Crystallographic orientation analysis of cleavage facets adjacent to a fracture trigger in low carbon steel

摘要

The microstructure and crystallographic orientation under a cleavage crack trigger point, which was detected on a fracture toughness specimen of low carbon steel, were investigated. SEM fractographs of an etched cleavage facet reveal that a flat cleavage facet between facet ridges spreading from the trigger point is divided by a grain boundary between ferrite and pearlite. Even a different phase boundary does not occasionally create any steps and ridges on the cleavage facet: it suggests this phase boundary is not an obstacle to cleavage cracking. Electron diffraction analysis for the thin foil sample milled out from this phase boundary demonstrates that the crystallographic orientation of the ferrite grain is consistent with that of the ferrite in the adjacent pearlite block. It is strictly examined that the ferrite/pearlite boundary does not act as a local resistance to cleavage crack growth when the crystallographic orientation of the ferrite in the pearlite block is aligned with an adjacent ferrite grain.