Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

The effect of pearlite on the X-ray diffraction peak reflected from ferrite phase in ferrite-pearlite steel was investigated using normalized carbon steels with different volume fraction of pearlite and a hypereutectoid steel with various pearlite lamellar spacing. The lattice strain in ferrite phase, which causes the broadening of X-ray diffraction peak, was increased in proportion to both of the volume fraction of pearlite and the inverse of pearlite lamellar spacing. As a result, the lattice strain in ferrite-pearlite steel can be simply formulated as functions of them. On the other hand, TEM observation reveals that pearlite has low-density dislocation in ferrite phase. This result suggests that the misfit between ferrite and cementite in pearlite generates the significant amount of elastic strain, which leads to the increasing in lattice strain. Therefore, the dislocation density must be overestimated in carbon steels with pearlite, if it is estimated from the experimental lattice strain directly.