Abstract
In this paper, it was intended to evaluate ductile fracture characteristic of cryogenic steel, especially focused on the effect of trace oxygen, which is important factor as impurity of practical steel, on the Charpy impact energy of 12% nickel steel. Since the ductile fracture characteristics were less investigated as for cryogenic steel such as 9% nickel steel, while the brittle crack initiation and propagation property have been extensively researched. 12% nickel steels were used in this investigation to avoid the difficulty of fracture phenomenon interpretation by accompanying brittle fracture. With decreasing oxygen content from 0.0024 to 0.0009 wt%, Charpy impact energy at the temperature of 77K and 273K, increased in the range of dozens of joule. This tendency reveals that the ductile fracture characteristic of 12% nickel steel is sensitive to the change of oxygen content compared to those of the other steels. With reducing oxygen content, the number concentration of Al2O3 descends, and it leads to the enhancement of resistance to ductile crack propagation, in which Al2O3 acts as the initiation site for void.
