Bi-modal toughness behavior of pipeline steels with excellent Charpy impact toughness properties

  Bi-modal toughness behavior of pipeline steels Copyright: IEHK Schematic representation of the bimodal toughness behaviour of modern pipeline steels.

During the last decade, the European steel industry has made impressive progress in providing pipeline steels with excellent mechanical properties. Continuous improvement of cleanness and tailored thermomechanical treatments led to damage tolerant microstructures. These materials offer high strength as well as impressive Charpy impact toughness properties with upper shelf values of more than 300 J. However, these steels quite often show the bi-modal toughness behavior, characterized by high upper shelf values, but missing transition values. Instead, a temperature regime of approximately 30-40 K is observed where samples fail either completely brittle or completely ductile. This unknown behavior leads to difficulties for several reasons. First of all, no transition temperatures can be defined, but they are an important element of the safety assessment procedures. Secondly, it still remains unclear whether this behavior could also be seen on the component level. The project will therefore explore by the underlying damage and damage fracture mechanisms, which are responsible for the unusual toughness behavior. Besides microstructural investigations by using SEM and atom probe tomography, a combined, numerical model will be developed, which describes brittle and ductile fracture. Additionally, microstructural simulation on representative volume element (RVE) will be performed. In the last part of the project it will be investigated how serious the bi-modal behavior influences the component behavior.