Abstract:
A composite barrel for use in extrusion or injection molding is disclosed. The composite barrel includes an outer housing having a cylindrical bore that extends throughout the length of the outer housing. A wear-resistant lining is disposed on an interior surface that defines the cylindrical bore. The lining is fabricated from an alloy that includes a base metal and phosphorus and the lining may contain hard abrasion-resistant particulate, such as tungsten carbide. The base metal is nickel or cobalt or a mixture of nickel and cobalt. The alloy is typically applied by centrifugal casting and can be cast in a nitrogen-rich atmosphere without creating undesirable lining porosity. Such linings can be made for a fraction of the cost of comparable linings that must be cast under vacuum or in an atmosphere of argon.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to materials and methods for protecting metal surfaces, and more particularly to materials and methods for fabricating wear-resistant linings of composite barrels used in injection molding or extrusion. 
     2. Discussion 
     Injection molding and extrusion are widely used methods for shaping plastic articles. In single-screw extrusion, solid polymer granules are supplied at one end of a screw that rotates within a cylindrical bore of a temperature-controlled barrel. As the screw rotates, polymer granules advance along the barrel between flights of the screw and an inner wall that demarcates the cylindrical bore. Because of high temperatures and pressures, the polymer granules liquefy as they travel downstream within the barrel. Liquid polymer exits the barrel through a die, rapidly cools and solidifies. Depending on the application, the polymer is cut, rolled or undergoes a second forming operation. 
     Like extrusion, injection molding employs a screw that rotates in a temperature-controlled barrel to liquefy and consolidate polymer granules. However, in many injection molding machines, the screw also serves as an injection ram. During a plasticating step, the screw rotates and fills, with liquid polymer, a cavity in the barrel located adjacent to the screw tip. A small port, which is sealed with a plug of solidified polymer, connects the cavity with a mold. Pressure generated during the plasticating step is insufficient to dislodge the polymer plug; but during a subsequent injection step, the screw stops rotating and moves forward, forcing liquid polymer into the mold. A check ring located near the screw tip prevents liquid polymer from travelling backwards between flights of the screw. 
     During extrusion or injection molding, internal barrel pressure can reach 100 MPa or higher. To withstand these high pressures, barrels are typically made from carbon steel, alloy steel or stainless steel. The clearance between the rotating screw and barrel inner wall is small—about 10 −2  cm—and therefore, polymer undergoing processing within the barrel exerts extremely high shearing forces on the bore surface. These high shearing forces, along with high barrel temperatures, corrosive polymer components and abrasive additives, such as TiO 2 , clay, silica, and carbon fiber, can rapidly erode conventional steel alloys. For this reason, many barrels have a wear-resistant layer or lining that is bonded to the bore surface. Barrels comprised of a steel outer housing and a wear-resistant lining are known as composite barrels or bimetallic barrels. 
     Composite barrel linings are typically applied using a process known as centrifugal casting. In this process, components of the lining—metal alloys and hard abrasion-resistant particles, for example—are placed in the cylindrical bore of the steel outer housing along with an amount of flux needed to minimize oxidation. Ends of the outer housing are capped to enclose the lining components. In some cases, the inner bore is evacuated and purged with argon gas. The barrel is placed in a high temperature furnace or induction coil and heated to a temperature sufficient to melt the metal alloy components of the lining. During heating, the barrel is often rotated slowly to evenly disperse the lining components. After the metals are melted, the barrel is removed from the high temperature furnace and is rapidly rotated to evenly distribute the lining components on the inner wall of the housing. As the outer housing cools, the metal solidifies and bonds to the inner wall of the outer housing, forming a wear-resistant layer. After the caps are removed, the bore of the composite barrel is honed to a desired diameter. 
     Although centrifugal casting is widely used to make wear-resistant linings for composite barrels, the method is expensive and its success depends on many factors. For example, the uniformity, morphology and hardness of the lining may depend on the form of the lining charge, the heating and cooling rates of the barrel, the amount of flux used, and whether the bore is evacuated or purged with argon gas during heating. Moreover, the use of argon dramatically increases the method&#39;s cost. Nitrogen gas, though much cheaper than argon, is unsuitable for centrifugal casting using known lining materials because its presence results in unacceptable lining quality, as evidenced by porosity, voids, oxidized particles, inclusions, etc. Because centrifugal casting depends on many factors, barrels are often re-spun or rejected, which increases the cost of the method. 
     The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention provides an alloy for fabricating wear-resistant linings. The alloy can be applied to metal surfaces in a nitrogen-rich atmosphere without creating undesirable lining defects such as porosity. The wear-resistant linings of the present invention can be made for a fraction of the cost of comparable linings that must be cast under conventional controlled atmospheres comprised of argon. 
     One aspect of the present invention provides a composite barrel that comprises a housing having a first end and a second end and an interior surface defining a cylindrical bore extending from the first end of the housing to the second end of the housing. The composite barrel includes a wear-resistant lining disposed on the interior surface of the housing. The lining is made from an alloy of a base metal and from about 0.5 wt. % to about 12 wt. % phosphorus. The base metal is nickel or cobalt or a mixture of nickel and cobalt. Typically, the alloy also contains about 0.1-26 wt. % chromium, 0.1-3 wt. % boron, 0.1-5 wt. % silicon, 0.1-0.8 wt. % carbon, and 0.1-5 wt. % molybdenum. The alloy may also include up to about 2 wt. % iron, 5 wt. % niobium, 3 wt. % copper, and 8 wt. % tungsten. To improve wear resistance, the lining generally includes one or more hard abrasion-resistant compounds. These include tungsten carbide, vanadium carbide or chromium carbide, either alone or in combination. Tungsten carbide is especially useful and may comprise as much as 60 wt. % of the charge. 
     A second aspect of the present invention provides a method of making a composite barrel comprised of an outer housing having a cylindrical bore extending throughout its length. The method includes the step of forming a wear-resistant lining—typically by centrifugal casting—on an interior surface that defines the bore. The wear-resistant lining is comprised of a metal alloy and optional hard abrasion-resistant compound, which are described in the preceding paragraph. During the forming step, the cylindrical bore is purged with a nitrogen-rich atmosphere. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side view of a composite barrel. 
     FIG. 2 shows an end view of the composite barrel shown in FIG.  1 . 
     FIG. 3 is a photomicrograph of a cross-section of a metal alloy containing phosphorus that is cast in a nitrogen-rich atmosphere. 
     FIG. 4 is a photomicrograph of a cross-section of a metal alloy-tungsten carbide mixture that is cast in a nitrogen-rich atmosphere. 
     FIG. 5 is a photomicrograph of a cross-section of a non-phosphorus metal alloy that is cast in a nitrogen-rich atmosphere. 
     FIG. 6 is a photomicrograph of a cross-section of a non-phosphorus metal alloy-tungsten carbide mixture that is cast in a nitrogen-rich atmosphere. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG.  1  and FIG. 2 show, respectively, a side view and an end view of a composite barrel  10 . The composite barrel  10  comprises an outer housing  12  having a first end  14  and a second end  16 . The outer housing is typically fabricated from carbon steel, an alloy steel or a stainless steel. An interior surface  18  defines a cylindrical bore  20  that extends from the first end  14  to the second end  16  of the outer housing  12 . A wear-resistant layer or lining  22  is disposed on the interior surface  18  of the outer housing  12 . Although the composite barrel  10  will usually have a cylindrical exterior surface  24 , the outer housing  12  may have any shape. 
     The wear-resistant lining  22  comprises an alloy of a base metal and from about 0.5 wt. % to about 12 wt. % phosphorus, though typically, phosphorus comprises about 6 wt. % or less of the alloy. The base metal is nickel or cobalt or a mixture of nickel and cobalt. Generally, the alloy also includes from about 0.1 wt. % to about 26 wt. % chromium, from about 0.1 wt. % to about 3 wt. % boron, from about 0.1 wt. % to about 5 wt. % silicon, from about 0.1 wt. % to about 0.8 wt. % carbon, and from about 0.1 wt. % to about 5 wt. % molybdenum. Ordinarily, chromium comprises about 10 wt. % or less of the alloy. In addition, the alloy may also contain up to about 2 wt. % iron, up to about 5 wt. % niobium, up to about 3 wt. % copper, and up to about 8 wt. % tungsten. 
     The metal alloys can be manufactured by well known powder manufacturing methods, including gas atomization. Details of the preparation of such alloys, including a nickel-phosphorus alloy containing boron and silicon are provided in U.S. Pat. No. 5,234,510, which is herein incorporated by reference. 
     To improve wear resistance, the lining  22  generally includes one or more hard abrasion-resistant compounds. These materials remain substantially unmelted throughout lining fabrication. Examples include tungsten carbide, vanadium carbide, or chromium carbide; of these, tungsten carbide is especially useful. The hard abrasion-resistant compounds are available as finely divided powders ranging in size from about 200 microns to less than about 45 microns. The term tungsten carbide refers to all forms of commercially available tungsten carbide powders including, but not limited to, tungsten carbide cemented with cobalt, nickel or a mixture of cobalt and nickel. The lining  22  may contain as much as 60 wt. % tungsten carbide. 
     Referring again to FIG.  1  and FIG. 2, the wear-resistant lining  22  is normally formed on the interior surface  18  of the outer housing  12  by centrifugal casting. In this process, components of the lining  22 , along with an amount of flux needed to minimize oxidation, are placed in the cylindrical bore  20  of the outer housing  12 . The lining components are supplied to the cylindrical bore  20  in various forms such as powders, cast ingots, bars, rods and the like. The first  14  and second  16  ends of the outer housing  12  are capped with steel plates (not shown) to enclose the lining components. Next, the cylindrical bore  20  is optionally evacuated through one or more ports in the steel plates and purged with nitrogen. The barrel is placed in a high temperature furnace or induction coil at a temperature sufficient to melt the metal alloy components of the lining, which is typically between about 1550° F. and 2300 ° F. While in the high temperature furnace, the barrel is rotated at about 15 rpm to evenly disperse the lining components. After the metal alloy in the charge is melted, the barrel is removed from the high temperature furnace, placed on rollers and rapidly rotated to evenly distribute the lining components on the inner wall  18  of the housing  12 . Typically, this corresponds to a barrel angular velocity that produces about a 70-G centripetal force on the charge. As the outer housing  12  and lining components cool during rotation, the alloy solidifies and bonds to the inner wall  18  of the outer housing  12 , forming a wear-resistant layer or lining  22 . Once the alloy solidifies, the composite barrel  10  is placed in a insulating media-sand, vermiculite and the like—to ensure that the lining  22  cools at a rate slow enough to prevent it from cracking. After the composite barrel  10  cools to room temperature, the steel plates are removed, and the bore  20  of the composite barrel  10  is honed to a desired diameter. 
     The disclosed method is more robust and inexpensive than conventional casting processes. For example, the disclosed method allows centrifugal casting in nitrogen-rich atmospheres, which is simpler and cheaper than casting in a vacuum or in an inert environment, such as argon. Surprisingly, the presence of 0.5 wt. % to about 12 wt. % phosphorus in the lining suppresses porosity that normally accompanies centrifugal casting of conventional alloys in a nitrogen-rich environment. Although not bound to any particularly theory, it is believed that phosphorus reduces the formation of boron nitride, which increases melt viscosity and thus porosity of the alloy. 
     Besides its use in composite barrel linings, the disclosed phosphorus alloys can be used to fabricate other wear-resistant surfaces that ordinarily require furnace processing in a nitrogen-free atmosphere. Examples of such processes include, but are not limited to, high temperature brazing and fused hard phase overlays of nickel base alloys. 
     EXAMPLES 
     The following examples are intended as illustrative and non-limiting, and represent specific embodiments of the present invention. 
     Example 1 
     One hundred grams of a powder sample of a metal alloy containing 2.2 wt. % phosphorus, 1.2 wt. % boron, 0.2 wt. % carbon, 4.0 wt. % chromium, 2.8 wt. % silicon, 3.0 wt. % molybdenum, and the balance nickel, is placed in an alumina crucible and heated to 2850° F. under a nitrogen atmosphere purge of 2500 cc/min and held for 10 minutes. Heating is carried out in an electric resistance, bottom-loading furnace. Following cooling, the resultant casting is weighed and sectioned. The casting weighs about 100 grams indicating that the metal alloy cast uniformly with no unmelted particles. 
     FIG. 3 is a photomicrograph of a cross section of the casting  30  at 100 times magnification. The casting  30  comprises a uniform layer of metal alloy  32  with few pores; the pores appear as dark areas  34  in the photomicrograph. 
     Example 2 
     Sixty grams of a powder sample of a metal alloy containing 2.2 wt. % phosphorus, 1.2 wt. % boron, 0.2 wt. % carbon, 4.0 wt. % chromium, 2.8 wt. % silicon, 3.0 wt. % molybdenum, and the balance nickel, is mixed with 40 grams of tungsten carbide. The resulting mixture is heated in a horizontal belt furnace (speed 14 in/min) with about an 85/15 N 2 /H 2  gas mixture at 2005 ° F. for nearly 1 hour. Following cooling, the resultant casting is weighed and sectioned. The casting weighs about 100 grams indicating that the metal alloy cast uniformly with no unmelted particles. 
     FIG. 4 is a photomicrograph of a cross section of the casting  40  at 100 times magnification. The casting  40  comprises tungsten carbide particles  42  dispersed in a uniform matrix of metal alloy  44 . Pores, which appear as dark areas  46  in the photomicrograph represent less than 2% of the cross-sectional area of the casting  40 . 
     Comparative Example 1 
     One hundred grams of a powder sample of a metal alloy containing 3.2 wt. % boron, 7.0 wt. % chromium, 4.8 wt. % silicon, and the balance nickel, is placed in an alumina crucible and heated to 2850° F. under a nitrogen atmosphere purge of 2500 cc/min and held for 10 minutes. Heating is carried out in an electric resistance, bottom-loading furnace. Following cooling, the resultant casting is weighed and sectioned. The casting weighs about 75 grams indicating that about 25 wt. % of the crucible charge did not alloy. Examination of the casting shows substantial amounts of unmelted, powdery material. 
     FIG. 5 is a photomicrograph of a cross section of the casting  50  at 100 times magnification. When compared to FIG. 3, the casting  50  comprises a non-uniform layer of metal alloy  52  with a large number of pores; the pores appear as dark areas  54  in the photomicrograph. 
     Comparative Example 2 
     Sixty grams of a powder sample of a metal alloy containing 3.2 wt. % boron, 7.0 wt. % chromium, 4.8 wt. % silicon, and the balance nickel, is mixed with 40 grams of tungsten carbide. The metal alloy-tungsten carbide mixture is heated in a horizontal belt furnace (speed 14 in/min) with about an 85/15 N 2 /H 2  gas mixture at 2005° F. for nearly 1 hour. 
     FIG. 6 is a photomicrograph of a cross section of one resultant casting  60  at 100 times magnification. When compared to FIG. 4, the casting  60  comprises a non-uniform matrix of metal alloy  62 ; a few tungsten carbide particles  64  are visible. Pores, which appear as dark areas  66  in the photomicrograph account for about 42% of the casting  60 . 
     It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reading the above description. The scope of the invention should therefore be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.