Effect of Retained Austenite on Notch Toughness in the Synthetic HAZ of 5.5 % Ni Steel —Studies on Retained Austenite in Cryogenic Steel Welds (Report 3)—

This investigation has been made of the influence of precipitated and retained austenite on impact toughness in the synthetic HAZ of 5.5% Ni cryogenic steels. Specimens were heated and cooled through the selected thermal cycles by induction heating and then the instrumental Charpy test was employed for the determination of crack-initiation and crack-propagation energy. From the result, the sufficient requirment is that the ductile fracture with plastic deformation occurs, that is, the temperature of impact test should be above the ductile-to-brittle transition temperature (DB-TT) in order to present a beneficial effect of retained austenite in HAZ on impact toughness. So the beneficial effect range of retained austenite on toughness in synthetic HAZ of 5.5% Ni steel represents the schematical diagram, 1st cycle peak temperature vs. DBTT. The instrumented Charpy test reveals that the austenite phase produced by wled thermal cycles takes part in the energy of crack-propagation rather than crack-initiation.