(a) to (c) OM and (d) TEM analyses of the steel at 873 K. The isothermal holding periods are 1 h in (a), 10 h in (b), and 100 h in (c) and (d). The OMs in (a) to (c) illustrate Widmanstätten plates and pearlite nodules in the austenite. (d) The BF image shows a portion of a (C + α) Widmanstätten plate in the austenite matrix.