June 9, 2009
Welding to repair damaged components in broken equipment involves three stages, each of which must be performed meticulously to ensure that the machinery runs properly and the repair lasts a reasonable length of time. What are these stages, and what equipment do you need to complete them?
Broken equipment—whether it's a large excavator in a gravel pit or a conveyor belt in a processing plant—eats away at your profits. You can almost hear the money blowing away in the deafening silence of an idle machine.
Repairing broken steel components in the field requires mastery in three areas:
The first step in welding repair is to remove the damaged metal. This can be done with oxyfuel or plasma cutting or carbon arc gouging. Oxyfuel and plasma typically are better for cutting throughmetal, whereas carbon arc gouging is better for gouging out a crack or defect without completely severing the part.
Oxyfuel torches, one of the most common tools for cutting, usually can be found on most service trucks. Plasma cutters, however, produce a smaller kerf (cut width), a smaller heat-affected zone (HAZ), and typically are faster than oxyfuel torches. Plasma cutters also cut through all electrically conductive metals, whereas oxyfuel can't cut through aluminum or stainless steel.
Carbon arc gouging is another cutting/gouging option when using welding generators with 300 to 500 amps of output and a high duty cycle. Carbon arc gouging uses a carbon electrode to melt the defective area and blast away molten metal with a focused, high-pressure stream of air.
To begin the repair, 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 even the slightest remnant of a defect will continue cracking, even after a weld is laid over it.
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, ductility, wear resistance, impact strength, and tensile strength. An exact material match ensures weld quality and longevity and helps to prevent premature failure and unwanted downtime.
The downtime for repair also provides an excellent opportunity to reinforce trouble spots. A part that breaks in the same place more than once might need 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 beveling your edges, by grinding along the surface until the bottom portion is about the thickness of a nickel.
Welding joint cleanliness 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 before welding; failure to do so will lead to a failed or weakened weld.
Once the piece is in place, it may be necessary to preheat the weld area. 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 typically is required on all material thicknesses when the carbon content of mild steel exceeds 0.40 percent. Consult your material supplier for specific material and process requirements.
To preheat, use an oxyfuel 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 the material being heated, change color when the target temperature is reached. Again, consult your material supplier for specific material and process requirements.
The two most common processes for field welding repair are shielded metal arc welding (SMAW), or stick, and flux cored arc welding (FCAW), or flux cored. Stick electrodes are self-shielded, as are many flux-cored wires designed for this application. Self-shielded processes reduce 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 in which wind interferes with shielding gases is more achievable using either the stick or flux-cored process.
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. Another option is E71T-8JD H8. These wires are all-position, multipass 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 with the former, there typically is no need to switch between wire types or sizes for the same repair. This allows you to lay bead after bead while stopping only to remove slag.
Selecting the right machine for stick welding is based largely on the diameter of electrodes to be used. A 1/8-in. electrode welds up to 145 amps, while a 5/32-in. rod performs optimally at about 180 amps. Therefore, a welding generator with a 100 percent duty cycle at 250 amps 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 also is necessary for short-circuit gas metal arc welding (GMAW) 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 or voltage-sensing capabilities. A remote control 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 you. A voltage-sensing wire feeder, however, works with any welding generator and is easy to hook up with no additional cables. The only real downsides to a voltage-sensing feeder are the lack of voltage control at the feeder and a little extra weight from its mechanical contactor.
On the opposite end of the size spectrum are portable, all-in-one machines (Figure 1) for repair in hard-to-reach locations (deep inside a plant, high up on scaffolding). These machines offer GMAW and flux-cored capabilities up to 150 amps and can plug into any 115- or 230-V power. This provides a portable, remote field welding solution for jobs for which it may be tough to get a truck near the weld. You can even perform repairs on stainless steel and aluminum by using the self-contained gas cylinder and by adding a spool gun with the all-in-one machines.
To perform carbon arc gouging, you need to make sure your machine is rated for the carbon diameter you want to run.
Contractors have come to expect the dual welding and power generation capabilities of engine-driven welding generators (Figure 2). 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. Some machines have 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.
For heavy-duty repairs and space savings on maintenance trucks, fleet managers might consider options that also include self-contained rotary screw air compressors for running air tools and plasma cutters.
Another factor to consider when selecting an engine drive is fuel. Most welding generators are available in gasoline and diesel. Gas engines offer a lower product cost, reduced weight, and smaller size. Diesel engines use 20 to 35 percent less fuel, have longer engine lives, and are required on certain sites. Choose whichever fuel option best suits your needs and work environment.
This article merely scratches the surface of the many repair variables and equipment options available. When in doubt, don't hesitate to contact your local welding supply expert for equipment and application tips. Taking the time to fix something right the first time will prevent it from breaking again—and that will save you money.