AU2001277412B2 - A method of manufacturing metallic composites and composites produced thereby - Google Patents

Description

WO 02/13996 PCT/AU01/00974 1 A METHOD OF MANUFACTURING METALLIC COMPOSITES AND COMPOSITES PRODUCED THEREBY Field of the Invention The present invention relates to a method of manufacturing a wear resistant composite material by a vacuum casting process and composites produced by the same process.

Background of the Invention In the field of ground engaging tools and industrial processing plants wear resistant materials are significant 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.

The environment in which wear resistant materials work affects the service life of these products. Typical environments that are encountered may produce conditions of abrasive wear, impact loading, temperature variation, vibration, and corrosion, all of these factors combining to reduce the service life of the components. The high cost in terms of downtime and replacement parts has lead to a plethora of methods and materials used in combating wear problems in industrial plants and for ground engaging tools.

There is a range of materials available with microstructures suitable for various applications. 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 These materials are characterised by hard carbides in a metallic matrix.

Generally it can be considered that these materials, although possessing good to excellent abrasion resistance, are not particularly easy to work with. They tend to be difficult if not practically impossible to weld. As these materials are brittle they tend to fracture when attached to the application with mechanical fasteners and fail catastrophically when subject to high impact loads.

In order to obtain the wear resistant properties compromises have been made in the area of formability, machineability and weldability. As a result of WO 02/13996 PCT/AU01/00974 2 these properties, problems of 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 machinable substrate. The processes for coupling the abrasion resistant product to the tough substrate can range from mechanically interlocking to full metallurgical bonding.

These composite products include Tungsten Carbide tiles silver soldered to carbon steel. The main problem associated with this procedure is the bond strength. The bond strength is limited because of the predominantly mechanical joint resulting and the need for close tolerances between the mating faces.

Hardfacing processes range form oxyfuel gas welding to the various types of arc welding and the 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 though some to a lesser degree. The thickness of the coating is 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 surfaces. The molten copper combines with the ferrous alloys to produce a columnar growth of copper/iron grains across the interface.

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 problems arise with the difference in thermal expansion of the two materials and the manufacturing technique does not readily lend itself to the manufacture of complex shapes. These problems limited the application of this process to simple blocks of relatively small size ie less than 500mm in length.

Carbon steel is often the substrate for these composite materials because it is easy to work with basic tooling and is cheap. Carbon steel also has the user- WO 02/13996 PCT/AU01/00974 3 friendly properties of being easily weldable in the field using commonly available techniques. This allows the wear product to be held in place by welding of the substrate to the application or by the welding of 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 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.

Statement of the Invention It is therefore the object of this invention to minimise the problems associated with the prior art by producing a matrix alloy/substrate composite material by the use of a suitable manufacturing technique.

With this object in view, the present invention includes 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 solidius temperature of the second material; heating the first and second materials at less than atmospheric pressure at sea level to a temperature above the liquidus temperature of the said first material; maintaining the temperature of the first and second said materials above the liquidus temperature of the first material for a predetermined period of time so that the first material at least partially fuses to the second material Following the manufacturing process, the products may be subject to a post production heat treatment to optimize the final product properties for anticipated service. Through suitable control of the cooling cycle it is possible to eliminate the need for this post production heat treatment.

WO 02/13996 PCT/AU01/00974 4 The matrix alloy can be chosen from a range of materials that exhibit at least partial solid solubility with the substrate material including iron, aluminium, nickel and titanium alloys when used in conjunction with a ferrous substrate. The selection of which matrix alloy would depend on the material characteristics required of the final composite.

In another embodiment of the present invention the matrix alloy has a composition within the following ranges in weight percent.

Element Minimum Maximum Carbon 1 Chromium 7 32 Manganese 1 Nickel 0.5 6 Silicon 0.3 4 Other Ti, Nb, B, Mo, COMBINED Sn, W, Cu) Another embodiment of the present invention includes a matrix alloy with a liquidus temperature of between 65000 and 1350C00. The liquidus temperature of the matrix alloy being at least 100C less than the solidus point of the substrate.

The matrix alloy can be manufactured in a separate process prior to the vacuum casting. When preparing the matrix alloy conventional foundry techniques can be employed although advanced techniques such as atomization, forging, diecasting may also be employed.

The substrate material is selected from a range of material that exhibits at least partial solid solubility with the matrix alloy. Predominantly, this would be a ferrous alloy but could include nickel or titanium base alloys. The actual analysis of this material can vary and would be chosen to balance the solidus of the alloy with the high temperature strength and solid solubility with the matrix alloy.

The substrate material can be chosen from a range of materials, especially from those materials that are weldable with common welding apparatus such as mig, tig and stick welding.

WO 02/13996 PCT/AU01/00974 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 alloy.

The furnace is held at a temperature above the liquidus temperature of the matrix alloy for some time. The furnace may be held above the liquidus temperature of the matrix alloy for a minimum of 10 minutes for every 50mm of cross section of the product.

The inert atmosphere is at a partial pressure of between 100mbar and 500mbar.

The inert atmosphere is preferably nitrogen although argon, argon/helium, inert gas, reducing atmosphere or any other gas suitable for welding may be employed.

Another embodiment of the invention provides composite products that are sufficiently wear resistant and tough, manufactured using the above described method with the addition of hard carbide or ceramic material being substituted for a proportion of the matrix alloy. Such products could be used in applications involving extreme abrasion where the component is subject to impact loading or where a complex shape is required. This process could also be used for the repair of large wear components suffering extreme localised wear such as slurry pump components.

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 of in combination with any other carbide, oxide, nitride, boride wear resistant material.

In another preferred embodiment of the current invention, the shell used to form the shape of the component manufactured using the vacuum casting technique, is a non-consumable item. This shell may be coated with a refractory compound applied to the remelt bar being placed in the shell. In this embodiment the substrate may be placed on top of the remelt with some weights so as to keep the substrate in positive contact with the remelt alloy. This assembly is then heated in a vacuum furnace as previously described. It will be understood that WO 02/13996 PCT/AU01/00974 6 various arrangements are possible using this technique and the geometry of the arrangements whereby the substrate is placed on the matrix alloy could vary significantly.

In other preferred embodiments, the substrate may be arranged to engage the matrix alloy during the heat treatment process such that when the matrix has solidified after processing a composite product is achieved. Such embodiments could include the non-consumable mould being of such a geometric configuration that the remelt material can be melted and contained in such a manner as to form a composite product with the substrate acting as an insert or partial mould for the remelted material.

In such an embodiment the final product could consist of a number of zones: a) the prefabricated component that forms the bulk of the substrate; b) the transition between the prefabricated component and the wear material, where the wear material is fused to the substrate (in some arrangements the substrate could be an insert or could be in the form of a shell with the matrix alloy being located inside the substrate); c) the matrix alloy or cast formed portion. This part could be formed in a non-consumable mould in such a way as to engage the substrate material.

WO 02/13996 PCT/AU01/00974 7 Brief Description of the Drawings The following description illustrates one explanatory embodiment of the method of the invention when used in relation to 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 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 sequence of events in the practice 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 1175-12750C; Figure 4 Typical temperature and pressure profile for a product produced with a consumable steel shell; 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 alloy; Figure 6 is a schematic diagram illustrating production of a composite in accordance with a method of the invention including a non-consumable mould; Figure 7 Typical temperature and pressure profile for a product produced with a ceramic-coated mould; and Figure 8 is a graph showing the relationship between temperature and time and acceptable product quality.

Detailed Description of Preferred Embodiments It will now be convenient to describe the present invention with reference to the accompanying drawings. The process for producing composite materials has a series of steps as shown in the general flow diagram of Figure I, and reference to Figures 2-5 is required to illustrate certain features of the invention.

WO 02/13996 PCT/AU01/00974 8 Initially, a trial matrix (Figure 2- 12) material was cast using conventional open air casting methods (Figure 1-1) of approximate composition of carbon 4%, chromium manganese nickel silicon This material was cast into sand moulds to produce a base matrix allow 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 (Figure 3) the liquidus line (11) being that line where the material changes from ausenite or M 7

C

3 carbide and liquid to liquid.

The substrate (Figure 2- 14) in this example was manufactured (Figure 1- 2) using a conventional open-air casting process the same as for the matrix material. The substrate (Figure 2-14) was in the form of a shell nominally of 0.2% carbon steel or 1% carbon tool steel. After manufacture of the matrix alloy (12) and the substrate (14) both are prepared for further processing by using highpressure water and dried, or with mild grit blasting to remove any oxidation and surface scale In other experiments the substrate (Figure 2-14) was fabricated using standard steel sections.

To ensure the correct quantity of matrix material is used; a calculation is performed on the volume of the substrate Using the known density of the matrix alloy the weight of matrix alloy required to fill the substrate when the matrix alloy is molten is determined The prepared substrate (14) containing the required amount of matrix material (12) is placed in a furnace capable of having atmospheric composition and pressure conditions changed in this case the furnace used was a vacuum furnace.

The vacuum furnace is evacuated and purged with an inert gas (in this case nitrogen) to remove oxygen from the chamber The vacuum furnace was filled with a partial pressure of nitrogen to 208-mbar It was surprisingly found that greater vacuum did not result in improved bond quality but a honeycomb type structure due to the vaporisation of some elements. At higher pressure, it was found that the bond quality was compromised due to interface reactions.

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 hold time required to obtain satisfactory product.

WO 02/13996 PCT/AU01/00974 9 Including the steps of: heat-up to 12500C to 13000C; (6a) hold for 60 minutes and cool to 700'C it being noted that nucleation and crystal growth can be manipulated by control of the heating and cooling rates.

The heat treatment is shown in Figure 4 in combination with the pressure cycle.

After the completion of the heat treatment program, the furnace is opened The substrate and now remelted matrix alloy composite products (10) are removed from the furnace and allowed to air cool to room temperature prior to final finishing processes.

The final product was then sectioned and examined to assess the bond interface Figure 5 shows 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 consisted of a matrix alloy (12) in this case a wear resistant low melting point white iron an interface/bond (13) and a substrate in the form of a consumable shell of approximately 0.2% to 1% carbon steel The metallurgical bond had an interface size (13) of approximately 10 microns. Adjacent to the interface (13) in the zone that was molten during processing; there is a carbide depleted zone This carbide depleted zone (15) can be manipulated based on cooling rate and material composition.

The substrate (14) microstructure consisted of pearlite 16 with the formation of intergranular carbides These are seen as light areas at the grain boundaries on the micrograph.

The matrix alloy consists of austenite (18) with eutectic chromium carbides The 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 and 15). It can be seen that the bond layer (13 and 15) between the matrix alloy (12) and the substrate (14) is relatively porosity free with some migration of the matrix alloy (12) into the substrate (14).

WO 02/13996 PCT/AU01/00974 The finished product was measured for dimensional accuracy and it was found that the composite product had not undergone significant dimensional change.

The temperature and pressure 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. The shell or mould used is reusable (Figure 6) and non-consumable. In this example the mould (26) is prepared and coated with a refractory substance. The matrix alloy, or remelt ingots, are charged into the mould (26) and a substrate or in this case an original worn part is arranged so that when the matrix alloy (12) melts, it fills the mould (26) and also comes into intimate contact with the substrate (30) so as to fuse or partially fuse with the substrate In order for the mould (26) not to fuse to the matrix material a coating system for the mould (26) was used.

A steel shell (26) can be used as a non-consumable mould providing a suitable barrier coating (27) is applied to the mould prior to use.

A number of trials were conducted with commercially available Foundry style refractory coatings for use as barrier coatings and the best results were obtained with a two coat system involving a kaolin type ceramic coating and a magnesite coating.

1. The steel shell mould is first cleaned using high-pressure water and dried, or with mild grit blasting, to remove any residual ceramic coatings or scale from previous use.

2. Apply a base coat of Kaolin type refractory coating suspended in water using either a spray, flow coat or dipping method.

3. Optimum results are obtained by the application of at least two thin coats of the Kaolin based refractory.

4. Ensure the coating is completely dry between application of subsequent coats.

5. The topcoat of magnesite based refractory is applied using either spray, flow coat or dipping. Only one coat of the magnesite based refractory is applied.

The standard coating is suspended in alcohol.

6. The final coat is allowed to thoroughly dry prior to the mould being used.

WO 02/13996 PCT/AU01/00974 11 The kaolin based refractory coating used on its own was inadequate in stopping the molten alloy from bonding to the steel shell.

The magnesite based refractory used in a single coating was inadequate in stopping the molten alloy from bonding to the steel shell.

Various other types of coatings were trailed individually and in combination, and the system used above was proven to be the most efficient barrier coating (27) system for the current casting process.

A series of experiments was conducted testing the variables time, and temperature above liquidus temperature (11) of the matrix material The results of the experiments are represented in Figure 8.

Referring now to Figure 8, it was found that there was a relationship between the holding time and the temperature above the liquidus temperature (11) of the matrix materials (12) and these are significant factors in the quality of the finished components.

The liquidus temperature of the matrix alloy (12) was determined either by thermal analysis during manufacture by using the phase diagram (Figure 3) for the particular matrix material The matrix material (12) used in this example had a liquidus temperature 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 and a range of problem resulted including: porosity of the matrix alloy lack of bonding, uneven melting and surface finish. Other problems encountered included the incomplete filling of the non consumable mould (26) or substrate (14).

The temperature to be reached above the liquidus temperature (11) follows an inverse relationship with time of soak. The higher the temperature of the heat treatment above the liquidus temperature the shorter the 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 If the WO 02/13996 PCT/AU01/00974 12 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 (5)

14. A method of producing a wear resistant composite product of Claims 12 to 13 wherein the refractory coating is a kaolin based material. A method of producing a wear resistant composite product of any one of Claims 12 to 14 wherein the refractory coating is a magnetite based refractory. WO 02/13996 PCT/AU01/00974

16. A method of producing a wear resistant composite product of any one of Claims 12 to 15 wherein the refractory coating is a combination of kaolin and magnetite based refractories.

17. A product of any one of claims 1-16 wherein at least one material is iron based.

18. A product of any one of claims 1-16 wherein at least one material is steel.

19. A product of any one of claims 1-16 wherein a least one material has a composition within the ranges of: Carbon 1.0-4.5 wt% Chromium 7.0-32.0 wt% Manganese 1.0-5.0 wt% Silicon 0.3-4.0 wt%.