consolidated using a gas tungsten arc (GTA) button melting process. All melting was conducted under an argon atmosphere, so the introduction of impurity elements (oxygen, nitrogen) was unlikely. Each button was remelted three times to ensure homogeneity. The buttons melted using this procedure showed little evidence of oxidation. The nominal composition of Alloy 690 and resulting compositions after the Nb and Mo additions are shown in Table 1. Note that only the major elements are listed in this table. The compositions of a select number of buttons were determined using SEM/EDS and showed that the calculated compositions in Table 1 were a good approximation of the actual compositions. Computational Modeling The ThermoCalc™ Scheil module (Ref. 37) was used to do the following: 1) determine an approximate starting point for this study in terms of the levels of niobium, 2) calculate mass fraction values of eutectic phase for each composition, and 3) calculate the average niobium and molybdenum levels at the end of the eutectic reaction. The TTNi7 database, including liquid, FCC, NbC, and Laves phases, were used for the calculation. The Scheil simulations were run using two values of fraction solidified as an end point for the mass fraction calculations, namely 0.95 and 0.98, in order to determine how the end point affected the calculations relative to solidification temperature range and fraction eutectic. These values of fraction solidified were selected based on the range that is commonly used by other investigators for determining solidification temperature range and/or fraction eutectic (Refs. 38–42). The mass fractions were determined by plotting the solid phase fractions as a WELDING RESEARCH 232-s WELDING JOURNAL / JULY 2016, VOL. 95 A B C Fig. 8 — SEM micrographs depicting crack backfilling in cast pins at cracking threshold. A — 4Nb0Mo; B — 6Nb0Mo; C — 8Nb0Mo. Fig. 7 — Optical micrograph of a network of backfilling and some open cracks in a 2Nb4Mo pin that exhibited surface cracking. Table 2 — Calculated Solidification Data for Fe30Cr Alloys with Nb and Mo Additions Fraction Solid Fraction Solid 0.95 0.98 Measured Alloy kNb kMo Volume Final Wt. Final Wt. Mass STR Mass STR Fraction Fraction Fraction Fraction (C) Fraction (C) Eutectic Nb Mo Eutectic Eutectic Alloy 690 0.16 – – 65 0.0192 108 0.003 ± 0.001 – – 2Nb0Mo 0.17 – – 215 0.0255 239.5 0.014 ± 0.006 0.201 – 2Nb2Mo 0.17 0.66 – 225.5 0.0268 254 – 0.191 0.0656 2Nb4Mo 0.17 0.68 – 237.5 0.0287 264.5 – 0.183 0.118 4Nb0Mo 0.18 – 0.0659 212.3 0.0959 221.9 0.075 ± 0.01 0.217 – 4Nb2Mo 0.18 0.67 0.0686 220.8 0.0986 237 – 0.206 0.0700 4Nb4Mo 0.18 0.69 0.0727 228 0.103 247.8 – 0.199 0.124 6Nb0Mo 0.19 – 0.152 191 0.182 199.5 0.144 ± 0.03 0.223 – 6Nb2Mo 0.19 0.68 0.156 201 0.186 216.3 – 0.212 0.0736 6Nb4Mo 0.19 0.69 0.160 207.8 0.190 226.5 – 0.204 0.129 8Nb0Mo 0.20 – 0.253 168.3 0.283 176.3 0.239 ± 0.018 0.227 –
Welding Journal | July 2016
To see the actual publication please follow the link above