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Vacuum-furnace hot zones are typically classified into one of two main categories based on the construction of their insulated walls or shielding.
The “insulated hot zone” is regularly constructed of rigid graphite board, graphite felt material or a combination of board and felt. Occasionally, high-purity-alumina ceramic fiber may be used as a substitute for the felt. The “all-metal hot zone” is constructed out of shields made from molybdenum, stainless steel and, for higher-temperature applications, tungsten. “Hybrid” construction, which uses components from both categories, is also used in the industry.
Of the hot zones that are used in single-chamber vacuum furnaces it is estimated that 75-80% are insulated, whereas the remaining 20-25% are of all-metal construction. Although all-metal hot zones are typically found to be initially more expensive than graphite-insulated hot zones, the benefits that the all-metal design provides make them a highly valued entity in the heat-treating world.
All-metal hot zones are used in vacuum furnaces throughout industry and are preferred in many applications for a variety of reasons. These include:
- High durability/long life
- Process cleanliness (graphite-free environment)
- Faster quench times due to better heat-transfer coefficients
- Faster pump-down speeds (due to not being hygroscopic)
- Higher ultimate vacuums (due to using no fibrous/porous material)
- Faster heating rates (due to high ratio of heating radiating surfaces)
|Fig. 1. Low-mass hearth|
All-Metal Hot-Zone Design Advantage
There have been many design advances in all-metal hot-zone construction over the years. It is due to these advances that all-metal hot zones are being provided into the market today with greatly reduced energy consumption as compared to the old-style hot zones that have been around since the mid-1960s.
If we take a look at a standard heat- treating all-metal hot zone comprised of molybdenum and stainless steel, we can identify the places where energy can be saved. These include:
- Reducing the mass of the hot zone itself
- Improving the shielding efficiency of the all-metal hot zone
- Using composite shielding with ceramic-fiber paper
As an example, PLANSEE has incorporated these features in its highly efficient “ENERZONE.” With the use of the ENERZONE, total energy consumption in an all-metal hot zone can be reduced by 20-25%. This hot zone not only saves energy, however, it also performs better than standard all-metal hot zones. The ENERZONE heats up faster and cools down faster and, therefore, it provides faster cycle times.
|Fig. 2. Furnace components|
Reducing the Hot-Zone Mass
The mass of the hot zone can be reduced in a number of ways. One method involves the use of a low-mass hearth. The typical hearth used in batch vacuum furnaces includes posts, caps and rails. The caps and rails used are usually of thick-diameter/heavy-thickness construction, whereas the low-mass hearth eliminates the need for caps and uses a thinner rail that is bent into a “U” shape to maximize strength and functionality. Not only is the mass of the hearth greatly reduced, but the U-shape rails are found to be much easier to remove for maintenance purposes since they maintain their shape and straightness run after run. PLANSEE’s low-mass hearth design is quickly becoming the industry standard and is shown in Figure 1.
Other design and fabrication approaches for reducing the mass in the hot zone include redesigning the element supports. When standard metallic – molybdenum or lanthanated molybdenum (ML) – strip heating elements are supplied, the traditional element support bracket consists of two posts with a bridge running between them, which provides the support to the element strip. The mass-reducing approach involves providing a one-piece bent element support assembly. Not only is there the benefit of the thinner material used in the element support, but since the part is a stand-alone assembly, there is the added benefit of having only one penetration through the shield wall at each element support location. With fewer penetrations through the hot-zone walls there are less conductive losses through the walls. Two of the PLANSEE element supports can be seen in Figure 2.
Another way to reduce mass of the hot zone is through the use of technological advances in the materials. With the use of doped molybdenum, we can reduce the thickness of the materials being used for racks and/or trays as well as the posts and rails that make up the hearth. The preferred hearth material of TZM is two to three times stronger than pure moly. Less material is therefore required to provide the same strength.
|Fig. 3. G-force elements|
Improving Shielding Efficiency
With regard to the structure of the heat shields themselves, the longitudinal orientation of the spacers between the shield layers has always dictated a design with thick shields. The thicker shields were necessary for strength and to help the hot zone retain its cylindrical shape. Recent innovations have brought about the development of circumferential spacers. This spacer orientation provides a more inherently sturdy shield assembly and allows lighter-gauge shields to be used without the concern of the hot-zone wall integrity.
Usage of high-strength stainless steel for the support frame also adds to the sturdiness of the shield assembly, thereby further allowing the use of the lighter-gauge shield material.
When addressing the issue of the actual shield selection, it should be noted that the efficiency of one layer of molybdenum is equivalent to five stainless steel layers. Making this substitution allows fewer shields to be used and, therefore, less mass.
With regard to improving the shielding efficiency of the hot zone, it is important to note that, although it is key to keep the overall mass of the hot zone down by the methods described above, it can be just as important to add another sheet of moly to the shield pack to aid in reducing radiation losses through the hot-zone walls. For instance, a 2400°F shield design would typically include one layer of ML, two layers of moly and one layer of stainless steel. To improve energy conservation, we would design it with one layer of ML, three layers of moly and one layer of stainless steel. One additional layer of molybdenum has a significant effect with regard to containing heat losses and conserving energy.
Limiting line-of-sight radiation losses also aids in reducing power consumption. Corner shields are used where the walls meet the floor and ceiling to provide better insulating seals. Washers (insulated from the hot zone) are used to limit the amount of radiation heat loss at power feed-throughs.
If we look at the results from converting a 24-inch x 24-inch x 36-inch standard all-metal hot zone into a low-mass hot zone, we see that:
- There is a reduction of total hot-zone weight by approximately 15-20%.
- Ramp rates up to 20% faster are obtainable in an empty furnace.
- Heat loss is decreased by approximately 15 KW.
- There is a lower energy demand per cycle for heating up and cooling down of 25-30 kWh.
- The quench performance is improved by at least 15%.
|Fig. 4. Large hot zone|
In order to further decrease the energy consumption of an all-metal hot zone, PLANSEE offers an option of using a composite shielding where a thin layer of a special alumina paper is added to the low-mass shielding design. The alumina paper quality and thickness is selected in such a way that the characteristics desired in an all-metal hot zone – such as superior heating and cooling speeds, short cycle times and the carbon-free environment – remain untouched. But the energy consumption is reduced by 10-15% in a very economical way. The paper is produced from pure alumina and is without organic content.
In order to achieve these energy-saving results without negatively impacting the life of the hot zone, a deep understanding and knowledge in terms of material properties, fabrication capabilities and design aspects is required. Unfortunately, in most cases the application of such advanced solutions does not reduce the unit price in the same manner due to the usage of high-performance materials, but it clearly increases the performance and reduces the operational costs and, therefore, presents better value for many applications.
PLANSEE incorporates at least some of these design features into every hot zone it builds in an effort to make each one as energy efficient as possible. Other key design features used in PLANSEE hot zones include the use of PLANSEE ML as the material of choice for hot-zone heating elements and key shields. Its increased ductility and creep strength make ML the perfect option for applications requiring dimension and shape stability.
The “G-Force” heating element (Fig. 3) is one of the newest innovations from PLANSEE. The element’s design combines superior element support with perfect expansion allowance, making this element’s life unmatched in the industry. The G-Force heating element is used throughout the large, recently built hot zone shown in Figure 4. Industries that are supported with PLANSEE hot zones include commercial heat treating, metal injection molding (Fig. 5), aerospace, lighting, electronics and medical. IH
|Fig. 5. MIM hot zone|
For more information: Contact Ray O’Neill, product manager – Furnace Components; Plansee USA LLC., 115 Constitution Blvd., Franklin, MA 02038 (USA); tel: 508-918-1234; fax: 508-553-3823; e-mail: firstname.lastname@example.org; web: www.plansee.com