This article provides an understanding of requirements that should be considered when designing alloy furnace components such as muffles and retorts.

The performance of consumable alloy furnace components in an atmosphere heat treating furnace will directly affect the operating cost of that furnace. Extending the service life, reducing cost, or improving thermal efficiency of such a component can reduce the overall operating costs of the furnace. In this article, furnace muffles and retorts operating in the 1200 F to 2100 F (650 C to 1150 C) temperature range are the primary focus.

These alloy components perform several functions inside the furnace. They act as the barrier between the furnace atmosphere and the ambient atmosphere and/or combustion products, and also are the primary heat transfer mechanism through which the heat source supplies radiant heat to the product load. These basic functions allow muffles and retorts to be affected by most operating variables. Opera-tional and financial concerns such as product quality, production rate, and furnace operating costs drive the need to maximize performance of these alloy components.

Analyses of atmosphere heat treating furnace operating costs show that the performance of alloy components directly affects several separate costs. Increasing performance of furnace components reduces or eliminates these costs. Although the cost of an individual alloy component may actually increase as a result of an optimized design that maximizes performance, the overall operating costs will decrease.

Furnace operating costs related to alloy components include:

  • Alloy Component Cost - higher performance alloys may reduce overall operating/life-cycle costs

  • Installation and Removal Cost - amortized costs decrease with service life

  • Maintenance Cost - eliminate or reduce repetitive repairs

  • Procurement and Spare Component Cost - reduced as service life increases

  • Furnace Downtime Cost - increased productivity with reduced downtime

  • Atmosphere Loss Cost - structural integrity can reduce this cost

  • Product Rework Cost

Furnace Design
Furnace design and operation should be evaluated prior to considering modification to alloy components. Not every component can be improved by changing its design. Some improvements can be gained by changing furnace design or operating parameters.

The design of the furnace must provide adequate clearance between the muffle or retort and the furnace cavity to allow for the expansion and contraction that will take place during the cyclical operation of the furnace. For example, a muffle that is twenty feet long at room temperature, constructed of 330 type stainless steel, will expand approximately 4 inches when heated to 1800 F. This expansion must be considered in the design of the furnace. It is important that the furnace support structure and any attachments will allow the muffle to expand freely as this thermal expansion is taking place.

In the case of a muffle, it may also act as the support surface for the conveying mechanism used to transport the product load through the furnace. The maximum performance of alloy components can only be achieved if the component load is transferred to the furnace support structure. Structural support of the alloy component and the appropriate consideration for expansion and contraction must be an integral part of the furnace design. This support should be uniform and designed to provide the lowest weight per square foot as is economically feasible.

This multitude of functions often requires that the design engineer balance different features of the component for optimum life and performance. For example, the heat resisting alloys used to manufacture these components are generally poor conductors of thermal energy. Therefore, a thin wall may perform better for many applications. When environmental attack is present, such as carburization, oxidation, or other type of chemical corrosion, a heavier wall may be required.

Adequate distance from the heat source to the alloy component surfaces is important to avoid over-temperature conditions. Also, product load clearances to alloy components and length of different temperature zones, such as preheat, high heat and cooling zones, should considered.

Furnace Operation
Considerations for furnace operation that may effect the life of alloy components include temperature uniformity, heat transfer rates, product loading, atmosphere control and cyclic operation.

Care should be taken to minimize temperature variance within the furnace cavity. Temperature variants cause localized expansion and contraction that create thermal stresses, deform the alloy, and cause thermal fatigue leading to failure. Temperature uniformity is greatly dependent upon the method of heating the furnace, use of circulating fans, length of temperature zones and product load.

Furnaces using muffles and retorts are typically heated with gas burners or radiant electric heating elements. When gas firing is used, the burners are normally located above and below the muffle or retort firing in opposite directions. This circulates the hot gases around the muffle or retort to promote heat transfer. High velocity type burners will create more turbulence in the furnace chamber and provide temperature uniformity. The burners generally have more heating capacity than can be dissipated, thus, care must be taken to prevent the flame from impinging on the alloy component.

In electrically heated applications, it is important that the upper and lower heating elements provided uniform heating of the muffle and retort. It is common to see applications where the top of a muffle is heated faster than the bottom causing distortion because the top has expanded greater than the bottom. Separate temperature controllers for above and below the muffle may be required.

Product loading can have a great affect on component service life. Constant temperature and steady flow of product is the ideal situation for muffles and retorts. Consider the thermal stresses developed in a batch rotary retort. If the furnace operates at a temperatures in excess of 1700 F and a load of steel parts is loaded into the retort at room temperature, a rapid cooling will take place on one end. When allowed to happen frequently, this rapid localized change in temperature can cause failure due to thermal cycling.

Proper control of the furnace atmosphere is also important to the life of alloy components. Poor control of the dew point in aluminum brazing applications can allow excess moisture and condensation of highly corrosive fluxing agents leading to failure due to corrosion. Control of carbon potential is important in carburizing furnaces. Excessive carbon will permeate the grain structure of the alloy and cause embrittlement and eventual component failure. Preheat or burn-off muffles especially require good atmosphere control to flush out these contaminants.

Cyclical operation of a furnace also has a great impact on the life of alloy components. Heat resisting alloys form a protective coating designed to resist oxidation. When the component expands or contracts due to cyclic operation, this coating tends to spall and oxidation continues, eventually thinning the component wall. The frequency of cycles should be reduced if possible.

Structural Design
A number of factors form the basis for structural design of the alloy components. The goal of enhancing structural design to maximize performance of alloys components is generally to minimize total weight of the components while maintaining its structural integrity or to minimize deformation of the component. This goal can be achieved by reducing the wall thickness of the component and selecting different fabrication designs such as corrugated walls, reinforcing gussets and controlled shape of the component to maintain or increase strength.

One design criteria used in the structural design of an alloy component is the product work package, which determines the overall geometry of that component. Fig. 1 illustrates the products work package in a muffle for a continuous belt furnace. The new product work package is the cross-sectional area required to adequately process the product within the muffle or retort. The product work package is determined using interrelated parameters that include maximum product dimensions and clearances to the inner surface of the alloy component.

Fig. 2 Comparison of cross-sectional designs.

In a new furnace design, once the furnace type is selected, the end users' product dimensions establish the product work package. This product work package is the controlling factor for the alloy component and the furnace chamber. However, when retrofitting an existing furnace, the product work package, furnace chamber, placement of piping, control thermocouples, and heating elements must all be considered in order to design the alloy component.

Significant differences in allowable stresses with respect to temperature provide a problem to the alloy component designer. The designer must compensate for the loss of materials strength at elevated temperature. This can be accomplished by increasing the material thickness, increasing the cross-sectional area using fabricating designs or upgrading to a higher strength material. Fig. 2 and Table I compare five different cross-sectional designs with respect to "moment of inertia". The area moment of inertia is a characteristic of a cross-sectional plane area that relates the ability of that cross-sectional area to resist bending. The larger the moment of inertia of the cross-sectional area, the greater the resistance to bending is for that cross-section.

In this example, all plates are 304.8 mm (12 in.) wide type 330 stainless steel and are assumed loaded uniformly across the span and are simply supported on both ends. Also assume all plates are exposed to a temperature of 1600 F (871 C) and the allowable stress of the design is determined using the 1% secondary creep stress at 10,000 hours. The bending stress, s, for each plate according to the equation[Ref. 1].

s = Mc/I

M is the bending moment (WL^2/8, W = material weight and L = length), c is the distance from neutral axis, and I is the moment of inertia, is compared in Table I. The designer is interested in the material with the lowest stress (the lower the stress, the less deformation will occur over time at elevated temperature).

Bending stress calculations show that doubling the flat plat thickness will increase section strength by a factor of two. While this is a significant gain, it requires twice the material, which increases the material cost. In this case budget constraints may play a role in the furnace design. While optimizing the structural design may increase one-time costs for materials, these costs may be overcome if service life is substantially increased by the choice of material.

Construction Materials
The selection of materials is a critical design option that can independently determine the success of any alloy component. However, the goal for material selection should be to maximize the alloy's strength while at the same time minimizing overall material cost. To achieve this goal, the selected alloy must meet minimum material requirements that include temperature and atmosphere resistance.

When considering an alloy for a specific application, it is important to have a basic understanding of the composition of the alloy and the function of each element in its composition so that detrimental material/atmosphere reactions are avoided. Table II outlines the major elements used in heat resistant alloys and the function of each element.

Process atmosphere and temperature affect materials selection and the relative strength of the selected material. Since an alloy's strength generally decreases with respect to time and temperature, the creep strength or average stress to rupture value should be used as the design service temperature. Creep is defined as the time-temperature deformation that occurs after the application of a load at an elevated temperature. Table III illustrates the creep strength and average stress to rupture at 10,000 hours for RA-330 Alloy.

Using a 1.0% secondary creep strength in 10,000 hours as an allowable design stress, a comparison can be made at different temperatures for the cross-section of a hypothetical alloy component. For example, a cross-section could develop a stress of 21 ksi (145 MPa) at 1000ÝF (538ÝC) prior to failure. However, the same cross-section could only develop a stress of 2.1 ksi (14.50 MPa) at 1600ÝF (871ÝC) prior to failure. Therefore, the same cross-section operating at 1600ÝF (871ÝC) could only carry 10% of the load it could carry operating at 1000ÝF (538ÝC).

Alloy selection should also be considered in conjunction with structural design options. For example, a corrugated plate may have considerably higher strength than that of a flat plate of the same material. Using another type of material may result in an even greater strength advantage, however, cost justification is also another important part of material selection. Justifying the cost of using the higher strength alloy may be difficult if a significant advantage in service life cannot be gained. Good material selection will account for all factors that could possibly impact the design.

Consideration must also be given to the forming process used to make the component. Cast components have certain characteristics that have advantages over wrought components for specific applications. Likewise, wrought components have some advantages in certain applications over cast components. Castings of similar composition typically have higher creep strength than wrought materials because of higher carbon content and coarser grain structure. Components with special shape and thickness requirements are sometimes only available in cast form. Wrought materials on the other hand typically have fewer internal and external defects and are available in thinner sections. The ductility of wrought materials and fine grain structure decreases cracking with better resistance to thermal fatigue.

Fabrication Methods
A number of fabricating methods can be used to maximize alloy component performance. These methods typically impact alloy component strength, quality and resistance to process atmosphere. Form-ing methods affect the integrity of the material of the alloy component. Forming dies should be selected to maintain bend radii equal to or greater than the material thickness. Single and continuous "hits" should be used to form material for minimal work hardening. Forming tolerances should be achieved to provide accurate weld joint design during component assembly

After an alloy component has been fully designed, perhaps the most important fabrication method is the proper use of quality assurance. A great deal of resources will be wasted on the design of an alloy component and the cost of materials if the proper welding procedures, in process inspection checkpoints and final inspection specifications are not followed. A simple quality assurance plan should include verification of material type, inspection of any welding processes including joint preparation, joint fit, filler metal, final dimensions, a leak test if used in a controlled atmosphere application, and any documentation necessary to fulfill customer needs.


This article has presented design considerations for maximizing the performance of alloy components. Identifying furnace operating costs is key to achieving an alloy component design that maximize performance. A system approach should be used to determine the alloy component design that will decrease these costs. This requires the combined consideration of several design criteria:

  • thermal expansion allow-ance, alloy component support, operating parameters, heating methods, product load requirements and atmosphere;

  • furnace chamber size, product work package and component plate designs;

  • environmental resistance, strength at elevated temperature;

  • fabrication methods, cast or wrought, welding, joint design/alignment/location, forming tolerances.

When the above are considered, the designer will strive for the optimum design for an original alloy component. Replacement alloy component design will consider failure analysis of previous components as well as the above criteria. Finally, balancing the cost of a component with service life is an important consideration. IH