Energy Conservation Through Proper Selection of Heat-Resistant Alloys
Energy conservation has become one of the primary issues for the heat-treating industry in the new millennium. It is important in this new business climate to look at all opportunities for increased furnace efficiency.
In this article we will look at improving the weight ratio of part loading to cast or fabricated alloy tooling and improving the efficiency of other internal furnace parts.
Reduced Weight vs. Service Life
A thorough evaluation is necessary prior to making any decision to reduce the weight of heat-resistant tooling. It is particularly useful to work with a qualified product design engineer when conducting an evaluation. The designer can verify that the lighter-weight design will provide satisfactory service life so that the savings in increased efficiency will not be offset by shortened service life. The designer can also look at various materials to present the best solution that will provide the longest service life possible with the lightest-weight tooling.
|Fig. 1. Comparison of a 36x48-inch batch-furnace tray design with redesign|
Evaluation of Design
The designer will run a careful stress evaluation based on operating conditions and loading data provided by the customer. Once the design has been evaluated from a strength and material standpoint, it must also be evaluated by the supplier’s manufacturing personnel to verify a quality product can be produced.
In the case of castings, many times the design stress calculations will provide a lighter-weight metal section than can be cast without having hot tears, internal voids or a missrun. Hot tears and internal voids are a main concern in heat-treat applications since they will greatly shorten the service life of the castings. In many cases, due to the size and shape of the design, it is more practical to fabricate the part or provide a hybrid assembly that consists of both castings and fabrications.
|Fig. 2. Stacking fixture compared to redesigned lighter-weight fixture|
Reducing Weight of Trays, Baskets and Fixtures
By evaluating existing tray designs and applying loading and operating temperatures, it is often possible to design a lighter-weight tray by taking into consideration the application requirements and features of the overall design. Some of the application requirements that can be reviewed are applied-load locations, part loading, support points and handling methods. Some design features that can be reviewed are metal section versus depth of casting, location of ribs, elimination of heavy sections and applying additional strength only in the areas where needed.
Figure 1 shows a standard batch-furnace tray design and a redesigned tray based on the actual application. If the new lighter-weight tray is strong enough for the application and does not present any casting difficulty, it is possible to obtain the same or even increased service life while reducing the weight by 20% or more. When looking at a redesign of an existing tray, it is vital to look at the mode of tray failure so that, in addition to providing a lighter-weight tray, the failure points can be minimized.
There are a variety of opportunities for weight reduction with fixturing. The designer will need to review the application based on the operating conditions, loading, handling and sizing of the fixturing. The mode of failure needs to be reviewed to determine if the tooling can be redesigned to provide a lighter-weight fixture with longer service life. Stacking fixtures or baskets can be greatly improved by changing the method of support to provide much lighter-weight tooling that will not warp, bow, crack or distort early in the service life. Figure 2 shows the original fixture design and a redesigned lighter-weight fixture that will provide longer service life. The primary mode of failure on the original design was premature sagging due to lack of internal support on the upper fixture layers.
Fabricated baskets and fixturing normally provide the lightest-weight option and may well be considered when practical. When properly designed, lightweight stacking baskets offer great potential energy savings. With any basket or fixture design, it is important to use internal supports if at all possible. In most cases it is possible to cut the weight of a basket or fixture by 25% or more by using one or more internal supports. With fabricated baskets, it is especially important to run stress calculations on the bottom support ribs as the wrought material in round bar form may not have sufficient strength to support the required loading. The initial cost and service life of wrought versus cast should be considered when determining total cost savings.
|Fig. 3. Typical hybrid basket (23x35 inch)|
Hybrid baskets and other tooling afford the designer an opportunity to reduce weight based on using castings where needed for greater strength and using wrought materials in other areas that do not see the same stresses. This is especially true for stacking baskets and some fixtures. The most common way to provide strength where needed is by using hybrid stacking baskets where the bottom is cast and the sides are partially or completely fabricated.
Cast components produced using HU or HT material are more than two times stronger than type 330 wrought material due to the higher carbon used in cast materials. The higher carbon and grid-style design provide the additional strength needed to keep the basket bottom from bowing down and pulling in the sides. Figure 3 shows an example of typical hybrid baskets. Many types of assemblies can be reviewed for possible weight reductions based on a cast/wrought design solution.
Weight Reduction on Furnace Components
Internal furnace components such as radiant-tube assemblies, chain guides, furnace rails and work supports can be evaluated for reduced weight based on the application. The designer can run a stress evaluation and review failure modes to determine if a weight reduction is feasible. In addition, options are available in material selection for the purpose of adding strength and reducing weight.
Radiant-tube assemblies afford the best opportunity for weight reduction and improved efficiency. If the metal section can be reduced on the tubes and cast components, it will not only reduce weight but will improve heat transfer from the burner system to the furnace chamber. Many older furnaces use up to ½-inch metal sections on cast tubing. Most new systems have reduced metal sections – in some cases less than 1/8 inch.
Many suppliers of cast radiant-tube assemblies use 3/16-inch metal section on the tubing and ¼-inch metal section on the cast components in order to provide the best ratio of lightweight components and casting quality. Based on mechanical stress, the section modules of a thin-wall tube are stronger in bending over an unsupported span than a heavier-wall tube, taking into consideration the difference in tube weight.
The main advantage of a heavy-wall tube for longer service life is that the heavier wall will provide additional metal section for flame impingement and corrosion. One way to offset this issue on thinner-wall tubing is to go with a superalloy in the areas of the assembly that are most prone to failure. Many companies use NC11C, NC14 or a comparable material on the firing end of the assembly to provide longer service life. In many cases, the firing-end material can be upgraded while using a lower-grade material for the exhaust end of the assembly.
Fabricated wrought assemblies can be produced with 10- to 12-gauge material and will provide the greatest energy efficiency. There are several wrought materials now on the market (e.g., 602CA, H230 and I601) that provide greater strength and corrosion resistance. However, initial cost and service life needs to be weighed against the actual energy savings. In most cases a cast assembly will cost less and, depending on material selected, will last longer than a similar fabricated wrought assembly.
Material Selection for Energy Conservation
Material selection can provide a means for reducing weight in the furnace if a superalloy or other high-strength alloy is chosen. This will provide the strength needed in the lighter-weight component. This is especially true in furnace parts operating in the 1750-2200°F range. As an example, a tube support in HT material has a rating of 2,000 PSI at 1800°F based on 1% creep in 10,000 hours. If the tube support is cast in NC11C material, the rating increases to 3,200 PSI. This is a 60% increase in strength and may allow the designer to reduce the weight of the tube support from 70 to 45 pounds. In many cases, a considerable reduction in weight is possible by changing to a higher-strength material.
When evaluating materials, it is important not only to look at the PSI rating but to also evaluate the resistance to corrosion, oxidation, thermal cycling and other atmosphere conditions that could attack the alloy. Prior to making any material changes, it is useful to consult with a heat-resistant alloy supplier to determine the best solution for your application.
|Fig. 4. Solidification software|
Tools such as 3-D solid modeling software, solidification-simulation software and stress-analysis software greatly improve the design and the end product.
3-D solid modeling allows the designer to see exactly how the customer’s parts and other components will integrate with the new design. In addition, the customer can look at the new design more closely to verify form, fit and function. Solidification software (Fig. 4) allows the designer and manufacturing to observe the casting process to verify that the new design will produce high-quality castings prior to the sampling process. Stress analysis software allows the designer to run a thorough stress analysis based on the loads, handling and operating conditions the casting will be subjected to.
It is important to verify the alloy supplier selected is a qualified source for your design requirements. The supplier should be qualified in the following three areas: state-of-the-art design tools; experience, with an excellent track record in product design; and the ability to produce a quality product in a timely manner at a competitive price. IH
For more information: Contact Richard J. Grimm, sales and technical director, Wirco, Inc., 105 Progress Way, P.O. Box 609, Avilla, IN 46710; tel: 260-897-3768; fax: 260-897-2525; e-mail: firstname.lastname@example.org; web: www.wirco.com
SIDEBAR: New Alloy Technology
Heat-resistant alloy suppliers are responding to the energy concerns of recent years by coming out with new lightweight products for use in industrial heating furnaces. With the help of qualified and experienced designers, improved manufacturing techniques, new design software and improved materials, alloy suppliers have the ability to provide customers with lightweight products that provide longer service life. The key is to provide your alloy suppliers with the information needed so that they can make a full evaluation of your requirements.