Effect of Magnesium Content on Crack Tendency in 300 mm × 1200 mm Slab Ingots

The following data shows the influence of magnesium content on the crack tendency of 300 mm × 1200 mm slab ingots.

1. Why Is Chemical Composition Adjustment Necessary?

The adjustment of the chemical composition of molten primary aluminum is mainly required for the production of aluminum ingots for remelting, such as Al99.90-Al99.00, Al99.7E and Al99.6E; refined aluminum ingots for remelting, such as Al99.996-Al99.95; and industrial pure aluminum ingots for pressure working, namely the 1xxx series.

The aluminum content of 1xxx industrial pure aluminum ingots corresponds to the purity range from refined aluminum ingots for remelting to ordinary aluminum ingots for remelting, that is, Al99.996-Al99.00. These aluminum ingots can all be produced directly from molten primary aluminum.

The chemical composition of molten primary aluminum in electrolytic cells is not always completely consistent. However, the grades of cast finished products are different, and each grade has strict requirements. In particular, industrial pure aluminum ingots have stricter chemical composition requirements to ensure that the processed aluminum products meet the required performance standards. Therefore, the chemical composition of molten primary aluminum must be properly adjusted.

Industrial pure aluminum has no tendency to form cold cracks. However, within the specified chemical composition range, if the composition and impurity content are not properly controlled, hot cracks may form due to high hot brittleness.

For industrial pure aluminum, the main method for reducing hot brittleness is to control the contents of iron and silicon impurities. Under a normal casting system, when the titanium content in the alloy is 0.01%-0.02%, production practice shows that if the silicon content is less than 0.3%, controlling the iron content to be 0.02%-0.05% higher than the silicon content is sufficient to eliminate the hot-cracking tendency of cast ingots.

When the silicon content is greater than 0.3%, the influence of the iron-silicon relationship on hot cracking becomes less obvious. This may be because, as the total amount of iron and silicon increases, the amount of non-equilibrium eutectic formed during crystallization also increases.

A common explanation of the influence of the iron-silicon ratio is that when the iron content in the alloy is higher than the silicon content, crystallization proceeds within a relatively narrow temperature range in the form of a peritectic transformation.

Additional Batching After Intermediate Analysis

During batching, if the product grade needs to be improved, high-grade aluminum ingots should be added. If the product grade needs to be lowered, low-grade aluminum ingots should be added.

After batching, if the intermediate analysis value cannot meet the specified value, additional batching must be carried out. The calculation method is the same as above. The commonly specified center value is used as the calculation standard for correction.

Let:

• a = amount of primary aluminum charged into the furnace
• b = standard value
• c = intermediate analysis value
• d = amount of master alloy added for correction
• e = alloying element content in the master alloy used for correction

The amount of master alloy d required for correction is calculated as:

d = [a(b - c)] / (e - b)

2. What Factors Affect Batching Accuracy?

The factors affecting batching accuracy mainly include the following:

1. The grade of molten primary aluminum in the electrolytic cell is determined according to the pre-analysis result from the day before aluminum tapping. If the purity of the molten primary aluminum in the electrolytic cell fluctuates sharply, the purity of the aluminum liquid after batching may easily exceed the specified value.
2. When aluminum is tapped from multiple electrolytic cells into one ladle, a large error in the tapping amount from each individual cell will affect batching accuracy.
3. Errors in the amount or purity of the added master alloy may cause the purity of the aluminum liquid after batching to exceed the specified value.
4. Fluctuations in the chemical composition of returned scrap will also affect batching accuracy.

3. How Should Excessive Aluminum Liquid Purity Be Corrected When Producing Aluminum Ingots for Remelting?

When producing aluminum ingots for remelting, if the purity of the aluminum liquid after batching exceeds the standard and the mixing furnace has not yet been filled, the amount of molten primary aluminum added can be adjusted according to the charged aluminum liquid amount and the intermediate analysis value of the chemical composition.

The content of the relevant element in the additionally added molten primary aluminum should be lower than the standard value. The amount of molten primary aluminum to be added can be calculated as follows.

Let:

• a = amount of primary aluminum charged into the furnace
• b = standard value
• c = intermediate analysis value
• d = amount of primary aluminum to be added
• e = purity of the added primary aluminum

Then the equation is:

ac + de = ab + db

After rearranging the equation, the formula is:

d = [a(c - b)] / (b - e)

Modern hot strip production has increasingly adopted continuous casting slabs as a replacement for traditional ingot-based rolling processes. As steelmakers pursue higher productivity, lower energy consumption, and improved product quality, slab reheating technology has become a critical part of the rolling line.

A notable development occurred in 1965 when McLouth Steel Corporation in the United States explored new methods for reheating continuously cast slabs before rolling. These efforts ultimately contributed to the development of advanced induction slab heating systems capable of handling cold, warm, and hot slabs in continuous production environments.

This article summarizes the principles, design requirements, and operating performance of induction reheating systems for steel slabs.

Why Secondary Reheating of Slabs Is Necessary

Before entering the rolling mill, continuously cast slabs typically require secondary reheating. Depending on their condition, slabs may be:

• Cold slabs
• Warm slabs (approximately 200–500°C)
• Hot slabs directly from continuous casting (typically 500–700°C)

The objective of secondary reheating is to:

• Achieve uniform rolling temperature
• Improve deformation characteristics
• Reduce rolling forces
• Enhance product quality
• Support continuous rolling operations

Early studies investigated pusher-type reheating furnaces and walking-beam furnaces. However, these systems encountered limitations when processing mixed batches containing cold, warm, and hot slabs simultaneously. Operational complexity and high costs encouraged the search for alternative technologies.

Development of Induction Slab Reheating

To overcome these challenges, Ross of Ajax Magnathermic proposed a reheating method based on a large rectangular induction coil.

Working Principle

The system operates as follows:

1. Slabs are positioned vertically.
2. Mechanical handling equipment lifts the slabs.
3. The slabs pass through a fixed rectangular induction coil.
4. Electromagnetic induction generates heat directly within the slab.
5. The slab reaches the required rolling temperature before entering the mill.

Because steel slabs may warp during heating, sufficient clearance must be maintained between the slab and the induction coil. While this reduces the power factor slightly, it allows safe and reliable operation.

Design Requirements for the Slab Heating Installation

The installation was designed according to the following production objectives:

Production Capacity

• Annual production target: approximately 2,200,000 tons

Slab Thickness

• Maximum slab thickness: 300 mm

Available Slab Widths

• 0.9 m
• 1.1 m
• 1.3 m
• 1.5 m

Operational Requirements

The system was expected to:

1. Process slabs of varying widths.
2. Achieve an average throughput of 276 tons/hour for general operation.
3. Reach 550 tons/hour when processing 1.5-meter-wide slabs.
4. Handle mixed charging of cold, warm, and hot slabs.
5. Support fully continuous production.
6. Provide high operational flexibility.
7. Be compatible with computerized process control.
8. Deliver large-scale production capability.
9. Minimize plant floor space requirements.
10. Reduce labor requirements.

According to reported results, the final installation successfully met all these design objectives.

Induction Heating Line Configuration

When processing the largest slabs:

• Thickness: 300 mm
• Width: 1.5 m
• Length: 8 m

The plant employed:

• 6 heating lines
• Production rate per line: 100 tons/hour

Each line contained three induction heating sections:

Total Power per Line

• 35,000 kW

Total System Power

• 210,000 kW

These values represent maximum rated capacities. In actual production, simultaneous operation at full power across all heating sections is rarely required.

Energy Consumption and Heating Performance

Published operating data indicated excellent performance characteristics.

Power Consumption

Energy consumption remained below:

358 kWh per ton

regardless of slab dimensions.

Oxidation Loss

Scale formation was controlled below:

0.25%

This represents a significant advantage over many conventional fuel-fired reheating furnaces.

Temperature Uniformity

The induction heating process achieved:

• Excellent temperature consistency
• Uniform slab heating
• Reduced thermal gradients
• Improved rolling quality

Advanced Coil Design

To accommodate different slab widths, the induction coils were designed with adjustable sections.

Key features included:

• Modular coil construction
• Adaptability to multiple slab dimensions
• Improved heating efficiency
• Reduced energy waste

Power regulation was achieved through specially designed controllable switching systems. The switching devices connected or disconnected power at the zero-crossing point of the AC waveform, minimizing electrical stress and improving system reliability.

Advantages of Induction Slab Reheating

Compared with traditional reheating furnaces, induction slab heating offers several important benefits:

Higher Productivity

• Continuous processing
• Rapid temperature rise
• Reduced waiting time

Improved Energy Efficiency

• Direct internal heating
• Lower heat losses
• Reduced fuel consumption

Better Product Quality

• Uniform temperature distribution
• Lower oxidation rates
• Reduced scale formation

Greater Automation

• Easy integration with computer control systems
• Real-time process monitoring
• Reduced operator intervention

Smaller Footprint

• Compact installation
• Reduced plant space requirements

Environmental Benefits

• Lower emissions
• Cleaner production process
• Improved workplace conditions

Conclusion

The adoption of induction reheating technology marked a significant advancement in hot strip mill operations. By enabling efficient reheating of cold, warm, and hot continuously cast slabs, induction systems provide exceptional flexibility, productivity, and energy efficiency.

The McLouth Steel installation demonstrated that large-scale induction slab reheating can successfully support annual production exceeding two million tons while maintaining low oxidation losses, excellent temperature uniformity, and energy consumption below 358 kWh per ton.

Today, induction slab heating remains an important technology for steel producers seeking higher efficiency, improved product quality, and more sustainable manufacturing practices.

1. Furnace Body and Auxiliary Devices

The furnace body of a vacuum induction furnace and the auxiliary devices installed on it generally include the equipment required for charging, ramming, melting, pouring and other processes. Figure 2-41 shows the furnace body and auxiliary devices of a horizontal vacuum induction furnace.

The furnace shell is divided into two parts: the movable furnace shell and the fixed furnace shell. The entire smelting process is carried out inside the furnace shell, making it a major component of the furnace.

The furnace shell of a vacuum induction furnace must withstand the strong pressure formed by the internal vacuum, so it must have sufficient structural strength.

Small furnace shells adopt a double-layer structure. The outer layer is made of ordinary steel plate, while the inner layer is welded from non-magnetic austenitic stainless steel plate, with water cooling between the two layers.

Large furnaces partially use a double-layer structure, while most parts adopt a single-layer steel plate structure with external water pipes for cooling.

The contact surface between the movable and fixed parts of the furnace shell, as well as the connecting surfaces between the devices installed on the furnace shell and the furnace shell, must be sealed with vacuum rubber parts.

The internal structure of a horizontal vacuum induction furnace is shown in Figure 2-42. Inside the furnace shell, there are devices such as an inductor, crucible support, ingot casting device, charging chute, and coaxial rotating electrode.

Outside the furnace shell, there are devices such as a charger, ramming rod, temperature measuring and sampling device, and observation window.

The power supply device is also installed on the furnace body. Small furnaces use water-cooled coaxial rotating electrodes, while large furnaces use water-cooled cables insulated from the furnace shell.

Figure 2-43 shows the structure of the water-cooled coaxial rotating electrode used in a small vacuum induction furnace. The water-cooled coaxial rotating electrode is divided into inner and outer layers.

Figure 2-43 Water-Cooled Coaxial Rotating Electrode

1. Outer electrode
2. Inner electrode
3. Rotating handle
4. Bearing
5. Wilson sealing ring
6. Sealing ring
7. Water pipe joint
8. Furnace shell

It is made of copper tubes. The outer layer is called the outer electrode, and the inner layer is called the inner electrode. An insulating layer cast from epoxy resin and quartz sand material is used between them.

Both the inner electrode and the outer electrode are cooled by water.

The functions of the water-cooled coaxial rotating electrode are as follows:

1. It transmits medium-frequency current to the inductor. The current circuit is: outer electrode → inductor → inner electrode.
2. It supplies cooling water to the inductor.
3. It tilts the crucible.

It can be seen that the water-cooled coaxial rotating electrode is a key component of small furnaces. It must be carefully used and maintained, and spare parts should be prepared when necessary.

Furnace models using the water-cooled coaxial rotating electrode structure include ZG-0.01, ZG-0.025, ZG-0.05, ZG-0.2 and other types.

The parts of the furnace body that need cooling include the furnace shell, ramming rod, temperature measuring device, sampling device, inductor, large flange, coaxial rotating electrode, cable and current collecting device. The pressure and flow rate of cooling water should ensure normal smelting.

In addition, large furnaces are also equipped with auxiliary equipment such as a charging chamber and vacuum unit, furnace shell moving device, and ingot charging and discharging device.

3. Vacuum System of Vacuum Induction Furnace

The smelting vacuum degree of the furnace is usually within the range of 10^-2 to 10^-3 Torr. The smelting vacuum degree of large furnaces is lower than that of small furnaces.

To maintain the smelting vacuum degree, a suitable vacuum unit must be selected to meet the vacuum requirements.

The vacuum system of the furnace includes the vacuum chamber, furnace shell, vacuum unit, pipelines, vacuum valves, measuring instruments, sealing elements and other components.

The main vacuum parameters of a vacuum induction furnace include:

1. Smelting Vacuum Degree

This refers to the vacuum degree maintained by the furnace during operation. It can also be divided into melting vacuum degree, refining vacuum degree and pouring vacuum degree.

The smelting vacuum degree is the main parameter indicating the quality of the furnace vacuum system.

2. Ultimate Vacuum Degree

This refers to the highest vacuum degree that the furnace can reach when it is empty at room temperature.

It is an important indicator for evaluating the pumping capacity of the vacuum unit and the air leakage and gas release of the furnace system.

3. Pumping Speed

This refers to the amount of gas extracted from the furnace by the vacuum unit per second.

The faster the pumping speed, the shorter the time required to reach the needed vacuum degree, and the higher the efficiency of the vacuum unit.

4. Pressure Rise Rate

This refers to the value by which the vacuum degree in the furnace body or other vacuum chamber decreases per unit time.

It is an indicator of the leakage condition of the furnace.

Plasma induction furnaces represent an advanced class of metallurgical smelting and refining equipment. By combining the rapid, uniform heating of conventional induction fields with the concentrated high-temperature energy of a plasma arc torch, these systems offer unparalleled refining capabilities. This technology is highly valued for manufacturing ultra-low carbon stainless steels, precision alloys, and high-temperature master alloys containing reactive elements like aluminum and titanium.

1. Core System Configuration and Schematics

A professional plasma induction furnace installation relies on an integrated, multi-system architecture to maintain both the electrical induction fields and the ionized plasma stream.

The Fundamental Structural Building Blocks

As illustrated in industrial furnace engineering layouts, a complete plasma induction furnace consists of four core technical assemblies:

1. The Furnace Body Assembly: The physical vessel including the crucible, outer structural framework, and surrounding induction coil array.
2. The Induction Power Supply Network: Provides alternating current to generate the electromagnetic fields needed to heat and stir the primary metal charge.
3. The Plasma Gun Torch Assembly: The mechanical assembly that injects the inert working gas and houses the primary negative electrode.
4. The Direct Current (DC) Plasma Arc Power Unit: A dedicated power supply that drives the high-voltage ionization and maintains a stable DC plasma column.

2. Physics of the Direct Current (DC) Plasma Arc

To operate a plasma induction furnace, technicians must understand gas ionization physics. Under ambient conditions, gas molecules exist in an uncharged, non-conductive state. Introducing a strong external force—such as a high-voltage electrical discharge or focused electron bombardment—forces these neutral gas molecules to dissociate into electrons and positively charged ions.

Quantifying Degree of Ionization ($\alpha$)

The relative ratio of ionized particles within a gas volume is defined as the Degree of Ionization ($\alpha$), which is mathematically expressed by the formula:

$$\alpha = \frac{n}{N}$$

Where:

• $n$ represents the absolute number of ionized, charged particles.
• $N$ represents the total cumulative number of gas particles within the defined domain.

Thermal Classifications of Plasma Streams

When a gas is fully ionized, it forms a plasma stream containing an equal balance of positive and negative charges, making it globally neutral. In industrial engineering, these streams are divided into two distinct thermodynamic classes:

3. Step-by-Step Functional Ignition Sequence

The basic heating principles of a plasma induction furnace match those of standard induction units, but the ignition of the plasma torch follows a strict sequence to ensure safety and system stability.

[AC Power Input] ➔ [Transformer & Rectifier] ➔ [High-Voltage DC Field Generated]
                                                            │
[Plasma Arc Initiated] ➔ [Anode Transferred to Melt] ➔ [High-Frequency Spark Discharged]

1. Primary DC Rectification: High-voltage alternating current ($\text{AC}$) from the main grid passes through a step-down transformer, line reactors, and a heavy-duty rectifier assembly. This converts the grid power into a low-voltage, high-current DC output.
2. Biasing the Electrodes: This modified DC voltage is applied across the primary cathode negative electrode (6) and a secondary auxiliary anode (7).
3. Gas Injection and Ionization: An inert working gas, typically pure Argon ($\text{Ar}$), is fed into the narrow gap between these electrodes. Simultaneously, a high-frequency spark generator (10) delivers a brief, high-voltage pulse that ionizes the argon gas molecules.
4. Arc Pilot Formation: This ionization creates a localized, conductive path, forming a stable pilot plasma arc between the cathode tip and the auxiliary anode nozzle.
5. Transferred Arc Sequence: As the plasma gun is lowered toward the crucible, the main plasma column extends downward until it touches the metallic raw materials. Once contact is made, the auxiliary anode circuit opens via a DC contactor (5). This transfers the main arc path so it runs from the cathode directly to the raw metal charge, which now acts as the primary anode.
6. Dual-Energy Refining: With the transferred arc established, both systems run at the same time: the plasma torch delivers concentrated thermal energy from above, while the induction coils heat and stir the melt from the sides. This dual-energy approach increases melting speeds and prevents "bridging"—where un-melted scrap forms a solid crust over the liquid metal. Once the charge is fully liquid, operators can lower the plasma torch power to save energy while maintaining standard induction refining cycles.

4. Detailed Component Engineering

A. Core Mechanical Components

As shown in industrial engineering blueprints, the furnace body uses a specialized multi-component layout:

• Plasma Torch Assembly (1): Positioned vertically through the center of the roof to deliver the primary plasma arc.
• Inspection Port (2): A dedicated viewing window that lets operators safely monitor melting progress and check bath conditions.
• Pouring Spout (3): The exit channel used to pour out the refined liquid metal once a heat is complete.
• Induction Coil Array (4): Heavy-duty water-cooled copper coils wrapped around the crucible to provide electromagnetic induction heating and melt stirring.
• Crucible Vessel (5): The primary refractory lining designed to hold the high-temperature molten metal bath.
• Bottom Anode Return Electrode (6): A heavy-duty electrode built directly into the bottom of the crucible to complete the DC circuit through the molten metal.
• Outer Furnace Shell (7): The main structural steel enclosure that supports and protects the internal components.
• Plasma Arc Column (8): The high-temperature ionized gas stream that transfers energy from the torch tip to the metal bath.
• Gas Injection Nozzle (9): Manages the flow and distribution of the inert shielding gas around the arc.
• Material Charging Hopper (10): A sealed feeding system used to introduce alloys or raw materials without breaking the furnace seal.

B. Advanced Refractory Roof & Shell Design

The furnace roof uses a water-cooled, double-walled steel structure lined with high-purity refractory materials. The central opening for the plasma torch is sealed with heat-resistant asbestos seals to prevent gas leaks. To allow safe inspection during active refining, the roof features quartz glass viewing ports equipped with integrated cleaning brushes or protective high-speed flash shutters.

Depending on the application, the outer furnace shell is built in either a fully sealed configuration for strict atmosphere control, or a semi-sealed design that isolates the upper crucible area while leaving lower components accessible.

5. Process Advantages Over Mineral Fuel Furnaces

Compared to traditional gas or mineral-fuel fired furnaces, plasma induction technology offers clear operational and metallurgical benefits:

• Elimination of Combustion Hazards: Because the system generates heat through electricity and gas ionization rather than open combustion, it avoids the risk of gas explosions or fuel leaks.
• Atmospheric Purity: The inert argon gas environment prevents oxygen, hydrogen, and carbon contamination from fuel combustion products, resulting in cleaner metal batches.
• Precise Temperature Control: Operators can adjust power levels to control bath temperatures across a wide range, making it ideal for processing complex alloys under strict thermal tolerances.

Conclusion

Modern plasma induction furnaces represent a major advancement in high-performance metallurgy by combining the benefits of induction heating with the intense energy of a plasma arc. Understanding gas ionization physics, following proper arc ignition steps, and maintaining high component standards allows modern smelting facilities to achieve excellent alloy purity and consistent operational safety.

To understand the refining capabilities of a plasma induction furnace, one must analyze both the microscopic physics governing plasma arc generation and the macroscopic structural engineering required to contain these extreme thermal environments.

1. The Thermodynamic Principles of DC Plasma Arc Formation

The Nature of Plasma Material States

Under typical conditions, gas molecules exist as electrically neutral, non-conductive particles. However, under the influence of external energy fields—such as high-voltage electrical discharges or targeted electron beams—gas molecules dissociate and ionize into free, electrically charged ions.

The substance that results from this process is a plasma jet/stream, which is technically defined as a fully or highly ionized gas mass where the total count of positive ions equals the total count of negative electrons. Because these charges balance perfectly, the total net electrical charge displayed by the moving plasma body remains zero.

The Degree of Ionization ($\alpha$)

The relative quantity of gas molecules that successfully transform into free ions is defined as the degree of ionization, mathematically represented by the variable $\alpha$:

$$\alpha = \frac{n}{N}$$

Where:

• $n$ represents the total number of ionized, electrically charged particles.
• $N$ represents the total cumulative number of gas particles present in the system.

High-Temperature vs. Low-Temperature Plasma

The Microscopic Ionization Chain Reaction

Direct current (DC) plasma arcs are sustained within a localized DC electric field.

1. Once an initial high-frequency spark ionizes the carrier argon ($\text{Ar}$) gas, the positively charged argon ions accelerate at high velocities toward the cathode.
2. Upon impact, they activate the cathode surface, triggering a massive, sustained emission of high-energy electron streams.
3. As these high-energy electrons collide with oncoming neutral argon molecules, they ionize them on impact.
4. These newly ionized particles then cascade at high speeds to collide with additional neutral molecules, sustaining a continuous, steady plasma arc column.

2. Structural Engineering of the Furnace Assembly

A plasma induction furnace is split into two primary engineering systems: the induction melting furnace body and the plasma arc generator.

The furnace body itself is comprised of three core structural elements:

A. Induction Coils and Crucible Configurations

The primary induction furnace assembly can utilize line-frequency, triple-frequency, or medium-frequency power supplies depending on production scale.

• Coil Design: The induction coil layout includes both heating coils and specialized stirring coils. Small-capacity furnaces only require a basic heating coil array. Large-capacity configurations, however, require dedicated electromagnetic stirring coils alongside the heating lines to homogenize the melt and optimize the refining reactions.
• Crucible Innovation: While the general geometry and refractory material selections match conventional induction furnaces, there is one critical engineering difference: a specialized water-cooled anode is embedded directly into the absolute bottom of the crucible lining. This bottom anode serves as the ground terminal required to establish and anchor the transferred DC plasma arc.

B. The Water-Cooled Furnace Lid Assembly

To maintain a controlled atmosphere and handle intense radiant heat from above, the furnace lid uses a heavy-duty, double-layer steel plate structure featuring active internal water cooling.

• Refractory Lining: The underside of the double-walled lid is lined with high-grade refractory insulation materials.
• Central Sealing: A precision bore is located in the exact center of the lid to allow the vertical entry of the plasma gun body. The clearance gap between the moving gun body and the furnace lid is packed with highly heat-resistant, tight asbestos-based sealing products to prevent gas leakage.
• Auxiliary Ports: The lid contains integrated charging hoppers with sealed, gas-tight hatches for introducing trace alloying elements, deoxidizers, or highly reactive, easily oxidizable modifiers during smelting.
• Process Observation: To safely monitor the slag and melt progress, the lid features a strobe-synchronized viewing window or a dedicated quartz glass observation port equipped with an integrated mechanical surface cleaning brush. The critical interface where the lid meets the upper furnace rim contains water-cooled channels lined with high-elasticity sealing rings to eliminate any atmospheric infiltration.

C. Furnace Shell Architectural Variations

The protective metal furnace shell is fabricated using two primary design methodologies:

1. Fully Enclosed Shell Architecture: The entire furnace structure, including the crucible and support frames, is completely sealed inside a gastight pressure vessel to maximize gas control.
2. Semi-Enclosed Shell Architecture: A more cost-effective design where only the upper sections positioned directly above the crucible rim are tightly sealed against the atmosphere, as depicted in standard manufacturing system drawings.

Conclusion

The plasma induction furnace relies on a precise balance of low-temperature plasma physics and robust mechanical design. By anchoring a $30,000\text{ K}$ gas arc column via a crucible-bottom anode while simultaneously running medium-frequency electromagnetic stirring coils, this design gives metallurgical plants unmatched control over alloy purity and refining speeds.

After the prepared charge is loaded into the furnace, the temperature of the melting furnace should be strictly controlled. The purpose of controlling the melting furnace temperature mainly includes three aspects:

1. To meet the suitable first refining temperature requirements of various aluminum alloys.
2. To meet the suitable casting temperature requirements of various aluminum alloys.
3. To maintain a reasonable holding temperature so as to reduce oxidation loss on the surface of molten aluminum.

Before charging, the furnace chamber temperature must not be lower than 700°C.

According to production practice, the melting temperature of commonly used wrought aluminum alloys is usually controlled within the range of 50–100°C above the alloy liquidus temperature.

For aluminum alloys such as 3A21, 2A70, and 2A80, because they have a relatively strong tendency to form primary crystals of intermetallic compounds, the melting temperature is generally controlled at 720–760°C to ensure full dissolution of alloying elements.

For eutectic 4xxx series aluminum alloys, when silicon is added in the form of master alloy or crystalline silicon, the melting temperature should preferably be controlled at 750–780°C.

For other aluminum alloys, the temperature is generally controlled at 700–750°C.

In electrolytic aluminum plants, because the temperature of liquid primary aluminum is very high, solid return materials should first be used to reduce the temperature of the molten aluminum to the upper limit of the melting temperature. Then the alloying charge materials should be added.

After all charge materials have been loaded, the molten aluminum temperature should first be measured. When it is not lower than 750°C, the first refining should be carried out. When the furnace temperature drops to 730°C, slag removal should be performed.

During the second refining, the refining temperature should not be lower than 5°C above the upper limit of the casting temperature.

The casting temperature varies Depending on the product, the casting temperature for pure aluminum is 690~710°C. For aluminum alloys, the melting temperature during furnace cleaning is as high as 850°C. After heating for a certain time, the slag is allowed to soften before cleaning. During furnace drying, a wider temperature range is controlled, from 100°C to 980°C, with holding at different temperature points for a certain time. In resistance-heated mixing furnaces, since the heating elements are generally located above the surface of the molten aluminum, heat transfer mainly relies on radiation. The furnace temperature is usually the furnace chamber temperature, and the measured furnace chamber temperature varies depending on the thermocouple placement. In resistance-heated mixing furnaces, thermocouples are usually placed at the furnace top. The measured furnace chamber temperature is usually higher than the molten aluminum temperature. In production practice, the heating power should be determined according to specific circumstances and is usually variable. Lower power is used in resistance-heated mixing furnaces, while higher power is used when high temperatures or rapid heating are required. When maintaining heat, never use high-power heating; otherwise, the switching frequency of the power switch will be too fast, which may damage the electrical equipment.

A High-Frequency (HF) Induction Furnace typically operates at power frequencies ranging from 10 to 500 kHz, utilizing a high-frequency vacuum tube oscillator as its primary power source. Due to limitations in power supply capacity, these furnaces are generally restricted to a melting capacity of under 100 kg. Consequently, they are primarily used in laboratories for scientific research and experimental smelting.

The Transition to Medium Frequency (MF)

Currently, high-frequency furnaces are being progressively replaced by Medium Frequency (MF) induction furnaces in industrial settings. This shift is driven by several factors:

• Complexity: HF power supply circuits are significantly more intricate.
• Efficiency: They exhibit lower electrical efficiency compared to modern solid-state systems.
• Safety: HF systems present higher safety risks due to high-voltage operation.
• Interference: High-frequency electromagnetic waves can cause significant interference with radio communications.

However, as ultra-small experimental furnaces—capable of smelting anywhere from a few grams to several hundred grams of metal—high-frequency induction technology remains widely utilized in specialized research environments.

1. Components of High-Frequency Induction Equipment

A complete high-frequency induction furnace system consists of the following essential components:

1. Filter Unit: To stabilize the input.
2. High-Frequency Power Supply: Often referred to as an electronic tube frequency conversion device. This is the core of the system and includes:
   • Power Transformer
   • High-Voltage Rectifier
   • Vacuum Tube Oscillator
   • Electrical Control System
   • Water-Cooling System
3. Furnace Body: The crucible and coil assembly where smelting occurs.
4. Water-Cooling System: To protect the induction coils and power components from overheating.

These components are typically integrated into a unified metal cabinet to form a standardized high-frequency induction heating unit.

2. Technical Specifications (Standard Series)

Domestic high-frequency induction heating equipment is standardized into several power ratings to suit different laboratory needs:

• Power Ratings: 3.5, 8, 10, 20, 30, 60, 100, and 200 kW.
• Operating Frequency: Standardized between 200 to 250 kHz.

3. Installation and Environmental Safety Requirements

Due to the unique physical properties of high-frequency waves and high-voltage electricity, strict installation protocols must be followed:

Proximity and Layout

To minimize power loss, the distance between the high-frequency induction furnace and its power source should be kept within 3 meters.

Electromagnetic Shielding

To prevent electromagnetic interference (EMI) with radio communications, the room housing the equipment must be shielded. Standard shielding methods include:

• Material: Lining the walls and ceiling with 0.5 mm thin steel sheets or steel wire mesh (with a grid size of approximately $50 \times 50 \text{ mm}$).
• Grounding: All shielding materials (sheets or mesh) must be connected to a robust grounding system.

Insulation and Operator Safety

Because both the power supply and the furnace operate under high voltage, the following safety measures are mandatory:

• Enhanced Insulation: Electrical grounding and insulation for the entire equipment suite must be reinforced.
• Insulated Tools: All smelting tools and operational handles must be equipped with high-voltage insulation grips.

FAQ Section

Q: Why is vacuum tube technology still used in high-frequency furnaces?

A: While solid-state (IGBT) technology is taking over, vacuum tubes are still favored for specific ultra-high frequency lab applications due to their ability to handle high frequencies at very small scales with relatively simple (though high-voltage) architectures.

Q: What happens if the shielding is inadequate?

A: Inadequate shielding results in "noise" across the radio spectrum, which can disrupt local telecommunications, emergency signals, and laboratory instrumentation.

Q: Can HF induction furnaces be used for gold and silver smelting?

A: Yes. Their ability to melt very small quantities (grams) makes them ideal for precious metal analysis and dental alloy preparation.

1. Furnace Body Structure of Medium Frequency Induction Furnaces

The furnace body structure of a medium frequency induction furnace is basically the same as that of a power frequency induction furnace. The furnace body consists of the furnace frame, furnace body, including frame type and cylindrical shell type, furnace cover, tilting mechanism, cooling water system, and power input system.

Figure 2-19 shows the furnace body and hydraulic tilting device of a 150 kg medium frequency induction furnace. Figure 2-20 shows the furnace body and hydraulic tilting device of a 500 kg medium frequency induction furnace.

2Figure 2-20: 500 kg Medium Frequency Induction Furnace

The main structural difference between a medium frequency induction furnace and a power frequency induction furnace lies in the relative position of the crucible and the inductor, as well as the cross-sectional size of the inductor.

Figure 2-21 shows the relative positions of the inductor and liquid metal in medium frequency and power frequency induction furnaces. It can be seen from the figure that the liquid metal in a medium frequency induction furnace is usually completely surrounded by the inductor, while in a power frequency induction furnace, the metal liquid level is higher than the inductor.

This arrangement is used to reduce the stirring effect of electrodynamic force in the power frequency furnace. Since the electrodynamic force of a power frequency furnace is greater than that of a medium frequency furnace, the liquid column height must be increased to reduce the hump height of the liquid surface and weaken the scouring effect of molten steel through gravity.

Figure 2-21: Relative Positions of the Inductor and Liquid Metal in Induction Furnaces

The relationship between the inductor height H and the liquid column height HL is as follows:

Medium Frequency Induction Furnace

H ≈ (1.1–1.3)HL

Power Frequency Induction Furnace

H ≈ (0.8–0.9)HL

In summary, the liquid column height of a medium frequency induction furnace is 1–2 turns lower than the inductor, while that of a power frequency induction furnace is 1–2 turns higher than the inductor.

In terms of the cross-sectional size of the inductor, the tube wall used for a medium frequency inductor is thin. This is because the skin effect of current is strengthened as the frequency increases, and because the terminal voltage of the medium frequency inductor is high while the current in the oscillating circuit is small.

The cross-sectional dimensions of the medium frequency inductor are shown in Figure 2-22.

Figure 2-22: Cross-Sectional Shapes of Inductors Used in Medium Frequency Induction Furnaces

The structural shape and insulation treatment of the inductor in a medium frequency furnace are the same as those of a power frequency induction furnace.

IV. Tilting Mechanism

The capacity of medium frequency induction furnaces is usually below 10 tons. Most furnaces currently in operation in China are below 1 ton. The tilting mechanisms of small and medium-sized medium frequency induction furnaces mainly include the following four types.

1. Lifting Type Tilting Furnace

This is the oldest tilting method. A crane or a special small hoist is used to lift the lifting rings installed on the furnace body, causing the furnace body to tilt and complete the tilting process. This method has gradually been eliminated.

2. Screw Drive Tilting Mechanism

This mechanism uses a motor to drive the screw lifting device through a reducer, completing the tilting process of the furnace body. This mechanism is only suitable for small furnaces.

Figure 2-23 shows a 150 kg medium frequency induction furnace using a screw drive tilting mechanism.

Figure 2-23: Screw Drive Tilting Mechanism

3. Worm Gear and Worm Mechanism Tilting Furnace

The worm gear and worm are installed on the horizontal shaft that rotates the furnace body, and the tilting action is completed by motor drive. A medium frequency induction furnace using this tilting mechanism is shown in Figure 2-24.

Furnaces of about 1 ton can use this mechanism for tilting.

4. Hydraulic Device Tilting Furnace

Using a single-cylinder or double-cylinder hydraulic system to complete furnace tilting is the most widely used method.

At present, most aluminum processing plants produce billets for pressure processing by remelting solid aluminum ingots. There are generally four main melting methods Melting Process:

• Batch melting
• Semi-batch melting
• Semi-continuous melting
• Continuous melting

Batch Melting Method

The batch melting method means that the metal is discharged completely after the processes of charging, melting, slag removal, and refining. No metal remains in the furnace.

This method is mostly used to produce finished aluminum alloys with high quality requirements. It can better ensure the uniformity of the chemical composition of the ingot.

Semi-Batch Melting Method

The semi-batch melting method is basically the same as the batch melting method. The main difference lies in the amount of liquid metal retained in the furnace.

In semi-batch melting, after tapping, about 1/5 to 1/4 of the liquid metal remains in the furnace. The charge for the next heat is then added for melting.

The advantages of this method include:

• The newly added charge can be immersed in liquid metal, reducing metal burning loss.
• Some settled inclusions can remain at the furnace bottom during tapping.
• Inclusions are less likely to mix into the liquid metal during casting.
• Furnace temperature fluctuation is small, which helps extend furnace service life.

This method is suitable for melting pure aluminum.

Semi-Continuous Melting Method

The semi-continuous melting method is similar to the semi-batch melting method. After each tapping of about 1/3 to 1/4 of the molten metal, the charge for the next heat can be added.

The difference is that in semi-continuous melting, only a small amount of molten metal is tapped each time, while most of the melted liquid metal remains in the furnace. This allows tapping and charging to continue alternately.

This method is suitable for melting scrap in a double-chamber furnace. Since the added charge is immersed in liquid metal, it not only reduces burning loss but also increases melting speed.

Continuous Melting Method

The continuous melting method means that charge materials are added continuously, while tapping is carried out intermittently.

This method has poor flexibility and a limited application range. It is mainly suitable for melting pure aluminum and is also used for melting aluminum scrap, aluminum chips, and similar materials.

Non-metallic inclusions in molten aluminum are mainly oxides. The harmful effects of oxides are mainly reflected in the following aspects:

1. Impact on Casting Performance

Oxides reduce the fluidity of aluminum alloy melt and weaken its ability to fill the mold. They also increase the tendency of the alloy to form dispersed shrinkage porosity and ingot cracks.

When there are many non-metallic inclusions in molten aluminum, the refining and degassing process during melting becomes more difficult. This can easily increase the number of inclusions in aluminum ingots or castings.

Oxides may directly form slag inclusions and oxide film defects in ingots or castings, reducing the yield of finished products.

2. Impact on Aluminum Product Quality

Oxides are an important cause of delamination and many surface defects in aluminum processed products. Their presence can seriously affect the quality of aluminum processing materials.

3. Impact on Heating and Heat Treatment

During heating and heat treatment, oxides can promote the formation of secondary porosity and bubbles.

4. Impact on Mechanical Properties

As the contamination level of aluminum alloy increases, the tensile strength and elongation of the alloy decrease. The reduction is especially serious in transverse elongation and dynamic mechanical properties, such as:

• Impact toughness
• Fatigue strength
• Fracture toughness

In addition, oxides can also reduce the stress corrosion resistance of aluminum alloys.

Why Is the Sodium Content High in Liquid Primary Aluminum?

In electrolytic aluminum production, cryolite is used as the main electrolyte raw material. Cryolite contains sodium fluoride. During electrolysis, sodium is decomposed and released.

Therefore, liquid primary aluminum in aluminum electrolytic cells contains a relatively high amount of sodium.

In electrolytic cells using strongly acidic electrolytes, the sodium content in liquid primary aluminum is relatively low.