Effect of Intercritical Quenching on Reversed Austenite Formation and Cryogenic Toughness in QLT-Processed 9% Ni Steel Original Research Article

In this work, a group of intercritical quenching was carried out by heating to different elevated temperatures ranged from 600 to 700°C to investigate its effect on the formation of reversed austenite and cryogenic toughness in the QLT-processed 9%Ni steel. The detail microstructural evolution in the process was observed by optical microscope (OM) and transmision electron microscope (TEM), and the reversed transfromtion of austenite transision during subsquent intercritical tempering was further exmamined by Differential Scanning Calorimetry (DSC). Finally the Charpy-V-Notch (CVN) impact testing performed at 77K was carried out to evaluate the cryogenic toughness of the QLT-processed 9%Ni steel. It is indicated that the intercritical quenching led to a typical lamellar mixture of highly-dislocated fresh marteniste and well-recovered original martensite as the intercritical heating temperature increased to 660°C and 680°C. This “dual phase” lamellar structure caused reverse martensitic α→γ transformation to take place in two stages during subsequent intercritical tempering, which was consistent with two different lower critical temperatures (Ac 1). The final morphology of reversed austenite formed during intercritical tempering was either granular or thin film in shape surrounded by martensitic laths. The results of X-ray diffraction (XRD) analysis further revealed that the volume fraction of reversed austenite increased with the intercritical quenching temperature, and arrived at a peak value around 660 °C, then decreased subsequently. Furthermore, the enhanced cryogenic toughness in the final structure was suggeted as a result of formation of the reversed austenite with high thermodynamic stability remained down to impact testing temperature of 77K.