Selecting Refractories for PM and MIM Sintering Furnaces: Part 2
Selecting the appropriate furnace refractory and kiln furniture for furnaces used to process powder metallurgy (PM) and metal injection molded (MIM) parts can be made easier by following some general guidelines and considerations. Furnace refractories (which can provide insulation, support and wear resistance) include bricks, muffles, beams and hearth plates that line a furnace (Fig. 1). 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.
Factors to consider when selecting a refractory are how will the material be used and which 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.
Four methods typically used to produce refractory products are: pressing (hand tamp and mechanical and hydraulic press), slip casting, extrusion and injection molding. A multitude of possible refractory mix variations are derived from five base materials including alumina (Al2O3), mullite (3Al2O3-2SiO2), zirconia (ZrO2), cordierite (2MgO- 2Al2O3-5SiO2) and silicon carbide (SiC). Properties of oxide refractories used in the PM and MIM industry are listed in Table 1.
The following example illustrates the selection process for a pusher-furnace application. Most pusher furnaces fire stainless steel parts at a sintering temperature of 2450F (1345C) in very dry hydrogen atmospheres (as low as -80F, or -60C, dew point). These conditions require the use of refractory packages radically different from those used in mesh-belt furnaces.
High purity (99%+) alumina furnace refractory is required under these extreme high-temperature, reducing conditions to minimize contamination inside the furnace. Silica, soda and potassia oxides present in most refractories are reduced under these conditions. Even tramp amounts of these oxides can be reduced in the high-temperature zones and reoxidized and/or deposited in the cooler, more humid zones near the furnace entrance. This reaction can contaminate parts, or simply be a nuisance to an operator trying to control furnace conditions. High-purity (99%+) dense or insulating "bubble" alumina should be used as hot-face, and in some cases, back-up insulation. To minimize the reduction and reoxidation of silica, refractory containing anything more than tramp amounts of silica should be avoided on the hot face, and used as backup only when the interface temperature is lower than 2100F (1150C).
Although 99%+ alumina muffle sections are used in some smaller pusher furnaces, poor thermal shock resistance limits their use in furnaces requiring 12-in. (300 mm) wide plates. Use of SiC sections typically is not considered feasible in dry pusher furnaces operating at temperatures higher than 2460F (1350C). SiC and associated impurities in SiC sections can flux PM parts and contaminate high temperature furnaces.
PM and MIM parts typically are sintered on ceramic pusher plates or setters in these furnaces. Common opening dimensions of pusher furnaces have resulted in the use of three standard pusher plate sizes: 12 in. long by 12 in. wide by 1 in. thick (300 by 300 by 25 mm), 12 by 6 by 0.75 in. (300 by 150 by 19 mm), and 8 by 8 by 0.875 in. (200 by 200 by 22 mm). Pushers are made of 91% alumina. Most part manufacturers would like to use 99% alumina pushers, but these pushers fail catastrophically under the fast furnace cycling conditions-typically a three to four hour cycle with forced cooling at the exit end of the furnace. Alumina-mullite-zirconia (AMS) pushers also have been evaluated, but have a shorter life than 91% alumina due to their higher friability. In instances where 91% alumina plates are failing due to thermal shock, AMS plates should be considered due to their higher thermal shock resistance. The larger 91% alumina pushers typically last three to four months when cycling about six times per day with a single layer of parts. Factors influencing pusher plate life include load, push rate, atmosphere and maximum temperature. Plates eventually develop cracks from cycling, and often simultaneously begin to warp; such plates should be replaced to minimize or eliminate furnace "jam ups."
It is not necessary, as some users believe, to "condition" (cycle without parts) as-received pusher plates, or to periodically flip them. Most refractory suppliers fire plates at temperatures higher than those used in the PM and MIM industry, so conditioning does not ensure performance or clean the plates. Flipping pusher plates can contaminate the parts being sintered as the bottom of the pushers can collect debris that builds up in the furnace.
Most pusher plates are square or rectangular. However design options are unlimited, and can include bosses, dimples and other built-in fixtures, slots, bevels and cropped corners (Fig. 2). While most designs are created to support specific part requirements, users often question (independent of part consideration) whether standard plates should or should not have slots. Slots are designed in pusher plates to eliminate or minimize catastrophic failure by allowing the plates extra room to expand and contract during the cycle. So, while slots may help prevent initial failure, they also can promote crack formation, which leads to failure as the plates age. Therefore, switching from solid plates that are working satisfactorily to slotted plates is not recommended, as flatness and service life may be sacrificed.
Kiln furniture in pusher furnaces is not limited to pusher plates. Setters and stacking plates (Fig. 3) are used to increase capacity, to ensure dimensional stability among layers and to accommodate specific compatibility requirements. However, it should not be assumed that increasing furnace capacity is as simple as adding layers of ceramics. Stacking plates can increase temperature variability within the stack, so post design, plate thickness, part spacing, push rate and time at temperature become critical variables in obtaining consistent PM and MIM parts.
Ceramic posts, preferably having a chemical composition similar to that of the plates, should be placed in either three- or six-point configurations to stabilize and support the load effectively. Pressed-steel posts should be avoided, as they can cause localized stresses in the plates and possibly even lead to catastrophic failure. Experience shows that 91% alumina posts can last for years, making it a worthwhile investment. Both posts and plates can be interlocked by designing posts with male/female parts, which prevent kiln wrecks. Unlike pusher plates, it is important to periodically flip stacking plates and setters to prevent sagging and maintain flatness. This should be considered when finalizing the stack design.
The correct plate size
Determining the correct stacking plate size is not straightforward in most applications because cycling creates stresses in the plates leading to fracture. Therefore, selection should take into consideration past experience of users and suppliers. Generally, 0.5-in. (13 mm) thick 91% alumina plates can be used to handle loads less than 7 lb (3 kg). The 91% alumina plates have good thermal shock resistance and are chemically compatible with most PM and MIM parts. For parts sensitive to silica contamination, zirconia- and alumina-coated 91% and 99% alumina plates are options. Coated 91% alumina plates offer thermal shock resistance and a reaction barrier for the parts. Because high alumina compositions are sensitive to thermal shock, 99% alumina setters typically are manufactured in less than 6-in. (150 mm) square sections.
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.
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: adam.j. email@example.com.