US2237244A - Method of making corrosion resistant metal tubes - Google Patents

Description

Patentecl Apr. 1, 1941 METHOD OF MAKING CORROSION RESIST ANT lVIETAL TUBES Richard A. Wilkins, Rome, N. Y., assignor to Revere Copper and Brass Incorporated, Rome, N. Y., a corporation of Maryland No Drawing. Application January 26, 1940,

Serial No. 315,861

- 4 Claims.

My invention relates to methods of making corrosion resistant metal tubes.

The invention is particularly concerned with making corrosion resistant tubes of phosphorbronze alloy which is so compounded, worked and heat treated that the metal of thetubes has improved physical properties and a greater resistance to chemical corrosion than the metal of the original cast alloy, and, what is of particular importance, especially with thin walled tubes within the range of commercial sizes, a greater resistance to stress corrosion than the metal of the origin-a1 casting.

Herctofore copper has been alloyed with tin to harden and toughen the copper. In connection with such alloys it has been common to add small amounts of phosphorus to the melt for deoxidizing it, the bulk of the phophorus however burning ofi, or going ofi in the slag, so that in the cast alloys only such minute fractions of a percent of phosphorus remain as exert no appreciable effect on the chemical properties of the alloys. It also has been proposed to add higher amounts of phosphorus to tin-bronzes for improving the' free flowing and casting qualities of the metal. In these latter castings it has been found, that although the phosphorus therein in itself tends to increase the corrosion resistance of the alloy, that resistance as a matter of fact is not increased.

Applicant has found that the above mentioned castings having relatively high amounts of phosphorus, even when consisting wholly of a mixture of copper, tin and phosphorus, contain one or more brittle secondary mixtures, which latter, existing as crystals or other segregates of difierent chemical composition from the mass of the metal of the casting, reduce the resistance of the cast metal to corrosion, particularly when subjected to stress.

Applicant has found that these crystals or other segregates, which for the most part apparently are a brittle compound represented by the formula CllsSna and brittle compounds having a higher phosphorus content than other portions of the metal, form with the remaining tougher metal of the casting galvanic couples, which couples at the surface of'the casting, when said surface is exposed to the action of corrosive media, form fool for initiation of corrosion of the metal, resulting, when the metal is subjected to vibration, bending, shock or other strains, in the corrosion working into the metal along the boundaries of these crystals or segregates. This defect is so pronounced under such conditions that the alloy is markedly subject to pitting, and to breakage or other failure, when subjected to a combination of corrosive action and stress, particularly the stresses occasioned by the repeated bendingor other fiexure to which a tube is commonly subjected when in use.

Applicant, however, has found, that by properly proportioning the tin and phosphorusto each other and to the copper, the cast metal may be worked-by operations and treatments hereinafter described, into the form of tubes in which these crystals or other segregates of different chemical composition from the rest of the metal are eliminated by dissolving them into the rest of the metal to form a single solid solution of copper, tin and phosphorus. By having all the metal in the form of such a solution, the above mentioned galvanic couple effects are entirely eliminated despite the metal, largely considered, being an aggregation of crystals, for when the deleterious crystals or segregates are eliminated all the crystals are of the same chemical composition and each is of uniform chemical composition. In this way,full advantage may be taken of the action of the phosphorus in increasing the resistance to corrosion, and a product obtained having a resistance to corrosion markedly greater than that of the cast ternary alloy.

This increase in the corrosion resistance obtained by eliminating the above mentioned deleterious segregates by dissolving them into the rest of the metal is particularly marked because, at the same time, the resistance of the tube to any stress to which it may be subjected also is increased by the working of the metal, which working acts to decrease the size of the grains or crystals of the metal and intimately interlock them. On account of its increase in resistance to stress and to chemical corrosion per sethe improved tube is particularly useful in situations where it is subjected to so-called stress corrosion, that is to say, to stress and the action of a corrosive medium under conditions such that the stress tends to cause the material mechanically to open up minutely and permit the corrosive medium gradually to eat into it.

The metal of the improved tubes according to the invention contains approximately 5 to 10% tin, 0.1 to 0.5% phosphorus, with the balance.

substantially all copper. The addition of small amounts of other metals such as vanadium, lithium, calcium, boron, manganese and other similar scavenging and deoxidizing agents however, is not precluded so long as they do not materially eliminate the characteristic corrosion resisting properties imparted by the combination of copper, tin and phosphorus. I

The three constituents of' the basic ternary alloy maybe melted and mixed according to usual metallurgical practice, the phosphorus being introduced preferably in the form of phosphor-copper or phosphor-tin. To prevent buming' off of the phosphorus, or it going off with the slag, the molten metal, while being handled, is preferably covered by a molten layer of neutral flux such as a mixture consisting of 50% each of fused borax and diatomaceous earth. In pouring the metal into the mold preferably the metal is poured in a known way from beneath this layer of flux so as to prevent the flux entering the mold.

Generally speaking, the present process consists in forming a casting of the above mentioned composition and hot extruding it materially to reduce its cross-sectional area and form a tube and then alternately cold drawing this tube to elongate it without reducing its volume,

- and heat treating it. The hot extrusion, which it has been found causes solution of a material part of the deleterioussegregates, so conditions the metal that substantially complete solution of all these segregates may be effected with ordinarily no more than two cold drawing opera-.

tions. Although applicant has found it is DOS-1 sible to cause this solution of the segregates wholly by alternate cold drawing and heat treatment, the present process effects marked economic'savings over such operations in that it permits the solution to be effected with a minimum of working of the metal, and in many cases with less total reduction of the casting, all of which will be appreciated from the fact applicant has found that when alternate cold drawing and heat treatment alone are employed a total of at least six cold reductions each of the same amount as with the present method, ordinarily will be necessary, which in some cases is economically prohibitive and mechanically impossible.

In making the tubes the metal mixture conveniently, but without limitation thereto, is cast in the form of a cylindrical billet, for example, one about 6 to 9 inches in diameter and about 70 inches long. This billet may then be sawed into shorter lengths, for example about 12 inches, adapted for use in the extrusion press, the end portions of the original billet preferably .being discarded, A short billet, so formed, may then be heated to about 1150 to 125 0 F. (about 620 to 680 C.), and while at that temperature, all but the outer surface portion and the stub end of the billet may be expressed through the die opening of the press, the shape of this opening being such as to cause the extruded metal to be in the form of a tube. At this'temperature approximately maximum plasticity of the billet is secured, and most of the undesired'segregates are at about their melting point or just under it, there being some tendency of the segregate considered as a whole to melt. The mass of the metal, being a solid solution of copper,.,tin and phosphorus of the composition represented by the proportions of these elements added to the melt, is well below its melting point although in a highly pla'stic condition.

. To secure effective results the size of the annular die opening through which the metal is expressed ordinarily should be such as to form a tube of such cross-section that it represents ,are-

duction of about 60 to 92% in the cross-sectional area of the billet. For example,- a billet '7 inches in diameter may be reduced in this way to form a cylindrical tube of about 1 inch inside diameter and 2.5 inches outside diameter, representing a reduction of about 89% in the cross-sectional area of a 7 inch diameter billet when all but a surface layer about 0.1 of an inch thick is expressed through the die. A reduction of 60 to 90% will ordinarily result in solution of about 60 to 70% of the undesired segregates, apparently caused by the flowing of the metal through the die with the segregates at a temperature at or slightly below their melting point. Complete solution, however, it has been found cannot be effected by any increase in reduction caused by reducing the size of the die' opening within the limits possible to be employed.

The outer portions of the cast billet ordinarily are imperfect and liable to contain impurities, and thus by not expressing those portions through the die these impurities are eliminated from the extruded tube. Although it is possible to remove these imperfections and impurities by machining the billet prior to extruding it, such operation because of its cost ordinarily would be economically prohibitive. I

The tube formed by extrusion is then, according to the improved method, drawn while cold by pulling'it longitudinally through an annular die opening to elongate it by reducing its wall thickness and .both its inside and outside diam-. eters without decreasing its volume. The particles of deleterious segregates present, which have been decreased in size by reason of the dissolving action on them during the extrusion process,'-are brittle at room temperature'and thereabout, and the mechanical action of the walls of the annular die opening acts to crush and divide them, while elongating the metal acts to separate or difiuse the divided particles. This breaking up and diffusing of the brittle segregates permits their further solution as a result of subsequent heat treatment to which the tube is subjected before further cold drawing.

The cold drawn tube may be heated, preferably in an annealing furnace. The nature of the metal is such that it has been found it can be solved and the tube will be annealed to condition it for further cold drawing.

By repeating the cold drawing and heat treating operations on the tube the segregates can be practically all dissolved, provided the tube is sufliciently reduced in cross-section as a result of each cold drawing operation. Ordinarily, if

- the cross-section is reduced about 20 to 25% of each cold drawing, only two such reductions, each followed by a heat treating operation, will be necessary to secure complete solution. In no case that has come to applicant's attention will.

1.8 inches outside diameter,-and from 1 to say about 0.75 inch inside diameter, if each of the two cold drawings reduces the cross-sectional area of the tube about 25% by reducing both its inside and outside diameters and wall thickness.

If desired, the tube, after the segregates are r dissolved, can be readily further reduced by alternate cold drawing and annealing in the usual manner, but under such conditions much greater reductions in cross-section may be made between anneals, say in the order of 70%, because of the absence of segregates which when present act to make the metal brittle; In practice satisfactory tubes with wall thicknesses of about 0.02 of an "inch have been made.

. sion press the cross-section of the metal could be reduced, if desired, .to the maximum extent that can be secured without fracture of the metal, so

asto secure a maximum breaking up or division of the segregates and their diffusion, it being understood that cold drawing causes embrittlemen-t of the bulk of the metal and will cause it to fracture if too great a reduction in cross-section is attempted without annealing it to condition it for further cold drawing. However, the commercial significance of increasing the reduction by cold drawing over about would be very slight, and no matter what the reduction at least two cold drawings in any ordinary case would be necessary to secure the desired results. Ordinarily such elongation as will reduce the thickness of the metal about 35 is the maximum that can be obtained with assurance that no fractures in the metal will result from the first cold drawing,'al-

though a slightly greater reduction, say about 40%, can be made as a result of the second cold drawing if the first reduces the metal about Smaller reductions in thickness could be employed, but they would necessitate a disproportionately unfavorable increase in the total number of cold drawing and heat treating operations,

by use of a plunger which is pushed entirely through the billet axially thereof toward the die opening which determines the outer tube diameter, the plunger acting to push the core out of thebillet through that opening and then remaining stationary in the billet with its projecting forward end in the opening so as to form with the latter an annular opening and enable the ram of the press to push the remaining portion of the billet, except its outer surface layer and its butt end, through the annular opening to form the tube.

The tube drawing operations are. preferably performed in a so-called plug bench, that is to say, one which pulls the tube through a die opening and over a normally stationary plug in that opening. so as to elongate the tube by reducing its internal and external diameters and wall thickness. It has been found that, type of drawing acts effectively to break up and difluse the brittle segregates.

The elimination of the segregate to a satisfactory degree can be determined by polishing and etching the surface of a specimen of the metal strip or tube in the usual manner and examining it at 75 diameters with a metallurgical microscope. Absence of the segregate when the metal is so examined indicates that it has been dissolved to a satisfactory degree even though traces of it may be observed at higher magnifications.

The tube as above described, it has been found, possesses the requisite toughness, tensile strength,

- and shock and abrasion resisting properties to and reductions of less than about 20% do not seem effectively to break up the segregates throughout the entire thickness of the tube wall unless the latter is rather thin, in which, latter case reductions as low as 10% may sometimes be made.

However, as above mentioned, it has been found, that after the segregates 'arerpractically all dissolved, the tube can be reduced by cold drawing, in any of the usual manners, as muchas 30% without fracture or it being necessary to anneal it to condition it for further cold drawing.

In respect to dissolving the segregates by heat treatment after each cold drawing. effective results will be obtained if the metal is-heated to about 550m 650 C. and maintained at that temperature from 60 to 90 minutes, the time depending upon the thickness of the metal, the greater the thickness the longer the time necmsary to secure maximum solution of the segregates. Longer times than 90 minutes do not seem to resuit in any increase in the amount of segregate dissolved. Shorter times than those specified could be employed in some cases; but would increase the total number of drawing operations a necessary, andin many instances would make it impossible to secure practical total solution of the segregate especially 11' small reductions in the cross-sectional area of the slab were caused by the drawing operations. I

Any suitable sort of extrusion press may 'be employed for forming the tube from the cast billet, for example, and preferably, one of the known typeswhich discards the central core of the billet enable it to stand up when subjected to combinations of stress and corrosion. The solution of the segregates enables full advantage to be taken of the phosphorus in resisting corrosion, while the mechanical properties Just mentioned act markedly to prevent any corrosion which may occur from working into the metal at its grain boundaries or along any minute cracks or surface scores that may exist therein, for example. those occurring as the result of abrasion. In all these respects the metal of the tube is markedly better than that of the casting, and may be machined with greater ease and less wear on the tools. Further, the tube may be cold worked by bending, pressing, etc., to form various machine parts and other articles, which cannot be done with the metal of the casting because the segregates render it brittle and refractory.

ment and facilities commonlypossessed by non-- ferrous mills, and that any 01 the above men-' tioned reductions made by drawing the tube need not be made in one operation, for example, if the equipment employed demandsit, each reduction may be made, for instance, in two drawing steps, and the material in such case need not be heat treated until after each second step.

It will be understood that, within the scope of the appended claims. wide deviations may be made from theforms of the invention above describedwithout departing from the spirit-of the invention,

I claim: 1. The method of making a corrosionresistant phosphor-bronze tube from a cast metal containing 5 to 10% tin, 0.1 to 0.5% phosphorus, balance substantially copper, and having secondary mixtures of two or more of these constituents existing as segregates in the metal, which comprises die expressing the cast metal at elevated temperature through an opening of such size and shape as will materially reduce its cross-sectional area and form a tube, subjecting the tube to an elongating cold drawing operation efiective materially to reduce its wall cross-sectional area without fracturing the metal, heating the cold drawn tube to about 550 to 650 C. and maintaining it at that temperature for a substantial period of time, and repeating the elongating cold drawing and heat treating operations specified until all.

the copper, tin and phosphorus in the metal exist substantially as a single solid solution.

2. The method of making a corrosion resistant phosphor-bronze tube from a cast metal containing 5 to tin, 0.1 to 0.5% phosphorus; balance substantially copper, and having secondary mixtures of two or more of these constituents exist- 3. The method of making a corrosion resistant phosphor-bronze tube from a cast metal containing 5 to 10% tin, 0.1 to 0.5% phosphorus, baling as segregates in the metal, which comprises die expressing the castmetal at elevated temperature through an opening of such size and shape as will reduce its cross-sectional area about 60 to 92% and form a tube, subjecting the tube to an elongating cold drawing operation eflective materially to reduce its wall cross-sectional area without fracturing the metal, heating the cold drawn tube to about 550 to 050 C. and maintaining it at that temperature for a substantial period of time, and repeating the elongating cold drawing and heat treating operations specified until all the copper, tin and phosphorus in the metal exist substantially as a single solid solution,

no reduction by cold drawing being less than a about 10% or more than about ance substantially copper, and having secondary perature for about to minutes, and repeating the elongating cold drawing and heat treating operations specified until all'the copper, tin and phosphorus in the metal exist substantially as a single solid solution as determined by absence of segregates when a polished and etched surface of the tube is microscopically examined at about "I5 diameters magnification.

' .4. The method according to claim 2, comprising reducing the cast metal such percentage in die expressing the tube, and reducing the die expressed tube such percentage by each cold after thesecond heat treatment.

RIC ARD A. wnxms.

drawing, that the single solid solution will exist