Ending the debate: how to thermally process silica refractory linings.
In this article, typical refractory manufacturer's recommendations for sintering and restarting a coreless furnace are analyzed. In addition, a detailed case study of one foundry's thermal processing practices is presented. While this article is directed toward ferrous melting in coreless furnaces lined with dry vibratable refractory, the concepts are applicable to the majority of refractory heating situations.
Most thermal processes during coreless furnace operation can be classified as sintering, cold restart or normal melting processes. Normal melting operations normally do not affect the thermal history of a refractory lining, so the focus will be the first two categories.
Sintering (to harden and densify) is the term used for the thermal processes that result in refractory hotface densification and development of the desired strength profile from the hotface to the coil grout. Sintering generally is used to refer to the initial heating of the refractory lining prior to exposure to hot metal. A cold restart or cold start refers to bringing the refractory back to working temperature after the furnace has been cooled for maintenance or production scheduling. Both of these operations require heating of the refractory lining in a controlled fashion.
The heat needed to sinter (harden and densify) a refractory may be supplied by gas or induction heating of a starter block or dense charge. In the sintering operation, two processes are happening simultaneously.
First, the temperature of the hotface (which is the surface of the refractory that will be in contact with molten metal) is raised. The amount of heat required to raise the temperature of the refractory depends on the heat capacity of the refractory and the efficiency of the heating process. Thermal losses, such as those resulting from heat escaping from the top of the furnace, will somewhat limit the rate at which the hotface may be heated. Still, it is possible to heat the hotface refractory at a rate in excess of what may be recommended by the refractory manufacturer.
The question that needs to be answered is, "If the refractory will heat at some rate, why shouldn't I heat it as fast as possible?" The answer lies in the second process, thermal conduction.
The main reason for heating rate (ramp) control during sintering is because materials have different rates of thermal conductivity. Thermal conductivity is a material property that describes how rapidly heat can be transferred through a unit of refractory, metal, liquid, gas or other substance. Unlike pressure, heat is not transferred instantaneously. The heat capacity of a material controls how hot a surface will become when exposed to a given amount of heat energy. Thermal conductivity controls how long it will take for a section of material at some distance away from the heated surface to reach some arbitrary temperature.
Thermal conductivity is the flow of heat through a material. This requires that a material to have both a 'hot' side and 'cold' side so that heat can flow through. This is the situation in a coreless furnace.
The hotface, whether heated by a gas flame, radiation from a starter block, or conduction from molten iron, is the heat source. The water-cooled power coil, which is essentially a heat sink, is the cold side. If a coreless furnace is filled with molten iron, the hotface temperature is about 2600F (1427C). The water temperature from the coil drains typically will range between 120-170F (48-77C).
Given these two fixed surface temperatures, it is apparent that after some period of time the temperatures in the refractory will stabilize. The condition at which the temperatures at arbitrary points in the lining have reached a maximum and do not change with continued exposure time is termed 'steady state.' The steady state condition is what refractory manufacturers are trying to obtain when designing sintering schedules.
Sintering schedules are key because it takes time for the refractory to reach steady state conditions. The refractory vendor designs sintering schedules not only on the basis of the temperature of the hotface, but also on the distribution of temperatures in the refractory cross-section, which takes time to stabilize. This distribution of temperatures must be reached to obtain the desired results with respect to bonding and strength development in the refractory. Insufficient heat or time during the sinter or restart will tend to adversely affect refractory performance.
For a graphical example of the sintering process, consider Figs. 1-4.
In Fig. 1, a cross-section of a cold refractory lining in a coreless furnace is shown at time 1. The hotface is to the left and the coils of the furnace are to the right. It is assumed the entire refractory system is at room temperature--the hotface refractory has the same temperature as the cold face. This is a steady state system.
As heat is introduced into the system at a constant rate, several things happen. First, the temperature of the hotface will rise until it reaches some maximum. This maximum temperature will be limited by the output temperature of the heat source, distance, amount of air flow and other factors. For this example, the surface temperature is 2400F (1315C). The temperature inside the lining will no longer be the same at all points, but instead will slowly rise in response to the elevated surface temperature. The rate of temperature rise is again constrained by the specific heat and thermal conductivity of the refractory lining components.
Figure 2 shows the lining at some point during the sintering cycle after the hotface has been exposed to some heat. The hotface of the refractory, which is closest to the heating source, will have the highest temperature. The cold face will have the lowest temperature.
Inside the refractory, there are locations (which would be cylinders in a coreless furnace sidewall) that have a constant temperature. These locations can be compared through an example by picking two temperatures (T1 and T2) and designating T1 as a higher temperature than T2. The cold face will have a temperature that cannot be below the temperature of the cooling water in the coils. Because the grout is a thermal insulator, the temperature typically will be in the range of 300500F (149-260C). As heat is pumped into the hotface, the locations of the T1 and T2 zones will move farther into the lining. This case is seen by examining the locations of the T1 and T2 lines in Fig. 3, which are closer to the cooling coil than in Fig. 2.
One might ask at this point, "If I continue to heat the hotface won't the entire lining eventually reach that temperature?" The answer is no, it will not. At some point, the system will reach thermal equilibrium. This is the result of the thermal characteristics of the refractory and the presence of cooling coils. Remember that the refractory is a thermal insulator, and it only can transmit a given amount of heat per unit of time.
Second, note that the cooling coil effectively is pushing 'negative heat' (cooler temperatures) back into the lining during the sintering process. The end result of this is shown in Fig. 4. The refractory lining has constant hotface and coldface temperatures, the same as Figs. 1-3. The difference is that the system now has reached thermal equilibrium--steady state. The locations of the temperatures corresponding to T1 and T2 are stationary because of the balancing effects of the heating and cooling sources.
What does all this have to do with sintering schedules? Remember that the reason for a sintering schedule is to allow the refractory to develop a dense, low porosity hotface, and to let the lining develop sufficient strength to withstand the thermal and mechanical abuse it will experience in service.
It is easy to raise the temperature of the hotface to working temperature. However, if it is not held at that temperature long enough for the sintering and densification to finish, a breach in the hotface will result in unsintered, loose refractory spilling into the bottom of the furnace. Also, if the hotface is not fully densified, metal and slag penetration will be high, which will result in degradation of refractory life.
Cold restarts of a previously sintered furnace lining address a different set of issues than does the initial sinter, but the thermal issues are similar. On a restart, the refractory cross-section has already been developed. Instead, the concern is closing cracks that may have occurred during cooling of the refractory lining. This process takes time.
Because refractories expand when heated, it is necessary to heat the refractory enough to close the cracks. This must be accomplished prior to the introduction of molten metal into the furnace. If molten metal enters the cracks before they close, fins in the lining will result, possibly leading to failure by ground faulting.
The time required to transfer sufficient heat into the lining to close the cracks is less than that required for sintering because the focus is the temperature level in the inch or so of refractory near the hotface, not the 3-6 in. required to heat during a sinter. As a result, the temperature can be increased faster during a cold restart than during an initial sinter.
Points to Remember
Refractory sintering and restarts are complicated issues, but remembering these key points can help your furnace's refractory:
1. Always consult the refractory vendor for its recommendations on temperature rates per hour, suggested hold temperatures and times, and final hold time and temperature.
2. To control furnace temperature accurately, it must be measured. This means that the foundry operator must use thermocouples to track the temperature of the form and/or charge during the sintering or re-heat cycle. Remember that a thermocouple measures the temperature only at the location of the thermocouple junction. If an average furnace temperature is required, more than one thermocouple must be used with a chart recorder to obtain a direct copy of the actual sintering cycle. Further, it is quite likely that the temperatures will vary by location, that is, the floor of the furnace may be significantly cooler than the sidewalls.
3. The refractory manufacturer's recommended sintering and cold re-start schedules are designed to ensure proper development of strength and bonding in the refractory cross-section.
4. There is a balance in terms of time spent heating refractory linings versus refractory life. In some cases, it is possible that shortening a sinter will result in no decrease in refractory life. In other cases, shortening the sinter cycle will shorten campaign life.
5. Written procedures must be developed for sintering and re-start operations. Implement a check-off system to ensure that each sinter or restart is performed the same way.
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For More Information
Introduction to Heat Transfer, Incropera, F.P., and DeWitt, D.P., 3rd Edition, John Wiley and Sons, New York, 19%.
Handbook of Industrial Refractories Technology--Principles, Carniglia, S.C., and Barna, G.L., Types, Properties, and Applications. Noyes Publications, Park Ridge, NJ, 1992.
RELATED ARTICLE: Sintering & Restart Methods at Brillion Iron Works
Brillion Iron Works, a 1000-employee gray and ductile iron foundry in Brillion, Wisconsin, uses three different procedures for sintering its coreless induction furnaces with silica refractories. Following is a look at those methods.
Starter Block with Cold Metal Charge Backcharging
1 Starter blocks should be a diameter that is within 6 in. of the lining form and have a flat top and bottom.
2 Starter blocks should be placed in the furnace until the furnace is 67-75% full. When molten, the blocks should fill the furnace to 33% of the normal operating level. Care should be taken to ensure that the blocks do not touch the lining form and that the stack is stable and will not topple.
3 Thermocouples should be placed at the bottom, middle and top of the furnace. Temperature control is required to 2000F (1093C).
4 Heat the starter blocks via furnace power. The rate of increase should be between 100-200F (37-93C), depending upon the size of the furnace and recommendations by the refractory supplier.
5 Continue the heat-up until the temperature in the furnace is between 1600-2000F (871-1093C) and hold for 2 hr at that temperature. Consult your refractory supplier for hold temperature and duration.
6 On completion of the hold, continue the heat-up until the starter blocks are 90% melted. Make regular visual observations during this step to ensure bridging does not occur.
7 Cold charge can now be added to the furnace. Do not try to put too much charge into the furnace at one time. If the melt temperature drops too far, bridging will occur, Continue to add cold charge to the furnace until it has been filled to the operating level. Try to maintain a temperature of 2400F (1315C) during the back-charging step.
New Lining or Shave Repair--Continue to raise the bath temperature at a rate of no more than 400F/hr until 2900F is reached. Hold for 1-2 hr and allow the temperature to drift back down to the normal operating temperature.
Cold Restart--Continue to raise the bath temperature at a rate not to exceed 400F/hr until the bath is at normal operating temperature.
Starter Block Sintering with Hot Backcharging
1 Follow the procedure for starter block with cold charge backcharging until you reach the point where the starter blocks are 90% melted and the furnace is ready for charging.
2 Molten metal for charging should be tapped from a second furnace between 2500-2600F.
3 Fill the furnace with molten charge to its normal operating level as rapidly as possible. The furnace power should be off during this step. Care should be taken to avoid allowing the metal stream to impinge directly on the sidewall during filling.
4 Apply furnace power at the lowest setting. Observe the furnace ground resistance reading. Increase the power setting as the ground reading allows to obtain a 200F/hr temperature rise (consult refractory supplier).
New Lining or Shave Repair--Continue to raise the temperature at 200F/hr until the bath reaches 2900F. Hold for 1-2 hr. Let the temperature drift back down to operating temperature.
Cold Restarts--A 400F/hr temperature rise should be used to elevate the bath to its normal working temperature.
Gas Torching with Hot Backcharging
1 Starter blocks are placed in the furnace. The bottom block is placed on four pieces of pig iron to allow hot air to circulate to the floor of the furnace. Pig iron spacers are placed between the upper blocks as well. The starter blocks are to act as heat sinks and chill the molten metal as it is poured into the furnace.
2 Two thermocouples are placed in the furnace on the top of the uppermost starter block. One is used to control the torch valve, the other is a spare.
3 Light the torch and hold temperature between 250-350F for an hour, (This temperature is based on the coldest temperature that can be maintained with a given torch).
4 Switch the torch to automatic mode, allowing the programmable controller to run the cycle.
5 Set the controller to raise the temperature 200F/hr to a temperature of 2050F.
6 Hold at 2050F for 5 hr. Tap molten metal for charging at 2500-2600F. Set the furnace power to its lowest level.
7 Add molten metal to the furnace. Continue to backcharge until the furnace is at its production level.
8 Increase the furnace power level to obtain a temperature ramp of 200F/hr.
New Lining or Shave Repair--Continue the 200F/hr ramp to 2900F. Hold for 1-2 hr. Let the temperature drift back down to normal operating temperature.
Cold Restarts--Shorten the hold at 2050F to 1-2 hr. A 400F/hr temperature rise is used after backcharging. The bath is elevated at this rate to normal operating temperature.
About the Authors
Timothy M. Green is senior research mineralogist at Allied Mineral Products, Inc., Columbus, Ohio. For the past eight years, he has been involved in design and troubleshooting of monolithic refractory systems, with an emphasis on dry vibratables for melting steel in coreless furnaces.
Domenic G. Boncci I melting superintendent t Brillion Iron Works, Brillion, Wisconsin. He has 25 years experience in the foundry industry, 15 years of that at Brillion Iron Works. He is responsible for all molten metal production from 3 coreless and 2 channel furnaces, furnace and idle refractories, ductile iron treatment process, and iron delivery.
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|Comment:||Ending the debate: how to thermally process silica refractory linings.|
|Author:||Bonacci, Domenic G.|
|Date:||Jul 1, 2002|
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