The use of precast refractory is a growing specialty in the refractory industry. Forging and heat-treating furnaces are prime examples of applications in which improved quality can be achieved through the use of precast refractory shapes, resulting in better lining performance, greater furnace reliability and reduced maintenance costs.
The design and manufacture of precast refractory shapes has become a specialized field within the refractory industry in recent years. As demands increase for greater refractory lining performance and lower maintenance costs, forge furnace operators are finding that one effective way to achieve those goals is to incorporate a broader use of precast refractory shapes into their lining systems. The applications for precast shapes are limited only by the imagination, and almost invariably, their use will result in better performance and true cost savings. This article will discuss the design and manufacture of precast refractory shapes and the benefits to be gained from both improved refractory performance and simplified installation logistics.
Applications in the Forge Shop
For most forge-shop operations with forge preheat furnaces and heat-treating furnaces, typical applications for precast refractory shapes would include:
- Front sill shapes
- Hearth blocks
- Door jambs
- Lower sidewalls
- Door bottom edge shapes
- Pier blocks
- Burner blocks
- Flue sections
- Curb sections
- Flue dampers
Precast Shape Design and Manufacturing
Initial design details are extremely important. In order to realize the true benefits to be gained from the use of precast shapes, a thorough knowledge of how the shape system will be used and installed in the field is an absolute requirement during the design phase. The successful design and manufacture of a high-performance refractory shape system requires a unique understanding of refractory materials, manufacturing, anchoring systems and construction practice. Factors that must be well known before the shape system is designed include: dimensional tolerances, construction sequencing, lifting and handling capabilities at the site; anchoring facilities; and the actual service demands within the furnace environment.
Precast shape manufacturing inherently requires the use of a mold or pattern to form the shape. There are several methods for mold-making that are routinely employed, and the type of mold construction and materials used depends on the size, complexity and dimensional tolerances required in the shape and sometimes the quantity of shapes required. For simplistic shapes with loose dimensional tolerances (1/16 inch), plywood forms or metal fabricated forms can be used. Toward the other extreme, some shapes may call for very tight tolerances, which require the use of a more sophisticated mold made from wood, plastic or metal. These molds may be of the type made by a foundry pattern-maker or machine shop.
Another factor in the design of a precast shape has to do with the schedule and sequencing of the actual field installation. The shape design must take into account job accessibility, what other lining components will already be in place when the shapes are to be installed and how the shape can be handled physically on the job site. Weight and lifting limitations must be considered and planned for, as well as the type of available access into the furnace or vessel. If necessary, lifting lugs, holes or other fixtures can be incorporated into the shape design.
The design of the anchoring system to be used in the shape is of tremendous importance. In addition to the normal considerations of alloy type and anchor size, the precast shape design must also consider all alternatives for attaching the shape to the furnace structure. Numerous methods can be used, including threaded-stud attachments through the wall; floor or structural elements; welded fixtures that attach to the furnace steel; or bolted assemblies that penetrate through the furnace steel.
Perhaps most importantly, the proper refractory material must be selected to suit the demands of the application. Factors such as the desired temperature profile through the lining, expected mechanical stresses, potential chemical attack on the lining, erosion mechanisms and expansion allowance must all be understood prior to selecting a material to be used in the precast shape.
A well-equipped precast manufacturing facility should include high-energy, large-capacity mixers, automated mixing stations with conveyors for material delivery, vibration tables, digitally controlled water addition, mixing-time controllers and adequate lifting capabilities for large shapes. The firing of shapes is accomplished with a digitally controlled furnace with burners capable of firing to at least 1300°F. In-house mold/pattern fabrication capabilities and CAD-generated drawings for design assistance should also be expected.
Benefits from Material Property Enhancement
Regardless of how complex or sophisticated the refractory castable selected for an application is, the actual in-the-field physical properties of the material can be drastically reduced if care is not taken during the mixing, pouring and curing processes. Particularly with the use of more complex refractory castables to solve specific wear issues, installation variables become even more critical to the performance of a lining. Unfortunately, lining quality is often compromised by field conditions during material placement. Project schedules, crew skill levels, equipment availability, job cost pressures or other demands can sometimes have an impact on proper refractory installation. Improper water addition, mix-time variations, over- or under-vibration, and improper curing can all drastically affect material quality. With precast shapes cast in a controlled shop environment, the physical properties of a castable will be more fully optimized.
The initial drying and firing of a refractory castable is a critical installation variable that can influence lining performance. Precast shapes are typically fired in a digitally controlled furnace prior to shipment, ensuring that the refractory manufacturer’s recommended bake-out schedule is closely followed. Since the shapes are fired slowly from all sides, the moisture is removed through the entire thickness of the shape in a controlled manner. Depending on the temperature to which the shape is fired, this can optimize the physical properties of the material through the entire thickness of the shape, not just the hot-face surface. This results in a truly homogeneous lining. Micro-cracking within the shape, which is often introduced during field bake-out but can go unnoticed, may also be reduced since the initial firing is more controlled.
In service, linings comprised of precast shapes often see less stresses and cracking due to the independent “floating” nature of the lining. The performance of the lining can also be more predictable, resulting in better opportunities to plan for maintenance and repairs.
Benefits from Installation Logistics
Other major benefits to be gained from the use of precast refractory shapes are related to simplified installation and repair logistics, which can lead directly to reduced costs and shorter furnace downtimes. Forming labor, materials, equipment costs, actual placement time and expense, and the associated costs during form removal, curing and cleanup are all eliminated with the use of precast shapes. These costs are shifted back to the manufacturer, who can absorb them much more efficiently when spread over their overall production capacity.
Whenever any portion of refractory repair work can be completed prior to crews being on site, costs are automatically reduced. With the use of precast shapes, crew sizing can be minimized. Speed of installation is another obvious benefit to both the installer and the owner, resulting in reduced costs due to shorter job duration. Material usage is also reduced when compared to other installation methods, such as guniting or ramming. Environmental hazards (such as dusting) and tripping hazards associated with equipment and hoses are also substantially reduced, if not eliminated.
Future repairs also become much more economical and quicker to accomplish. Repair areas can often be isolated to just the immediate wear area within the boundaries of a shape. Anchor attachment points can typically be reused. Replacement shapes, purchased early and kept as spare parts on site, can be easily installed in a fraction of the time required for conventional repair methods. Repairs become more of a mechanical maintenance job rather than a refractory installation job requiring a specialized crew and equipment.
The initial on-site bake-out of a new refractory lining can be a very expensive and time-consuming component of a refractory repair project. The use of precast and prefired refractory shapes can sometimes reduce or even eliminate the need for an extensive initial bake-out. If an entire repair is made with a prefired system, then normal furnace startup schedules can be used without the fear of steam spalls or other damage during the initial heating. Bake-out of multi-component linings, which may include a combination of precast shapes and other materials placed in the field, can often be reduced by the prefiring of the castable shapes, particularly if that material would have been the critical item determining the bake-out schedule. This can have a positive impact on job costs and reducing downtime.
Precast refractory shapes will continue to be a growing specialty in the refractory industry in coming years. Forging and heat-treating furnaces are prime examples of applications where improved quality can be achieved through the use of precast refractory shapes, resulting in better lining performance, greater furnace reliability and reduced maintenance costs.
TFL, Inc. a supplier of refractory materials and related services located in Houston, Texas, and is a leading manufacturer of custom-designed precast shapes. Author Paul Fisher is vice president of TFL. He may be reached at 281-590-8500 or firstname.lastname@example.org. For more information, visit www.tflhouston.com.