Effect of Heating Rate Before Tempering on Reversed Austenite in Fe-9Ni-C Alloy

The alloy was reheated to 580 °C for tempering at rates of 2, 5, 10, 20, and 40 °C/s, respectively, after quenching. The amount, distribution, and stability of reversed austenite were investigated by X-ray diffraction (XRD) and electron back scatter diffraction (EBSD). The microstructure and cryogenic impact energy were studied by scanning electron microscope (SEM), transmission electron microscope (TEM) and Charpy V-notch (CVN) tests. The results showed that when the sample was heated at 10 °C/s, the volume fraction of reversed austenite exhibited maximum of 8%; the reversed austenite was uniform along all kinds of boundaries; the reversed austenite contained higher concentration of carbon which enabled it to be more stable. The cryogenic toughness of the alloy was greatly improved when heated at 10 °C/s, as the fracture surface observation showed that it mainly fractured in ductile rupture mode, which was consistent with the results of cryogenic impact energy.