Improving Air Melting with Metal Degassing

Vacuum Cap (VCAP) furnace technology combines air melting and vacuum refining into a hybrid process that produces many metallurgical advantages compared to traditional air melting. Air melting has limitations with regard to the purity of the alloy (low vapor-pressure tramp elements), tight composition control, and other metallurgical issues like gas content (oxygen, nitrogen, and hydrogen levels) and high carbon content.

In cases where full-vacuum melting is not required, VCAP furnaces take advantage of efficient charging and melting in air, and then a vacuum cap is applied to improve refining of ferrous and nonferrous metals. This also reduces the scatter in product properties, ultimately resulting in less rejections.

Air induction melting technology is a very common manufacturing method, used all over the world, that makes it possible to produce complex castings in many different types of alloys.

Quality and performance requirements on alloys are becoming more stringent for end-users in industries like aerospace, medical, power generation, oil and gas, specialty automotive, and trucks. These requirements include few if any defects and better mechanical properties.

However, these properties are difficult to attain using standard air-melting techniques.

These are the main challenges of the air melting technologies:
-  They may suffer from limited control of alloy-composition cleanliness.
-  Oxygen- and nitrogen-level heat-to-heat variability.
-  Current practice to decrease gas content is to add virgin material, because deoxidation with aluminum only could result in non-metallic inclusions.
-  Low vapor-pressure tramp elements like Pb, Bi, Zn, etc., can be reduced only via dilution.
-  Carbon reduction is a metallurgical challenge in air melting.

VCAP technology is a solution to address all these challenges because of the advantages unique to the vacuum induction melting process, such as excellent control over the entire alloy chemistry, not only the desired alloy composition but also the beneficial trace elements and harmful impurities. Additionally, the reproducibility of precise composition control from heat-to-heat is exceptional and results in a remarkable consistency of material properties at high levels.

VCAP technology, shown in Figure 1, is a hybrid process that combines air melting and vacuum induction melting techniques (VIM.) Essentially, VCAP involves an air-melting furnace that incorporates a cap that can be placed on the top of the induction melting coil, making it possible to carry out vacuum degassing cycles once the alloy is fully melted in air.

Figure 2 depicts the basics of VCAP technology as a hybrid of an air-melting furnace (left) and vacuum induction melting (right.) The process variables also influence capital investment, process complexity, and quality of the material produced. Overall, VCAP furnaces offer a powerful combination of precision, purity, and flexibility, making them an ideal choice for producing high-performance alloys and other advanced materials.

VCAP highlights

VCAP technology includes the following main highlights that make it an advantageous option compared to air melting technology:

It is available in a wide range of sizes, from 50 to 30,000 kg, so customers may choose furnaces sized for small pour weights or research purposes, and also for large pour weights.

Vacuum levels range from 100 to 0.01 mbar. Vacuum levels highly depend on the C and O composition and the following decarburization reaction.

Argon partial pressure atmosphere melting also is available.

Argon/nitrogen porous plug systems are available for additional agitation within the melt, and then promote good mixing, degassing, and overall metallurgical reactions.

VCAP can be used to process almost all metals:
-  It is available to melt and mix selected raw materials/revert.
-  Ferrous and non-ferrous alloys.

Some different metallurgical processes may be achieved:
-  Reduction of hydrogen, oxygen and nitrogen (vacuum degassing.)
-  Reduction of low vapor-pressure tramp elements like Pb, Cd, Bi, Zn.
-  Deoxidation using combination of vacuum and C-O reaction.
-  Decarburization-intensified C-O reaction at low pressure, enabling excellent decarburization for extra-low carbon levels (improved alloy workability/machinability.)
-  Desulphurization (limited.)
-  Use of reducing slags and / or powder injection in air or controlled atmosphere

As a result, better micro-cleanliness is achieved due to strong carbon deoxidation and smaller residual inclusions, and subsequently:
-  Increased metal fluidity, which improves mold filling.
-  Significant improvement of mechanical properties.
-  Improvement of technological characteristics like hot workability, weldability and machinability.
-  Significantly reduced scatter in product properties and characteristics, so fewer rejections.

Process steps

VCAP furnaces offer a compelling option for high-quality foundry applications that require an improvement in air melting and do not require full vacuum melting. VCAP furnaces are available to suit a wide variety of applications in sizes ranging from 50 kg (110 lbs) to 20 tons. Typical applications feature low- and high-carbon steels, stainless steels, cobalt alloys, tool and die steels, nickel-based alloys, and nonferrous alloys.

The VCAP furnace is designed for induction melting of a solid charge in air or vacuum, with a final degassing stage under vacuum. The final pouring of the metal is performed in air or under a protective atmosphere of inert gas. Configuration is based on the Inductotherm range of steel shell induction furnaces, which are fully adapted by Consarc for vacuum treatment of liquid metal.

The furnace shell is fully sealed for vacuum operation and a sealing flange/apron is provided on top of the unit. Following the air melt operation (or vacuum/inert gas if required), a water-cooled vacuum lid is placed on top of the furnace, either by factory crane or optional lift/swing pivot arm. The vacuum chamber is connected to a multi-stage, mechanical vacuum pumping system that can evacuate the atmosphere above the molten bath.

The induction melting coil is powered from an Inductotherm VIP^® Power supply with the power and frequency matched for fast melt rates (high productivity), and optimal stirring (metallurgical quality) in the liquid state. The stirring frequency ensures that the alloy is fully homogenized, and that fresh liquid metal is cycled to the surface of the bath to aid the degassing procedure.

Once the atmosphere is evacuated, the degassing procedure and intensified CO reaction allows removal of undesirable gases, hydrogen, nitrogen, and oxygen to much lower levels than would be possible in air.

At the end of the degassing sequence, the vacuum lid can be removed, and a protection ring is placed around the sealing flange. Then, the furnace is ready for tilt pouring into a transfer ladle or molds. Usually, the pouring process is carried out in air, but options for pouring under protective atmosphere also are available. Figure 3 summarizes the process flow.

Case studies

Consarc has conducted a number of trials at Brno University of Technology in the Czech Republic, working with an 80-kg Consarc VCAP furnace installed there.

Table 1 presents some examples of the trials conducted and benefits obtained from these VCAP tests. It includes different experiences with different alloys, and with different metallurgical objectives for each one of them.

Figure 4 shows the evolution of oxygen and nitrogen during the VCAP trial melt at Brno University.

Table 2 summarizes the results of studies conducted at some customers’ production sites.

Conclusions

VCAP technology combines the simplicity of an air melting process with the benefits from vacuum melting, such as reduction of deleterious gases like hydrogen, oxygen (by deoxidation), and nitrogen, and low vapor-pressure tramp elements like Pb, Cd, Bi, Zn.

Moreover, the VCAP process performs decarburization and desulphurization reactions to reduce carbon and sulphur, respectively.

As a result of the benefits of VCAP technology, foundries can achieve:

Better chemical control of the alloys cast, better micro-cleanliness, clear improvement of mechanical and foundry properties, and an improvement of technological characteristics like hot workability, weldability, and machinability.

Process improvements and significant reduction of scatter in product properties and characteristics. And as a result of that, VCAP provides a more consistent casting process, and fewer rejections on cast parts.

Iñaki Vicario is a casting technology specialist with Consarc Corp.