SEM micrograph of the banded structure in the weld nugget on the advancing side. (b and c) SEM and TEM micrographs of the corroded region inside the banded structure containing sigma phase precipitates. (d and e) SEM and TEM micrographs of the un-corroded region outside of the banded structure.
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Sigma phase precipitates were found not only in the banded structure located in the advancing side of the weld nugget, but also in lower amount in the weld center. Fig. 6 shows the TEM micrographs of the sigma phase precipitates in the weld nugget center at the welding speed of 80 mm/min. It can be seen that the sigma phase is formed both in the austenite and delta ferrite matrix. The width of sigma phase particles is less than 200 nm. The reasons for sigma phase formation will be described in greater details below.