Weld-overlay coatings applications

Banovic, S. W., DuPont, L. N., Marder, A. R. (1998), Study of Low-Aluminum Fe-Al Weld Overlay Coatings for Applications in Reducing Environments, Paper No. 4-3 in Proc. Twelfth Anna. Conf. Fossil Energy Materials, CONF-980561, ORNL/ FMP-98/1. 

As production demands have led to increasingly higher operating temperatures and pressures, protection of carbon steel waterwah tubes has involved studding, application of weld overlay or thermal spray, or the use of composite (stainless-clad carbon steel) or chromized (diffusion-coated carbon steel) tubes to provide resistance to sulfidation and oxidation on the fireside. 

Most welding processes are used for application of surface coatings and on-site deposition can be more easily carried out, particularly for repair purposes. Weld overlays are very versatile because a large number of commercially available alloys can be selected to provide protection from a wide range of environmental degradation mechanisms. 

The metal temperatures of the waterwall tubes (WWTs) are relatively low, approximately 230-330 C, depending on the steam pressure (2.9-9.8 MPa). Here CRCs such as metal spray coatings, weld overlays, or claddings with high CRMs on the outer surface are applied onto the base material of carbon steel as shown in Fig. 19.12. Table 19.3 shows examples of the application of various CRCs to WTE boilers. 

Chromium carbide is important in powder preparations designed for thermal spray applications of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing rods and electrodes for weld-applied overlays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Cr3C2 are listed in Table 2. 

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