Metal fibers obtained by bundled drawing

Abstract: Metal fibers obtained by the bundled drawing of a composite bundle of metal wires embedded in a metal matrix followed by the electrolytic removal of the matrix so that the surface layers of the fibers contain only a negligible amount (≦ 0.2 % at) of the matrix metal. Furthermore, a description is given of a process and an apparatus for the continuous electrolytic removal of the matrix material. Thereby, the composite bundle acts as an anode. Stainless steel fibers according to the invention have an average Cr/Cr+Fe+Ni content at their surface between 1 % and 15 %.

Description: The present invention relates to metal fibers obtained by the bundled drawing of wires embedded in a matrix which consists of a different metal than the fibers. After the drawing operation, the matrix material is removed, leaving a bare bundle of fibers. Specifically, the invention also comprises a process and an apparatus for the continuous electrolytic removal of said metal matrix, using the embedded bundle as an anode. U.S.A. patent 3.379.000 describes the manufacture of stainless steel fibers by bundled drawing, i.e. starting with a bundle of wires embedded in a metal matrix which differs from the wire metal, e.g. in copper sheaths. After the drawing, the copper is stripped in a nitric acid solution. The fibers which are obtained using this patented method still show some traces or remnants of the matrix material (copper) at their surfaces. To turn the stripping of the matrix metal in HNO₃ into an ecologically sound process, considerable sums must be spent on the neutralizing of the generated nitrogen oxide fumes and on converting the used stripping fluid into disposable waste. Apart from that, some remnants of the metal matrix are left on the fiber surface. Thus, this surface is somewhat contaminated and this can be a disadvantage in certain applications. With the present invention it is now possible to manufacture metal fibers, using the bundled drawing method as described above and yet avoid this contamination of the fiber surface. The average concentration of matrix metal in the surface layers of the fibers thus obtained is at the most 0.2 % at. The average copper content in the surface layers of standard metal fibers, obtained by applying the HNO₃ stripping process to a copper matrix, is more than 2 % at. The thickness of the surface layer under consideration is about 50 Å 5 nm. The metal fibers obtained by applying the present invention may be stainless steel fibers with a chromium content of at least 10 % by weight. Specifically, the fibers will contain at least 16 % Cr and also Ni. Furthermore, the invention can be used to manufacture refractory fibers containing Fe, Cr, Al and, optionally, Y or rare earths (as is described, for example, in U.S. patent 4.139.376) and fibers from Ni/Cr alloys, Hastelloy®, Inconel®,, titanium or Carpenter®20cb3. It is also an object of the invention to provide stainless steel fibers of the kind specified above and having a reduced average Chromium content (a lower Cr/Cr+Fe+Ni-ratio) at their surface, i.e. with a Cr/Cr+Fe+Ni-ratio between 1 % and 15 % wherein the Cr, Ni and Fe-contents are expressed in at %. Even if said fibers retain more than 0.2 % at of matrix metal at their surface, the lower Cr-content offers the advantage of a better corrosion resistance as will be explained further on. The invention also comprehends a process and apparatus for the continuous electrolytic removal of the matrix material from a drawn composite bundle. Thereby, the bundle serves as an anode and the embedded bundle is transported continuously through successive electrolytic baths at a temperature of over 20°C. A method is known from the published German patent application No. 2.812.576 and from the published British patent application No. 2.032.959 for continuous electrolytic descaling of steel wire by non contact current flow. The steel wire remains entirely submerged in the electrolyte when passing from anode to cathode and thus no transition zones are present interbetween wherein the current is forced to run only through the wire. The Luxembourg patent No. 43.371 deals with the electrolytic removal of a metallic coating from a metallic piece wherein the coated piece is branched as anode. Contrary to the process and apparatus in such conventional electrolytic stripping installations, transition cells are present between the electrolytic baths. During the process, as defined in claim 1, the bundle is supported at the level or in the vicinity of these transition cells. The arrangement and the distances between the various cells or baths are such that in the spaces between the electrolytic baths and the cathodic transition cells the current is conducted by the bundle. During the process, at least part of the matrix material is deposited on the cathodes facing the bundle in the electrolytic baths. All these measures contribute to the development of a more economical process with the additional advantage of a higher quality fiber product. The fibers are less damaged as will be shown further and some of their characteristics are more constant, i.e. display less variation than in case of standard bundled drawn fibers. These points will now be explained in more detail on the basis of an embodiment of the invention, illustrating the unexpected additional advantages. Figure 1 is a diagram of a processing installation for the continuous removal of the matrix material from the bundle. Figure 2 shows the composition profiles for quantities of Cr and Ni close to the surface of a stainless steel fiber for a bundle obtained by a standard method and for a bundle obtained by applying the present invention. For comparison, figure 3 illustrates the variation of the nitrogen content throughout the fiber thickness (close to the surface) of the same two types of fibers. A series of composite bundles 1, obtained by the usual process of bundled drawing, consisting of several thousands of metal fibers embedded in a body of copper and surrounded by an iron jacket, are transported continuously through an apparatus in accordance with the present invention and specifically through a series of electrolytic baths 2 and 4 for the removal of the metal matrix, i.e. the iron jacket and the body of copper. As schematically represented in figure 1, the iron jacket of bundle 1 is removed by dissolution in a first series of electrolytic baths 2. Subsequently, the bundles 1 pass through a rinsing apparatus 3 and the copper matrix is removed in a next series of electrolytic baths 4. During the process, the copper is recuperated at least in part and preferably in full by deposition on the cathodes 5. This prompt recuperation of metal is an important advantage compared to the earlier treatment with HNO₃. In accordance with the invention, cathodic transition cells 6 are placed between the successive baths 2 resp. 4, in which anodes 7 have been mounted - e.g. made of lead - facing the passing bundles 1. On the other hand, in baths 2 resp. 4, the cathode plates 8, resp. 5 have been placed at a distance of several centimeters from the path of the bundle. As a result, current carrying contact elements may be omitted. This was found to be an advantage because, among other things, the current transmission to the bundles by way of mechanical contact (e.g. via rolls) can become increasingly irregular as more matrix material disappears from the bundle. In general, current transmission by mechanical contact elements causes an additional tensile strain on the bundle as well. As the total processing apparatus can reach a considerable length (especially when aiming for a high and therefore productive processing speed) the bare bundle (as a result of the installation of contact rolls) would have to overcome yet an additional tensile strain at the exit. This would increase the chance of fiber or bundle fracture. The broken off pieces of the fibers might then wind themselves around the contact rolls which could impede the regular transmission of current even more and which could damage the bundle. In order to minimize current leakage at the transitions between baths and cells, and hence to minimize energy consumption, the overflow sections 9 of successive baths and cells are placed at a sufficient distance from each other so that at least a major part of the electrical current is forced to flow through the bundle in these transition zones 10. Apart from that, this measure promotes the controllability of the electrolytic process. Preferably, the temperature of the electrolytes in the various baths and cells should be higher than room temperature (over 20°C) ; e.g. 50-60°C, in order to increase the efficiency of the matrix removal. In principle, quite some compositions are possible for the electrolytic bath, acidic as well as alkaline. For example, a bath containing sulfuric acid can be used in the section for removing iron (2) as well as in the section for removing copper (4). Obviously, if the metal matrix contains only copper, a copper removal section (9) will suffice. In this case, a suitable electrolyte might contain H₂SO₄ and CuSO₄. In the baths 2, lead cathodes 8 can be used. However, in the baths 4 it is preferable to use cathodes 5 made of a stronger material (metal) and with less adhesive affinity with respect to the matrix material which has to be deposited. This facilitates the mechanical removal of the layer of metal deposit from these cathodes 5. Naturally, the installation is equipped with pumps 11 and pipes 12 for the circulation of the fluids from the various collectors 14 to the baths 2, 3, 4 and cells 6 and to the respective overflow sections 9. At regular distances in the installation, the bundles are supportd by e.g. ceramic cross-bars or combs 13. Preferably, these wear resistant means of support 13 should be mounted at or near the place of the transition zones 10. It is advisable to use current stabilized rectifiers 15 for the current supply. Current densities between 5 and 75 A per dm² of bundle surface were found to be suitable for the iron removal baths. Preferably, the sulfuric acid concentration should be between 200 and 400 g/l. In order to realize an iron removal efficiency of more than 100 % in the baths 2, passivation of the iron jacket must be prevented. This can be achieved by using a relatively low current density (e.g. less than 30 A/dm²) in the first bath(s). It was also found that this high efficiency can be obtained by limiting the increase of the molar product of the iron ions with the sulfuric acid concentration in the electrolyte. A suitable value for the molar product is, for example, 2.5. The efficiency can become higher than 100 % because, apart from the electrolytic dissolution process of the iron jacket, a simultaneous chemical iron dissolution process occurs as well. In order to keep the local current density variations within acceptable limits in the electrolytic baths 2 or 4, it turned out to be advisable to select a bath length in the transport direction of the bundle of less than 75 cm. A practically uniform current density distribution in the baths has the advantage of permitting a higher total current without negative effect on the efficiency. Naturally, the cathodic transition cells can be much shorter. Furthermore, it was found advantageous from the viewpoint of the lowest possible energy consumption and the realization of a uniform current density distribution, to install successive power supply circuits for successive series of baths and to separate these from each other. This separation could be effected, for example, at the level of the cathodic transition cells 6 which are situated between one series of baths and the next. A series of baths may consist of one or more baths. In order to dissolve as little copper as possible in the last electrolytic bath 2, the current here (A/dm²) will have to remain relatively low. The copper removal baths may have the same composition as the usual copper sulfate/sulfuric acid baths for the electrolytic deposition of copper. Furthermore, the average current densities, normally used in this kind of electrolysis (direct current or pulsating current) were found to be suitable for the invention. In the discontinuous process for electrolytic removal of the matrix metal from the composite bundle, again the composite acts as an anode. Therefor, the bundle is stored on a metallic supporting frame which is anodically polarized. A convenient frame is e.g. a spool of steel wire equal or similar to that disclosed in U.K. patent No. 1.502.924 onto the core of which the composite bundle is wound in a substantially cylindrical layer. The layer thickness is preferably small in view of permitting a sufficient penetration for the electrolyte which has to dissolve the matrix material during the electrolysis process. The frame with the bundle stored on it is submerged in a bath containing as electrolyte a solution of H₂SO₄ above room temperature. In view of accelerating the dissolution process the electrolyte is either continuously stirred or circulated by means of a pump so as to force on a continuous basis a fresh solution interbetween the neighbouring windings in the cylindrical layer. Metal plates are suitably arranged in the bath as cathodes thereby facing the outside and/or inside of the cylindrical layer. The plate design and their disposition is of course choosen to avoid a substantial obstruction of the fluid flow through the bath. The electrical current to the electrodes is supplied by a voltage stabilized rectifier. The voltage is set at a value below 2,5 V. A suitable maximum current is e.g. 20 A per kg of composite to be treated. In this way, the matrix material is completely removed after a run of several hours with an electrolyte at a temperature of almost 50°C. Example A composite bundle of stainless steel fibers with a fiber diameter of 12 µm of the type AISI-316L, embedded in copper and surrounded by an iron jacket, was treated in the apparatus and according to the continuous process described above. The various values of the current densities, bath lengths, bath concentrations, temperatures etc. were kept within the above mentioned limits. The resulting fiber bundle, and in particular the composition of its surface layer, was compared to the same bundle 316L which had been stripped in HNO₃ in the standard manner. The average tensile strength of the fiber obtained by applying the invention was 8.85 % higher than that of the standard stripped fibers, while the variation in the value of the tensile strength over its length was considerably smaller. This is presumably due to the fact that the nitric acid affects the very thin fibers in a more aggressive, irregular and penetrating way than a well regulated electrolytic process. The results of an analysis of the composition of the surface layer of both types of fibers (Scanning Auger Multiprobe) have been summarized in table 1. The percentages are averages. surface layer (0.75 µm) N % at Cr/Ni % Cr/Cr + Fe + Ni % Cu at % fiber obtained with invention 1 70 7 0 fiber obtained by standard technique 3 220 22 2.3 Figure 2 shows the variation of the Cr/Cr+Fe+Ni-content throughout the fiber thickness for both types of fibers. Curve 17 applies to the fiber bundle which was stripped in HNO₃ while curve 16 applies to the fiber bundle which was treated in accordance with the invention. When HNO₃ is used, the Ni at the fiber surface will be depleted faster than the Cr, while the application of H₂SO₄ has the opposite effect. Therefore, the ratios as shown in figure 2 and table 1 confirm the expected composition changes for both removal processes. It was even established that to strip composite bundles with copper matrixes and fibers from Fe/Cr alloys (possibly with a very low Ni-content) such as AISI-430 types, in HNO₃ is quite difficult. A possible explanation could be the (almost complete) lack of Ni at the fiber surface. However, with the electrolytic stripping process in H₂SO₄/CuSO₄-baths in accordance with the invention, the copper between these fibers can be removed much faster, probably because of the presence, and thus the depletion possibility, of Cr (16-18 % by weight). This means that the present invention permits specifically the manufacture of stainless steel fibers made from alloys which contain Ni and at least 16 % Cr by weight whereby the average Cr/Cr+Fe+Ni ratio in the surface layers of these fibers ranges between 1 % and 15 % and wherein the Cr, Ni and Fe contents are expressed in at %. Preferably, this ratio should be less than 10 %. Moreover the average value of the Cr/Ni ratio in the surface layer should be less than 80 %. The above mentioned average ratio for Cr/Cr+Fe+Ni of less than 10 % as well as said accompanying average ratio for Cr/Ni of less than 80 % is also achievable when the fibers retain more than 0.2 % at of matrix material in their surface layer. In analogy the chromium at the surface of FeCrAl-fibers will be depleted (will decrease) more according to the process of the invention than when stripping the bundle in HNO₃. This means that the FeCrAl-fibers according to the invention have a lower average Cr-content at their surface than conventional FeCrAl-fibers. Similarly in relatively Ni-rich alloy fibers as Hastelloy®- and Inconel®-fibers stripped according to the invention, the Ni-content at their surface will rise somewhat on the average compared to the same fibers stripped in HNO₃. It is immediately apparent from the table that, unlike the fibers which were stripped by a standard method, the fiber which was treated in accordance with the invention, no longer shows any detectable quantities (0 %) of copper at its surface. Furthermore, the nitrogen content in the surface layer of the fiber treated in accordance with the invention, is considerably lower than in case of the standard treatment. Curve 18 in figure 3 shows the variation of the nitrogen content in at % from the fiber surface (0 nm) to a depth of 30 nm for a fiber treated in accordance with the invention. Curve 19 represents the nitrogen variation for the fiber which was treated with HNO₃. It is remarkable that, as shown in figure 3, the relatively higher nitrogen content in case of standard treatment (curve 19) is also maintained a little further (deeper) below the fiber surface. This could suggest the higher aggressiveness of HNO₃ in comparison to the electrolytic stripping in an H₂SO₄ environment. After all, it was found that the value and variation of the sulfur content at the fiber surface as well as deeper into the fiber was comparable for both types of fibers. If it had been found that a fiber treated in accordance with the invention displayed higher sulphur contents than a fiber treated in the standard way (in HNO₃), we would have to decide on an aggressive attack by H₂SO₄ as well. However, the test results show that this is not the case. Apparently, we can conclude that the electrolytic process in accordance with the invention offers a gentler, less aggressive treatment for very thin fibers. Therefore, it is a further characteristic of the bundled drawn metal fibers in accordance with the invention., that, on average, they display a lower nitrogen content in their surfaces than the fibers which have been stripped in HNO₃ in the standard way. Therefore, the metal fibers obtained by applying the invention, in particular the stainless steel fibers, will show, on average, a nitrogen content of at most 1.5 at % close to their surfaces. Finally, both types of fibers were subjected to a corrosion test (Strauss test ASTM standard A 262-86 part E). The weight loss, after remaining 72 hours in a boiling copper sulfate solution, was 23 % for the fiber treated in the standard way and only 15 % for the fiber treated in accordance with the invention. Ergo, the fibers treated in accordance with the invention show a higher resistance to corrosion as well.

Claims: A process for removing of a matrix material from a bundle of metal fibers embedded in it, said matrix comprising a metal which differs from the metal of the fibers and whereby, the metal matrix is removed by an electrolytic process and in which the embedded bundle acts as an anode characterized in that the embedded bundle (1) is continuously transported through successive electrolytic baths (2, 4) containing an electrolyte at a temperature of at least 20°C without making mechanical contact with current carrying contact elements and in which cathodic transition cells (6) are present between these baths and in which the current runs through the bundle (1) in transition zones (10) between these baths (2, 4) and cells (6) and in which at least part of the matrix material is deposited on cathodes (5) which are facing the bundle. A process according to claim 1, in which the metal matrix consists of copper and the electrolyte contains H₂SO₄ and CuSO₄. A process according to claim 1, in which the metal matrix consists of copper surrounded by a steel jacket and in which the steel layer is removed in H₂SO₄ baths in a first series of cells (2) while the Cu is removed in H₂SO₄/CuSO₄ containing baths in a next series of cells (4). A process according to claim 1, wherein the bundles are supported at the level or in the vicinity of the transition zones (10) where most of the current is conducted by the bundle (1). A process according to claim 1, wherein the current supply for the electrolysis is regulated with a stabilized current and with current densities between 5 and 75 A/dm² of bundle surface. A process according to claim 5 wherein the current is supplied by way of successive separated power supply circuits each of which serves one of the successive series of baths, and wherein a series of baths may contain one of more baths (2) resp. (4). An apparatus for carrying out the process according to any of the claims 1 up to and including 6, characterized in that it contains successive electrolytic baths (2, 4) provided with cathodes (8, 5) and transition cells (6) present between the baths (2, 4) provided with anodes (7), means (11, 12) for the recirculation of the bath fluids to overflow sections (9) and power supplies (15) for the electrical current to the anodes and cathodes which are separated per series of baths and also wear resistant means (13) at the level or in the vicinity of the transition zones (10) for supporting the bundle being processed. An apparatus according to claim 7, wherein the anodes (7) are made of lead. An apparatus according to claim 7, wherein the cathodes (5) have a small adhesive affinity with respect to the deposited matrix material. An apparatus according to claim 7, wherein all baths (2, 4) have lengths of not more than 75 cm. Metal fibers obtained by the process of claim 1, characterized in that the average concentration of matrix metal in a surface layer of the fibers with a thickness of 5 nm is at most 0.2 % at. Metal fibers according to claim 11, wherein they are stainless steel fibers with at least 10 % Cr by weight. Stainless steel fibers according to claim 12, wherein they contain Ni and at least 16 % Cr. Stainless steel fibers according to claim 13, wherein the average Cr/Cr+Fe+Ni ratio in their surface layers is between 1 % and 15 %, wherein the Cr, Ni and Fe contents are expressed in at %. Fibers according to claim 14, wherein the average Cr/Cr+Fe+Ni ratio is less than 10 %. Stainless steel fibers according to claims 14 or 15, wherein the average Cr/Ni ratio in the surface layers is less than 80 %. Metal fibers according to claim 11, wherein they are refractory fibers containing Fe, Cr, Al, and optionally rare earths or Y. Metal fibers according to claim 11, wherein their surfaces contain an average nitrogen content of at most 1.5 at %. Stainless steel fibers obtained by the process of claim 1, characterized in that the average Cr/Cr+Fe+Ni ratio in the surface layers of the fibers is between 1 % and 15 %, wherein the Cr, Ni and Fe contents are expressed in at %. Stainless steel fibers according to claim 19 wherein the average Cr/Cr+Fe+Ni ratio is less than 10 %. Stainless steel fibers according to claims 19 or 20 wherein the average Cr/Ni ratio in the surface layers is less than 80 %.

Inventors: FRANCOIS ROGER, FRANCOIS, ROGER, François, Roger
Applicants: BEKAERT SA NV, N.V. BEKAERT S.A.
Application year: 1989
Main classification: C25F5
IPC codes: B21C37, C22C38, C25F5, B22F1, D01F9, C25F7