WELDING RESEARCH ple a specimen for transmission electron microscopy (TEM) observation of the eutectic structures. Selected area electron diffraction (SAED) was carried out to identify the types of eutectic constituents. Hot ductility tests were conducted on a Gleeble-3500 thermal simulator, and protected by an argon atmosphere from oxidation of the fracture appearance. The rod specimens for hot ductility tests were 121.5 mm in length and 10 mm in diameter. The peak temperatures were 1350ºC (labeled 1) and 1250ºC (labeled 2), and the displacement load was 0.5 mm with a velocity of 0.1 mm/s at peak temperatures. The fracture appearance of ruptured specimens was observed by SEM. Results Observation of the Laves Phase in Virgin FB2 Steel In virgin FB2 steel, some micronsized particles were found, as shown in Fig. 1. All the particles were distributed randomly and inhomogeneously, and most parts of the matrix were free of the particles. Based on our experience, the particles could only be found when the entire specimen was traversed by SEM with about 1000 magnification. Most of the particles were located within austenitic grains and were much larger than conventional precipitates, such as M23C6 or the MX phase. The results of energy-dispersive x-ray (EDX) show the particles were enriched in boron, oxygen, molybdenum, vanadium, and chromium compared with the nominal chemical composition of the FB2 steel listed in Table 1. The main alloy elements in the particles were molybdenum, chromium, and ferrite, suggesting they may be some kind of intermetallic compound. In addition, the mole ratio of (Fe + Cr) and Mo in the particles is nearly 2. All of the characteristics mentioned were A A B B C D E F Table 1 — Chemical Composition of FB2 Steel (wt%) C Si Mn Ni Cr Mo V Nb N Co B Fe 0.13 0.05 0.40 0.15 9.3 1.5 0.2 0.05 0.02 1.0 0.01 Balance 258-s WELDING JOURNAL / JULY 2016, VOL. 95 Fig. 1 — A — The Laves phase in virgin FB2 steel observed by SEM. B — Chemical composition of the Laves phase obtained by EDX. Fig. 2 — Microstructures observed by SEM in FB2 steel after thermal simulations: A — Tp = 1150˚C; B — Tp = 1200˚C; C and D — Tp = 1250˚C; E and F — Tp = 1350˚C.
Welding Journal | July 2016
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