FORTY-FIRST CONFERENCE PERFORMANCE OF TUNGSTEN CARBIDE TIPPED DETACHABLE WEAR PADS FOR SHREDDER HAMMERS BY E. L. HORNIBLOW and F. JOHNSON Cattle Creek Co-operative Sugar Milling Association Limited Introduction Original development in providing wear protection to shredder hammers started in 1964. In 1966, Shawn and McWhinney reported hammer tests including some results on tungsten carbide inserts. Experimentation was confined to small hammers up to 31.75 mm wide, and protection was provided by thin sections of cemented carbide some 3.2 mm thick. The incidence of failure was high, due to shattering of the tips through impact with stones. While some minor success was achieved, the manufacturer terminated experiments in face of overwhelming evidence reported by Jacklin (1972). This report supported the use of electro casting chromium carbide onto hammers contained in a water cooled mould. Between the years 1965 and 1971, a considerable improvement had been made in the methods of harvesting cane. The level of entrained tramp material had significantly decreased in most areas. This improvement provided a climate wherein the re-introduction of tungsten carbide-faced hammer experimentation appeared to be justified. Reasons for Using Replaceable Wear Pads The project was revised in the light of experience gained during original tests and from field information supplied by engineers in the Mackay district. Merit was seen in providing a replaceable/disposable head, faced with tungsten carbide, and proto-types were placed in service at Pleystowe mill. The reasons for the change from solid hammer to replaceable wear pads are: It was seen that by brazing (silver soldering) carbide pieces to hammer bodies, operational difficulties were bound to occur even where the best re'suits were obtained. The hammers would need to be reconditioned by the manufacturer. This would entail transportation over 620 to 650 kilometres each season. Where odd failures occurred, the hammer face would wear and require major reconditioning to accommodate further pieces of carbide or, alternatively, be sent to scrap. Original tests used thin sections of carbide, which were not shock. resistant. This method was obviously used to minimize the mass of carbide employed, with a view to reducing initial cost. The result was a wear piece most likely to shatter under adverse conditions.
152 FORTY-FIRST CONFERENCE Benefits to be Obtained from Using Wear Pads Unlike hardface welding, which is dependant upon the skill of the welder, carbide plate protection is reproducible. Replacement of worn pads can be made with minimum loss of mill crushing time. Continuous crushing can be obtained with no loss of shredding efficiency. Cane shredding is freer, particularly under wet cane conditions. Hammers require balancing no more than once per season. First installation requires only one set of hammers plus a few spares, as against two sets of hammers where weld hardfacing is employed. Hammer life is related to wear in the "boss" of the hammer only. Shredder maintenance costs are reduced approximately 50 per cent. Replaceable Wear Pads-Original Design The design of the wear pads seated in 1972 was changed. A solid piece of carbide 12.7 mm thick was silver soldered to the top of the wear pad. Studs of carbide were silver-soldered into holes drilled below the main wear piece. Two styles were tested, a single row and double row pattern of wear studs were used. By using studs, the carbide was presented to the work fully supported by the steel body of the pad, i.e., in the strongest presentation to resist shatter due to impact against tramp material. Hammer bodies were machined to accept a wear pad held in position by high tensile socket headed "un brako" bolts. Additional security was provided by introducing a spigot fastening on the wear pad top. To provide a wear resistant surface, the steel body was oil-quenched in the post-braze period. Figure 1 shows the wear pads tested on a Pleystowe hammer. Fig.-l Pleystowe trial 1972
Fig. 2-Pleystowe trial 1973. Results of Tests The results of the limited tests were sufficiently encouraging to warrant wider tests during 1973. Precise records of all wear pad performance were not obtained because, where minor chipping of carbides occurred, the pads were removed from the mill and returned to the manufacturer for examination. Several pads from the limited number supplied crushed some 250 000 tonnes of cane before the season terminated. Significant Findings The stud pattern was successful in that no losses were encountered due to failure from impact with tramp materials. Towards the end of the season, the wear area became apparent, and was seen to extend from the outer edge of the hammer extending down about 38 mm. This indicated the total area requiring protection. Carbide wear was excellent, but the steel backing eroded away between the studs, indicating the need to use a more wearresistant steel or different wear pattern protection The steel in the hardened condition proved to be on the brittle side and, where carbide failure occurred, the steel si~nllarly failed against major impact, i.e., when weld hardfaced hammer heads become detached. On the Pleystowe hammer (Figure 2), the wear pad was not fully supported by the hammer backing. The wear pads protruded beyond the narrow section hammer body. When this style of hammer was used, it became apparent that a tougher wear pad steel was necessary. The prototype was over-designed, and modifications were seen to be necessary. Difficulties were anticipated in providing the spigot at mills where machine stop facilities were limited. Experience showed that chamfers on the pad sides were not necessary and tended to weaken the wear pad.
154 FORTY-FIRST CONFERENCE 1974 7. The carbide grade used showed good wear properties. However, under adverse conditions where steel tramp metal was included with the cane or resulted from detached hammer heads, failure occurred. The tungsten carbide used was of medium wear resistance in relation to other carbides available, i.e., composition 16.0 per cent Cobalt binder with a 2 micrometer Tungsten Carbide base. Modifications Made for and During 1972 Season Changes to Wear Pad Style The hammer wear pad was modified to include a keyway instead.. of a spigot. All necessarv chamfers were deleted. These changes vrovided easier machining of the wear pad and hammer head,~he keyway allowed engineers the choice of using a fixed key milled into the hammer or using a loose key. To prevent erosion of the steel backing, a wedge-shaped carbdie tip was introduced. This was incorporated on advice received from Cattle Creek mill. Provision was made to accommodate a 5 degree positive rake through neutral to a 5 degree negative rake according to engineers' preference in relation to carbide tip location (refer to Figure 2.) Steel Backing To provide better wear resistance in the steel body, a change was made to a special air hardening steel, Comsteel Orbit grade. This steel has a through hardness capability rather than the skin hardening properties obtainable from the previously used steel. A typical analysis of this steel is C0.7%, S.15%, Mn2.0%, Cr 1.0%, Si0.30%, MO 1.35% Carbide Grades To determine optimum carbide performance, three grades of tungsten carbide were placed under test. a. YO grade. This is a tough grade of carbide commonly used in a fully supported steel casing in cold heading operations for making steel bolts. b. PO grade. A wear and impact resistant grade commonly used for lamination die punches and dies. c. DO grade. A wear resistant grade commonly used in rotary percussion drills in hard rock mining. TABLE I-Composition of test carbides Grade Tantalum carbide Cobalt binder Base carbide size YO 2.0% 25.0% 2-3 micrometer PO 16.0% 2-1 micrometer DO II.Ox 6.0 micrometer Photo micro graphs of the various grades are shown in Figure 3. Photo micro graphs of the various grades are shown in Figure 3.
1974 FORTY-FIRST CONFERENCE 155 TABLE Il-Typical physical properties of test carbides Grade S.G. Hardness Abrasion Transverse Coercive V.P.N. index rupture strength force YO 13.15 850 275 350 min. 70 PO 13.85 1140 170 350 min. 95 DO 14.38 l l85 95 350 min. 100 Tests were carried out at various mills in the Mackay area, with the most comprehensive test being conducted at Cattle Creek. Cattle Creek mill has a high incidence of stones in the cane supply, and any success recorded at this mill could be related to the rest of the industry. Modifications Resulting from Field Tests 1. In view of the incidence of steel cracking when using "Orbit" steel, the manufacturer has altered the post-braze heat treatment cycle and the steel pads are tempered for two hours at 200 C. 2. Radii in keyways and tip recesses have been increased to eliminate "stress raisers" and to provide additional insurance against any further crack problems. 3. North Eton used a 5 degree positive rake on their hammer and achieved improved results and a better wear pattern. 4. North Eton also used a red lead paste to seal the wear pad, preventing any possibility of cane intrusion under the pad, and to provide easy removal of the worn wear pad at the end of the season. Wear Pattern The wear pattern shows that the highest incidence of wear is in the top of the wear padltip. In the 3.175 mm-long wedge-shaped tip provided, the most severe wear is from the thick end working in the area nearest the strike bar, diminishing progressively to the thin end of the wedge. By inclining the tip forward, a more even weat is experienced over the life of the carbide. However, some initial loss in impact resistance could be expected and inclination should not exceed 5 degrees. This is shown in Figure 4. Results of Insert Trial--Cattle Creek Initially, 110 tips were acquired for trial purpose, consisting of 50 hard inserts (DO) designated type A; 40 medium inserts (PO), type B and 20 soft inserts (YO), type C. An extra 20 type A inserts were acquired later when it became apparent that type C insert was unsatisfactory (they were too soft). In all, 84 insert hammers-32 type A, 32 type B and 20 Type C-were randomly arranged in four complete rows, 21 hammers per row, the remaining tips being held for replacements. The first two rows were installed on 11 August 1973 and the two remaining rows were installed on 18 August. A total of 52 inserts-22 type A, 27 type B and 3 type C-completed the season without replacement. In many instances, replacement of the remaining insert hammers was due to breakage of the tungsten carbide tip rather than to wear. Table 3 lists the average hammer life for each type in terms of the average tonnes crushed per hammer and the percentage of original
1 56 FORTY-FIRST CONFERENCE 1974 YO grade. Impact-resistant grade. PO grade. Tough, wear-resistant grade (fine grain).
1974 FORTY-FIRST CONFERENCE 157 DO grade. Tougher grade, with increased wear resistance (course grain). Fig. 3-Micro photographs of cemented tungsten carbide used at Cattle Creek Mill magnification : 1 SOOX. inserts to complete the season. The average life has been defined as throughput (in tonnes) divided by number of hammers. These values suggest that type B insert was the best, because the hard type A tip failed more from breakage caused by foreign material, such as rocks. Figure 5 shows the hammers in various stages of wear. Item A is a new insert; B is a typical worn insert after treating 256 000 tonnes; Item C shows the tip breakage due to foreign material, and D shows the wasting of the hammer head that can occur below the insert. Wasting of the hammer head highlights the need for hard-facing the head of the hammer if large through-puts tonnages are to be expected from the hammer. Maintenance Costs During the 1973 season, the shredder was run with a total of 168 hammers and 8 rows, 4 rows of the conventional hard-faced hammers and four rows of the trial insert hammers. Maintenance costs, based on material and labour costs, for each group of hammers were $1.10 cents per tonne cane for conventional hammers and 78 cents per tonne cane for the insert type. TABLE Ill-Average life of inserts TypeA TypeB TypeC Average hammer life (tonnes cane hammer) 194000 217000 124000 Per cent original hammers to complete remainder of season (approx. 256 000 tonnes) 68 84 15
158 FORTY-FIRST CONFERENCE 1974 Fig. 4-Wear pattern. Results of Insert Trial-North Eton The inserts were placed under test during the second week of the season and were in service at the end of the season. Inspection shows that two to three weeks of service life remains in worn hammers. The grade of tungsten carbide in service was Type B (PO Grade). Eficiency The cost per tonne cane crushed (based on 120 tonnes per hour average) for conventional hard-facing is $0.0257 per tonne canefor tungsten carbide hard-facing. $0.0185 per tonne cane. Breakage rate of carbide inserts was 10 per cent over the season. The total crop crushed was 350 000 tonnes. The wear pad cost is based on original installation cost plus 10 per cent replacement, divided into tonnes of cane crushed. No adjustment in costs have been made for further wear life available in 1974. Results of Insert Trials-Pleystowe Mill Based on the results of limited tests conducted with tungsten carbide wear pads in 1972, an expanded list was arranged for the 1973 season. Because of errors in manufacture related to the number of bolts, holes, and the necessity for re-supply, the test was delayed until the end of the season. In 1972, one tip returned 360 000 tonnes crushed before the hammer was lost through a failure in the hammer eye. Tests for the 1973 season indicate that this throughput life repeatable in that about 100 000 tonnes were crushed with no significant wear (see figure 6). The cost per tonne of cane crushed was not precisely determined for wear pads. However, an estimated 50 per cent reduction was obtained in shredder maintenance costs. Eficiency Based on the first 400 000 tonnes of cane crushed, the cost of hammer maintenance (arc hard-facing) was $0.0215 per tonne while estimated
FORTY-FIRST CONFERENcE Fig. 5-Hammer wear comparison. A-A Fig. 6--Shows wear comparison. new wear pad on a Cattle Creek 51 mm hammer. B-Pleystowe style hammer and 70 mm wear pad after crushing approximately 10 tonnes of.cane. C-North Eton hammer and wear pad which crushed 325 000 tonnes approximat
160 FORTY-FIRST CONFERENCE 1970 cost for tungsten carbide-faced pads was 1.06 cents per tonne. The cost for arc hard-facing does not include the requirements to purchase a duplicate set of hammers or the removal and replacement costs involved in routine maintenance. These costs are: duplicate hammers, approximately $7 000, hammers $6 000, and removal and replacement $1 000). Comments The cost information supplied is only approximate, having been based on the limited information available from three mills. More definite information will not be obtained until several mills have completed a season with a full complement of tungsten carbide wear pads. Acknowledgements Thanks are accorded to the Chief Engineers of Pleystowe and North Eton for their interest and assistance in providing details of their tests and the management of Cattle Creek mill for permission to publish this paper. REFERENCES Jacklii, G. D. The electro-casting of shredder hammer tips. Proc. Qd. Soc. Sug. Cane Technol. Thirty-ninth conf. 95. Shann, D. S. and McWhiuney W. (1966). Cane shredder hammer performance. Proc. Qd. Soc. Sugar Cane Technol. Thirty-third conf. 151.