US3140108A - Process and product of metallurgically joining zirconium to ferrous metal - Google Patents

Description

J. L. KLEIN ETAL R CT OF METALLURGICALLY JOINI IRC UM TO FERROUS METAL July 7. 1964 y PROCES 4 sheets-sheet `2 Original Filed May 25 S AND P mm vm INVENTOR` Loewe/Islam, /f/e/o BY Kauf/afm Altar/leyV JUlY 7. 1964 J. L. KLEIN ETAL 3,140,108

PROCESS AND PRODUCT 0F' METALLURGICALLY JOINING ZIRCONIUM TO FERROUS METAL Original Filed May 25, 1960 4 Sheets-Sheet 3 j l', i l\ a I *il y "Il .il *l f H INVENTOR.; Laewens/em, /rle/'l/ BY Kauf/nam Allamey- July 7 1964 .J.1 KLEIN ETAL 3,140,108

PROCESS AND PRODUCT OF METALLURGICALLY JOINING ZIRCONIUM TO FERROUS METAL Original Filed May 25, 1960 4 Sheets-Sheet 4 Alfa/nay- United States Patent O 3,140,108 PRQCESS AND PRODUCT F METALLURGICALLY JOINING ZlRCONIUM TO FERRGUS METAL Joseph Lester Klein, Arlington, Albert R. Kaufmann, Lexington, and Paul Loewenstein, South Lincoln, Mass., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Continuation of application Ser. No. 31,786, May 25, 1960. This application July 28, 1960, Ser. No. 46,041 4 Claims. (Cl. '287-119) The present invention relates in general to the provision of metallurgically bonded joints between zirconium or high-zirconium alloys and ferrous metal. More particularly, it relates to an improved method and resulting product of joining such predominantly-zirconium metal to stainless steel, especially austenitic stainless steel, in the form of rods, tubes, and the like. As is known, metallic zirconium and high-zirconium alloys have become quite promising as specialized materials of construction. In the chemical processing industry, their excellent resistance to corrosion by substantially all dilute mineral acids, hot concentrated aqueous caustic, and high temperature pressurized water, and their further general ability to withstand nitric acids in hot concentrated solutions, are markedly advantageous for service as inert process piping. More prominently in the nuclear energy art, zirconiums exceptionally low neutron absorptivity combined with strength at elevated temperatures has made it outstanding for mechanical structure directly Within the chain fission reactive cores of high-temperature thermal power-productive neutronic reactors. In fact, the designs of several such neutronic reactors currently being engineered for construction each employ a bundle of elongated, co-extensive, spaced-slightly apart, re-entrant, zirconium alloymore particularly Zircaloy-Z-tubes as the principal pressure-retentive structure of its core; a multiplicity of fissionable fuel elements arrayed within the essentially neutron transparent tubes cooperatively affords self-sustained chain fission reaction, whereupon circulating pressurized coolant fluid conducted through the tubes in heat-transfer relationship with those elements removes useful heat generated.

As a cardinal impediment in such applications, though, a series impasse has heretofore been encountered in making any satisfactory unitary joint between predominantlyzirconium metal and ferrous metal, especially stainless steel. It is now rather generally accepted in the art that fusion welding, by any of the familiar techiques, is ineffectual therefor; resulting weldments have proven to be, at best, very poor in mechanical properties and corrosion resistance. The want of such a joint has been particularly adverse in the case of zirconium pipes and tubes, Where connections to ferrous metal vessel nipples, tubing runs, and the like must often be absolutely uid-tight in view of the extra hazardous radioactive or corrosive character of the fluids carried. Various mechanical jointsgasketed couplings, rolled and other frictional connections, and the like--investigated have, in common, left much to be desired; leakage, albeit sometimes only temporary, attending temperature excursions, especially upon rapid heating or cooling, stands as a notable shortcoming. Indeed, in said contemporary power-productive neutronic reactor design, the need for the advent of some reliable, unitary joint to so couple the ends of each in-core zir- ICC conium pressure tube to contiguous stainless steel extension tubes outside the reactors core region has become crucial to the basic technical feasibility of the pressure-tube-type reactor.

Accordingly, an object of the present invention is to provide a rigid, strong, unitary metallurgically-bonded joint between zirconium as Well as high-zirconium alloys, and ferrous metal, especially austenitic stainless steel.

Another object is to provide such a joint particularly suited for buttwise connection of pairs of members both similarly in the form of rods, tubes, or like configurations of substantially uniform axial cross-section.

A further object is to provide such a joint particularly suited to joining contiguous tubes in a reliably fluid-tight relationship.

Still another object is to provide such a tube joint which does not require encircling outside collars, inside sleeves, or other bulky member either exceeding the outside diameter of the tubing or obstructive of the tubing interior.

Still a further object is to provide an improved method of fabricating such joints.

Yet another object is to provide such a method readily and simply adaptable to fabricating such a joint integrally and simultaneously with the fabrication of members of substantially uniform axial cross-section to be joined.

Yet another object is to provide a method adaptable to fabricating a contiguous series of such joints as a single product readily severable into individual unitary nipples each featuring one extremity of the predominantly-zirconium metal and the other extremity of the ferrous metal.

Again another object is to provide such a method of appropriate fitness and suitability for large-scale metalworking application.

An especially important object is to provide such a joint which is markedly corrosion resistant.

Other objects will become apparent hereinafter.

In accordance iwth the present invention, a metal constituted predominantly of zirconium is metallurgically joined to a predominantly ferrous metal by arraying in tandem a mass of said predominantly ferrous metal followed by a substantially contiguously abutting mass of said predominantly zirconium metal both within a susbtantially-vacuum-tight malleable ferrous metal can, and, while establishing and maintaining substantial evacuation of gas from the interior of said can, hot extruding said enveloping can and concomitantly, in axial tandem, the therein contained said ferrous mass followed by said contiguously abutting predominantly-zirconium mass, such arraying and extrusion being effected after rst degassing said malleable ferrous metal can and all other metal which is afforded communication with said zirconium metal during hot extrusion and contains nitrogen subject to additional susbtantial evolution in vacuo at the temperature of extrusion, said degassing being effected by protractedly heating without melting and conjointly drawing a substantial vacuum thereupon. Upon said extruding, the leading face of the zirconium billet penetrates as a slender wedge deepely into the trailing face of the ferrous mass, affording a soundly metallurgically bonded juncture of extensive area and high strength. Typically, upon so extruding, in tandem, billets of degassed type 304L, 347, or 321 stainless steel followed by Zircaloy-2 with planar mating faces at about 1600-l650 F. and 6:1 cross-sectional area reduction ratio with about several feet per minute ram speed, all within a degassed 16 gage SAE- 1015 can, extruded rods result having conical interfacial junctures between the two metals of slender taper of axial length approximating five times the rod diameter; across the juncture the metals prove to be soundly bonded with manifested tensile strength normal to the interface of the order of 35,000 to 55,000 p.s.i.-is some cases greater than 60,000 p.s.i.-and similar strength in shear along the interface. This compares quite favorably with the about 70,000 p.s.i. ultimate tensile strength of the Zircaloy-2 itself.

As applicant has discovered, the crucial key to this unique realization of metallurgical bonding is the rigorous degassing of the ferrous metal ca n and also every ferrous metal billet subject to deleterious nitrogen evolution upon being heated to the extrusion temperature. The ferrous components are heated and held for an extended periodsay several hours-to a temperature best higher than the hot extmsion temperature under a good dynamic vacuum. Merely maintaining a vacuum upon the assembled billets during extrusion proves inadequate. Indeed, without such preliminary protracted hot degassing, the tandem extrusion operation, even though conducted in vacuo, exhibits much the same unsatisfactory results as have characterized previous fusion welding attempts; representative tensile strengths of the poor order of 6,000 to 9,000 p.s.i. are suffered. It is thought that the zirconium component, upon heating, tends to act as a getter in removing residual traces of elemental nitrogen from the abutting heated ferrous metal and can, and the consequent elevated-temperature nitriding of zirconium at the zirconium-ferrous interface is a prime adverse mechanism to which the unsound bonding otherwise experienced is attributable. Thus, tenacious nitrogen in the ferrous components, albeit in minute amount, is regarded as the agent primarily obstructive to bonding. In the instant process, not only does the critical hot degassing evidently purge the ferrous metal effectively, but then the extrusion operation, propitiously being of a mechanical nature admissive of maintaining an evacuated condition throughout, is seen to cooperate by affording substantial exclusion of such obstructive agent during the entirety of hot joining. However, in view of uncertainty respecting the complete mechanism responsible for the sound bonding achieved, it is not intended for this invention to be limited to any particular theory as to the precise phenomena involved.

The particular zirconium materials amenable to the instant process are subject to wide variation. Both principal types of elemental zirconium of current significance in the art-viz., raw zirconium metal produced by magnesium reduction of zirconium tetrachloride (i.e., Kroll Process), and refined, crystal-bar zirconium derived by hot-wire decomposition of zirconium iodide vapor (i.e., van Arkelde Boer Process)are suitable. In addition, applicability extends generally to alloys constituted mostly of zirconium; typical are the binary alloys of the order of 0.5 to 2.0 weight percent of such metals as aluminum, chromium, copper, hafnium, iron, manganese, molybdenum, nickel, niobium, tin, titanium, tungsten, and vanadium. Further, it has been notably successful in the case of those multicomponent high-zirconium alloys which now stand as the forms of zirconium of primary commercial importance for structural service, particularly the ductile, corrosionresistant Zircaloy-2: zirconium plus approximately 1.2 to 1.5% (by weight) tin, 0.07 to 0.2% iron, 0.05 to 0.15% chromium, and 0.03 to 0.08% nickel, and the also hydrogen-embrittlement-resistive Zircaloy-4; Zr plus approximately 1.2 to 1.5% Sn, 0.07 to 0.2% Fe, and 0.05 to 0.15% Cr. Likewise, diverse types of extrudible ferrous metal may be used. In addition to mild and carbon steels and myriad steel alloys, the stainless steels, both ferritic and austenitic along with martensitic, are suitable. Among these, stainless steel types 347, 321 and 304L (American Iron and Steel Institute type numbers and compositions) all austenitic with basic composition of about 18% Cr plus about 8% Ni-have been found quite satisfactory. It is desirable, of course, that the selected ferrous metal should not be subject to deleterious metallurgical reaction under the degassing and extrusion conditions employed. For instance, during heating up to and holding at approximately 1700 F. of Type 304 stainless steel for degassing, some adverse intergranular carbide precipitation has been noted; however, Type 304L-an identical composition other than for a lower carbon content-has given consistently good bonding results with no manifestation of damaging carbide precipitation and hence represents a superior selection.

A mass of the selected ferrous metal is shaped in the form of an appropriate billet. Being an extrusion procedure, the present process specializes in the production of the joints in configurations of individual rods, tubes, and like elongated members of generally uniform axial crosssection. For such products, each billet is best of an axial cross-section roughly similar in shape to that of the desired product but of several-fold larger area. Thus, for a rod the billet is normally a solid cylinder, while for tubes a thick-walled, hollow, open-ended cylinder is in order. It is advantageous for the rear axial extremity of the ferrous billet to be axially symmetrical-say cropped in an axially perpendicular plane-to facilitate subsequent mating with a zirconium billet. In particular accordance with the present invention, such ferrous metal can and billet are thoroughly degassed by heating over an extended period while continuously drawing a high vacuum upon it. The temperature is preferably at least as high as, or higher than, any to be used throughout the extrusion operation. Normally, though, extrusion temperatures are limited to maximum values somewhat below 1710" F., the melting point of the 17% iron-83% zirconium eutectic; accordingly, a ferrous degassing temperature of the order of 1700 F. is particularly well suited. The higher the vacuum, the better; a vacuum below a small fraction of a micron of mercury absolute pressure is particularly favored. It is beneficial to continue the heating and dynamic evacuation until a substantial equilibrium is obtained, with virtualy no further traces of gas evolving. A degassing period approximating 3 to 4 hours is frequently ample. Similar hot degassing of the zirconium metal, though, is inadvisable; the heating, rather than serving to eliminate residual nitrogen, would tend to promote its nitriding with the zirconium. By the same token, the degassing of the ferrous metal should well be out of the presence of the zirconium billet, to avoid capture of the freed nitrogen by the gettering action of the zirconium. All degassing may well be conducted in a conventional vacuum furnace or heated autoclave.

The degassed ferrous components are then permitted to cool, whereupon the vacuum may be broken, preferably with a noble gas; helium is particularly preferred. Alternatively, it is acceptable to break the vacuum by admitting air although the billet should well not be reheated while in contact with the air. The ferrous billet is thereupon axially aligned with a predominantly-zirconium metal billet having much the same axial cross-section; the front axial extremity of that zirconium billet should preferably mate closely with the abutting rear face of the ferrous billet, to avoid burdening the extrusion with large interfacial gaps remaining to be closed. To facilitate evacuation of gases from the billets for the extrusion, it is convenient to dispose the aligned billets in a closelyfitting vacuum-tight malleable metal can provided with a sealable evacuation aperture, e.g., a tube or nipple. Not only Will such a metal can generally extrude with the billets as a thin enveloping sheath, but upon selection of a relatively soft metal for the can, such as mild steel, the can will further serve as a suitable plastically deformable material appropriate for easing the passage of the billets through the extrusion die. In instances where the degassing vacuum has been broken by noble gas or other protective atmosphere, all billet alignment and canning operation can profitably and conveniently be effected in the same atmosphere.

The aligned billets are thereupon evacuated of contacting gas; in cases where the billet pair is canned, a high vacuum is drawn through the evacuation aperture therein whereupon the aperture is sealed. Again, the higher the vacuum, the better; evacuation to a small fraction of a micron or lower is favored. The evacuation billet pair is then heated to extrusion temperature. The range of 1500 to 1650 F. is preferred. Temperatures much higher are better avoided in order not to chance exceeding the aforementioned 1710 F. eutectic melting point. Temperatures much lower than 1500 F. become disadvantageous because stiifening of the metals, especially the stainless steel or other ferrous component, with decreasing temperatures would necessitate inordinately high extrusion force. 1600 F. is the apparent optimum.

The heated billet pair is thereupon placed into an extrusion press billet chamber-an elongated massivewalled tube of uniform cross-section, normally cylindricalclosed by a die defining an aperture of substantially smaller cross-sectional area. By advancing a close-fitting ram into the opposite open extremity of the billet chamber, the billet pair is then promptly hot extruded, with the ferrous metal leading and the zirconium metal following; upon so being axially pressed from the chamber through the die, the metals form a long, continuous shape of the same cross-section as the die opening and emerge enveloped in a thin sheath of the can material. To facilitate streamlined flow of the metal into the die aperture, the die should well define an approximately conically converging surface from chamber to aperture. Cross-sectional area reduction ratios between billet and die aperture of the order of :1 to 6:1 and up to as great as 10:1 have been found satisfactory with little difference in resulting bond strength. Reduction ratios as low as 4:1 to 2:1, though, evidently sacrifice some bond strength. 6:1 is the apparent optimum. Extrusion speeds produced by ram velocities from as low as 13 inches to as high as 55 inches per minute have afforded good results, suggesting that the choice of speed can be made consistent with the practical speed range limitative of the particular extrusion press employed. At very slow velocities, it is often beneicial to preheat the interior of the billet chamber to minimize the rate of heat loss from the slow-moving billet pair to the chamber walls, ram, and die.

As the resulting extrusion issues from the die, no significant difference in bond strength has been found to obtain between cases where the eflluent extrusion is water quenched as it passes out of the die and where it is simply left to air cool quietly. The sheath of can material is removed by mechanical peeling, if thick enough, or machining and/or grinding; more conveniently, such materials as soft iron, copper, and brass may often be quickly and cleanly removed by pickling in aqueous nitric acid, which does not excessively attack zirconium, said Zircaloys, or said stainless steels.

The extremities of the desheathed extrusion may then be cropped or cut straight and square, thereby providing, as the ultimate product, a member of uniform cross-section featuring an extremity constituted solely of the zirconium metal, the other extremity constituted solely of the ferrous metal, and a sound metallurgical bond therebetween. The product may be made of virtually any overall length, beyond that required for the juncture, as admitted by the capabilities of the particular extrusion press by simply resorting to billets of commensurate length. Thus, if a long stainless steel rod joined to a long zirconium rod is desired, the instant process not only serves to effect the joining but simultaneously to fabricate the two rods. On the other hand, individual short couplings-say only a few diameters in length-can be produced; inasmuch as the zirconium metal is readily fusion welded to like zirconium metal and the ferrous metal to like ferrous metal, an elongated member of each metal may be welded, either in the shop or in the field, to its respective end of such a coupling to accomplish their connection.

In the extrusion product obtained, the area of bonded juncture generally assumes the configuration of a rather precise conical surface symmetrical with the extrusion axis and apexed toward the leading extremity of the extrusion. With extrusions produced from billet pairs abutted across a planar interface perpendicular to the extrusion axis, the length of the resulting conical juncture is generally several times the radial thickness of the metal in which it is located; a juncture slope of 1:10 is typical for such cases. Provided there is no pressing requirement for maintaining the length of juncture as short as possible, however, the area of juncture over which the disruptive force is distributed may beneficially be sizeably enlarged by resorting to a billet interface in the configuration of an axially symmetrical conical surface with apex in the direction of the leading extremity. Upon extrusion, the resulting conical juncture obtained is proportionately longer and hence more expansive. Representatively, a billet interface of conical surface angled 35 from the longitudinal axis has produced juncture slope of the order of 1:30 to 1:35, 30 produced 1:35 to 1:40 slope, and 25 a 1:45 to 1:50 slope. In addition to increasing generally the cumulative strength of joined area, such elongation of the juncture affords a second beneficial result. That is, upon extrusion with the flatter billet interfaces, the extrusion tends to sink somewhat into the feathered edge of extruded ferrous metal at the outer periphery of the juncture, and, in the case of tubes, into the feathered edge of zirconium metal around the inside periphery. Upon varying the billet interface angle with the axis in 15 steps starting with 90, it has been found that the sinking of the can becomes monotonically more slight, the more acute the angle, thus mitigating the extent of the annular depression which might be left, upon removal of the sheath, as a discernable imperfection in the otherwise smooth surfaces. Typically, a 30 angle for the conical billet interface is evidently adequate for largely eliminating all such depression, without need for resort to any more acute angle. A propos where joint length must be minimized, use of a sawtooth cross-sectioned billet interfacedefining a multiplicity of concentric sharp-edged ridges and valleys in the mating faces of each billet-beneficially affords not only expanded area, but a circumferentially-corrugated keyed configuration of juncture for enhancing its strength notably against axial tensile stress.

It has been found essential that the ferrous metal precede the zirconium in the extrusion operation. Reversal of the sequence has proven generally inoperative. With a planar billet interface, attempts at extruding Zircaloy-2 ahead of Type 347 stainless steel have resulted in gross void formation at the area of intended juncture. Resort to forward-apexed conical billet interfaces has been unavailing; so acute an angle as 20 was necessary merely to eliminate gross voids, but still left much of the interface abutted but unjoined.

As an additional special aspect of the present invention, a thin barrier layer of specifically titanium or niobium may be incorporated between the abutting faces of the ferrous and zirconium billets to complement the ferrous degassing operation in promoting bond strength. A foil of either titanium or niobium of the order of 5 mils in thickness and shaped to conform to the mated interface is normally sufiicient; it is Well to degas the barrier along with the ferrous billet before extrusion. Upon extrusion,

the barrier follows the tapered contour of the juncture, representing an unbroken transition layer at the bonded interface. In some instances, use of such barrier layers affords somewhat greater unit tensile strengths of bond; however, the resulting joints appear more susceptible to corrosive attack. For example, after several days to one week immersion of typical joints in pressurized water at approximately 300 to 360 C., oxide formation and some spalling became discernable at the interface periphery. Superiority in bond strength is believed ascribable to the function of the layer in serving to bar any residual nitrogen, still remaining in the ferrous billet after degassing, from readily diffusing into the zirconium, while at the same time sustaining sound bonding of its surfaces to both the ferrous and zirconium metals. Empirical investigation evidences that titanium and niobium are extraordinary in accomplishing such dual function; witness: moylbdenum sheet, niobium-vanadium alloy sheet, molybdenum sheet plus niobium sheet (Mo next to stainless steel billet) all have exhibited outright failure in bond ing, while the results with certain other layers were at best uncertain or anomalous. In any event, sound bonds obtain with the titanium and niobium, such that use of these specific interlayers, at least, represents a valuable optional procedure aiming toward enhanced bonding in those situations where the possible compromise of corrosion resistance is acceptable.

Representative apparatus and articles for conducting the present process are illustrated, along with resulting product embodiments, in the appended drawings.

In the drawings,

FIG. 1 is a cross-sectioned elevation of the billet chamber region of an extrusion press including a canned pair of billets ready for extrusion.

FIG. 2 is a partially diametrally-sectioned view of a rod, sans sheath, resulting from tandem extrusion of the FIG. 1 billet pair.

FIG. 3 is a cross-sectioned elevation of such a billet chamber region showing a joint of tubular configuration in the course of extrusion.

FIG. 4 is a partially diametrically-sectioned view of a tube, sans sheath, resulting from completion of extrusion of the FIG. 3 tubular joint.

FIG. 5 is a diametrally-sectioned view of a canned billet pair featuring a barrier layer disposed therebetween.

FIG. 6 is a partially diametrally-sectioned view of a tube, after sheath removal, featuring a bonded barrier layer at the juncture.

FIG. 7 is a cross-sectioned elevation again of a billet chamber region showing a rod featuring a multiplicity of successive joints in the course of extrusion.

Referring to FIG. 1, a thick-walled, reentrant, horizontal, hollow-cylindrical extrusion container inner liner l, defining as its interior a billet chamber 2, is disposed snugly within a thicker-walled, re-entrant, horizontal, hollow-cylindrical extrusion container outer liner 3, in turn disposed within a massive extrusion container 4. Coaxial with the billet chamber 2, a conically-convergent, circular-annulus die 5, backed by a bolster 6, and fitted tightly in the recess of a die holder 7, is urged against one extremity of the billet chamber 2, by a massive die head 8. The extrusion container 4 and die head 8, as well as the die holder 7, are stationarily anchored in a structural frame (not shown) and locked in place thereto by acceptance of locking pins (not shown) in receptacle wells 9, 9', and the tightfit of a thrust-transmissive shim ring 10 located between the die head 8 and said frame. Into the opposite extremity of the billet chamber 2, a close-fitting, solid, cylindrical ram 11, extends, wherein it is centered by a sliding fit through a centering ring 12. Within the extrusion chamber 2, is positioned a heated tandem extrusion billet pair comprising a solid, right cylindrical mass of hot degassed ferrous metal 13, at the fore, followed by a predominantly-zirconium billet 14, aft; both billets are sealed in a vacuum-tight malleable metal can 15, from which gas has been substantially evacuated through an evacuation tube 16, which has been subsequently sealed by pinching flat. In operation, the ram 11 is advanced into the billet chamber 2, to force the canned billet pair 13, 14, through the die 5.

The FIG. l operation results in a simple, elongated rod sheathed in the metal of the can 15. After removal of that sheath, the produced rod, referring now to FIG. 2, comprises a leading length of the ferrous metal 21, into the rear extremity of which a trailing length of the predominantly-zirconium metal 22, penetrates in the form of a slender spire. At the axially-symmetrical, rather precisely conical juncture 23, the two metals have becomc soundly bonded metallurgically.

Referring to FIG. 3, the system described in FIG.1 is modified to produce tubular joints by substituting a different, axially-bored ram 31, extending into the extrusion chamber inner liner 1; reciprocably supported from within the bore of the ram 31, a slender, solid cylindrical mandrel 32, extends in spaced relationship through the aperture of the die 5, and on forward beyond the die head 8. A heated tandem extrusion billet pair comprising a leading mass of degassed ferrous metal 33, and a following mass of predominantly-zirconium metal 34-which had, at the outset, been disposed within the extrusion chamber inner liner 1, in the configuration of thick-walled, hollow, right cylindrical billets sealed in an annular, evacuated, malleable metal can 35, slidably impaled upon the mandrel 32-are progressively becoming squeezed through the die 5, by the steady powered advance of the ram 31, into the configuration of an elongated tube 36, covered inside and out with a sheathing 37, of the malleable metal of the can 35, pressed firmly about the mandrel 32. The tube resulting from the FIG. 3 operation, after removal of the sheathing 37, comprises, referring now to FIG. 4, a leading length of the ferrous metal 41, and a trailing length of the predominantly-zirconium metal 42, soundly bonded together at an axially-symmetrical, forward-apexed frusto-conical juncture 43.

Referring to FIG. 5, a more intricate billet pair adapted to improve extrusion of a connected rod comprises a forward, horizontal, cylindrical billet of degassed ferrous metal 51, defining an axially-symmetrical cavity 52, in its trailing extremity. Spaced slightly apart therefrom, a trailing, solid, coaxial cylindrical billet of predominantlyzirconium metal 53, defines as its forward extremity an axially-symmetrical spire 54, of shape mating closely with the cavity 52. Within -the interspace therebetween is disposed a thin barrier layer of niobium or titanium metal 55, of conforming conical configuration. All are sealed in a vacuum-tight malleable metal can 56, from which gas has been evacuated through an evacuation tube 57, subsequently sealed by pinching. Casual gaps 58, 58', between the billets and can walls provide sufficient spacing for gas egress to permit thoroughgoing evacuation. Upon extrusion, this billet pair is adapted to produce a conical juncture considerably longer and more acute than in FIG. 2, and containing a thin interlayer of the titanium or niobium soundly bonded between the two principal metals.

A billet pair (not shown) similar to that in FIG. 5, but constituted rather of thick-walled hollow cylindrical billets and a correspondingly truncated barrier layer is adapted to produce a corresponding tubular joint. Referring to FIG. 6, in such a tube connection, the frusto-conical interlayer 61, of titanium or niobium, is soundly bonded to the forward ferrous metal mass 62, and to the trailing predominantly-zirconium mass 63.

In FIG. 7, a composite billet comprising a multiplicity of alternate discs of degassed ferrous metal 71, 71, and predominantly-zirconium metal 72, 72', appropriately disposed in an evacuated malleable metal can 73, is being extruded into a sheathed rod 74, comprising alternate lengths of the ferrous metal 75, 7S', and zirconium metal 76, 76', while the resulting bonding between trailing ferrous faces and leading zirconium faces is sound, the junctures between trailing zirconium faces and leading ferrous faces are very poorly bonded, if at all. The desheathed rod may be cut apart to provide a series of nipples, each bearing one of the well-bonded junctures, while the alternate unsound joints are discarded. In this way, a goodly number of sound joints can be extruded in a single operation, and the resulting composite rod represents a cartridged or nested supply of nipples severable as needed.

Further illustration of the quantitative aspects and preferred conditions and procedures of the present method and product is provided in the following specific examples. In Example l, two comparative tandem extrusions of rods are outlined, one without hot degassing of the ferrous component, and one with degassing in accordance with the present invention. In Example 2, a succession of comparative tandem extrusions, all in accordance with the present invention, of rods demonstrates the effect of varying different individual process parameters upon the obtaining bond strength. In Example 3, production of tubular joints is exemplified, while in Example 4, the integrity of resulting joined tubes under internal hydraulic pressure is shown. Finally in Example 5, the resistance of resulting joints to the corrosive effects of high-temperature pressurized water is assessed.

EXAMPLE 1 A right cylindrical billet of Type 304 stainless steel and a similar billet of Zircaloy-Z were inserted end-to-end into a closely-litting 16 gage mild steel can 2 inches in outside diameter. Gas was evacuated from the assembled can to a vacuum of 1 micron. The canned billet pair was heated to 1600 F. and promptly extruded, stainless steel end first, through a 0.833 inch circular-apertured die, employing extrusion apparatus generally corresponding with that illustrated in FIG. 1, at ram speed of 13 inches per minute. After quietly cooling in air to approximately room temperature, the resulting extruded rod Was stripped of the obtaining mild steel sheath, and several axially perpendicular cuts Were made through the region of conical juncture to yield several 5716 inch thick discs. Each disc comprised an outer ring of stainless steel surrounding an inner disc of zirconium metal. Two stainless steel studs were welded at diametrically opposite locations to the ring periphery, and two radial saw cuts were made through the ring approximately 1/8 inch above and below the stud. In each case, the strength of the bonding per unit bond area between the meltals was determined by pulling the studs outward with progressively increasing tensile stress and noting the minimum tensile stress needed to rupture the ferrous-zirconium bond. For comparison, the entire operation was repeated with the exception that the ferrous billet and the mild steel can were held at 1700 F. under a dynamic Vacuum which reduced pressure to 1 micron for approximately 4 hours, whereupon they were permitted to cool under vacuum, and then the vacuum was broken by admitting air, all before insertion of the billets into the can. The minimum values of tensile stress which ruptured the ferrous-zirconium bond were as follows:

P.s.i. First run (without hot degassing) 6,000 Second run (with hot degassing) 53,000

EXAMPLE 2 In a succession of runs, a multiplicity of connected rods were individually produced by hot tandem extrusion in general accordance with the procedure of the second run of Example 1, including hot degassing of the ferrous metal billet and can before nal billet assemblage in each case. The diameter of each canned billet pair was nominally 2 inches. Among the runs, different extrusion conditions were employed as a means for evaluating the effect of each principal process parameter upon bonding strength. In some instances in the tensile strength tests, the metal parted at the ferrous-zirconium juncture while in others it failed elsewhere, principally at the weld between a stud and the stainless steel ring. Particular operating conditions andthe ultimate imposed stresses reached upon metal failure in representative runs are set forth in Table 1 below.

10 Table 1.-Eject of Derent Parameters Upon Stainless Steel-Zircaloy-2 Bond Strength Type of Extru- Reduc Extru- Stainless sion tion sion Rod Extru- Bond Strength Steel Teru- Ratio Speed Cooling sion (1,000 p.s.i. ten- (AISI perature (areal (in./ N o. sion) type) F.) area) min.)

EFFECT OF TEMPERATURE 347 1, 500 6:1 13 air 23156 48, 27, 48 48, 36, 48 347 l, 600 6:1 13 alr 23110 37, 34, 40, 51 347 1, 600 6 1 13 air- 23261 14, 38, 3 347 1, 600 6:1 13 air 23262 51, 40, 53 304L 600 6:1 13 air 23112 32, 48, 37 304L 1, 500 6 1 13 air. 23263 40, 40, 35 304L 1, 600 6:1 13 air- 23264 53, 43

EFFECT OF REDUCTION RATIO 347 1, 600 6:1 55 air 23123 54, 33, 57,

33, 60 304L 1, 600 6: 1 55 air. 23268 38, 48, 40 347 l, 600 10: l 55 air- 23163 55, 35, 304L 1, 600 10:1 55 air 23272 36, 32, 56

EFFECT 0F EXTRUSION SPEED 347 1, 600 6:1 13 air 23110 37, 34, 40,1

5 347. 1, 600 6:1 13 a1 23261 14, 38, 35 7 6:1 13 air. 23262 51, 40, 53 6:1 13 air. 23112 32, 48, 47 6:1 13 air. 23263 40, 40, 35 6:1 13 air 23264 51, 53, 43 6 1 55 air 23123 54, 33, 57, 33, 60 6: 1 55 air. 23267 13, 35, 57 s; 1 55 air. 2325s 38,421,411

EFFECT OF TYPE OF STAINLESS STEEL AND ROD CO OLIN G 304 1, 600 6:1 13 air 23158 54, 51, 53, 48 304 1, 600 6:1 13 Water- 23125 6, 29, 3 6:1 13 air 23112 32, 48, 37 6:1 13 aia--- 23263 40, 40, 35 6:1 13 air. 23264 51, 53,43 s z 1 13 waren 23125 49, 40, 52 6:1 13 Water 23266 32, 38, 39 6:1 13 ail 23113 13, 23,9 6: 1 13 water.. 23127 43, 26, 39 6:1 13 air 23110 37, 34, 40, 51 6: 1 13 air- 23261 14, 38, 35 6: 1 13 air- 23262 51, 40, 53 6:1 13 Water- 23265 49, 60 50 EFFECT OF INTERLAYERS (included 5 mil niobum 35, 48

interlayer) 347 1, 600 6:1 13 l air 23116 13,111,211

(included 5 mil titanium interlayer) Footnotez Metal parted elsewhere than at ferrous-zirconium interlayer (principally at Weld between stud aud stainless steel).

EXAMPLE 3 A set of live tubulal joints were individually extruded employing a system in general accordance with FIG. 3, commencing in each case with one hollow cylindrical billet 3%. inches outside diameter and 11/2 inches inside diameter of Type 347 stainless steel and one of Zircaloy-2 respectively and a 16 gage mild steel annular can. The stainless steel billets and cans were first degassed in the manner detailed in Example l. In every case the stainless steel-Zircaloy interface was of frusto-conieal shape with conical surface angled 30 to the longitudinal axis of the billet pair and apexed toward the stainless steel billet. Extrusion conditions were 1600 F. extrusion temperature, approximately 6:1 area reduction ratio, 13 inches per minute extrusion speed, and air-cooling after eX- trusion. After stripping the steel sheathing from the resulting extruded tube, longitudinal strips about 1/2 inch wide were cut from three of the tubes and subjected to various tests to characterize their bond strentgh. Two

1 1 strips from each cut tube were pulled in tension. None of the strips failed preferentially along the joint. In all cases, the strip necked and fractured either across the all- Zircaloy section or across the joint near the all-Zircaloy section. Single strips from two of the three extrusions were used for bend tests. These strips were bent to form a complete loop over a 2-inch mandrel and no separation occurred at the point. Single strips from two of the three extrusions were also rolled in the longitudinal direction to give about a 13% elongation. No failure of the joint resulted from rolling. These strips were subsequently pulled in tension and the fracture was across the all- Zircaloy section. Stud pull tests normal to the interface were performed on strips from one of the three extrusions. These gave values of 17,000, 23,000 and 41,000 p.s.i. These values are somewhat lower than the general level obtained with rod extrusions in Example 2, but the geometry of the sections from the strips was such that cuts around the stud had to be made through the joint manually with a hacksaw; such inelegant cutting is, of course, quite severe and probably mechanically separated the junction layers somewhat around the sawed periphery.

EXAMPLE 4 To one of the remaining uncut joints from Example 3, which is approximately 2 inches in outside diameter with an approximately 1A cut wall thickness, was welded end caps; the resulting closed container was filled with Water through a pressure nipple provided, and subjected to progressively increased internal hydraulic pressure, with capped extremities essentially unrestrained from axial motion. At 18,900 p.s.i. pressure (representing a hoop stress approximating 78,000 p.s.i.) a 1,46 inch bulging of the all-Zircaloy section of tubing was detected, but with the juncture remaining quite intact. The test was thereupon terminated without actually consummating the apparently inevitable blow-out of the all-Zircaloy wall.

EXAMPLE 5 One of the remaining uncut joints from Example 3 was immersed in pressurized water at 360 C. for 28 days, without indication of significant corrosion attack at the juncture or elsewhere.

It is estimated that the Example 5 test was equivalent in severity to 8 years of exposure in 250 C. waterthe realm of interest for power-productive nuclear reactor service. Further preliminary corrosion tests in aqueous nitric acid have shown no significant preferential attack at the joint.

As defined in the Metal Handbook, 1948 ed., edited by T. Lyman, pp. 307, 554, American Society for Metals, 1948: SAE-1015 steel is a carbon steel comprised of iron together with the following minor constituents in substantially the indicated proportions by weight-caribou 0.13 to 0.18%, manganese 0.30 to 0.60%, phosphorus 0.040% (maximum), sulfur 0.050% (maximum). Likewise, American Iron and Steel Institute Nos. 304, 321, and 347 stainless steels are there defined to be austenitic stainless steels comprised of iron togther with the following minor constituents in substantially the indicated proportions by weight:

No. S04-carbon 0.08% (maximum), chromuim 18.0 to 20.0%, nickel 8.00 to 11.00%, manganese 2.00% (maximum).

No. 321-carbon 0.08% (maximum), chromium, 17.0 to 19.0%, nickel 8.00 to 11.00%, titanium at least 5 times the carbon percentage.

No. 347-carbon 0.08% (maximum), chromium 17.0 to 19.0%, nickel 9.00 to 12.00%, columbium 10 times the carbon percentage.

As defined in Alloy Digest, p. SS-55, Engineering Aliloys Digest, Inc., No. 304-L stainless steel is essentially the same as No. 304, supra with the sole significant exception that the proportion by weight of barbon is more rigorously limited to 0.03% (maximum). As defined 12 in Metallurgy for Engineers, J. Wolff et al., p. 60, Wiley, 1952, mild steel is regardable as simple steel containing less than 0.30% by weight carbon and less than 1.0% silicon and manganese.

Although this invention has been described with particular emphasis upon joining simple rods and tubes, it is inherently of much wider applicability. Other extrudable sections, for example rectangular-sectional conduit, I-beams, tubes featuring internal and/or external ribs, straight or spiral, and the like may be joined. Other extrusion techniques can be applied; a piercing mandrel procedure may be applied instead 0f the described floating mandrel style of tube extrusion. Impact, rather than press, extrusion may be in order. The saw-tooth and conical interface configurations for the billet pair suggest varieties of other extended surfaces-eg., threaded spindle and socket-for enhanced keying of the juncture against axial tension. Extension to other materials, especially to heavy metals, similarly difficult to join metallurgically is promising. Diverse additional applications of the hereinbefore-disclosed methods and products will become apparent to those skilled in the art. It is therefore to be understood that all matters contained in the above description and examples are illustrative only and do not limit the scope of the present invention.

This patent application is a continuation of our copending patent application Serial No. 31,786, filed May 25, 1960, now abandoned, likewise entitled Process and Product of Metallurgically Joining Zirconium to Ferrous Metal, in the names of Joseph Lester Klien, Albert R. Kaufmann, and Paul Loewenstein.

What is claimed is:

1. A method of metallurgically joining a metal constituted predominantly of zirconium to a predominantly ferrous metal which compises arraying in tandem a mass of said predominantly ferrous metal followed by a substantially contiguously abutting mass of said predominantly zirconium metal both Within a substantiallyvacuum-tight malleable ferrous metal can, and, upon establishing and while maintaining substantial evacuation of gas from the interior of said can, hot extruding said enveloping can and concomitantly, in axial tandem, the therein contained said ferrous mass followed by said continguously abutting predominantly-zirconium mass, such arraying and extrusion being effected after first degassing said malleable ferrous metal can and all other metal which is afforded communication with said zirconium metal during hot extrusion and contains nitorgen subject to additional substantial evolution in vacuo at the temperature of extrusion, said degassing being effected by protractedly heating without melting and conjointly drawing a substantial vacuum thereupon.

2. The method of claim 1 wherein an intermediate layer of metal selected from the group consisting of niobium and titanium is placed between the abutting surfaces of said masses of predominantly zirconium and predominantly ferrous metals so that said intermediate layer is maintained between said masses throughout the extrusion step.

3. A metallurgically bonded point between predominantly zirconium and predominantly ferrous metals produced by the process of claim 1.

4. A method of metallurgically joining a metal comprised predominantly of zirconium to be predominantly ferrous metal which comprises degassing said predominantly ferrous metal and a ferrous can of size accommodative of said masses in tandem, by heating said predominantly ferrous metal and said can for a period of 3 to 4 hours at substantially 1700 F. and conjointly drawing a substantial vacuum upon same, thereafter arrayng in tandem the resulting degassed predominantly ferrous metal followed and contiguously abutted by said mass of zirconium both contained in substantially-vacuumtight envelopment by and within said ferrous can, and, upon establishing and while maintaining substantial evacuation of gas from contact with said masses in the interior of said can, hot extruding said enveloping can, and concomitantly, in axial tandem, the therein-contained said predominantly ferrous metal followed by said contiguously abutting predominantly zirconium, said extrusion being carried out after said masses are heated to the range of 1500 to 1650 F. and effecting a crosssectional area reduction between the canned billet and die aperture of ratio within the range of 5:1 to 10: 1.

References Cited in the le of this patent UNITED STATES PATENTS Re. 3,744 Shaw Nov. 23, 1869 1,569,954 Donaldson et al Jan. 19, 1926 1,771,620 Ehrmann July 29, 1930 2,023,498 Winston Dec. 10, 1935 2,254,516 Farr Sept. 2, 1941 2,932,885 Watson Apr. 19, 1960 2,932,887 McCuaig et al Apr. 19, 1960 14 2,975,893 Johnson Mar. 21, 1961 2,986,273 Bandgett May 30, 1961 FOREIGN PATENTS 744,313 Great Britain Feb. 1, 1956 OTHER REFERENCES

Claims (1)

1. A METHOD OF METALLURIGICALLY JOINING A METAL CONSTITUTED PREDOMINANTLY OF ZIRCONIUM TO A PREDOMINANTLY FERROUS METAL WHICH COMPRISES ARRAYING IN TANDEM A MASS OF SAID PREDOMINANTLY FERROUS METAL FOLLOWED BY A SUBSTANTIALLY CONTIGUOUSLY ABUTTING MASS OF SAIS PREDOMINANTLY ZIRCONIUM METAL BOTH WITHIN A SUBSTANTIALLYVACUUM-TIGHT MALLEABLE FERROUS METAL CON, AND, UPON ESTABLISHING AND WHILE MAINTAINING SUBSTANTIAL EVACUATION OF GAS FROM THE INTERIOR OF SAID CAN, HOT EXTRUDING SAID ENVELOPING CAN AND CONCOMITANTLY, IN AXIAL TANDEM, THE THEREIN CONTAINED SAID FERROUS MASS FOLLOWED BY SAID CONTIGUOUSLY ABUTTING PREDOMINANTLY-ZIRCONIUM MASS, SUCH ARRAYING AND EXTRUSION BEIG EFFECTED AFTER FIRST DE-

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