Electroslag Remelting

ESR - The Process

In brief, the Electroslag Remelting process involves the gradual melting of the lower end of a cast or forged electrode through a layer of molten slag into a water-cooled copper mould.

The slag which is usually calcium fluoride, with addition of lime, magnesia, alumina and other oxides, serves several functions. In its molten state, it is electrically conductive, acting somewhat like a resistance heating element when a high current is passed between the electrode and the mould. The slag becomes superheated with the continued passage of electric current, causing the electrode tip immersed in it to become molten. Any inclusions in this molten metal are dissolved by and become part of the slag, leaving a purified metal to pass as droplets through the slag layer into the bottom of the mould.

After the process has continued for some time, a purified ingot having a controlled solidification structure is built up in the mould. The slag, which also acts to prevent oxidation of the molten metal in the usual case of operation in air atmosphere, floats on top of the liquid metal at the top of the ingot due to its relatively low density.

Towards the end of the process, the power is gradually reduced. This reduces the volume of liquid metal on top of the ingot. At the same time, additional metal is added from the electrode at a reduced rate to compensate for solidification shrinkage. The result is a sound ingot top with a high yield of useful product.

Controlled Solidification - The Principle Factor in the ESR Process

The main reason to operate a modern ESR is to obtain controlled solidification. Ladle metallurgy and vacuum induction melting processes can efficiently produce large volumes of liquid metal of high cleanliness and excellent chemistry. When this metal is poured into moulds and allowed to solidify the segregation and solidification shrinkage often make the resulting ingots unusable. This is due to the excessively long solidification times associated with ingots of industrially useful size.

In ESR, this unusable metal is remelted and chemically homogenised across the cross section of the electrode. Then, because the melting rate is controlled so that only a relatively thin layer of metal on top of the ingot is molten at one time, the metal is relatively quickly re-solidified.

This avoids macro segregation effects. Also, because there is always liquid metal on top of the metal which is freezing (and shrinking), the formation of shrinkage cavities, pipe or porosity is avoided. The result when the process is conducted properly, is a sound chemically homogeneous, high yield ingot.

Background - Consarc ESR Technology breakthroughs

In the 1960’s Consarc’s engineers conceived a new approach to electroslag remelting, quite different from others in use at the time. The resulting improvements in the efficiency and performance of the process established Consarc as the leading manufacturer of ESR systems worldwide.

Coaxial ESR Furnaces

Consarc did not invent electroslag remelting but in 1966 conceived a different version of the process, one which is still in use today producing metals used in the most demanding aerospace and other applications.

What Consarc’s engineers did was to combine the use of a "high fill ratio" electrode (an electrode whose diameter approached that of the crucible) with an AC line frequency furnace design in which the main power conductors were arranged coaxially around the melt zone. The high fill ratio electrode provides significant benefits including:

reduced power consumption
increased ingot quality
improved ingot surfaces

The coaxial conductor system allowed the use of smaller, more efficient power supplies and decreased undesirable magnetic stirring of the melt. Moreover, it virtually eliminated the stray field eddy current heating of surrounding steelwork which had characterised all prior ESR furnace designs and allowed magnetic interaction between adjacent melts.

The principles of this coaxial design are incorporated in all ESR furnaces produced by Consarc.

ESR configurations (Static / Electrode Exchange / Combination)

The majority of ESR furnaces operated today are of the single head two melt station variety. However Consarc can also supply twin head electrode exchange furnaces melting into either a central collar mould / ingot withdrawal station or a static mould.

Combination furnaces with twin heads, central electrode exchange and outboard static melting stations in one plant have also been supplied by Consarc to provide the flexibility remelt a wide variety of electrode feedstock sizes.

ESR Product Development

Consarc followed up its initial innovations in the ESR arena with additional product and process enhancements. Many features that have become "standard" on modern ESR installations were initially developed and installed on Consarc ESR systems. These include:

Utilisation of load cells to continuously weigh the electrode during the melting process to more accurately measures melt rate
A PC/ PLC based melt control system that fully automates the melting process, eliminating the older manual operation. The resulting precise process control gives a consistently high quality final product
Ability to adjust the electrode tip position horizontally at any point in the melt (X-Y axis adjustment) to keep the process electrically and thermally centred in the crucible
Development of process and equipment for the melting of large round ingots up to 110T and 1800mm diameter and large slab ingots up to 750mm by 80 inches (2000mm in cross section with weights up to 30T
The use of dry air above the melt zone to virtually eliminate hydrogen pickup in large steel ingots while maintaining an ability to de-sulphurise the metal

Other features of a modern Consarc ESR furnace include:

Inert Gas Melting Systems (IGMS). The use of inert gas (argon) above the melt zone to provide an oxygen and nitrogen free melting environment can improve the cleanliness of remelted steel grades and also greatly assist the control of reactive metal (Al, Si) concentrations to tight tolerance levels.
PC controlled vibratory Slag Feeders to automate and provide consistency of start up practices during the slag melting phase.
Fully electromechanical drives systems including single servo motor ram drive for precise control of ram and electrode during the remelt process.

Flexible starting practices (Hot Slag start / Cold Slag start)

Consarc furnaces can operate with either a cold slag / dry start practice with prefused slags or with a hot slag start using premelted slags depending on process requirements and customer preference.

In the majority of cases cold slag starting is the preferred choice. Consarc provide PC controlled vibratory Slag Feeders to automate and provide consistency of start up practices during the slag melting phase.

Consarc can also design and build a slag melting furnace if required for the customers' application.

Reliability assured by proprietary power supply (Reactor based)

Consarc are the only supplier of ESR furnaces that design and produce our own power supplies to meet the exact requirements of the specific melting application .The Consarc ESR power source is a rugged, water-cooled, reactor based single phase AC power supply. These long-life units take up very little floor space and have exceptional reliability and low operating/maintenance costs.