ELECTROSLAG REMELTING (ESR) TECHNOLOGY- RECENT DEVELOPMENTS AND TRENDS

Vinay K. Sethi, INTECO Special Melting Technologies

Abstract The history of Electroslag Remelting technology is briefly reviewed in this paper. The paper also describes the different ESR plant concepts which are common now a days. The recent development of new generation ESR technologies developed internationally are introduced including Electroslag Rapid Remelting (ESRR) and large scale electroslag remelting (+100 T ESR) technology. The paper gives merit of ESR ingots vis-a-vis normal ingot casting and forging and recent trends in application of ESR products in Energy sector. The Electroslag Remelting (ESR) Process has been developed and introduced as a production process for high grade steel ingots in 1958 in the Soviet Union, and is being applied in Western Europe since 1966. In the 1960‘s, independent research in the field of ESR has been done in Austria and Great Britain. At the same time, the first ESR plants have been installed in Western Europe and the US on an industrial scale basis. Since then the production capacity for ESR-steel has reached more than 1,200,000 tons per year. Although secondary liquid metal refining processes have been able to replace the ESR process to some extent for a variety of products, the ESR-process is still gaining increasing importance when control of solidification in regard to macro- and microstructure as well as higher cleanliness levels is required. Another considerable advantage of ESR compared to conventional ingot casting is the increased yield, which gains importance with an increasing ingot diameter. Principle: In Electro-Slag Remelting, a new remelted ingot is formed in a water cooled copper mould by remelting a consumable electrode in a superheated slag bath. The heat required to melt the electrode is produced when current passes through the liquid slag by its electrical resistance. The melting rate of the consumable electrode is controlled by the power input to the slag pool.

First Generation ESR and Present Generation ESR

1. Main column 2. Support Structure 3. Electrode carriage 4. Electrode arm 5. Stub 6. Electrode 7. Collar mold 8. ESR Ingot 9. Ingot carriage 10. Base plate 11. Base plate carriage 12.Loading carriage 13. Loading arm 14. Pit 15.Main plateform 16. Preheating furnace

ESR Parts

By choosing suitable conditions, the depth and shape of the liquid metal pool can be controlled as such that homogeneous and segregation free ingots are obtained. The liquid metal droplets formed at the tip of the electrode pass through the superheated, metallurgically active liquid slag, thereby creating excellent conditions for slag-metal reactions, such as removal of sulphur and oxygen.

WHY ESR INGOTS: COMPARISION TO CONVENTIONALLY CAST INGOTS

• Dense structure of ingot without inner shrink hole or porosity. • Higher cleanliness (less inclusions). • Homogenous structure and chemical analysis over the whole ingot length. • Higher yield of remelted material to final product. • General improvement of mechanical properties. • Smooth surface finish. • Improved macro- micro structure (controlled solidification).

Two ingots of the same chemical composition left: conventionally cast and forged (microstructure contains large, blocky carbides right: ESR remelted material (very fine, uniformly distributed carbide particles).

Surface finish of ESR Ingot

In principle, for standard ESR operation the following basic configurations are presently being built, each having its particular fields of application

• Plant with short collar mould / retractable baseplate • Plant with static mould • Combined concepts (short collar mould • and static mould)

• Pressure ESR

In addition to the above mentioned, specialized versions of the process are available, for example:

• Electroslag Rapid Remelting ESRR® • Current Conductive Mould (CCM)

Plants with Short Collar Mould / Retractable Baseplate

This plant configuration consists of a fixed collar mould arranged at the level of the working platform, an automatic electrode change system and a retractable baseplate with ingot car to withdraw the remelted ingot from the mould and to remove it from underneath the plant.

Picture : Plant with Short Collar mould.

SHORT COLLAR MOULD OPERATION : Following are the salient points:

• The ingot length is independent of electrode length. With multi electrodes long ingots can be produced.

• Moderate investment cost for a number of different ingot sizes

• High productivity for long ingots

• High yield due to remelting of multiple electrodes in one long ingot

• Break -out possible

• Relatively high investment cost

• Slag selection depending on surface quality aiming for minimized power consumption

• Treatment of ingot bottom during remelting possible (insulation, heating, forced cooling)

• Sampling during remelting possible

• Surface of ingot visible during remelting

Plants with Static Mould

Static mould / single electrode plants are preferably installed when comparably short ingots up to max. 2.5 m length are required. In order to reduce down time, such plant is equipped with two melt stations and one swingable pylon with the electrode carriage. In that case, remelting can proceed in one melt station, while in the second melt station the ingot is removed and the station is prepared for the next melt. Operation with Static Mould: Following are the salient features:

• Ingot length limited by max. electrode length • Production of long electrodes difficult • Crucible stock rather expensive • High productivity at moderate investment cost • Low yield due to limited ingot length • Break -out impossible • Moderate investment cost • Slag with high amount of CaF2 required • For crack -sensitive grades ingot length is limited. • No sampling possible • Surface condition can be examined only after stripping.

Picture : ESR Plant with Static mould

Combined Plants (short collar mould and static mould)

For applications where great flexibility regarding electrode dimensions, ingot dimensions, and steel grades is required, combined furnace concept makes sense. One central melt station (short collar mould or static mould) is installed, for the production of larger ingots using multiple electrodes. As electrode change is applied, two furnace heads are required. One or two outer static mould melt stations are installed besides the central melt station, within the range of the respective furnace head.

Picture : Plant with combined concept (Short collar mould and Static mould)

Pressure Electroslag Remelting (P-ESR)

Nitrogen, besides hydrogen and oxygen, is a gaseous element which is soluble in iron, nickel and cobalt and its alloys. While oxygen and hydrogen are not desirable, nitrogen is used as an alloying element in particular cases for enhancing steel properties. A direct transfer from the atmosphere into the liquid metals is not possible in ESR, as the liquid metal at the electrode tip as well as at the pool surface is covered by the liquid slag and shielded against a direct contact with the gas phase. Due to the extremely low nitrogen solubility in standard slags applied in ESR, a mass transfer of nitrogen from the gas phase to slag and further on to metals (in contrary to hydrogen) does not occur. Therefore nitrogen has to be supplied continuously during remelting by means of solid nitrogen-containing additives (e.g. Si3N4 or FeCrN). A major possibility to increase the solubility in liquid steel is to increase the atmospheric pressure in the system. The pressure level applied in the P-ESR process mainly depends on the requested nitrogen content of the remelted ingot.

Picture: Pressure ESR

RECENT ADVANCEMENT IN ESR TECHNOLOGY

ESRR® – Electroslag Rapid Remelting

The ESRR process has been developed to enable the production of near net shaped billets from segregation prone alloys at comparably high melt rates (compared to ESR) and without center soundness defects (compared to continuous casting). The ESRR® process generates near net shaped billets instead of ingots, with the same excellent material properties of ingots from conventional ESR.

The consumable electrode, like in the standard ESR process is melted in the super heated slag bath, when electric current is passed through. The liquid metal droplets dripping off the electrode tip are collected in the narrow, lower part of the mould, where the initial solidification takes place and the re-melted billet is continuously formed. In case of ESRR® operation, the electric current passes from the electrode tip into the slag pool and is returned partially through the mould walls by the application of uncooled conductive elements as well as through the remelted ingot. The power distribution is as such, that approx. 80-90 % of the total current is transferred through the mould walls and only 10-20 % through the remelted billet. Combining the specially designed T-shaped mould with the described current split, results in a metal pool shape which is independent from the total power input. Hence, high melt rates can be applied and a controlled way of solidification, positively affecting the ingot structure, can be obtained at the same time. In continuous operation the remelted billet is withdrawn by means of a separate withdrawal and cutting equipment, similar to as applied for the continuous casting process.

Picture: ESRR

CCM®, Current Conductive Mould

A new current conductive mould concept, offers a variety of new possibilities which are yet to be explored. The possibility to directly heat the slag pool, by means of a current conductive element, offers an independent and separate control of the temperature of the slag and the rate of remelting. Depending on the chosen split of power input through the electrode respectively through the slag pool, a shallower liquid metal pool can be achieved.

LARGE SIZE ESR PLANTS :

Market Trend

Within the last two years, a huge demand for ESR – furnaces, designed for the production of ingots with comparably large diameters and weight, has evolved. Especially, producers of equipment for the power generation industry (e.g. turbines, rotors, pressure vessels) have discovered the advantages of ESR – remelted semi-finished goods for their applications. Until today, it is common practice to use ingots produced by conventional ingot casting as input material for the forging process and the sequential production steps.

A comparison of conventional ingot casting and ESR shows that ESR is favorable in regard of the following aspects:

Picture : Large scale ESR plant.

• In ESR, a considerably higher yield of > 90% can be achieved, compared to ingot casting (approx. 70%).

• The ESR process improves cleanliness and mechanical properties of the produced ingot.

• Furthermore, ESR ingots are more dense and homogenous. This leads to smaller hot forming factors as opposed to conventionally cast ingots.

Typical Sizes in a Large size ESR plants are

• Ingots with a diameter of 1600 – 2600 mm

• Ingots with a weight of 100 – 250 tons

Plant Concepts are following :

• Static mould design

Multiple electrode remelting / electrode change

• Short collar mould design

Multiple electrode remelting / electrode change

• Combined plants (for higher productivity)

• Application of CCM – concept

The Large ESR plants have following main applications of the Products:

For Power Generation

Alternative, nuclear and conventional

Pressure vessels

Gas, wind and water turbines

Hydraulic turbines and generators

Generator shafts

For Marine

Crank shafts

Paper Industry

Rolls

On and Off-Shore Oil and Gas Industry

Piping systems, wells heads, valves

Why large ESR Ingots?

For example in a Forging Shop, INPUT: approx. 55000 t/a ingots (from conventional ingot casting), the Yield: approx. is 70% and loss of approx. 16500 t/a.

There is Loss from mechanical and thermal processing which in above case is approx. 15000 t/a.

Therefore, the final OUTPUT is approx. 20000 – 25000 t/a.

While ESR plants have an advantages of ESR ingots, the Yield is approx. 90% – 93%. Therefore, the loss is of approx. 3850 - 5500 t/a in comparison to 16500 t/a in ingot casting in the forge shop. The main advantages can be summed up as:

Loss is reduced by 70% with ESR.

Better preconditions for hot forming resulting from a homogenous and dense ingot structure, Higher cleanliness.

Better mechanical properties.

Conclusion:

ESR Ingots have thin, fully refined and homogeneous microstructure, without sharp segregation and significant reduced porosity and solidification shrinkage. Whatever the size of the ESR ingot and where ever in the ingot, such ESR microstructure is far easier to forge than the one from core area from conventional ingots, which require powerful straining as to “break” the solidification sharp microstructure. Therefore, forging Electro-Slag Remelted ingots for large rotor requires most of time less press power / “simple” forging sequence than for conventional ones of same weight. Electro-Slag Remelting provides an opportunity to achieve complete consolidation with a lower level of forging technology “know how”. It provides a very effective short cut for new suppliers to the power generation industry to quickly enter a demanding market and compete effectively with long established forging suppliers with minimum product development time.