Thermal Stability of Retained Austenite and Properties of A Multi-Phase Low Alloy Steel

• Published in: Metals (Basel ) 2018-10, Vol.8 (10), p.807
• Main Authors: Xie, Zhenjia, Xiong, Lin, Han, Gang, Wang, Xuelin, Shang, Chengjia
• Format: Article
• Language: English
• Subjects: low alloy steel; multi-phase microstructure; retained austenite; thermal stability
• ISSN: 2075-4701; 2075-4701
• DOI: 10.3390/met8100807

In this work, we elucidate the effects of tempering on the microstructure and properties in a low carbon low alloy steel, with particular emphasis on the thermal stability of retained austenite during high-temperature tempering at 500–700 °C for 1 h. Volume fraction of ~14% of retained austenite was obtained in the studied steel by two-step intercritical heat treatment. Results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicated that retained austenite had high thermal stability when tempering at 500 and 600 °C for 1 h. The volume fraction was ~11–12%, the length and width remained ~0.77 and 0.21 μm, and concentration of Mn and Ni in retained austenite remained ~6.2–6.6 and ~1.6 wt %, respectively. However, when tempering at 700 °C for 1 h, the volume fraction of retained austenite was decreased largely to ~8%. The underlying reason could be attributed to the growth of austenite during high-temperature holding, leading to a depletion of alloy contents and a decrease in stability. Moreover, for samples tempered at 700 °C for 1 h, retained austenite rapidly transformed into martensite at a strain of 2–10%, and a dramatic increase in work hardening was observed. This indicated that the mechanical stability of retained austenite decreased.