AU2006317507A1 - A method of manufacturing metallic composites in an inert atmosphere and composites produced thereby - Google Patents

Description

WO 2007/059568 PCT/AU2006/001762 1 A METHOD OF MANUFACTURING METALLIC COMPOSITES IN AN INERT ATMOSPHERE AND COMPOSITES PRODUCED THEREBY Field of the Invention The present invention relates to a method of manufacturing a wear resistant metallic composite material and to products produced by the method. Background of the Invention In the field of ground engaging tools and industrial processing plants, wear resistant materials factor significantly in the cost of construction and maintenance. Consequently, the useful life of the wear materials used to prevent damage of the structures is an important economic consideration in design. It is therefore desirable to maximise the useful life as much as possible, so as to keep the accompanying costs to a minimum. The working environment in which wear resistant materials are utilised typically affects their service life. Typical working environments that are encountered often produce conditions such as abrasive wear, impact loading, temperature variation, vibration, and corrosion or combinations thereof. Each of these factors, both singularly and in combination, acts to effectively reduce the service life of the components. The associated high cost in terms of downtime to replace parts and the cost of the replacement parts themselves upon the parts becoming worn, has lead to the development and use of many methods and materials in an attempt to combat such wear problems in industrial plants and for ground engaging tools. These typically attempt to extend the working life of the wear materials. A range of materials are known to be available and suitable for use in various applications in wear environments. Some of the materials available for use in severe wear environments can be grouped in the following categories: * Chromium White Irons * Tungsten Carbide Composites * Cobalt base alloys * Nickel based alloys Each of these materials are characterised by hard carbides in a metallic matrix. Generally, it is recognised that these materials, although possessing good WO 2007/059568 PCT/AU2006/001762 2 to excellent abrasion resistance, are not particularly easy to work with. They typically tend to be difficult, if not practically impossible, to weld. As these materials are brittle, they tend to fracture when attached to an application with mechanical fasteners, and typically fail catastrophically when subjected to high impact loads. In order to obtain the requisite wear resistant properties for an application, compromises are often made in the areas of formability, machineability and weldability. As a result of these compromises, problems associated with fabrication and fixing of these materials often accompany the use of wear resistant materials. One of the ways to obtain a compromise in the material properties is to form a composite product. The composite products often have an extremely wear resistant product coupled to a weldable or machineable substrate. The process or the manner in which the abrasion resistant product is coupled to the tough substrate can range from mechanically interlocking to full metallurgical bonding. An example of these composite products includes tungsten carbide tiles, silver soldered to carbon steel. The main problem associated with this product is the bond strength. The bond strength is limited because the resulting joint is predominantly mechanical and there is a need for close tolerances between the mating faces. Hardfacing processes range from oxyfuel gas welding, to the various types of arc welding and also include the more advanced techniques of plasma transferred arc and laser welding. These hardfacing processes have some similarity in that a surface is coated using consumables. The consumables are selected so that the resulting coating has the desired chemical and microstructural properties. All of the hardfacing techniques suffer similar problems to varying degrees. The thickness of the coating is often limited and cracking of the coating is common, due to the significant thermal and shrinkage stresses placed on the applied surface and the substrate. Vacuum brazing has been used successfully to join white -irons to mild steel through the use of a copper-brazing alloy. The parts are heated to a temperature above the melting point of the copper to allow the copper to wet both WO 2007/059568 PCT/AU2006/001762 3 surfaces. The molten copper then combines with the ferrous alloys to produce a columnar growth of copper/iron grains across the interface. Other furnace brazing techniques have been used in the past and all of these techniques require the use of a vacuum or partial vacuum. The use of a vacuum or partial vacuum has been considered imperative for minimizing oxidation of the products during the brazing heat treatment cycle and to ensure integrity of the joints between the two materials. Problems arise with this process because of the close tolerances required for the mating faces and the difference in thermal expansion between the two materials. Further, this manufacturing technique does not readily lend itself to the manufacture of complex shapes. These problems have limited the application of this process to the manufacture of simple blocks of relatively small size, i.e. usually less than 500mm in length. Carbon steel is often the substrate of choice for these composite materials because it is easy to work with basic tooling and is relatively cheap. Carbon steel also has the user-friendly properties of being easily weldable in the field using commonly and readily available techniques. This allows the wear product to be held in place by directly welding the substrate to the application, or by welding studs to the substrate and then bolting the wear product to the application. There are, however, significant limitations to the preparation of composite materials using these conventional manufacturing techniques including: . Limited orientations of the wear product. * Limits to the thickness of weld metal deposit. * Close machining tolerances required for vacuum brazing. * Limit to size of vacuum brazed components. * Limits on size of end product due to the differences in thermal expansion of the wear material and the substrate. . Limits to the complexity of shapes that can be produced. * Cracking of the wear resistant materials during manufacture. * High capital cost requirements for vacuum furnaces.

WO 2007/059568 PCT/AU2006/001762 4 STATEMENT OF THE INVENTION According to a first aspect of the present invention, there is provided a method of producing a wear resistant composite product, including the steps of: contacting a first material with a second material, the first material having a liquidus temperature that is lower than a solidus temperature of the second material; heating the first and second materials in an inert gas atmosphere at a pressure greater than atmospheric pressure to a temperature above the liquidus temperature of the first material; maintaining the temperature of the first and second materials above the liquidus temperature of the first material for a predetermined period of time to at least partially fuse the first material to the second material. Preferably, the first material is a matrix material and the second material is a substrate material. Following the manufacturing process, the products may be subjected to a post production heat treatment to optimize the properties of the final product for the anticipated service or for the particular requirements. Through suitable control of the post production cooling cycle, it may also be possible to eliminate the need for this post production heat treatment. The matrix material can be chosen from a range of materials that exhibit at least partial solid solubility with the substrate material. These materials include iron, aluminium, nickel and titanium alloys, when used in conjunction with a ferrous substrate. The selection of an appropriate matrix material is largely dependent on the material characteristics and properties required of the final composite. In another embodiment of the present invention, the matrix material has a composition within the following ranges in weight percent.

WO 2007/059568 PCT/AU2006/001762 5 Element Minimum Maximum Carbon 1 4.5 Chromium 7 35 Manganese 1 10 Nickel 0.5 6 Silicon 0.3 4 Other (V, Ti, Nb, B, Mo, COMBINED COMBINED30 Sn, W, Cu) A further embodiment of the present invention includes a matrix material having a liquidus temperature of between 650 0 C and 1350'C, the liquidus temperature of the matrix material being at least 100 C less than the solidus point of the substrate. The matrix material can be manufactured in a separate process prior to the composite product manufacturing process. When preparing the matrix material, conventional foundry techniques can be employed although advanced techniques such as atomization, forging and diecasting are also suitable. The substrate material is selected from a range of materials that exhibit at least partial solid solubility with the matrix material. Predominantly, this would be a ferrous alloy,, but could include nickel or titanium base alloys. The actual analysis of this material can vary and is selectable so as to balance the solidus of the alloy with the high temperature strength and solid solubility with the matrix material. The substrate material can be selected from a range of materials, in particular from those materials that are able to be welded with common welding apparatus such as mig, tig and stick welding. A range of manufacturing techniques such as forging, fabricating or casting can be used to produce the substrate. The substrate can be in the form of a shell. The furnace temperature can operate in the range of between 500C and 2500C above the liquidus temperature of the matrix material. The furnace is held at a temperature above the liquidus temperature of the matrix material for a predetermined period of time. The furnace is typically held WO 2007/059568 PCT/AU2006/001762 6 above the liquidus temperature of the matrix material for a minimum of 10 minutes for every 50mm of cross section of the product. The inert gas atmosphere of the furnace is at a pressure greater than atmospheric pressure. The inert gas atmosphere is preferably nitrogen, although argon, argon/helium, inert gas, reducing atmosphere or any other gas suitable for welding may be employed. According to a further embodiment of the invention, there are provided composite products, manufactured according to the above described method. In this embodiment, hard carbide or ceramic material is substituted for a proportion of the matrix material, which results in products having properties of wear resistance and durability. Such products are useful in applications involving extreme abrasion, where the component is subject to impact loading, or where a complex shape is required. This process is also suitable for the repair of large wear components suffering extreme localised wear, such as slurry pump components and ground engaging tools. The hard carbide or ceramic material includes carbides, nitrides and borides of Ti, W, Cr, Mo, Ta, V, Nb, and B. The ceramic material may include oxides, nitrides and titanates of Si, Al, Mg, Ti, V, B, and Nb either individually, or in combination with any other carbide, oxide, nitride or boride wear resistant material. In another preferred embodiment of the current invention, there is provided a shell or mould, used to form the shape of the component manufactured according to the inert gas casting method of the present invention. The shell is a non-consumable item and is preferably coated with a refractory compound, applied to the shell. In this embodiment, the substrate may be placed on top of the matrix material and weighted thereupon, so as to keep the substrate in positive contact with the matrix material. This assembly is then heated in an electric furnace. It will be understood that various arrangements are possible using this technique and the geometry of the arrangements whereby the substrate is placed on the matrix material may vary significantly. In other preferred embodiments, the substrate may be arranged to engage the matrix material during the heat treatment process such that when the matrix WO 2007/059568 PCT/AU2006/001762 7 material has solidified after processing, a composite product is achieved. Such embodiments could include the non-consumable mould being of a desired geometric configuration, such that the matrix material is melted and contained therein to form a composite product. In such an instance, the substrate is able to act as an insert or partial mould for the matrix material. In this embodiment, the resulting final product may consist of a number of zones, as listed below: a) the prefabricated component, forming the bulk of the substrate; b) a transition between the prefabricated component and the matrix material, where the transition is fused to the substrate c) the matrix material, which may be formed in a non-consumable mould in such a manner so as to engage the substrate or transition material. BRIEF DESCRIPTION OF THE DRAWINGS The following description illustrates one explanatory embodiment of the method of the invention when used in relation to the bonding of a matrix alloy to a ferrous substrate. It will be convenient to further describe the present invention with respect to the accompanying Figures. The Figures illustrate the sequence of the invention and show a possible arrangement of the matrix alloy and substrate. They have been selected for convenience only and are not intended to limit the scope of the invention in any way. It would be clear to one skilled in the art that the compositions quoted are typical only and could vary considerably and still achieve fundamentally the same result. In the figures: Figure 1 is a flow chart showing the practical sequence of events of the method of the present invention; Figure 2 is a schematic diagram illustrating production of a composite material in accordance with a method of the invention; Figure 3 is a phase diagram of matrix material showing liquidus range of 1175-1275C; Figure 4 is a typical temperature and pressure profile for a product produced in accordance with the invention, using a consumable steel shell; WO 2007/059568 PCT/AU2006/001762 8 Figure 5 is an optical micrograph of a composite product produced in accordance with the thermal and pressure profile of Figure 4, showing the interface produced between the substrate and matrix material; Figure 6 is a schematic diagram illustrating production of a composite in accordance with a method of the invention, including use of a non-consumable mould; and Figure 7 is a typical temperature and pressure profile for a product produced with a ceramic-coated mould. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT It will now be convenient to describe a preferred embodiment of the present invention, by way of example only, with reference to the accompanying drawings. The method for producing composite materials according to the present invention has a series of steps as shown in the general flow diagram of Figure I, and with reference to Figures 2-5 illustrating certain features of the invention. Initially, a trial matrix material was cast using conventional open air casting methods, the material having an approximate composition of carbon 4%, chromium 9%, manganese 1.6%, nickel 1% and silicon 1%. This trial matrix material was cast into sand moulds to produce a base matrix material 12 for further experiments. The liquidus temperature 11 for the matrix material 12 was determined by thermal analysis and cross checked with the phase diagram for the alloy system, such as that of Figure 3. In this trial, the liquidus line 1 leis that line where the matrix material 12 changes from ausenite or M 7

C

3 carbide and liquid to liquid. The substrate 14 in this example was manufactured using a conventional open-air casting process, substantially the same as for the matrix material 12. The substrate 14 was in the form of a shell, nominally comprised of 0.2% carbon steel or 1 % carbon tool steel. After manufacture of the matrix material 12 and the substrate 14, both are prepared for further processing by the application of high pressure water and subsequently dried, or with mild grit blasting so as to remove any oxidation and surface scale. In other experiments the substrate 14 was fabricated using standard steel sections.

WO 2007/059568 PCT/AU2006/001762 9 To ensure the correct quantity of matrix material 12 is used, a calculation is performed on the volume of the substrate 14. Using the known density of the matrix material 12, the weight of matrix material 12 required to fill the substrate when the matrix material 12 is molten is determined, referred to as step 4 of Figure 1. The prepared substrate 14 containing the required amount of matrix material 12 is placed in a furnace having the capacity for modification of atmospheric composition and pressure conditions. In this case, the furnace used was an electric furnace fitted with Kanthal elements capable of reaching a maximum temperature of 13000C. The electric furnace sits within a mild steel casing which is capable of being pressurized during the heating cycle of the process. However, it should be understood that any suitable furnace may be employed, without departing from the scope of the invention. The heat treatment furnace is then purged with an inert gas to remove oxygen from the chamber. Preferably, the inert gas is nitrogen. The heat treatment furnace is then filled with a positive pressure of the inert gas to achieve a pressure above atmospheric pressure. In this case, the furnace was filled with a positive pressure of nitrogen to 40kPa. The furnace is then set to run through a predetermined heat treatment program based on the liquidus temperature 11 of the matrix material 12 and the predetermined hold time required to obtain a product having the requisite properties for the particular application at hand. The heat treatment program typically includes the steps of: a) heat-up to 12500C to 14000C; b) hold for 60 minutes; and c) cooling to 7000C , it being noted that nucleation and crystal growth can be manipulated by control of the heating and cooling rates. The heat treatment process is represented graphically in Figure 4. After completion of the heat treatment program, the furnace is opened. The substrate 14 and now remelted matrix material 12 composite products 10 are removed from the furnace and allowed to air cool to room temperature prior to final finishing processes.

WO 2007/059568 PCT/AU2006/001762 10 The final product was then sectioned and examined to assess a bond interface 13 between the substrate 14 and matrix 12. Referring to Figure 5, there is shown an optical micrograph of the resulting microstructure from which it was determined that the bond was fully fused and metallurgical in nature. The composite product 10 consisted of a matrix material 12, in this case a wear resistant, low melting point white iron, an interface/bond 13 and a substrate 14 in the form of a consumable shell of approximately 0.2% to 1% carbon steel. The metallurgical bond 13 had an interface size of approximately 10 microns. Adjacent to the interface 13, there is a carbide depleted zone 15 within the zone that was molten during processing. This carbide depleted zone 15 can be manipulated, based on cooling rate and material composition. Referring again to Figure 5, the substrate 14 microstructure consisted of pearlite 16 with the formation of intergranular carbides 17. These may be observed as light areas adjacent the grain boundaries on the micrograph. The matrix material 12 consists of austenite 18 with eutectic chromium carbides 19. A bond layer 20, consisting of the bond interface 13 and the carbide depleted zone 15 has altered morphology showing a depletion of chromium. This is identified by the lack of carbides in these zones 13, 15. It can be seen that the bond layer 13 between the matrix material 12 and the substrate 14 is relatively free of porosity, with some migration of the matrix material 12 into the substrate 14. The finished product was measured for dimensional accuracy and it was found that the composite product 10 had not undergone significant dimensional change. The temperature profile is shown in Figure 4. This cycle is based on the liquidus temperature of the matrix material and time. Figure 6 illustrates another aspect of the invention, utilising a shell or mould 26. The shell or mould 26 is preferably reusable and non-consumable. In this example, the mould 26 is preferably prepared and coated with a suitable refractory substance. The matrix material 12 is charged into the mould 26 and a substrate 30, is arranged therein so that when the matrix material 12 melts, it fills the mould 26 and comes into intimate contact with the substrate 30 so as to fuse or partially fuse with the substrate 30. In this embodiment, the substrate can WO 2007/059568 PCT/AU2006/001762 11 advantageously comprise an original worn part 30. In this manner, the worn part is essentially recycled, thereby assisting in cost recovery and minimisation. In order to prevent the mould 26 from fusing to the matrix material 12, it is preferred that a coating system for the mould 26 be used. In a preferred embodiment, the mould or shell comprises a steel shell 26. This provides a non-consumable mould 26, providing that a suitable barrier coating 27 is applied to the mould 26 prior to use. A number of trials were conducted with commercially available Foundry style refractory coatings for use as a barrier coating 27, and the best results were obtained with a two coat system involving a kaolin type ceramic coating and a magnesite coating. This barrier coating was prepared by a method including the following steps: 1. The steel shell mould is first cleaned using high-pressure water and dried, or with mild grit blasting, so as to remove any residual ceramic coatings or scale from previous use. 2. A base coat of Kaolin type refractory coating suspended in water is applied, using either a spray, flow coat or dipping method. 3. Preferably, at least two thin coats of the Kaolin based refractory are applied, to obtain optimal effect. It is preferred that the coating is allowed to completely dry between application of subsequent coats. 4. The topcoat of magnesite based refractory is then applied, using a suitable method of application, such as spray, flow coat or dipping. Preferably, only a single coat of the magnesite based refractory is applied. The standard coating is then suspended in alcohol. 5. The final coat is then allowed to dry thoroughly prior to the mould being used. It was found that the kaolin based refractory coating on its own, was inadequate in preventing the molten matrix material 12 from bonding to the steel shell. Similarly, a single coating of the magnesite based refractory was found to be inadequate in preventing the molten matrix material 12 from bonding to the steel shell.

WO 2007/059568 PCT/AU2006/001762 12 Various other types of coatings were trialled individually and in combination. However, the system used above was found to be the most effective and efficient barrier coating 27 system for the casting method of the present invention. The liquidus temperature of the matrix material 12 was determined either by thermal analysis during manufacture, or by using the phase diagram (Figure 3) for the particular matrix material 12. The matrix material 12 used in this example had a liquidus temperature 11, of approximately 1190-12000C. Once the liquidus temperature 11 of the matrix material 12 is established, the temperature above the liquidus temperature 11 to be reached and time required at this temperature during the heat treatment can be established. It was found that if the maximum temperature in the heat treatment was not high enough, wetting of the substrate 14 was not satisfactory. This resulted in a range of problems, including: porosity of the matrix material 12, lack of bonding, uneven melting and uneven surface finish. Other problems encountered include the incomplete filling of the non consumable mould 26 or substrate 14. The temperature above the liquidus temperature 11 that is required is inversely proportional with time of soak. That is, the higher the temperature of the heat treatment above the liquidus temperature 11, the shorter the soak time required. However, the temperature above the liquidus temperature 11 that the process can be run at is limited by the solidus temperature of the substrate 14. If the operating temperature is too high, the substrate 14 or mould 26 will not have sufficient strength to hold the molten matrix material 12 Modifications and variations of the composite casting method and product composite of the invention are possible as will be appreciated by a skilled reader of this disclosure. Such modification and variations are within the scope of the invention.

Claims (25)

1. A method of producing a wear resistant composite product, the method including the steps of: contacting a first material with a second material, the first material having a liquidus temperature that is lower than the solidus temperature of the second material; heating the first and second materials within an inert gas atmosphere at a pressure greater than atmospheric pressure to a temperature above the liquidus temperature of the first material; maintaining the temperature of the first and second materials above the liquidus temperature of the first material for a predetermined period of time to at least partially fuse the first material to the second material.

2. The method of claim 1, wherein the first material is a matrix material and the second material is a substrate material.

3. The method of claim 2, wherein the matrix material is selected from .materials that exhibit at least partial solid solubility with the substrate material.

4. The method of any of claims 1 to 3, wherein the inert gas atmosphere is reducing.

5. The method of any one of claims 2 to 4, wherein the liquidus temperature of the matrix material is at least 100"C less than a solidus point of the substrate material.

6. The method of any one of claims 2 to 5, wherein the matrix material has a liquidus temperature of between 6500C and 1350'C.

7. The method of any one of claims 1 to 6, wherein at least one material is iron based. WO 2007/059568 PCT/AU2006/001762 14

8. The method of any one of claims 2 to 7, wherein the substrate is iron based.

9. The method of any one of the preceding claims, wherein at least one material is steel.

10. The method of any one of claims I to 9, wherein at least one material has a composition within the ranges of: Carbon 1.0-4.5 wt % Chromium 7.0-35.0 wt% Manganese 1.0-10.0 wt% Nickel 0.5-6 wt% Silicon 0.3-4.0 wt% with V,Ti,Nb,B,Mo,Sn,W and Cu being less than 30 wt% combined.

11. The method of any one of Claims 2 to 10, wherein at least a portion of the matrix material is substituted with a material that has greater wear resistance than the matrix material.

12. The method of claim 11, wherein the material substituting the matrix material is a carbide, boride or nitride of Ti, W, Cr, Mo, Ta, V, Nb or B.

13. The method of Claim 11 or 12, wherein the material substituting a portion of the matrix material is a ceramic.

14. The method of any one of claims 2 to 13, wherein the matrix material is manufactured in a separate process prior to manufacture of a composite product produced by the method.

15. The method of claim 14, wherein the matrix material is prepared by foundry techniques, including atomisation, forging and diecasting.

16. The method of any one of Claims 1 to 15, wherein the second material is in the form of a receptacle for the first material. WO 2007/059568 PCT/AU2006/001762 15

17. The method of any one of Claims 1 to 16 wherein the second material is in the form of an insert and the first material is contained in a non consumable mould.

18. The method of Claim 17, wherein the non consumable mould is coated with a refractory material.

19. The method of Claim 18, wherein the refractory material is a kaolin based material.

20. The method of Claim 18 or 19, wherein the refractory material is a magnesite based refractory.

21. The method of any one of Claims 18 to 20, wherein the refractory coating is a combination of kaolin and magnesite based refractories.

22. A product produced by the method of any one of claims 1 to 21, wherein at least one material is iron based.

23. A product produced by the method of any one of claims 1 to 21, wherein at least one material is steel.

24. A product produced by the method of any one of claims 1 to 21, wherein at least one material has a composition within the ranges of: Carbon 1.0 - 4.5 wt% Chromium 7.0 - 35.0 wt% Manganese 1.0 - 10.0 wt% Nickel 0.5 - 6 wt% Silicon 0.3 - 4.0 wt%.

25. A non-consumable mould, coated with refractory material, for use in the method of any one of claims 17 to 21.