Stabilization of retained austenite by the two-step intercritical heat treatment and its effect on the toughness of a low alloyed steel

• Published in: Materials in engineering Vol. 59; pp. 193 - 198
• Main Authors: Xie, Z.J., Yuan, S.F., Zhou, W.H., Yang, J.R., Guo, H., Shang, C.J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2014
• Subjects: Austenite; Austenite reverted transformation; Ferrite; Fracture toughness; Heat treatment; Intercritical partition; Low alloy steels; Low temperature toughness; Retained austenite; Scanning electron microscopy; Stabilization; Structural steels; Austenite reverted transformation; Retained austenite; Low temperature toughness; Intercritical partition
• ISSN: 0261-3069
• DOI: 10.1016/j.matdes.2014.02.035

•Fine film-like stable retained austenite was obtained in a low alloyed steel.•Stabilization of retained austenite was studied.•Intercritical partition of C, Mn and Ni was revealed by TEM study.•Effect of retained austenite on toughness was investigated.•Fracture process of the steel was studied by instrument impact test. Fine film-like stable retained austenite was obtained in a Fe–0.08C–0.5Si–2.4Mn–0.5Ni in weight percent (wt.%) steel by the two-step intercritical heat treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite, which is called as “reverted structure” and “un-reverted structure”, respectively. The second step of intercritical tempering is beneficial for producing film-like stable reverted austenite along the reverted structure. The stabilization of retained austenite was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry and X-ray diffraction (XRD) analysis. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Stable retained austenite is not only beneficial for high ductility, but also for low temperature toughness by restricting brittle fracture. With 10% (volume fraction) of retained austenite in the steel, high low temperature toughness with average Charpy impact energy of 65J at −80°C was obtained.