September 1, 2009
Stud welding, an often-overlooked process, can have a significant effect on the life-cycle cost of a product.
In coal-fired power plants, so much relies on the little things. Smooth, cylindrical studs 3/8 inch in diameter by 0.25 in. long are placed in furnaces operating at 1,600 degrees F or more. The studs transfer heat from the hot side (the furnace) into the boiler tube. A refractory coating, which protects the tubes from erosion, is applied over the studs. As the stud wears, so does the refractory coating. At some point the studs become so short that the furnace has to be shut down, the studs rewelded, and the refractory coating reapplied.
For decades plants have used 430 stainless steel studs, a material that has to be replaced about every 12 months. This can be an expensive endeavor, especially if unplanned. The entire furnace must be shut down so workers can access the components, remove the studs (usually by this point worn down completely to the tube), and reweld new ones. Increasingly, though, another corrosive-resistant material—a mix of aluminum and iron—has been found to extend service life; some early tests show by three to four times, depending on the application. Moreover, the stud runs cooler, so it transfers heat more efficiently than a stainless steel stud can, thereby allowing everything to run more efficiently.
This development exemplifies one of the advantages of stud welding, a process that can have a significant effect on the life-cycle cost of a product.
Wire processes rely on the welder's steady hand and, in automated environments, optimal fixturing to ensure the best joint access and quality weld. Stud welding, however, is different. The end product may look a bit like resistance projection welding; a stud is analogous to a fastener put in place by a press-type resistance welder or automated system. But stud welding doesn't need two-sided access, which is especially valuable in automated situations. In manual setups, workers bring a hand tool to the work. Like wire welding, stud welding relies on an electric arc to fuse metal between the stud and base metal (see Figures 1 and Figure 2). But unlike wire welding, the stud process uses a single burst of energy, measured in microseconds, to make the weld (see Figure 3). And since most of the welding process happens automatically with a push of the trigger, it need not be performed by a highly skilled, specialized operator.
What exactly happens between the stud and base metal depends on the stud welding process. In traditional electric arc stud welding, the stud is welded to the base plate using a ceramic ferrule to hold the molten metal in place until the bind is formed. (A process variant, gas arc stud welding, uses a shielding gas instead of a ferrule.) This produces a dense weld as strong as the fastener and base plate. The best bond forms when the base plate is heavy enough to support the full strength of the welded fastener, though the process can be used for lighter-gauge applications as well. Welding currents are from 200 to 2,400 amps, and weld cycles are from 0.1 to 1.5 seconds, depending on the diameter of the fastener and the material being joined. It uses a specialized power source that in some respects is similar to the DC power sources used for shielded metal arc (stick) welding.
Capacitor discharge (CD) stud welding, however, uses an electrostatic storage system as a power source—in essence, a bank of batteries. Studs are engineered with a small projection, or tip, with high electrical resistance. The burst of electricity vaporizes the tip, which creates molten metal both on the base material and the stud. The spring in the gun then pushes the stud down into that molten metal, which solidifies quickly. This all takes place in 6 to 10 milliseconds. The process limits the heat generation and has a low penetration level, so that studs can be welded to extremely thin and coated material. It also uses no ferrule or flux. The weld quality, though, does depend on the geometric consistency of the stud's tip. If the tip dimension varies, so does weld quality. If the tip is too small, the energy liquefies it too quickly; if it is too large, the tip may not totally liquefy for complete fusion.
Drawn-arc capacitor discharge stud welding, however, controls the time of current flow independent from the stud geometry. It uses the same principle of storing energy in capacitors, but in this process the stud doesn't require a tip. Instead, the weld is created by pulling the stud off the workpiece and drawing an arc between the stud and the plate. Immediately after, a spring plunges the stud down into the molten material to create the weld. Unlike conventional CD stud welding, its drawn-arc variant is a controlled process. The operator adjusts the settings on the welding power source to control the stud's drawing action as well as the energy put into the weld.
While drawn-arc CD stud welding can be controlled, it isn't a closed-loop process. If some unforeseen variation in stud size occurs, the system can't compensate on-the fly. Only the latest stud welding technology—inverter drawn-arc stud welding—can sense changes and compensate for them within a millisecond, to ensure the process uses the correct current and time for the stud actually being welded. Such inverters weld with up to 1,500 amps for 1 second and less, giving that burst of energy required for fusing the stud to the base plate. (A drawn-arc variant called short-cycle stud welding is optimized for automated stud feeding and high-productivity environments.)
CD stud welding, though older, still has its place in the market. The process leaves no backside marking and is particularly suited for thin-gauge work that requires cosmetically important finishes. Most other applications, however, have transitioned to an inverter stud welding process.
Applications for stud welding run the industrial gamut. The automotive sector, for instance, has embraced stud welding as a way to attach fasteners to sheet metal in the car body to hold cable harnesses and to provide grounding for electrical systems. By replacing projection welding of small stamped brackets with stud welding, manufacturers have reduced tool and fixture costs. The process also uses smaller fasteners, which reduces weight.
At the other end of the application spectrum, power plants require studs to endure extreme heat, oxidation, corrosion, and erosion. And in these environments, inadequate material performance can lead to some significant failures.
For instance, in 2003 one of the largest federally owned coal-fired power plants in the country had a premature failure of newly replaced components, and part of the investigation concluded that the stainless steel 430 used in the studs was not a robust material for these extreme environments. Managers began a search for alternatives, including stainless 310, INCONEL® alloys, and a number of other exotic materials. Some performed fairly well, but they were too expensive.
In the end, plant managers chose a specialized aluminum-iron stud composition. The aluminum holds the secret to its performance. When the stud reaches its service temperature inside the furnace, between 1,600 and 1,800 degrees F, the aluminum content spurs the formation of aluminum oxide (the same hard material used in some cutoff and grinding tools) on the surface of the stud. This coating helps prevent corrosion, erosion, and oxidation. Most important, it has greatly extended the boiler tubes' service life.
This aluminum-iron stud material could work in other high-temperature applications, including electric arc furnaces in steel mills. Like in power generation, these furnaces have casings lined with a refractory material, and today the steelmaking industry has the same stud durability issues as the power generation market.
In these applications and others, a simple change can make a significant impact. When it comes to maintenance and operations costs, a 0.75-in. welding stud literally can save millions—a fact that says a lot about an often overlooked joining process.
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