Electroslag Welding 4y5q5v

ELECTROSLAG WELDING 1. Introduction Single- welding of heavy plates has long been desired as a means of avoiding multi- welding techniques. Before 1900, graphite molds were placed on each side of a space between vertical plates to contain molten metal created by graphite electrodes, fusing the edges to form the weld. Graphite molds were replaced by copper or ceramic molds, and conventional welding arcs, gas torches and Thermit mixtures were devised for generating the molten metal with a degree of superheat sufficient to obtain uniform coalescence. In the early 1950's, Russian scientists from the Paton Institute of Electric Welding in Kiev announced the development of machines that employed the principle of an electrically conductive slag to make single- vertical welds. An electro slag unit was introduced in the United States in 1959. Since then, many refinements and modifications have been made, resulting in production machines capable of meeting the standards of our industry.

2. Fundamentals 2.1 Principle of Operation Electro slag Welding (ESW) is a welding process producing coalescence of metals with molten slag that melts the filler metal and the surfaces of the work pieces to be welded. The weld pool is shielded by this slag, which moves along the full cross section of the t as welding progresses. The process is initiated by an arc that heats a granulated flux and melts it to form the slag. The arc is then extinguished by the conductive slag which is kept molten by its resistance to electric current ing between the electrode and the work pieces. Usually a square groove t is positioned so that the axis or length of the weld is vertical or nearly vertical. Except for circumferential welds, there is no manipulation of the work once welding has started. Electro slag welding is a machine welding process, and once started, it continues to completion. Since no arc exists, the welding action is quiet and spatter-free. Extremely high metal deposition rates allow the welding of very thick sections in one . A high quality weld deposit results from the nature of the melting and solidification during welding. There is no angular distortion of the welded plates. The process is initiated by starting an electric arc between the electrode and the t bottom. Granulated welding flux is then added and melted by the heat of the arc. As soon as a sufficiently thick layer of molten slag (flux) is formed, all arc action stops, and the welding current es from the electrode through the slag by electrical conduction. Welding is started in a sump or on a starting tab to allow the process to stabilize before the welding action reaches the work.

Heat generated by the resistance of the molten slag to age of the welding current is sufficient to fuse the welding electrode and the edges of the workpiece. The interior temperature of the bath is in the vicinity of 192S°C. The surface temperature is approximately 1650°C. The melted electrode and base metals collect in a pool beneath the molten slag bath and slowly solidify to form the weld. There is progressive solidification from the bottom upward, and there is always molten metal above the solidifying weld metal. Run-off tabs are required to allow the molten slag and some weld metal to extend beyond the top of the t. Both starting and run-off tabs are usually removed flush with the ends of the t.

2.2 Process Variation There are two variations of electro slag welding that are in general use. One variation uses a wire electrode with a non-consumable guide () tube to direct the electrode into the molten slag bath. This variation will be referred to as the "conventional method." The other variation is similar to the first, except that a consumable guide extends down the length of the t. This variation will be called the "consumable guide method." With the conventional method, the welding head moves progressively upward as the weld is deposited. With the consumable guide method, the welding head remains stationary at the top of the t, and both the guide tube and the electrode are progressively melted by the molten slag. Conventional Method The conventional method of electro slag welding is illustrated in figure. One or more electrodes are fed into the t, depending on the thickness of the material being welded. The electrodes are fed through non-consumable wire guides which are maintained 2 to 3 in. (50 to 75 mm) above the molten slag. Horizontal oscillation of the electrodes may be used to weld very thick materials.

Water-cooled copper shoes (dams) are normally used on both sides of the t to contain the molten weld metal and slag bath. The shoes are attached to the welding machine and move vertically with the machine. Vertical movement of the welding machine is consistent with the electrode deposition rate. Movement may be either automatic or controlled by the welding operator. Vertical movement of the shoes exposes the weld surfaces. There is normally a slight reinforcement on the weld, which is shaped by a groove in the shoe. The weld surfaces are covered with a thin layer of slag. This slag consumption must be compensated for during welding by the addition of small amounts of flux to the molten slag bath. Fresh flux is normally added manually. Flux-cored wires may be used to supply flux to the bath. The conventional method of electro slag welding can be used to weld plates ranging in thickness from approximately 1/2 to 20 in. (13 to 500 mm). Thicknesses from 3/4 to 18 in. (19 to 460 mm) are most commonly welded.

2.3 Equipment The equipment for ESW process methods is the same except for the design of the electrode guide tubes and the requirements for vertical travel. The following are major components of electro slag welding equipment: a) Power supply b) Wire feeder and oscillator c) Electrode guide tube d) Welding controls e) Welding head f) Retaining shoes (dams)

3. Applications 3.1 Base Metal Many types of carbon steels can be electro slag welded in production, such as AISI 1020, AISI 1045, ASTM A36, ASTM A441, and ASTM A51S. They generally can be welded without post-weld heat treatment. In addition to carbon steels, other steels are successfully electro slag welded. They include AISI 4130, AISI 8620, ASTM A302, HY80, austenitic stainless steels, ASTM A514, ingot iron, and ASTM A387. Most of these steels require special electrodes and a grain refining post-weld heat treatment to develop required weld or weld heat-affected zone properties.

3.2 t Design There is one basic type of t, which is the square groove butt t. Square edge plate preparations can be used to produce other types of ts such as corner, T-, and edge ts. It is also possible to make transition ts, fillet welds, cross-shaped ts, overlays, and weld pads with the ESW process. Typical ESW t designs and the outlines of the final welds are shown in figure. Specially designed retaining shoes are needed for ts other than butt, corner, and T-ts.

t Design for Electro slag Welding Line Shows Depth of Fusion Into the Base Metal

3.3 Applications and other typical uses a) Structural b) Machinery c) Pressure vessels d) Ships e) Castings

4. Advantages and Limitations The ESW process offers many opportunities for reducing welding costs on specific types of ts. Process advantages are the following: 1) Extremely high metal deposition rates; ESW has a deposition rate of 35 to 45 lbs per hour per electrode. 2) Ability to weld very thick materials in one ; there is one equipment setup and no inter- cleaning since there is only one . 3) Preheating is normally not required, even on materials of high hardenability. 4) High-quality weld deposit; the weld metal stays molten for an appreciable time, allowing gases to escape and slag to float to the top of the weld. 5) Minimum t preparation and fit-up requirements. Mill edges and flame-cut squares edges are normally employed. 6) High-duty cycle; the process is automatic and once started continues to completion; there is little operator fatigue. 7) Minimum materials handling; the work needs to be positioned only to place the axis of the weld is vertical or near-vertical; there is no manipulation of the parts once welding has started. 8) Elimination of weld spatter, which results in 100 percent filler metal deposition efficiency. 9) Low flux consumption; approximately 1 pound of flux is used for each 20 pounds of weld metal. 10) Minimum distortion; there is no angular distortion in the horizontal plane; distortion is minimum in the vertical plane, but this is easily compensated for. 11) Minimum welding time; ESW is the fastest welding process for large, thick material. The limitations of the ESW process are the following: 1) The ESW process welds only carbon and low alloy steels, and some stainless steels. 2) ts must be positioned in the vertical or near vertical position. 3) Once welding has started, it must be carried to completion or a defective area is likely to result. 4) ESW cannot be used on materials thinner than about 3/4 in. (19 mm). 5) Complex material shapes may be difficult or impossible to weld using ESW.