Electro slag welding

by Owen Gorton

Description

Electro slag welding is a very efficient, single pass process carried out in the vertical or near vertical position and used for joining steel plates/sections in thicknesses of 25mm and above. It was developed by the Paton Institute in the Ukraine in the early 1950s and superseded the very high current submerged arc process for making longitudinal welds in thick-walled pressure vessels.

Unlike other high current fusion processes, electro slag welding is not an arc process. Heat required for melting both the welding wire and the plate edges is generated through a molten slag's resistance to the passage of an electric current.

In its original form, plates are held vertically approximately 30mm apart with the edges of the plate cut normal to the surface. A bridging run-on piece of the same thickness is attached to the bottom of the plates. Water cooled copper shoes are then placed each side of the joint, forming a rectangular cavity open at the top. Filler wire, which is also the current carrier, is then fed into this cavity, initially striking an arc through a small amount of flux. Additional flux is added which melts forming a flux bath which rises and extinguishes the arc. The added wire then melts into this bath sinking to the bottom before solidifying to form the weld. For thick sections, additional wires may be added and an even distribution of weld metal is achieved by oscillating the wires across the joint. As welding progresses, both the wire feed mechanism and the copper shoes are moved progressively upwards until the top of the weld is reached. See figure 1.

Fig.1. Electro slag welding

The consumable guide variant of the process uses a much simpler set-up and equipment arrangement which does not require the wire feed mechanism to climb. In this case, the wire is delivered to the weld pool down a consumable, thick-walled tube which extends from the top of the joint to the weldpool. Support for the molten bath is provided by two pairs of copper shoes which are moved upwards, leapfrogging each other as welding progresses. The tubular guides can be further supplemented by additional consumable plates attached to the tube. Generally, as the thickness of plate increases, the number of wires/guides increases, approximately in the ratio of one wire per 50mm of thickness, see figure 2.

Fig.2. Consumable guide welding

Current status

In the fabrication industry, the process continues to be used for thick walled pressure vessels which are post-weld normalised and for structures such as blast furnace shells and steel ladles which are used at above ambient temperatures. The process is also extensively used for the welding of railway points.

Important current issues

Considerable interest was shown in electro slag welding during the 1970s when ideas for increasing welding speed were investigated. This was seen as an important parameter for increasing productivity and as a way of reducing heat input to improve HAZ and weld metal impact properties.

However, since that time little has been done by way of development. Those developments that have taken place have been limited to the tuning of parameters and tailoring techniques for specific applications.

Benefits

The principal benefits of the process are:

speed of joint completion; typically 1 hour per metre of seam, irrespective of thickness
lack of angular distortion
lateral angular distortion limited to 3mm per meter of weld
high quality welds produced
simple joint preparation, i.e. flame-cut square edge
major repairs can be made simply by cutting out total weld and re-welding

Risks

Electro slag welding is not one of the major welding processes because the high heat input generates large, coarse grained weld metal and HAZs which lead to poor fracture toughness properties in these areas. Toughness improvements can only be achieved by post-weld normalising treatment. Additionally, the near parallel-sided geometry of the weld, combined with the coarse grains, can make it difficult to identify defects at the fusion boundary by standard ultrasonic NDT techniques.

The process has considerable potential for increasing productivity. However, its use has been limited because of relatively poor understanding of the process and, for specific applications, the significance of the fracture toughness values. As a result, use of the process has been restricted to a few niche applications.

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