Mobile equipment: Face-lift
January 26, 2009 By: Carl Peters Pit & QuarryAre you looking for ways to extend the service life of your construction equipment? One of the techniques you can use is called hardfacing, which is depositing wear-resistant surfaces on metal components to extend their life. Hardfacing can be used to restore worn parts to useable condition and also to overlay new components before being placed into service. Hardfacing is beneficial as it decreases the need for replacement parts, increases operating efficiency and reduces overall costs.
Hardfacing is commonly performed on metal edges that scrape the earth or crush other materials, such as excavator buckets and tractor blades.
Operators first learning to hardface may not achieve the desired results and assume the machine or electrodes are not working properly. In many of these instances, though, the equipment operator did not take the necessary preparations before welding or has chosen the wrong process, parameters or consumables. Here, we will discuss various hardfacing applications relevant to the construction industry, ways to identify metallurgy as well as basic hardfacing and welding procedures, and safety.
In many cases, rebuilding equipment is less expensive than the cost of replacement, making hardfacing an attractive option. There are many different items that could potentially benefit from hardfacing that can be placed into three basic “wear” categories -- abrasion, impact and metal-to-metal.
Abrasion is the most common type of wear. This includes all earth-engaging implements, such as tractor buckets, blades and teeth. Under the impact heading, you will find equipment used to pound and smash, such as crusher hammers. Metal-to-metal occurs when steel parts roll or slide against each other on such items as crane wheels, pulleys, idlers on track-drives, gear teeth and shafts.
Basic metallurgy
Before you can hardface or weld, you first need to identify the parent metal by following one of the testing methods described below.
The first is a magnetic test. If a magnet will stick to the implement, it is likely iron-based. A magnet that will not stick indicates probably a manganese or stainless product.
The spark test involves taking a grinder to the item. If you get a 30-in. long, moderately large volume of yellow sparks with just a few sprigs and/or forks, this indicates the metal is a mild steel. If, however, you achieve a 25-in. long, slight-to-moderate volume of yellow, orange sparks, a few forks with intermittent breaks but few if any sprigs, this indicates an alloy steel. If the sparks are 15-in. long, short and red in large volume with numerous and repeating sprigs, this is a sign of a high carbon metal.
The chisel test helps to indicate the type of metal as well. If the metal fractures in large chunks when you take a chisel to it, this means you have cast iron, which can be difficult to weld unless using special high-nickel electrodes and heat-treating. On the other hand, if the chisel yields corkscrew-like shavings, you are looking at a weldable steel.
Identify the method
We will now examine three types of welding methods, which differ by speed and cost. Please remember that the methods are available to all welding and hardfacing products. However, specific products often have properties that are somewhat unique and not exactly duplicated when utilized by a different process.
• Stick welding: Manual or stick welding requires the least amount of equipment and provides maximum flexibility for welding in remote locations and in all positions. Typically, each rod permits welding for about one minute. In seconds, one can change from mild steel to stainless to hardfacing or from a small to a larger diameter electrode for small or large welds. Although this type of welding is the most simplistic, it takes the greatest operator skill.
• Semiautomatic: This type of welding uses wire feeders and continuously fed electrodes. The welding gun is hand-held by the operator. The gun continuously feeds wire as long as the trigger is depressed, making it much easier for an operator to learn than stick welding. Semiautomatic welding increases deposition rates over manual welding because there is no need to stop after burning each rod.
• Automatic: Requiring the greatest amount of initial setup, automatic welding has the highest deposition rates for maximum productivity. The welding gun is carried by a mechanized carriage and the welding operator simply pushes a start button. This type of welding is common at repair centers for heavy equipment.
Welding procedures
There are five basic steps when hardfacing or welding that must be followed.
• Proper preparation: You first need to ensure that the metal you are hardfacing or welding is clean and dry. Remove rust, dirt, grease, oil and other contaminants by wire brushing. If not removed, these contaminants can cause porosity, cracking and poor weld deposit quality. You must also remove badly cracked, deformed or work-hardened surfaces by grinding, machining or carbon-arc gouging.
• Proper preheat: The combination of alloy content, carbon content, massive size and part rigidity creates a necessity to preheat in many welding or hardfacing operations. Most applications require preheating, as a minimum to bring the part to a room temperature of 70°-100° F. Medium to high carbon and low alloy steels will require higher preheat to prevent underbead cracking, welding cracking or stress failure of the part. Preheating can be done with a torch, oven or electrical heating device. Special temperature-melting crayons can help you verify proper preheat. Too much heat can often ruin alloy materials.
• Correct welding procedure: Identify the correct wire feed speed (amperage), travel speed, size of weld, polarity, etc. Make sure the completed weld meets your expectations in regards to size and appearance. Welds should be smooth and uniform, free from undercut or porosity. If possible, watch a video showing the type of welding you will be doing so you know how the welding procedure and final weld should appear.
• Proper cooldown: Preheating is the most effective way of slowing the cooling rate of massive or restrained parts, which are inherently, crack sensitive. Insulating the part immediately after hardfacing or welding with dry sand, lime or a glass fiber blanket also helps minimize residual cooling stresses, weld cracking and distortion. Never quench a weld with ice or water as this will lead to greater internal stresses and potentially weld cracking.
• Post weld heat treatment: Some items may require tempering or heat-treating. What this means is that you warm the item with your torch after welding and allow it to slowly cool.
Safety
There are a few rules you should follow as you are welding or hardfacing:
• Protect yourself from fumes and gases: Always hardface or weld in an open, well-ventilated room and keep your head out of the fumes -- especially with hardfacing.
• Wear protective clothing: Protect your eyes and face with a welding helmet designed for arc welding, not just gas-welding goggles. In the same manner, protect your body from weld spatter and arc flash with woolen or cotton clothing, a flameproof apron, gloves and boots. Also make sure to protect others around you from the arc rays as well.
• Beware of electric shock: Do not touch live electrical parts and make sure your welding machine is properly grounded. Never weld if you are wet or if your gloves have holes in them.
• Fire/explosion hazard: Never weld in an enclosed space or near materials that could be ignited by welding sparks. Never weld alone. Always have a buddy nearby in case of an emergency.
Carl Peters is director of technical training at The Lincoln Electric Co. He can be reached at carl_peters@lincolnelectric.com.
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