June 27, 2002
A trained plasma technician can tell a lot about the health of a plasma system if he learns how to inspect the electrode, understands normal wear patterns, and knows how to spot signs of trouble. This article shows the difference between good and bad wear in air and oxygen plasma systems.
Electrodes for high-powered plasma cutting systems are highly engineered, consumable parts, similar in design, material, and function to an automotive spark plug. Like spark plugs, electrodes emit high-voltage electricity in a high-temperature environment.
The materials in electrodes must withstand the temperatures of plasma arc emissions; endure swirling, high-velocity gas jets; and provide an airtight seal for high-pressure gases and fluids. The electrode, like a spark plug, is the hardest-working component in the system.
A good mechanic can tell a lot about the health of a combustion engine by looking at the spark plugs. A trained plasma technician can do the same for a plasma system by learning how to inspect the electrode, understanding normal wear patterns, and knowing how to spot signs of trouble.
The electrode carries DC power from the plasma power supply to the metal plate. Typically, it comprises a copper or copper-silver composite holder that contains an emissive element of hafnium—a high-melting-point metal that can sustain an arc in air and oxygen cutting environments. The emitting element is eroded away slowly by the heat of the arc and the high-velocity plasma gas stream. Most of this wear occurs at the start and stop of a cut, when the molten hafnium material quickly heats up and cools down, melting and then resolidifying.
During normal wear, a small, concave pit forms in the end of the part that wears away steadily, a few thousandths of an inch at a time, to a depth of 0.040 to 0.125 inch, depending on the torch and consumable design and materials (see Figure 1).
When the pit becomes too deep, the arc attaches to the holder material and melts it. The electrode fails when it no longer initiates and sustains an arc. If molten material from the electrode is deposited downstream into the bore of the nozzle, it causes a blowout—catastrophic failure of both the electrode and nozzle.
Normal electrode life for oxygen plasma systems is one to two hours of arc-on time and 200 to 300 pierces. Air systems typically can achieve twice this life, 400 to 600 starts, because the nitrogen component of air makes it less reactive with the electrode. Oxygen plasma systems with inert start gases and current ramping can reach 1,000 or more starts before an electrode change is necessary.
Knowing the difference between good and bad electrode wear can help you improve your system's performance.
New Condition. Figure 2shows a picture of a new electrode. This electrode is a welded copper-silver composite with silver on the front and copper on the back.
Normal Wear. Figure 3shows an electrode with a normal wear pattern. The hafnium pit is well-centered and uniform in shape, indicating good alignment of consumables and a proper plasma gas swirl. The depth of the pit is approximately 0.100 in. The front edges of the electrode are sharp and distinct, and the silver has no severe discoloration. Some grayish-colored oxides on the front surface of the part are normal.
Normal Wear Half-life. The electrode in Figure 4shows a normal wear pattern that has been pulled prematurely for another reason, such as the torch riding the plate; torch crash; voltage change (when the voltage rises as the distance between the torch tip and plate increases); or a change in cut quality, such as slagging or excessive beveling. The pit depth is 0.078 in. Although this part looks consumed, it may burn another 100 starts or more and proceed to a depth of 0.100 or 0.140 in. before approaching failure.
Off-center Burn. Figure 5exhibits an off-center burn. This problem can be easy to spot. It usually indicates a severe gas flow problem—such as a broken or clogged swirl ring—or a torch part misalignment caused by assembly errors or fit-up problems. If completely changing torch parts doesn't correct the problem, the torch probably is damaged.
Moisture on Start. Figure 6shows that moisture was present during the arc start. This electrode has a rough swirling arc track from the wrench flats—the side of the part where the wrench fits—down to the face of the electrode. Preflow gas moisture can cause the high frequency to attack the silver material. The front edges of the silver aren't sharp; instead, they're smoothed over with a sandblasted surface condition.
Check preflow gas for signs of moisture. One quick check is the paper towel test: Hold a clean paper towel under the torch with gas flowing through the system (in the test or gas check mode only). No sign of moisture or contamination should be present.
Coolant Leak. As shown in Figure 7, coolant leaks produce severe arcing of the electrode face and sides, characterized by pitting and pocks in the electrode surface. The front surface is rough and black with shiny melted spots of holder material.
This problem often is caused by cut O-rings, insufficient O-ring lubrication, or loose or misaligned parts. The paper towel test can detect a coolant leak, as can a mirror, on which mist will form.
Low Preflow. Figure 8illustrates how insufficient gas during arc initiation produces a "lazy start"—the arc takes too long to travel from the start point, usually a sharp corner like a wrench flat, to the emitting element. These parts have a fairly uniform ring of molten holder material surrounding the pit. The surface may look like a solder splash or weld puddle has formed along the front of the part.
Blowout. Figure 9shows an electrode that has been run to catastrophic failure. Because the electrode is upstream, it will damage the nozzle when molten material is blown out of the end of the part and deposited into the nozzle interior.
Low Plasma Gas (Snuffing). Low gas flow is indicated if the electrode has small pockmarks all over its end and there is corresponding damage to the interior of the nozzle (see Figure 10). Low gas flow produces uncontrolled arcing between the nozzle and electrode.
Check the gas flow rates to the torch. The best way to do this is with a flowmeter (0 to 400 cubic feet per hour) and a hose placed on the outlet of the torch with the system in test mode.
If a flowmeter isn't available, a quick check is to feel the gas flow at the outlet of the torch with only plasma gas turned on. You should feel a swirling flow of gas that actually has a suction force.
High Gas Flow. If the nozzle is in good condition but the electrode has a deep, concentric pit, the plasma gas flow rate may be too high (see Figure 11). If the plasma gas swirl is too intense, the element erodes quickly. This can cause a rapid, deep wear pattern. Check the volumetric flow rate of the plasma gas.
Learning how to identify different types of electrode wear can help you anticipate failure, save money on parts, and improve system performance.
David Cook is technical service director, Kirk Ferland is technical service group leader, and Jason Start is technical service representative of Centricut LLC, Two Technology Drive, West Lebanon, NH 03784, phone 800-752-7623, fax 800-317-0438, e-mail firstname.lastname@example.org, Web site www.centricut.com. Centricut is a manufacturer of consumables for thermal cutting torches.
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