Selecting Refractories for PM and MIM Sintering Furnaces: Part 1
Although applications are unique and conditions vary, there are some general guidelines and considerations that can be used to select the appropriate furnace refractory and kiln furniture for sintering furnaces used to process powder metallurgy (PM) and metal injection molded (MIM) parts. Even if the refractory and/or furniture is specified by the equipment builder, it still is important to understand how the refractory was selected and what its life expectancy is.
Furnace refractories include bricks, muffles, beams and hearth plates that line a furnace; they can provide insulation, support and wear resistance. Kiln furniture, including batts, plates, slabs, boats, saggers, setters, fixtures and posts, generally is used to support the product being sintered in continuous and batch furnaces, and often is considered as a consumable.
Manufacturing conditions often are proprietary and vary among manufacturers. However, four methods typically are used to produce refractory products: pressing (hand tamp and mechanical and hydraulic press), slip casting, extrusion and injection molding. Throughout the industry, standard lead times to produce furnace refractory and kiln furniture start at six to seven weeks, even though actual production typically takes two to four weeks. Large orders and refractory shapes that require new tooling can increase delivery times to between 12 and 14 weeks.
Ceramic refractory compositions include alumina (Al2O3), mullite (3Al2O3-2SiO2), zirconia (ZrO2), cordierite (2MgO-2Al2O3-5SiO2) and silicon carbide (SiC). There are countless mix variations based on these five base materials. Purity level, grain size and shape, manufacturing technique and refractory properties and characteristics are among the many variables that can make selecting the optimal refractory a challenge (figure 1).
Properties of SiC refractories used in the PM and MIM industry are listed in Table I.
Define application need and conditions
Factors to consider when selecting a refractory are how will the material be used and which of the many refractory properties, such as flatness, dimensional integrity, wear resistance, chemical compatibility, thermal shock resistance, creep resistance, thermal conductivity, hot strength, surface finish and refractoriness, are critical in a given application. Operating conditions also must be defined, including furnace atmosphere, dew point, maximum operating temperature, firing cycle time, maximum heating and cooling rates, and heating method. In addition, it is important to know the location of the thermocouples so that the temperature measurements are meaningful. The conditions seen by the parts being processed, the thermocouples and the refractories may be very different.
The following example illustrates the selection process for a mesh-belt furnace application. Mesh-belt furnaces often are used to sinter ferrous PM parts at a temperature around 2050F (1120C) in a controlled nitrogen-hydrogen atmosphere having a dew point as low as -60 to -80F (-50 to -60C). These furnaces are heated using either resistance heating elements or gas-fired burners, and most belt furnaces use either high-temperature alloy or SiC ceramic muffle sections (figure 2) to control sintering conditions and to separate the fibrous refractory linings from the parts being sintered.
While the initial investment is higher for ceramic muffles than for alloy muffles, the thermomechanical stability of SiC ceramic muffles is better than that of alloy muffles, which can provide benefits that offset the initial higher cost. For example, the use of SiC sections can eliminate costly downtime associated with replacing warped, fatigued alloy muffles. Most SiC muffle sections are guaranteed for up to five years, but many sections survive even longer. Long service life, high temperature strength and wear resistance and good thermal conductivity make SiC muffles an ideal choice for these furnaces.
Traditionally, SiC sections are made of relatively coarse-grained oxide-, nitride- and oxynitride-bonded SiC. Choice of the material type is not critical because all of the coarse grained SiC mixtures seem to work satisfactorily. SiC sections having smooth surfaces and consistent dimensions work best. A smooth surface finish can improve mesh belt life by reducing wear, and consistent dimensions ensure hassle-free replacement of old, worn out sections. SiC beams (figure 3) and hearth plates often are used as structural supports for the muffles, while the furnace typically is lined using kaolin grade fiber and insulating brick.
Most muffle sections must be large enough to accommodate 18 to 36-in. (460 to 915 mm) wide belts. Therefore, manufacturing large sections from alumina-mullite and alumina-mullite-SiC mixes presents a challenge due to their limited green strength. These materials also have lower thermal conductivity and lower resistance to thermal shock, long-term creep and wear compared with SiC, which limit their use in large belt furnaces. However, small alumina sections often are required for use in some high-temperature pusher designs.
Many parts are placed directly on the mesh belts in these furnaces, while some parts must meet special flatness tolerances or they require a reaction barrier, which calls for the use of kiln furniture such as ceramic setters and fixtures. Setters usually are manufactured from cordierite or graphite when the maximum furnace temperature is less than 2050F (1120C). Above this temperature, the performance of cordierite is unreliable, while graphite setters need a reaction barrier (for example, alumina spray coatings or other materials) to prevent softening or melting of the PM parts. For sintering most PM parts, 91% alumina offers a stable, reaction free, high strength alternative setter material to both cordierite and graphite. In some special cases, a zirconia or alumina coating must be used on 91% alumina setters to prevent unwanted chemical reactions. It is important to quantify initial flatness requirements for setters as each manufacturer has its own allowable warpage tolerances.
This article is based on a presentation given at the 2000 International Conference on Powder Metallurgy and Particulate Materials, sponsored by Metal Powder Industries Federation; proceedings available from MPIF. Part two of this article covering refractories selection for pusher furnaces will appear in the May issue.
For more information: Adam J. Osekoski is applications engineer, Saint-Gobain Industrial Ceramics Inc., Ceramic Systems, 1 New Bond St., PO Box 15136, Worcester, MA 01615-0136; tel: 508-795-5707; fax: 508-795-5011; E-mail: email@example.com.