Study of creep damage in creep exposed martensitic high chromium steel weldments

Precipitation, grain-boundary and sub-boundary hardening are the most important strengthening mechanisms in creep resistant martensitic high chromium steels. In recent years, a design concept for the stabilisation of the microstructure (precipitates, grain boundaries and martensitic lath structure) by addition of boron and nitrides was developed. This so called MARBN steel (MARtensitic steel strengthened by Boron and Nitrogen) combines boron strengthening (solid solution strengthening) with nitride strengthening (precipitation strengthening). Welding trials showed no uniform formation of a fine grained region in the heat affected zone HAZ, which is considered critical for creep strength due to Type IV cracking. First creep test results of crosswelds are very promising. In this work, an improved MARBN steel was developed and investigated. Uniaxial creep tests of base material as well as of welded joints have been carried out at 650°C up to 25.000 hours at different stress levels. The creep strength of base material and that of welded joints was analysed and the evolution of damage was investigated using synchrotron micro-tomography and electron back scatter diffraction in order to understand the creep damage mechanism. The combination of long-term creep testing data with a 3-D damage investigation by using synchrotron μCT allows a completely new view on the basic failure mechanisms at elevated temperatures in the heat affected zone of welded joints.