With the push for alternative energy technologies, the wind-energy field has seen significant growth over the last 10 years. Projections for the next 10 years indicate an even larger requirement for wind turbines to reduce dependence on fossil fuels.

With this increased need in the wind-turbine market, there has been a significant increase in component sizes, i.e. gears, bearings, pinions, etc. Existing gearbox manufacturers and gear heat-treating facilities do not have the required equipment sizes or production capacity to accommodate these larger parts and higher volumes associated with the deep case-depth requirements of these components. Due to the large load volumes and weights, the primary installed base expansion in furnace technology has been in pit furnace systems. Furnaces sized to process gears 2-3 meters in diameter are not uncommon with load weight requirements of 15-20 tons.

As these large components are under severe duty cycles and not easily replaced, the quality of the heat-treatment process is critical to final part and gearbox function. Of primary concern are quality of the carburized case and intergranular oxidation (IGO). All of these attributes are functions of the heat-treating process and the process equipment designs.

Two leading furnace technologies are used for this process. One technology utilizes an atmosphere-tight retort, while the other utilizes conventional refractory-lined furnace technology. The differences in furnace technology will be discussed with emphasis placed on how system design affects the carburized case and IGO. In addition, a comparison of processing atmospheres is provided between generated endothermic gas and nitrogen/methanol atmosphere. A description of both furnace designs is provided.

Large-Component Furnace Technologies

Retort-Muffle DesignBy definition, a muffle-furnace design utilizes an atmosphere-tight, metallic muffle to isolate the work from both the heating system and furnace refractory. In practice, the design most commonly used in large pit carburizing furnaces is a hybrid design with furnace top and bottom refractory exposed to the workload.

The primary component of the system is the muffle (Fig. 2). In the muffle-system design, there is a seal at both the top and bottom. Typically, the top seal is a water-cooled metallic flange arrangement utilizing an o-ring seal for atmosphere integrity. The bottom seal can be of multiple designs, including sand, ceramic fiber or oil. With the furnace sealed at both ends, expansion of the retort is critical. In most applications, an alloy bellows is added to the lower portion of the retort above the lower seal to allow for expansion. Materials of construction are typically wrought 330 SS. As the muffle is the primary atmosphere seal in the system, muffle maintenance is critical to process performance. Through-carburizing of the muffle within the hot zone as well as metal dusting of the muffle near the top and bottom seals are of concern.

A benefit of the muffle design is incorporation of reduced-cost electric heating systems. As the heating elements are not exposed to the carburizing environment, metallic rod overbend elements can be used without risk of short circuits from carbon deposit or carburization of element lead-in connections. These heating systems are generally lower in cost than comparable radiant-tube furnace designs. However, this cost saving is often offset by the higher operating costs associated with electric-furnace designs.

One area of concern with the muffle concept is temperature and atmosphere uniformity within the working chamber. In larger furnace designs, heat losses from the furnace cover and floor are not compensated for within the furnace design because the muffle allows for heating only from the side walls of the furnace.

One improvement to the design is the addition of an internal baffle inside of the muffle to direct atmosphere flow between the baffle and muffle, ensuring that atmosphere is circulated through the full depth of the furnace. The disadvantages of this approach are additional alloy content within the system and the possibility of reduced heat transfer from the shielding effect of the second retort because the primary form of heat transfer is re-radiation from the heating elements through the retort.

Internal Baffle Design
This furnace design is substantially the same as the muffle-furnace design with several key differences. An atmosphere recirculation baffle replaces the muffle and both the heating elements, and all of the refractory is exposed to the furnace atmosphere. With this design, there is only one atmosphere seal provided at the top of the furnace. It is typically made of ceramic fiber and without the use of cooling water. The internal baffle is constructed of wrought 330 SS material in a similar fashion as the muffle design.

The baffle resides entirely within the hot zone of the furnace, making it less susceptible to metal dusting. Additionally, any holes that may develop in the baffle are not critical to atmosphere containment in the process.

The major area of concern for atmosphere integrity is the radiant-tube heating system. Both gas-fired and electrified radiant tubes can be utilized in this design. Risk of short circuit from carbon buildup on feed throughs is eliminated by the use of special alloy heating elements. As this is a mature furnace technology used in carburizing processes, maintenance programs are well established.

Unlike the muffle-furnace design, atmosphere is continuously recirculated from the top to the bottom of the furnace based on the design of the internal baffle. This includes wiping of the radiant tubes and is accomplished without the addition of a secondary baffle. The major benefit of this baffle is improved temperature uniformity throughout the work envelope.

A typical baffle system is shown in Figure 1.

Temperature Uniformity

Two of the major factors affecting part quality are system temperature uniformity and atmosphere composition. The system temperature uniformity is a function of the heating and atmosphere recirculation system designs. The localized temperature in the furnace chamber has a direct effect on the achieved case depth within the parts. This includes both inconsistent case of multiple parts in a load and within the same large part. For a typical 1750°F boost/diffuse carburizing cycle operating on a 12- and 24-hour cycle time at temperature respectively, the predicted effective case depths are indicated in Table 1.

These variations in case depth are strictly temperature-dependent and do not account for additional variations that arise from part geometry or atmosphere variations within the furnace. Temperature spread within the working envelope of the two furnace designs was measured by a typical nine-point temperature survey. Temperature spread during soak conditions at 1750°F is displayed in Table 2 along with the expected case-depth variations for the two furnace types.

By improvement of the temperature uniformity within the furnace chamber, measurable improvement in the case-depth uniformity can be achieved. This improvement can lead to more precise case-depth range specifications, allowing for shorter carburizing times and improved cycle-to-cycle repeatability.

The second factor in the case-depth uniformity within the furnace is the atmosphere consistency throughout the working chamber. Assuming that both furnace systems utilize the same carbon-potential control system, including oxygen probe and infrared gas-analyzing equipment, the ability to control the atmosphere within the furnace chambers is equivalent. Variations in atmosphere composition between endothermic and nitrogen/methanol-based atmospheres are essentially eliminated by addition of an enriching agent (typically natural gas). The enriching gas is controlled based on the measured oxygen content and/or CO/CO2 ratio in the system.

Microstructure analysis (SEM) of various furnace cycles is provided in Figures 3-5 as defined by Table 3. The additional factor controlling the atmosphere consistency in the system is the recirculation system provided. In both furnace designs, large recirculation fans are typically located in the furnace cover. Atmosphere circulation in the baffle furnace and muffle with internal baffle are equivalent. In muffle designs without internal baffle, atmosphere variance is greater from top to bottom within the furnace because there is no assurance that the atmosphere flow reaches the bottom of the furnaces.

Conclusions

As can be seen by the SEM photos, all samples show varying amounts of IGO independent of furnace type or processing atmosphere. Depth of IGO seen is strictly dependent on time and temperature, as would be expected for endothermic-based atmospheres.

As both furnace technologies are capable of producing parts meeting the required specifications, other factors should be used in determining the appropriate technology.

Total cost of ownership, including utility costs, should be strongly considered. Gas-fired designs will typically operate at 50–60% total utility cost of an equivalently sized electric design. This fact coupled with a lower initial investment cost for baffle-furnace designs leads to a significantly reduced cost of ownership over the lifetime of the equipment. IH

For more information: John W. Gottschalk, director, special products, Surface Combustion, Inc., 1700 Indian Wood Cir., Maumee, OH 43537; tel: 419-891-7145; fax: 419-891-7151; e-mail: info@surfacecombustion.com; web: www.surfacecombustion.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: intergranular oxidation, endothermic gas, nitrogen/methanol, pit carburizing furnace, muffle furnace, temperature uniformity, case depth, SEM

Fig. 5. 18CrNiMo7-6 in a baffle furnace (see Table 3)