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Fig. 1. Typical workload (bell annealer)[6]


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Fig. 2. Retort being lowered into position[6]

 

Annealing can be performed in either batch or continuous furnaces. Box, pit and car-bottom furnaces are examples of batch units, while mesh-belt and roller-hearth styles are examples of continuous equipment. For annealing of steel coils, however, the most common type of furnace used is a bell-type furnace (Figs. 1-3). Let’s learn more.



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Fig. 3. Heating bell being lowered in place[6]

Annealing Equipment

Bell annealing (Fig. 4) heats batches of metal that are placed on a base assembly, enclosed by an inner cover and covered by the heating bell (furnace). An overhead crane is used to load the base and move the heating bell that is suspended from the crane. The base assembly normally includes a fan (optional) to provide a source of convection to enhance the heat transfer to the charge. The inner cover contains the desired atmosphere and protects the charge from the heating source. Direct fired, tangentially fired, radiant tube and electric elements are common heating methods. After annealing, cooling is performed by removing the heating bell but leaving the inner cover in place to maintain the protective atmosphere. If a bright finish is desired, the metal must be cooled to near ambient temperature before opening the workload to air. A gas-to-water heat exchanger or forced-cooler system is often used to shorten the cooling times, especially at low temperatures. The forced cooler replaces the heating bell at the end of the heating cycle and circulates air or sprays a fine water mist to accelerate the cooling of the outside of the inner cover.



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Fig. 4. Sectional view of a typical bell annealing furnace for wire products[7]

Annealing Problems

Microstructural Issues
In heat treating, changes happen to the internal structure (or microstructure) of the material and are not readily apparent to the naked eye. This is one of the reasons why process- and equipment-induced variables must be tightly controlled. For example, for many cold-heading components, process anneals are used to try to obtain a spheroidized microstructure (as opposed to a more lamellar structure). If lamellar, the more open (i.e. further apart) the lamellar structure, the softer it is and the more easily it is worked. A typical low-carbon steel microstructure (Fig. 5) consists of ferrite (white areas) and pearlite (dark areas). Pearlite is a combination of ferrite and iron carbide (Fe3C).

Properly distributed carbides are acceptable in most wire-drawing operations. However, some process annealing cycles (Fig. 6) produce agglomerated carbides (white globular regions) outlining the grain boundaries (necklacing). The result is that the wire will be more difficult to draw down to a smaller size without internal tears or breakage.

The presence of non-metallic inclusions, as well as the matrix microstructure, is also important considerations. The carbon content and cleanliness of the steel influence the volume fractions of ferrite, pearlite and carbide – what form the carbide is in (fine vs. coarse) as well as the morphology of those particles – and their size, shape and distribution. For example, fine wire drawers (e.g., tire wire, valve spring wire) are adamant about excluding nondeformable oxides, sulfides, carbides and nitrides for forming and fatigue reasons. Equally undesirable is carbon segregation, especially along the centerline of the wire, because it creates islands of martensite.



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Fig. 5. Process-annealed microstructure – low-carbon steel[8] (Photomicrograph courtesy of Aston Metallurgical Services Co., Inc.)

 

Decarburization
By definition, decarburization is a result of (usually unintentionally) removing carbon from the surface of steel when it is heated and held at temperature or when in contact with certain types of furnace atmosphere (Fig. 7). The result is a change in mechanical properties at the surface, most notably lower surface hardness and loss of fatigue resistance. Decarburization can be total (100% ferrite or “free” ferrite) or partial, and it can vary in depth from a few ten thousandths to several thousandths of an inch. In most cases, it is no more than 2% of the total thickness of the material. Carbon restoration can be used to reverse the effects of decarburization by driving carbon back into the surface of the rod or wire.



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Fig. 6. Process-annealed microstructure – low-carbon alloy steel[8] (Photomicrograph courtesy of Aston Metallurgical Services Co., Inc.)

 

Soot
A carbon-bearing furnace atmosphere can become out of control and cause sooting (carbon deposits) to occur on the surface of the wire (Eq. 1). This Boudouard (or gas producer) reaction may occur in a CO-rich furnace atmosphere in the critical range of 500-300°C (932-572°F). Of equal concern is carbon pickup, which in some cases results in surface cracking.
(1) 2CO ® CO2 + C



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Fig. 7. Decarburized surface microstructure

Summing Up

The choice of furnace and control of the furnace atmosphere is critical to the success of the annealing process. Each step in the process must be well understood in order to achieve the proper microstructure, grain size and mechanical properties in the annealed wire. IH