Effects of local-brittle-zone (LBZ) microstructures on crack initiation and propagation in three Mo-added high-strength low-alloy (HSLA) steels

Seok Gyu Lee, Bohee Kim, Seok S Sohn, Woo Gyeom Kim, Kyung Keun Um, Sunghak Lee

Research output: Contribution to journalArticle

1 Citation (Scopus)


Fracture toughness of intercritically-reheated coarse-grained HAZ (ICRCGHAZ), i.e., local brittle zone, is severely reduced at low temperatures by the presence of undesirable microstructural features such as coarsened prior austenite grains, bainitic microstructures including bainitic ferrite (BF), granular bainite (GB), and acicular ferrite (AF), and martensite-austenite constituent (MA). In this study, effects of the microstructural features on critical CTOD and crack initiation and propagation process were investigated in the ICRCGHAZ of three Mo-added high-strength low-alloy (HSLA) steels. The Mo addition caused the increased volume fractions of GB, BF, and MA and the resultant deterioration of critical CTOD. According to the observation of cracking process, most of hard MAs, particularly MAs located along prior austenite grain boundaries (PAGBs), provided initiation sites of {100} cleavage cracks. In addition, a large hardness difference between PAGB-MA and matrix led to an easy MA/matrix interfacial separation and an increase in crack initiation sites, thereby resulting in very low critical CTOD in the high-Mo-containing steel having the highest fraction of MA. The detailed microscopic observation of cracking process at MAs and macroscopic MA/matrix hardness difference provide plausible ideas for reliably evaluating critical CTOD and for designing HSLA steels in consideration of ICRCGHAZ microstructures.

Original languageEnglish
Pages (from-to)125-133
Number of pages9
JournalMaterials Science and Engineering A
Publication statusPublished - 2019 Jul 8



  • Crack-tip opening displacement (CTOD)
  • High-strength low-alloy (HSLA) steel
  • Intercritically-reheated coarse-grained heat-affected zone (ICRCGHAZ)
  • Local brittle zone (LBZ)
  • Martensite-austenite (MA) constituent

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

Cite this