AU5848298A - Abrasion resistant surface coatings and method of forming same - Google Patents

Description

"ABRASION RESISTANT SURFACE COATINGS AND METHOD OF FORMING SAME" TECHNICAL FIELD

This invention relates to abrasion resistant surface coatings and a method of forming same.

This invention has particular but not exclusive application to abrasion resistant surface coatings for sugar mill rollers, and for illustrative purposes, reference will be made to such application. However, it is to be understood that this invention could be used in other applications where mill rollers or the like are improved by having abrasion resistant surface coatings, or where an abrasion resistant surface is required other surfaces . BACKGROUND ART In the sugar industry, sugar cane is crushed between rollers arranged in pairs or in groups of three. The rollers have a grooved surface with the grooves extending circumferentially about the roller. The grooved arrangement provides for gripping of the sugar cane fibre as it passes between the rollers, the groove of an upper roller being operatively aligned with the ridges on its opposing lower roller or rollers.

Sugar mill rollers are usually constructed as a cast roller mounted on bearings and turned by drive means. Wear resistant material has on occasion been provided on the caps of the ridges of grooved rollers, but only at intermittent locations around the circumference of the roller. This approach has been taken partly. It was believed that the intermittent application of the abrasion resistant material produced the best gripping surface for the milling of sugar cane.

Typically, conventional hardfacing matrix is used, including, for example, high speed steels, austenitic manganese steels, austenitic high chromium irons, cobalt or copper based alloys, or nickel-chromium-boron alloys according to the practices set forth in Jones, F D, & Horton, H L, Machinery's Handbook, ISBN 0-8311-1155-0. Abrasion resistant material is laid up in the matrix to form a hardfacing composite.

After a period of use, sugar mill rollers require refurbishment due to the hardfacing composite wearing smooth, causing slippage of the cane and/or bagasse between the rollers. Accordingly, in order to refurbish such a roller according to traditional technology, the entire roller is removed from the mill, broken up and recast into a new roller.

This approach is costly in terms of replacing the entire roller and requires a significant downtime for the sugar mill in having the roller replaced. There are also costs for heavy transport for removing and delivering such rollers to site.

The present invention aims to alleviate one or more of the above disadvantages and to provide a roller having a wear resistant surface and method of forming same which will be reliable and efficient in use. DISCLOSURE OF INVENTION With the foregoing in view, this invention in one aspect resides broadly in a hardfacing for a sugar mill roller, said hardfacing including a wear resistant material supported by a mild steel matrix and applied to the outer surface of the sugar mill roller. In a further aspect, this invention resides broadly in a method of hardfacing a sugar mill roller comprising the steps of forming a mild steel weld pool on the surface of said roller; impinging a particulate wear resistant material on said weld pool, and allowing said weld pool to solidify. In another aspect, this invention resides broadly in a method of applying a hardfacing capping to a ridge profile of a sugar mill roller comprising the steps of progressively laying up by welding a hardfacing composition comprising a mild steel matrix material and a particulate abrasion resistant material continuously impinged thereon.

Preferably, the ridge profile includes a base portion and one or more elongate projections extending away from the base portion, and one or more beads of hardfacing composition are applied to the base portion against the sides of the or each projection in successive layers until the height of the or each projection is substantially reached. It is further preferred that at least one further bead of hardfacing composite is applied to the or each respective projection to substantially encapsulate same.

It will be appreciated that the hardfacing may be applied to substrates other than sugar mill rollers.

The projection may comprise a plurality of tongues or tenons extending outwardly. Preferably, the projection is an elongate projection and is located interposed between two land areas formed on the roller ridges, each land area being of a width sufficient for receiving a molten hardfacing composition and said projection being of a thickness sufficient to support the building of successive layers of hardfacing composition on one land area against collapse onto the other land area due to erosion of the projection by the welding process.

In another aspect, the invention resides broadly in a method of forming a capping of hardfacing composite in the form a rib, wherein a ridge profile is provided having a base portion and one or more elongate projections extending away from the base portion in the form of a wall, the method including welding one or more beads of hardfacing composite to the base portion and against the sides of the or each projection in successive layers until the height of said wall is substantially reached. Preferably, one or more respective final beads of hardfacing composite are provided to substantially encapsulate the or all of the respective projections with hardfacing composite.

The method of applying the hardfacing composite may be metal inert gas (mig) welding a mild steel rod and injecting the wear resistant constituent into the molten weld pool prior to solidification of the weld. However, other methods of welding may be used, such as, for example, tungsten inert gas (tig) welding, metal arc welding, or such like.

In another aspect, this invention resides broadly in a roller assembly including:- a mounting shaft; a plurality of shell portions, each said shell portion being mountable to said mounting shaft and shaped for abutment together to form a roller cap; each said shell portion having an outer perimeter and an inner diameter of a dimension permitting tight engagement of said roller cap onto said mounting shaft, and a wear resistant surface applied to the outer perimeter. The roller caps may have a substantially smooth outer surface for receiving the hardfacing, but preferably, the roller caps include a plurality of grooves extending circumferentially about the outer perimeter. The roller assembly is preferably circular in section, although it will be appreciated that other sections may be utilised, such as elliptical or dog-boned interengageable sections. The wear resistant surface is preferably in the form of a wear resistant weld applied to the outer perimeter as hereinbefore described. Preferably, at least some of the ridges between the grooves include a cap portion with an abrasion resistant material on each cap portion.

The roller cap may include shell portions connectible to form an annular shell element, or be comprised of a unitary annular shell element, each shell element being abuttable to form the roller assembly such as for example, a sugar mill roller.

Preferably, the wear resistant surface is applied by welding a band of weld material containing hardfacing composite along each respective ridge between each successive groove. In sugar milling applications, the groove profile is selected to be similar in size and shape of cross-section to the groove profile of cast sugar mill rollers, that is generally between 15 mm and 80 mm pitch, but more commonly, either 25 mm, 37.5 mm or 50 mm pitch.

Preferably, the groove profile includes a ridge profile which includes an elongate projection and a land area on each side of the elongate projection such that successive layers of weld may be applied sequentially, allowing sufficient time between the respective layers for cooling of the weld. Suitably, the first layer of weld on each side of the projection is applied to the respective land areas. The elongate projection is of a suitable thickness to perform as a dam preventing the weld from falling onto the other land area. The combination of application of weld layers is adapted to produce a weld profile which when solidified is of substantially triangular cross-section.

The combination of welding torch angle, pre-machined roller tooth profile, rotational speed of the roller (weld surface speed), raw materials, preheat & specific welding consumables provides the finished profile for a sugar mill roller to be used in an as-welded condition. This is preferably done whilst the roller is rotated with its longitudinal axis in the horizontal plane. In another aspect, this invention resides broadly in a method of applying a hardfacing composite to a sugar roller, said method including rolling said roller at a circumferential speed suitable for the application of the hardfacing composite whereby said hardfacing composite is applied continuously around the entire circumference of said roller. Preferably, a plurality of beads of hardfacing composite are applied about the roller. It is also preferred that the beads of hardfacing are applied in parallel relationship along the distal edge portions of the ridges normally provided on sugar mill rollers. The roller preferably includes shell portions as hereinbefore defined.

The shell portions may be formed by machined solid metal, such as steel, rolling or casting. Where the shell portion is formed by rolling, the hardfacing composite may be applied before or after the forming.

Preferably, the wear resistant hardfacing is made by using ARM-2 weld as manufactured and supplied by Abrasion Resistant Materials Pty Ltd, at a suitable current, typically 200 to 260 amps, and a suitable potential, typically 22 to 26 volts in a mig atmosphere at a linear rate sufficient to coat the required net area required for the weld. This wire conforms to BS2901 Part 1 1970A18 and AWS A5.18 E70S-6.

The grit may be added into the molten weld pool during the welding process, by any suitable means, and is preferably to mesh size 12/35. It has been surprisingly found that abrasion resistant hardfacing material provided as described above provides a superior gripping surface for sugar cane milling. Moreover, the grip improves with use, and this is believed to be due at least in part to the acidic juices preferentially eroding away part of the matrix and providing a more pronounced profile of the hard tungsten carbide particles which grip onto the fibre mat upon contact. There is also believed to be a contribution to the erosion from frictional wear of the bagasse on the matrix. In a further preferred form suitable for use as a sugar mill cane crushing roller, the roller cap includes fixing means for fixing to the mounting shaft. For example, where the roller cap is in the form of arcuate segments, pairs of semi-circular sectional pieces or triplets each having segments with internal radii at 120°, and so forth may be provided. The segments are abuttable to form a roller end cap which is removable and replaceable on the mounting shaft to form the sugar cane roller. The mounting shaft is adapted suitably to receive the segments by provision of complementary fixing means operatively engagable with the fixing means on the respective segments.

The fixing means may for example include one or more flange portions disposed circumferentially about the internal arc of circumference and at each respective end of each roller cap. Each flange portion may further include an aperture extending therethrough in a coaxial direction and through which fastening means, such as a bolt, screw or such like may be passed to fasten the roller cap to the mounting shaft through corresponding apertures on the mounting shaft. Preferably, however, a sleeve with a flattened portion extending axially along its entire length is provided, the sleeve being adapted to fit on the shaft and receive the hard-faced shells which include a complementary internal land on their respective internal bores. The roller end caps are fixed, such as by bolting, onto the ends of the sleeve. In another aspect, this invention resides broadly in a method of forming a roller assembly including: - providing a mounting shaft; forming a plurality of shell portions; mounting each said shell portion to said mounting shaft in abutting relationship to form a roller cap, and applying a wear resistant surface to the outer perimeter of at least some of the shell portions.

Preferably, the wear resistant surface is applied to the shell portions prior to mounting on the mounting shaft. The shell portions may be raised in temperature relative to the temperature of the mounting shaft in order to provide tight engagement of the shell portions on the mounting shaft. For example, the shell portions may include an inner diameter of a dimension permitting tight engagement of the shell portion onto the mounting shaft by heating each said shell portion to a temperature sufficient to expand said shell for engagement about the shaft, whereby a plurality of said shells may be abutted along the shaft.

Where the shell portions are provided in arcuate portions, they are suitably connected circumferentially to form respective annular shells. In a preferred embodiment, each said shell portion includes a plurality of grooves extending circumferentially about the respective outer perimeter as hereinbefore described. BEST MODE FOR CARRYING OUT THE INVENTION In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention and wherein: -

Fig. 1 is an exploded view of a hard-faced roller assembly mounted onto a bearing shaft;

Fig. 2 is an end view of the roller assembly of Fig. 1; Fig. 3 is a section on A-A of Fig. 2 showing part of the roller assembly;

Fig. 4 is a sectional detail view of abrasion resistant material applied to caps of the grooves of the roller of Figs. 1 to 3;

Fig. 5 is a sectional detail view of an alternative application of the abrasion resistant material of Fig. 4;

Figs. 6 to 9 are sectional detail views of alternative preferred embodiments of ridge profiles for application of hardfacing composite;

Fig. 10 is a sectional detail view of the ridge wear profile of Fig. 6 after a typical part cycle of use, and Fig. 11 is a diagrammatic collective representation of self-roughening of the wear profile of Fig. 10.

Referring to Figs. 1, 2 and 3, a roller assembly 10 includes a plurality of roller shell sections, one of which is shown typically at 11 in Fig. 1, each mounted on a collar 18 which in turn is mounted on a shaft 12. In order to hold the roller shell sections 11 on the shaft 11 and sleeve 18, there are provided two respective end shell sections 13.

Each end shell section 13 and shell section 11 includes seven co-axial apertures shown typically at 15, the location of which are better displayed in Fig. 2, where the respective apertures 15 have respective bolts inserted, as shown typically at 14. Each end of the sleeve 18 includes respective blind apertures shown typically at 17 which are threaded for receipt of the respective bolts 14.

Each shell section 11 and end shell section 13 also includes a central bore 16 through which the shaft 12 is passed. The shell sections 11 include a land 19 which engages with a complementary key 20 on the sleeve 18 to prevent the shell sections 11 rotating on the sleeve 18.

Referring to Fig. 3, each end shell section 13 and shell section 11 includes a plurality of ridges shown typically at 21 and interposed therebetween, a plurality of grooves shown typically at 22 extending circumferentially about their respective outer perimeters.

Referring to Fig. 4, a portion of an end or centre shell section 13 or 14 is shown in sectional view. Each ridge 21 includes a ridge cap portion 31 upon which a plurality of abrasion resistant welds 33 are provided to provide abrasion weld cap 32. In this embodiment, the grooves 22 are not capped with weld material.

Referring to Fig. 5, the ridges 21 and grooves 22 are provided with a plurality of abrasion resistant welds 33 in a similar fashion to those provided on the abrasion resistant weld cap 32 shown in Fig. 4. However, the entire surface of the ridges 21 and grooves 22 are provided with the abrasion resistant welds 33 to form an abrasion resistant weld coating 34.

Referring to Figs. 6 to 9, a ridge profile 23 is of a variety of shapes as shown. In Figs. 6 to 9, weld material is represented by a triangle and dot fill pattern. The following combinations regarding welding process are fundamental to weld profile creation so that when the molten weld solidifies the final weld shape is of substantially triangular cross-section.

The combination of welding torch angle, pre-machined roller tooth profile, rotational speed of the roller (weld surface speed), raw materials, preheat & specific welding consumables provides the finished profile for a sugar mill roller to be used in an as-welded condition. This is preferably done whilst the roller is rotated with its longitudinal axis in the horizontal plane. The shape shown in Figs. 4 and 5 are very difficult to form any other way with inert-gas metal-arc (Mig) carbide hardfacing except by elevating the roller from the horizontal and inclining it on its longitudinal axis.

It will be appreciated that the preferred pre-machined shapes may vary from those shown in Figs. 6 to 9. It is preferred that the ridge profiles 23 are substantially symmetrical, however, it will be appreciated that in some applications, where a non-symmetrical weld profile is required or desired, a non-symmetrical ridge profile may be appropriate. However, the ridge profile 23 includes a base portion 24 and one or more projections 25. The ridge profile 23 shown in Fig. 7 is the preferred option for general applications, however, ridge profile 23 shown in Fig. 6 is preferred for larger pitch rollers. In each case, weld layers 1 to 9 are applied successively as required to the ridge profile 23. Another way of describing the ridge profiles 23 in Figs. 6 to 9 is as a base portion 24 having one or more stepped or sloped reductions in cross-section.

It is believed that tungsten carbide dispersal throughout the matrix and multiple pass layer weld adhesion both to the roller and to each layer give an advantage to each of the ridge profiles 23 shown in Figs. 6 to 9.

Clearly, the dimensions could be proportioned up in scale to accommodate larger profiles of larger rollers. The larger pitch rollers would also need the same profiles machined in the teeth to achieve similar results. The ridge profile 23 in Fig. 6 is a minimum nine weld pass profile; the ridge profile 23 in Fig. 7 is a minimum seven weld pass profile, and the respective ridge profiles 23 in Figs. 8 and 9 are for a minimum seven or nine weld pass profile, or a minimum five weld pass profile respectively.

Although the successive beads of weld are generally applied in numerical order as given in Fig. 7, the order may be changed to 1, 4, 2, 5, 3, 6 and then 7. A similar method may be applied to Fig. 8 with successive beads of weld applied on alternate sides of the projection 25.

The ridge profiles 23 described above have been applied to a standard grey cast iron roller, in spite of the expected difficulties in the relatively high carbon content of the cast iron affecting the re-welded profile by diffusion of carbon into the weld. Referring to Fig. 10, the ridge profile 23 has worn down to a worn surface shown at 26, with more of the base portion 24 wearing than the cap portion 27. An undercut portion 28 has been found to form in experimental trials of the invention on milling of sugar cane after milling about one million tonnes of cane.

Indeed, it has also been surprisingly found that the wear resistant coating of the present invention increases its grip on the bagasse, since it appears that the substrate in which the carbide is embedded partially wears away from the surface of the cap portion 27, exposing the angular grits of carbide material for gripping the bagasse.

Accordingly, as shown in Fig. 11, a sugar mill roller portion 40 has a ridge profile shown generally at 41. A hardfacing composition of the present invention is applied to the ridge tops shown typically at 42. A magnified portion 45 is shown in separate views at 51, 52, 53 and 54. magnified portion includes a surface 46 of mild steel embedded with wear resistant tungsten carbide grits, some of which are exposed and shown typically at 47. As the surface of the hardfacing wears, some of the grits 47 are broken away from the matrix, the original surface being shown in dotted outline at 48 in views 52, 53 and 54. The surface 46 wears or erodes away to expose new grits as shown at 50 in views 53 and 54.

In order that this invention may be more readily understood and put into practical effect, reference will now be made to an example of the invention having the welding process specifications as follows:

The rotating speed of roller to apply weld layers was in the range of from 21.7 mm to 22.3 mm per second for the butter layer weld and hardfacing layers. The rotating speed of roller to apply final capping weld was 19.35 mm to 20.02 mm per second. The welding torch angle was 18° from the vertical when applied to the uppermost (that is, horizontal) perimeter of the roller. This can also be varied up to 25° to weld the bottom layers in pre-machined ridge profile 23 as shown in Fig. 6.

The current was in a range of from 200 A to 300 A and the voltage from 22 V to 26 V on ARM - 1 and 2 wires. These wires are 0.9 mm wires which conform to BS2901 Part 1 1970A18 and AWS A5.18 E70S-6. The preheat temperature of the grey cast iron rollers was 100 °C ± 5°C prior to welding. ARM 1 wire is mild steel and ARM 2 wire is 309L stainless steel.

A further technique may be utilised where ARM - 2 wire is used as a butter layer without ARM STD 1 tungsten carbide grit in the weld. Then ARM - 1 wire with ARM STD 1 tungsten carbide grit is immediately welded onto the first butter layer weld bead and dilution of the first butter layer by the second layer creates a unique matrix and weld metallurgy. This affects weld bead shape and cooling characteristics and appear to affect the solidification (as opposed to melting) of the tungsten carbide in the weld pool. This produces a superior bond to grey cast iron rollers and in particular reduces the problem of longitudinal underbead cracking in the heat affected zone.

In use, the abrasion resistant surface of this invention may be applied to a sugar mill roller. Although it will be appreciated that the hardfacing composite may be applied to a solid roller in accordance with the present invention, instead of replacing the entire sugar mill roller when it wears out, the roller caps may be unbolted from a roller drum and replaced by replacement cylindrical portions. The worn out roller caps may be recycled by separating the hardfacing composites and the substrate according to any known process.

On occasions, one or more shell sections of a roller may wear more than the remainder. In such a case, only the shell sections so worn need be replaced, reducing the cost of maintaining the roller.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as claimed in the following claims.

Claims (14)

ΓÇö CLAIMS --

1. A hardfacing for a sugar mill roller, said hardfacing including a wear resistant material supported by a mild steel matrix and applied to the outer surface of the sugar mill roller.

2. A hardfacing as claimed in claim 1, wherein said hardfacing is applied in the form of a weld.

3. A hardfacing as claimed in claim 2, wherein said weld includes a plurality of successive layers of weld laid upon one another in succession to produce a weld profile of substantially triangular cross-section.

4. A hardfacing as claimed in any one of the preceding claims, wherein said sugar mill roller includes a ridge profile having a base portion and one or more elongate projections extending away from said base portion, and said hardfacing comprises one or more beads of hardfacing composition substantially to the height of the or each said projection.

5. A hardfacing as claimed in claim 4, wherein said hardfacing composite substantially encapsulates the or each respective said projection.

6. A method of hardfacing a sugar mill roller comprising the steps of: forming a mild steel weld pool on the surface of said roller; impinging a particulate wear resistant material on said weld pool, and allowing said weld pool to solidify.

7. A method of applying a hardfacing capping to a ridge profile of a sugar mill roller comprising the steps of progressively laying up by welding a hardfacing composition comprising a mild steel matrix material and a particulate abrasion resistant material continuously impinged thereon.

8. A method as claimed in claim 6 or claim 7, wherein said roller includes a ridge profile having a base portion and one or more elongate projections extending away from said base portion, and one or more beads of hardfacing composition are applied to said base portion against the sides of the or each said projection in successive layers until the height of the or each said projection is substantially reached.

9. A method as claimed in claim 8 , wherein at least one further bead of said hardfacing composite is applied to the or each respective said projection to substantially encapsulate same.

10. A method of forming a capping of hardfacing composite in the form a rib, wherein a ridge profile is provided having a base portion and one or more elongate projections extending away from the base portion in the form of a wall, the method including welding one or more beads of hardfacing composite to said base portion and against the sides of the or each said projection in successive layers until the height of said wall is substantially reached.

11. A method as claimed in claim 10, wherein one or more respective final beads of hardfacing composite are provided to substantially encapsulate the or all of the respective said projections with said hardfacing composite.

12. A roller assembly including: a mounting shaft; a plurality of shell portions, each said shell portion being mountable to said mounting shaft and shaped for abutment together to form a roller cap; each said shell portion having an outer perimeter and an inner diameter of a dimension permitting tight engagement of said roller cap onto said mounting shaft, and a wear resistant surface applied to the outer perimeter.

13. A roller assembly as claimed in claim 12, wherein said wear resistant surface is in the form of a hardfacing composite as claimed in claim 1.

14. A roller assembly as claimed in claim 12 or claim 13, wherein said roller cap includes respective shell portions connectible to form an annular shell element.

15 A method of forming a roller assembly including: - providing a mounting shaft; forming a plurality of shell portions; mounting each said shell portion to said mounting shaft in abutting relationship to form a roller cap, and applying a wear resistant surface to the outer perimeter of at least some of said shell portions.