A B C D consistent with the Laves phase in CB2 steel reported by Kasl (Ref. 11). Therefore, it can be speculated that the particles observed in virgin FB2 steel were the Laves phase. Evolution Behavior of the Laves Phase during the Welding Thermal Cycle The results of thermal simulation show the Laves phase in specimens experiencing WELDING RESEARCH peak temperatures below 1200ºC (including 1200ºC) stayed unchanged, as shown in Fig. 2A and B, while in specimens experiencing higher peak temperatures (1250º and 1350ºC), some microstructures with morphologically distinct forms instead of the Laves phase particles were found, as shown in Fig. 2C–F. Based on the morphology, it can be speculated they were eutectic structures. It is noteworthy that in Fig. 2C–F, the eutectic microstructure observed by SEM is just one eutectic constituent and the other eutectic constituent located in the intervals of the eutectic microstructures might be electrolytic etched. In specimens experiencing a peak temperature of 1250ºC, the net-like eutectic microstructures (indicated by solid arrows) are surrounded by some separated particles (indicated by dashed arrows), and they all locate within austenitic grains, while in specimens experiencing a peak temperature of 1350ºC, the morphology of eutectic microstructures is more typical, and the size was much larger than that in Fig. 2C and D. The net-like eutectic microstructures (indicated by solid arrows) locate at triple grain junctions with “tails” along grain boundaries (indicated by dashed arrows). The chemical composition of the eutectic microstructures listed in Fig. 3 shows the content of Mo was lower than that in the Laves phase, and the contents of other elements were similar to those of the Laves phase. Based on the differences in morphology and chemical composition, it can be speculated that the eutectic microstructure was a new phase rather than the Laves phase. The Results of Hot Ductility Tests In the hot ductility tests, specimen 1 (Tp = 1350ºC) ruptured without necking, and the peak force was 16,000 N. Specimen 2 (Tp = 1250ºC) did not rupture with a slight necking of about 0.4 mm. The result shows that the zero ductility temperature (ZDT) of FB2 steel is between 1250º and 1350ºC. The fracture appearance of specimen 1 presents a typical liquefied morphology, as shown in Fig. 4A. In Fig. 4B, some microstructures exist on JULY 2016 / WELDING JOURNAL 259-s Fig. 3 — The chemical composition of eutectic microstructures obtained by EDX. Fig. 4 — The fracture appearance of a ruptured specimen in hot ductility tests: A — Macroscopic fracture appearance; B and C — microscopic fracture appearance; D — chemical composition of eutectic microstructures in B and C obtained by EDX.
Welding Journal | July 2016
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