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How does a small or medium-size construction company organize to perform metal repair on its construction equipment? What sort of equipment does it need in a welding and fabrication shop? What kinds of repairs are done there? How does hardfacing fit in? By Gene Dallaire |
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To get a realistic view of how a welding shop works, we talked to a typical medium-size grading and excavating contractor in the Midwest doing grading and excavating of roads and highways and asphalt paving. It also owns and operates a quarry that produces aggregate for both itself and other contractors. This company, which chooses to remain anonymous and therefore we will refer to as XYZ Construction Company, has 150 pieces of construction equipment in its fleet, including bulldozers, loaders, backhoes, excavators, graders, and rock-crushing equipment. Virtually all maintenance is done in-house. Many repairs of construction-equipment metal components are done in the field when the day's shift is over. Other repairs, especially major ones, are done in XYZ's central maintenance facilities. Why does the company do the vast majority of repairs in-house? Primarily, says a company spokesperson, "to minimize downtime of construction equipment-to make hay while the sun shines." There is a long winter in this northern region, with only so many months a year to do construction. Accordingly, the company doesn't have the luxury to wait for a dealer or other outside maintenance facility to repair its equipment. Basic Equipment Needed for a Welding and Fabrication Shop What sort of central maintenance facilities does XYZ have; what sort of shop for repairing metal on construction equipment? XYZ makes a clear physical separation between its engine and equipment repair facilities and its welding and fabrication shop. The latter, separated from the former by a solid wall, consists of five bays. The welding shop is sharply separated from engine repair because: (1) there is often much dust in the air in the welding shop, harmful to engine repair, and (2) the intense light from arc welding might cause eye damage to ungoggled mechanics working in adjacent areas. What equipment does XYZ have in its welding and fabrication shop? Most important are the power sources for doing electric-arc welding. The shop has three-phase AC electric power coming in from an outside electric utility. That power is used to operate 20 arc-welding power-supply units. All these units are DC; XYZ does not use AC for arc welding. These DC power-supply units are of varying sizes, with outputs ranging from 300 amps to 600 amps, and are priced new from $6,000 to $8,000 (including gasoline or diesel engine). XYZ uses these units in the shop and in the field. The three leading suppliers of arc-welding power-supply units are ESAB, Lincoln Electric, and Miller Electric. In the shop, XYZ uses both stick electric-arc welding and MIG (metal inert gas) electric-arc welding with wire feed (see the "Metallurgy and Welding 101" article in the November/December 2000 issue). In stick welding, a flux coating on the surface of the electrode stick melts during welding, covering the weld pool with a protective layer, which prevents atmospheric oxygen and nitrogen from contaminating the weld pool. With MIG welding, on the other hand, the weld pool is protected from direct contact with the atmosphere by the release of an inert gas, which hovers over the weld pool. The trigger of a welding gun initiates both the feeding of the wire electrode and the release of the protective inert gas. For many shop-welding applications, XYZ prefers MIG to stick electric-arc welding. MIG welding is much faster, for the consumable wire electrode (with many feet wound onto a nearby large reel) is fed automatically and rapidly through the welding gun. With stick welding, the welder must stop often to clamp on a new consumable welding electrode, slowing down the welding process. Also, with MIG welding, since no flux is used, no slag covering forms on top of the weld pool, as in stick welding. In stick welding, after the weld pool hardens, the welder must first take the time to chip off this slag surface coating before laying down an additional layer of weld material. Also Needed: Torches, Grinder, and Overhead Crane In addition to welding metals, XYZ also cuts steel plates-for example, when welding abrasion-resistant plate onto excavator buckets. For cutting, it uses an oxyacetylene torch. Incidentally, such a torch is rarely used in the actual welding of construction equipment, for oxyacetylene welding is far too slow-it takes too long to heat the base metal to appropriate temperatures. Heating metals electrically, as in electric-arc welding, is far more focused and efficient. For cutting steel, XYZ also has in its shop a gas-plasma torch. This is much more expensive than an oxyacetylene torch-$10,000 versus $275 for the oxyacetylene hardware. Yet if a shop is doing a lot of cutting, oxyacetylene can become expensive, for a considerable amount of gases are burned. At $35 per tank for oxygen and $30 per tank for acetylene, that can add up fast. But the plasma torch consumes no gases; it merely operates off shop compressed air. If a shop does a lot of cutting, it may make sense to purchase a gas-plasma torch; although it has a high initial capital cost, operating costs are low. In repairing a metal crack, the welder must first gouge or widen it out to make room for the weld material to flow in. To that end, XYZ's welding and fabrication shop has air-arc gouging equipment, which runs off shop compressed air. Finally, XYZ's welding shop has band saws, drill presses, and grinders. Why this equipment? A major task is cutting steel plate to appropriate sizes-for example, cutting a plate for later welding onto inside or outside walls of an excavator bucket. Once the steel plate has been cut by an oxyacetylene or gas-plasma torch, its rough edges must then be smoothed-the reason for having a grinder. A shop also needs a band saw to cut steel-shaft material to appropriate lengths to be pivot-hole pins. This well-equipped construction-company welding shop also needs an overhead crane for moving heavy objects around the shop. Often, for instance, a bucket needing major metal repair may be removed from an excavator in the field and lifted onto a trailer by a truck-mounted crane. The truck would then pull the trailer to the welding shop, backing the trailer into a bay. The bay overhead crane would now lift the bucket off the trailer and place it in an appropriate welding position. An overhead crane for a construction-equipment welding shop, says XYZ, needs to have a 10-ton lifting capacity, an item with a price tag of about $25,000. Among leading producers of such cranes are North American Crane (Philadelphia, PA) and P&H Crane (Wakashaw, WI). A well-equipped welding shop also needs a 10,000-lb.-capacity forklift (which costs $18,000) for unloading steel sheet and plate off trucks and moving them into the welding shop. To do numerous repairs on construction equipment, XYZ regularly purchases a variety of steel plate from a regional steel warehouse, in this case Leeco Steel in Chicago, IL. Typically it buys two types of steel plate, T1 steel and abrasion-resistant (AR) steel, in 8- x 10-ft. sizes, ranging in thickness from 3/8 to 2 in. Excavator Buckets: Protect With Both Steel Plate and Hardfacing With the above equipment and materials in its welding and fabrication shop, XYZ can do a wide range of metal repairs on its construction equipment. One of its biggest problems is excavator buckets. To protect bucket surfaces from wear, XYZ workers weld AR steel plates onto bucket inside and outside surfaces, then add hardfacing on top of these welded-on plates. Specifically, XYZ welds AR steel plates onto both the inside and outside surfaces of excavator buckets. Yet there are areas of bucket surfaces where it is not possible to weld on plate-for instance, across the bucket teeth and along the top of the bucket. In such uncongenial areas, XYZ applies hardfacing to protect the bucket base metal. On the outside bottom surface of an excavator bucket-a region of especially high abrasion-XYZ not only welds on AR plate, it also adds a grid pattern of hardfacing ridges on top of the AR plate itself. Without the hardfacing grid, the company, when working in highly abrasive soils, might have to replace the AR plate in less than a month. But by hardfacing the surface of the AR plate, XYZ can protect it indefinitely. After a few months, it will have to rehardface the surface of the AR plate, which remains intact. Actually, how often the bottom surface of a bucket has to be rehardsurfaced depends on the character of the soils being excavated. If a site contains an abrasive granite quarry or soil, rehardfacing might have to be done every four to six weeks; if it has limestone-laden earth, the bucket might be able to go up to six months without being rehardfaced. In applying hardfacing, XYZ adds not a continuous coating but a pattern (either grid or hatch) of ridges. These welded ridges are anywhere from 1/4 to 3/8 in. high. The ridges must be oriented so that they lie perpendicular to the direction that the earth will "flow" over the bucket surface. In this way, the earth will no longer flow directly in contact with the original T1-steel surface of the bucket. Instead, it will slide over the hardfacing ridges, thereby protecting the bucket base metal. XYZ does most of its hardfacing in the field using either self-shielded-stick or wire electric-arc welding. The choice depends on the welder. If the bucket can be placed in a good welding position in the field, many welders prefer using wire welding, for it is much faster than stick welding. Many of XYZ's maintenance trucks are equipped with the wire reels needed to do the wire welding. Hardsurfacing the bottom of a bucket might typically take five hours. Protect Blades With Steel Liners Another major activity done in XYZ's welding and fabrication shop is the repairing of bulldozer blades. The bottom cutting edge, bolted onto the 4-ft.-high blade, must be replaced fairly frequently. It is merely a matter of removing the old cutting edge, purchasing a new cutting edge from an aftermarket dealer, then bolting it onto the blade, a repair readily done in the field. But XYZ has a program to protect the entire 4-ft.-high surface of the blade itself. And to do this, it does not use hardfacing because a grid-shaped pattern of hardfacing ridges on the blade surface causes soils, especially those with a high clay content, to build up on the blade surface itself. This caking of soil on the blade inhibits the smooth lateral flow of earth during bulldozing. Accordingly, to provide protection of the T1 alloy-steel dozer blade (subjected to both tensile forces and abrasion) while at the same time sidestepping the dozer-blade caking problem, XYZ welds a T1 alloy-steel liner over the entire dozer-blade surface. The company buys 1/2- to 5/8-in.-thick steel plate from a steel vendor and has it bend the steel to the actual curvature of the 4-ft.-high dozer blade. In its own shop, XYZ now welds this steel liner onto the bulldozer-blade surface. The smooth, shiny surface of the liner ensures that soil glides over it during dozing without surface caking. The liner usually lasts for two or three years, then is replaced with a new liner. Gene Dallaire is a former feature article writer for Chemical Engineering and Civil Engineering magazines. He currently teaches history at Lansing (MI) Community College.
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