Observation and Quantification of Crack Nucleation in Ferrite-Cementite Steel

Abstract

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 a SEM observation and an image analysis. Tensile tests using circumferential notched round bar specimens were conducted. A trace analysis of cleavage surface of cementite using the EBSD analysis indicated a possibility that the cleavage surface coincides with (010) plane. Distributions of cementite particle crack lengths 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.

Effects of States of Carbon and Solute Nitrogen on Toughness of Ferritic Steel

“…After analyzing the crystal orientation of cementite at several locations, we inferred that these surface defects are defects within the cementite because the cementite has almost the same crystal orientation and can be considered as a single crystal. Although it is unclear whether these face defects were caused by impact forces or small-angle sub-grain boundary formation associated with cementite growth, the results are consistent with those of Shibanuma et al 24) in the sense that subcracks in cementite occur mainly in the θ -Fe 3 C (010) plane as a cleavage fracture surface of cementite.…”

3D Observation of Micro-cracks as Cleavage Fracture Initiation Site in Ferrite-pearlite Steel

“…3,[24][25][26] As a result of application of the model to the fracture toughness test of the steels having various size of microstructures, the prediction results showed good accuracy with the actual experimental ones. The model was limited in an application to the steels having simple ferrite-cementite microstructures, so that the applicability of the model particularly to the steels actually used is a new issue to be solved in the future.…”

Investigation of Micro-crack Initiation as a Trigger of Cleavage Fracture in Ferrite-pearlite Steels

“…The specimen configuration is the same as those in the author's previous study 16) as shown in Fig. 13.…”