Long Lining Life and Long Coil Life Go Hand-in-Hand

Do you want to maximize lining life and coil life in your induction furnace? Of course you do. Many people don’t realize both factors are interrelated.

Melt shops can become complacent with maintenance of their lining procedures as well as routine maintenance of their melting equipment as the shine on their new equipment fades. Short lining life and coil life result in increased downtime, costs, and frequent coil changes.

This article explains the reasons for short lining life and short coil life and their effects on the melt shop, and outlines methods to save thousands of dollars in repairs and lining costs per ton of melted metal.

We will focus on a coreless furnace application melting ferrous materials using silica refractory as the working lining. Thoughts expressed in this article pertaining to ferrous materials are similar to those expressed for nonferrous materials, using an aluminabased refractory.

Over the years, melt shops have reported coil life as short as one year or less while others report coils lasting three to five years and even longer. Generally speaking, melt shops reporting longer coil life cycles experience good lining life campaigns as well.

Let’s review reasons for a shorter lining life span and, in turn, its effect on the coil life.

Selecting refractory material
The refractory material selected must be suitable for the material to be melted and its pouring temperature. Refractory manufacturers customize the amount of binding agent or binders added to refractory, depending on the desired pouring temperature. The melt shop should consult with the refractory manufacturer for proper binder content in the refractory they purchase. Too much binder can cause the lining to sinter all the way to the coil while too little binder will not give the refractory a sufficiently thick hot face. Short lining life due to finning or severe erosion is often a result of improper refractory material being selected for the melt operation.

Installation Procedure
The recommended procedure used for the installation of the refractory should be followed precisely in accordance with the refractory manufacturer’s instructions. Silica linings are installed using a number of vibrating tools to compact the refractory between the coil and melt-out lining former. The wall thickness of the melt-out form needs to be of a proper thickness to ensure that the form does not buckle or deform during the vibration process—or prematurely during the sinter. The thickness and shape of the form should be in accordance with the furnace OEM’s recommendation. Charge material or weights should be placed inside the form to prevent it from lifting during the vibration process which otherwise can cause a void under the form. Not following the proper installation procedure can result in low density of the silica, causing metal penetration and excessive erosion. Improper installation procedures will result in short lining life. Many melt shops have automated their lining installation procedures using refractory installation equipment.

Drying coil grout before lining
If the coil grout cement is replaced, it is important that this new grout material be dried properly before installing the lining. Electric heaters can be used for this purpose. Consult with the grout refractory manufacturer to obtain the proper dry-out schedule. Improper dry-out may cause the grout to lose strength and allow premature cracking. Failure to dry out the grout can cause excessive moisture to be present at the coil that can lead to coil arcing or grounding during the lining sintering procedure.

Sintering procedure
The sintering procedure for the lining must be followed precisely. Rushing the sinter and dry-out procedure will not allow sufficient time for moisture in the refractory to escape, causing excessive moisture next to the coil which leads to coil arcing or the coil going to ground. A proper procedure ensures the binder in the refractory creates a proper working face. Many furnace power supplies have computer controls that provide the operator with a program to sinter the refractory in accordance with the refractory vendor’s sintering schedule. Not following the sintering schedule will result in short lining life.

Taking temperature of the bath
The pouring temperature of the molten metal in the furnace must always be measured with a calibrated immersion dip thermocouple rather than by eye or by guessing the temperature. Overshooting the pouring temperature by 100°F beyond a target pour temperature of 2800°F, for example, will cause a much shorter lining life span than pouring at the correct temperature. The higher temperature will cause higher refractory wear, resulting in a short refractory life.

Ground leak detection system
The furnace ground leak detection system needs to be functioning and tested daily for proper operation. Under no circumstances should a coreless furnace be operated without a fully functioning ground and leak detection system. Furnaces without a proper ground leak detector risk having metal penetration to the induction coil that, if undetected, can cause coil arcing with resultant water leakage and possible catastrophic explosion. A handheld furnace ground leak detector tester is available for Inductotherm coreless furnaces to verify that the furnace molten metal bath is properly grounded. This test must be performed at every heat to assure safe operation of the furnace.

Improper charging techniques
Improper charging techniques can damage the refractory. Dropping extra-large pieces into the furnace can crack the lining, creating a run-out situation during the melting procedure. Bridging can also occur due to improper charging, causing the molten metal under the bridge to superheat beyond the refractory’s maximum temperature capability.

Superheating under a bridge condition can result in a runout and cause a molten metal-water explosion. Many melt shops use automated charging systems with conveyors that keep up with the melt rate of the furnace. Not keeping up the charge procedure with the melt rate can cause excessive temperature in the molten bath, resulting in erosion of the refractory and short life. An improper charging technique can cause damage to the refractory, resulting in a short lining life.

Improper cold-start procedures
Improper cold-start procedures can lead to short lining life due to molten iron getting into lining cracks. Melt shop operators should consult with the refractory manufacturer and obtain the proper cold-start procedure for their particular furnace size. Many induction power supplies incorporate an automatic cold-start feature that takes the guesswork out of this task.

Lining removal systems
Today, many melt shops use a mechanical lining removal device to push out the refractory lining. Others with older furnaces still use manual chipping chisels to break up and remove the lining.

This older procedure is very time consuming and exposes the worker to refractory dust. If the worker is not careful when using the chisel hammer while removing the lining, the induction coil tubing can be damaged to the point of a water leak. Improper lining removal will cause short coil life.

Furnace coil sweating
Modern melt shops use closed water-cooling and recirculating systems to conserve water and reduce costs. Some of the older cooling systems don’t have controls that keep the cooling water to the furnace and power supply above the dewpoint. When the cooling water in the system gets below the dewpoint, sweating can occur inside the power supply and on the induction coil. Droplets of moisture can form on electric conductors in the power supply, which can cause arcing across the components such as SCRs, diodes, transformers and capacitors. This arcing can cause component failure.

The sweating that occurs on the furnace coil can also allow electrical arcing across the coil turns and to ground. Arcing can be severe enough to cause a turn-to-turn short with a resultant water leak.

Newer closed-loop cooling systems incorporate a temperature diversion valve placed in the recirculating system in such a way that the cooling water bypasses the heat extraction device (cooling tower, dry air cooler, etc.) to allow the water to maintain a temperature above 75°F which is typically above dewpoint. Coil sweating and resultant coil arcing obviously affect overall coil life span.

Preventive maintenance
Many melt shops have periodic preventive maintenance procedures in place while others do not. The furnace manufacturer should be contacted to obtain the recommended preventive maintenance schedule and procedure for the power supply, furnace and associated equipment in the melt department.

If the furnace assembly is not properly maintained, short coil life will result. If magnetic shunts are used in the furnace design, they need to be torqued periodically to ensure they stay in the proper location supporting the coil. The torque procedure should be in accordance with the furnace manufacturer’s recommendations. Loose shunts can move during operation and come in contact with water connection pipes and hose barbs causing arcing and coil grounding.

Power lead and hose connections need to be checked for leaks during the preventive maintenance procedure. Water leaks can cause coil arcing, resulting in short coil life. In addition to power leads and hoses, other furnace components such as top cast blocks, Faraday rings, tie rods, furnace covers and cylinders should be checked during the preventive maintenance procedure.

Monitoring lining wear
Most melt shops check the wear of the refractory lining on a regular basis. Many actually take physical measurements of the lining on the weekend after the lining has cooled off or between shifts.

Some melt shops also have computer controls which monitor and record furnace lining wear over time. This information along with physical dimensions is useful in predicting when the lining needs to be changed.

Melt shops that don’t have a program in place to monitor lining life risk having a run-out that would no doubt cause coil damage.

Lining life vs. coil life cost-per-ton
The following charts illustrate the approximate refractory and coil change-out repair cost per ton of melted metal based on a 2,500 kW, 200 Hz batch melting system melting iron on a two-shift, five-day basis, 50 weeks per year. It is assumed that the iron pouring rate is about four tons of iron per hour for the comparison.

As shown in these charts, melt shops experiencing both short lining life and short coil life can have a cost per ton more than three times those which experience long lining and long coil life campaigns. It is recommended that those experiencing “life on the low side” consider all of the above possible areas that contribute to that poor life. Reviewing current procedures may uncover areas for immediate improvement in both lining life and coil life, which go hand-in-hand.