A B Fig. 10 — Mechanical properties of the deposited metals: A — Tensile tests; B — impact tests at 77 K. In addition, the impact value at 77 K of the CaF2-CaO-SiO2 type was above 80 J, which was distinctly larger than that of the TiO2-SiO2-SrO type, the latter was below 70 J in the impact value — Fig. 9B. Such a difference in the cryogenic impact properties were believed to be contributed from the impurities, e.g., S, P, and O, in the deposited metals. Since the S, P, and O were in relatively lower levels in the deposited metal of the CaF2- CaO-SiO2 type than in that of the TiO2-SiO2-SrO type (as shown in Table 3), the impact value at 77 K of the former was reasonably larger than that of the latter. Although the impurities are generally in very low levels, they are detrimental to the impact properties. For example, the S can easily form a low-melting-point eutectic Ni + NiS (melting point about 917 K); and the P can easily form a low-melting-point eutectic Ni3P + Ni (melting point about 1153 K). These low-melting-point eutectic phases tended to distribute along grain boundaries, which deteriorate the grain boundary strength and increase cracking susceptibility (Refs. 12, 13). The weak grain boundaries should more easily initiate and propagate cracks, leading to the lower cryogenic impact values. The S and O could also be trapped in the deposited metal in the form of sulfides and oxides due to fast solidification during welding. The higher contents of S and O led to the larger volume fractions and number densities of the inclusions. It was determined that these inclusions were the sulfides and/or oxides of Al, Ti, and Mn as indicated — Fig. 8. Although there was no direct evidence to show the effect of these inclusions in this study, it was believed that they were detrimental to the toughness due to their easily debonding from the metal matrix (Refs. 36–39). Because the TiO2-SiO2-SrO type has higher S and O (Table 3), it is reasonable that the deposited metal of this type of electrode exhibits lower cryogenic impact values — Fig. 10B. Although both deposited metals exhibited cryogenic impact values in different levels, their fractographies all reveal the typical dimple fracture mode — Fig. 11. The plastic fracture mode at 77 K is generally favorable to the cryogenic application. Since the two electrodes (all are in accordance with the ENiCrFe-9 Standard) investigated were developed for welding 9% Ni steel for construction of the LNG tanks, the matching in cryogenic mechanical properties between the weld metal and the 9% Ni steel should be considered. Nowadays, the Charpy impact values at 77 K of the 9% Ni steel are almost above 200 J (Refs. 40–42). The heat-affected zones of the 9% Ni steel can generally reach above 100 J (Refs. 41, 42). However, the deposited metals of the TiO2- SiO2-SrO type electrodes exhibited only 66–70 J of the cryogenic impact WELDING RESEARCH Table 6 — Chemical Compositions of the Intergranular and Grain Boundary Precipitates as Marked on Fig. 9 Analyzed by EDS on SEM (wt%) No. C O Ni Cr Fe Mo Mn Nb Ti Al 1 17.11 – 14.13 6.54 1.79 4.64 1.39 54.4 – – 2 17.73 – 5.70 3.09 1.40 5.39 0.59 66.10 – – 3 11.08 6.99 8.50 5.68 1.14 – 1.19 57.08 8.34 – 4 11.42 6.51 3.05 4.77 – – – 63.62 10.63 – 5 13.70 – 7.20 7.98 2.43 7.10 1.31 60.28 – – 6 17.51 – 12.50 8.78 3.23 4.23 2.42 51.33 – – 7 12.75 5.82 8.46 5.83 – – 1.81 54.88 10.45 – 8 8.43 10.70 8.33 6.95 2.12 – 2.73 51.79 6.50 2.45 9 18.23 2.10 5.73 5.73 – 7.70 1.64 58.87 – – 10 9.74 7.59 10.67 5.92 2.02 6.93 3.19 50.41 3.53 – 476-s WELDING JOURNAL / DECEMBER 2016, VOL. 95
Welding Journal | December 2016
To see the actual publication please follow the link above