US2138614A - Sheathing electric cables - Google Patents

Description

Patented Nov. 29, 1938 UNITED STATES PATENT OFFICE 2,138,614 SHEATBING ELECTRIC CABLES New Jersey No Drawing. Application November 10, 1937 Serial No. 173,854

5 Claims.

This invention is directed to the sheathing of cables with lead and tothe sheathing of cables with lead alloys, such as lead-tin alloys, for example.

In the sheathing of electric cables heretofore trouble has been experienced with drossing and oxidation of the lead and while many attempts have been made to overcome these difficulties it has been impossible with the methods so far employed entirely to eliminate flow lines, lamina.- tions, pitted holes, spongy lead, etc., in the finished sheath.

Broadly speaking, in the sheathing of electric cables commercial lead is melted in a melting .pot, the molten lead being fed by charges to the cylinder of a lead press from which it is extruded by a suitable ram through a die about the insulated conductor in the form of a sheath. It is customary to extrude the lead in charges, that is to say, after one charge has been extruded from the press, the press ram is withdrawn, the cylinder press refilled with a fresh charge, and the second charge extruded, these operations being repeated until the desired length of cable has been sheathed.

At the end of each operation of the press a remnant of the old charge remains in the press, and on the next cycle of operation the incoming new charge must be welded to or united with the remnant of the old charge. This has been found exceedingly difficult because in the interim between the end of the extrusion stroke of the press ram and the refilling of the press cylinder and the beginning of the next extrusion stroke of the ram, the old charge cools. The result of all this is that along the length of the sheath where any two charges have been joined the sheath may be weakened.

Not only have the cable manufacturers experienced trouble in this direction, but they have also experienced trouble with flow lines, pitted holes and spongy lead, as above pointed out. Various methods have been tried. in an endeavor to overcome all of these difiiculties.

It has been suggested, for example, that the presence of oxides, due to oxidation of the metal while in the melting pot and the further oxidation which takes place when the metal is run into the press cylinder, is largely responsible for some of these troubles and one method which has been tried in an attempt to eliminate the oxides is the use of a reducing flame of hydrogen so disposed that the molten lead as it passes into the lead press charging cylinder from the melting pot is subjected to the action of this flame, with the idea that as the molten lead passes through the flame the oxides already present and those which are likely to form during the passage of the molten lead from the melting pot to the press cylinder will be reduced. This method, however, 6 is not successful owing to the speed at which the molten lead necessarily flows into the press.

Another method which has been tried is to maintain the lead press under a vacuum but while this method is efiective in preventing oxidation in the press it cannot reduce the oxides and other deleterious impurities present in the commercial lead as received from the refinery and the oxides which form in the melting pot and are to some extent in solution in the molten lead therein.

Another method which has been used employs an inert gas. The lead press cylinder is thoroughly washed with this gas before the charge of molten metal enters the cylinder and the gas 20 is allowed to flow into the press cylinder as the molten lead is being admitted thereto. This gas, obviously, cannot remove the oxides in solution already present in the'molten metal, and while it may inhibit further oxidation in the press cylinder, it causes foaming of the molten metal and the occlusion of some of the inert gas due to the rapid cooling effect of the gas on the mass of molten metal. This prevents obtaining a perfect weld between the old charge in the press and 30 the incoming new charge, thereby producing a laminated structure in the finished sheath, as above pointed out, and in addition blistering and sponginess will result from the presence of the occluded gas in the metal of the finished sheath. 35

I have discovered that remarkable results are obtained so far as elimination of the difficulties, above pointed out, heretofore experienced in the production of good cable sheaths are concerned, if the commercial lead, that is, lead as it is received from the refinery, is first pretreated with a small quantity of sodium before it is extruded into a sheath.

Sodium has a strong aflinity for oxygen, and, apparently, has the ability to rid the lead not only of the oxides present but of other deleterious impurities as well, such as sulfides, for example, and the ability to stabilize the lead so that the lead will not reoxidize, so that a sheath composed of lead so treated is remarkably free of deleterious impurities, has a uniform crystal structure and is free from inclusions, especially at the boundaries between crystals where the usual intercrystalline segregation and corrosion take place. I find also that I have uniform pliability through- 6.-

out a complete charge length of a material produced in accordance with my invention. It will be appreciated, of course, that by bringing about a better bonding between the crystals oi! the metal a uniform crystal structure with greater stability and less susceptibility to corrosion'is produced, all of which are very advantageous features with respect to lead sheaths for cables.

I have found also that the usual sluggishness of the lead at the pouring temperature is overcome by the marked increase in the fluidity of the lead when treated in accordance with my process and that the crystals of the lead cohere more firmly owing to the greater purity and plasticity of the cementing medium between the crystals.

In practicing my invention the sodium is employed in an insufficient amount to form an alloy with the lead. This is important in that it is my intention to produce a lead which is in a pure state, free of all oxides and other deleterious impurities, the presence of which, as above pointed out, as well as the presence of a lead-sodium alloy which would be present were sodium used in excess of that needed for freeing the lead of oxides and other deleterious impurities being detrimental to the production of the desired product.

In the commercial practice of my invention the sodium is preferably added to the molten com mercial lead as a sodium lead alloy, as I have found that in this manner the danger of too violent reaction is avoided. Upon the addition of the sodium lead alloy to the molten metal the alloy will be broken down into its component parts by the molten lead, the sodium thus released reducing any oxides present in the lead, combining with any oxygen present and decomposing such deleterious materials as sulfides which aregeneral- 1y present in commercial lead. The oxides and the deleterious impurities thus released float to the top of the molten mass from which they are readily skimmed off.

I have found that not only is a sheath composed of lead thus treated far superior to prior sheaths, all of which will be brought out hereinafter, but that it is unnecessary immediately to extrude the lead so treated, no drossing taking place even when the lead is allowed to stand for several days in the melting pot and the lead retaining its original silvery appearance.

I am aware that it has been suggested heretofore in the refining of impure leads, as distinguished from the treatment of commercial lead as received from the refinery-the material I am working with-to ad d alkali metals such as sodium to the impure lead for reducing the amount of tin, antimony, arsenic, etc., present.

These prior processes, however, are for a different purpose than the process of the present invention and in all cases the amount of added sodium is far in excess of that employed by me and sufficiently in excess to form an alloy with the lead and render the processes unfit for my purpose.

The use of sodium has been suggested heretofore in the making of lead-antimony-sodium alloys. Insuch processes the antimony is fused, lead added and when the mixture is fused metallic sodium is added. As an alternative the antimony and metallic sodium are first alloyed, the lead fused in a separate crucible and then antimony and sodium are poured into the lead crucible, the properties of the fiinished alloy depending upon the amount of alloying materials added. Such a method, however, is unsuitable for my purpose, in that I deliberately avoid any alloying of the sodium with the lead, and when the finished product is to be a lead alloy such as lead tin, for example, the alloying metal, that is to say, the tin must be added to the commercial lead after the lead has first been treated with sodium lead alloy.

I am'aware also that it has been suggested, prior to my invention, to produce an alloy for deoxidizing and coating iron plates wherein metallic sodium is employed. Such a process, of course, is foreign to the cable sheathing art,

but is mentioned because it involves the use of sodium. That process diflers from the present invention in that the lead, tin and zinc and sodium are all fused together, whereas in my process, as above stated, the commercial lead is first treated with sodium lead alloy and the alloying metal then added.

The addition of lithium in small amounts to lead for cable sheaths has been suggested, but I have found that while lithium and sodium are in the same periodic group they have an entirely different eiTect upon the lead. I have found, for example, that lithium, even when used in amounts restricted to the same limits as specified herein for sodium, forms an alloy with the lead, the characteristics of which are entirely different from that of commercial lead; the product resulting from the addition of lithium to commercial lead forms an alloy which is difilcult to extrude, is not of uniform composition and age hardens.

In the practice of my invention when treating commercial lead preparatory to making a sheath of lead as distinguished from a lead alloy sheath, sodium in the form of a sodium lead alloy is added to the commercial lead in the melting pot. The amount of sodium added is critical, ranging from 0.005% to 0.05% by weight of the lead being processed. Upon the addition of the sodium lead alloy to the molten lead the alloy will be broken down into its component parts by the molten lead, which is at a temperature of around 800 F., the sodium thus released reducing any oxides present in the lead, combining with any oxygen present and decomposing such deleterious materials as sulfides which are generally present in commercial-lead. The oxides and the deleterious impurities thus released float to the top of the molten mass from which they are readily skimmed off.

A trace of uncombined sodium remains in the lead. I have found this trace, approximately 0.001%, apparently has a stabilizing efiect upon the lead in that the treated lead may be allowed to cool and then be remelted without the necessity of again treating with sodium, the remelted lead having the same physical characteristics as the lead resulting from the first treatment. After this treatment the lead is fed to the cylinder of the lead press and extruded in the usual way.

When the sheath is to be a lead alloy, such for example as a lead-tin alloy, the same sodium range is employed as before which is insufficient to produce an alloying eflect with the lead or with the tin, but it is essential that the commercial lead first be treated with the sodium lead alloy, as above described, after which the alloying metal-tin-is added.

I have found that the addition of the sodium overcomes the natural sluggishness of the lead and lead alloys at the pouring temperature, which is of distinct advantage, as will be readily appreciated by those skilled in this art. I find also that a sheath made from my material is more elastic, has greater fatigue limit, is tougher and its life materially prolonged as compared with sheaths made by prior methods. I find too that my improved sheath is slower in deterioration and its creep characteristics are less as compared with sheaths made by prior methods, and that it is capable of standing much higher internal pressures and over longer periods of time. The crystal structure of the sheath too seems to be ideal in that there is a perfect bonding between the crystals of the metal, so that the crystal structure is uniform, stable and highly resistant to corrosion. I find also on examination of sheaths produced by my process and subjected to long time pressure tests in which internal pressures have been built up deliberately until the sheath ruptures that the point of failure will always be a knife edge break, which is an accepted indication of freedom from laminations and other forms of imperfections.

This application is a continuation-in-part of my copending application Serial No. 115,589, filed December 12, 1936.

It is to be understood that the procedure above described may be varied within the purview of my invention.

What I claim is:-

I. In the sheathing of electric cables with a lead alloy, the method which comprises introducing from 0.005% to 0.05% by weight of sodium into commercial lead while the latter is at a temperature of around 800 F. thereby to stabilize the same, and thereafter adding the alloying material to the stabilized lead.

2. In the sheathing of electric cables with a lead alloy, the method which comprises adding a sodium lead alloy to commercial lead which is at a temperature of around 800 F., the sodium added varying from 0.005% to 0.05% of the total Weight of the commercial lead and the lead of the added sodium lead alloy, thereby to stabilize the lead, and thereafter adding the alloying material to the stabilized lead.

3. The process of producing a stable lead for sheathing electric cables, which process comprises adding sodium to commercial lead in an amount varying from 0.005% to 0.05% by weight of the lead while the lead is at a temperature at which the sodium combines with any oxygen present in the lead, but below the red heat temperature of the lead, whereby approximately 0.001% sodium by weight is retained in the lead.

4. The process of producing stable lead for sheathing electric cables, which process comprises adding sodium to commercial lead in an amount varying from 0.005% to 0.05% by weight of the lead while the lead is at a temperature of around 800 F., allowing the mass thus produced to cool and then remelting to extrusion temperature but below the red heat temperature of the lead whereby approximately 0.001% sodium by weight is retained in the mass.

5. An electric cable sheath comprising lead stabilized with approximately 0.001% sodium.

' BERNARD B. REINITZ.