Prakt. Met. Sonderband 38 (2006) 213 
Ss stays 4. DISCUSSION 
Jed and 
clearly As shown by the quantitative evaluation, the My3;Ces particles coarsen continuously 
gure 5). (constant volume fraction, decreasing number density, increasing mean diameter). This 
process is found in the thermally aged and, even more pronounced, the creep loaded 
ses for samples. In contrary, the parameters of the VN precipitates show no clear tendency at first 
onstant — until the longest running samples A16000 and C16000. At this loading time of 16000h, 
e effect the number frequency drops dramatically. At the same moment, the V- and N- containing 
mod. Z-phase appears for the first time, consuming the nitrogen formerly bound in VN 
:ndency precipitates. The precipitates of this phase were found to have greater diameters than VN 
(by a factor of 2), and therefore producing a much lower number frequency, when 
consuming the same amount of Nitrogen. Conspicuously, the creep properties deteriorate 
at the same moment. 
This behaviour is also found in similar steels [21]. The mod. Z-phase is known to coarsen 
fast [26], and thus further degrading the creep properties. Numerical thermodynamic 
studies based on new databases [22, 23] show that this phase in fact is an equilibrium 
phase, and displacing VN in the long run. Kinetic simulations, carried out on the software 
MatCalc, also approve this behaviour [23]. The slow coarsening of M,3;Cs precipitates 
additionally contributes to the overall decrease of the precipitates number density. 
00 
N 5. SUMMARY 
The formation and interaction of precipitates in the course of heat treatment, thermally 
ageing and creep loading were measured successfully by TEM/EFTEM for the COST steel 
CB8. The precipitate types VN and M23Cs were studied in detail by evaluating the evolution 
of their size (distribution), volume fraction and number density. As most remarkable effect, 
the decrease in the number density of VN by a factor of 10-20 in the longest running 
samples was identified. This corresponded to the first appearance of V- and N- containing 
mod. Z-phase Cr(V,Nb)N. The formation of mod. Z-phase is found as the most likely 
reason for the drop of the VN number density, and therefore the drop of long-term creep 
resistance of the steel CBS. 
For future developments, the emphasis must be laid on suppressing the formation of mod. 
Z-phase as long as possible. For this goal it is inevitable to associate the microstructural 
investigations with thermodynamic kinetic simulations of the precipitate evolution, in order 
to find improved chemical compositions of 9-12% Cr steels. 
ACKNOWLEDGMENTS 
One of the authors (B. Sonderegger) wants to thank the Austrian Federal Ministry of 
Transport, Innovation and Technology and the Province of Styria for their financial support. 
REFERENCES 
[1] COST - Action 536: Alloy development for Critical Components of Environmental 
friendly Power planT (ACCEPT); http://cost.cordis.lu/src/action detail.cfm?action=536 
[2] G. Dimmler: Quantifizierung der Kriech- und Zeitstandfestigkeit von 9-12% Cr Stahlen 
auf Mikrostruktureller Basis, Ph.D. thesis. Graz University of Technology, 2003