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Title:
BRAZING METHOD AND COMPOSITES PRODUCED THEREBY
Document Type and Number:
WIPO Patent Application WO/2015/003201
Kind Code:
A1
Abstract:
A method of brazing or joining metallic materials, particularly ferrous based metallic materials to effect a metallurgical join is required. The method has advantage in that it removes or substantially minimises need for treatment of surfaces of mating surfaces of the metallic materials prior to joining. The method includes steps of placing a mating surface of a first metallic material adjacent to a mating surface of a second metallic material, the mating surfaces of the first and second metallic materials having a carbon-containing component placed therebetween. The metallic materials with carbon-containing component in between is subjected to heating for a period of time sufficient to effect a metallurgical bond between the first and second metallic materials.

Inventors:
HUGGETT PAUL (AU)
Application Number:
PCT/AU2013/001366
Publication Date:
January 15, 2015
Filing Date:
November 26, 2013
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
HUGGETT PAUL (AU)
International Classes:
B23K1/008; B23K1/19; B23K1/20; C23C8/22
Foreign References:
JPH067970A1994-01-18
GB368605A1932-03-10
Attorney, Agent or Firm:
Watermark Patent & Trade Marks Attorneys (Melbourne, Victoria 3001, AU)
Download PDF:
Claims:
CLAIMS:

1. A method of brazing or joining metallic materials, the method comprising the steps of:

placing a mating surface of a first metallic material adjacent to a mating surface of a second metallic material, the mating surfaces of the first and second metallic materials having a carbon-containing component placed therebetween; heating the first and second metallic materials with the carbon containing component therebetween until a predetermined minimum temperature is reached; and

maintaining temperature substantially at or above the predetermined minimum temperature for a predetermined period of time to effect a metallurgical bond between the first and second materials.

2. The method according to claim 1 wherein the first and/or second metallic material is ferrous based.

3. The method according to claim 1 or 2 wherein the carbon-containing component is graphite.

4. The method according to claim 3 wherein the carbon-containing component is a graphite sheet. 5. The method according to claim 3 wherein the carbon -containing component is a graphite powder.

6. The method according to claim 5 wherein the graphite sheet is of size and shape comparable to mating surfaces of the first and second metallic materials.

7. The method according to any one of the preceding claims wherein the first and second metallic materials are arranged adjacent to each other with a gap therebetween in excess of requirements for capillary action for brazing.

8. The method according to any one of the preceding claims wherein mating surfaces of the first and/or second metallic material are not machined or otherwise substantially treated prior to heating.

9. The method according to any one of the preceding claims wherein the first metallic material is a steel substrate.

10. The method according to any one of the preceding claim s wherein the second metallic material is selected from steel, stainless steel, cast iron or white iron.

1 1 . The method according to any one of the preceding claims wherein mating surface of the first and/or second metallic materials has flatness tolerance substantially between 0 to 1 .0mm substantially across the surface.

12. The method according to any one of the preceding claims wherein heating is conducted in a vaccum.

13. The method according to any one of claims 1 to 1 1 wherein heating is conducted in an inert gas atmosphere.

14. The method according to any one of the preceding claims wherein heating is conducted at a very high partial pressure.

15. The method according to any one of claims 1 to 1 3 wherein heating is conducted at a low partial pressure. 16. The method according to any one of the preceding claims wherein the predetermined minimum temperature in the furnace is from about 1 100°C.

17. The method according to any one of claims 1 to 15 wherein the predetermined minimum temperature in the furnace is about 1 140°C.

18. The method according to claim 5 wherein the graphite powder has particle size of 100 microns or less.

19. The method according to claim 14 or 15 wherein the graphite powder is used to form a paste. 20. The method according to any one of the preceding claims wherein three or more metallic materials are joined together, each adjacent and respective metallic material having a carbon component arranged therebetween prior to heating to effect metallurgical bonding of adjacent metallic materials.

21 . A metallic composite produced by the method of any one of claims 1 to 20.

Description:
BRAZING METHOD AND COMPOSITES PRODUCED THEREBY

FIELD OF THE INVENTION

The present invention relates to a method of brazing a composite material and to composites produced by the method. In particular, the method relates to a method of furnace brazing ferrous alloys utilising carbon diffusion into at least one ferrous based surface. The method finds particular utility in furnace brazing of ferrous alloys without requiring that joining surfaces be substantially treated prior to brazing. BACKGROUND TO THE INVENTION

Ferrous based composites consisting of a hard wear resistant component, such as white cast iron, metallurgically bonded to a tough and ductile component, such as a steel substrate, have been used in the wear protection of equipment such as earthmoving and industrial processing equipment for some time. There are a number of commercial manufacturers of these types of alloy composites, and the principal method of manufacture is vacuum brazing using copper as the braze alloy.

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 whet both surfaces. The molten copper combines with the ferrous alloys to produce a columnar growth of copper/iron grains across the interface. Vacuum brazing is particularly useful in this process as it minimises oxidation of the copper and of the surfaces to be joined.

The process by which the molten copper wets the surface of the steel substrate and white cast iron is driven by capillary action. In order for the capillary process to successfully occur and to obtain a high quality brazed joint, the surfaces to be joined must be closely fitted and be exceptionally clean. In most cases the gap between the surfaces to be mated typically must be no more than 0.20mm and is often required to be less. Further, the surfaces must also have sufficient flatness, typically within about 0.1 mm, otherwise the capillary action required for bonding will not occur. Problems can therefore arise with this process because of the close tolerances required for the mating faces.

In most cases, in order to ensure this close fit, the mating faces to be joined must be surface treated, such as by machining, to ensure that the gap therebetween is small enough substantially across the entirety of the mating surfaces and that each surface is sufficiently flat. In the production of wear resistant composites where a steel substrate is bonded to a white cast iron, machining of the steel substrate is a relatively inexpensive process. However, machining of the white cast iron, which is inherently very hard, requires a higher cost surface grinding process and is often time consuming, which adds significantly to overall costs.

In the production of vacuum brazed ferrous composites, the machining process represents a significant cost to the manufacturer. For example, it is known than in at least some applications, this surface machining process constitutes som e 20% of the overall manufacturing cost. It also adds another process step where process variables can be introduced that can result in rejects and other quality associated issues.

It is therefore advantageous at least in terms of cost minimisation and qua lity control if the surface machining or treatment process can be reduced or dispensed with completely.

SUMMARY OF THE INVENTION

With this in mind, it is an object of the present invention to provide a method of brazing or joining together metallic components which reduces or substantially removes the need to machine or otherwise treat the surfaces of the components to be bonded.

With the aforementioned in view, the present invention provides in a first aspect a method of brazing or joining metallic materials, the method comprising the steps of: placing a mating surface of a first metallic material adjacent to a mating surface of a second metallic material, the mating surfaces of the first and second metallic materials having a carbon-containing component placed therebetween; heating the first and second metallic materials with the carbon -containing component therebetween until a predetermined minimum temperature is reached; and maintaining temperature substantially at or above the predetermined minimum temperature for a predetermined period of time to effect a metallurgical bond between the first and second metallic materials. The predetermined minimum temperature is preferably about 1 140°C.

The present invention is based on the principle of diffusion of carbon into a metallic surface. The present invention finds particular utility in brazing or joining of ferrous based metallic materials. The first metallic material may therefore be a ferrous based substrate such as a steel. The second metallic material may a lso be a ferrous based metallic material. In an embodiment of the invention where the method is utilised for the formation of wear resistant metallic composites, the second metallic material may be a ferrous plate, white iron casting or other similar material.

The first and second metallic materials, having layer of carbon -containing component therebetween, are heated in a controlled atmosphere furnace where oxygen is substantially excluded or reduced. An example is in a vacuum furnace or inert gas furnace. As the temperature rises and increases towards and beyond the predetermined minimum temperature, the temperature of the first metallic material also rises and diffusion of carbon into the surface of the first metallic material also increases. This causes increase of carbon concentration at the surface of the first metallic material and the relative melting point of the surface decreases.

As the temperature exceeds the predetermined minimum, the surface of the first metallic material has sufficient dissolved carbon that the surface of the first metallic material begins to melt, leading to a liquid surface area of the first metallic material whilst remainder remains solid.

The presence of the liquid surface area acts to further accelerate diffusion of the carbon containing component into the first metallic material. The creation of this liquid layer on the first metallic material facilitates joining with the second metallic material since the carbon containing component also diffuses into the second metallic material in substantially the same manner as it diffuses into the first metallic material. Mating surfaces of both the first and second metallic material absorb the carbon containing component via diffusion, resulting in both mating surfaces melting locally and creating liquid surfaces. These liquid surface layers enable joining together of the first and second metallic materials in a manner which is analogous to brazing, though without necessitating use of a braze alloy between the mating surfaces of first and second metallic materials.

Advantageously, application of carbon containing component between the surfaces of the first and second metallic materials produces surfaces with a lower melting point. Diffusion of carbon into the surface lowers the melting point for both the first metallic material, typically steel substrate and second metallic material, typically white cast iron, so that bonding occurs.

The first and second metallic materials are subsequently cooled, resulting in a complete metallurgical bond being created between the two materials.

It is desirable and an advantageous feature of the present invention that surfaces of the first and second metallic materials that are to be mated together can be arranged adjacent to each other with a gap therebetween which is in excess of requirements for capillary action for standard ferrous based brazing alloy methods. Advantageously, neither mating surface of either the first or second metallic material need be substantially surface treated, such as by machi ning before they are mated together using the method of the present invention. The presence of the carbon containing component provides a reducing environment on the mating surface of both the first and second metallic materials. This provides optimum surface wetting conditions during the process. Since the carbon containing component diffuses substantially evenly across the surface of each metallic material and the resulting liquid metal areas have relatively low fluidity, the method of the present invention advantageously enables relatively wide gap joints to be achieved.

Advantageously, application of the carbon component enables diffusion of the carbon into the surfaces of each of the first and second (or additional) metallic materials when subjected to furnace processing, lowering melting points of the surfaces so that effective bonding can occur. Usefully, application of the carbon component between the surfaces of the first and second metallic material assists in creating a stronger metallurgical bond by, for example, reducing porosity of the bond.

Following the manufacturing process, the resulting bonded composite product 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. Selection of type and quantity of the carbon containing component is important. It is preferred that the carbon-containing component is graphite. Graphite is an ideal selection for purposes of joining together two ferrous based metal alloys. It is preferred that the carbon component is a graphite powder or paste and can be sourced from foundry grade graphite typically used as an addition for melting cast irons. The graphite is crushed to a fine particle size, ideally of less than 100 microns and can either be dusted onto the surface of the steel substrate to form a thin loose layer, or premixed with petroleum jelly to form a thick paste prior to application.

The graphite may be provided in a sheet form or as a graphite powder or graphite powder paste. In the event that a graphite sheet is used, the graphite sheet is first cut to substantially match dimensions of the first and second mating surfaces to be joined. Care must be taken not to use excessive amounts of carbon containing component. If excessive amounts are used, 'burning' of the surfaces occurs and can result in significant material loss from the resulting joint. Residual oxygen level in the furnace desirably should also be monitored and controlled since this too can affect the efficacy of the method. If there is excessive residual oxygen inside the furnace, excessive amounts of the carbon containing component will likely be consumed, resulting in insufficient diffusion of the carbon containing component and inadequate melting of the mating surfaces, leading to an inferior or unsuccessful join.

The present method is particularly useful for brazing or joining of a steel substrate to a wear resistant component such as a white cast iron. However, the present method finds application in joining together any combination or number of ferrous alloys, such as but not limited to steel, cast iron and stainless steels. The present method may advantageously be used to join multiple layers of metallic materials, joining together any combination of ferrous based metallics. For example, the present method could be utilised to join together a first, second, third or more metallic materials. In such an instance, carbon containing component is placed between respective layers of metallic material prior to heating, so as to effect metallurgical bonds to join each metallic material to an adjacent metallic material. When two, three or more layers of metallic materials are joined, according to the method of the present invention, each layer of metallic material has a suitable quantity of carbon containing component placed therebetween prior to furnace treatment so as to effect the metallurgical bond.

It is preferred that the first and second metallic materials with carbon containing component therebetween is heated in an inert gas atmosphere or a vacuum, preferably with a very high partial pressure or low positive pressure. The inert atmosphere is typically a positive pressure within the nominal range of 20 to 100kPa. The inert atmosphere is preferably argon, although other inert gases such as nitrogen, or argon/helium, may be employed. A reducing atmosphere or any other gas suitable for the exclusion of oxygen may also be employed. It has been found that a joint at an interface of the ferrous -based metallic materials using the present method has a full bond across substantially the entire mating surface. Advantageously, the present method can be used to join various types, shapes and forms of metallic ferrous alloys, including plates, bars and castings. In each instance, the full bond substantially across the entirety of the mating surfaces advantageously ensures integrity and strength of the join. According to a further aspect of the invention there is provided a metallic composite produced by the method of the present invention. In one embodiment, the metallic composite is a wear resistant composite material. In such an embodiment, the metallic composite is a composite comprised of a steel substrate and a wear resistant component, desirably a white cast iron. However, the composite may comprise any number of metallic materials, joined together by metallurgical bonding as per the method of the present invention .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following description made with reference to the following Figures in which:

Figure 1 is a graphical representation of an iron-carbon phase diagram, illustrating the relationship between melting point of a ferrous based alloy and carbon concentration;

Figure 2 is a simplified graphical representation of iron-carbon liquidus, representing melting point of a ferrous based alloy at a particular carbon concentration and indicating that as the amount of carbon increases in a surface of a metal material, the melting point of the surface decreases;

Figure 3 is an optical micrograph showing a typical metallurgical bond achieved between two ferrous based metallic materials, using the method of the present invention; Figure 4 is a graphical representation of tensile stress/strain for a composite material having three layers of metallic material, as produced by the method of the present invention;

Figure 5a is a representation of a three layered composite material manufactured according to the method of the present invention, prior to tensile stress testing; Figure 5b is a representation of an optical micrograph of the three layered composite product of Figure 5a after tensile stress testing;

Figure 5c is an upper perspective of Figure 5b;

Figure 5d is a side view of the fracture faces of the three layered composite product of Figures 5a to 5c;

Figure 6 is a representation of first and second metallic materials (iron substrate and white iron) prior to joining by the method of the present invention;

Figure 7 is a representation of the first and second metallic materials of Figure 6, with carbon containing component added to mating surface of first metal material prior to treating by the method of the present invention;

Figure 8 is a representation of first and second metallic materials with carbon containing component therebetween, loaded into a vacuum furnace prior to treatment by the method of the present invention; and

Figure 9 is a representation of resulting composite product produced by joining components of Figures 7-8 and in accordance with the method of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION It will now be convenient to describe the present invention with reference to the accompanying drawings. It should be understood that following description is illustrative of explanatory embodiments of the present invention and should not be taken as limiting the scope of the invention to any one embodiment. Further, the Figures depicted in this specification have been included for illustrative purposes only and are not intended to be limiting. It will be understood by the relevant skilled addressee that any compositions and/or materials quoted are typical and illustrative only and can be varied without departing from the scope of the invention.

Substitute Sheet

(Rule 26) RO/AU The method of the present invention includes a series of steps to create a metallurgical bond to join a first metallic material and a second metallic material. In the present embodiment, the first material is a first ferrous alloy, such as a steel substrate 12 and the second material a second ferrous alloy such as a white cast iron 14. However, it should be understood that the present invention finds application in creation of a metallurgical bond between any combination of ferrous based materials, such as but not limited to, steel, stainless steel, cast iron or white iron. In this embodiment, the steel substrate may be a flat steel bar or plate, typically having about 0 to 1.0mm flatness tolerance across a surface that is to be joined with the second metallic material. The second metallic material, which may be any ferrous material in any form such as plate, casting or bar, similarly also has a 0 -1.0mm flatness tolerance across the surface to be mated to the first metallic material. In this respect, neither the mating surface of the steel substrate, nor the mating surface of the second metallic material requires no machining or significant cleaning or other such treatment prior to undergoing the present method to effect a metallurgical bohd between the two materials. A carbon containing component is placed adjacent to and between the two mating surfaces. In this embodiment, the carbon containing component is a sheet of pure graphite 16. The sheet 16 is of a thickness suitable for the particular materials to be joined. The graphite sheet 16 may have a nominal thickness of about 0.25mm. Care must be taken not to provide a graphite sheet 16 having excessive thickness, as this can lead to burning of the mating surfaces and can result in material loss from the resulting joint. Similarly, if the graphite sheet 16 is too thin, insufficient graphite may be present to effect a structurally sound joint. In any event, in order to ensure formation of a metallurgical bond across substantially the entirety of the mating surfaces, the graphite sheet 16 should have size and shape comparable to the mating surfaces to be joined. The graphite sheet 16 should be cut to match the dimensions of the mating surfaces.

Substitute Sheet

(Rule 26) RO/AU Cutting can be achieved by any suitable means, such as using scissors, utility knives or guillotines.

The cut and sized graphite sheet 16 is then placed between the mating surfaces of the first and second materials to be joined. This forms a composite sandwich arrangement consecutively comprising first metallic material (steel substrate), graphite sheet and second metallic material. Optionally, further portions of metallic materials can be stacked, increasing the size and layers of the composite sandwich. Importantly, a graphite sheet, similarly sized to fit purposes, is placed in between each adjacent layer of metallic material.

The composite sandwich of metallic materials and one or more graphite sheets 16 is then placed inside a controlled atmosphere furnace. The furnace can be either a vacuum or inert gas atmosphere furnace. Regardless of the type of furnace, residual oxygen within the furnace ideally should be monitored and minimised so as to avoid consumption of excessive amounts of the graphite, which would lead to insufficient diffusion of the graphite and inadequate melting of the mating surfaces. The vacuum furnace is evacuated and purged with an inert gas, in the present case argon, to remove oxygen from the furnace chamber. The vacuum furnace is then back filled with a high partial pressure of inert gas to a suitable pressure, typically about 900-mbar. It has been advantageously found that positive pressure of inert gas facilitates good bonding between respective mating surfaces.

When inserting the entirety of metallic materials with one or more graphite sheets 16, care should be taken to maintain the stacked arrangement such that mating surfaces and graphite sheet(s) 16 therebetween are in alignment with each other so as to substantially represent the final intended form of composite product produced by joining of the metallic materials. Figure 8 illustrates examples of such stacks in a vacuum furnace.

The furnace is then set to increase the heat to at least a predetermined minimum temperature. The minimum temperature will depend at least in part upon the composition of the metallic materials to be joined, as well as the composition of

Substitute Sheet

(Rule 26) RO/AU the carbon containing component. Minimum predetermined temperature is typically nominally about 1140°C. Temperature is maintained at or just above this predetermined minimal temperature for an amount of time sufficient to obtain satisfactory metallurgical bonding between the respective metallic materials.

Holding the temperature at or just above the predetermined minimal temperature permits diffusion of the graphite into the adjacent ferrous alloy components. Once the predetermined minimum temperature inside the furnace has been held for the predetermined minimum time, the furnace is permitted to cool. After the furnace has cooled, the composite sandwich can be removed from the furnace and a full metallurgical bond will have been achieved across substantially the entirety of each mating surface.

In the present invention, use of the carbon containing component, in this case, a graphite sheet 16, is particularly beneficial in respect to the initial state of the mating surfaces. Tests have confirmed that the present method can be used on as-cast ferrous alloy surfaces without need for machining or any other significant surface treating. It can even be used when the steel substrate is in a scaled surface condition, as would be the case when the steel is supplied as 'black' bar or place, where the surface has a residual scale present from the rolling processes.

Advantageously, no flux is required to clean the respective mating surfaces during the brazing process, since the graphite sheet between the two mating surfaces produces the fluxing action.

The presence of the carbon (graphite) provides a reducing environment on the surfaces being joined, thereby eliminating or substantially reducing presence of surface oxides and of any scale present. This provides optimal surface wetting conditions during the joining or brazing process. Since the graphite diffuses evenly across the mating surfaces, the resulting liquid metal areas have low fluidity, enabling joining of mating surfaces despite there being a relatively wide therebetween.

Substitute Sheet

(Rule 26) RO/AU The final bonded composite product was sectioned and examined to assess the bond interface. A typical metallurgical bond achieved using the method of the present invention is shown in detail in Figure 3, clearly demonstrating that the bond produced joins two ferrous based components together. A further example is illustrated in Figure 9, showing a resulting external appearance of the bonded surfaces between first and second metallic materials following treatment by the present method. The present invention can be further understood with reference to the following non-limiting example of the working of an embodiment of the invention method.

Example 1

As discussed above, the applicant has discovered that diffusion of carbon into surfaces of two components to be metallurgically bonded, such as steel substrate and white cast iron, lowers the melting points of the surfaces so that bonding can occur.

In this example, a 0.2% plain carbon steel plate, having thickness of 8mm and width of 40mm and having scale on the surface intended to be mated with a second ferrous based alloy was used as the first metallic material. That is, the first metallic material is a steel substrate. A sheet of pure graphite, having nominal thickness of 0.25mm was placed on top of the carbon steel plate, the sheet of graphite having first been cut to have dimensions substantially the same as the mating surface of the steel plate.

A white iron casting with a nominal composition of Fe-15Cr-3C was selected as the second metallic material. The white iron casting was placed on top of the steel plate, with the graphite sheet therebetween, creating a composite sandwich of consecutively, steel plate, graphite sheet and white iron casting on top. The composite sandwich was placed inside a vacuum furnace, whilst retaining the stacked arrangement such that surfaces of the steel plate, graphite sheet and iron casting remained in alignment. The vacuum furnace was heated to a

Substitute Sheet

(Rule 26) RO/AU temperature of about 1180°C and this temperature was maintained for a period of three hours.

At the temperature of 1180°C, the vacuum furnace was purged with high purity nitrogen gas until the internal furnace pressure was 800 Torr. The furnace was held at the temperature of 1180°C for a period of one hour and subsequently permitted to cool to a temperature of 950°C. The metallic materials were then removed from the furnace. The resulting bond created between the steel plate and white iron casting is shown in Figure 3. Example 2

In this example, three layers of 3.0mm thick steel, in the form of steel plates, were bonded together using the present method. A first layer was overlaid with a sheet of 0.13mm pure graphite so that the graphite sheet was contiguous with the mating surface of the first layer of steel. A second layer of steel plate was placed atop the first sheet of graphite and a second sheet of graphite placed atop the second layer of steel plate. A third layer of steel plate was placed on top, thereby creating a composite sandwich comprising three layers of steel plate and two layers of graphite sheet. The composite sandwich was placed into a vacuum furnace and heat treated substantially as described above. After heat treating, the furnace was permitted to cool to about 950°C and the steel component removed from the furnace.

The resulting component showed an ultrasonic thickness measurement of 9.3mm, representing a solid and metallurgically sound product. That is, metallurgical bond was effected between the first and second steel plates and between the second and third steel places.

The resulting product, being essentially a three-layered steel laminate, was subjected to tensile strength testing and it was found that the measured tensile strength of the steel laminate was 620Mpa. Significantly, this is a considerably higher tensile strength compared to the normal tensile strength of the original steel plates, which was measured at 480Mpa. The graph of Figure represents tensile strength/strain curve for the resulting three layered steel laminate

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(Rule 26) RO/AU produced using this method. Photographs of the resulting steel laminate are depicted in Figures 5a and 5b, which show the steel laminate before and after tensile testing. It will be appreciated that whilst the preceding example demonstrates metallurgical bonding or joining of three separate layers of ferrous alloy, components comprising any number of layers of ferrous based alloy could be manufactured using the present method and without necessitating any prior surface treatment or machining prior to joining.

Modifications and variations of the present method and product composite of the invention are possible as will be appreciated by a skilled reader of this disclosure. Such modifications and variations are within the scope of the invention.

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(Rule 26) RO/AU