TWI Knowledge Summary

Strip cladding

by Gene Mathers

Strip cladding is a fusion welding technique used to deposit a strip of weld metal on to a component to achieve the desired dimensions or properties, replacing lost metal or providing a wear or corrosion resistant surface. This enables, for example, a creep resistant material such as 2 ¹ / ₄ Cr 1Mo steel to be provided with a corrosion resistant surface layer of stainless steel for use in corrosive media at high temperatures. The process is usually confined to relatively large and thick components which need to be manipulated to enable welding to be carried out in the flat position. The technique finds its widest application in the oil, gas and related industries and in the nuclear power generation field where it is used for surfacing the internal surfaces of pressure vessels and large diameter pipe and in the reclamation of steel mill rolls.

The welding process employed to do this may be either submerged arc or electroslag, with the wire filler metal being replaced by a flat strip of the surfacing material. The strip widths commonly used vary from 15mm to 240mm, with a thickness of 0.5mm in Europe and 0.4mm in Japan, enabling a band of the strip width and some 3mm to 5mm in depth to be deposited in a single pass. Deposition rates can be very high and vary according to the process, strip width and welding current. For example, the deposition rate from a 60mm wide strip can be as high as 36kg/hr for electro-slag and 20kg/hr for submerged arc welding. In addition to achieving higher deposition rates than submerged arc cladding, electroslag cladding has the capability of providing far lower levels of substrate dilution into the surfacing material. This means that the required chemical analysis can often be achieved in a single layer deposit where submerged arc cladding may need two layers.

Materials available for surfacing include martensitic, austenitic and duplex stainless steels, nickel based alloys, copper nickel alloys, cobalt based and carbide containing hardsurfacing alloys. The majority of the parent materials on to which the cladding is deposited are carbon or low alloy steels, selected on the basis of cost or for their specific mechanical properties, as in the example quoted above. Conventional submerged arc welding equipment can be used for both processes, the only modification being the provision of a welding head designed to feed the selected width of strip.

The main limitations of both processes are the need to weld in the flat or downhand position and to provide adequate access for the welding head. The overlap between adjacent beads must be controlled to avoid lack of fusion defects. The chemical composition of the strip must be carefully selected to take into account the level of dilution, and welding parameters must be closely controlled to ensure that the predicted level of dilution is consistently achieved.

Copyright 2000, TWI Ltd