Laves phase precipitation behavior in the simulated fine-grained heat-affected zone of creep strength enhanced ferritic steel P92 and its role in creep void nucleation and growth
The fine-grained heat-affected zone (FGHAZ) specimens for P92 steel weld were produced by welding thermal simulator, then the simulated specimens were aged at 923 K for 0¿5000 h. The Laves phase parameters of FGHAZ were measured using SEM-back-scattered electron, and a two-dimensional (2D) precipitate/matrix model was used to calculate the stress-strain concentration at Laves phase/matrix interface by ANSYS. The results indicate that the maximum precipitated quantity of Laves phase in both FGHAZ and base metal (BM) is around 1.0%; however, the time to the saturated value is about 500 h shorter in the FGHAZ than in the BM, meanwhile. In fact, the differences in precipitation and coarsening of Laves phase between the FGHAZ and BM are closely correlated to the grain size. Furthermore, the matrix of FGHAZ is softened due to its polygonal sub-grains with low dislocation densities replacing the martensitic lath structure. The concentration of stress-strain at the Laves phase/matrix interface is larger in the FGHAZ than in BM, resulting from its softened matrix and coarser Laves phase, which contributes to the formation of creep voids on grain boundaries. Among these two factors, the softening of matrix has more significant influence on the formation of creep voids over the coarsening of Laves phase.