Proper practices for optimizing silica refractory campaign life.

Following sound refractory practices can lengthen the life of your coreless induction refractory linings for a more cost-effective, productive melt department.

Foundries utilizing coreless induction furnaces often miss production and cost-savings opportunities because they must shut down operations to replace the furnace's refractory lining, either due to a problem or a regularly scheduled reline. In many cases, this activity is premature, as the refractory lining could have delivered a longer production campaign to the foundry, if only it had received the proper care and attention.

Many practices impact the silica refractory lining life of a coreless induction furnace. These influences include form design and construction, installation, sintering, cool down and reheat. To extend the life of your refractory lining, it's vital that foundry employees understand the impact of each of these practices on the operation.

This article summarizes practices for maximizing campaign life and explains the theory behind each. Ensuring that these practices are followed can prevent premature lining loss and improve your melt operation's bottom line impact.

FORM DESIGN AND CONSTRUCTION

Objective: Maintain form rigidity throughout installation and sinter.

The form has two key functions: to transfer vibrational (mechanical) energy into the refractory for densification (during form vibration) and to remain rigid during the sinter until sufficient strength is developed before the form melts.

To achieve these objectives, a number of form design details must be considered, including:

* proper form thickness;

* smooth welds;

* cross bracing of the form;

* form construction;

* form placement.

Proper Form Thickness: For furnaces smaller than 5 tons, a consumable form can be constructed from 0.25 in. (6 mm) plate steel. For larger furnaces, a form thickness of 3/8 in. (10 mm) must be used to maintain rigidity. The outside dimension of the form is the same as the finished lining configuration, as recommended by the furnace manufacturer. If a removable form procedure is used, it must be 3/8 in. (10 mm) thick with a taper to facilitate easy removal. Removable forms with thicknesses smaller than 3/8 in. tend to deform and become out-of-round due to repeated heating, cooling and removal stresses.

Smooth Welds: Form welds should be ground smooth for easy form removal and for a uniform monolithic refractory lining. Any form irregularities will result in irregularities in the refractory lining.

Cross Bracing of the Form: For a removable form, horizontal and vertical bracing is critical to keeping the form sides straight and rigid during vibration and sinter. Braces should be located no more than 24 in. (610 mm) apart to maintain form rigidity. Internal bracing must be located so that it does not interfere with the mounting or use of a form vibrator. Due to the need for charge placement, consumable forms cannot use cross bracing; however, they still require internal stiffening rings to maintain their roundness. The bracing of the form becomes more significant in larger furnaces due to the increased size and weight of the form.

Anchoring of Form: Removable and consumable forms are anchored to the furnace structure to prevent movement during refractory installation and vibration. If not anchored, a form will tend to be forced upward during vibration, resulting in a higher-than-desired refractory floor height and possible defects in the floor refractory, which may result in subsequent spalling.

Form Construction: The form must be concentric to provide an even lining configuration. This helps prevent the refractory from experiencing uneven erosion and also ensures that the lining is equally affected by the electromagnetic field. The form bottom also must be perpendicular to the form centerline so that it sits square after the installation of a properly leveled bottom refractory.

A removable form is usually tapered (1-2 [degrees] from top to bottom) or collapsible. Due to its shape, a tapered form allows for easy removal after preheating. A collapsible form also allows for easy removal. but caution must be taken to ensure that it is rigid and contains no open seams that may allow refractory to enter, which would result in low density surface defects.

Precaution must also be taken when using segmented forms. As with the collapsible form, the seams must be tight to prevent refractory from entering. Also, all forms must have a "flat bottom" to rest on the floor with full welds that keep refractory from entering the form and ensure full contact with the bottom refractory. Any gaps caused by a concave or convex form could result in bottom spalling soon after sintering is completed.

Form Placement: The form must be properly installed in the furnace to achieve a uniform wall thickness. It must be centered at both the bottom and the top of the furnace and rest level on the refractory floor. An unleveled form results in uneven sidewall thickness. In addition, an unleveled removable form can damage the refractory during removal.

INSTALLATION

Objective: Optimize refractory density. The main objective when installing the refractory is to optimize the installed density to prevent refractory saturation by molten metal and slag. Saturation alters the entire chemical, thermal and mechanical behavior of the refractory lining and results in less than optimal refractory performance.

Two methods of installing silica refractory in a coreless furnace are commonly used: vibrating tamper and form vibration.

Installation by Vibrating Tamper: Important factors to consider when using the vibrating tamper include the ramming foot design, refractory layer thickness, "de-airing" procedure and tools, and ramming technique.

Ramming Foot Design - When vibrating the floor, a 6 in. (152 mm) diameter foot with 1/8-in. (3 mm) diameter holes spaced on 1 in. (25 mm) centers is standard. The holes in the foot are required to assist in the de-airing of refractory during vibration [ILLUSTRATION FOR FIGURE 1 OMITTED]. A circular ramming foot design is useful during floor installation to facilitate ramming along the inside circumference of the furnace.

When vibrating the sidewall, a curved ramming foot that matches the radius of the form and sidewall is used. This allows for easy and complete ramming along and around the form and next to the slip plane. The curvature of the ramming foot is dependent upon the furnace size and radius. The foot also is designed to have 1/8-in. (3 mm) diameter holes for de-airing of refractory. The recommended width of a sidewall ramming foot is 66-75% of the sidewall thickness. A ramming foot that is too wide won't fit easily between the form and sidewall. Thus, the potential exists to destroy the slip plane material while ramming due to the tight fit between the form and slip plane.

The tamper feet for both the floor and sidewall should be flat and heavy so that maximum pressure can be evenly applied to the refractory layer.

Refractory Layer Thickness - Dry silica refractory should be installed in no more than 5 in. (125 mm) thick loose layers. A 5-in. loose layer will densify to about 4 in. after proper compaction. Layer thicknesses greater than 5 in. have been shown to reduce the effectiveness of the vibrating tamper, resulting in low installed density and possible laminations.

"De-Airing" Procedure and Tools - Forking of the refractory is done to de-air and begin the densification process prior to ramming with the tamper. Forking of each loose layer is completed by a forking tool or a spade making four passes around the form to adequately de-air the refractory. The length of the forking spikes depends on the loose refractory layer thickness. For example, a 5-in. (125 mm) refractory layer requires a fork with 5-in. spikes to ensure that the entire refractory layer is de-aired and also helps knit adjacent layers [ILLUSTRATION FOR FIGURE 2 OMITTED].

After vibrating each layer, the fork also is used to scratch the rammed refractory surface before adding new material. When scratching the newly rammed surface, the fork should penetrate 0.75 in. (19 mm) into the refractory. This is done to redistribute the coarse particles that have segregated around the ramming foot and also to prevent laminations between layers.

Ramming Technique - When ramming the floor, start in the center and ram peripherally. The entire floor should be rammed at least four times to ensure compaction. The sidewall should be rammed at least four passes around the form. A downward force should also be applied to the tamper to ensure that maximum vibrational energy is transferred into the refractory. Use light pressure with the first pass to avoid burying the foot in the refractory, and increase the pressure on each subsequent pass as the material becomes denser.

For example, if the floor is 10 in., then two layers of 5-in. loose fill and one layer of 3-in. loose fill should be sequentially installed, de-aired, rammed, scratched and then bottom-scraped level to where the form will sit. The area between the form and the furnace coil must be scratched before starting the sidewall installation. Each sidewall layer is 5-in. loose with four passes on layer.

Installation by Form Vibration Methods: Form vibration is frequently used on medium and large furnaces. As with the tamper method, careful attention to detail is necessary to achieve maximum refractory density. The most commonly used form vibrating systems include: standard vibrating rig with pneumatic or electric vibrator, a vibratory cross system and a three-legged system.

Regardless of the system used, the process of installing the floor, setting forms, and adding and de-airing the refractory material is the same. When using form vibration, the refractory floor is generally installed with a vibrating tamper. An alternative method for compacting the bottom refractory is to attach a vibrator to a rigid steel plate that is about 2.5 in. smaller than the inside diameter of the grouted furnace. A larger plate will damage the grout and possibly the coil, while a smaller plate tends to dig into the refractory instead of riding level on the bottom surface. The vibration plate must have 1/8-in. (3 mm) de-airing holes drilled on 3-in. (75 mm) centers. This ensures that air is not trapped in the floor.

Once the bottom has been installed and leveled, the steel lining form is set. The top of the lining form is then restrained with wedges, or by welding a steel bar across the top of the form and to the furnace superstructure. Do not use internal weights such as scrap or starter blocks in the form during form vibration as they will dampen the vibration energy.

Next, scratch the space between the form and furnace. The dry refractory is then introduced into the space between the form and the furnace in 5-in. loose layers. A large funnel can be used to help reduce dust and segregation when pouring the material behind the form. Each layer should be forked to reduce segregation and to de-air the material before the next layer is introduced. Using a forking tool with tines that are the same length as the layer thickness helps mix the adjacent layers together as the material is de-aired.

Vibration Procedure - Vibration begins once the space behind the form is filled. Continue to add more material to the top of the furnace as the refractory settles and densifies. Two simple measurements of the effectiveness of the installation are the total pounds of dry silica used and the vibration frequency. Vibration frequency can be easily monitored using a simple reed-type tachometer to verify that sufficient vibration energy is being imparted to the form. Follow the minimum acceptable frequency recommendations for the vibrating device used.

Standard Vibrating Rig - A material handling-type pneumatic vibrators is often used with a square-type steel rig welded or bolted to the lower portion of the form in four locations. To facilitate the transmission of vibration energy from the rig into the material, the rig should be made as stiff as possible. In order to assure that all areas of the furnace receive equal treatment, the vibrator should be operated for at least 10 min on each leg of the rig. Larger furnaces will require relocating the rig at higher levels in the form, then repeating the vibration procedure. Any loose connection will manifest excessive noise and movement, accompanied by a loss in vibrations per minute (VPM). The minimum acceptable frequency on most properly operating vibrators is 3200 VPM.

Vibratory Cross System - With this system, the vibration rig can be relocated from above, eliminating the need for a person to climb down into the form to move a vibrator. Four wooden battens, made from a hardwood (such as oak or hickory), are placed in the form to serve as guides for the mounting shoes of the cross. The cross should be positioned about one-third of the way up from the bottom of the form, then tightened against the battens, using the hydraulic ram. The cross must press equally against all four battens. If it does not, the battens can be adjusted or shimmed to equalize the pressure of the cross against all four sides of the form. In large furnaces, compaction occurs best when the material is vibrated in two positions inside the form, one-third the distance up from the bottom and one-third the distance down from the top.

When the vibratory cross is in position, tighten it against the battens while running the vibrator at low speed to reduce friction. Increase the vibrator to full speed, and check its tightness again several times during the installation. The vibrator should be operated for 8-15 min at a minimum frequency of 4600 VPM for silica refractories. Note that this is a considerably higher frequency than with traditional vibration. For large furnaces, the vibration time is extended up to 20 min at both the top and the bottom positions.

Three-Legged Vibrator Systems - Another option for form vibration is the expanding leg vibrator. Usually, this device uses three legs, although similar devices with four or more legs have been sold. Impact-type or rotary vibrators are mounted on each of the legs, which are held tightly against the form by a spring or other mechanism.

The vibrator assembly is pulled upward during operation so that the vibration is transferred to both the top and the bottom of the form. While it has been used effectively in foundries for many years, this method has several disadvantages compared to other methods, including:

* more moving parts, resulting in higher maintenance requirements;

* difficulty in controlling the vibrator rig in the taper section of the form;

* a less rigid system due to the lack of a positive attachment to the form, reducing the overall efficiency of vibration transmission into the refractory;

* careful crane operation is required to properly control the rate of vibrator removal from the form;

* frequency cannot be monitored, since this system uses multiple vibrators operating independently of one another.

In spite of these disadvantages, it is possible to achieve good results if the vibrator is properly used and maintained.

SINTERING

Objective: Develop a refractory hot face.

Properly sintering a silica refractory helps minimize metal penetration through the development of a good hot face. A proper hot face is achieved by following a sinter schedule that allows for phase changes of the silica grain. Illustrated below are the temperatures at which a silica grain transforms into different polymorphs, resulting in a volume change of the refractory.

Crystal Transformations (Density in g/[cm.sup.3])

[Mathematical Expression Omitted]

Proper heating rates and hold times are used to accommodate the transformation of the refractory. Particular attention to the heating rate and hold time is necessary to achieve proper hot face development. Hold times are generally applied at 1600-2000F (870-1093C) for 2 hr to accommodate the large volume expansion (12%) during the transformation from beta quartz to tridymite. This volume expansion of the refractory reduces the open porosity, thus reducing the potential for molten metal penetration. A conservative ramp rate allows for transfer of heat into the refractory and assists in the phase transformations. Because refractories are generally insulators, more time is needed for heat transfer.

A final temperature hold is applied 100F (38C) above the maximum tap temperature. This final hold allows for the transformation from tridymite to cristobalite. which again results in a volume expansion, closing off open porosity and also adding strength to the refractory. Silica refractories are rarely held above 2900F (1595C) due to their temperature limitations.

The most commonly used and recommended sintering approach is the conventional method. While accelerated and liquid sinter methods also are used. they will often decrease refractory lining performance. A conventional sinter schedule takes advantage of the ramp rates and hold times to ensure phase changes occur for the development of the hot face. An accelerated sinter schedule uses increased ramp rates and a reduction or exclusion of hold times. This reduces hot face development and greatly enhances the potential for metal penetration and reduced refractory campaign life due to the lack of hot face development.

COOL DOWN PRACTICE

Objective: Minimize the depth and size of thermal cracks.

This technique minimizes the effect of thermal cracking on refractory lining performance. For operators who find it necessary to completely shut their furnaces down for maintenance or weekend shutdowns, it is critical that they understand the effect this has on refractory performance.

Obviously, it is best to keep all refractories at a constant operating temperature to avoid thermal stress and, ultimately, thermal cracking. This may not be practical for all operations due to scheduling and/or furnace size. When shutdown is an absolute requirement, it is important to recognize that it is best to rapidly cool the refractory lining.

Contrary to what many operators believe, rapidly cooling a refractory hot face helps control crack development. By rapidly cooling the hot face, the development of numerous small cracks is promoted. However, these cracks are shallow in depth, and thus are easier to close upon subsequent reheating than larger and fewer thermal cracks that can form during slow cooling.

Details of the procedure involve completely emptying the furnace after the last heat: An air fan or air hose should then be placed opposite the spout (often called the 6 o'clock position) and turned on immediately to facilitate airflow down the sidewall. across the floor and up the spout side. !t is helpful to chip off and remove any slag or metal adherence between the refractory and spout. By chiseling this area free, you will ensure the refractory lining does not adhere to the spout and crack horizontally. Figure 3 outlines the typical configuration and procedure.

Table 1. Cold Startup Procedure for Silica Refractory Linings

                       Time to 200F (110C)
Iron Capacity          Below Melting Point       Hold Time

1000 lb or less                0.75 hr             1 hr
1000-3000 lb                   0.75 hr             1 hr
3000-5000 lb                      2 hr             1 hr
5000 lb to 4 tons                 3 hr             1 hr
4-15 tons                       7-8 hr             2 hr
15 tons                       11-12 hr             3 hr

When it isn't necessary to cool the furnace to ambient temperatures, an alternative to rapid cooling is the use of a gas torch to keep the refractory in thermal equilibrium above 1500F (815C). As shown in Fig. 4, cristobalite, which is the major component of the hot face, does not dramatically shrink until temperature is below 1500F (815C). If the refractory is maintained below 1500F (815C), the furnace can be back charged and placed into operation immediately.

REHEAT PROCEDURE

Objective: Ensure metal does not infiltrate thermal cycling cracks.

After the refractory has been cooled below 1500F (815C), heat is required to expand the refractory lining and seal thermal cracks. All cracks must be completely closed prior to liquid metal exposure. The length of time required to completely close thermal cracks depends on furnace size. The larger the furnace, the larger the mass of refractory and consequently the longer the time required to reach thermal equilibrium.

The procedure involves charging the furnace with clean, densely packed cold charge. A minimum of one thermocouple must be used to monitor temperature. Two thermocouples are essential in furnaces larger than 10 tons. Normally, the furnace is then heated at 300F/hr (165C/hr) until it reaches 200F (110C) below the melting point of the metal charge. (Smaller furnaces can be heated at a more rapid rate due to the reduced refractory volume, while larger furnaces require slower heating times.) The furnace must be held at this temperature for the time outlined in Table 1. This will ensure all thermal cracks are sealed prior to liquid metal exposure. This procedure will reduce the potential for short refractory campaigns due to metal finning.