Effect of Manganese on Cryogenic Toughness of 7% Nickel-added Steels with Intermediate Heat Treatment

The effect of the manganese content of 7 % nickel-added steel on cryogenic toughness was investigated. Charpy impact tests on steels with manganese contents ranging from 0.05% to 2.16% revealed that the absorbed energy at -196 °C increased with decreasing manganese content. Two steels with manganese contents of 0.2 % and 0.8 % were selected for a more detailed investigation of the dependence of toughness at -196 °C on the intermediate heat treatment temperature, fracture behavior and retained austenite. Although the 0.2 % manganese steel stably exhibited a high absorbed energy at -196 °C at all intermediate heat treatment temperatures examined in this work, it was necessary to select an appropriate intermediate heat treatment temperature for the 0.8 % manganese steel in order to achieve high absorbed energy at -196 °C. The volume fraction, size, and nickel content of retained austenite were quantified using XRD and SEM/EDS, and the characteristics of the retained austenite in the steels with low manganese alloy designs were investigated. The cleavage-type brittle fracture that appeared in some specimens was discussed from the viewpoint of the stability of retained austenite.