Preventive Maintenance Keeps Induction Systems In Peak Condition: Part 2
Ignoring original equipment manufacturers' (OEM) recommended maintenance schedules for induction heating equipment can create problems that are not apparent from just looking at the system. Following a regular maintenance schedule can minimize equipment-breakdown surprises. This article offers recommended maintenance procedures for heat-treat coils and bus work, heat treat fixtures, quenching apparatus, melting furnaces and leads, forging coils and water-cooled skid rails and air and hydraulic components. Recommended maintenance of water-cooling systems, cooling towers and power supplies and heat stations was presented in Part 1 of this article (IH, June 2001). This information is not intended to replace OEM equipment manuals, but it provides easy-to-use checklists for regular system maintenance.
Heat-treat coils and bus work
Induction coils require periodic cleaning. Normal high-temperature water exiting coils can cause internal calcium scale build-up within the water-cooling paths. The scale, appearing as white, tan and green powder and sludge, functions as a thermal insulator, reducing thermal conduction and causing coils to fail prematurely at solder joints. The following schedule of preventive maintenance can minimize or eliminate this problem.
- Clean the exposed coil surface each work shift using a clean damp rag to remove any carbon and scale build-up, which can cause an inaccurate ground leak-detector fault. It may be necessary to clean the coils more often if excessive oil or steel shavings are on the parts being processed.
- Check bolts for tightness on all bus extensions and coils, including those not easily accessible. Bolts and washers on the bus and coil connections should only be nonmagnetic stainless steel or silicon bronze. Never use carbon-steel washers and bolts. Hardware should include a flat washer, lock washer and a bolt long enough to engage at least three full threads. The recommended torque for a 0.375 in. (10 mm) brass bolt is 23 to 25 lbf ft (31 to 34 N ?m), 45 to 50 lbf ft (61 to 68 N ?m) for stainless steel and 40 to 45 lbf ft (54 to 61 N ?m) for silicon bronze. Use of plain brass hardware should be avoided. Follow the manufacturer's recommended torque specifications. Do not overtighten.
- Inspect integral quench coils and quench barrels having removable quench clean-out plates. Inspection frequency may need to be greater depending on how clean the quench fluid is. Some coils may need cleaning every shift, while others require weekly or monthly cleanings of the small quench spray holes. Plugged quench holes can result in an uneven quench or a barber-pole (spiral soft rings) condition. Coils and quench barrels that do not have removal covers and plates are the most difficult in which to keep the holes clean. However, they still need cleaning, usually by inserting a stiff wire (such as a paper clip) into the holes to push the debris back into the quench pocket. This generally is only a quick fix because the debris cannot always be flushed out. An oxyacetylene torch having a very small torch tip can be used to carefully burn out debris from the holes. This must be done using great care so as not to melt or damage the coil body by overheating.
- Inspect all quench and coil hoses for crimps, which can reduce flow, and replace at least every other year. Tie wraps can be used to neatly route the hose out of the way.
- Immerse the entire coil in humidifier cleaner or a solution used to dissolve hard-water deposits and soak for a few hours. Wear safety glasses with side shields. A bubbling reaction removes calcium deposits inside the coil opening up cooling-water flow paths. Do not to leave the coil in this solution too long or permanent damage (etching) will occur. Do not submerge a coil that has any type of flux concentrator or lamination stack built into it. This type of coil must be cleaned by pumping (using a low-pressure pump) the cleaner through the interior passages of the coil for cleaning, so as not to damage the flux concentrator and lamination stack. The pump should be placed inside a plastic bucket containing humidifier cleaner. Pump cleaner through the coil and back into the bucket for several hours and then flush using fresh water. Clean the laminations and concentrator using hot water to remove carbon steel and oil build-up. Check for good contact between the laminations and flux intensifiers and the copper. Check the concentrator for good contact and replace with contact cement if necessary.
Each time a coil is taken out of service, clean it using hot water to remove carbon scale, polymer and oil build-up. Use a nonmetallic abrasive sponge to remove all deposits; removal is very easy when the coil is first removed from service, but becomes very difficult if the deposits are allowed to dry. Immerse the inductor in hot water for one hour before cleaning if debris is dried on the part.
Fixtures include scanners and devices used to position a workpiece within the induction coil, such as lift and rotate and walking-beam devices. They also can include a ball screw, acme thread screw, rack and pinion, air cylinders, etc. OEM recommended maintenance should be followed for moving parts. Use only the recommended oils and grease on all moving and wear surfaces. Most maintenance procedures are performed on an annual basis, with the exception of chain-drive idler roll adjustment (as necessary) and workpiece centers and locating tooling, which should be inspected every six months and replaced if necessary:
- Check the accuracy of up and down positioning and rotation total indicator reading (TIR) accuracy by cycling the machine using a dial indicator to judge positioning repeatability. This test usually shows if the ball screw and nut, rack and pinion, etc., have excessive wear.
- Check the over-travel limit switches for function and protection; protect tooling that could be damaged by an over-run during the test.
- Check the condition and function of safety sensors, such as ground detectors, limited-access devices (door switches) and light curtains.
- Inspect and replace worn and frayed rubber belts used for rotation.
- Check the low-level sensor on the quench tank for proper function.
- Check brushes in motors and drives; these typically have a service life of five to ten years.
Melting furnaces and leads
Maintenance of coreless induction furnaces is critical due to the close proximity of molten metal and water in the cooling coil. Safety is of the utmost importance. A furnace lining should be inspected for erosion and mechanical damage in the high wear areas, such as metal/slag interface, after each tap/pour when the furnace is empty but still hot. Items to look for include dross build-up at the molten metal line and dark streaks and dark areas, which indicate irregular cooling usually due to refractory failure. Repair or replace the lining per the refractory supplier's recommendation.
Easy-to-perform daily maintenance is as follows:
- Keep the melt deck clean and free of hot slag and charge materials that are not needed for the work in progress, and check the molten-metal run-out pit for moisture and debris.
- Inspect the furnace and water-cooled leads for water leaks and condensation.
- Test the ground detector for proper operation and protection function.
- Inspect hydraulic connections (use only nonflammable fluid in the system).
- Make sure that the back-up emergency cooling system for the furnace is in good working condition.
Perform the following maintenance steps on a regular monthly basis or during a furnace reline, whichever comes first:
- Remove deck plates and inspect the furnace shunts, coil, leads and bus connections.
- Examine the copper coil closely for discoloration, arcing and overheating, indicated by a blue or red color on the copper. Black scale on the copper indicates that the coil has been subjected to extremely high temperatures; this is a danger sign.
- Clean out metal slag or chips inside the furnace enclosure, and look for indications of molten metal penetration through the coil.
- Clean the cylinder rods and housings used to lift the furnace using a rag, and check the tightness of all fittings.
- Inspect water-cooled leads for wear and cracks and check connections for tightness and for discoloration, which indicates loose connections (if in doubt, repair the leads).
- Tighten shunt and tie rod bolts using a torque wrench per OEM recommendations.
- Check cooling-water and hydraulic filters and clean or replace.
- Test the ground detector before putting the unit back in service.
Use two independent methods of support when lifting the furnace for inspection in case there is an unexpected dropping of the furnace due to a loss of hydraulic pressure.
Forging coils and water-cooled skid rails
Coils used to heat a workpiece for forging either are lined with cast refractory or have a ceramic liner. The following maintenance should be performed on a daily basis:
- Blow all loose scale out of the coil opening and check coil connections for tightness, daily. If scale is not removed, it builds up inside the coil and works its way into microcracks until it contacts the copper surface, which causes overheating and arcing, and results in coil failure. Wear safety goggles when using compressed air.
- Inspect the exit end of the coil where the highest temperature occurs. Patch as needed to eliminate direct exposure to heat radiation from the part and to reduce the chance of arcing. Remove loose scale and refractory that is cracked or contaminated by grease and oil. Rough up any area that has a glassy appearance using a file. After installing the patch, use a heat lamp for several hours to cure the refractory.
Follow the OEM casting and patching procedures. Flush the copper tubing using humidifier cleaner as described earlier. Never use refractory that is out of date. Refractory and the water used to mix it should be used at the same specified temperature. Premix an entire bag to get even consistency. Reseal partially used bags in an airtight container to keep out moisture. Cover freshly poured refractory using plastic to allow moisture to weep (usually overnight is sufficient), then dry in an oven at progressively higher temperature per refractory instructions. If an oven is not available, leave the plastic in place an extra day, remove and let air cure for 12 hours, then apply the heat lamp.
Every week, inspect water-cooled rails for wear and inspect water-cooled leads for wear and connection tightness. Replace rails as needed. The OEM should recoat (hardface) the rails. The supplier often can rebuild defective leads at a lower cost than purchasing new leads.
Air and hydraulic components
Air-operated doors, workpiece locating centers and solenoid quench valves (easily forgotten parts of a system) and hydraulic devices require regular maintenance. The filter, lubrication and regulator (FLR) should be maintained per the OEM manual. Excessive water in the compressed air can cause rust and other corrosion and premature wear of pistons and delicate seals. Some downstream components, such as cylinders and actuators, are sealed for life. Excessive seal failures usually are due to improper lubrication of the FLR. Some FLRs have a filter or water collection bowl, which requires regular draining. Failure to drain collected water can affect air-cylinder and solenoid performance and reduce their service life.
PLC and PC controls
Changes made to recipe programs should be backed up on floppy disc to ensure that all recipe data is current. Follow OEM manual recommendations for back-up battery replacement, and back up all data on disc before changing the battery. Inspect fuses for overheating and arcing on a yearly basis. If a computer is used for controls, defrag the hard drive and empty the wastebasket at least once per year.
Check door safety latches for proper function yearly. Wipe the interior using hot water and fan dry before operation. Keep water off vital components. Control boards should be cleaned using an electrical cleaner. A clean, dry paintbrush also works well to remove dirt from fragile components.
AcknowledgmentThe authors thank all Ajax Magnethermic Engineering, R&D, Sales, Customer Service Department, Park Precision and Ajax Induction Services personnel and customers who contributed to this article.
SIDEBAR: Quenching Means Everything!
Quenchants usually are water, polymers mixed with water, and water-soluble oil mixed at various concentrations depending on steel types being processed. Quenchants have a finite life depending on many factors such as carryout rate and contamination by other incoming fluids, and break down over time. An induction system should not be used as a parts washer because heavy oil will contaminate the quenchant. Also, chips cause arcing and damage to the coil, and can clog the spray holes on the quench apparatus.
The polymer quenchant or soluble-oil solution must be checked daily using a refractometer or viscosity tube after the quench has been on and recirculating for a few minutes to ensure the composition is correct. This should be done prior to heat treating any parts to ensure that every part processed "sees" the same quench solution. As water evaporation occurs, the polymer concentration may increase. Also, as pieces are processed, carryout may reduce the amount of available quench. Fresh water and polymer or soluble oil need to be added to adjust the solution to the proper concentration.
Quench filters also must be checked and cleaned daily. A regular schedule based on the amount of scale generated (debris on the workpieces) should be established. If there is no quench filter, it is necessary to remove sludge and scale from the bottom of the tank. Failure to clean the tank can result in damage to the quench heater if the heater comes into contact with the built up scale. Sludge build-up also can inhibit the function of the low quench level switch. Further, excessive plugging of the quench spray holes can occur. Because it is difficult to predict how often the tank should be cleaned due to many factors, cleaning is recommended every three to six months.
Never place rag-type shields in the quench tank, as the polymer attacks and dissolves the rags, plugging the small holes in the quench barrel. The point at which the quenchant impinges the part is of utmost importance. The smaller the diameter of the work piece the more important impingement point is! A single hole in the quench head directed at the wrong angle can cause a barber-pole effect. Sometimes foreign matter can become lodged in the hole and cause the same barber-pole effect.
Quenchants have a tendency to breakdown chemically over time and they begin to smell bad after extended use. Experience indicates that a quenchant is past its useful life when it starts to have a foul odor. Most quenchant manufactures offer a service to check the remaining useful life of a quenchant.
Many quench tanks are made of mild carbon steel coated with an epoxy on the interior surfaces. The coating eventually breaks down (between five and ten years), and larger pieces collect in the "Y" strainer and smaller particles plug the quench spray holes of the coils. Check with the OEM for recommendations to clean, strip and recoat the tank.