Study of the microstructure of a weld bead P91 steel by transmission electron microscopy

P91 steel offers a good weldability, but the mechanical properties of the welded joints are found to be inferior compared to the base metal. The heat affected zone (HAZ) of these types of steels is ~ 4 mm making it difficult to extract carbon replicas and thin foil to study the microstructure generated in each sub-zone of the ZAC: coarse grained ZAC (CGZAC), fine grained ZAC (FGZAC) and intercritical ZAC (ICZAC).

Nev-ertheless, it was possible to extract replicas from each region to identify precipitates and using a focused ion beam (FIB-SEM), thin films could be extracted from a specific area to characterize the matrix microstructure. The precipitates present in each subzone of the ZAC were characterized as well as how the dissolution of M23C6 affects the matrix. At low peak temperatures and low heating / cooling rates such as those characterizing the fine grain zones (ICHAZ and FGHAZ) the M23C6 carbides were found to be partially dissolved and in contact with retained austenite.