On the Effect of Nickel Alloying for Improving the Brittle Fracture Resistance of Structural Plate Steels

Nickel is an important and widely used alloying element in carbon steels which can significantly improve toughness, especially at low temperatures. This effect is explicitly used in alloys for cryogenic steels where nickel contents of up to 9 mass % are used. In structural plate steels with minimum yield strength of 355–460 MPa, the nickel alloy content is limited per specification to 0.7 (DIN EN 10225) or 1.0 (Norsok) mass %. Nevertheless, even comparably low nickel additions can be beneficial to toughness performance of structural plate steels. Particularly in heavy-gauge plates where rolling deformation in the center area and hence microstructural refinement is limited, nickel alloying may be the only metallurgical means to achieve toughness improvement. The evaluation of brittle fracture resistance of structural plate steels for either onshore or offshore constructions predominantly relies on the fracture mechanic protocol of crack tip opening displacement. This criterion characterizes the local deformation capacity of the steel close to the crack tip and is valid for the considered structural plate steels, due to their elastic-plastic material behavior.