Electroslag Remelting Process: Part One

Abstract:

Electro Slag Remelting (ESR) is a process used for remelting and refining of steels and special alloys which are used for critical applications in aircraft, thermal and nuclear power plants, defense hardware, etc.
The prime attribute of the process which differentiates it from other hosts of secondary refining processes is its capability to control both solidification structure and chemical homogeneity simultaneously. The ESR technology is of interest not only for the production of smaller weight ingots of tool steels and superalloys, but also of heavy forging ingots up to raw ingot weights of 165 tons.

Electro Slag Remelting (ESR) process has been known since the 1930's, but it took approximately 30 years before it became an acknowledged process for mass production of high-quality ingots. It is a process used for remelting and refining of steels and special alloys which are used for critical applications in aircraft, thermal and nuclear power plants, defense hardware, etc.

The prime attribute of the process which differentiates it from other hosts of secondary refining processes is its capability to control both solidification structure and chemical homogeneity simultaneously. The ESR technology is of interest not only for the production of smaller weight ingots of tool steels and superalloys, but also of heavy forging ingots up to raw ingot weights of 165 tons.

To describe the process briefly, the material to be refined by ESR is first obtained in the form of an electrode which is essentially an ingot with no or minimum taper. The electrode is suspended from a mast assembly which can vertically move at a controlled rate. A reactive slag bath is contained in a water cooled copper crucible. The tip of the electrode is kept dipped in the slag pool which is heated and kept molten by passing a high ampere, low voltage current through the same. The temperature of the slag bath is about 200°C higher than the melting point of the electrode material.

As a result a thin film on the tip of the electrode melts. The liquid metal drops are formed which pass through the slag and deposit on the other side in the liquid metal pool which solidifies progressively. The liquid metal in the film and in the droplets is in contact with reactive slag and thus gets refined. The solidification rate of the liquid metal is controlled by the melting rate and water cooling.

The above is a general description of the basic process, but a number of new developments have taken place. A short collar mould is most commonly employed now-a-days in place of a full length copper crucible. The power supply could be AC or DC. The inductance of power circuit is reduced by using bifilar electrode arrangement or by coaxial leads or by using low frequency power supply. Charged solid or premelted slags can be used.

During the ESR process, due to presence of an active slag which is essentially a mixture of CaF2, CaO and Al2O3, sulfur removal from the liquid metal takes place rapidly. There is usually no change in chemical composition of the alloying elements but minor composition adjustments can be done during ESR melting. However, removal of hydrogen is difficult during ESR melting. Hydrogen has to be controlled by restricting, hydrogen content of the starting electrode. This has been one of the important limitations of the ESR process.

As a mentioned above, ESR process is used for remelting of special Steels, superalloys and low alloy steels An estimation of the current usage of ESR in developed countries is given in Table 1.

Table 1: Usage 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

Metallurgy of the Electroslag Remelting Process

Due to the superheated slag that is continuously in touch with the electrode tip, a liquid film of metal forms at the electrode tip. As the developing droplets pass through the slag, the metal is cleaned of non-metallic impurities which are removed by chemical reaction with the slag or by physical flotation to the top of the molten pool. The remaining inclusions in ESR are very small in size and evenly distributed in the remelted ingot.

Slags for ESR are usually based on calcium fluoride (CaF2), lime (CaO) and alumina (Al2O3). Magnesia (MgO), titania (TiO2) and silica (SiO2) may also be added, depending on the alloy to be remelted. To perform its intended functions, the slag must have some well-defined properties, such as:

  • Its melting point must be lower than that of the metal to be remelted;
  • It must be electrically efficient;
  • Its composition should be selected to ensure the desired chemical reactions;
  • It must have suitable viscosity at remelting temperature.

In spite of directional dendritic solidification, various defects, such as the formation of tree ring patterns and freckles, can occur in remelted ingots. It is important to note that white spots normally do not occur in an ESR ingot. The dendrite skeletons or small broken pieces from the electrode must pass the superheated slag and have enough time to become molten before they reach the solidification front. This prevents white spots.

The ingot surface covered by a thin slag skin needs no conditioning prior to forging. Electrodes for remelting can be used in the as-cast condition.

Electroslag Remelting Furnaces

Significant advances have been made over the years in plant design, coaxial current feeding and particularly in computer control and regulation with the objective of achieving a fully-automated remelting process. This in turn has resulted in improved metallurgical properties of the products. A fully coaxial furnace design is required for remelting of segregation-sensitive alloys in order to prevent melt stirring by stray magnetic fields.

Shielding of the melt space with protective atmosphere has been the latest trend in recent years. Remelting under increased pressure to increase the nitrogen content in the ingot is another variation of ESR. ESR furnaces can be designed for remelting of round, square and rectangular ingots.

Finally, in the German company ALD, computer controlled process automation has been developed: the ALD's automatic melt control system (AMC). ALD's electrode immersion depth control into the slag is based on slag resistance or slag resistance swing. Using the resistance parameter automatically decouples the immersion depth and remelting rate control loops which are otherwise cross-influencing each other.

AMC benefits include ease of operation and accuracy and repeatability of producing ingots with excellent properties, including:

  • Homogeneous, sound and directionally solidified structure;
  • High degree of cleanliness;
  • Free of internal flaws (e.g. hydrogen flakes);
  • Free of macro-segregation;
  • Smooth ingot surface resulting in a high ingot yield.

An ESR furnace is basically a movable copper mold that contains a basic slag. The heat of the slag is used to melt the as-cast ingot. The ingot melts droplet-by-droplet and the dense steel falls through the slag and re-solidifies at the base of the mold. The slag acts as a filter, absorbing sulfur. In addition, the relatively fast re-solidification results in a material with a relatively low level of segregation. The result is steel with a low inclusion level and a homogeneous microstructure (see Figure 1).


Figure 1: A schematic representation of the ESR unit (left)

The slag contained within the copper mold is heated and used to remelt the as-casted ingot. As the as-casted ingot is consumed the mold moves upward, leaving behind an ESR-ingot with superior properties. The photograph on the right is an actual ESR furnace in operation.

Electroslag Remelting of Heavy Forging Ingots

At the end of the 1960's, the concept of using ESR plants to manufacture large forging ingots gained acceptance. Increasing demand for larger electrical power generating units required forging ingots weighing 100 tons or more for manufacturing of generator and turbine shafts. ALD's largest ESR furnace, commissioned in the early 1970's, allows manufacturing ingots of 2,300 mm diameter and 5,000 mm length weighing up to 165 tons. The furnace operates with ingot withdrawal employing four consumable electrodes remelted simultaneously in the large diameter mold and replacing the consumed electrodes with subsequent ones and as many as necessary to produce the desired ingot weight.

Directional solidification must be ensured over the entire ingot cross-section and length to avoid interior defects, such as macro-segregation, shrinkage cavities and non-uniform distribution of inclusions. By maintaining the correct remelting rate and slag temperature, directional solidification can be achieved for ingot diameters as large as 2,300 mm (Figure 2). Accordingly, the ESR ingot is free from macro-segregation in spite of the large diameter and on the other hand the cleanliness and homogeneity result in excellent mechanical properties as compared to conventionally cast steel ingots.


Figure 1: 165 ton ESR ingot, 2,300 mm diameter x 5,000 mm long

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