7126Weld Overlay Wallpapering

Overlay welding of carbon steel or low-alloy steel with the high-performance corrosion-resistant nickel alloys has seen increased activity in recent years such as in overlay welding of thick-walled tube sheets and flanges. Since the weld contains microsegregation of alloy constituents, its corrosion resistance may be slightly inferior to a fully wrought structure. This inferiority can be further compounded by base metal dilution or use of the improper flux. In recent years, electro slag overlay welding has been successfully employed providing excellent results. Other methods are submerged arc strip cladding and gas metal pulse arc overlay welding. Each method had its own advantages and disadvantages and is discussed in greater detail elsewhere [50]. Use of two- or three-layer deposits to achieve a desirable deposited chemistry is needed for optimum corrosion resistance and in most cases depends on the process and welding parameters used. However, with very low iron containing Ni-Cr-Mo alloys, the same desired chemistry and corrosion resistance can be obtained with one layer as is the case with Ni-Cr-Mo alloy 59, which has an iron content of typically less than 1%. However this will depend on the intended application, and it may be necessary to use two-layer deposit if the environment is very corrosive.

If large areas are to be protected, a technique known as wallpapering has been successfully used in the industry, especially in the power plant flue gas desulfurization systems. In these systems a 0.063-in.-thick sheet of corrosion-resistant alloy of the Ni-Cr-Mo family is applied over carbon steel structure by the "wallpapering" technique, which has proven to be very reliable and cost effective. Details of this method are described elsewhere [51].

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