Electroslag Remelting

an electrometallurgical process in which a metal in the form of a consumable electrode is re-melted in a bath of an electrically conducting synthetic slag by the action of heat released upon the passage of electric current through the slag. The method, which significantly increases the quality of the metals and alloys produced, was developed in the early 1950’s at the E. O. Paton Institute of Electric Welding of the Academy of Sciences of the Ukrainian SSR on the basis of the electroslag welding process (seeWELDING). The consumable electrode may be manufactured by casting, rolling, or forging metal produced in an open-hearth, arc, or vacuum induction furnace or oxygen converter.

Figure 1. Electroslag remelting: (a) with one consumable electrode, (b) with two consumable electrodes; (1) consumable electrode, (2) slag bath, (3) metal bath, (4) ingot

During electroslag remelting, the temperature of the slag, which consists of various components (CaF₂, CaO, SiO₂, Al₂O₃, and others), exceeds 2500°C. Drops of the molten electrode metal pass through the slag layer to form a layer of metal underneath; subsequent solidification of the molten metal in a water-cooled crystallizer produces an ingot (Figure 1). As melting progresses, the consumable electrode continues to be introduced into the slag layer to maintain the volume of crystallizing metal. The slag functions as a refining medium. Electroslag refining of a metal occurs in the film of molten metal on the melting tip of the electrode, in the drops of molten metal passing through the slag bath, and on the boundary surface of the slag and metal baths.

Selective refining is achieved by changing the composition of the slag and the temperature regime of the process. As a result of electroslag refining, the sulfur content in the metal can be lowered by a factor of 2–5, and the content of oxygen and nonmetallic inclusions can be lowered by a factor of 1.5–2.5. The ingot produced is characterized by a compact, ordered microstructure that is free of casting and shrinkage flaws. The chemical and structural homogeneity of the ingot ensures isotropy of the physical and mechanical properties of the metal in the cast and deformed states.

Electroslag remelting is used to produce ingots weighing from several tens of grams to 200 tons in virtually any required shape, as determined by the shape of the crystallizer. In addition to intermediate ingots (for rolling shaped sections, pipes and tubing, and sheets) and forging ingots (for conventional forging, extrusion, and sheet-metal forming and forging), the technique is also used to produce shaped castings (for crankshafts, fastening hardware, pressure vessels, and gear teeth). Electroslag remelting is used in ferrous metallurgy (for the production of ball-bearing, structural, stainless, and tool steels and heat-resistant alloys), nonferrous metallurgy (for the production of chromium bronze and nickel-copper alloys), and heavy machine building (for the production of heat-resistant, high-strength die and roller steels). The process has been patented and is used under Soviet licenses in many countries.