Electroslag Remelting Process: Part One

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The Evolution and Significance of Electroslag Remelting Technology

The Electroslag Remelting (ESR) process, while discovered in the 1930s, took three decades to emerge as a mainstream industrial process for producing high-quality metal ingots. This sophisticated refining method has revolutionized the production of critical components for aerospace, nuclear power, and defense applications, offering unprecedented control over both solidification structure and chemical composition. The technology's versatility allows for processing both small tool steel ingots and massive forging ingots exceeding 165 tons, making it indispensable in modern metallurgical operations.

Fundamental Principles of the ESR Process

The ESR process operates through a carefully controlled metallurgical mechanism. At its core, the process begins with a specially prepared electrode - an ingot with minimal taper - suspended from a precision-controlled mast assembly. This electrode interfaces with a reactive slag bath contained within a water-cooled copper crucible. The process utilizes high-amperage, low-voltage current to maintain the slag bath at approximately 200°C above the electrode material's melting point. This temperature differential creates a controlled melting environment where the electrode tip forms a thin liquid film.

The metal undergoes three critical phases during processing:

1.   Initial melting at the electrode tip
2.   Droplet formation and passage through the reactive slag
3.   Progressive solidification in the water-cooled mold

Modern implementations have enhanced this basic configuration through several technological advances, including the adoption of short collar molds, variable power supply options (AC or DC), and improved current delivery systems such as bifilar electrode arrangements and coaxial leads.

Metallurgical Refinement and Slag Characteristics

The ESR process achieves superior metal refinement through precise control of slag chemistry and process parameters. The slag system, primarily composed of calcium fluoride (CaF₂), lime (CaO), and alumina (Al₂O₃), serves multiple critical functions in the refining process. During operation, this reactive slag system facilitates both chemical purification and physical cleansing of the metal.

Key slag requirements for optimal ESR processing include:

• Melting point below the target metal's melting temperature
• Appropriate electrical conductivity characteristics
• Chemical composition suited for desired refinement reactions
• Optimized viscosity at operational temperatures

One notable limitation of the ESR process concerns hydrogen control. Unlike other elements, hydrogen removal during processing proves challenging, necessitating strict control of hydrogen content in the initial electrode material. However, the process excels at sulfur removal and maintains excellent control over alloying element compositions.

Current industrial applications focus primarily on four major categories:

• Tool and Die Steels (37.5% of usage)
• Stainless and Nickel-based alloys (25.0%)
• High-Strength Constructional Steels (25.0%)
• Superalloys (12.5%)

Table 1. The usage breakdown of ESR process in developed countries

Grades Melted	%Usage
Tool and Die Steels	37.5
Stainless and Nickel base alloys	25.0
High Strength Constructional	25.0
Super alloys	12.5
Total	100

 

Advanced ESR Furnace Technology and Process Control

Modern ESR furnace design represents a significant evolution in metallurgical processing equipment. Recent technological advances have focused on three key areas: plant design optimization, coaxial current feeding systems, and computerized control integration. These developments have culminated in the achievement of fully-automated remelting capabilities, resulting in superior metallurgical properties.

The ALD's Automatic Melt Control (AMC) system exemplifies modern process control sophistication. This system's innovative approach to electrode immersion depth control, based on slag resistance parameters, effectively decouples the traditionally interdependent immersion depth and remelting rate control loops. This advancement delivers several key benefits:

• Consistent production of high-quality ingots
• Enhanced operational efficiency
• Improved metallurgical outcomes, including:
  • Homogeneous, directionally solidified structure
  • Superior material cleanliness
  • Elimination of internal flaws
  • Reduced macro-segregation
  • Optimal surface finish for improved yield

Figure 1: Schematic representation of the ESR unit and actual furnace in operation

The basic ESR furnace configuration, utilizing a movable copper mold containing basic slag, demonstrates the elegant simplicity of the process while delivering sophisticated metallurgical results. The controlled droplet-by-droplet melting process, combined with slag filtration, produces steel with exceptional cleanliness and microstructural uniformity.

Large-Scale ESR Applications and Heavy Forging Innovations

The late 1960s marked a pivotal expansion in ESR technology with its application to large-scale forging ingot production. This development was primarily driven by the power generation industry's growing demand for massive components, particularly generator and turbine shafts exceeding 100 tons.

ALD's pioneering achievements in large-scale ESR technology culminated in the development of their largest furnace facility, capable of producing:

• Ingots up to 2,300 mm in diameter
• Lengths reaching 5,000 mm
• Maximum weights of 165 tons

The advanced process employs a sophisticated multi-electrode system, featuring:

• Simultaneous operation of four consumable electrodes
• Sequential electrode replacement capability
• Controlled ingot withdrawal mechanism

Critical to the success of large-diameter ESR processing is maintaining directional solidification across the entire ingot cross-section and length. This careful control prevents common internal defects such as:

• Macro-segregation
• Shrinkage cavities
• Non-uniform inclusion distribution

The result is a superior product that combines excellent mechanical properties with exceptional structural uniformity, surpassing conventionally cast steel ingots in quality and performance.

Figure 2: 165 ton ESR ingot, 2,300 mm diameter x 5,000 mm long