"METHOD OF PRODUCING A HARD FACING MATERIAL'

Technical Field

[0001] The present invention relates to a method of producing a hard facing material. Background Art

[0002] It is common for a number of components commonly used in industry to be exposed to both chemical and physical situations where the life of the component is adversely affected, for example, industrial plant used in a high pressure acid leaching (HPAL) process or Pressure Oxidation (POX) process in a mining environment.

[0003] All components of the plant used in a HPAL or Pox process are susceptible to wear and corrosion due to the nature of the process being undertaken, wherein ore slurry is mixed with highly concentrated sulphuric acid at high temperature and pressure. Damage done to plant is a common cause of shutdowns for maintenance as well as breakdowns in the plant, both of which lead to massive cost to industry as the plant is shut down while repairs and maintenance are carried out.

[0004] To help protect components from the highly corrosive and abrasive compounds used in a HPAL or POX process, it is widely known to either manufacture components from materials of increased hardness and low reactivity, such as titanium. Alternatively, surface treatments that increase the hardness and chemical reactivity of the underlying metal are used. One such example of a surface hardening treatment is the process of creating transition metal alloys such as Titanium Nitride (TiN) through methods of Chemical Vapour Deposition.

[0005] Components that are treated with a surface coating of transition metal alloys by chemical vapour deposition are still known to fail under the conditions inside a HPAL or POX plant as the total thickness of the surface layer is still only in the range of 100 nanometres. Such failures normally occur due to failure of the relatively thin surface coating wherein it is removed or damaged and the less wear and corrosion resistant material under the surface coating is quickly abraded or eaten away.

[0006] It is possible to create transition metal alloy surface layers in the range of 1 - 3 millimetres are possible using the TIG torch method. Wherein, under controlled conditions the transition metal is heated until it is molten. Once molten the transition metal will react with nitrogen which may be fed into the inert gas mix under which this is undertaken, thereby allowing an alloy of the transition metal and the non inert gas to be formed.

[0007] Crack formation in the surface is a known disadvantage to this method that as greater depths of material are deposited. Crack formation occurs in the surface of the deposited material due to a large residual stresses that occurs as the transition metal alloys formed solidifies. As the molten transition metal reacts with the gas in the shielding gas to form a transition metal alloy the resulting material has a higher melting point and hence quickly solidifies.

[0008] The cracks that form in the solidifying surface layer of the transition metal alloy not only leave the resulting material more susceptible to erosion and corrosion, but in the case of valve bodies receiving this treatment, reduce the hydrostatic pressure at which they may effective operate.

[0009] The present invention attempts to overcome at least in part the aforementioned disadvantages of previous surface hardening treatments.

Summary of the Invention

[0010] In accordance with one aspect of the present invention there is provided a method of producing a hard facing material comprising the steps of;

Pre-heating a work piece to a working range of temperatures; Attaching at least one resistance heater to maintain the work piece at a desired temperature;

Applying a TiN surface layer to a surface of the work piece; and

Heating the work piece to a desired temperature for a desired period prior to allowing the work piece to cool to ambient temperature.

[0011] Preferably, the method further includes the step of machining the work piece to remove surface imperfections after the work piece has cooled to ambient temperature.

[0012] Preferably, the method further comprises the step of applying a surface coating treatment to fill any further surface imperfections after the machining step.

[0013] Preferably, the surface coating treatment is in the form of a thermal spray process.

[0014] Preferably, the surface coating treatment applies a mixture of metal oxides or carbides to form a ceramic compound.

[0015] Preferably, the surface coating treatment consists of an oxide or carbide of Titanium.

[0016] Preferably, the surface coating treatment consists of an oxide or carbide of Tantalum.

[0017] Preferably, the surface coating treatment consists of an oxide or carbide of Nickel.

[0018] Preferably, the surface coating treatment consists of an oxide or carbide of Chromium.

[0019] Preferably, the surface coating treatment consists of an oxide or carbide of tungsten molybdenum. [0020] Preferably, after the thermal spray process is completed a further step of machining the work piece is undertaken to remove excess material from the work piece is undertaken.

[0021] Preferably, the step of machining the work piece is a process of lapping.

[0022] Preferably, the step of heating the work piece after the application of the TiN layer is undertaken for a period of between 2 and 5 hours.

[0023] Preferably, the desired temperature to which the work piece is heated during the heating process after the application of the TiN layer is between 400 and 550 degrees Celsius.

[0024] Preferably, the desired temperature the work piece is pre heated to is in the range of between 250 and 500 degrees Celsius.

[0025] Preferably, the resistance heaters applied to the work piece are arranged to maintain the work piece in a range of between 200 and 350 degrees Celsius.

Brief Description of Drawings

[0026] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a micrograph of a surface layer of TiN track applied to a substrate.

Figure 2 is a macrograph of a cross section of TiN track applied to a substrate.

Figure 3 is a top view of a TiN track applied to a substrate.

Figure 4 is a photograph of a work piece after the method of the present invention has been applied.

Best Mode for Carrying out the Invention

[0027] Referring to the Figures, there is shown a method of producing a hard facing material. The present invention will be described in relation to the formation of titanium nitride (TiN) although the same approach could be used to form surface coatings comprising other transition metal nitrides as well.

[0028] Figure 1 shows a micrograph of the results of a layer of TiN applied to a substrate in accordance with the prior art practice as discussed above. A number of surface features are noticeable in the resulting track of TiN material deposited on a substrate. Significant rippling is visible in the track; the rippling appears to occur in two different directions, radially along the electrode travel direction and perpendicular to the radial direction. The ripples observed affect the overall evenness of coverage of the surface coating. ¹

[0029] Crack formation in the surface coating of TiN deposited on the substrate is shown in figure 1. These cracks propagate along the entire length of the nitrided tracks. The tendency toward crack formation is more in tracks that are placed using increasing arc

1 .

energy density.

[0030] Further improvement on the invention, it was developed a method of reducing and in some cases supressing cracking issue. This is done by implementing a specific heat treatment procedure.

[0031] The procedure consists in various stages of heat treatment throughout the entire

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process: pre, during and post nitriding. During the nitriding process, a heat treatment machine is directly attached to the job.

[0032] Figure 2 demonstrates the depth of surface features in a TiN track laid in accordance with the method of the present invention. Demonstrated here are the surface features that are visible after the initial build-up of material using the TIG torch method. Surface features as shown increase the chance of wear and reduce the sealing capability of materials covered with the surface treatment.

[0033] Figure 3 is a top view of a TiN track laid in accordance with the present invention. The ripples common with this method of deposition are clearly visible. [0034] Figure 4 is a photograph of a work piece that has been treated using the method of the present invention. Cracks in the TiN layer can be seen traveling in a generally concentric manner on the surface of the work piece. These cracks have been treated using a thermal spray process and are noticeable due to the darker colouring of the titanium dioxide material used in this particular example. As can be seen in the photograph, the overall surface finish of the work piece is very high.

[0035] In use, the method of the present invention relates to a method of producing a hard facing material comprising the steps of pre-heating a piece in accordance with a heating profile, depositing a TiN alloy onto the surface of a substrate; reheating the work piece in accordance with a heating profile and allowing the work piece to cool to ambient temperature

[0036] The work piece to which a hard facing surface layer is to be applied to, is first place into a furnace for a period of time to raise the temperature of the work piece to the range of 250 to 500 degrees Celsius.

[0037] More preferably, the work piece is heated until it about 300 degrees Celsius. The length of time taken to heat the work piece will vary in accordance with the volume and the type of material that the work piece consists of.

[0038] The work piece once reaching the required temperature is removed from the furnace and positioned to allow the TiN process to be undertaken. Preferably, a number of resistance heaters are attached to the work piece. The resistance heaters are activated during the TiN process to maintain the temperature of the work piece.

[0039] Preferably, the work piece is maintained at a temperature in the range of between 200 and 350 degrees Celsius.

[0040] More preferably, the work piece is maintained at a temperature of 250 degrees Celsius +/- 30 degrees. [0041 ] In accordance with the method of the present invention a TiN alloy is created on a surface of a substrate by surface melting titanium under a TIG torch in a nitrogen rich environment.

[0042] It has been discovered by the inventor that by adjusting welding parameters and/ or making multiple passes over an area using the TIG method that it is possible to increase the thickness of the surface layer to be within the range of between 1-10 millimeters.

[0043] Once the TiN layer has been completed the work piece is again returned to the furnace. Preferably, the work piece is then heated to a temperature in the range of 400 to 550 Degrees Celsius. More preferably, the work piece is heated to be around 450 degrees Celsius.

[0044] The work piece is left in the furnace for a period, the furnace then being switched off, and the work piece being allowed to cool to ambient temperature inside the furnace. Preferably the furnace is allowed to run for a period ranging from 2 to 5 hours. More preferably, that furnace is allowed to run for 3 hours before being switched off.

[0045] Advantageously, the inventor has discovered that the process of heating and cooling of the work piece after the application of the TiN layer cause the minimization of residual stresses in the surface layer of the hard facing material. The residual stress in the surface layer of the material being one of the causal factors for crack creation.

[0046] Ifi accordance with another preferred embodiment of the present invention the work piece is then machined to remove as many of the surface imperfections as possible. This process normally involves a machining process (turning) or grinding of the surface of the material to remove as many of the ripples in the surface layer of the TiN coating as possible. This machining process aims to smooth the surface of the substrate thereby reducing variations in the depth of the surface layer to as little as possible.

[0047] In accordance with another preferred embodiment of the present invention, cracks in the substrate that are deeper than the machining process is capable of removing will be treated using a preferred surface coating technology to fill the cracks. The treatment aims to fill the cracks in the TiN layer with a metal ceramic compound.

[0048] This can be achieved using a thermal spray process. The method involves passing a powdered transition metal or transition metal oxide through a high temperature flame, heating and accelerating the powder toward the substrate. The melted powder impacts the substrate and cools rapidly to form a coating.

[0049] Preferably, the process may use a powered transition metal or transitions metal oxides or carbides to form ceramic compound within the cracks. More preferably, the process uses mixtures of powdered transition metals or their oxides such as Titanium, Tantalum, nickel, chromium, tungsten molybdenum and their oxides or carbides. Even more preferably, the process uses a mixture of Titanium Oxide and Chromium Oxide sprayed by the thermal spray process.

[0050] In accordance with a preferred embodiment of the present invention titanium dioxide is used as the transition metal oxide in the thermal spray process.

[0051] Once the process has been applied the item is then lapped or ground to remove excess material, so that the required surface finished is produced. Preferably the lapping or grinding reduces excess material to a point where a contiguous surface layer of TiN and titanium dioxide covers the substrate.

[0052] In accordance with one preferred embodiment of the present invention, the substrate that the TiN coating is applied to a valve's trim. The surface treatment in accordance with the present invention is applied to all components of the valve trim. By applying the method of the present invention to all the components of a valve's trim, a higher tolerance surface finish can be achieved, thereby increasing the hydrostatic pressure that the valve can withstand. [0053] Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.