ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Investigation of Micro-crack Initiation as a Trigger of Cleavage Fracture in Ferrite-pearlite Steels
Kazuki Shibanuma Yoshiki NemotoTakashi HiraideKatsuyuki SuzukiShuji Aihara
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2017 Volume 57 Issue 2 Pages 365-373

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Abstract

This study presents investigations of the micro-crack formation in pearlite microstructure as a trigger of unstable cleavage crack propagation in ferrite-pearlite steel. In order to clarify the micro-crack formation mechanism, a trace analysis was conducted to compare the direction of crack surface with those of cleavage and slip planes of ferrite in pearlite. It is 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 probability that the micro-cracks in pearlite are formed by the shear fracture on slip planes in ferrite. The condition of unstable propagation from a micro-crack in pearlite into a neighbor ferrite grain was investigated. Effective surface energy was estimated by the crack length obtained by SEM observation and the local stress calculated by finite element analysis. The result showed the estimated effective surface energy of a propagation from micro-crack in pearlite into ferrite matrix is larger than that of cleavage crack propagation across boundary between ferrite grains. A probability of the micro-crack formation in pearlite was quantified by the measurement of micro-cracks in steels having various ferrite-pearlite microstructures and finite element analysis. As a result, the probability of micro-crack formation could be effectively estimated as a function of only the equivalent plastic strain, independently from temperature, volume fraction of pearlite and loading condition.

EBSD images near micro-cracks together with {100} plane or {110} plane traces. Fullsize Image
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© 2017 by The Iron and Steel Institute of Japan
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