A B C D Fig. 7 — Schematic of the Laves phase evolution during welding, Tp = 1250˚C: A — Before heating; B — the interface between the Laves phase and (Fe) melted first during heating; C — the central part of the Laves phase melted subsequently; D — during cooling, the Chi phase and austenite precipitated from the liquid. Fig. 6A was sampled by FIB, and in the bright field image, the eutectic constituent observed by SEM can also be seen clearly, indicated by white arrows. Because the specimen was not etched, the other eutectic constituent in intervals can also be seen, indicated by black arrows. The four SAED patterns in Fig. 6B labeled 1–4 correspond to the areas in Fig. 6A labeled by 1–4, respectively. The results of SAED indicate that the eutectic constituent observed by SEM had a body-centered-cubic (BCC) structure, consistent with the Chi phase, and the lattice parameter of the eutectic constituent was 0.906 nm (see the appendix for the calculating process), close to that of the Chi phase. The other eutectic constituent also had a BCC structure, and the lattice parameter was 0.271 nm. It’s acknowledged that martensite has a body-centered-tetragonal (BCT) structure, of which the lattice parameter ‘a’ is equal to ‘b,’ but is not equal to ‘c’ because of the distortion during martensitic transformation. However, the lattice parameter ‘a’ is almost equal to ‘c,’ so the diffraction patterns of martensite are almost the same as those of BCC, such as ferrite. In addition, the lattice parameter of martensite was almost equal to that of ferrite at 0.2866 nm. Based on the structure and lattice parameter, the other eutectic constituent can be preliminarily judged as martensite. Considering that martensite was transformed from austenite, the eutectic reaction in cooling was most likely to be L → Chi + (Fe). Based on this analysis, the morphology difference of eutectic microstructures in Fig. 2C–F can be explained. As to the specimen experiencing a peak temperature of 1250ºC, it was mentioned in the constitutional liquation resulting from the eutectic reaction between the Laves phase and austenite section, the interface between the Laves particles and matrix dissolved first, as shown in Fig. 7B. The chemical composition of the liquid deviated from the eutectic composition, biased toward austenite, because the solute (Cr and Mo atoms) was diluted by the matrix. Subsequently, the rest of the Laves particles dissolved, as shown in Fig. 7C. Because there was no direct contact between the central part of the Laves phase and matrix, and because the mobility of Cr and Mo atoms was limited, the solute in the central part could not be diluted adequately like that at the interface. Thereby, the chemical composition of liquid in the central part was biased toward the Laves phase enriched in Mo and Cr. During cooling, austenite precipitated from liquid at the interface as the predominant phase and merged with the matrix, leaving a very small amount of Chi phase alone, which formed divorced eutectic. The divorced eutectic is indicated by dashed arrows in Fig. 2C and D. While in the central part, the amount of liquid was larger than that at the interface, so the Chi phase and (Fe) could precipitate alternatively, forming net-like symbiotic eutectic, labeled by solid arrows in Fig. 2C and D. The divorced and symbiotic eutectic is illustrated in Fig. 7D. In the case of the specimen cooled from 1350ºC, solute atoms diffused adequately in the liquid because of the higher temperature. At grain boundaries, the liquid film was thin and (Fe) precipitated from the liquid film and merged with the matrix, forming tail-like divorced eutectic, which is labeled by dashed arrows in Fig. 2E and F. The amount of liquid at triple grain junctions was larger, so (Fe) and the Chi phase could precipitate alternatively, forming symbiotic eutectic, which is labeled by solid arrows in Fig. 2E and F. The grain boundaries surrounded by the eutectic structures became “ghost boundaries,” of which the strength was impaired and crack initiation would prefer to take place at this site. Conclusions In the present work, constitutional liquation resulting from the eutectic reaction between the Laves phase and (Fe) was found, suggesting a liquation crack tendency in FB2 steel during welding. Then the origin and evolutionary behaviors of the Laves phase in the welding thermal cycle were analyzed. There are four conclusions obtained as follows: 1) The large particles of the Laves phase in virgin FB2 steel formed in the casting process, which was attributed to dendritic segregation. 2) The fast heating rate of welding led to constitutional liquation in areas where the temperature was above 1250ºC (including 1250ºC), and the eutectic reaction between the Laves phase and (Fe) should account for the constitutional liquation. 3) The areas where constitutional liquation occurred would act as crack initiation when some load was present at 1350ºC, leading to intergranular fracture. At 1250ºC, the limited mobility retarded liquid wetting grain boundaries, so hot plasticity was present. 4) The Chi phase was found as a eutectic constituent during cooling. The mobility of atoms and liquid influenced the final morphology of eutectic microstructures, and divorced eutectic as well as symbiotic eutectic were observed in specimens experiencing different peak temperatures. WELDING RESEARCH 262-s WELDING JOURNAL / JULY 2016, VOL. 95
Welding Journal | July 2016
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