US3196052A - Prestressing wire and method of manufacturing the same - Google Patents

Description

July 20, 1965 K. G. HANN 3,196,052

PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME Filed June 28, 1961 5 Sheets-Sheet 1 5 m0 WIRE AFTER PRocEssme 6 E g \wms BEFORE PRocEssms. a

E 40 I G- 3 PERCENTAGE OF ELONGAT\DN.

July 20, 1965 K. G. HANN 3,196,052

. PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME Filed June 28, 1961 5 Sheets-Sheet 2 Jig. Z);

July 20, 1965 NN 3,196,052

K. G. HA PRESTRESSING WIRE AND METHOD OF MANUFACTURING THE SAME Filed June 28, 1961 5 Sheets-Sheet 5 Z a O 0 DQocEssED AT 290 C. QELAXED AT 20 G.

g mmAL LOADING norms/5 \b 9 5 0 PROCESSED AT 200 C. 03 \QELAXED AT 20C m INITIAL LQADING lOO TON/5QJN,

*- 0 2000 4000 woo 8000.

DUQATION United States Patent Office 3,l%,052 Patented July 20, 1965 3,196,952 PRESTRESSEIG WEE AND METHGD OF MANUFACTURING THE SAME Kenneth Graeme Hann, Hendrescythan, Creigiau, near Cardifi, 'Glarnorgan, Wales, assignor to Somerset Wire Iompany Limited, Bridgwater, England, a British company Filed lune 28, 1961, Ser. No. 12%,441 Claims priority, application Great Britain, lune 1, 1%3, 15,235/53; Aug. 10, 1953, 22,055/53 8 Claims. (Cl. 143-12) This application is a continuation-in-part or" my application Serial No. 43 1,965, filed May 24, 1954, now abandoned, for Prestressing Wire and Method of Manufacturing the Same.

This invention relates to a new or improved method of straightening metal wire.

The present invention is concerned with the straightening of cold drawn high tensile steel wire which is formed of plain medium carbon and high carbon steel as opposed both to alloy steel, and also to mild steel, the tensile strength of which latter steel is too low for it to be useful for the purposes herein described.

The invention is concerned in particular with medium and high carbon steel wire having a carbon content within the range of 0.35% to 0.9%. In such medium and high carbon steels, manganese and silicon are present by reason of having been added as deoxidising agents in accordance with ordinary steel manufacturing practice, the manganese content usually not exceeding 0.7% and rarely, if ever, exceeding 1.0% and the silicon content usually not exceeding 0.4% and rarely, if ever, exceeding 0.8%, the sulphur and phosphorous contents of which usually do not exceed 0.04% and 0.03% respectively, and the maximum permissible upper limit of these two latter elements being in general 0.06% in each case; with iron and the usual residual, i.e., commercial impurities forming the balance of the composition. Medium and high carbon steel having a composition as described in this paragraph, in particular a carbon content within the above range of 0.35% to 0.9% is herein referred to as plain carbon steel.

The subjection of plain carbon steel wire to a straightening operation is advantageous for a variety of reasons, namely:

(i) Ease of manipulation, i.e., no allowance need be made for varying degrees of curvature in the subsequent manipulation operations to which the wire may be subjected.

(ii) In certain particular applications it is very desirable that the wire should be as straight as possible. One specific case is that of wire spokes for wheels.

A further specific and very important application of the invention is in the production of pre-stressing wire for reinforced concrete construction wherein it is of especial importance that the wire should not only be of a markedly straight configuration but should also possess particularly good tensile properties including a high ultimate tensile stress and the maximum resistance to creep under a continued tensile loading. That is to say in such application to pre-stressing wire, it is most important that the wire should possess a low stress relaxation, that is to say, the minimum of reduction or falling off of its tensile stress from some initial maximum, during the continued tensile loading of a piece of wire, the length of which is kept constant. In other words, the strain in the wire therein is maintained at a constant value of progressively reducing the tensile loading, i.e., the tensile stress in the wire and over an extended period of time.

As a pro-stressing wire is required to maintain a predetermined tensile loading on the concrete, such stress relaxation characteristic is a measure of the, usefulness of the wire as a pro-stressing wire.

(iii) In certain specific operations in which wire is fabricated the straighter the wire the simpler and easier the fabrication operation, for example, in the coiling of wire in the production of coiled springs.

It is already known to straighten cold drawn plain carbon steel wire by advancing the wire in the cold state, i.e., at room temperature, through oifset dies in a rotary straightening machine. Such a method in which the wire is straightened cold results in significant impairment of certain of the mechanical properties of the wire. In particular there is a marked lowering of the proof stress and some lowering of the ultimate tensile strength of the wire.

For instance the following test was carried out by myself on plain high carbon steel wire having the following composition:

Percent Carbon 0.8 Manganese 0.7 Silicon 0.2 Sulphur and phosphorus 0.05

iron and the usual commercial impurities the re mainder.

1 Maximum in each case.

This wire of diameter 0.2" was advanced in the cold state through a rotary straightening machine as aforesaid, and I found a reduction in the 0.1% proof stress of between 10 and 15% and a reduction in the ultimate strength of the wire by substantially 5%.

A principal object of the present invention is the provision of a new or improved method of straightening cold drawn plain carbon steel wire, which method avoids the aforesaid impairment of certain of the mechanical properties of the wire as occurs if the wire is straightened in the cold state by the existing technique above referred to, which method in accordance with this invention serves incidentally to improve substantially certain of the mechanical properties of the wire, namely in particular to improve substantially its creep resisting properties un der tension as well as to increase the elongation of the wire before failure under tension.

A further object of the present invention is to provide an improved method of manufacture of a plain carbon steel pro-stressing wire by which the wire is characterized by a particularly low stress relaxation.

A further object of the present invention is to provide a new or improved method of manufacturing a plain carbon steel pro-stressing wire which is characterized by having a particularly high ultimate tensile strength.

Other objects of the present invention will become apparent from a consideration of the specification, including its accompanying drawings and the appended claims.

The present invention in its essentials comprises a method of straightening a cold drawn plain carbon steel wire and having a carbon content within the aforementioned range of 0.35% to 0.9%, and preferably within the range of 0.4% to 0.8%, wherein the wire after the cold drawing operation is heated to a tempering temperature within the range of 150 C. to 500 C., and preferably within the range of 200 C. to 400 C., and a controlled tension is applied to successive lengths of the so heated wire, such tension being of magnitude sufiicient to effect a permanent elongation and straightening of the wire without subjecting the wire to such a high tensile stress as to cause it to fracture.

The foregoing method, in addition to producing a straightening effect on the wire, also results in relieving local stresses by the subjection of the wire to a tempering operation, and improves significantly the tensile properties of the wire, notably by increasing its ultimate tensile stress and improving its creep resistance and stress relaxation under continued tensile loading.

In order that the meaning of certain expressions em ployed in this specification may be clearly understood, these are defined in the following:

DEFINITIONS H (1) By the expression permanent elongation employed herein is meant that the elongation imparted to the wire when heated to a tempering temperature as defined below is at least partially retained after the wire has cooled to normal, i.e., room temperature.

In other words the elongation of a given length of wire which is produced is greater than that expected from the sum of the strain in the wire due to the applied tensile load and the linear expansion due to the rise in temperature.

(2) By the expression tempering temperature is meant a tempering temperature as customarily understood in the art of heat treating the plain carbon steel to which the invention is applied, that is to say it is a temperature within the tempering range to which the steel would be heated after a quenching operation for the purpose of increasing its ductility.

Such temperature will necessarily, of course, be below the lowest transformation point in the iron carbon diagram and will necessarily vary in accordance with the analysis of the particular steel concerned. In the case of medium or high carbonsteel having a carbon content within the range of 0.35% to 0.9% the tempering temperature would, as is well understood, be within the range of 150 C. to 500 C., and as hereinafter explained would preferably be within the range of 200 C. to 400 C.

(3) By the expression straightening as used herein is meant that the shape of the wire which is subjected to the straightening operation is made to conform more closely to an absolutely straight configuration, but such expression is not herein intended necessarily to imply that'the wire is so shaped by the method according to this invention as to be perfectly straight, although it is believed that in fact by the method according to this invention it is possible to produce wire. in a form which closely approximates to that of absolute straightness.

(4) 0.1 proof stress.-The tensile stress required to produce a permanent elongation of 0.1% in the wire. Such proof stress is commonly measured during an ordinary tensile test as customarily practised.

The primary advantages of the present invention may be summarised as follows:

(a) It provides a method of straightening wire formed of cold drawn plain carbon steel in which the wire can be straightened to a form comparable with that obtainable by a cold straightening method using a rotary straightening machine as above described, wherein the impairment of certain of the mechanical properties of the wire attendant on such hitherto known method as above indicated are avoided.

(b) The wire is much less liable to creep, i.e., much 4 more stable under conditions of long period or continuous tensile loading.

(0) The elongation of the wire before failure is increased as compared with an otherwise similar wire produced by normal drawing processes.

(d) The elastic limit or proof stress of the wire can be markedly increased in comparison with that of an otherwise identical wire but produced by normal drawing processes.

(e) The impact resisting properties of the wire are improved.

Although the invention is capable of a general application of which one very important application is in the production of wires for use in the manufacture of wire spokes the invention is especially applicable to the production of wires for use in pre-stressed concrete as not only does the particularly straight configuration of the wire facilitate its use for this purpose, but as hereinbefore explained under paragraphs (b) to (d) inclusive above, the improvement in the tensile properties make it especially suitable for this work.

At present it is customary in pre-stressed concrete to employ as the pre-stressing members, wire formed of plain carbon steel as herein defined, and produced by ordinary cold drawing operations, and these entail the disadvantage that when they are subjected to a tensile load equal to half or more than half of the breaking load, the elongation of the wire increases with time and concurrently the stress in the wire held at a constant strain relaxes with time. These are phenomena which are particularly undesirable in pre-stressed concrete construction, which disadvantageous phenomena are avoided, or substantially avoided, by the present invention.

As above explained the present invention is essentially concerned with a method of straightening cold drawn wire formed of plain carbon steel, and although the method may be applied to such wire which has previously been subjected to a cold drawing operation, the cold drawing operation and the straightening method in accordance with the present invention are preferably combined, so that the drawing down die or the reducing rolls employed in the cold drawing operation may co-act with the wire pulling means employed in producing the required tension, such pulling means comprising various known forms of gripping devices, for example caterpillar gripping jaws, or a power driven rotating block or capstan, around which the wire would be wound continuously after its straightening.

Thus the apparatus required for carrying out the method the subject of this invention would then constitute an auxiliary part of the drawing down plant.

The preferred method of heating the wire or the like is by direct resistance heating, i.e., by passing an electric current through that part of the wire or the like which is being subjected to the tensile loading, but other methods of heating may be used such as induction heating, or by passing the Wire through the lead orsalt bath, or through some form of furnace.

The invention may be applied to wire or the like of plain configuration, or it may be applied to wire which is of indented configuration peripherally, and the indenting operation may be combined with the straightenin method the subject of this invention. a

As above set forth the invention is applicable to plain, medium and high carbon steels as distinct from alloy steels and comprising a carbon content within the range of 0.35% to.0.-9%. The minimum'valuepf this range is determined by the fact that with a carbon content below 0.35% the ultimate tensile strength of the steel is too low for the steel to be commercially useful in the production of wire required to possess marked tensile strength, as in the case, for example, of pre-stressing wire, and for the foregoingreason 0.4% carbon represents the minimum useful carbon content for many applications of plain carbon steel wire. The upper limit of 0.9% for the carbon range is determined by the fact that 0.8% carbon is the eutectoid composition for iron carbon, and carbon in amount greater than 0.8% tends during the production of the wire to form at the grain boundaries pre-eutectoid cementite which reduces the ductility of the metal and makes the drawing operation more difficult. For this reason 0.8% carbon is the preferred upper limit for the carbon range.

In order that the invention may be more fully understood, reference is directed to the accompanying drawings, wherein:

FIGURE 1 shows a schematic layout of one form of plant for carrying out one specific method in accordance with the present invention;

FIGURES 2, 3 and 4 illustrate modifications of the plant depicted in FIGURE 1 for carrying out respectively three modified forms of the said method;

FIGURE 5 shows a graph illustrating the improvement in certain physical properties obtained in the finished wire when subjected to the straightening method forming the subject of this invention;

FIGURES 6, 7, 8 and 9 are graphs depicting the relaxation characteristics of pro-stressing wires of a number of different analyses, and which wires have been subjected to the method of this invention.

Referring firstly to FIGURE 1 of the drawings, the invention is here depicted as applied to the straightening of plain carbon steel wire so as also to improve the tensile properties thereof and to render the wire particularly useful for pre-stressed concrete construction. The wire is first advanced in the cold state through one or more drawing down or reducing dies, one only of which dies of conventional form is indicated at 10 in the drawing, such die or dies serving to effect reduction in the cross section of the wire and thereby effectively to grip it. The wire is advanced continuously through this die 10 by means of a continuously rotating power-driven capstan or block 11, around which the wire is coiled by the continued rotation of the capstan or blocked in the known manner. This capstan or block 11 is generally of conventional form but has a diameter which is substantially larger than that normally used, a particularly convenient diameter to be employed being 350 to 500 times the wire diameter, i.e., a diameter of 100" in the case of wire of 0.2 diameter, which 100" diameter is substantially four times larger than that normally used.

The length of the wire advancing continuously from the drawing die to the capstan or block is depicted at 12.

The drawing die 10 is connected through lead 13 to one side of a motor generator of known form indicated diagrammatically at 12 and adapted to supply alternating or direct current at a low voltage, conveniently 20 to 40 volts, and at a substantial amperage, for example, of the order of 200 to 500 amperes.

The other side of this motor generator is connected by lead 15 to a plurality of spring loaded current collecting brushes 16 which are in electric conducting engagement with the wire 12 at a position intermediate the drawing die 10 and the capstan or block 11, these brushes being so arranged as to permit of the wire advancing freely in relation thereto as it is coiled around the capstan or block.

The arrangement is such that a heavy current of the value above indicated is thereby passed along that part of the wire 12 which is between the brushes 16 and the drawing die 10.

The side of the motor generator 14 which is connected to the drawing die 13 is further connected by another lead 17 through adjustable resistance 10 and brush gear 19 to the rotating capstan or block 11 so as thereby to pass current along that part of the wire 12 which is between the capstan or block 11 and the brushes 16. The adjustable resistance 18 is adjusted so as to maintain the wire 12 as it advances from the brushes 16 towards the capstan or block 11 at a substantially constant temperature, and a convenient current to pass along the wire between the brushes 16 and the capstan or block 11 for this purpose is one of the order of 50 to amperes, i.e., of magnitude substantially less than that passed along that part of the wire which is between the drawing die 10 and the brushes 16.

The arrangement is in fact such that a heavier current is passed along that part of the wire which is advancing from the drawing die to the brushes so as to raise this part of the wire relatively rapidly to the desired temperature, and the wire is subsequently maintained at such temperature for a predetermined period of time by the passage of the lesser current therealong between the brushes 16 and the capstan or block 11.

The maximum temperature to which the wire is heated is in fact a tempering temperature, namely, a temperature within the range of to 500 C., the preferred value of this temperature within the foregoing range being dependent upon a number of different factors as follows:

(a) The composition, in particular the carbon content of the plain carbon steel employed. The lower the carbon content, the lower the tensile stress necessary to effect the desired permanent elongation and straightening of the wire, particularly at the lower temperatures within the above range of 150 C. to 500 C.

(b) The desired tensile properties of the wire. In general the application of a higher tensile stress with a heating temperature towards the lower end of the temperature range produces a particularly high ultimate tensile stress in the wire.

(c) The tension which is applied to the wire by means of the rotating capstan or block. The higher the temperature, the lower the tensile stress necessary to produce a given permanent elongation in the wire. Conversely, the greater the tension the lower the temperature which can satisfactorily be employed, it being understood that if the tension is too great then failure of the wire under tension will occur.

Set out below is the result of some experimental measurements of the tensile stress necessary to produce a permanent elongation of 1% and also 2% in plain carbon steel wire heated to varying temperatures within the temperature range above mentioned:

1% elongation 2% elongation The analysis of the wire employed in the foregoing tests was as follows:

Percent Carbon 0.76 Silicon 0.23 Manganese 0.63 Sulphur 0.02 Phosphorus 0.02

Iron and residual impurities the remainder.

(d) The desired stretch, i.e., permanent elongation of the wire. The minimum usefulpermanent elongation is 1%, and the maximum permanent elongation which I find to be advantageous is in general about 5%.

As applied to wire formed of plain carbon steel, I find that satisfactory results are obtained if the wire is heated to a maximum temperature which is within the range of 150 to 500 C., and preferably within the range of between 200 to 220 C. as the lower limit, and 400 C. as the upper limit, one especially preferred temperature range where the carbon content is approximately 0.8% being 350 to 400 C.

The definedlimits of 150 C. and 500 C; are determined as follows. The lower temperature limit is determined by that fact that:

(i) Below this limit the maximum ultimate tensile strength of the straightened wire commences to fall, the ultimate tensile strength being at a maximum in the case of a carbon content of about 0.8% where the heating temperature is around 150 C. to 200 C.

(ii) 150 C. represents the lowest practical temperature limit at any rate with the higher carbon contents, e.g., 0.7 to 0.9% at which the wire under commercial conditions of operation can be stretched without fracture, to produce a useful permanent elongation, i.e-., of at least 1%.

The defined limit of 500 C. represents the maximum temperature at which the stretching of the wire can be eifected consistent with the retention of a sufficiently high ultimate tensile strength for the process to be useful.

- In Table D which appears later herein there 'is set out certain test results at various temperatures within the foregoing range, and which substantiate the selection of the foregoing maximum temperature limits of 150 C., to 500 C., as well as the foregoing preferred lower limit of 200 to 220 C. below which temperatures the ultimate tensile stress commences to decrease, while quite a marked decrease in ultimate tensile stress occurs where the temperature at which the wire is stretched is above 400 C.

In order to prevent the wire heated to the foregoing temperature on subsequent cooling contracting on to the block or capstan prior to its removal therefrom, so as seriously to impair the removal of the wire, I provide means for cooling the wire immediately prior to its advancement around the capstan or block.

Such means comprise a tube 20 through the interior of which the wire passes, the tube having a bore substantially greater than the diameter of the wire and the end of the tube nearest to the capstan or block being partially closed 'so as to provide therein merely a central hole 21 of bore slightly greater thanthe diameter of the wire so as to provide a guide for the wire and maintain it substantially central within the tube. Adjacent this end of the tube 20 this is provided with an inlet 22 for cold Water which is fed continuously through the tube, the water flowing out at the opposite end 23 thereof, and thereby serving to cool the wire down to a temperature at which it can be coiled without fear of'it gripping the capstan or block unduly tightly.

The tensile load is applied to the wire by the action of the power driven capstan or block 11 in advancing the wire through the drawing die and the reduction in cross section at the drawing die must be sufliciently great as to enable this to exert the necessary grip on the wire for this purpose, so that the capstan or block may exert on the wirea tensile force sufficiently great as to produce the elongation in the length of the wire heated to the temperature aforesaidwhich is greater than the sum of the strain in the wire due merely to the tensile load and the linear expansion due to the rise in temperature. Otherwise the desired straightening of the wire would not be effected, and concurrently the above referred to improved creep resisting and elongation characteristics of the wire under tension would not be obtained. v

In the case of a plain carbon steel wire having a carbon content of 0.76% which is heated to a temperature within the particularly preferred limits of 300 C. to 400 C., the

tensile loading which is applied to the wire may be within the range of approximately 35% to 70% of the breaking load, and preferably is between 35% to of the breaking load, i.e., with a carbon steel .wire having a carbon content of 0.76% and a breaking load of approximately 100 tons per square inch the tensile loading applied may correspond to' a tensile stress in the wire of approximately 50 tons per square inch. a

1 Although as above described it is preferred to carry out the method of the subject of this invention as a continuation of the cold drawing down process, in that the reducing die or. rolls employed provide the necessary anchorages for the one end of the wire which is being processed, the invention as shown in FIGURE 2 may be similarly applied to wire which has previously been drawn down by taking a length of such drawn down wire, winding it around a feeding drum 24 provided with a brake or other means for retarding its rotation, the wire being fed from this feeding drum 24 to thepower driven capstan or block 11 and subjected in so doing to heating, tension, and cooling by means arranged in exactly the same way as in the case of the plant depicted in FIG- URE 1, as will be apparent from this FIGURE 2 of. the drawing; I In applying the invention to galvanised carbon steel wire, the brushes 16 by reason of the slidable engagement' of the heated wire therewith would impair the galvanised coating, and to avoid this in the case of galvanised wire, the arrangement depicted in FIGURE 3 is adopted in which the carbon brushes I6 and lead 21 are omitted entirely and the lead 15 is connected direct to the brush gear 19, that is to say the heavy current of, for example, 300 to 500 a'rnperes fiows from the whole of the wire which is between the drawing die 10 and the capstan or block 111.

The arrangement shown in FIGURE 3 may advantageously be applied to wire which is not galvanised in that this arrangement is a simpler one than that of FIGURE 1 by its omission of the brushes 16.

The invention may also be applied to indented wire which, after the conclusion of the drawing down operation, is indented in the known manner by passing it, as shown in FIGURE 4, between a pair of indenting rolls 25 of known form provided in the usual way with a series of peripheral indentations or recesses. Such indented wire is advantageous as applied to pre-stressed concrete.

In this latter arrangement depicted in FIGURE 4, the

I drawing down die 10 is replaced by one or more sets of reducing rolls indicated diagrammatically at 26, which serves to elIect equal reduction of the wire, and these reducing rolls may replace the drawing down die 10 with the arrangements depicted in FIGURES 1 and 3.

In this lastarrangement depicted in FIGURE 4, one lead 13 from the motor generator set 14 is taken to the indenting rolls 25, otherwise thearrangement is identical with that depicted in FIGURE 3. I 7

With both arrangements depicted in FIGURES 3 and 4 in which the full current flows along the entire length of the wire, including that part thereof which is between the cooling tube and the capstan or block, it i important that the'cooling tube 17 should be disposed as close to the capstan or block 11 as is practicable so as to reduce the length of the cooled wire between the tube 17 and the capstan or block, through which this full current is flowing, and thereby to avoid undesirable reheating thereof.

With any of the above arrangements, if the reduction of area of cross section produced by the drawing die 10 or drawing down rolls '26 is unduly low it may be necessary to augment the. load reaction thereby put on the encased wire in various known Ways, namely, by passing the drawn down Wire through rolls to which is attached a brake, or by passing the wire through rolls, which cause reverse bending, or by using a plurality of drawing down dies or sets of drawing down rolls arranged in series with one another. The object, of course, in using any of such devices is simply to increase the tension in the wire sufiiciently to obtain wire which is as straight as possible.

Further, in place of the motor generator set 14 there may be used a transformer for supplying low voltage alternating current at the desired current density.

Three specific examples of pro-stressing wire formed of cold drawn plain carbon steel and processed, i.e., subjected to the method of this invention, using the particular plant depicted diagrammatically in FIGURE 3 of the accompanying drawings, are as follows:

Table A Analysis, percent Examplel l Example H i Example III 0. 70 O. 40 0. 51 0. 23 0. 061 0. 203 0. 021 0. 037 0. D45 0. D24 0. 028 O. 029 Manganese. 0. 63 0. 69 0. 61 Iron and re dual impurities t) 1 The remainder.

.2 3 6 inch.

.202 inch.

Comparative tensile tests were performed on wire in accordance with each of the three examples, these comparative tests being performed in the case of each of the three examples on wire subjected to the method the subject of this invention, under the conditions specified in Table A, and hereinafter referred to as processed wire, together with otherwise identical wire, i.e., of the same analysis manufactured under the same conditions and cold drawn to the same final diameter of 0.202"; such latter wire in each case being not, however, subjected to heating under tension in accordance with the present invention. Such latter wire for convenience in description is herein referred to as unprocessed wire.

The results of these tensile tests are set out in the following table B.

The improvement in the 0.1% proof stress, in the ultimate tensile stress, and in the percentage elongation of 10 the processed wire in each of the three examples as compared with the otherwise identical unprocessed wire is significant in every case.

In the case of Example I, the improvement of the tensile properties obtained with the processed wire as compared with the unprocessed wire, is further illustrated by the graph depicted in FIGURE 5 of the accompanying drawings, wherein is depicted two stress-strain curves for wire of the foregoing specific composition cold drawn down in each case by the final drawing down die to a diameter of .202, wherein the dashed line depicts the stressstrain curve obtained with the wire after drawing down before processing, and the continuous line depicts the stress-strain curve of this same wire after both drawing down and processing in accordance with the foregoing specific example.

Wire in accordance with each of the three foregoing examples, was subjected to a series of stress relaxation tests which were performed at both room and elevated temperatures, the -results of which tests are set out in the graphs forming FIGURES 6 to 8 inclusive, and which refer respectively to Examples 1, II, and HI.

These stress relaxation tests were carried out both with processed wire and also for comparative purposes with unprocessed wire in accordance with each of these three Examples I, H, and III.

In carrying out the tests on the processed and unprocessed wire according to the foregoing three examples, a test length of 29" was taken in each case and the test specimen was subjected to tension in a known form of creep testing machine embodying a heating furnace surrounding the wire so that the test could be carried out at both room and elevated temperatures, the latter being checked continuously by three thermocouples spread over the gauge length of the specimen.

in performing the test, the metal of the wire was stressed to a value of between 50% and of the ultimate tensile strength of the wire at room temperature, and after loading the Wire to the determined percentage within the above range of the ultimate tensile strength, the extension of the wire under the tensile loading was noted and was thereafter kept constant by periodically removing a portion of the tensile load applied to the wire, namely, by removing Weights from the beam of the tensile testing machine.

The foregoing test was carried out at room temperature, i.e., at ordinary atmospheric temperature in the case of each of the three examples, l, H, and III, and further tests were performed at an elevated temperature of C. in the case of Example I.

Each of the foregoing elevated temperature tests as well as each of the foreoing room temperature tests were performed with the processed and unprocessed wire in each case.

Readings of the applied tensile loading were noted periodically and plotted on a time basis on each of the graphs forming FIGURES 6, 7 and 8.

These three graphs show in every case a reduction in the tensile stress with the length of the wire maintained constant, i.e., the wire was subjected to the designed conditions of use of a pro-stressing wire, in which the wire is embedded in a surrounding matrix of concrete.

Accordingly, the stress relaxation tests .the results of which are depicted inthe graphs, FIGURES 6, 7 and 8, are a measure of the behaviour of the wire under service conditions, i.e., as a pro-stressing wire in concrete constructions, and the reduction in the tensile stress both at room and elevated temperature is a measure of the ability of the wire to maintain its desired pro-stressing characteristics, i.e., its initial tensile loading both at room temperature and also when subjected to moderate heat as may occur during the initial stages of a building fire.

Obviously, the usefulness of the wire as a pre-stressing wire .is denoted by the percentage of the stress reduction or stress relaxation over a given period of time from the initial value; the smaller the reduction the greater the utility of the wire.

In performing the tests carried out at room temperature, both the processed and unprocessed lengths of wire, in the case of Example I, having the higher carbon content, namely, .76%, were loaded to an initial loading of 80 tons per square inch. This initial loading was found too great in the case of the tests which were performed with Example I at elevated temperatures, and lower initial loading values were employed corresponding to the limit of proportionality in the tensile test, the initial loading selected for each elevated temperature being the same in the case of the unprocessed wire as well as for the processed wire at the particular temperature involved.

In the case of the Wire according to Examples II and III having a lower carbon content, namely, 0.40% and 0.51% respectively, the initial loading of the wire was again of a lesser degree than that employed in Example I at'room temperature, the initial loading being 60 tons per square inch in the case of Example II and 66 tons per square inch in the case of Example III.

The stress relaxation for both the processed and unprocessed wire in each of the three examples at the end of 1000 hours for both room temperature and also for the particular elevated temperature tests above referred to, is set out in the following Table C, in which table is set out both the initial stress to which the wire was subjected, and alsothe final stress at the end of 1000 hours while maintaining the length of wire constant. In the table is also set out the percentage reduction in stress necessary to maintain the constant length of the test specimen during the test, which percentage reduction stress is herein referred to by the term percentage stress relaxation. In this Table C, the abbreviation R.T., is used to indicate room temperature.

Reference to the foregoing Table C and to the three graphs FIGURES 6, 7 and 8, clearly show that over the period of each test, namely, 1000 hours, in every case the stress relaxation of the processed wire is less than that of the unprocessed wire.

Table C Initial Final stress Percentage stress, alter 1,000 stress tons/sq. hours, relaxation inch tons/sq. inch Example I:

Processed R.T 80 78 2. Unprocessed RT 80 74. 3 7. 1 Processed 100 C 69 62. 7 9.1 Unprocessed 100 C. 69 55.8 19. 1 Example II:

Processed R.'I 60 58. 3 1 2.8 Unprocessed RJI 60 55 8. 3 Example 111:

Processed R.T 66. 5 65. 7 1.2 Unprocessed R.T 66.5 61. 7 7. 2

1 Less than 3.

From the foregoing table, as well as from the three Further extremely important data which was noted, but which cannot readily be denoted on the maximum permissible size for the accompanying graphs, is that even after 1000 hours, the two curves for the processed and unprocessed wires of each example at each test temperature, were diverging relatively, that is to say the stress relaxation of the processed wire was continuing to decrease at a faster rate than was the case with the unproc essed wire; the increase in stress relaxation which was occurring with the processed wire at 1000 hours being in every case negligible at room temperature. Thus as will be seen from the graphs, the curve for the processed wire in every case is virtually horizontal after the expiration of 1000 hours, although not quite horizontal in the case of the unprocessed wire. i

A further series of tensile tests and stress relaxation or creep resistance tests were carried out on a number of lengths of wire processed in accordance with the invention, in which the lengths of wire, following the cold drawing thereof, were stretched by differing amounts at various temperatures including a number of temperatures selected within the range of to 500 C.

These wires were all of the same analysis as that of Example I so as to have a carbon content of 0.76% and a' diameter prior to the processing operation of 0.201".

The so processed wire was subjected to tensile tests in which measurements were taken of the ultimate tensile stress and the 0.1% proof stress.

The various lengths of wire processed by stretching the lengths by different amounts at varying temperatures, as above referred to, were subjected to stress relaxation tests in which the lengths of wire were stretched at room temperature, increasing the tensile load by 0.1 ton increments until the 0.1% proof stress was reached. At this point the tensile stress and strain figures were noted, and it was observed that the'wi-re was now creeping, i.e., increasing in length under the applied tensile stress, and the strain value was now kept constant and over a period of 20 hours by decreasing the tensile stress during that pe: riod, a measurement being taken of the tensile stress at the end of the 20 hour period with the strain, i.e., the length of the wire, maintained constant during this time.

After the expiration of the 20 hour period the samples were subjected to, maximum tension so as to obtain a measurement of the ultimate tensile stress and also of the ductility as measured by the. percentage reduction in area at the fracture of the specimen.-

The results of'the foregoing series of tests with lengths of wire according to Example I, i.e., a carbon content of 0.76% are set out in the following Table D, the explanation of the abbreviations used in which is as follows:

C%tress relaxation test carried out as above described,

followed by tensile test on stress relaxed specimen.

TO'rdinary tensile test on specimen not subjected to stress relaxation test.

U.T.S.Ultimate tensile stress.

SO0.1% proof stress. a V

SF-Stress in specimen at the expiration of 20 hours after commencement of the stress relaxation test, maintaining the strain of the wire constant following the attainment of the 0.1% proof stress value.

The ratio of SF to S0 in each of the stress relaxation tests, i.e. creep tests, was evaluated as providing an indication of the reduction in tensile stress necessary to maintain the length of the specimen unchanged under the tensile loading.

All of the stress figures given are in tons per square inch.

The whole of the stress relaxation tests, the results of which areset out in Table D, were carried out at room temperature.

Table D Percent Wire Temp. 01% Stress Nature Stretch, during Proof after Ratio Red. of Tensile i.e. Perma- Stretching, U.T.S. Stress, 20 hrs. SF: 50 Area,

Test nent E1on C. SO Creep, Percent gation SF O 1 100 'l 1 100 C- 1 150 'l 1 150 C 2 150 T 2 150 C l 200 C 2 200 '1 2 200 C 200 T 5 220 C 1 250 T 1 250 C 2 250 T 2 250 C 5 250 T 5 250 C 1 300 T 1 300 C 2 300 T 2 300 C 5 300 T 5 300 C 1 400 '1 2 400 C 2 400 T 4 400 C 5 400 C 1 500 C 2 500 C 5 500 1 Unprocessed.

An extended stress relaxation test was performed with a length of wire according to Example I, i.e., with a carbon content of 0.76%, and stretched at a temperature of 200 C. so as to produce a permanent elongation, i.e., stretch of 2% at that temperature. Two specimens which were respectively so processed were taken and initially loaded to a tensile stress of 100 and 110 tons per square inch at room temperature and subjected to stress relaxation tests at this room temperature, which was actually C., in a manner similar to the tests already described. The result of this further series of tests is set out in the graph forming FIGURE 9.

As will be seen from this figure, the tensile stress in the more highly stressed specimen fell at a more rapid rate during the first few hours than the specimen stressed to the lesser initial value and took considerably longer to reach a zero relaxation rate. Nevertheless the more highly stressed specimen attained a zero relaxation rate corresponding to total cessation of further creep under a stress in excess of 100 tons at the expiration of less than 8000 hours, while the specimen stressed initially to the lesser value attained zero relaxation at a tensile load of no less than about 93 tons per square inch after the expiration of a little more than 6000 hours.

This result makes plain the great utility of such processed wire as a pro-stressing wire in reinforced concrete construction.

It is further plain from each of the foregoing test results that the processed wire, i.e. wire which has been subjected to the method the subject of applicants invention, possesses significantly improved stress relaxation properties as compared with otherwise identical wire not subjccted to the method the subject of the invention, so that in the art of pro-stressing wire for concrete, the present invention is of considerable importance.

What we claim then is:

1. A method of increasing the resistance to creep under tension of Wire for use in reinforced concrete construction, said method comprising subjecting plain carbon steel wire having a carbon content within the range of to .9% to a cold drawing operation by advancing the same through drawing means, applying tension to the drawn wire to effect its advancement through the drawing means,

heating the wire after its emergence from the drawing means and while subjected to said drawing tension to a tempering temperature within the range of 220 to 500 C. and maintaining the drawing tension at such value as having regard to the time during which each wire length is maintained at the tempering temperature and the value thereof as to impart a permanent elongation to the wire of not more than about 5% and increase the creep resistance thereof without at the same time applying a tensile force suificiently great to the wire as to effect failure of the wire under the applied tensile load.

2. A method of increasing the resistance to creep under tension of plain carbon steel wire as herein defined for use in reinforced concrete construction, said method comprising heating cold drawn plain carbon steel wire having a carbon content within the range of 0.35% to 0.9% to a temperature within the range of to 400 C., while simultaneously applying a controlled tension to the heated wire, and producing in the wire a permanent elongation of between 1% and 2%.

3. A method of increasing the resistance to creep under tension of plain carbon steel wire as herein defined for use in reinforced concrete construction, said method comprising heating cold drawu plain carbon steel wire having a carbon content within the range of 0.4% to 0.9% to a temperature within the range of 200 to 400 C., while simultaneously applying a controlled tension to the heated wire so as to produce in the wire a permanent elongation of between 2% and 5%.

4. A method of increasing the resistance to creep under tension of plain carbon steel wire as herein defined for use in reinforced concrete construction, said method comprising subjecting plain carbon steel wire having a carbon content within the range of 0.7% to 0.8% to a cold drawing operation by advancing the same through drawing means, applying tension to the drawn wire to effect its advancement through the drawing means and While sub jected to said drawing tension heating said wire to a temperature within the range of 350 to 400 C., and maintaining the drawing tension at such value as having regard to the time during which each wire length is maintained at the drawing temperature and the value thereof as to impart a permanent elongation to the wire and in 15 crease the creep resistance thereof without at the same time applying a tensile force sufficiently great to the wire as to efiect failure of the Wire under the applied tensile load.

5. A method of increasing the resistance to creep under tension of plain carbon steel Wire as herein defined for use in reinforced concrete construction, said method comprising subjecting plain carbon steel wire having a carbon content within the range of 0.4% to 0.8% to a cold drawing operation by advancing the same through drawing means, applying tension to the drawn wire by means of a power rotated capstan or block around which the wire is coiled to effect its advancement through the drawing means, heating the wire after its emergence from the drawing means and while subjected to said drawing tension to a temperature within the range of 250 to 400 C., and maintaining the drawing tension at such value as having regard to the time during which each wire length is maintained at the drawing temperature and the value thereof as to impart a permanent elongation to the wire and increase the creep resistance thereof without at the same time applying a tensile force sufficiently great to the wire as to effect failure of the wire under the applied tensile load, and cooling the wire after it has been heated to the tempering temperature and before it is coiled around the capstan or block for the purpose specified.

6. A method of increasing the resistance to creep under tension of carbon steel wire as herein defined for use in reinforced concrete construction, said method comprising subjecting plain carbon steel wire having a carbon con-- tent within the range of 0.7% to 0.8% to a cold drawing operation by advancing the. same through drawing means, applying tension to the drawn wire by means of a power rotated capstan or block around which the wire is coiled to effect its advancement through the drawing means, heating the wire after its emergence from the drawing means and while subjected to said drawing tension to a tempering temperature within the range of 350 to 400 C., and maintaining the drawing tension at such value as having regard to the time during which each wire length is maintained at the drawing temperature and the value thereof as to impart a permanent elongation to the wire and increase the creep resistance thereof without at the same time applying a tensile force sufliciently great to the 'Wire as to efiect failure of the wire under the applied tensile load, and cooling the wire after it has been heated to the 1% tempering temperature and before it is coiled around the capstan or block for the purpose specified.

7.'A method of making straightened, stress-relieved, cold drawn steel wire having its'relaxation properties improved which comprises subjecting cold drawn carbon steel wire having a carbon content of about 0.4% to about 0.9% to sufiicient tension to effect a uniform permanent elongation of the wire (of about 1 to 5%) While the wire is heated to a temperature between 150 and 500 C., and cooling the wire to room temperature in the straightened condition.

8. A flexible tension member suitable for use in prestressed concrete structures which comprises cold drawn plain carbon steel wire. having a carbon content within the range of 0.4% to 0.9% and having its relaxation properties improved by heating the member subsequent to the cold drawing and under sufiicient tension to impart without rupture a permanent elongation of not more than about 5% to the member, to a temperature within the range of 150 to 500 C. such that the member is characterized by having a tensile strength of from to more than long tons per square inch, depending upon the carbon content and the temperature of heating, and a stress relaxation of less than 3% when subjected at room temperature to aninitial load of at least 70% of its tensile strength and thereafter maintained at a constant length for 1000 hours.

References Cited by the Examiner UNITED STATES PATENTS 2,281,132 4/42 Young 148-- 12 2,589,881 3/52 Sims et al 14812 2,767,836 10/56 Nachtman 148-12 2,816,052 12/57 Hoff et al 14812 OTHER REFERENCES DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, ROGER L. CAMPBELL,

Examiners.

Claims (1)

1. A METHOD OF INCREASING THE RESISTANCE TO CREEP UNDER TENSION OF WIRE FOR USE IN REINFORCED CONCRETE CONSTRUCTION, SAID METHOD COMPRISING SUBJECTING PLAIN CARBON STEEL WIRE HAVING A CARBON CONTENT WITHIN THE RANGE OF .35% TO .9% TO A COLD DRAWING OPERATION BY ADVANCING THE SAME THROUGH DRAWING MEANS, APPLYING TENSION TO THE DRAWN WIRE TO EFFECT ITS ADVANCEMENT THROUGH THE DRAWING MEANS, HEATING THE WIRE AFTER ITS EMERGENCE FROM THE DRAWING MEANS AND WHILE SUBJECTED TO SAID DRAWING TENSION TO A TEMPERING TEMPERATURE WITHIN THE RANGE OF 220* TO 500* C. AND MAINTAINING THE DRAWING TENSION AT SUCH VALUE AS HAVING REGARD TO THE TIME DURING WHICH EACH WIRE LENGTH IS MAINTAINED AT THE TEMPERING TEMPERATURE AND THE VALUE THEREOF AS TO IMPART A PERMANENT ELONGATION TO THE WIRE OF NOT MORE THAN ABOUT 5% AND INCREASE THE CREEP RESISTANCE THEREOF WITHOUT AT THE SAME TIME APPLYING A TENSILE FORCE SUFFICIENTLY GREAT TO THE WIRE AS TO EFFECT FAILURE OF THE WIRE UNDER THE APPLIED TENSILE LOAD.

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