In structural steels, high crack arrest toughness is required to stop brittle cracks from propagating in order to prevent eventual fracture of the structure. It has been known that Ni can reduces the risk of cleavage fracture of α-Fe as it is in the solid solution as substitutional alloying element. Also, Ni can enhance the stability of γ-Fe between martensite-lath which leads to improved toughness. In this study, deformation induced transformation of γ-Fe and stress relaxation effect are investigated as a toughnening mechanism of 9% Ni steels during brittle crack propagation tests.
Recently the demand of liquefied natural gas has increased, and the construction of liquefaction plant for efficient transport near gas-producting area and of receiving/re-gasification terminal near gas-consumption area is planned. Both liquefaction plant and regasification terminal would need LNG storage tanks with cylindrical double shell structure. 9% Ni steel has been utilized as the construction material for inner storage tank for more than half a century primary due to its high toughness at −162 degree C. Generally it has been known that Ni reduces the risk of cleavage fracture of alpha-Fe in a state of being substituted solid solution. In addition, Ni stabilizes γ-Fe in microstructure in particular between martensite-lath and it has been reported that the stabilized γ-Fe leads to improved toughness.
The authors summarize the past finding about the effect of retained austenite of Ni-added steel on the toughness of the material. Brophy (Brophy et al, 1948) made the use of the knowledge of Armstrong (Armstrong et al, 1940) and conducted the research and development to apply Ni-added steel to the steel for LNG storage tanks. It is well known that 9% Ni steel has been established as the standard material of the steel for LNG storage tanks. Brophy (Brophy et al, 1948) revealed the tempering temperature and keyhole Charpy characteristic in 8.5% Ni steel produced by NNT process. It is said that the austenite phase after twice normalizing (899 deg.C −787deg.C) is extremely unstable and a large amount of stable austenite can be retained by performing an appropriate following tempering treatment. It is in the agreement with the condition that impact property is the highest. While Charpy impact values mainly correspond to the initiation characteristics of brittle crack, there are reports that retained austenite also contributes to brittle crack propagation.
