strip wire cloth, out in the machine direction, which strip is 1 /2 inches wide and is under a tension of 10 pounds. Each reversal represents two flexures in both the upward and downward directions.
The fatigue life of strands was measured on a machine comprising a rotating spider carrying four l-inch diameter spindles which are free to rotate and are equally spaced around and parallel to the central axis of the spider. A group of strands may be tested at one time. Each strand is fastened to the machine frame at one side of the spider, passes around the four spindles, hangs down, and is fastened to a 140 gram weight at the other side of the spider. The spider is rotated at 93 rpm. and, at each revolution, each strand contacted by the spindles is flexed four times through over the spindle. The fatigue strength is measured by the time it takes for a strand to break.
Performance In one instance a fourdrinier wire, made according to the teachings of the present invention, ran for 48 days as opposed to an average life of 12 days for bronze wires. Upon removal, the wire was worn about one-tenth as much as a phosphor bronze wire would have been at the end of its life, except for an area oflocal damage which had worn into a hole.
In another instance a wire made according to this invention ran fnr 24 davs in contrast to a nnrmal life nfalmm 7 Have While in the foregoing there have been described preferred embodiments of the invention, various modifications may become apparent to those skilled in the art to which the invention relates. Accordingly, it is not desired to limit the invention to this disclosure, and various modifications and equivalents may be resorted to, falling within the spirit and scope of the invention as claimed.
1. In a woven fourdrinier beltcomprising warp and weft strands of corrosion-resistant wire, the warp strands:
a. composed of austenitic stainless steel in which the chemical composition of said warp strands comprises 0.08 percent maximum carbon, 8.00-'-10.00 percent manganese, 0.06 percent maximum phosphorous, 0.03 percent maximum sulfur, 1.00 percent maximum silicon, 19.0021.5 percent chromium, 5.57.50 percent nickel, 0.l-0.40 percent nitrogen, and the remainder substantially of iron;
h. drawn and annealed to have a tensile strength with the range 1 10,000 to 170,000 p.s.i.; and having an elongation at fracture within the range of 20- -55 percent the improvement wherein said warp strands are of flattened cross-sectional shape, said flattening having been accomplished before annealing to give a ratio of width to thickness of between 1.3:1 and 1.6:1 to facilitate interweaving with said weft strands.
2. A woven fourdrinier belt as defined in claim 1 in which said warp strands have been subjected to approximately 16 drawing reductions and then annealing at a temperature of between 1,600" and 1,900 for between 6-18 seconds.
3. A woven fourdrinier belt as defined in claim 1 having a mesh range of from 40 to 100 strands per inch, and in which said warp strands have been subjected to approximately 16 drawing reductions and subsequently to flattening and then annealing at a temperature of from l,700 F. to l,900 F. for from 6 to 18 seconds.
4. A woven fourdrinier belt as defined in claim 3 in which said belt is woven in semitwill weave.
5. in a woven fourdrinier belt comprising warp and weft strands of corrosion-resistant wire, the warp strands:
a. composed of austenitic stainless steel in which the chemical composition of said warp strands comprises 0.08 percent maximum carbon, 3.00-4.00 percent molybdenum, 1s.o0 20.00 percent chromium, 11.00-15.00 percent nickel, and the remainder substantially of iron,
. drawn and annealed to have a tensile strength within the range of 1 10,000 to 170,000 p.s.i.; and
. having an elongation at fracture within the range of 20- -55 percent, the improvement wherein said warp strands are of flattened cross-sectional shape, said flattening having been accomplished before annealing to give a ratio of width to thickness of between 1.3:] and 1.6:1 to facilitate interweaving with said weft strands.
6. A woven fourdrinier belt as defined in claim 5 in which said warp strands have been subjected to an 80 percent reduction by drawing and then annealing at a temperature of between 1,200 F. and l,750 F. for between 6 seconds to 30 seconds.
7. A woven fourdrinier belt as defined in claim 5 having a mesh range of from 40 to 100 strands per inch, and in which said warp strands have been subjected to approximately 16 drawing reductions and subsequently to flattening and then annealing at a temperature of from 1,200 F. to l,750 F. for
, from one-tenth to 30 seconds.
8. In a woven fourdrinier belt comprising warp and weft strands of corrosion-resistant wire, the warp strands:
a. composed of austenitic stainless steel in which the chemical composition of said warp strands comprises 0.15 percent maximum carbon, 5.507.50 percent manganese, 0.06 percent maximum phosphorous, 0.03 percent maximum sulfur, 1.00 percent maximum silicon, 16.00- l8.00 percent chromium, 3.505.50 percent nickel, 0.25 percent maximum nitrogen, and the remainder substantially of iron;
. drawn and annealed to have a tensile strength within the range 110,000 to 170,000 p.s.i.; and
having an elongation at fracture within the range of 20- -55 percent, the improvement wherein said warp strands are of flattened cross-sectional shape, said flattening having been accomplished before annealing to give a ratio of width to thickness of between 1.3:1 and 1.6:1 to facilitate interweaving with said weft strands.