Observation and Quantification of Crack Nucleation in Ferrite-Cementite Steel

Abstract

序論Kazuki Shibanuma, Shuji aihara and Shigeru OhtSuka Synopsis : 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.

Prediction model on cleavage fracture initiation in steels having ferrite–cementite microstructures – Part I: Model presentation

“…As the fracture condition of Stage I, the probability of the nucleation of a crack at cementite particle was employed. It was quantified based on the experiment and finite element analysis in the author's previous study [21]. As the fracture condition of Stage II, we employed a local fracture stress criterion formulated by Petch [3].…”

“…Some experimental investigations thus were conducted to understand the mechanism of the crack nucleation in the past [18][19][20]. We employ a probability of crack nucleation at cementite particle as the condition of Stage I, which was quantified by the author's recent study [21]. The quantification was based on the experiment and finite element analysis.…”

“…Maximum principal stress in cementite particle and surrounding ferrite matrix, [r Y = 500 MPa, r r = 0.0[21].…”

“…Strain and stress distributions obtained by finite element analysis in the circumferential notched specimen, 10LM, R15, À80°C[21]. SEM image of cracked cementite particles[21].The relationship described in Eq.…”

“…SEM image of cracked cementite particles[21].The relationship described in Eq. (3) is shown by the red 1 line inFig.…”