PSI - Issue 53

Daniel R. Galán-Rivera et al. / Procedia Structural Integrity 53 (2024) 407–415 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.4. Material 4 Material 4 is the most widely used of the wear resistant cobalt-based alloys and exhibits good all-round performance. It is regarded as the industry standard for general-purpose wear resistance applications, has excellent resistance to many forms of mechanical and chemical degradation over a wide temperature range, and retains a reasonable level of hardness up to 500°C (930°F). It also has good resistance to impact and cavitation erosion. WD Co6 is ideally suited to a variety of hardfacing processes and can be turned with carbide tooling (Fig. 7). High hardness differences obtained between the base metal and the coating material often resulting in weld cracking after cooling process (Table 2). At first stages of repairing (first deposited layers), presence of cracking appeared in both, base metal and overlay material. To address the problem of cracking, a preheat treatment in the temperature range between 100 and 120 ° C were carried out in the tramway rail. Also, a strict control in the cooling cycles between subsequent layers was applied. Non-metallic inclusions also appeared when present powder material is injected. These problems have been successfully resolved by decreasing the powder flow deposition and change de orientation of the nozzle. As a rule, this metal powder works properly by increasing the powder flow injection, while maintaining the laser power at similar values than other powders. Macrographic preparation of cross sections of the deposited layer revealed the presence of cracks in the heat affected zone (HAZ), at both ends of the coating. A new section of coating obtained in a different region of the sample, also show presence of a similar defect. Apparently, the defects (cracks) are continuous throughout the entire coating. The results of the hardness map show a hardness that doubles the value in the coating material and in the HAZ respect the base material. Along with high differences existing in the chemical compositions of materials, contribute to the appearance of cracking caused by high thermal gradient produced during the cooling stages from repairing (high) temperatures.

Fig. 7. Macrograph and detail of the tram rail repaired with “Material 4 ”

3.5. Material 5 This material was designed as a corrosion resistant CoCr alloy, and rapidly found application as a biocompatible hip implant and denture alloy. This material consists of CoCrMo alloy matrix containing dispersed hard carbides which strengthen the alloy and increase its hardness, but also decrease the ductility properties. The type, shape, size, and distribution of the carbides is strongly influenced by the processing history of the alloy, and for this reason the mechanical properties of this powder are very dependent upon the manufacturing route and any subsequent heat treatments. Due to the low volume fraction of carbides, the Co-based alloy matrix dominates the wear and corrosion properties. Material 5 has excellent cavitation, galling and metal-to-metal sliding wear resistance, but is not recommended for severe hard particle abrasion.