WASTE WEAR- EXTENDING LANDFILL EQUIPMENT LIFE

Transcription

1 WASTE WEAR- EXTENDING LANDFILL EQUIPMENT LIFE Doug Fleming, Sulzer Metco (Canada) Inc., Garry Liddell, TRANZWELD Ltd. Abstract Abrasive wear occurs on ground engaging and earthmoving equipment typically found in the mining industries. Depending on the material being moved, wear on the equipment can be minimal to very severe. Very severe wear conditions can mean thousands of dollars in repair and replacement costs of worn out parts and components. Operations that are experiencing very severe wear on their equipment usually use some form of wear protection on these expensive components in order to extend their wear life and keep the equipment in the field working in the most economical and efficient manner. When we think of wear and abrasion we think of mining and quarry operations but believe it or not abrasive wear is also a significant problem for the waste handling industry. The heavy equipment such as compactors, dozers and loaders used to handle the thousands of tonnes of waste at landfills and transfer stations experience tremendous abrasive wear from "garbage". Maintenance costs on this equipment are another expense that adds to the rising cost of refuse disposal. Landfill and transfer station operators want to reduce wear related maintenance costs and keep their equipment in the field operating for as long as possible. These reduced maintenance costs are the short term gains operators are always striving for. Another aspect in landfill operation is long term gains such as making the most efficient use of the space they have. This is where the importance of maximum compaction density comes into play. Properly designed and well maintained compactor tips will optimize compaction density to make the most of the landfill airspace resource. Sulzer Metco (Canada) Inc.'s innovative Tuffstudds Wear Protection System can help meet the cost reducing requirements of these operators giving them both short and long term gains. The paper will explain the types of wear, metallurgical considerations, current practices and the Tuffstudds concept. It will also demonstrate results through actual case study examples showing how Tuffstudds, installed by TRANZWELD Ltd. (our exclusive NZ Tuffstudds distributor) on various types of waste management equipment working in New Zealand, can help to achieve the overall goals of lower waste management costs. Types of Wear The OECD (organization for Economic Cooperation and Development) defines wear as: "The progressive loss of substance from the operating surface of a body occurring as a result of relative motion at the surface" [1]. Commonly recognized wear categories and their respective estimated shares of heavy machinery wear are shown in Figure 1. [2] Metal to Metal 22% Abrasive 52% Corrosive 10% Impact 16% Figure 1. Illustration of wear category shares in heavy machinery. 1

2 Usually there are various types of wear that work at the same time on heavy ground engaging equipment found in landfills. The most common types of wear in these kinds of environments are abrasive and impact wear. Impact Wear Heavy equipment such as compactors and loaders can be subjected to impact wear. Impact wear is a battering, pounding type wear that breaks, splits or deforms metals surfaces. It is a slamming contact of metal surfaces with other hard surfaces or objects. Toughness of the metal surfaces can be defined as the capacity of a material to absorb energy by deforming plastically before fracturing. Abrasive Wear Abrasive wear occurs when non-metallic materials rub, grind, scrape or roll under pressure across a metallic surface. The type of abrasive wear can be determined by: The properties of the wear materials, The properties of the abrasive materials The nature and severity of the interaction between the abrasive and the wear material. Abrasive wear can be classified as: Gouging wear: The resulting wear can be extreme when high-stress or low-stress abrasions are accompanied by some degree of impact and weight. The metal surface receives prominent gouges and grooves when massive objects (often rock) are forced with pressure against them. A low velocity example of this is when a dragline bucket digs into the earth and a high velocity example would be rock crushing. In both instances the action of the material on metal is similar to that of a cutting tool. Gouging abrasion also places a premium on toughness. Sometimes this is at the expense of harder and more abrasion resistant alloys. Carbide containing alloys are used successfully, when supported by a tough alloy, preferably austenitic manganese [10]. High stress grinding abrasion: This is more intense than simple scratching. It happens when small, hard, abrasive particles are forced against a metal surface with enough force to crush the particle in a grinding mode. Most often the compressive force is supplied by two metal components with the abrasive sandwiched between them. Sometimes this is referred to as three-body abrasion. The surface becomes scored and surface cracking can occur. There are examples of softer, tough alloys outperforming harder alloys in grinding abrasion applications. The successful range of alloys includes austenitic manganese, martensitic irons and some carbide containing alloys (usually smaller carbides, like titanium carbide) in a tough matrix [10]. Low stress scratching abrasion or erosion: This is normally the least severe type of abrasion. Metal parts are worn away through the repeated scouring action of hard, sharp particles moving across a metal surface at varying velocities. The velocity, hardness, edge sharpness, angle of introduction and size of the abrasive particles all combine to affect the amount of abrasion. Alloys containing carbide (particularly chrome-carbide) are used successfully to resist low-stress abrasive wear. Due to the absence of impact the relatively 2

3 brittle high carbon-chromium steel alloys are well suited for low-stress abrasive applications [10]. In abrasive wear there are two extreme mechanisms of material removal, one in which plastic deformation plays a dominant role and the other in which fracture with limited plastic deformation dominates. Metallurgical Considerations to Combat Abrasive Wear It is not possible to define the various conditions experienced in such work areas such as landfills with one simple type of abrasive wear mechanism. Various factors are at work which could bring all types of wear mechanisms into play at some time or another on a component. In a general review of hardfacing alloys for highly abrasive environments, Farmer [3] recommended high carbon, high chromium iron based hardfacing materials. The dependence of wear rate on the relative hardness of the working surface and mineral component is shown in Figure 2, reproduced from Dawson and Craig [4]. From this it is seen that chromium carbide is a suitable wearing surface. The consensus of several authors [3, 5, 6, 7, 8] is that a high volume fraction (45-50%) of M7C3 type chromium carbides in an iron based alloy containing typically2-4% C and 10-28% Cr is one of the preferred wear resistant materials for highly abrasive environments [9]. Figure 2. Relative wear rate vs. abrasive hardness. Zum-Gahr [5] showed that in a pin abrasion test the abrasive wear rate of a white cast iron against 150 mesh garnet decreased continuously as the chromium carbide volume fraction increased from 5-50%. However he reported an increasing wear rate with increasing carbide volume when wearing against 180 mesh silicon carbide, a much harder abrasive, due to the 3

4 brittleness of the coarser primary chromium carbide grains. Rohrig [6] also reported similarly for white cast irons that for each mineral a minimum abrasion loss exists at certain carbon content and that this minimum is displaced to lower carbon contents with increasing mineral hardness. The heat treatment of the high carbon, high chromium white cast irons has been studied extensively by Dodd and Parks [7] and by Maratray and Usseglio-Nanot [8]. Dodds and Parks pointed out that the carbon content of the matrix must be lowered by the precipitation of secondary carbides during cooling to allow the austenite to transform to martensite, and Maratray and Usseglio-Nanot showed that the addition of a small amount of molybdenum (3%) permits a much higher carbon content to be retained without the undesirable transformation to pearlite. What is required in a wear resistant material is a high volume of relatively fine primary M7C3 chromium carbides grains in a matrix of secondary carbides and martensite. [9] It should be noted that there can be a misconception in hard surfacing. Greater hardness does not always mean greater abrasion resistance or longer wear life. Several alloys may have the same hardness rating but vary greatly in their ability to withstand abrasive wear. For example, many of the best surfacing alloys derive their high abrasion resistance from very hard carbides dispersed throughout a softer, tougher matrix. Bulk hardness tests (Rockwell or Brinell) that measure the average hardness of both the carbide and matrix together, over a relatively large area, often register the same hardness as other conventional metals. However, in actual performance, a carbide-containing surfacing alloy has a substantially better abrasive wear resistance. When equally comparing several surfacing alloys with each other high bulk hardness ratings are not the only factor assuring resistance to wear. Resistance (especially to low and highstress abrasion) depends on a combination of both hardness and the metallurgical microstructure of the alloy. The microstructures of alloys vary according to the ratio of carbides to matrix and the type of carbides in the alloy. The alloy with the hardest and most evenly dispersed carbides, along with the highest percentage of carbides, will have the best resistance to low-stress and high-stress abrasion [10]. Current Hard Surfacing Practices Wear protection on heavy ground engaging equipment is achieved by the attachment of alloy steel plates or bars, cast alloy plates or some variation of surfacing or welding. Each method has advantages under certain conditions and all are used extensively. Hard surfacing by welding may comprise a complete overlay or may consist of stringer or rosettes in selected areas. Unfortunately hard surfacing by welding for wear protection has certain disadvantages [9]. Because the desired hardfacing properties are only achieved with the right microstructure, obtaining the correct composition is critical. In this respect, metal arc welding is too operator dependent and highly variable. The welder has a strong influence on the quality of the weld overlay; overheating of the weld puddle can significantly effect the composition of the overlay. The extent of the dilution from the base can be up to 50% and is very dependent on the individual welder. At the same time essential alloying elements such as carbon and chromium can be lost in the arc to an uncontrolled extent. This makes it difficult to achieve a weld overly with the desired composition and microstructure even when multiple layers are deposited. Furthermore the extensive heating and penetration into the base metal, which is 4

5 usually a high strength alloy steel can destroy the metallurgical properties of the base metal and can cause cracking and subsequent failure [9]. Manual arc deposition is slow, typically less than 2 kg/h and is expensive due to the skill required by the welder. Finally the presence of fumes and gases in the welding environment is of concern from the point of view of the health of the operator. What is required is a system for depositing reproducibly a high carbon, high chromium alloy with the required microstructure, without adversely affecting the base material at a high rate and low cost, preferably such that selected areas can be protected in the same manner as by welded beads or rosettes. This latter requirement is important because as pointed out by Farmer[3], an intermittent pattern of beads or rosettes will retain a static bed of the mineral being worked and will result in an effective wear resistant surface with a relatively small amount of hardfacing alloy [9]. The Tuffstudds Concept The Tuffstudds concept is a wear protection system that meets the above mentioned requirements. In this method studs of hardfacing alloy are welded to component surfaces in a spaced array by an electric arc stud welding technique. The studs are typically mm diameter and mm high and are spaced 5-10 mm apart. Stud welding technology is used in many industrial situations to weld fasteners or anchors onto structural components or manufactured parts. The same basic principles and metallurgical concepts used in any arc welding procedure are utilized. Since its inception during the Second World War as a method of applying fasteners in the shipyard industry, electric arc stud welding has developed into an efficient, versatile and automated process. The welding sequence, shown in Figure 3 is controlled by a solid-state circuitry to provide maximum reproducibility. A more complete description of the sequence is as follows: Tuffstudds and arc shield placed in contact with the work surface. (Figure 4) Pull the trigger. A pilot arc in initiated. Tuffstudds is automatically lifted. Main welding current melts a portion of the stud and the work surface. Within a second the Tuffstudds is plunged into the molten pool. The arc shield retains the molten metal in the weld area for maximum weld strength and safety. Figure 3. Stud welding sequence A strong metallurgical bond is developed at the weld interface. 5

6 Weld parameters are dependent on the diameter of the Tuffstudds and the compositions of both the stud and the work surface. Typical weld cycles are 0.6 to 1.1 seconds, with weld currents in the range of amps for studs of mm diameters. Figure 5 shows the stud welding gun used for Tuffstudds applications and the sequence of loading the gun. Figure 5 a) the stud welding gun b) load a Tuffstudds c) load an arc shield d) position and pull the trigger Two chemical compositions of Tuffstudds are in commercial use. Both are white cast iron, one containing volume percent of primary M7C3 type chromium carbides in an eutectic matrix of secondary carbides (Fig 6 (a) and the second containing molybdenum, comprising of a network of fine primary carbide dendrites for increased toughness (Fig 6 (b). A considerable effort was necessary to develop a casting technique to produce the required microstructure in a stud of the required dimensions with the close tolerances necessary for conventional electric arc stud welding guns. Figure 6 (a) M7C3 type chromium carbides Figure 6 (b) molybdenum modified The stud welding procedure was also the subject of considerable study to develop a technique which will permit the extremely hard and highly alloyed cast stud to be welded reproducibly to the various alloy steel surfaces encountered in such ground engaging equipment as compactors. Laboratory testing included impact tests in a modified izod machine and the welding procedure was optimized to make the impact strength of the weld exceed that of the stud. 6

7 A significant advantage of the Tuffstudds concept over conventional electric arc hardfacing is that much faster deposition rates are possible, up to 250 studs per hour resulting in 9 kg/h for the larger studs compared with 1-2 kg/h for manual arc welding. Usually the studs are spaced about 5-10 mm apart resulting in area coverage of up to 0.33 m 2 /h with the larger 22 mm diameter Tuffstudds. The spacing also appears to be optimum for the development of the entrapped static bed of crushed debris, a desirable wear resistant situation for which the upright studs are much superior to rosettes or beads of weld metal. (See Fig. 7) Figure 7. Mineral debris packing in between the Tuffstudds creating a "deadbed effect" further enhancing the wear resistance. Another feature of Tuffstudds is that through the quick and easy stud welding process each stud has an excellent metallurgical fusion weld to the wear surface and the upper region or each stud retains its original chemistry, characteristics microstructure, hardness and shape. Because the stud welding process is so simple and automatic it is not so operator dependent therefore eliminating the effects of dilution or overheating and possible lose of essential alloying elements such as carbon and chromium to an uncontrolled extent. With the Tuffstudds process the metallurgical characteristics are maintained through the welding cycle. Another benefit is the minimal heat input into the base metal that is produced during the welding cycle compared to other hardfacing processes. This reduces the risk of cracking or distortion due to heat build up. Another very important benefit of the Tuffstudds system that needs to be mentioned is that the process produces minimal smoke and fumes compared to manual hard surfacing techniques. The all important issues related to having a safe and healthy working environment for the operator can be addressed with the smokeless process it has to offer. Landfill Compactors The basics for utilizing compactors in landfills is simple- get the maximum density and therefore creating more airspace for long term longevity of the site. At the same time the landfill operators are looking for ways to reduce their operating and maintenance costs of their compactors on a short term basis. Tuffstudds wear protection used on compactor tips can provide operators with both long term (maximum density) and short term (reduced operating costs) gains. 7

8 Packing Density- Long Term Gains A compactors ability to provide maximum compaction is one of the most important factors landfill operators consider when operating their equipment. Attaining maximum packing density efficiency increases the airspace resource of the landfill. Many variables come into play in achieving maximum compaction including wheel and tip design along with increased compactor weights. Packing densities of 830 kg/m 3 can be achieved as a result of better compactors [12]. The packing tip shape is designed to effectively compress the material downwards to attain maximum density in that area. (See Figure 8) In order for the tip to perform at its maximum packing efficiency it needs to maintain its OEM designed shape for as long as possible. As discussed earlier wear and predominantly different forms of abrasive wear are going to deteriorate the edges on the packer tip making the tip's packing action less effective. Packing tips that are worn and rounded will cause diffused packing action which will not achieve maximum compaction densities. (See Figure 9). Figure 8 Uniform packing pattern action Figure 9 Worn tip creating diffuse packing action Maintaining the designed shape of the compactor tips is a very worthwhile step towards the long term gains of increased packing density which is buying time for the landsite. The better the packing the density the longer the life of the site (increasing the airspace) and the long term costs are reduced. So doing short term little things like protecting compactor tips so they pack better, pays off in long term gains. Packer Tip Protection- Short Term Gains As mentioned above packer tip designed profiles need to be maintained in order to get the maximum packing efficiency from them. Garbage, trash, debris and "cover" material does cause abrasive wear on the tips making them lose their packing effectiveness. Tuffstudds wear protection applied to compactor has proven to be an economical method of prolonging the life of expensive packer tips. Figure 8 shows an example of a tip applied with Tuffstudds. Here are some of the benefits of Tuffstudds wear protection on compactor tips: Retain edge longer for more effective packing action Packs to density faster resulting in less passes. Breaks up trash and garbage better for quicker compacting. Tuffstudds offer a long lasting, thick layer of wear protection. The short term gains the operator will attain through cost reductions by achieving maximum packing density by maintaining compactor ideal tip packing shape through the use of Tuffstudds wear protection is: Lower fuel costs and less time wasted with less passes. 8

9 Lower compactor tip replacement and maintenance labour costs. Less equipment downtime (an increase in percentage for machine availability). Figure 10 below shows an example of badly worn and rounded packer tips which would create very inefficient packing action. Figure 11 shows packer tips with Tuffstudds wear protection. Notice the material packing in between the studs further enhancing the wear protective barrier. Figure 10 Worn Compactor tips Figure 11 Tuffstudded Packer tips Tuffstudds Field Experience- Landfill Equipment TRANZWELD Ltd. has been the exclusive Tuffstudds distributor in New Zealand for the past 8 years. TRANZWELD supplies and installs Tuffstudds with a totally mobile stud welding system powered by a diesel generator. They have made extensive Tuffstudds applications on such waste management equipment as compactors, bull dozers and grapple clams. The use of Tuffstudds on wearing components of these pieces of equipment has saved landfill operators thousands of dollars in reduced maintenance costs. A few examples of these cost savings will be demonstrated. Compactors Tips Field tests have shown that Tuffstudds applied to compactor tips can extend the life of the component by at least 2 times versus if left unprotected. Figure 12 shows a tip after 6250 hours of operation. Plenty of Tuffstudds material remaining for many more hours of wear protection. Other examples of Tuffstudds wear protection on compactors are shown in figures 13 and 14. Fig. 12 after 6250 hours Fig. 13 after 2400 hours Fig. 14 after 8000 hours 9

10 Figure 14 shows a tip after 8000 hours in the field. Without protection these tips are normally replaced after 7000 hours. It is estimated by the operator this Tuffstudded tip will last 14,000 hours; extending it's operating life by double. Figures 15, 16, and 17 shows examples of compactors tips that are badly worn and need the costly procedure of either completely replacing them or rebuilding them. Fig. 15 Badly worn tip Fig. 16 Extreme wear Fig. 17 Worn tip requires costly rebuild Costs Savings Example- Rebuilt Compactor Wheel Drum- New Compactor Tips with Tuffstudds Wear Protection Installed Cost to supply and install brand new tips on a typical compactor: Tip Cost: $485/ea $835 X 35 tips per Wheel Total Cost: Tip Installation Cost*: $350/ea X 4 Wheels per Compactor $116,900 *Tip installation costs consists of removal of old worn tip, clean drum surface, weld preheat procedure, weld new tip on to drum. Cost to apply Tuffstudds wear protection to compactor tips: Tuffstudds Installation Cost/tip: $265 X 35 tips per Wheel Total Tuffstudds $265/tip 4 Wheels per Compactor $37,100 Total Initial Outlay to Supply and Install Tuffstudded Compactor Tips: $154,000 Tuffstudded Compactor Tips can last 2 Times longer than unprotected tips. Conclusion: A $37,100 initial investment in Tuffstudds Wear Protection can save $116,900 in Compactor Tip and Maintenance Costs. 10

11 Figure 18 and 19 shows a brand new compactor tip before and after Tuffstudds installation. Figure 20 shows the complete wheel. Notice also in Figures 19 and 20 Tuffstudds applied to wheel edges which do have a very high wear rate. Figure 18 New compactor tip Figure 19 New compactor tip with Tuffstudds Figure 20 Tuffstudds installed on new compactor tips and Wheel edges Rebuilt Compactor Tip with Tuffstudds The following is an example of a compactor wheel where the tips have been completely rebuilt then Tuffstudds were applied. Figures 21 shows the rebuilt tip with Tuffstudds applied and figure 22 shows the completed rebuilt compactor wheel. Figure 21 Rebuilt Tuffstudded tip Figure 22 Completely rebuilt wheel with Tuffstudds Figure 23 below shows the comparison between a new compactor tip and one that has been in the field operating for 1,550 hours. Notice the significant wear on this tip after a relatively short operating time. The important factor is for the operator to identify the wear problem now before it is too late. Rebuilding the tip and further protecting it with Tuffstudds is an option at this point but leaving it to wear further may cause this option not to be economically viable and the whole tip will have to be replaced. A costly procedure as illustrated previously. 11

12 Figure 23- New tip versus worn tip (1,550 hours) Other Tuffstudds Landfill Equipment Applications Compactor wheels are just one of the critical components where Tuffstudds wear protection is used on waste management equipment. Other landfill equipment experiences severe abrasive wear in the day to day operations. It should be noted that just like any ground engaging operation wear rates and severity or wear will vary from landfill to landfill. Factors such as travel distance of the equipment and the type of "cover" material used at the landfill site will affect wear on components at different rates. The following figures show some examples of other Tuffstudds applications on landfill equipment. Figure 24 Compactor Blade Figure 25 Scrapper Bars Figure 26 Inside Wheel edge Figure 27 Dozer Blade Figure 28Dozer Side Arm Figure 29 Grapple Clam 12

13 Summary and Conclusions Abrasive wear on equipment working in the landfill and waste management environments can result in significant maintenance costs for the repair and replacement of worn out components. Improvements in wear protection systems can result in short term cost reductions through reduced component costs, less equipment downtime and lower labour costs. Wear protection improvements can also result in long term gains through overall improved operation efficiencies such as attaining maximum packing density with well maintained compactor tips. The Tuffstudds Wear Protection System can help meet the cost reducing requirements of these operators giving them both short and long term gains. The Tuffstudds concepts offers the desirable high carbon, high chromium alloy that can be conveniently and economically be applied by an electric arc stud welding technique. Significant cost savings have been demonstrated with the application of Tuffstudds on high wear components such as landfill compactor tips. Other areas of application for Tuffstudds in landfill and waste management that were highlighted were compactor blades, scrapper bars, wheel edges, dozer blades, side arms and grapple clams. Acknowledgements The authors thank the Management of Sulzer Metco for permission to publish this paper. References [1] Yumaguchi Y. Tribology of Plastic Materials, Elsiver 1990, pp [2] WTIA Technical Note 4. The industry guide to Hardfacing for the Control of Wear. [3] Farmer, H.N., Factors Affecting Selection and Performance of Hardfacing Alloys. Amax Symposium, Materials for the Mining Industry, Vail, Colorado, [4] Dawson, R.J. and Craig, G.B., Tribology: Abrasive Wear. Engineering Digest, vol. 27, No. 8, 1981, p. 23/27. [5] Zum Gahr, K.H. Abrasive Wear on Metallic Materials, Metallurgical Aspects of Wear, p. 73/104. Deutsche Gesellschaft für Metallkude, [6] Rohrig, K., Cast Abrasion Resistance Iron Base Alloys; Metallurgical Aspects of Wear, p. 269/289. Deutsche Gesellschaft für Metallkude, [7] Dodd, J. and J.L. Parks, Factors Affecting the Protection and the Performance of Thick Section High Chromium-Molybdenum Alloy Iron Castings. Climax Molybdenum Company, Greenwich, Connecticut, U.S.A., 1980 [8] Maratray, F. and Usseglio-Nanot, R., Factors Affecting the Structure of Chromium and Chromium-Molybdenum White Irons. Climax Molybdenum S.A., Paris, [9] Clegg, M.A., Cook, R.C., Fraser, R.W., A New Method of Wear Resistant Surfacing by Welding, p. 1, Sherritt Gordon Mines Limited, Fort Saskatchewan, Alberta, Canada, [10] Technical Library-Wear Factors, Available: (Accessed 2005, Sept 9) [11] Cook, R.C., Silins V., Tameling, J., Tuffstudds- A New Wear Protection System, CIM Bulletin, vol. 75, No. 846, 1982, p. 2. [12]Bolton, Neal, Compactonomics, Available: (Accessed 2005, Sept 6) 13