This article was originally published on March 10, 2015.

Retorts, muffles and calciners are all examples of heating systems that employ heat-resistant alloys and accompanying heat-resistant welding consumables in construction.

The key to designing pieces of equipment that will last for at least the expected life cycle is dependent on proper alloy selection and design criteria.

Retorts

Retorts are round furnace vessels that are closed on one end while the other end rests on a base.^[1] Retorts are for batch operations. Sometimes the closed ends are on the top, and the bottom sits on a sand seal or water-cooled organic seal ring. Such furnace arrangements are called bell furnaces. Bell furnaces have an inner cover inside of which the parts to be treated sit. The inner cover seals in the desired atmosphere and protects the parts from the heating source.^[2] The outer cover (the bell) goes over the inner cover and performs the heating operations.

There are also retorts where the closed ends are on the bottom and the seal is on the top, which is completed by lowering a cover. Gas nitriding is often performed in such a retort. An external chiller cools both the retort and the atmospheric gas.^[3]
Common processes performed in retorts include annealing, carburizing, brazing, nitriding and vapor deposited coatings.

Alloy selection for retorts must take into consideration:

•   Actual metal temperature that the retort will see
•  Creep and/or rupture strength of the material at the above temperature
•  The ability to withstand the heat-treating atmosphere

Figure 2 is a guideline of temperature limits of various alloys.^[4] Please note that for alloy 600, some sources specify an oxidation limit of 2000°F (1093°C) instead of 2100°F (1149°C).

Creep Resistance

It is important to note that the driving parameter is the temperature that the retort material sees, not the operating temperature within the furnace. Depending on the heat source, this temperature can be close to the operating temperature or a good deal above the operating temperature. A problem that operators encounter is an unexpected, premature creeping or bulging of a retort. Though they believe that they have not changed anything, product loads increase to accommodate increased production demands, and this causes heating sources to work harder and the retort to get hotter. Even though the process temperature has not changed, the process dynamics have, and the heating causes the retort to get significantly hotter. This results in an unexpectedly shorter life.

Creep-strength tables can be found on websites of various manufacturers and distributors. The most common way of reporting creep strength is the stress required to produce 1% minimum creep rate in 10,000 hours. A good design keeps the stress well below this threshold and adds a reasonable safety factor. Rupture strength is the minimum stress required to cause the material to rupture or break in 10,000 hours. Creep can be visualized by the photograph in Figure 3.^[4]

In this case, strips of four different alloys have been fabricated into perfect cylinders, welded to a bottom plate and placed in an air furnace at 1832°F (1000°C) for 24 hours. The samples of 321 and 310 have sagged from their own weight and have little to no creep strength at this temperature. The RA 253 MA® and the 601 have retained their shape and still have at least some creep resistance at this temperature.

Atmosphere Resistance

Finally, the material must be able to resist the atmosphere. Consider the case of carburizing as an example. The purpose of the process is to establish a carburizing atmosphere sufficient for carbon to diffuse into the material being treated. The carbon will form a compound, and the surface will get significantly harder than the unreacted core. The retort is also a metal and subject to the same atmosphere. This means that over time the retort itself will begin to carburize. The key to carburization resistance is the protective oxide coating on the retort surface.^[1] The key to protection is the ability of the oxide to remain thermodynamically stable compared to the carbide.

The main oxides include chromia, silica and alumina. The latter two oxides are crucial in imparting carburization resistance, especially with increasing temperature. Nickel content is also quite important. Nickel lowers the solubility of carbon in the alloy. The 35% Ni content of RA330^® is enough to impart reasonable carburization resistance. Higher-nickel alloys will further improve resistance but at a higher price. These same factors guide the decision for nitriding and ferritic nitrocarburizing.

In the case of inert gas or hydrogen-containing reducing atmospheres, the creep strength will be the major consideration followed by oxidation resistance. The oxidation resistance is still a factor but only on the outer surface since the inner walls will be protected by the protective atmosphere.

Alloy Choices

Given all these factors, most retorts are constructed primarily from RA330. This alloy has oxidation resistance to 2100°F, 35% Ni and 1% minimum Si to impart resistance to both carburization and nitride resistance in case-hardening applications. It also has creep resistance to just over 1800°F (982°C).^[5] These retorts also have sections made from RA333^® in areas that are subject to metal dusting and nickel alloy 600 in the areas closest to the heat sources because of the increased ductility from the high nickel content of this alloy.

For higher-temperature applications, nickel alloy 601 is the material of choice because of its higher creep strength to higher temperatures^[5] and its oxidation resistance up to 2200°F (1204°C). Serious consideration should be given to newer alloy RA 602 CA® because it has oxidation resistance up to 2250°F (1232°C), excellent creep strength^[5,6] and has lasted up to four times as long in the same environment in head-to-head comparisons to nickel alloy 601.^[7]

The trade-off here is a higher initial cost. Any economic justification needs to consider entire life cycle. In such cases, the more expensive RA 602 CA often is the economical choice if the consumer is patient to take it to the end of life.

Muffles

Muffles are continuous furnace shells typically with a flat bottom, straight walls and curved roof. A typical muffle is shown in Figure 4. The muffles are contained within an outer box. Muffles are usually heated from above, below or both. A moving belt conveys parts through the furnace. Atmospheres are piped into the muffle, and protective fire blankets are maintained to keep out air.

Design Considerations

Processes typically performed in muffle furnaces include bright annealing, carburizing and copper brazing. Factors for consideration in the construction of muffles are the same as they are for retorts, but there are some additional considerations. Since some muffles can be rather long, they have more of a tendency to bow. Therefore, a great deal of effort is spent designing a muffle to keep straight. The two primary methods for doing this include transverse corrugation and built-in stiffeners to keep the muffle supported. Obviously, alloy selection and creep strength are that much more critical. For extremely high-temperature applications, RA 602 CA once again should be considered, and its high cost should be weighed against anticipated total life.

Some operators will support the weight of the muffle with a silicon-carbide hearth plate. A more apt description would be a silicon/silicon-carbide composite refractory. The manufacturing process is not consistent, and sometimes there is free silicon in the plate.^[1] In the event that there is enough free silicon, it will undergo a eutectic reaction with the nickel in the muffle alloy at 1767°F (964°C). Once that starts, the silicon will attack and eat through the metal. The only known way to prevent this attack is to prevent contact between the muffle and the hearth plate. This can be accomplished by a physical separation between the two surfaces with alumina or zirconia cloth, board or plaster slurry applied to the silicon carbide. The appearance of such an attack is not consistent. Long periods of time can elapse between appearances.

The protocol for alloy selection is similar to that of retorts. RA330 is typically the material of choice up to 1850°F (1010°C). In this case, the entire muffle shell is made from this material. As temperature increases, the material of choice becomes nickel alloy 601. Once again, if the 601 alloy starts warping and creeping at the highest temperatures of 2200°F and beyond, RA 602 CA should be considered as an alternative due to its high creep strength even at high temperatures.

RA 253 MA can be considered for use in hydrogen and/or nitrogen atmospheres up to 2000°F. This material is a stainless steel alloy with nitrogen to enhance the creep strength, and it actually surpasses the creep strength of both RA330 and 601 (until 1800-2000°F). This alloy with only 11% Ni will be far more cost effective than 601. It should not be used in carburizing atmospheres, and it may be marginally more tolerant of spilled copper than higher nickel alloys. Finally, 309 stainless steel can be used up to almost 1800°F (982°C) in neutral or reducing atmospheres. It is used for carbon-fiber production because it will exhibit some resistance to sulfidation due to its low Ni content.

Calciners

Calcining is a thermal-treatment process in absence of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition, phase transformation or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials.

Alloy selection is wide open depending on temperature and process gas. They would include 309, 310, RA 253 MA, RA330, 800 H/AT, 601, RA333, HR230, 617 and RA 602 CA.^[1] The stainless steels are subject to sigma-phase precipitation in the range of 1100-1600°F (593-871°C). The onset of this precipitation occurs fastest when the metal temperature is closer to the middle of the range. Calciners are typically rotary, and they are constantly subjected to bombardment by material falling and striking the calciner surface as a result. In the event that the calciner is cycled on and off, care needs to be taken at room temperature if the vessel is one of the stainless grades because the metal can be quite brittle if the operating temperature is in the sigma precipitation range.

RA 602 CA has been used successfully up to and above 2100°F (1149°C) in a 60-inch-diameter calciner for production of high-purity alumina. Rotary calciners that continue rotating at temperature sag in one direction, then another. During operation, care should be taken to ensure that the filled vessel is always moving, even at a very slow speed. The product has weight and exerts stress on the vessel wall where it sits. By maintaining even a slow rotation, the product is constantly moving and prevents any prolonged stress moments on any particular point. If hot product is allowed to sit on a hot vessel that is not moving for some reason, the net effect is to increase the pressure on the vessel, which in turn will accelerate creep if the overall stress is high enough.

Considerations for Welding

The welds of these materials should result in a weld that is stronger than the base metal and more oxidation resistant than the base metals. Materials suppliers typically supply pertinent information on how to weld their alloys, including the correct welding consumables, shielding gases, welding parameters and more. Since this article deals primarily with Ni-based alloys with 20% or more nickel, these comments pertain only to such alloys. Here are some general guidelines to pay attention to when welding.^[1]

1.  Use welding consumables with restrictions on the content of embrittling elements.

2.  Cleanliness is extremely important. Remove all oils, paints, lubricants and other contaminants.

3.  Welding technique is critical. Use techniques that yield reinforced convex beads.

4.  Do not use oxygen in shielding gases, especially when using GMAW.

5.  When in doubt it is acceptable to over-alloy provided that the weld is stronger than the base metal.

There are more considerations that are specific to rotary calciners. Where possible, weld the flights only in areas that will be in the cooler zones of the calciner. If the design will allow, do not weld the entire length to allow freer expansion and contraction. This will allow the cooler flights to move more freely to minimize the risk of cracking when the hotter shell expands more than the flight.

References

1. James Kelly, Heat Resistant Alloys, Printed and Bound by Art Bookbindery, Canada, 2013

2. Daniel H. Herring, Atmosphere Heat Treatment, BNP Media II, LLC, Troy, MI., 2014

3. Heat Treaters Guide: Practices and Procedures for Iron and Steel, Edited by Harry Chandler, ASM International, Materials Park, Ohio, 1995

4. Marc Glasser, Rolled Alloys, Private correspondence

5. High Temperature Alloy Properties. Rolled Alloys, Temperance, MI. 2011

6. VDM Metals, Altena Germany, Private correspondence

7. Case History – RA 602 CA, Rolled Alloys, Temperance, MI, 2014

8. Wikipedia, Calcination