Weld-overlay coatings

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. 

Process Approximate deposh tfakkness (min) nwi DeposMon rate, kg/h IMlntioD silisle l er, % lypkaluses 

Oxyacetylene (OAW) 1.5 1 1-5 Small area deposits on light sections 

Shielded metal arc (SMAW) 3 1-4 15-30 Multilayers on heavier sections 

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Figure 6-18. Example of the protection afforded to T-22 by a weld-overlay coating of Fc3Al after 50 h at 450-460 °C in a simulated coal-fired boiler flame impingement test (Tortorelli et al., 1998).

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. 

Erosion resistance of materials is very dependent on the erosion conditions, the effects of which are dominated by a number of variables including particle size, shape, composition, and velocity angle of incidence and temperature. Unlike wear properties, the erosion rate of weld-overlay coatings generally increases with increasing hardness (Fig. 8). However, the erosion resistance of weld-overlay alloys depends on whether the... 

Thermal spraying is a generic term for a group of processes that apply a consumable in the form of a spray of finely divided molten or semi-molten droplets to produce a coating. A number of extensive reviews of the topic can be found in Ref 47 to 51. The characteristics that distinguish thermal spray processes from weld-overlay coatings are indicated as follows (Ref 51) ... 

Substrate adhesion, or bond strength, is dependent on die materials and dieir prop es and generally is characterized as a mechanical bond between the coating and the substrate, unlike the metallurgical bond found in weld-overlay coatings. 

Table 14 Compares the characteristics of various weld overlay coatings...

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