Vacuum furnace users encounter unique maintenance issues. Hot-zone longevity has been an issue that can be improved by using carbon composite (CC) products. CC plate is used for hot-face protection and CC profiles for edge protection. A complete CC hot zone has been developed that provides an extremely hard-wearing surface designed to withstand gas erosion and accidental operator damage.
One of the biggest pitfalls in existing hot-zone design is the use of molybdenum and its inherent problems with high-temperature embrittlement and fragility when conducting repairs or replacement. Molybdenum has been used for many years in insulation-board fixings, element-support assemblies, elements, heat shields and edge protection. Replacing molybdenum products with CC products wherever possible reduces hot-zone maintenance and repair.
The plug-and-play option is invaluable where furnace downtime is a major issue. A reline to an existing outer cage could see the furnace out of production for up to three weeks, but a plug-and-play hot zone can shorten this time to only two to three days.
CC Tee-Bolt Board Fixings
Molybdenum insulation-board fixings with molybdenum butterfly clips have been the standard within the high-temperature furnace industry. Users experience butterfly clips and the actual board fixing breaking, which leaves an extremely hard and brittle stud in the fixing nut and makes replacement difficult. This often results in the hot zone being removed from the furnace vessel to access the outside of the hot-zone cage.
Following extensive trials with respected customers, these molybdenum fixings are now being replaced on all hot-zone relines using M10 CC tee-bolt fixings. Should breakage occur due to accidental damage or operator error, the CC fixings are easily replaced from inside the hot zone within a matter of minutes. Any CC stud left inside the fixing nut is very simply tapped out and a new fixing inserted.
The CC board fixings are used with a CC washer to prevent damage to the hot face when tightening (Fig. 2). This approach has resulted in a major improvement in furnace downtime and saved many maintenance hours in replacing broken molybdenum fixings. It is recommended that checking the tightness of the fixings is included in the operator’s daily, weekly and monthly maintenance tasks.
CC Element-Support Fixings
Building on the success in replacing molybdenum insulation-board fixings with CC, the same approach has been adopted for element-support fixings, with all molybdenum fixings replaced with CC. The fixings are available in sizes from M10, M16 and M20. When looking at improved element-support design, the existing OEM design is examined for obvious weakness or maintenance issues, and a simplified redesigned system is offered wherever possible. By using a mixture of standard ceramic parts and CC fixings, an easily replaced low-maintenance element-support system can usually be offered.
Existing molybdenum eyebolt element-support hangers as used by many original equipment manufacturers (OEMs) are now replaced with CC eyebolt fixings. These element-support assemblies have the obvious advantage of ease of replacement from inside the furnace hot zone.
CC Double-L Profile
Many OEMs use a raised face on the hot-zone door assembly to provide better insulation properties and prevent line-of-sight radiation to the outside of the hot zone. This raised face also provides a sealing edge for the door to seal onto the leading edge of the hot zone. The edges of this raised face have for many years suffered from damage due to gas erosion and thermocouple positioning damage. The sacrificial skin fitted to the door insulation has also suffered from coming loose due to vibration during furnace quench.
The development of a patent-approved CC double-L profile to picture-frame the door insulation has created a unique system that will protect all edges of insulation from gas erosion. The sacrificial skin on the hot-zone door is completely captivated, and all fixings are kept inside the sealing edges.
The double-L profile provides complete door-edge protection against accidental operator damage (Fig. 3) and also a hard-wearing sealing face between the door and the hot-zone leading edge. This double-L profile can be used for both CC plate and graphite-foil sacrificial skin and is available to suit both 20- and 40-mm raised faces.
CC Gas Grids
Machined-graphite gas grids have been used for many years within the heat-treat industry, and they are usually very heavy, quite fragile and susceptible to damage by falling parts. Replacement gas grids are typically a long lead-time item due to the size of the plate required prior to manufacture and extensive machining required. Graphite gas grids are usually quite bulky and are usually fitted in the roof, base and sides of the hot zone. This large bulk of graphite has to be heated up along with the furnace load every time the furnace is operated and obviously has extra power-usage implications.
Standard CC gas grids are lighter, stronger and easier to handle and install. When coupled with CC profiles protecting the aperture, an extremely long-lasting system, which is less susceptible to gas erosion, is obtained.
CC grids can be offered on much shorter lead time, and replacing graphite gas grids with CC gas grids has had no effect on furnace temperature surveys or cooling parameters.
CC Sacrificial Skin
Graphite perforated-foil sacrificial skin is used throughout the industry to protect the hot face of the insulation board from gas erosion and accidental damage. Because the product is heavy and difficult to handle, it is labor-intensive to fit.
If the foil face is damaged or torn, gas pressure can get behind the foil, damaging the protective barrier very quickly. Extensive damage to the insulation board from gas erosion can occur once the sacrificial skin/barrier has been broken. Once the integrity of the insulation board is compromised, temperature-survey problems can occur as well as damage to the hot-zone outer cage.
Extensive use of CC plate (usually 1.5 mm thick) as a sacrificial skin/barrier has reduced hot-zone reline time. A very hard-wearing face that offers protection from gas erosion and accidental damage is achieved, and the use of CC strips covering the joints of the sacrificial skin offers a major improvement in hot-zone longevity. With CC plate sizes available in sheets of 3,000 mm x 2,000 mm, most sides of the hot zone can be covered in one sheet to prevent any unnecessary joints.
Should the hot face get coated by chrome buildup, it can be lightly scraped off and vacuumed out. Buildup of chrome can result in arcing and shorting between elements and fixings.
CC plate sacrificial skin coupled with CC profile edge protection can add many years to the lifespan of the hot zone and keep hot-zone maintenance to a minimum.
CC L and U Profiles
CC profiles are used extensively to protect all apertures of the hot zone including bung and gas-grid apertures and also door-front leading edges. The combination of L and U profiles can be used to replace most molybdenum edge-shield protection. The use of profiles greatly reduces damage by gas erosion and accidental damage and provides a hard-wearing sealing face.
Any round apertures (bung or bulls-eye) in the hot zone are converted to flat-sided to enable the use of U or L profiles to hold sacrificial skin in place and prevent gas erosion. Also, any round doors are converted to flat-sided so CC edge profiles can be used. This approach helps to protect the edge of the door from accidental damage or gas erosion and gives a hard-wearing sealing face.
With L and U profiles being readily available in longer lengths, aperture edge protection is made simpler by the use of one-piece edges. With increased widths available to accommodate sacrificial skins, all-round protection can be added to any leading edge or application.
The use of CC profiles for edge protection coupled with CC sacrificial skin has increased hot-zone life considerably.
All survey and load thermocouple ports are protected by removable plugs manufactured from CC or graphite products to prevent damage to the insulation by the constant inserting and removing of the load and survey thermocouples.
The overtemp and control thermocouple ports are always protected by graphite bushings. Once the plug becomes damaged, they can be readily replaced to prevent ongoing damage to the insulation. Therefore, front-edge protection of the base and sealing face has to be renewed, not the complete insulation (Fig. 4).
Another option is to have availability of plugs designed for different applications. For example, one would be used for the standard temperature monitoring during normal production and another with more feedthrough possibilities for monthly general-functionality checking.
All edges of insulation board (even edges protected by profiles) are painted and cured with graphite paint to prevent damage and wear by gas erosion. The curing of the paint has proven to reduce pump-down times on bake-out due to outgassing.
All joints in barrel-stave-construction hot zones are also painted and cured.
Fabricated Outer Cages
With most companies trying to keep furnace downtime to an absolute minimum, a complete, all-new, fully insulated hot-zone outer cage can be supplied. This allows the existing hot zone to be removed and the new one installed within a few days. This approach sees the furnace downtime reduced from two to three weeks for an outer-cage reline to two to three days to install and bake out the new one.
One of the main advantages to manufacturing a new outer cage is that the existing one can be repaired and even relined and stored as a spare should anything happen to the installed hot zone. This approach is useful for customers with only one or two furnaces that cannot afford to lose a furnace for up to three weeks for reline.
Graphite Heater and Hearth Sets
All relines are offered with complete top-quality graphite heater and hearth set assemblies. Arrangement drawings of the heater and hearth sets are supplied with the hot-zone documentation listing individual spare-part numbers for ease of replacement. Complete traceability of all materials fitted within the hot zone is available.
Safety and Health
When disassembling a worn-out hot zone, dust from the dissolving insulation contains deposits from every heat-treated material. For a skilled disposal and to secure the health of every person involved, a converted spray booth that extracts the particles is used (Fig. 5) in addition to personal protective equipment.
For more information: Contact Jez Higham, Schunk Carbon Technology; e-mail: firstname.lastname@example.org; web: www.schunk-group.com. For U.S. inquiries, contact Bill Warwick at email@example.com; web: www.schunkgraphite.com.