In any manufacturing process involving a kiln or furnace, the need for consistency is key to ensuring right-first-time components, whether this applies to processing temperatures, atmospheric content or the overall firing process.
While the quality of the kiln and furnace infrastructure itself is crucial to achieving consistency, what is no less important is the correct selection and use of refractory products involved in the process, whether these be high-temperature insulation systems, kiln cars and furniture, or fired refractory shapes.
The Importance of Correct Material Selection
In all of these areas, poor product selection or incorrect matching of the material to the application has the potential to result in inefficient or ineffective processing, wasted energy and process time, and possibly even reputational damage if a crucial order deadline is missed due to an unresolved processing issue.
Above all, unnecessary downtime while failed materials (such as insulation systems) are replaced is unacceptable. Put simply, best practice in material selection, system design and installation will go a long way toward achieving optimum production efficiency and system uptime based on the amount of cycles that can be undertaken before these systems need replacing.
It is inevitable that products will at some stage need to be replaced. Perhaps for that reason, the focus among many individuals with responsibility for specifying or purchasing kiln insulation and furnace refractories has traditionally been on achieving the lowest unit cost. As with so many things, however, the best price does not necessarily mean best value. The focus should always be on finding the best solution rather than the cheapest product, which will almost inevitably require more frequent replacement (Fig. 1).
With the cost differential between a lower-grade insulation system and a premium product paling into insignificance compared with the cost of downtime while these systems are replaced, it is in an area where even a modest time investment or investigation of total life-cycle costs can pay major dividends.
Achieving the Best Insulation System
Insulation systems are integral to ensuring processing temperatures are not just high enough but consistent throughout the kiln or furnace and helping to ensure uniformity of parts. Minimizing the loss of heat through the sides and top of the kiln or furnace is not just key for processing. It also contributes to operator safety because external surfaces are not as hot (Fig. 2). Energy costs are reduced too because more heat is retained inside. While natural gas costs are relatively low currently, it cannot be assumed that this will always be the case. Many companies are taking commendable steps toward reducing their carbon footprint on a voluntary basis, irrespective of any legislative compulsion to do so.
Insulation systems need not just offer minimal heat transfer. There may also be a requirement for thermal-shock resistance in applications where rapid heating or cooling is employed. Resistance to corrosion is also often a key attribute, depending on the application, as are physical strength and flexibility (Fig. 3).
Depending on the process, a choice can be made between high-temperature insulation wools such as refractory ceramic fibers (Kaowool® or Cerafiber®), alkaline earth silicate fibers (Superwool®) and polycrystalline fibers or combinations of high-temperature fibers with insulating firebricks (IFB) or special-duty castables (Kao-Tab®, Kao-Tuff®).
Whatever solution is considered, specifiers and purchasers should take steps to satisfy themselves about the quality and consistency of the materials used and the integrity of the manufacturing process of these products. Inconsistency in either of these areas will likely result in substandard performance, with more frequent replacement also being necessary.
Rather than simply buying on price, therefore, investigations should first be made to ascertain how suppliers’ processes are benchmarked between different facilities as well as establishing the existence of quality plans and what use is being made of comparative testing methods. If the manufacture of the lining material can be proven to be consistent, then its performance in key areas such as heat transfer can be accurately predicted and relied on. This is vital where accurate temperature control is needed.
For processors seeking to reduce downtime even further, the use of a lining material suitable for a higher temperature than the actual process employs is more and more frequently considered. The latest polycrystalline fibers, for example, are manufactured by a chemical process rather than the melt and attenuation process more commonly used for other lining materials. However, the chemical and thermal stability of polycrystalline systems mean they can offer extended service life – up to 10 years is not uncommon – compared with refractory ceramic fibers. Polycrystalline systems can also deliver better performance in areas such as heat transfer.
Meeting the Unique Demands on Kiln Furniture
The story is similar when it comes to kiln furniture, which is subject to intense demands on its performance. These demands include the effects of rapid heating and cooling, which make material choice and design absolutely critical.
There are many systems available in various materials that are suitable for lower-temperature processing. However, demands from within the industry for kiln furniture led Morgan to develop a new high-performance, nitrite-bonded, silicon-carbide material that can withstand temperatures of up to 1500°C (2732°F).
The material, known as Halsic-N™, combines the properties of proven materials such as silicon carbide (SiC) and silicon nitride (Si3N4) in a microstructure that delivers strength, excellent refractory properties and resistance to oxidation and thermal shock. Standard designs include setter plates, beams and supports, while components can also be produced to individual customer specifications.
Thermocouple protection tubes made from Halsic-N are ideal for the melting of nonferrous metals such as aluminum and magnesium, where its strong metal-repelling characteristics help ensure long service life and optimized performance. With flexural strength of 160 MPa, Halsic-N can even be coated with a specially formulated coating for the firing of porcelain or technical ceramics.
The Shape of Things to Come?
When it comes to fired refractory shapes, the need to employ best practice is equally pronounced. While it is recognized that these products are generally used only once and need to withstand a single firing, they are generally subject to rapid heating and cooling and need to be strongly resistant to thermal shock. More and more frequently they are used with expensive cast metals and superalloys, an area where material loss as a result of a failed consumable is not acceptable due to the high material costs involved.
Once again, material quality is paramount. Even under high-temperature firing, it is crucial that oxidization of consumable products is kept to an absolute minimum to prevent contamination of the materials being processed. This means purity of consumable products is vital, so another area where rigorous investigation of suppliers’ raw-materials sourcing and manufacturing processes represents time well spent.
The Best Options for Inert and Vacuum Furnaces
As previously detailed for ceramic furnace consumables, high-temperature inert and vacuum furnaces pose similar challenges for purchasers and designers. In these ultrahigh-temperature furnaces, which are capable of reaching 3000°C (5432°F), designing the proper insulation solution of carbon or graphite felt and carbon or graphite rigid board is incredibly important since the costs of these heat-treating processes play an increasing role in the expansion of high-end technologies, such as solar and semiconductor products.
There are a variety of standard materials available, designed specifically for these applications. Many of these materials can also be enhanced in a number of ways to provide the process or furnace designer with additional options. For instance, felt products can be specifically designed to provide enhanced insulation performance or longer life.
The rigid-board products offered by Morgan include a standard material as well as a low thermal-conductivity material (RGB-LTC) for energy improvements across many applications. In addition, specially developed coatings can be applied to improve life or reduce friability for use in harsher environments.
Insulation purity is also vital for product performance. Many levels of purity can be supplied, offering the designer even more options. Whatever the application or need, providing a specific solution to solve a specific problem is key to delivering the best cost of ownership for the purchasing company (Fig. 4).
Product specification and system design are not areas where processors should feel they are on their own. The leading manufacturers of these systems are keen to be more than just suppliers and work in a strategic and collaborative way with customers. A deep understanding of their individual application requirements is gained. This assists with system design and recommending and supplying the most appropriate consumables, whether these are standard products or need to be developed on a contracted basis.
Manufacturers will even partner with installers to help ensure best design practices and that issues such as poor sealing around doors (a condition that can rapidly negate the investment made in a premium product) are avoided. A good installer will ensure the very best is obtained from the product selected.
Above all, consumables should not be treated as commodities. They are vital tools, and without them effective processing cannot take place.
The author acknowledges the following Morgan Advanced Materials’ contributors: Chris Johnson, Roger Patrick, Randy Bishop, Dr. Michael Rozumek.
For more information: Visit www.morganthermalceramics.com/kiln-furnace, or for more information on Morgan Advanced Materials, visit www.morganadvancedmaterials.com.