In modern mill roll manufacturing, the medium frequency induction furnace has become the dominant melting equipment for producing high‑performance rolls for hot strip mills, section mills, bar mills and plate mills. The stability of the melting process is the foundation for achieving reliable roll hardness, wear resistance, thermal fatigue resistance and accident‑free service in demanding rolling conditions.
This article focuses on the first critical step in mill roll production: the medium frequency furnace melting process. It follows the actual industrial sequence from charge preparation to tapping, combining physical principles with practical parameters that can be used as process reference in foundry operations.
1. Role of Melting in Mill Roll Performance
Mill rolls operate under cyclic thermal and mechanical loads, high contact pressure and severe surface wear. Whether the roll is alloy cast iron, high chromium iron, indefinite chill, high speed steel (HSS) or tool steel type, its microstructure is determined at the melting and refining stage. Subsequent spheroidization, inoculation, static casting or centrifugal casting can only fine‑tune what has already been established in the liquid metal.
For long campaign life and stable roll behavior in mill stands, the melting practice in a medium frequency furnace must ensure:
- Precise control of carbon, silicon and alloy elements (Cr, Mo, V, Ni, W, Nb, etc.).
- Low and stable gas content (O, N, H) to avoid pinholes and sub‑surface defects.
- Low oxide and non‑metallic inclusion index for good fatigue resistance.
- Proper superheat and good fluidity for sound feeding and shell formation in roll molds.
- Reproducible thermal history to ensure stable microstructure between heats.
For high value products such as work rolls for hot strip mills or HSS rolls for finishing stands, the melting process is often the decisive factor between an excellent and a problematic roll campaign.
2. Physical Principles of Medium Frequency Furnace Melting
A medium frequency induction furnace uses alternating current in a copper coil to generate a time‑varying magnetic field. This field induces currents inside the metallic charge, heating it volumetrically until it melts. The process is governed by two fundamental electromagnetic and thermal laws.
2.1 Faraday’s Law of Electromagnetic Induction
When alternating current flows through the inductor coil, the magnetic flux through the metallic charge changes with time. According to Faraday’s law, this varying flux induces an electromotive force (EMF) in the conductive charge material. The magnitude of the induced EMF is proportional to the rate of change of magnetic flux and the number of turns in the coil.
In a crucible‑type medium frequency furnace, the metallic charge itself forms a secondary “short‑circuited” conductor. The induced EMF causes eddy currents to circulate within the charge. These eddy currents are the direct source of heat generation in the metal.
2.2 Joule–Lenz Law (Resistive Heating)
The power dissipated by these induced currents is governed by the Joule–Lenz law:
Heat generated per unit time = I²·R,
where I is the induced current and R is the electrical resistance of the metal path. Inside the charge, this heat is released directly in the metal volume, causing its temperature to rise from ambient to the melting point, and further up to the required tapping temperature or superheat.
The medium frequency range (typically 250–1000 Hz in many roll shops) offers a good compromise between penetration depth of current (skin effect) and stirring intensity of the molten bath, ensuring reasonable heating uniformity for medium and large crucible diameters.
3. Typical Equipment for Mill Roll Melting
While exact specifications vary by plant, the following table gives representative data for furnaces used to melt alloy cast iron and HSS for mill roll production.
| Parameter | Furnace A | Furnace B | Furnace C |
|---|---|---|---|
| Nominal capacity | 5 t | 10 t | 20 t |
| Rated power | 3.0 MW | 6.0 MW | 10.0 MW |
| Frequency range | 400–800 Hz | 250–500 Hz | 250–400 Hz |
| Melting rate (alloy iron) | 2.0–2.5 t/h | 4.5–5.5 t/h | 8–10 t/h |
| Power consumption (alloy iron) | 540–580 kWh/t | 520–560 kWh/t | 500–540 kWh/t |
| Operating lining life (acidic) | 60–80 heats | 80–100 heats | 90–120 heats |
| Cooling water flow (coil + cabinet) | 40–50 m³/h | 60–70 m³/h | 90–110 m³/h |
Mill roll foundries often operate two or more furnaces in duplex or multi‑furnace configurations to allow flexibility in alloy adjustments and higher throughput for large back‑up rolls or multiple small work rolls.
4. Charge Materials for Mill Roll Melting
Proper charge selection defines both material cost and metallurgical cleanliness. Common components in a typical charge for alloy cast iron or HSS mill rolls include:
- Pig iron: base iron with low residuals, chosen according to required carbon and silicon range.
- Steel scrap: clean, low‑residual scrap to fine‑tune carbon, sulfur and inclusion level.
- Returns: gating and riser returns from previous roll castings, limited to control tramp elements.
- Ferroalloys: ferro‑chromium, ferro‑molybdenum, ferro‑vanadium, ferro‑tungsten, ferro‑niobium, ferrosilicon, etc.
- Carbon carriers: carburizers such as petroleum coke or artificial graphite.
For high chromium or high speed steel type rolls, alloy content can reach 10–20% or higher. Charge material planning must therefore consider not only target chemistry but also oxidation loss and recovery rate of each element during melting and holding.
4.1 Example Chemical Ranges for Typical Mill Roll Grades
| Roll Type | C (%) | Si (%) | Mn (%) | Cr (%) | Mo (%) | Ni (%) |
|---|---|---|---|---|---|---|
| Alloy indefinite chill cast iron roll | 2.8–3.4 | 1.1–1.6 | 0.5–0.9 | 0.8–1.5 | 0.2–0.5 | 1.0–2.0 |
| High chromium cast iron roll | 2.0–3.0 | 0.5–1.2 | 0.5–1.0 | 8.0–18.0 | 0.5–2.0 | 0.5–2.5 |
| High speed steel work roll | 1.6–2.2 | 0.3–0.8 | 0.3–1.0 | 3.0–6.0 | 3.0–5.5 | 1.0–4.0 |
These ranges illustrate why medium frequency furnace melting for mill rolls is not a simple cast iron operation but a complex alloy design and control task.
5. Step‑by‑Step Medium Frequency Furnace Melting Process
Although each foundry has its own internal standards, the following sequence reflects typical practice in industrial mill roll shops using medium frequency induction furnaces.
5.1 Furnace Preparation and Lining Control
The furnace lining, usually acidic (silica) or neutral/basic for more alloyed grades, must be inspected before each heat. Typical control items include:
- Remaining lining thickness, measured by steel bar or ultrasonic tools.
- Cracks, erosion around metal level and tap spout, and potential hot spots.
- Inductor coil leakage check and refractory between coil and lining.
For a 10‑ton furnace melting alloy cast iron for mill rolls, a safe remaining working lining thickness of 60–80 mm at the metal line is often maintained, depending on internal standards and risk analysis.
5.2 Charging Sequence
Charge addition sequence strongly influences melting rate, energy efficiency and furnace lining life. A common approach:
- Add heavy and dense materials first (low‑phosphorus pig iron, heavy scrap) to form a stable base.
- Add returns and medium‑sized scrap.
- Reserve most ferroalloys and carburizer additions for the molten stage to reduce oxidation loss.
- For high alloy grades, part of alloying elements may be added in solid charge to promote dissolution balance.
Charge cleanliness is critical. Oily scrap, rust‑covered pieces and strongly oxidized returns should be minimized, as they increase slag volume, oxygen content and nitrogen pick‑up, all of which deteriorate roll quality.
5.3 Power‑On and Melting Stage
After charging, power is applied and the furnace operates with increasing power factor up to a pre‑set level, balancing network capability and lining safety. Typical practice for a 10‑ton furnace:
- Initial power at 40–60% to avoid strong mechanical impact on still‑solid charge.
- After partial melting and bath formation, increase power to 90–100%.
- Maintain working voltage and current in the safe range specified by the equipment manufacturer.
The electromagnetic stirring effect of the medium frequency field helps homogenize temperature and chemical composition, but excessive stirring can damage the lining, so each foundry tunes the compromise based on experience.
5.4 Alloying and Temperature Control
Once the charge is fully molten and slag is skimmed, alloying additions are made based on calculated quantities and prior heat data. The following practical points are widely applied:
- Add high‑melting or strong‑carbide‑forming ferroalloys (for example, ferro‑tungsten, ferro‑vanadium, ferro‑niobium) earlier to ensure complete dissolution.
- Add more oxidation‑sensitive elements (for example, Si, Mn) later in the sequence, often as part of the deoxidation step.
- Adjust carbon with carburizer during holding, carefully monitoring sampling results.
Superheat must be high enough to guarantee smooth filling of large roll molds and good feeding, but not so high as to cause serious grain coarsening or excessive lining erosion. For many alloy cast iron and HSS roll grades:
- Typical tapping temperature is in the range of 1480–1570 °C.
- The exact value depends on casting temperature loss, mold preheat and casting method (static vs. centrifugal casting).
5.5 Deoxidation and Refining
Compared with arc furnaces, medium frequency furnaces have a different atmosphere and slag behavior. However, deoxidation remains essential to reduce inclusions and gas porosity. A typical practice for mill roll melting is:
- Use a combined deoxidation system with ferrosilicon, ferromanganese and aluminum or silicon‑calcium alloys.
- Apply cover slag based on silica, lime and fluorspar mixes, or specialized roll‑grade slags designed to absorb inclusions.
- Degassing operations (argon bubbling or vacuum treatment) may be implemented for premium grades requiring low hydrogen and nitrogen levels.
Stable deoxidation conditions contribute to improved cleanliness and lower risk of surface defects on the final mill roll.
5.6 Sampling and Composition Fine‑Tuning
Before tapping, spectrometric sampling is carried out to verify composition. Depending on the roll grade, the following target tolerances are widely used in industrial practice:
- Carbon: ±0.03–0.05% around the nominal value.
- Silicon: ±0.05–0.10%.
- Chromium, molybdenum, nickel: ±0.10–0.20%, depending on grade sensitivity.
Fine‑tuning is made using small additions of ferroalloys and carburizers, followed by adequate homogenization time (usually 3–8 minutes) before the final check sample. For large campaigns or critical high‑speed steel work rolls, some plants adopt double sampling as routine.
5.7 Tapping and Transfer to Casting
Once chemistry and temperature are confirmed, power is reduced to avoid over‑heating while keeping the bath fully molten. Tapping is conducted into preheated ladles or direct casting systems. Industrial best practice for mill rolls includes:
- Ladle preheat typically above 900 °C for high alloy molten metal.
- Strict control of slag carryover using well‑designed tap spouts and slag‑stopping devices.
- Minimum tapping time compatible with safe operation to limit temperature drop and re‑oxidation.
From here, the molten metal enters the next stage such as centrifugal casting of shell layers, static casting for roll necks or composite roll manufacturing, but the foundation laid in the medium frequency melting step dictates much of the final microstructure.
6. Process Parameters with Practical Reference Value
The following process window is representative for a 10‑ton medium frequency furnace melting an alloy indefinite chill cast iron used for hot strip mill work rolls. Values should be adapted to local conditions and roll specifications but provide an engineering reference.
| Item | Typical Value | Notes |
|---|---|---|
| Metal charge ratio (pig iron : scrap : returns) | 40 : 30 : 30 | High‑quality pig iron to limit S and P; returns controlled to reduce tramp elements. |
| Power on time | 90–110 min | From initial power‑on to tapping, including alloying and holding. |
| Slagging‑off time | 10–15 min before tapping | Combined with deoxidation and surface cleaning of the bath. |
| Oxygen (total) in molten metal | 25–45 ppm | Measured by immersion probes; lower values for premium rolls. |
| Nitrogen content | 60–90 ppm | Controlled by clean charge and short holding time at high temperature. |
| Hydrogen content | 1.5–3.0 ppm | Affected by refractories, slag treatment and possible argon degassing. |
Plants producing high‑performance rolling mill rolls frequently track these parameters in statistical form to link melting practice with scrap rate, grinding allowance and roll performance on the mill.
7. Metallurgical Impact on Mill Roll Microstructure
The medium frequency furnace stage determines the size, distribution and morphology of carbides, graphite and matrix phases that will form during solidification and heat treatment. For example:
- For alloy indefinite chill rolls, the balance between carbon and chromium strongly influences the depth of chill and graphite form, affecting both wear resistance and spalling risk.
- For high chromium iron rolls, carbon and chromium ratio controls the volume fraction of M7C3 carbides, critical to hot wear resistance.
- For high speed steel rolls, careful control of Mo, W, V and Nb promotes a fine dispersion of MC and M2C carbides, improving red‑hardness and thermal fatigue behavior.
These microstructural targets can be reached only when the molten metal leaving the furnace has consistent composition and low inclusion content. Poorly controlled melting leads to uneven hardness across barrel length, unexpected shell cracking, shell‑core separation or early surface spalling in service.
8. Advantages of Medium Frequency Furnaces for Mill Roll Production
Compared with older cupola or arc furnace routes, medium frequency induction furnaces provide several advantages that are particularly relevant for high‑grade mill roll manufacturing:
- Compact installation and flexible operation: short start‑up time and easy adaptation to different roll grades and batch sizes.
- High thermal efficiency: much of the electric power is converted directly into heat in the metal, reducing total energy consumption per ton.
- Accurate temperature control: in‑bath thermocouples and pyrometers allow close control of superheat, important for large‑section roll castings.
- Low atmospheric pollution: absence of combustion gases simplifies off‑gas treatment and improves working conditions.
- Excellent alloy control: reduced oxidation and better recovery of expensive alloying elements like Mo, V and Nb compared with air‑melting processes.
These features explain why many modern roll shops use medium frequency induction furnaces as the main melting unit, often supported by ladle refining or secondary processes for the most demanding applications.
9. Practical Recommendations for Stable Mill Roll Melting
Based on industrial practice and metallurgical fundamentals, the following recommendations help stabilize the first step of mill roll manufacturing:
- Standardize charge preparation with written rules for scrap acceptance, returns ratio and moisture limits.
- Maintain a detailed furnace log including power curves, melting time, slag practices and all additions.
- Regularly calibrate temperature and gas measuring instruments to ensure process data reliability.
- Correlate furnace data with roll inspection results (UT, hardness profile, microstructure) and mill performance to refine the process window.
- Train operators not only on furnace control but also on the impact of each melting decision on down‑stream roll behavior.
When the melting operation in the medium frequency furnace is treated as a controlled metallurgical process instead of a simple heating step, the resulting mill rolls exhibit higher consistency, extended service life and more predictable performance in the rolling mill.