The Role of Grain Boundary Orientation and Secondary Phases in Creep Cavity Nucleation of a 316H Boiler Header

Abstract

Cavity formation during creep of steels at high temperatures and stresses is closely related to the original and evolved microstructure, particularly the orientation between grains and precipitation at the grain boundaries. Understanding the initiation, growth and coalescence of creep cavities is critical to determining the operational life of components in high temperature, high stress environments such as an advanced gas-cooled nuclear reactor. However, accelerated laboratory-based testing frequently shows another kind of void within the microstructure, caused by plastic damage and ductile failure, particularly if a specimen fails during a test. This paper compares the type of voids and cavities observed in an AISI 316 stainless steel after extensive service in a gas-cooled nuclear reactor boiler header and after uniaxial creep testing of a similar material at higher stresses. The differences between the features observed and their potential mechanistic origins are discussed.