AU4299193A - Hard facing - Google Patents

Description

Title; "HARD FACING"

FIELD OF THE INVENTION

THIS INVENTION relates to hard facing of working surfaces comprised of materials such as metals to provide wear resistant properties to the working surfaces.

BACKGROUND TO THE INVENTION

Various techniques exist whereby surface coatings may be applied to metals. Coatings might be applied, for example, by vapour deposition; plasma spray apparatus; and lasers, which lasers are used to melt the surface of a metal to a certain depth, with materials added to the laser formed melt to form hardened coatings of particular compositions. When materials are added to melt pools it is known to add particles which have a desired wear characteristic which become immersed in the melt and bonded by it when the melt cools. It is known to add tungsten carbide to a melt formed in a steel substrate. Whilst the particles add to the wear resistant properties of the surface, the matrix which binds it, being base material is subject to wear and its degradation can lead to the particles being lost.

In one prior art system tungsten carbide is fed into a MIG generated melt pool. Problems can and do occur with this system. ^'A poor interface can occur between the hard facing and the parent material. Carbide particles added by the MIG approach (using Metal Inert Gas welding electrodes) tend to fall to the base of the weld pool giving an uneven distribution and having an adverse affect on the interface with the parent material to which the hard facing is being applied.

OBJECT OF THE INVENTION

It is a primary object of the invention to provide a hard facing comprised of particulate wear resistant material in a matrix of substrate material which contains therein a means sufficient to support the matrix in use of the hard facing. Other objects and various advantages of the invention will hereinafter become apparent.

NATURE OF THE INVENTION

The invention achieves it objects firstly in a work piece comprising: a base material with a layer of hard facing applied thereto; the layer of hard facing having particles of wear resistant particulate material bonded therein in a matrix of base material; characterized in that: said particulate material is bonded in said matrix with recrystallized particulate material therein following a degree of dissolution of the particulate in a melt pool in the base material.

Preferably, the hard facing comprises tungsten carbide particles bound in a matrix of parent material, the tungsten carbide particles being derived from fused tungsten carbide added to a melt pool formed in the parent material with recrystallised tungsten carbide existing in the matrix after a degree of dissolution and recrystallisation.

Secondly, the invention provides a novel method for hard facing surfaces with a particulate material added to a melt pool formed in the surface to be hard faced characterised in that the method comprises: creating a melt pool in an area of the surface which is to be hard faced using the arc of a TIG electrode; introducing particulate material at a determined rate into the melt pool; and moving the TIG electrode over the surface to progressively scan the melt pool across that area of the surface which is to be hard faced as particulate material is added to the melt pool. Thirdly, the invention provides an apparatus for hard facing surfaces comprising: means for forming a melt pool in a surface to be hard faced; and a metering means whereby particulate material can be introduced into the melt pool at a determined rate; characterised in that the apparatus comprises: a TIG electrode placed, in use, in operative disposition relative to said surface whereby to create the melt pool therein; a means to move the TIG electrode to scan the TIG electrode over said surface; and a means to power the TIG electrode which creates, in use, a molten pool in said surface to which the metering means supplies particulate.

The present proposal for hard facing is made more effective when the equipment to establish the melt pool is preferably fitted with a magnetic means whereby to remove any magnetic particles which might otherwise be attracted by a TIG electrode because of the magnetic properties of the arc established by the electrode.

Magnetic attraction of contaminants in the particle feed to the melt pool has been found to be the mechanism whereby a TIG electrode is rendered ineffective over a short period of time. The particulate material which is used to effect a hard facing might be cleaned of magnetic contaminates during its preparation, prior to use in the present process, or by a separator incorporated into the apparatus used to perform the present process. The apparatus is operated with particles such as fused tungsten carbide. The cobalt contained in sintered tungsten carbide makes presently available sintered carbide unusable. In a particular preferred form of the invention the coating application is carried out in a controlled manner which enables monitoring of the surface treatment process and so as to enable adjustment of the electrode amperage and particle feed rates to maintain a desired degree of hard facing, preferably using robotic means.

Two basic forms of tungsten carbide exist, fused and sintered. Sintered tungsten carbide comprises tungsten carbide particles bonded to establish grains of tungsten carbide at a useful size, the bonding material being cobalt. Fused tungsten carbide powder is formed by adding carbon to molten tungsten pouring it into a mould and then crushing it into useful size grains. In other applications of tungsten carbide hard facings, where proprietary products are used, the ratio of tungsten to iron can be 6:4 or 41% of the volume. The carbide in these systems is usually finer than what is used herein and it is prone to dissolution in the iron when applied by an electric arc system using MIG techniques. The tungsten iron alloy which is produced that way is inferior as a wear resistant surface.

The tungsten iron alloy which is produced using MIG approaches is inferior to what is produced in performance of the present invention. In performance of the present invention, dissolution of the particulate material is controlled, being minimised subject to the need to create a tungsten alloy matrix which is reinforced with re-deposited tungsten carbide. The tungsten carbide can be applied with a TIG system with less dissolution than with a MIG system.

Until now, all the tungsten carbides for TIG application have been supplied in a metal tube so that there is no free tungsten carbide that could contaminate the electrode. By the time the carbide is released from the tube, any cobalt associated with sintered carbide should be above the Curie temperature. At that point, it is no longer magnetic.

It can be important to maintain optimum conditions in the arc area. The degree of dissolution can be controlled and so to can the concentration of the redeposited tungsten carbide. In continuous practice, the temperature of the parent material can increase and this requires the adjustment of the current to the electrode producing the arc that generates the melt pool. Surface appearance at the pool gives a control factor allowing the current to be adjusted and this might be done manually by an operator monitoring the pool. However, this adjustment of the current to compensate for the temperature increase of the parent material can also be achieved automatically by using an infra red sensor focused on an area just ahead of the weld area. The infra red sensor indicates the temperature of the piece being treated and it is not affected by the heat or ultra violet rays from the arc of the welder. Having established the temperature it is an easy matter to convert this into the parameter that controls the welding current.

The use of automated control equipment enables both the amount of carbide added per sq cm to be controlled together with the electrode amperage which establishes the depth of the melt pool to which the particles are added. The condition of the metal surface could be monitored by a closed circuit television system and the amperage required to produce the desired hard facing could be controlled manually. However, control is preferably effected automatically by computer or processor control. Parameters for a desired coating could be preset and hard facing can progress automatically. A visual monitoring system can be used to enable supervision to ensure that the process is proceeding successfully.

Because of the particular conditions needed to activate the arc at the TIG electrode, it is simplest to maintain the arc over any flat intervening area not requiring hard facing if different hard facing positions are required. This can be achieved by programming the robot controlling the electrode so that at the end of a hard facing area the welding head moves at greater speed, for example, 100 mm/sec, to the next area for hard facing. At greater speeds such as this, very little heat is transferred to the parent material in maintaining the arc. Concurrently control can be exercised by variation of the amperage levels at the electrode. Reduced power reduces heat levels.

The above controls and conditions allow fused tungsten carbide (the hardest form of tungsten carbide particles) to be fed into a molten pool of metal with a concentration of carbide particles up to 80% in the hard facing area. The parent material can be any metal, alloy or composition, provided it is sufficiently conductive to support the TIG operation. Control or setting of desired parameters is particularly important where stainless steels are hard faced, to be used in either of or both of corrosive and erosive applications. Broadly the amount of carbide added will control the thickness of the hard facing which will influence the amperage needed for sufficient melt to bond the particles. In any hard facing system, there are a number of significant factors in achieving a good result. These include the need to achieve an excellent bond between the hard facing layer and the parent material. In the two photographs herewith, FIGS. 3 and 4, FIG. 3 shows a satisfactory sharp interface and FIG. 4 shows an unacceptable more diffuse interface between a layer of tungsten carbide added to a melt and the parent material underneath. Hard facing with control of particle size and heat input produces a tungsten alloy matrix with redeposited tungsten carbide. The photograph of FIG. 5 shows reformed carbide in the matrix mainly concentrated around the tungsten carbide particle. Also the process of applying a material to establish an abrasion resistant surface must be able to be controlled to enable selection of particular wear resistant properties. The thickness of the hard facing will be influenced by the intended application of the work piece.

The fact that a non-magnetic material such as fused tungsten carbide can be used successfully with a TIG operation (Tungsten Inert Gas) provides the means of designing simple, readily operated TIG based equipment using relatively standard TIG style electrodes. Such equipment, together with suitable controls, enables production of an ideal hard facing system to which robotic control is readily applied, the robotic control comprising use of program controlled means for moving a TIG electrode as required, as by a robot arm, relative to a work piece. The above described process is successful because the matrix of the hard facing is derived from the parent material and the tungsten carbide is evenly distributed therein. The added tungsten carbide particles form a concentrated layer of excellent wear resisting material in a thickness that can be controlled to suit the specific wear problem.

The surface of the weld pool indicates performance of the hard facing. Exposed particles of tungsten carbide can indicate insufficient heat to bond the particles together and these can be lost in use. A smooth surface indicates that the tungsten carbide has been dissolved into the matrix resulting in a softer and less wear resistant hard facing. The preferred size of particle is - 1.18 +1.0 mm, with up to 10% of - 1.2 + 1.18 permissable. Less than 1 mm can create a higher rate of dissolution which lessens the ability of the hard facing to resist wear. Particles of 420 microns and lower can be forced away from the melt area by the force generated by the arc. Of the two types of tungsten carbide, only fused carbide can be used in the present system. Fused carbide has a hardness from 2000 HV to 2400 HV whereas sintered carbide has a hardness of 1100 HV to 1300 HV.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to various preferred embodiments which are illustrated in the accompanying drawings, in which:

FIGS. 1 and 2 illustrate an embodiment of the invention; FIGS. 3 to 5 are photomicrographs showing sections through the interface between layers of hard facing material and substrates;

FIG. 6 is a detailed schematic showing how the electrode might be adapted to enable striking an arc in equipment in accordance with the invention as set out in FIGS. 1 to 3;

FIG. 7 is a view showing the components of a complete system in accordance with the invention; and

FIGS. 8 to 11 are details regarding initiation techniques. PREFERRED EMBODIMENTS

In a preferred form, the apparatus of the present invention comprises the following elements; a feeder that can be adjusted by a controller to give controlled rates of feed of particles such as fused tungsten carbide to the weld pool; a magnetic separator to eliminate any particles that could be influenced by a magnetic field so as to be attracted to the electrode, stopping them from entering the TIG area and saving the electrode from fouling; a standard type, water cooled argon supplied TIG torch; a dispersion unit that directs the tungsten carbide particles to centre of the weld pool; a robot arm that can control the movement of the TIG torch together with a controller, the controller acting to set parameters such as amperage, carbide feed rate, robot arm oscillation amplitude (which sweeps the electrode over a surface) , speed of the oscillation and forward travel speed; support means for the work piece and a closed circuit television system that enables an operator to monitor the condition of the weld pool so that corrections for the amperage can be made if necessary.

The TIG electrode can be traversed over the surface of a work piece which is to receive a hard facing in any pattern of movement which produces a good coverage of the surface. A simple line or zig zag pattern arising out of a combined sweep and forward movement is suitable.

For the most effective hard facing operation the system can be monitored by observing the condition of the surface of the weld pool. An observer is then able to effect any correction which might be required. Depending on the thickness of the parent material there is the need to change the amperage because the build up of heat from the weld requires less heat input to achieve the desired melt as the thickness of the work piece decreases. With control of heat input it is possible to apply a hard facing material to thin plate down to 3 mm thick with an ultra thin concentrated coating of long wearing tungsten carbide material or on the edge of a 1.5 mm piece of steel.

The appearance of the surface can be used to determine the level of amperage to be used. Insufficient amperage does not produce a sufficiently deep molten pool of metal and the carbide particles that sit on the steel can be seen to glow. If the amperage is correct and the pool depth is adequate, the particles sink into the pool and fill the pool. The particles on top are covered by a meniscus of molten metal. If the amperage is too high, the excessive heat causes the steel to dissolve the tungsten carbide and the surface becomes smooth. A reasonably large size particle is required for feeding to the melt pool, otherwise the particles are too light and they can be pushed by the TIG discharge to affect the distribution of particles in the hard facing.

In FIG. 1 a hopper 10 with sloped side walls 11 is mounted with its lower end open onto a metering screw in a suitable housing therebeneath to feed material to a collecting bowl 14 whose walls 13 cause material to flow into line 15. The metering screw can be powered by a chain 17 driven by variable speed motor 12. The hopper 10 feeds to the metering screw which is powered by chain 17 about gears 18 and 19 with motor 12 connected to gear 18 via a gear box 20. The apparatus is mounted on a suitable support 16. It will be clear to those skilled in the art that various means for metering material from a hopper such as 10, or other suitable supply, can be provided, it being desired to control the amount of material being flowed from the supply to the TIG apparatus as will become more clear below. A continuous weighing cell might be particularly useful.

In FIG. 1, the line 15 feeds the material from hopper 10 over a magnetic particle trap 21 into line 22. Magnetic particle trap 21 is provided to screen magnetic particles out of the supply stream in line 22 to avoid contamination of the TIG torch 23. The magnetic particle trap can consist of a bed of permanent magnets and it can be any of the known devices created for such an operation. The TIG torch 23 is mounted on a robot arm 25, or other suitable means whereby it can be moved over a surface to be treated such as 29 to which hard face 30 is being applied. Alternately, the metal piece 29 might be moved beneath the TIG torch. The robot arm engages the TIG torch 23 with a support block 26 attached thereto a feeder 27 which delivers material into a melt pool produced by tip 28 in surface 29. The supply lines 24 to the welder are suitably supported to permit movement of the torch by the robot arm 25. The tip of the TIG electrode is a point. The discharge it produces in use has a substantial foot print on a work piece and a useful sized melt pool is readily created.

The particulate feed to the TIG electrode area could be pretreated to separate magnetic particles or it can be caused to flow or pass over magnets within the equipment which will collect any particles that would be attracted in the magnetic field produced around the electrode to deposit on the electrode during welding. In FIG. 2 is seen a detail of the feeder 27 comprising a tube 35 provided with baffles such as 32 thereacross to slow down the flow of material through the feeder 27 whose flow 33 slides down chute 34 into the melt zone beneath electrode 28. The effectiveness of the hard facing system can be affected by the stream of tungsten carbide entering the molten pool. It is desirable that it is kept to a minimum area of spread and that it is directed to a determined position of the pool. A system of feeding has been developed using the above described unit for counteracting the effect of kinetic energy with an inclined chute to discharge the carbide into the weld pool. This chute must be inclined at an angle to prevent build up of material in the chute. At this angle the tungsten carbide is delivered in a spray pattern which may be unsatisfactory. By applying a vibrator 36 to the feeding leg, the angle of the chute can be altered so that the carbide is fed in a concentrated stream and can be accurately positioned to feed into the molten pool.

In the process of fusing the tungsten carbide into the parent metal, it is necessary to ignite the electric arc. There are various options by which to do this, for example:- (1) The electrode can touch the parent material and then be withdrawn to the operating distance with the power supply on (FIG. 6);

(2) The arc can be ignited at the operating distance by an imposed high frequency AC signal. However, the use of the high frequency AC to establish the discharge can be detrimental to any computer controlled equipment. Although a shielding can be provided, high frequency discharge is not recommended when used in conjunction with robots. Therefore, in the present process, it is preferable to use options, such as (1) above. With reference to FIG. 6, this is achieved by the robot arm 25 having electrode 28 advanced to touch the parent material, applying a 50 gram load via the compression spring 37, and then quickly returning to the operating level. This procedure ignites the arc. In FIG. 6, electrode holder 38 is screwed to sleeve 39 which supports compression spring 37 above retention ring 42 on the sleeve 39. The compression ring 40 is engaged by the robot 25 and its downward movement to press electrode 28 onto a work piece compresses spring.

Damage to the electrode tip is repaired by resharpening or fitting a spare electrode. This represents a serious interruption where repetitive work is planned. The point can be protected by programming a space between the electrode point and the material to be treated and providing a flexible material such as steel wool to act as the conductor. This requires that the material be placed in a strategic position for every weld start. A system has been devised where a piece of typically .005 brass shim is formed into a trough 44 with arms 45 and 46 and fitted to the electrode 28 (see FIGS. 8 to 11). FIG. 10 is a side view of the electrode of FIG. 8 with a shim as seen in FIG. 9 mounted thereto. FIG. 11 is a view of the electrode seen from the front. When the brass 44 makes contact with the material to be welded the arc is created and the brass is melted and destroyed. These brass igniting units could be stored in a magazine rack and the robot can be programmed to select one of igniting units prior to commencing a weld. This procedure will provide an efficient arc imitating system for continuous robotic applications. Alternately, it is possible to use a brass shim bent into a U-section 47 (see FIG. 12) and to program the robot to contact the electrode 28 to the brass of this little brass spring leaf on the work piece 29. This minimises the pressure on the electrode point to give a fairly long use from the resharpened point. However, it means placing the brass in an exact position and because of its lightweight, it can be moved out of the strike area by the carbide feed.

With the above proposed methods, the electrode point will never come into contact with any part of the system and therefore longer life can be expected. FIG. 7 shows an elevation of apparatus as used in the present invention. A support frame 1 is shown holding a particulate hopper above an electrode assembly on a robot 5. A closed circuit television monitor 2 and computer monitor 3 are mounted with a controller and computer unit 4. A table 6 supports a part being hard faced with a television camera 7 mounted thereover. The TIG electrode equipment 8 completes the apparatus.

In the above described hard facing, there are two basic ingredients. The tungsten carbide particle and the matrix. The quality of the tungsten carbide can vary but generally the quality of the supplied carbide appears consistent. The properties of fused tungsten carbide depends on physical structure. A high degree of martensite-type 'feather' structure adds to hardness. The matrix can be controlled by varying the degree of dissolution of the tungsten carbide in the parent material. This dissolution can be effected by the particle size of the carbide and the heat input. Because it is important to have both tungsten carbide particles and dissolved carbide, it is necessary to consider the particle size distribution. Because of the pressures of the arc, there is a limit on the particle size that can be used. The fine particles are selectively blown away from the arc area. To overcome this problem, fine tungsten carbide particles, 2-6 mu are desirable for maximum dissolution and these can be granulated either in an organic or inorganic substance provided it is not attracted to a magnetic field.

The TIG method as described herein will facilitate the development of a hard facing system with up to 80% tungsten carbide particles in a matrix that is reinforced with reformed carbides. These carbides are tougher and more wear resistant than normal tungsten carbides. This process allows the application of a system high in tungsten carbide content, to odd shapes, and surfaces which would approach the long wearing properties of tungsten carbide tiles but without the application problems and fragile nature of the tiles. It is essential that the arc is started on a clean surface. The presence of carbide particles cannot be tolerated. Whilst the fused carbide is not attracted to the electrode, it will weld onto the electrode if there is contact at the arcing point. Therefore, the procedure is to make the arc away from the starting point of the hard facing weld. When the arc is made, the electrode then proceeds towards the starting point and at some point the computer or controller brings in the carbide feed. The timing is arranged such that the electrode has a clean start but carbide is available at the beginning of the hard facing run.

Because of its simplicity and effectiveness this process can be applied to a range of applications such as mower blades, cane harvester blades, drill pipes, and ultra thin wear plate down to 3 mm thick. However, the technique is particularly useful to produce a hard facing mainly for ground engaging tools in all types of mining, earth moving, etc. Tests conducted on agricultural tines show considerably reduced wear rates when hardened by the above process.

Claims (17)

1. A work piece comprising: a base material with a layer of hard facing applied thereto; the layer of hard facing having particles of wear resistant particulate material bonded therein in a matrix of base material; characterized in that: said particulate material is bonded in said matrix with recrystallized particulate material therein following a degree of dissolution of the particulate in a melt pool in the base material.

2. A work piece as claimed in Claim 1 wherein: said base material is a steel or other conductive material, said particulate material is fused tungsten carbide and the melt pool is established by the arc of a TIG electrode.

3. A work piece as claimed in Claim 2 wherein: the layer of hard facing is applied to a predetermined area of the work piece by a process scanning the TIG electrode over the predetermined area with particulate added to the melt pool as the TIG electrode progresses over the surface.

4. A work piece as claimed in any one of Claims 1 to 3 wherein: the hard facing is first applied to plate or ribbon material and sections of said plate or ribbon material are bonded to the work piece as required.

5. A method for hard facing surfaces with a particulate material added to a melt pool formed in the surface to be hard faced characterised in that the method comprises: creating a melt pool in an area of the surface which is to be hard faced using the arc of a TIG electrode; introducing particulate material at a determined rate into the melt pool; and moving the TIG electrode over the surface to progressively scan the melt pool across that area of the surface which is to be hard faced as particulate material is added to the melt pool.

6. A method for hard facing surfaces as claimed in Claim 5 wherein: the degree of surface irregularity of the surface of the melt pool is observed to monitor the rate of addition of particulate material thereto with amperage to the TIG electrode reduced if the particulate material dissolves to leave a smooth surface.

7. A method for hard facing surfaces as claimed in Claim 5 wherein: the amperage to the electrode is increased if surface roughness of the melt pool indicates too little movement of particulate into the melt pool.

8. A method for hard facing surfaces as claimed in Claim 5 wherein: the surface of plate or ribbon material is hard faced and portions thereof are bonded to a work piece.

9. A method for hard facing surfaces as claimed in Claim 5 wherein: the amperage to the electrode or the electrode speed over the surface is varied to reduce heating between areas to be hard faced.

10. A method for hard facing surfaces as claimed in any one of Claims 5 to 9 wherein the added particulate is fused tungsten carbide which is treated to remove magnetic particles therefrom to avoid contamination of the TIG electrode.

11. An apparatus for hard facing surfaces comprising: means for forming a melt pool in a surface to be hard faced; and a metering means whereby particulate material can be introduced into the melt pool at a determined rate; characterised in that the apparatus comprises: a TIG electrode placed, in use, in operative disposition relative to said surface whereby to create the melt pool therein; a means to move the TIG electrode to scan the TIG electrode over said surface; and a means to power the TIG electrode which creates, in use, a molten pool in said surface to which the metering means supplies particulate.

12. An apparatus for hard facing surfaces as claimed in Claim 11 wherein: the TIG electrode is scanned under robot control in a continuous line or zig zag manner over the area to be hard faced.

13. An apparatus for hard facing surfaces as claimed in Claim 11 wherein: there is provided a magnetic trap by which magnetic particles in the particulate material are removed before delivery to the melt pool.

14. An apparatus as claimed in any one of Claims 11 to 13 wherein: the rate of travel of the electrode is controlled to maintain the melt pool in areas to be hard faced and increased when traversing areas between areas to be hard faced to maintain the arc.

15. An apparatus as claimed in any one of Claims 11 to 13 wherein the amperage of the TIG electrode is controlled and reduced if the surface irregularity of the melt pool indicates too high a rate of dissolution indicated by the degree of smoothness.

16. An apparatus as claimed in any one of Claims 11 to 13 wherein the amperage of the TIG electrode is controlled and increased if the surface irregularity of the melt pool indicates too low a rate of dissolution indicated by the degree of roughness.

17. An apparatus as claimed in any one of Claims 11 to 13 wherein: the particulate is delivered to the melt pool by a chute with a vibrator attached thereto to vibrate the chute during use.