Other Aspects of the Impact Fracture Toughness-Microstructure Relationship in Nano-bainitic Steels

Yoozbashi M. N., Zolfaghari R., Yazdani S., TEKELİ S.

Journal of Materials Engineering and Performance, 2024 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Publication Date: 2024
  • Doi Number: 10.1007/s11665-023-08971-6
  • Journal Name: Journal of Materials Engineering and Performance
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, PASCAL, Aerospace Database, Applied Science & Technology Source, Aquatic Science & Fisheries Abstracts (ASFA), Chemical Abstracts Core, Communication Abstracts, Compendex, Computer & Applied Sciences, INSPEC, Metadex, Civil Engineering Abstracts
  • Keywords: fracture surface analysis, impact fracture toughness, isothermal transformation, microstructure relationship, nano-bainitic steels
  • Gazi University Affiliated: Yes


The relationship between impact fracture toughness and the microstructure in a composition of nano-bainitic steel was investigated in this study. To achieve bainitic microstructures, samples were austenitized at 950 °C for 30 minutes and then austempered at 200, 250, and 300 °C for different times for completion of transformation. Mechanical properties were evaluated using hardness measurements and Charpy impact tests. Microstructure and fracture surfaces were analyzed using optical microscopy, image analysis software, thermodynamic modeling software, X-Ray diffraction (XRD) analysis, and scanning electron microscopy. The results showed that the comparison of the retained austenite volume fraction obtained through image analysis agreed well with the results from XRD pattern analysis. The values of bainitic ferrite calculated by the thermodynamic model also agreed well with the results obtained through the XRD method. The results also indicated that impact toughness was significantly influenced by the amount and mechanical stability of retained austenite, while the effects of martensite and bainite plate thickness were less significant. Fracture surface analysis further revealed a transition in fracture mode from numerous dimples and some cleavage to a brittle fracture mode predominantly dominated by cleavage facets as the transformation temperature decreased. In conclusion, the impact fracture mode can be described as quasi-cleavage.