2013 Volume 99 Issue 9 Pages 582-591
It is known that a cracking of brittle phase such as cementite works as a trigger of cleavage fracture initiation. This study shows a microscopic observation of cracked cementite and its quantitation of the cracking nucleation in ferrite-cementite steels. Seven steels with various sizes of microstructures are produced by laboratory scale vacuum melting and rolling. The cementite particle thickness was measured by SEM observation and image analysis. Tensile tests using circumferential notched round bar specimens were conducted. A trace analysis was carried out to identify crystal plane of cleavage surface of cementite using EBSD analysis. The results indicated a possibility that the cleavage surface is formed on (010)-plane. Distributions of length of cementite particle crack were measured for various strain and stress conditions. In order to understand a microscopic internal stress of cementite particle, a finite element analysis was carried out. An estimation formula of internal stress of the cementite particle from macroscopic stress and strain was developed based on the numerical results. A nucleation of cementite cracking should be quantitated based on a stochastic framework because of its uncertainties such as distribution, shape, orientation and so on. The measured distributions of cementite particle thickness and crack length were approximated by introducing a distribution function considering upper limit. Probability of nucleation of cementite cracking was formulated as a function of cementite particle thickness and macroscopic stress and strain, based on the approximated distribution function and the estimation formula of internal stress of the cementite particle.