Abstract:

The effect of Mn content on the microstructure and cryogenic mechanical properties of a 7% Ni steel was investigated within the Mn content range from 0.13% to 0.36%. The microstructure of the steel as determined by optical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscattering diffraction and X-ray diffraction was presented, and the low-temperature mechanical properties were given. The size of prior austenite grain did not change a lot as Mn content increased. Film-like reversed austenite, having high stability, was found mainly in the specimens with lower Mn content; however, in the specimen with the highest Mn content, the role of Mn was not obvious in stabilizing reversed austenite. Besides, with increasing Mn content, the amount of reversed austenite at grain boundaries gradually decreased. The variable Mn content had a significant effect on cryogenic toughness, but not apparent on cryogenic tensile strength or yield strength. An excellent combination of cryogenic tensile and impact properties was obtained when Mn content of steel was 0.13%.

Key words: 7% Ni steel, Mn content Microstructure, Reversed austenite, Cryogenic mechanical properties