Background and Significance
In recent years, the high-end cast and forged parts market has seen rapid growth, with increasing demand for premium-quality forgings. However, traditional melting and casting techniques often struggle to meet the stringent requirements of advanced forgings, such as bellows, 18-18 retaining rings, and H13 wear-resistant steels. These methods commonly lead to inclusion accumulation and severe segregation, compromising product quality and qualification rates.
To address this challenge, China Erzhong (Deyang) Heavy Equipment Co., Ltd. has introduced a 15-ton Electroslag Remelting (ESR) furnace alongside a tailored remelting process. This system significantly improves molten steel cleanliness and ingot solidification quality to meet the technical demands of advanced steel grades.
In 2018, a substantial upgrade of the 15t ESR furnace was carried out, covering both mechanical and electrical control systems. Additionally, the slag system was upgraded from mixed binary slag to pre-fused binary slag. To ensure the production of high-quality ingots, the team initiated process development and refinement efforts based on the new equipment and slag materials, focusing particularly on surface quality control.
Introduction to the 15t ESR Furnace
1.1 Technical Features of the 15t Furnace
The 15t ESR furnace is a dual-arm alternating system, offering several metallurgical and operational advantages:
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Improved Inclusion Removal
During both the droplet formation and descent phases, the steel droplets are in full contact with the slag. This allows non-metallic inclusions in the molten steel to be effectively absorbed by the slag.
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Directional Solidification & Surface Finish
The steel solidifies beneath a slag cover, with both the slag and molten pool acting as thermal insulators and feeders, promoting dense and uniform solidification. A thin slag shell forms on the mold wall during upward slag movement, resulting in:
- Smooth ingot surfaces
- Effective thermal insulation
- Bottom-up directional solidification
- Reduced composition segregation
Precise Process Control
- Stable remelting with minimal current fluctuation
- Real-time display of remelting parameters (e.g., current, voltage, water temp) via an industrial control system
- Accurate weighing through the base tank’s load system
- Optional inert gas shielding based on material needs
1.2 ESR Ingot Surface Quality Indicators
A high-quality ESR ingot should meet the following criteria:
- No visible slag grooves, especially at the base and around electrode swap zones
- Smooth remelting process with stable current and voltage
- Controlled and consistent melting rate
Surface Quality Control Measures for ESR Ingots
The quality of the ingot surface is primarily affected by:
- Furnace operating procedures
- Pre-fused slag selection
- Arcing and slag-forming methods
- Remelting power input control
2.1 Furnace Operation and Personnel Training
To ensure the correct operation of the upgraded 15t furnace, the mechanical and electrical teams received additional hands-on training. Key areas included:
- Mechanical system actions
- Computer interface navigation
- Power parameter setup
- Emergency and process adjustments
2.2 Selection of Pre-fused Slag
Pre-fused slag was introduced to replace traditional mixed slag due to its:
- Lower melting point
- Minimal moisture content
- More stable melting behavior (no “slag popping”)
However, the slag-forming electrical settings still required tuning. The technical team also recommended placing a thick steel plate over the base tank to protect it during arcing.
Currently, 7-3 pre-fused slag is used (7 parts CaF₂, 3 parts Al₂O₃), with performance validated through practical trials.
Bag of 7-3 pre-fused slag used for electroslag remelting, improving ingot quality and melting behavior.
Optimization of Arcing and Slag Formation Methods
2.3.1 Plan 1 — With Base Plate, Fast Slag Addition
- A large steel plate was placed on the base tank
- Arc ignition using an arc starter and a scrap metal electrode
- Full slag charge added within 50 minutes
- Gradual increase in current and voltage per curve
Normal remelting started at 1.5 hours
Results:
- Some damage occurred to the bottom cushion
- Good bottom ingot shape
- The base tank is mostly unharmed
- Slag melting was stable
- Overall, promising surface results
2.3.2 Plan 2 — Pre-layered Slag + Base Plate
- Steel plate on the base tank
- Arc ignition as before
- Pre-layered three bags of slag around the arc (but not over the arc starter)
- Lower initial power input
- Power not to exceed 10 levels (10,000A) before slag fully added
- Gradual increase in remelting current
Results:
- Good ingot base shape
- No damage to base tank
- However, chemical dilution was served in the bottom samples due to the steel plate melting
- Result: The Plate method was abandoned to protect the alloy composition
2.3.3 Plan 3 — No Base Plate, Pre-layered Slag
- Removed base plate
- Arc initiated using starter and scrap metal electrode
- Pre-layered slag
- Slag added within 45 minutes
- Power input strictly controlled (not over 10,000A initially)
- After 30 minutes: Increase to 15,000A and stabilize slag
- Reduced water flow by 20% to improve slag melting (due to stronger base cooling)
Results:
- Excellent bottom formation (See Figure 2)
- Good ingot composition
- No base tank damage
This method is deemed optimal for 7-3 slag
Clean the bottom formation of the ESR ingot after arcing without a base plate using 7-3 slag.
Remelting Electrical Input Control
2.4 Best Practices for Power Input
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End of slag formation:
- Increase the voltage to the upper limit of the remelting range
- Raise the current to lower the remelting threshold
- Ensures a smooth transition from slag forming to remelting
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Slag fully molten:
- Raise the current to the mid-level of the normal range
- Adjust voltage as needed (especially for large ingots >2.5t)
- Prevent slag groove formation at water inlet zone
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During full remelting:
- Monitor slag temperature and melting rate (See Figures 3 & 4)
- Adjust power settings to maintain a stable melt rate and avoid arcing exposure
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Electrode changeover:
- Keep swap time short
- The operator must be precisely positioned
- Commanded by the shift supervisor
- After the swap:
- Increase voltage to remelting max
- Drop current to remelting min
Rapidly raise slag temperature to avoid defects at the swap point
3. Chart showing slag temperature fluctuations during ESR remelting. 4. Plot of melting rate changes across normal ESR cycle for 15t ingot. 5. Safety and control zone during electrode exchange in the ESR furnace.
Conclusion
In 2023, with the adoption of the above arcing methods and electrical control protocols, the plant successfully produced over 1,500 tons of small ESR ingots for components like bellows.
Results:
- High qualification rate
- Clean surfaces, especially at the base and electrode exchange zones
- Stable chemical composition
- Minimal slag grooves or segregation
ESR ingot after demolding, showing a clean surface and uniform structure.
