Optical Micrographs Figure 13 shows the microstructure of single-bead cladding No. 47 (74.8 J/mm) and No. 50 107 J/mm). As shown by the transverse optical micrographs in Fig. 13A and B, the large tungsten carbide particles are those from the filler metal that were not melted completely during welding. Dendrites of Ni-rich primary solidification phase -Ni are visible in the matrix between large tungsten carbide particles. The secondary dendrite arm spacing is larger with cladding No. 50 than No. 47. It is well known that the higher the heat input is, the slower the cooling rate during welding (Ref. 19). It is also well known that the slower the cooling rate is, the more time is available for dendrite arms to coarsen and increase the secondary dendrite arm spacing (Refs. 19, 20). So, the higher the heat input is, the larger the secondary dendrite arm spacing can be expected. Thus, the higher heat input used for depositing cladding No. 50 can be expected to result in larger dendrite arm spacing. Local Composition Measurements by HighResolution EPMA Figure 14 shows a big tungsten carbide particle in cladding No. 9 (Fig. 11), and the various phases in it identified by EPMA. As mentioned previously, with the high-resolution EPMA used in the present study, the diameter of the electron beam was only 80–100 nm in diameter and the volume below the sample surface affected by the beam was 250 nm in diameter. WELDING RESEARCH The original electron image taken during EPMA has been replaced by a SEM image taken after EPMA. The SEM image, which is shown in Fig. 14, has a higher contrast, which is needed to show the different phases in the particles more clearly. WC exists along the edge of the big particle as shown by the compositions at Points 1, 2, and 3. The presence of WC along the edge is consistent with the observation of a WC “shell” around big tungsten carbide particles by Vespa et al. (Ref. 3), who used the same welding wire used in the present study, that is, PolyTung NiBWC. It is also consistent with the observation of a “halo” around big tungsten carbide particles by Choi et al. (Ref. 2), although they used a different welding wire, that is, an Arctec Tungcore FCS cored wire. Figure 14 shows that inside the big tungsten carbide particle WC exists at Points 4 and 5, and W2C exists at Points 10, 11, and 12. The presence of both WC and W2C inside tungsten carbide particles was also reported by Vespa et al. (Ref. 3) and Choi et al. (Ref. 2). According to Fig. 14, W3C2 also exists inside the big tungsten carbide particle, which was not reported by Vespa et al. (Ref. 3) or Choi et al. (Ref. 2). Further identification, e.g., by x-ray diffraction (XRD), is needed to confirm the presence of W3C2. It is interesting to note that Huang et al. (Ref. 21) showed XRD peaks corresponding to -Ni, WC, W2C, and WC1-x. If x = 1/3, WC1-x becomes WC2/3 or W3C2. Figure 15 shows the composition measurements in an interdendritic area in cladding No. 9. At Points 1, 2, and 3, the -Ni dendrites are Ni-rich as expected. However, they also contain significant amounts of W and C. The interdendritic eutectic is composite-like, consisting of a small lighter contrastphase like -Ni and a small darker-contrast phase. The darker-contrast phase at Points 4 and 5 appears to be Ni3B, but it also contains a significant amount of C. The lighter-contrast phase at Points 6 and 7 are similar to -Ni in composition. Figure 16 shows the x-ray diffraction pattern obtained by directing the x-ray over an area of 0.1 mm diameter on the transverse cross section of cladding No. 9 (Fig. 11). The diffrac- DECEMBER 2016 / WELDING JOURNAL 459-s A C B D E F Fig. 10 — Singlelayer square cladding No. 11 made by GMAWCSC. A — Pattern of motion of workpiece relative to welding gun; B — top view; C— transverse cross section; D, E, F — optical micrographs. Dilution = 4.6%.
Welding Journal | December 2016
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