WELDING RESEARCH more clearly. As shown, the fraction of WC is about 0.125. The effect of substituting Ni with Cr on the fraction of WC is shown as follows. Figure 20C and D shows the calculated results for 39.58 wt-% W, 0.34 wt Si, 0.67 wt-% B, 52.95 wt-% Ni, 1.08 wt-% C, 0.36 wt-% Fe, and 5 wt-% Cr or just Ni-39.58W-1.08C- 0.67B-0.36Fe-0.34Si-5Cr. In other words, 5 wt-% Ni is substituted with 5 wt-% Cr and the contents of other alloying elements such as C, B, Fe, and Si remain unchanged. As shown, WC still starts to form from the weld-pool liquid at about 1650°C. However, the formation of M6C carbide soon follows at about 1500°C. The amount of WC after solidification is only about 7.5 wt- %, instead of 12.5 wt-% as in the case without Cr. On the other hand, the amount of M6C is as high as 15 wt-%. With 10 wt-% of Ni substituted by 10 wt-% Cr, the composition of the weld-pool liquid becomes 39.58 wt-% W, 0.34 wt Si, 0.67 wt% B, 47.95 wt-% Ni, 1.08 wt-% C, 0.36 wt-% Fe, and 10 wt-% Cr or just Ni-39.58W-1.08C- 0.67B-0.36Fe-0.34Si-10Cr. As shown in Fig. 20E and F, WC no longer forms during solidification. Instead, M2C carbide forms at about 1630°C and M6C carbide about 1570°C. The resultant content of M6C carbide is as high as 30 wt%. These results suggest that Cr tends to form undesirable Cr-containing carbide M6C at the expense of WC. Liyanage et al. (Ref. 26) used three Ni-rich matrix alloys containing 0, 8.4, and 13.8 wt-% Cr in PTAW of Ni-WC cladding. It was shown that the WC volume fraction in the cladding was lower with than without Cr. The solidification paths of the three matrix alloys were calculated based on the Scheil solidification model. No fractions of solid phases were shown. It was pointed out that the solidification paths showed that the stability of WC is reduced when Cr is added, which reduces WC and promotes Cr-containing carbides. Landwehr (Ref. 27) also showed Cr addition increased WC dissolution. Ni-WC claddings were made by using a defocused laser-beam and adding Ni-WC powder to the weld pool. The cladding made with a powder containing 14 wt-% Cr showed significantly more WC dissolution than one made with a Cr-free powder. Although the DECEMBER 2016 / WELDING JOURNAL 463-s Fig. 16 — Xray diffraction pattern of cladding No. 9 (Fig. 11) identifying the presence of phases, including Ni, Ni0.09W, WC, W2C, WC1x and Ni3B. Fig. 17 — Tungstencarbon phase diagram (Ref. 22).
Welding Journal | December 2016
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