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For this primer on metallurgy and welding, the author interviewed an expert in the arc-welding field, Thomas J. Black, manager of hardfacing and high-alloy products for The Lincoln Electric Company in Cleveland, OH. Black, a metallurgical engineer, has more than 40 years’ experience in the welding field. A companion article titled "Dealing with Metal Wear on Construction Equipment" reviews in-depth histories that describe equipment dealers' and progressive grading and excavating contractor's most pressing metal-wear problems and how they're solving them. It appears in the January/February 2001 issue. By Gene Dallaire
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What are the main kinds of steel used in the fabrication of construction equipment? Black: Most of the components of construction equipment are made of some type of carbon steel. Carbon steel is the most widely used type of steel, used for fabricating structural steel for buildings, for automobiles, for washing machines, and so on. A carbon steel consists mainly of iron with some carbon - anywhere from 0.1 to 0.5% - and some silicon and manganese. There are no other alloying elements. If the carbon content of the carbon steel is low - 0.1-0.35% - it is referred to as a mild steel or a mild carbon steel. Most construction equipment is made out of a carbon steel with a carbon content ranging from 0.1 to 0.25%, a tensile strength of 30,000-50,000 psi, and a Rockwell hardness of 5 Rc. To compare that with something more familiar, the steel used for the skeletal structure of buildings, A36 steel, is quite similar - carbon content, below 0.25%; tensile strength, 30,000-50,000 psi. Besides carbon steel, two other types of steel often used to fabricate certain components of construction equipment are low-alloy steel and high-alloy steel. An alloy steel, besides consisting of iron and carbon, also has one or more alloying elements added - silicon, manganese, chrome, nickel, molybdenum, columbium, titanium, vanadium, or copper. T1 steel is a trade name for A514 steel, a high-alloy steel with a tensile strength of 100,000 psi, often used for certain components of construction equipment that need high strength, such as the arms supporting the bucket of a front-end loader. This A514 or T1 steel is also available in steel plate, which in the repairing of loader or excavator buckets is often welded to inside or outside bucket surfaces. There are some components of construction equipment - for instance, gears, the drums used for reeling in cables, and cable pulleys (as on drag lines) - that are made of carbon steel. While their base may be fabricated from a mild carbon steel, typically they are overlaid with a surface deposit of hard metal-to-metal tool steel, with a Rockwell hardness range of 40-55 Rc. There is very little stainless steel used in construction equipment. Given these different kinds of steels, please itemize more fully the major components of construction equipment and the type of steel each is made of. Black: Generally the type of steel selected by construction-equipment makers depends on what strength is needed for the particular component. The sheet steel of cabs is low-carbon steel, a mild steel. The chassis of a low-boy trailer for carrying construction equipment is typically made of carbon steel. But a trailer designed for carrying a large, heavy dozer might be fabricated of a high-alloy steel that has been quenched and tempered to bring the tensile strength up to about 100,000 psi. The booms and arms of backhoes and excavators, since they need considerable tensile strength, are often made of low-alloy steels that have been heat-treated to get the needed tensile strength - 50,000-100,000 psi - and toughness levels. The arms supporting loader buckets and the push arms holding dozer blades are often made of these same low-alloy, heat-treated steels. The buckets of loaders and excavators need to have both high strength and some abrasion resistance and thus are often fabricated of low-alloy or even high-alloy steels. Concerning the undercarriages of dozers and other tracked construction equipment, the sprockets, idlers, and rollers typically are made of low-alloy steel, iron, and carbon alloyed with chrome, nickel, and molybdenum (SAE 4130 or 4340). Often the surface of these components would be flame- or surface-hardened to provide metal-to-metal wear resistance. In the rebuilding of such undercarriages, workers often apply a hardfacing material that would be comparable to the original surface-hardened material, now worn off. What welding methods are most commonly used in the repair of construction equipment? Black: In today’s world, most construction work is done using some type of electric arc welding. Oxy-fuel welding, in which a high-temperature flame is created by reacting acetylene, propane, or some other gaseous fuel with oxygen, is not used to any great extent in the repair of construction equipment. This is because electric arc welding is much faster than oxy-fuel welding, an efficient welder being able to deposit 15 lb./hr. of weld material with certain arc-welding methods versus the mere 3 lb./hr. with the oxy-fuel approach. Electric arc welding is much faster because the electric current is much faster at melting the weld material than the oxy-fuel flame is. What really is electric arc welding? Black: Arc welding is the controlled use of electric current (amperage) to melt both the base and the consumable material. The major types are stick, continuous wire with automatic or semiautomatic feed, and tungsten inert gas, not widely used in construction-equipment repair.
What are the advantages of stick arc welding? Black: One major advantage is that the price of the equipment for stick welding–mainly, the field power source consisting of a diesel or gasoline engine coupled to a generator or an alternator - is low compared to other types of equipment. Also, this equipment is highly portable. Another major advantage to stick welding is that it is very easy to change the weld material. The weld material is the particular alloy of which the electrode is composed; it is a consumable electrode. It is an easy matter to unclamp the present electrode and clamp in a new one made of the weld alloy needed at that moment. Still another key advantage of stick arc welding over other methods of arc welding is that it can also be used for out-of-position welding - that is, for welding in vertical and overhead positions. This makes the stick-welding approach very suitable for doing hardfacing and other welding tasks in the field. Clearly, the power source is the main item needed for field arc welding. Please explain size ranges, weights, prices, and so on. Black: There is a wide range of electrical power sources available on the market for field welding. Such sources typically consist of either a diesel or a gasoline engine driving a direct-current generator or alternator with a capacity to produce an output direct current ranging from 100 to 600 amps, depending on the particular model. The smallest power supplies weigh as little as 50-60 lb., including weight of the engine. On the other hand, the higher-amp units can weigh 1,000-2,500 lb. and must be mounted in a pickup truck or on a trailer. The most popular-size power source with many construction companies doing field arc welding is something in the 200- to 250-amp range. Such a unit would typically weigh 400-600 lb., including engine weight. As for the price range of field power sources, the smaller gasoline-powered units, 125 amps, would sell for about $1,500, including the engine; a 250-amp unit for $5,000-$7,000; a 300-amp unit for about $8,000; and a 400-amp unit for $12,000. For arc welding, do contractors mainly use direct current (DC) or alternating current (AC)? Black: Construction-equipment maintenance people almost always use DC in field arc-welding applications because it gives them much better control over the stick-welding process. Furthermore, there is a much lower probability of electrocution with DC, as the body is able to present greater resistance to the flow of electricity when the electric current is flowing through it in only one direction. Of course, electrocutions, even with AC, are fortunately quite uncommon. Many construction companies also use DC power sources in their shops. A major reason for this is that these are often the same units they transport to the field. Thus, using the same power sources in shop and field gives them flexibility. It also allows them to use the same consumable electrodes for both shop and field applications. Finally, many construction companies will do stick arc welding in the field and semiautomatic-wire-feed arc welding in the shop, and such wire arc welding is done predominantly using DC power. On the other hand, for shop-only arc-welding power sources, some construction companies use AC models because AC is more energy efficient and thus more economical, lowering electricity bills about 20% compared with DC power. Besides stick welding, there are of course other types of arc welding, such as MIG welding. Please expound on what that is, where it is used, how popular it is, and its pros and cons. Black: The acronym MIG stands for metal inert gas. In MIG welding, instead of the welder using a stick electrode, he uses a consumable solid-wire electrode made of the alloy material. The wire can be automatically and continuously fed to the welding gun, making this a fast way to weld. In actual practice, many people have been misusing the term MIG welding, applying it to any kind of arc welding in which a wire electrode is fed continuously through a welding gun. But this is not correct. To speak correctly, MIG welding designates an arc-welding technique that employs a solid, small-diameter wire automatically fed through a welding gun and that uses an inert-gas shield to protect the molten weld area from the atmosphere.
In MIG welding, what is the purpose of the gas shield? What kind of gas is used? And what prevents the gas from floating away from the weld site? Black: The purpose of the gas is to protect the molten weld area from the atmosphere. Specifically, if there were no inert gas covering the weld area to protect it, then nitrogen from the atmosphere would dissolve in the molten, high-temperature metal of the weld pool and, as the liquid cooled, become much less soluble in the liquid at lower temperatures and bubble out of the molten metal, creating a weld area that would be porous, frothy, lacelike, and weak. The weld area would be further weakened by the reaction of atmospheric oxygen with the molten iron, creating iron oxides. It is to protect the molten weld area from these harmful effects of both nitrogen and oxygen that a weld pool must be protected from direct exposure to the atmosphere - either by an inert gas or by a flux over the weld area. As for the gases used to protect the molten weld area in MIG arc welding, typically they are carbon dioxide, a mixture of argon and carbon dioxide, a mixture of argon and oxygen, or a mixture of argon, carbon dioxide, and oxygen. Most MIG welding is done using some sort of argon mixture. Carbon dioxide gas is cheaper than argon but its use is more restrictive. The source of the gas for welding is some sort of pressurized gas cylinder. A tube conveys the pressurized gas from the cylinder through the welding gun onto the weld area. Pulling the trigger on the welding gun activates a solenoid valve in the gas line, delivering gas over the weld area at the rate of 20-40 ft.3/hr. As the old gas blanket drifts away from the welding area, it is continuously replenished with fresh gas. Please expand further on the solid wire used in MIG welding? Black: As I mentioned earlier, in MIG electric arc welding, the wire takes the place of the stick electrode. The wire is a consumable electrode, made of the particular steel alloy needed to make the specific weld. Such solid wire is available in diameters ranging from 0.02 in. up to 0.0625 in. The MIG wire feeder feeds wire off a reel through the welding gun continuously and automatically at a preselected feed rate. The operator controls starting and stopping with the welding-gun trigger. The welder would of course select a solid wire made of the appropriate steel alloy for the welding task. In addition, the diameter of the wire chosen would depend upon the size of the power source. The lower the amps, the smaller the wire diameter that would be chosen and the slower the flow of the protecting gas. The higher the amps, the bigger the wire that could be used and the greater the flow of protecting gas. So far you have discussed two main types of electric arc welding: stick welding and MIG welding using solid wire. Aren’t there other important types of electric arc welding that use tubular wire - so called metal-cored wire and flux-cored wire - instead of solid wire? Please discuss what they are, where they fit in, and their pros and cons. Black: Metal-cored wire is a tubular wire that contains a metal alloy in the core of the wire. There is no flux material placed in the core; thus, the weld area has to be protected by an inert gas or some other means. Flux-cored wire, on the other hand, is a tube of wire - diameters range from 0.035 to 0.125 in. - that contains both a metal alloy and a flux within the wire’s core. When melted during welding, the flux is released from the core, forming a protective coating over the weld beads. The flux protects the molten weld from the atmosphere during welding and, upon cooling and hardening, forms a shiny surface, enhancing the weld’s appearance. Said another way, flux-cored - wire welding is like doing stick electric arc welding "inside out." In stick welding, the slag is a coating on the outside of the electrode. In welding using flux-cored wire, the slag is in the inside core of the wire. Both solid-wire and metal-cored - wire electric arc welding are used less in the field and in out-of-position applications since there is nothing to support the weld pool. Their use is mainly for flat or horizontal welding. With flux-cored - wire welding, on the other hand, the flux supports the weld pool; thus, this self-shielded method is most suitable to field welding applications and is quite popular there because welders do not have to worry about a protective gas blowing away. Electric arc welding using self-shielded flux-cored wire has become a very common method for field applications. Why its popularity? (1) There is no need to transport gas cylinders to the field, (2) there is no need to erect welding curtains to keep the wind from blowing into the weld area, and (3) there is no need to worry about protective gas blowing away. As I mentioned, the flux-cored wire is self-shielding: The flux contained in the core flows over the molten weld beads, protecting them from direct exposure to the atmosphere. Nonetheless, in shop welding applications, solid-wire and gas-shielded flux-cored and metal-cored welding is extensively used. Indeed, some welders prefer gas-shielded welding in shop situations, especially with argon-gas mixtures; metal transfer is smoother, and there is less smoke. Incidentally, the same electric power supply and other equipment can be used for both flux-cored - wire welding and solid-wire MIG welding. In switching over from flux-cored wire to MIG welding, the worker does need to change both the wire and the welding gun, for the gun provides the arrangements for gas shielding. What, then, are the most popular welding methods used in the field for the repair of construction equipment? Black: Eighty percent of the welding on construction equipment in the field is done with either stick or self-shielded flux-cored electric arc welding. There is limited use of gas shielding in the field because gas shields can too readily blow away. The stick electrode and self-shielded flux-cored wire are self-shielded products: They contain the flux material that protects the molten weld either on the outer surface, as on the stick electrode, or in the wire’s core. What about in the shop? What are the most popular welding methods there? Black: Gas-shielded arc welding using solid wire or metal-cored or flux-cored wire is quite popular, accounting for perhaps 80% of shop welding. Furthermore, on the more sophisticated and extensive construction-equipment repair jobs–for example, welding repair work on tractor undercarriage sprockets and idlers and on cable drums and pulleys–there is a type of arc welding used not yet mentioned: fully automatic submerged arc welding. In this method, the gun feeds either a solid or tubular wire to the weld pool. A granular flux having the texture and appearance of kitty litter is either spread by hand, gravity-fed, or carried in a compressed airstream over the weld pool, forming a protective blanket. Since the arc is submerged beneath the flux layer, this method is called the submerged arc welding. The unfused flux material can be recirculated. Submerged arc welding is popular in the shop for such applications for these reasons: a high-quality weld, higher productivity compared to other arc welding methods, no smoke, and no visible arc light, the arc being submerged under the granular flux - thus, welding hood and protective clothing are not needed.
What is hardfacing? Black: Hardfacing is the application of a wear-resistant layer onto a surface that is to be exposed to wear in order to extend the life of that surface. It is the process of depositing that wear-resistant layer or pattern of ridges by one of various welding techniques. In hardfacing, say, the inside or outside surfaces of a loader or excavator bucket, why is it not common practice to coat the entire surface with the weld material? Why is it that welders generally create some sort of grid pattern composed of 0.125-in.-high "ridges"? Black: The approach you suggest, laying down a complete coating of hardfacing material over the bucket surface, would be the most effective surface protector. But that is rarely done because of cost and time required. Rather than laying down a continuous coating, it is much more economical of both hardfacing material and of a welder’s time to apply a grid of elevated ridges or other geometrical pattern. Using a grid pattern rather than a continuous layer also reduces weight. A typical hardfacing grid line that a welder would lay down would be two beads (0.25 in.) wide and one bead (0.125 in.) high. If working in a softer material, such as a loamy soil, the aim should be to lay down a grid pattern of hardfacing ridges that will tend to trap the soil on the steel surface, forming a layer of clinging earth that will protect the steel surface from further abrasion. This is best done by laying down a series of parallel ridges, perhaps 2 in. or less apart, that are 90º to each other. In other cases, say when operating in more rocky soils, the aim is not to trap soil on the surface - the rockiness of the soil would make that difficult - but to protect the underlying steel from abrasion caused by the movement of the rocky soil directly over the steel surface. This can be done by laying down a pattern of hardfacing ridges in the direction of flow, like rails, or even a pattern of hardfacing "dots" - anything that will prevent the rocky soil from coming into direct contact with the steel surface; the soil in effect slides over the tops of the ridges without coming into direct contact with the steel surface. Many construction-equipment welders seem to use a welding rod called E7018 for a wide range of welding applications. Is E7018 a sort of cure-all for a wide range of welding tasks? Black: Definitely not! Welding rods of the E7018 class are not a cure-all for all sorts of welding repair applications. Many welders are using E7018 not because it is the appropriate alloy for a particular task but because it is cheap. And that is a serious mistake because it means that the welding workmanship will not last very long. Consequently, the construction equipment will suffer further downtime, which can be very costly. E7018 is a welding alloy suitable for joining certain metals together; for example, welding a brace onto an excavator boom. But welders are misusing this material because it is cheap. Often they are using it for the buildup material that serves as the base for the hardfacing alloy that is subsequently laid on top of it. And sometimes they are using the E7018 for the hardfacing material itself. Such weldings won’t last and won’t have anywhere near the durability that they should have. This is because E7018 is a relatively soft material, unsuitable for applications calling for high abrasion resistance and high resistance to compressive loadings. In doing the hardfacing of an excavator bucket, a dozer blade, or a cable pulley, many welders often select both the wrong buildup material and the wrong hardfacing material. Is that your contention? Black: Yes! E7018 should not be used in hardfacing applications as a buildup material since it has a tensile strength of only 70,000 psi and a yield strength of only 58,000 psi, whereas the softest legitimate buildup alloys have tensile strengths of 100,000 psi and, more typically, 150,000 psi and a yield strength of 98,000 psi. In sum, at least half of the welders in the construction industry are incorrectly doing the hardfacing of loader and excavator buckets, bulldozer blades, pulleys, and other construction-equipment components. They are using the E7018-type welding rod or wire to construct the buildup layer, when that material is too soft and too weak for the application. And even if they do select an appropriate buildup material, they often select a hardfacing material that is not suitable. Sometimes they are using E7018 for the hardfacing layer, which is far too soft and weak. This is because they fail to consider what the specific wear environment is for that construction-equipment component and pick the product by hardness. Please provide some guidance as to how welders should go about selecting the appropriate rod or wire for hardfacing various construction-equipment components. Black: It is most important that the maintenance repairer ask himself, "What is the wear environment for the particular construction-equipment component I am about to hardface?" Is it metal-to-metal wear; for example, a cable winding over a pulley or onto a metal drum? Metal-to-metal wear plus impact? Severe impact, such as a crushing hammer? Metal-to-earth abrasion plus impact, such as dump-truck body surfaces, an excavator bucket and bucket teeth, a dozer blade? Or severe metal-to-earth abrasion plus impact, such as an excavator bucket and bucket teeth, a dozer blade, or a scraper used in especially abrasive soils, one requiring a tungsten carbide hardfacing material? What particular buildup material to use for what application? In some cases, the same material can be used for both buildup and hardfacing. In most cases, a buildup material with a Rockwell hardness of 20-35 Rc is needed. And that will also be the minimum hardness range needed for any hardfacing material. The hardfacing material in a metal-to-metal application needs to have a Rockwell hardness in the range of 38-58 Rc. If, for instance, this involved a small metal gear meshing against a large one, one should make the surface of the larger, more expensive gear harder than that of the smaller gear; that way, all the wear is on the smaller, less expensive gear. In hardfacing applications where there is moderate metal-to-earth abrasion and impact, the hardfacing materials would often be austenite and chrome carbide alloys with a hardness range of 28-53 Rc. For more severe metal-to-earth abrasion and impact, one would use austenite and chrome carbide alloys with a higher hardness range (49-59 Rc). And for severe-abrasion applications with little impact, one would use chrome carbide alloys containing as much as 5.5% carbon and 30% chrome, with hardness ranging from 55 to 70 Rc. The most severe-abrasion applications - the hardfacing of bucket teeth, blades, and scrapers in abrasive soils - would use a tungsten carbide alloy for a hardfacing material. In many cases, welders use a metal-to-metal welding material for applications that demand a material with good abrasion resistance - and the results don’t last because the abrasion resistance of metal-to-metal materials is poor. In applications where there is impact - such as rock crushers and rolls used in crushers - the hardfacing material needs to be made of Hadfield manganese, a steel alloy containing 14% manganese. In hardfacing a loader or an excavator bucket, a relatively soft alloy will work fine when handling soft, loamy soils. On the other hand, sandy soils demand the use of a hardfacing material with high abrasion resistance. To reiterate: Welders often select the wrong hardfacing material. For instance, they might select a material that has been designed for metal-to-metal wear when they should be selecting a material that has excellent abrasion resistance in a metal-to-earth environment. They are doing this because they fail to realize that there is a wide range of welding materials and that each is designed for use in a particular wear environment. They must: (1) identify the wear environment and (2) select the most appropriate welding rod or wire for that wear environment. In rebuilding construction equipment or in applying hardfacing to critical construction-equipment components, the overall goal is to minimize future equipment downtime. Too many maintenance people focus on the cost of the welding rods or welding wire, selecting alloy materials more for their low cost than for their suitability. Such an approach constitutes false economy. The cost of the welding consumables is not all that important. But what can be very costly is equipment downtime; a single piece of equipment being out of service frequently costs a contractor thousands of dollars per day. And when hardfacing is done using cheaper and inappropriate welding alloys, the certain result will be that the welding repair will fail sooner than if it had been done correctly, leading to costly downtime. You discussed how to select the most appropriate hardfacing material. But what are the guidelines for selecting the buildup material itself? Black: The first step in selecting an appropriate buildup material for a hardfacing application is for the welder to identify the base material he plans to weld to. Is it a carbon steel, a low-alloy steel, a high-alloy steel, a high-manganese material, or something else? Once the welder has identified the base material, he can then determine the most appropriate buildup material and the temperature to which the base material must be preheated before welding can begin. It is true that some welders do not preheat the base material - or do it incorrectly - an omission that increases the probability that the base material will crack during welding. A good rule of thumb on the temperature to which the base metal has to be preheated is this: If it is a 0.2% carbon steel, heat the base metal to 200ºF; if 0.3% carbon steel, preheat to 300ºF; if 0.4% carbon steel, preheat to 400ºF; and so on. The base metal must be kept at that temperature as a minimum, throughout the welding process, or cracking can result. But it would be OK to let the temperature rise 200º-300ºF above that minimum temperature. The base metal can be preheated using an oven, an oxy-fuel torch, or an electric-resistance heating blanket. How does a welder determine the character of the base metal to which he is welding; what sort of steel or steel alloy it is? Black: The first step is to find the manual on the construction equipment. This will often indicate the types of steel used for the various components. He might also call a dealer of that particular type of construction equipment and ask. If those approaches do not work, he could then run some simple tests to find out. If a magnet does not stick to the metal surface, that suggests that the base metal is either stainless steel or Hadfield manganese. If the magnet does stick to the surface and the surface is rough and very rusty, then the base material could be cast iron. If the magnet does stick and the surface is not rough or rusty, the next step would be to chisel off a chunk of the metal surface. If he is able to shave off the surface, that indicates the base metal is a carbon steel. If it comes off as a chunk, it is cast iron. If the chiseling creates a dent in the surface, and if when hit again nothing happens, that is an indication that the first blow work-hardened the dented area. Such metal behavior suggests that the base metal is a manganese steel. Now if these above tests suggest that the base metal is a carbon steel, the question then becomes, "What is the carbon content?" To find out, it is simply a matter of running a spark test by touching the base metal with a grinding wheel. If a large number of yellow sparks are thrown off, that indicates the carbon content of the steel is 0.25% or less. If, on the other hand, there are fewer sparks and they are of reddish color with tails trailing, and if the total length of the sparking is less than 2 ft., that suggests that the base metal is a steel with about 0.4% carbon. If there are no sparks at all or very few, then the base metal is likely a chromium carbide alloy. You said there is a wide range of hardfacing rod and wire on the market. In other words, they have materials available for a wide range of arc-welding techniques? Black: The vast majority of hardfacing materials are in the form of rod and wire that are self-shielded - that is, that contain fluxes for shielding the molten weld from the atmosphere. There is only a limited amount of hardfacing materials available on the market suitable for gas-shielded arc welding - and those are mainly used in shop applications. Gene Dallaire is a former feature-article writer for Chemical Engineering and Civil Engineering magazines. He currently teaches history at Lansing (MI) Community College. A companion article titled "Dealing with Metal Wear on Construction Equipment" reviews in-depth histories that describe equipment dealers' and progressive grading and excavating contractor's most pressing metal-wear problems and how they're solving them. It appears in the January/February 2001 issue.
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