Fig. 1 — Four curves that compare cracking susceptibility or “sensitivity” as a function of alloying content in Al alloys (Ref. 36). some evidence that it reduces susceptibility to solidification cracking in fully austenitic stainless steels (Refs. 4, 5, 14). It has been reported that in some materials, a large fraction of liquid of eutectic composition present at the end of solidification may promote a backfilling or “crack healing” effect (Refs. 7, 15–17). Backfilling is defined as a partial or entire healing of solidification cracks due to the flow of liquid metal into newly formed cracks or voids (Ref. 18). Both volume fraction eutectic and liquid phase characteristics such as viscosity, surface tension, and wettability of eutectic films determine whether the liquid will “heal” a forming crack (Ref. 19). It is known that some Nb-bearing, Ni-based alloys, such as Alloy 625, are resistant to solidification cracking, presumably due to a backfilling (crack healing) effect (Refs. 5, 7, 9, 20–23). The amount of eutectic constituent necessary to promote crack healing is likely alloy dependent, but in one study of Alloy 625 weld metal, an increase in the fraction of Nb-rich eutectic from 7 to 11.5 vol-% resulted in a significant decrease in cracking susceptibility (Ref. 20). Cracking susceptibility curves have been developed for Al alloys (Fig. 1) that illustrate a reduction in cracking susceptibility at high alloy content due to the crack healing effect. All of these binary (or ternary) systems exhibit a eutectic reaction. As alloy content increases, cracking susceptibility initially increases to a peak value and then decreases, presumably due to a crack healing effect. The variation in the location of the peak cracking susceptibility in these different Al-based systems is associated with the fraction of eutectic liquid present at the end of solidification at a given composition. At the peak, the liquid completely wets the solidification grain boundary and promotes cracking, while at composi- WELDING RESEARCH 230-s WELDING JOURNAL / JULY 2016, VOL. 95 Fig. 2 — Percentage of circumferential cracking for the Alloy 690 base composition and the Nbbearing alloys. Each point and error bar represents the average of at least 4 acceptable pins. The red circles indicate the critical cracking threshold or longest pin at which no cracking was observed. Table 1 — Calculated Composition (wt%) of Alloys Evaluated in this Study Composition Ni Cr Fe Nb Mo C Alloy 690 60.51 29.45 9.13 – – 0.0290 2Nb0Mo 59.29 28.86 8.95 2.00 – 0.0284 2Nb2Mo 58.09 28.27 8.76 2.00 2.00 0.0278 2Nb4Mo 56.87 27.68 8.58 2.00 4.00 0.0273 4Nb0Mo 58.09 28.27 8.76 4.00 – 0.0278 4Nb2Mo 56.87 27.68 8.58 4.00 2.00 0.0273 4Nb4Mo 55.66 27.09 8.40 4.00 4.00 0.0267 6Nb0Mo 56.87 27.68 8.58 6.00 – 0.0273 6Nb2Mo 55.66 27.09 8.40 6.00 2.00 0.0267 6Nb4Mo 54.47 26.51 8.22 6.00 4.00 0.0261 8Nb0Mo 55.66 27.09 8.40 8.00 – 0.0267
Welding Journal | July 2016
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