Patent Publication Number: US-2019169822-A1

Title: Wear part and method of making the same

Description:
FIELD OF THE INVENTION 
     This invention relates to component wear surfaces with embedded wear particles to increase hardness and limit erosion of the surface. 
     BACKGROUND OF THE INVENTION 
     Industrial applications put tools in cyclic contact with abrasive materials that remove the surface of the tool. Over the service life of the tool the abrasive materials wear away and erode the exposed tool surface until the tool has to be replaced. A harder surface for the tool can extend the service life of the tool by reducing the rate of wear during operation. 
     Wear tools can be manufactured by casting, powder metallurgy infiltration or other techniques. Incorporating wear resistant particles or materials into the tool during forming of the part can limit erosion during operation to provide increased service life. Manufacturing processes, such as infiltration or casting, can incorporate wear resistant particles into the molten matrix material to improve wear properties. Constituents of wear particles can dissolve in the molten matrix and degrade material properties around the wear particle on solidification. This can limit retention of particles in the matrix if the matrix material becomes locally brittle around the particles. A large volume of hard particles in the matrix can also decrease the toughness of the tool body. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the inclusion of hard particles or wear particles into wear parts. The particles are generally incorporated into the tool base metal in a molten phase. Constituents of the hard particle can leach into the molten alloy and can alter material properties of the matrix around the particles. Excess carbon from a tungsten carbide particle dissolving into a molten steel alloy can alter the steel to a white iron or other iron phase proximate the particles. This white iron is more brittle and tends to fracture under operating conditions which can limit retention of the particles and reduce service life of the surface with loss of the particles. The present invention enables hard particles such as carbides to be included in or on parts that are cast or produced by other manufacturing processes with limited degradation of the matrix material properties. 
     The particles can be coated to help protect the particle from heat and chemical attack. The coating can include a metal carbide layer or a metal nitride layer or both to enable inclusion in or exposure to molten metal of ferrous and non-ferrous based alloys. Coatings on the particles limit dissolution of the constituents. Alloying elements introduced locally with the wear particles can increase toughness of the matrix. Alloying elements can also limit development of deleterious phases in matrix materials. This use of such hard particles with local alloying can provide a longer useful life for all kinds of products exposed to abrasive wear. 
     In a first embodiment of the present invention a wear part includes coated hard particles and an alloying element along a wear surface to limit the effect of brittle phases in the matrix material. 
     In another embodiment of the present invention, a method of making a wear part includes coating a wear particle with a first layer, locating an alloying element proximate the particle, introducing a molten matrix material to envelop the particle and alloying element and cooling the molten matrix to form the wear part including the coated particle and alloying element. In another embodiment of the present invention the matrix material includes primarily iron. In another embodiment of the present invention the matrix material is ductile iron. In another embodiment of the present invention the method includes coating the particle with a second layer. 
     In another embodiment of the present invention, a mold for creating a wear component includes coated wear particles proximate the mold wall and alloying elements proximate the wear particles. In another embodiment of the present invention the wear particles have a first layer coating the particle of a first material and a second layer coating the particle of a second material distinct from the first material. 
     In another embodiment of the present invention, a method for increasing wear resistance in a wear part includes coating a wear particle with a metal carbide, introducing an alloying element proximate the wear particle, immersing the alloying element and coated particle in a molten matrix to create a concentration gradient of alloying element and coated particles at a working surface of the wear part. In another embodiment of the invention, the method includes depositing a metal nitride coating and/or a sub-stoichiometric metal nitride coating on the carbide coating to promote wetting by a molten ferrous matrix. 
     In another embodiment of the present invention, a method for increasing wear resistance includes positioning an encapsulated wear particle along a casting surface of a mold with a precursor carrier or other means together with a mitigating element prior to pouring of the molten metal, introducing molten metal into the mold, consuming the carrier material and the encapsulated wear particle in the molten material and dispersing the mitigating element proximate the wear particle to alloy the molten metal. 
     In another embodiment of the present invention, a wear part includes a body having an external wear surface. The body is composed of a metallic matrix material having a coated hard material and an alloying element in the matrix material along the wear surface. The alloying material limits the effect of brittle phases in the matrix material adjacent the hard material. 
     In another embodiment of the present invention, a method of making a wear part includes coating hard material with at least one protective layer, locating the coated hard material and an alloying element adjacent the coated hard material in a mold, introducing a molten metallic matrix material into the mold to envelop the coated hard material and the alloying element with the alloying element limiting formation of brittle phases in the matrix material adjacent the hard material, and cooling the molten matrix to form the wear part incorporating the coated particle and alloying element. 
     In another embodiment of the present invention, a method of making a wear part includes applying coated hard particles and an alloying element on or adjacent a surface of a mold, introducing a molten metal into the mold to envelop the coated hard particles and the alloying element with the alloying element dispersing into the molten metal to offset the formation of brittle phases in the metal adjacent the hard material, and cooling the molten metal to form the wear part incorporating the coated particle and alloying element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a wear surface with a hard particle encapsulated in two layers in a matrix material with an alloying element and a hard particle encapsulated in two layers partially exposed on the surface of the matrix. 
         FIG. 2  is a vertical cross section of a mold with a secondary encapsulated hard wear particle impregnated matrix prior to the metal pour. 
         FIG. 3  is a vertical cross section of a mold with encapsulated hard wear particle impregnated mesh layers on the inner mold surface prior to the metal pour. 
         FIG. 4  is a cross section of a wear surface with diamonds encapsulated in one layer and the hard wear particle is shown exposed on the surface and embedded in the surface. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Many industrial operations involve tools and other parts that engage abrasive materials. For example, tools in mining and drilling operations are quickly worn away by cyclic material contact. Downtime for replacement of worn tools and components during industrial operations can significantly increase operating costs. Increasing the service life of wear members by increasing surface hardness limits downtime and increases operation efficiency. 
     Incorporating hard particles in a matrix forming the surface of a member can significantly increase wear resistance. The term “particle” is used herein as a broad term to encompass hard material in many different forms including, e.g., spheroids, powder, whiskers, etc. The hard material can also be included in other forms such as preforms or other larger blocks. Tools and parts can be formed by infiltration where a molten matrix material fills spaces between hard particles such as tungsten carbide packed in a mold. In a part manufactured by casting, the tungsten carbide or other hard material can be incorporated into the molten material before it solidifies in a mold. Tungsten carbide or the like can also be incorporated in a surface by welding. These processes can expose the hard material to aggressive conditions of chemical reactions and/or heating. 
     Hard material such as titanium nitride, tungsten carbide or diamond can breakdown by contact with molten matrix metals such as iron, copper and nickel as well as other elements during processing. As an example, diamond exposed to oxygen at ambient pressure degrades at processing temperatures above 700° C. with the diamond structure converting to graphite which is softer and less wear resistant than diamond. In a vacuum or in inert reducing environments degradation occurs above 1500° C. To maintain the structural integrity and material properties of the hard material can include one or more protective layers. For example, when using hard particles (e.g., tungsten carbide or diamond particles), each particle is provided with one or more of the protective layers. The protective layers on the particle or other hard material provide protection against degradation of the particle at high temperature and/or from chemical attack by the constituents of the matrix. Examples and methods of encapsulated hard particles as disclosed in U.S. application Ser. No. 14/593,900, which is incorporated herein by reference in its entirety, can be used in connection with the present invention. Although hard particles are commonly discussed herein, they are examples and such references without noting other alternatives is not to be considered limiting. 
     Coated particles can wholly or partially dissolve during manufacture when exposed to long durations of heat and/or where the coating is incomplete or damaged. Breakdown of the coating can allow particle constituents such as nitrogen or carbon to dissolve into the molten matrix and degrade material properties, which can then result in reduced product performance. Introducing an alloying element to the matrix metal can limit formation of phases in the matrix material with poor mechanical qualities. For example, where the coating of the hard particle is not complete, carbon from tungsten carbide or diamond can dissolve in a molten ferrous matrix material. This excess carbon can form brittle graphite phases such as flakes of white iron locally around the hard particles. In an excavating tooth, for example, impact with rock and soil during use can fracture the white iron more readily than the balance of the steel matrix releasing the hard particles from the matrix. Such release reduces the useable life of the wear part. 
     Local alloying elements such as nickel, chrome or molybdenum can limit the effect of or formation of degraded ferrous structures to increase toughness around the hard particles. By dispersing the alloying elements locally proximate the hard particles, the alloying can be limited to the areas where it is needed, reducing the volume and cost of the alloying elements required and maintaining the material properties of the bulk matrix material. Wear particles in the matrix also decrease the toughness of the cast body. The toughness can drop exponentially with increasing particle volume. Alloying of the matrix material can compensate for the decreased toughness of the cast body. 
     A wear surface  10  with encapsulated wear particles  12  in an alloyed matrix  14  are generally shown in  FIG. 1 . Layers on the wear particle  16 , tungsten carbide for this example, are not drawn to scale. Metal carbides such as SiC or TiC can form a primary layer  18  on the particle surface that protects the tungsten carbide from degradation when exposed to the molten matrix metal. The carbide layer covering the tungsten carbide limits contact of molten iron or other elements of the matrix with the tungsten carbide surface. A secondary layer  20  of nitride on the carbide layer can further protect the carbide layer and tungsten carbide particle from chemical attack by molten steel, iron-based alloys or other matrix metals. A single or a plurality layers around the hard particle is possible. 
     An alloying element  22  is introduced into the matrix material to limit formation of deleterious phases of the matrix material constituents. The alloying element modifies the matrix material properties locally. The alloying of the matrix can be limited to the area of distribution of the hard particles near the surface to improve retention of the particles in the matrix. The alloying element can be introduced as part of a carrier which acts as a sacrificial structure to dissolve in the molten matrix. Migration of the alloying element can be limited by the viscosity of the molten matrix. Local alloying can be limited to the area proximate the sacrificial carrier. The sacrificial carrier for the coated hard particles could be a wire, a screen, a foil or a layer painted on the inner surface of a mold. Alloying elements can improve hardenability and abrasion resistance in the wear surface. 
     During operation the working surface of the tool or other part exposed to abrasive conditions continually wears away creating a new surface. Coated hard particles distributed through the working end of the wear tool continually expose new diamond particles over the service life of the tool. Particles in the mounting end of the tool are not generally effective in increasing the tool service life since the tool is usually replaced before the mounting end becomes a wear surface. Particles in the mounting end for this kind of tool tends to add unnecessary expense to the tool. Nevertheless, it may be beneficial for some parts to include diamond or other hard particles in the mounting end and/or throughout the part. 
     While the invention is discussed here in terms of specific types of particles, such as diamond or tungsten carbide, in a ferrous matrix, this is for the purpose of illustration. The process can be applied to other hard particles in other matrix materials that limit wear in an abrasive environment. Hard or wear particles useable in this invention include, for example, ceramic, ceramic fibers, ceramic platelets or metal compounds such as titanium carbide or cubic boron nitride. The present invention is suitable for use with other hard particles in non-ferrous alloys cast or melted in manufacturing processes. The invention is described in terms of a ground engaging tool (such as an excavating tooth) solely as an example, i.e., for the purpose of illustration and should not be taken as a limitation. 
     The metal element of the metal nitride and/or the metal carbide can be any of titanium, vanadium, chrome, silicon, boron, tungsten, niobium, tantalum, zirconium, hafnium, molybdenum, aluminum or other metal or alloy. The metal compounds produced can include silicon carbide, boron nitride, titanium nitride, titanium carbon nitride, vanadium nitride, chrome carbide and vanadium carbide. Metal compounds can also include more complex compounds such as titanium aluminum nitride. The listed elements and compounds are examples and should not be considered a limitation. 
     Protective layers can also promote wettability of the particle surface. Wetting of the particle surface allows the molten material to better adhere to the surface. If the molten metal does not wet the surface of the encapsulated particles, the particles may tend to segregate at a surface, or clump together instead of distributing through all or a portion of the part. Insufficient wetting of the hard particle in the molten matrix may lead to the hard particle not being retained on solidification of the molten metal, particularly during use. 
     Better adhesion limits extraction of the coated particle from the matrix when a portion of the particle is exposed and contacts impinging materials. This enables the wear particles to be better retained in or on the tool during use of the part such as in a digging operation. A volume of hard particles without adequate bonding to the matrix can also act as crack initiation sites. Coatings such as metal nitride can be difficult to wet with molten metal. Adjusting the composition of the layer can allow the surface to better interact with the liquid. A sub-stoichiometric metal nitride composition with the ratio of metal atoms to nitride atoms modified from the most stable form can significantly modify the surface properties to provide preferential wetting of the surface. 
     The encapsulated particle can, for example, be positioned in a mold assembly  30  at a surface of a mold in advance of pouring in molten metal. In the present example, the cast part is a point of an excavating tooth; a core box (not shown) will form the mounting cavity to mount the point on a nose of an adapter or cast lip. Other wear parts for earth working equipment or other components exposed to abrasive environments during use are also possible though not illustrated herein. The positioning of the hard particles can be accomplished in several ways. The encapsulated particle can be incorporated into a precursor matrix material that could be a wax or a paint or other carrier that binds the particles in place as shown in  FIG. 2 . The precursor matrix  34  can be painted onto the surface of the mold  32  so that the encapsulated particle  12  is retained on the selected surface. Similarly, the alloying element  22  can be incorporated into a precursor painted on the mold surface. 
     When the molten metal is poured into the mold, the precursor matrix vaporizes or oxidizes, releasing the diamond (or other hard particle) and alloying elements. The diamond disperses and migrates into the molten metal together with the alloying element to form the working surface. This process retains the diamond where it is needed most such as a working portion and limits migration of the hard particles to other regions (such as the mounting end) as the molten metal quickly becomes viscous on cooling which limits mixing and particle migration. The working surface of the part can include a concentration gradient of alloying element and coated particles. The surface can include a higher concentration of the element and particle than a layer spaced from the surface. 
     Alternatively, a hard particle can be incorporated in a sacrificial medium such as a mesh or cloth, a metal ribbon, metal foam or ceramic foam that lines the surface of the mold together with an alloying element as shown in  FIG. 3 . In a similar manner to the precursor matrix, the mesh or cloth  38  is consumed when the molten metal enters the mold and the encapsulated particle  12  and alloying element  22  are released to mix with the molten metal so that it is distributed through the wear surface. Two or more layers of mesh  38 A and  38 B can be used with multiple layers lining the mold. The diamonds (or other hard material) can be released progressively as the liners are sequentially consumed on introduction of the molten metal. Sequential release of the diamonds from spaced layers in the mold can provide better distribution of the diamonds in or on the surface of the wear member. Alternatively, multiple sequential layers can alternate coated hard particles and alloying elements  22  to sequentially release them into the molten matrix. Sequencing the release of materials can be a function of expected viscosity of the matrix material and desired distribution of the released materials. 
     Other placement methods can be used to preferentially distribute the hard particles and alloying materials through the melt or in a particular portion of the part. Encapsulated diamonds (or other hard particles) and an alloying element can be poured into the mold immediately before the pour or simultaneous with the pour. The method used for including the coated particles may be determined by the shape and size of the cast part, the casting process and/or the size of coated diamond particles and alloying element particles. More than one kind of alloying element and/or hard particle can be incorporated into a wear part. The alloying element could also overlay the protective coatings on each particle. 
     Layers can be deposited on the hard particle surface using any of a number of techniques. The method chosen can depend on the material being deposited and the substrate material it is deposited on. Generally, each particle is processed and the protective coatings are applied over the entire particle surface at a constant thickness, though the thickness and coverage can be dependent on the reactivity of the coating material with the crystal structure of the surface. Fluidized beds are frequently employed so that the grains are suspended in an aqueous or gaseous flow that allows even deposition of the coating material. Coatings may be applied using electroless, electrolytic, chemical vapor deposition (CVD), physical vapor deposition processes (PVD), pre-ceramic polymer pyrolysis or other techniques. 
     In one example, a wear resistant particle such as diamond, tungsten carbide, silicon carbide or titanium carbide is coated with an initial layer of metal carbide that can strongly adhere to the hard particle. This initial layer can protect the particle from thermal and oxidative damage such as graphitization or degradation. The initial layer of carbide can be a continuous coating which completely covers the hard particle to provide that protection or can be a partial coating that covers more than half the surface of the particle. Alternatively, the first layer of metal carbide can be a mixture of metal carbides that can enhance fracture toughness of the layer compared to a carbide coating of a single metal. 
     A second layer of metal nitride is adhered to the carbide layer of the coated hard particle to protect the carbide layer coated hard particle from oxidation and chemical reaction with the molten metal matrix. The second layer can be a carbonitride, such as SiCN and/or Ti(CN) or other carbonitride where the layer will adhere to the carbide layer coated hard particle and provide the particle protection from oxidation and chemical reaction. The carbon chemistry of the carbonitride layer can provide wetting and adhesion in a ferrous based molten metal matrix. Alternatively, the second layer can be an aluminum nitride as in TiAlN. Alternatively, the second layer of metal nitride can be a mixture of metal nitrides, such as Si3N4 with TiN. This Si3N4-TiN composite can have enhanced fracture toughness as compared to either nitride singularly. The second layer material can have a degree of solubility with the carbide initial layer which can promote adhesion of the layers to each other and produces a stronger multilayer coating. 
     To promote wetting and strong bonding with the molten metal matrix, a third layer can be applied to the protective metal nitride second layer. This third layer can consist of a substoichiometric metal nitride where there are not enough nitrogen atoms to make up the complete crystal structure. In effect the stoichiometry of the metal nitride can be changed to a substoichiometric crystal structure. 
     Both the materials used as a second layer of metal carbonitride or metal aluminum nitride can be used as a third layer with a substoichiometric chemistry to provide good wetting and adhesion to the metal matrix without sacrificing the protection from chemical reaction and dissolution with the molten metal matrix or oxidation protection. 
     Molybdenum and chrome are strong carbide formers and will pull carbon away from carbide particles reducing the carbide concentration around the hard particles. Nickel is not a strong carbide former but refines grain size during processing and increases hardness toughness of the matrix material to better retain hard particles in the matrix. 
     In one example chrome is the alloying element. Coated diamond and chrome particles are mixed in a liquid carrier. The liquid carrier coats the interior surface of a mold used to create a wear part. The liquid carrier is allowed to cure to a solid state and the mold is assembled with any inserts, sprues and vents required. The matrix material forming the wear part is poured into the mold and on contact with the carrier, the carrier material is consumed releasing the hard particles and the alloying elements. The hard particles disperse from the mold walls into the molten matrix and the alloying element can melt or dissolve in the matrix. Carbon from diamond or other elements dissolved from hard particles can migrate into the matrix material. The chrome in the matrix material increases toughness and limits the formation of degraded iron phases in the matrix to maintain retention properties of the matrix. 
     Alternatively, as another example, nickel from a carrier melted by the molten steel disperses in the molten steel. The molten steel once introduced into the mold cools most quickly at the mold surface. The time of exposure above the graphitization temperature for the diamond (or other hard particle) is short and conversion of the diamond to graphite and chemical degradation is limited. Formation of any carbon phases from migration into the molten steel is mitigated by the nickel locally dispersed near the mold surface. 
     The layers deposited on the hard particle surface preferably range from, for example, one micrometer to one millimeter but could be smaller or larger depending on the intended purpose. Hard particles preferably range from nanometer sized up to 5 millimeters, but other sizes could be used. 
     In an alternative embodiment, a hard particle  12 A can be coated with a single layer  18  as shown in  FIG. 4 . The single layer can be a nitride or carbide to protect the hard particle. The hard particle can be incorporated into a matrix material  14  with an alloying element  22  to limit formation of deleterious phases of the matrix material from dissolved elements of the hard particle. 
     It should be appreciated that although selected embodiments of the representative encapsulated wear particles and alloying elements are disclosed herein, numerous variations of these embodiments may be envisioned by one of ordinary skill that do not deviate from the scope of the present disclosure. The presently disclosed methods and configurations for encapsulated particles lend themselves to use for many types of wear particles, and the resulting hardened surfaces are well suited to a variety of applications beyond wear members. 
     It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.