Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.