Study of Brittle Fracture Surfaces at Low-Temperature in Relation to Microstructures of Low Carbon Steels

Abstract

The brittle fracture surfaces of the ferrite-pearlite and the martensite structure in low carbon steels were investigated with a scanning electron microscope. The direct correspondence between the fracture surface and the microstructure was made, and the dependence of the fracture mode on grain size was studied from the view point of the crystallographic orientation relationship.
The fracture surface prepared by the Charpy impact test revealed the discontinous fracture process; it consisted of the repetition of nucleation and propagation of the microcrack, and the cleavage plane was determined as {100} by the orientation measurements of the facet pit and the fracture surface. These microcrack propagation units could be considered as the fracture facets and those in ferrite-pearlite structure were composed of some adjacent grains in which the crystallographic orientation differences about fracture surface {100} were very small. The fracture facet was generally larger than the ferrite grain size, but it did not correspond to the austenite grain directly.
The martensite structure, both as quenched and tempered, had the domains in which the crystallographic orientation of laths was uniform and the crack also nucleated and propagated in {100}.