Microstructure and Phase Transformations in High-Strength Bainitic Forging Steel

ERİŞİR E., Ayhan I. I., Guney C., Alan E., Durger N. B., Un S.

JOURNAL OF MATERIALS ENGINEERING AND PERFORMANCE, vol.30, no.5, pp.3458-3467, 2021 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 30 Issue: 5
  • Publication Date: 2021
  • Doi Number: 10.1007/s11665-021-05689-1
  • 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
  • Page Numbers: pp.3458-3467
  • Keywords: carbide-free bainite, forging steel, phase transformations, prior austenite grain size
  • Kocaeli University Affiliated: Yes

Abstract

In this work, the microstructural evolution and the phase transformations on a 0.22 wt.% carbon bainitic forging steel are investigated using a deformation dilatometer. The microstructure evolution was characterized using light microscope (LM), scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and electron backscatter diffraction (EBSD). It was tried to predict the phase transformations via a continuous cooling transformation (CCT) diagram calculated by JMatPro using the results of prior austenite grain size measurement. In addition, critical temperatures were calculated using Thermo-Calc, JMatPro, and empirical formulations. Based on the dilatometric tests, hardness, and microstructural investigation, a deformation-CCT diagram was plotted. It was found that a bainitic microstructure can be obtained at cooling rates from 0.15 to 8 K/s. The microstructures varied from a mixture of bainite and blocky martensite/austenite islands at lower cooling rates to a bainitic ferrite with martensite at higher cooling rates. Finally, an industrially hot-forged sample cooled at 1 K/s was investigated. The microstructural evolution showed a microstructure of carbide-free bainitic ferrite along with film and island of retained austenite.