Microstructure of low alloyed steel close to the fracture surface

A transmission electron microscopy study of a tempered bainitic nuclear reactor pressure vessel steel was carried out. Cross-section thin foils from nickel electroplated Charpy V-notch specimens fractured in impact and quasi static loading at −30 °C were prepared in the ductile tearing and cleavage regions of the fracture surface. In the ductile tearing zone, the microstructure was very heterogeneous. Dislocation cells, shear bands, and fine heavily deformed subgrains were found. The deformation in the impact specimen was localized only in the vicinity of the fracture surface, where long thin subgrains situated along the fracture surface were often observed. In the quasi static three-point bend specimen, the strain localization was found also in deeper areas under the fracture surface. There were shear bands (bundles of long thin subgrains) mostly aligned in parallel to the fracture surface. Numerous areas of the shear band intersections (at ∼45° to the main shear band direction) were also observed. In the vicinity of cleavage zones of both types of specimens, no significant differences in the microstructure with respect to the bulk material were observed. Only an increased dislocation density was found.