2016 Volume 102 Issue 6 Pages 330-339
This study presents investigations of micro-crack initiation in pearlite microstructure as a trigger of unstable cleavage crack propagation in ferrite-pearlite steel. In order to clarify the micro-crack initiation mechanism, a trace analysis was conducted to compare the direction of crack surface with those of cleavage and slip planes of ferrite phase in pearlite structure. It was found that any directions of crack surface were not coincident with those of {100} planes. On the other hand, all of the directions of crack surfaces showed good agreement with those of {110} planes. The result showed a possibility that micro-cracks in pearlite structure are formed by shear fracture on slip planes of the ferrite phase. The condition of unstable propagation from a micro-crack in pearlite structure into a neighbor ferrite grain was investigated. Effective surface energy was estimated by crack length obtained by SEM observation and local stress calculated by finite element analysis. The result showed the estimated effective surface energy is larger than that of cleavage crack propagation across boundary between ferrite grains. A probability of micro-crack initiation in pearlite structure was quantified by measurement of micro-cracks in steels having various ferrite-pearlite microstructures and finite element analysis. As the result, the probability of micro-crack initiation could be effectively estimated as a function of only the equivalent plastic strain, independently from temperature, volume fraction of pearlite and loading condition.
