Wear resistant coating

Abstract

A method of forming a wear resistant and galling resistant coating for abrasive environments and a feed material for the method are disclosed. The feed material is for forming a wear resistant and galling resistant coating on a substrate by a welding process that heats the feed and the substrate. The feed material comprises 35 to 50 wt % titanium nitride particles and a balance of commercially pure titanium or titanium alloy particles and incidental impurities. The method involves delivering the feed material to a surface of a substrate and exposing the feed material and the substrate to sufficient energy to cause at least the commercially pure titanium or titanium alloy particles in the feed to melt and at least some of the titanium nitride particles in the feed to melt, thereby forming a melt pool. On solidification of the melt pool, at least some of the titanium nitride particles are embedded in a matrix formed from melt pool, thereby forming a wear resistant and galling resistant coating on the substrate. A wear resistant and galling resistant coating formed of the feed material is also disclosed.

Claims

1. A method of forming a coating on a substrate of titanium alloy, the coating being resistant to wear and galling in a corrosive and abrasive environment, the method comprising the steps of: (a) delivering a feed to a localized surface of a substrate by conveying the feed to the substrate in an inert conveying gas and controlling the flow of the conveying gas to control the feed rate of the feed, the feed consisting of: (i) 35 to 50 wt % titanium nitride particles; (ii) a balance of commercially pure titanium or titanium alloy particles with incidental impurities; and (b) heating the feed and the substrate to cause the commercially pure titanium or titanium alloy particles, at least some of the titanium nitride particles and the localized surface of the substrate to melt to form a melt pool; whereby, on solidification of the melt pool, at least some of the titanium nitride particles are embedded in a matrix formed from melt pool, thereby forming a wear resistant and galling resistant coating on the substrate.

2. The method defined in claim 1, wherein the substrate is a component of an autoclave.

3. The method defined in claim 2, wherein the autoclave component is an agitator.

4. The method defined in claim 1, wherein the corrosive and abrasive environment comprises autoclave processing conditions that extract valuable minerals from a mined ore.

5. The method defined in claim 4, wherein the corrosive and abrasive environment comprises autoclave processing conditions involving an elevated pressure in the range of 30 to 52 atm, temperatures in the range of 120 C. to 270 C. and acid addition to a slurry of ground ore and water of 200 to 500 kg/t of ore.

6. A method as defined in claim 1, wherein the method involves depositing one or more layers of the wear resistant coating on the substrate to build up the thickness of the wear resistant coating.

7. A method as defined in claim 1, wherein the method further comprises carrying out steps (a) and (b) while the substrate is exposed to the ambient atmosphere.

8. A method as defined in claim 1, wherein the method further comprises a step of pre-treating the substrate to remove contaminants.

9. A method as defined in claim 8, wherein the pre-treating step is carried out while the substrate is in contact with the ambient atmosphere.

10. A method as defined in claim 8, wherein the surface pretreatment step is selected to remove oxygen, iron and other contamination.

11. A method as defined in claim 8, wherein the pretreatment step involves removing a contaminated surface layer from the substrate.

12. A method as defined in claim 1, wherein the method further comprises controlling the temperature of the molten material to be between the melting temperature of titanium and the vaporisation temperature of titanium.

13. A method as defined in claim 1, wherein the method comprises controlling phases of titanium nitride formed upon solidification of the molten materials by controlling the time that the molten material remains molten.

14. A method as defined in claim 1, wherein the method includes preheating the substrate before steps (a) and (b) and maintaining the substrate temperature in a range of 50 C. to 150 C.

15. A method as defined in claim 1, wherein the method further comprises controlling conditions to form the wear resistant and galling resistant coating with a matrix of titanium nitride having a hardness in the range of 400 Hv to 550 Hv.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a cross-section showing the microstructure of a TiO.sub.2 wear resistant coating.

(3) FIG. 2 is a cross-section showing the microstructure of a TiN wear resistant coating.

(4) FIG. 3 is a cross-section showing an embodiment of a microstructure of a TiN wear resistant coating formed according to the invention.

(5) FIG. 4 is a perspective view of overlapping sections of the wear resistant coating in FIG. 3 formed on the surface of a titanium substrate.

(6) FIG. 5 is a schematic representation of an apparatus for forming a wear resistant coating.

DETAILED DESCRIPTION

(7) The description that follows is in the context of applying a wear resistant coating to a substrate of titanium alloy. It is important to appreciate, however, that the wear resistant coating may be applied to other materials that can be directly welded with titanium and other alloys by use of a suitable butter layer.

(8) An apparatus 1 for forming a wear resistant coating on a substrate 10 is shown in FIG. 5.

(9) The apparatus 1 comprises a spray nozzle 20 having an elongate body. The spray nozzle 20 includes a laser generator 22 that generates a laser 40. The laser generator 22 is aligned along a central longitudinal axis of the elongate body. A sleeve surrounds the laser generator 22 to form an annular feed flow chamber 24.

(10) The laser generator is linked to a power source 26 to generate the laser 40 with sufficient energy to melt small particles of titanium in the range of 20 to 170 m. The chamber 24 is linked via a conduit to a reservoir 28 of feed particles for forming the wear resistant coating. The reservoir 28 is supplied with argon gas from a gas source 30 to fluidize the particles and convey the entrained particles through the conduit and chamber 24 and then onto the substrate 10.

(11) The flow of particles and gas from the chamber 24 is controlled to converge from the annular opening surrounding the laser generator 22 in a flow stream (denoted by an arrow marked 50 in FIG. 5) that intersects the laser 40 at the surface of the substrate 10. Accordingly, the feed particles are subject to high temperatures at the surface of the substrate 10.

(12) The feed particles comprise a blend of titanium alloy particles and titanium nitride particles. The titanium nitride particles comprise 35 to 50 wt % of the blend. Both the titanium particles and the titanium nitride particles have a size in the range of 20 to 170 m.

(13) It will be appreciated that alternative configurations for supplying feed particles to the surface of the substrate 10 may be adopted. For example, the titanium alloy particles and the titanium nitride particles may be supplied from separate reservoirs and combined together in the chamber 24 so that a blend of feed particles is formed in the chamber 24 and is supplied as described above to the surface of the substrate 10.

(14) Alternatively, the blend of particles may be formed at the surface of the substrate 10 by supplying the titanium alloy particles and the titanium nitride particles through separate nozzles that direct the particles to the point on the surface of the substrate 10 that is irradiated by the laser.

(15) The applicant has observed that, although the laser melts the titanium alloy particles, the titanium nitride particles generally remain in a solid state and become embedded in the wear resistant coating by being surrounded in a matrix of titanium alloy dispersed with secondary titanium nitrides.

(16) The applicant has also observed that because the laser energy is selected to melt the titanium alloy particles only, a weld pool generated by the laser quenches so rapidly under the argon shield gas (powder gas) that oxygen is unable to react with the molten titanium. This results in a wear resistant coating that is generally free of oxygen.

(17) TABLE-US-00001 Substrate: Titanium grade 12 Substrate thickness: >25 mm Preheat: >50 C./<150 C. Ti particles: Amperit 155.093 Ti particle size/density: 90 to 125 m/4.51 g/cm.sup.3 Ti particle weight %: 58 TiN particles: Amperit K80 TiN particle size/density: 40 to 145 m/5.22 g/cm.sup.3 TiN particle weight %: 42 Substrate pre-cleaning: acetone wash Conveying gas and flow rate: Argon at 10 l/min Shielding gas & flow rate: Argon at 23 l/min Ti/TiN particle feed rate: 18 g/min Laser: Laserline LDF 6,000-100 Spot size: 8.5 mm

(18) An example of a microstructure for a wear resistant coating formed in accordance with these conditions is shown in FIG. 3. Discrete particles of titanium nitride are shown dispersed generally homogenously in a generally continuous matrix of titanium alloy with secondary precipitates of titanium nitrides. These secondary precipitates add both wear & galling resistance. The titanium alloy of the substrate is metallurgically bonded with the wear resistant coating. A wear resistant coating formed by a series of side-by-side laser passes is shown in FIG. 4. The feed rate of particles identified above produces a wear resistant coating thickness of 1.0 mm. However, it is possible with this process to build up the thickness of the coating by running subsequent laser passes and feed particles over already formed coating. In this manner, it is possible to build up the coating to any desired depth, but it is expected that thicknesses of up to 10 mm will be suitable for a wide variety of applications. For example, the wear resistant coating may be applied to agitator blades for autoclaves, diffuser cones, wear plates and valve components.

(19) In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word comprise or variations such as comprises or comprising is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

(20) It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.

(21) It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

(22) In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

(23) Furthermore, invention(s) have been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention(s). Also, the various embodiments described above may be implemented in conjunction with other embodiments, for example, aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments.