The impact of welding on the creep properties of advanced 9-12% Cr steels

The term "long-term creep properties" for heat resistant steels is mainly reflected by the 100.000 hour creep rupture strength at elevated testing temperature. Often, results of high stress, short-term creep tests are extrapolated to this 100.000 hour target value. Results of longterm creep tests are rather rare because of high testing costs and the time consuming testing procedure. Especially, long-running crossweld creep tests have not been performed in a sufficient extent so far, although, the heat-affected zone of crosswelds of ferritic chromium steels is known as a possible weak point. The long-term creep properties of crosswelds is linked to microstructure of the heat-affected zone of 9-12% chromium steels. The formation of heat-affected zone microstructures is studied by dilatometry, in-situ X-ray diffraction using synchrotron radiation, optical microscopy as well as most advanced electron microscopic methods. Results of crossweld creep tests up to duration of 40,000 hours are directly linked to the heat-affected zone microstructure at the location of fracture. The most predominant failure mechanism at lower stress levels and long term duration is Type IV cracking in the fine-grained heat-affected zone region of crosswelds. The failure mechanism is discussed in detail in this paper.