The Basics of Field Welding Repair on Construction Equipment
In the construction industry, nothing cripples a jobsite like a broken machine. When it comes to fixing broken steel in the field, there are three steps to master:
* Cutting and removal of the failed component.
* Preparation of the new joint/part.
* Welding and cleanup.
This article discusses these three stages and helps guide you to the right combination of equipment to meet your field repair needs.
|Stick welding, or SMAW, is the most commonly used process for field repair|
Cutting and Removal of the Failed Metal
The first step in welding repair is removal of the damaged metal. This can be done with oxy-fuel, plasma cutting or carbon arc gouging. Oxy-fuel and plasma are typically better for cutting through metal, whereas carbon arc gouging is better for gouging out a crack or defect without completely severing the part.
Oxy-fuel torches are one of the most common tools for cutting and are found on most service trucks. Plasma cutters, however, produce a smaller kerf (cut width), a smaller heat affected zone (HAZ), and are typically faster than oxy-fu el torches. Plasma cutters also cut through all electrically-conductive metals, whereas oxy-fuel won’t cut through aluminum or stainless steel.
Carbon arc gouging is another option when using welding generators with 250 to 500 amps of output and a high duty cycle. This process uses a carbon electrode to melt the defective area, and blast away molten metal with a focused, high-pressure stream of air.
To begin: cut away the damaged area and remove all rough edges to ensure proper fit-up of the replacement part. It is extremely important to fully grind out all cracks—even beyond what’s visible—because the slightest remnant of a defect will continue cracking even after a weld is laid over it.
Engine-driven welding generators feature 5,500 to 20,000 watts of generator power , depending on the model, for running grinders (pictured), drills and other tools.
Preparation of the Weld Joint
Choosing the correct replacement/filler material is critical. All components should be replaced with a material that meets or exceeds the strength of the parent material. Each application varies in mechanical properties such as required strength, ductility, wear resistance, impact strength, and tensile strength. An exact material match ensures weld quality and longevity, and avoids premature failure and unwanted downtime.
This downtime also provides an excellent opportunity to reinforce trouble spots. A part breaking in the same place more than once might mean it needs to be reinforced with additional steel.
Once you’ve obtained the right alloy, cut the steel to its required size and bevel the edges at a 30-degree angle for better welding penetration. For heavier sections of material it is recommended to leave a small “land” at the bottom of the joint. This can be done, after having beveled your edges, by grinding along the surface until the bottom portion is about the thickness of a nickel.
Cleanliness of the welding joint is critical. While some welding processes are more forgiving than others, it’s never wise to leave any contaminants behind. All rust, oils, and paints must be ground or wiped away prior to welding—failure to do so will lead to a failed or weakened weld.
Once the piece is in place, preheating the weld area may be necessary. Preheating is done to remove hydrogen and other gases, reduce the maximum hardness, minimize shrinkage stresses, and minimize distortion; all of which might cause cracking when an extremely hot welding arc is applied to cold steel. Preheating is typically required on all material thicknesses when the carbon content of mild steel exceeds .40 percent. Consult your material supplier for specific material/process requirements.
To preheat, use an oxy-fuel torch outfitted with a special "rosebud" tip that widens the flame. Preheating temperatures vary based on the material to be welded. A temp stick (or heat crayon) can be used to gauge the temperature as it changes. Temp sticks come in various temperature values and, when applied to material being heated, will change color when the goal temperature is reached. Again, consult your material supplier for specific material/process requirements.
Which Welding Process Should You Use?
The two most common processes for field welding repair are Shielded Metal Arc Welding (SMAW), or Stick; and Flux Cored Arc Welding (FCAW). Stick electrodes are self-shielded, as are many Flux Cored wires for this application. Self-shielded processes cut down on the amount of equipment needed—no need to haul in a gas cylinder, hose and regulator. Adequate protection of the weld bead in outdoor applications where wind interferes with shielding gases is more achievable using either the Stick or Flux Cored processes.
Common electrodes used in Stick welding are 6010, 6011, 6013, 7018 and 7024 with common diameters ranging from 1/8- to 5/32-in. Each of these electrodes offers all-position welding capabilities (except 7024). The first two digits of a stick electrode represent the “as welded” minimum tensile strength: 6010 provides 60,000 psi tensile strength, for instance.
A common wire for Flux Cored welding in repair applications is the self-shielded, general-purpose E71T-11 wire (such as Hobart’s Fabshield® 21B). Another option is E71T-8JD H8 (such as Hobart’s Fabshield® XLR-8). These wires are all-position, multi-pass wires with good impact properties at low temperatures. FCAW can replace and improve productivity over 7018 Stick welding in certain applications. Both wires offer higher deposition rates than Stick electrodes, and the slag removes easily. An added benefit of Flux Cored over Stick is that there is typically no need to switch between wire types or sizes for the same repair. This allows the welder to lay bead after bead while stopping only to remove slag.
Machines that include welders, generators and air compressors in the same unit take only half the bed space of a separate engine-driven air compressor and welder, freeing up 50 percent more room on your truck for equipment and supplies.
Welding Equipment Selection
Selecting the right machine for Stick welding is based largely on the diameter of electrodes used. A 1/8-in. electrode welds up to 145 amps, while a 5/32-in. rod experiences optimal performance at about 180 amps. Therefore, a welding generator with a 100 percent duty cycle at 250 amps (Miller's Bobcat™ 250) offers enough welding power to meet most Stick welding needs.
For Flux Cored welding, a welding generator with constant voltage (CV) output provides superior wire welding performance versus a constant current (CC) machine. A CV output is also necessary for short circuit MIG welding for general fabrication. Amperage requirements vary based on the type and diameter of wire you are using, but 250 to 350 amps is sufficient for most applications.
You also need to match your welding generator with a wire feeder for Flux Cored welding. There are two options for field work: portable suitcase wire feeders with either remote voltage controls (SuitCase® 12RC ) or voltage sensing capabilities (SuitCase X-TREME 12VS). An RC machine offers voltage and wire feed speed control at the feeder and no mechanical contactor, which lowers its weight. These machines require a welding generator with a 14-pin receptacle and an extra cord between the feeder and the welder. This limits this particular feeder to within 100 ft. of the welder. A voltage-sensing wire feeder, however, works with any welding generator, and is easy to hook up with no additional cables needed. The only real downside to a voltage-sensing feeder is the lack of voltage control at the feeder, and a little extra weight from its mechanical contactor.
Factors to Consider for Gouging, Power Generation and Air Supply
To perform carbon arc gouging, you need to size your machine to the carbon diameter you want to run. As an example, for maximum gouging performance, look for a welding generator that provides at least 200 amps for 3/16-in. carbons, 300 amps for ?-in, 350 amps for 5/16-in. and 450 amps for 3/8-in.
Contractors have come to expect the dual welding and power generation capabilities of engine-driven welding generators. These machines save space on maintenance trucks by eliminating the need for a stand-alone generator, and have the power to run grinders, drills, chop saws, lights, and air compressors. New machines feature two separate generators in the same unit—one for the welding arc and one for auxiliary tools. Keeping these generators separate allows a worker to fire up any tool off of the machine’s generator while another person is welding without affecting the performance of the welding arc. Manufacturers are also beginning to add battery chargers/jump starters to engine drives (Trailblazer 302 Air Pak) to give field mechanics another tool to combat idle equipment.
For heavy-duty repairs and space savings on maintenance trucks, fleet managers should consider a combination welder/generator/air compressor. These machines not only feature welders and generators, but also include self-contained rotary screw air compressors for running air tools and plasma cutters. The Trailblazer 302 Air Pak, for instance, offers 26 CFM of air (up to 160 PSI) at 100 percent duty cycle.
Another factor to consider when selecting an engine drive is fuel. Most welding generators are available with gasoline or diesel engines. Gas engines offer a lower product cost, reduced weight and a smaller size. New electronic fuel injected (EFI) welding generators with gas engines reduce fuel use by as much as 27 percent and harmful emissions up to 33 percent compared to carbureted models, offering contractors yet another way to go green. Diesel engines typically use 20- to 35-percent less fuel than gas engines, have longer engine lives, and are required on certain sites.