Prakt. Met. Sonderband 38 (2006) 207 
EFTEM INVESTIGATIONS ON PRECIPITATES IN A CREEP 
wischen RESISTANT 9-12% CR STEEL 
B. Sonderegger”, G. Kothleitner”, W. Rechberger”, H. Cerjak’ 
* Institute for Materials Science, Welding and Forming, Graz University of Technology, 
Kopernikusg. 24, A- 8010 Graz, Austria 
** Research Institute for Electron Microscopy and Fine Structure Research, Graz 
University of Technology, Steyrerg. 17, A-8010 Graz, Austria 
ABSTRACT 
9-12% chromium steels are well established materials for high temperature components in 
ultra efficient power plants. One important parameter for the creep resistance of these 
steels is the formation and interaction of different precipitate populations like NbC, VN, 
M23Ce, Laves phase and modified Z-phase. Energy filtered TEM (EFTEM) studies were 
carried out on samples of the European COST steel CB8. Enhanced analysis methods 
| , were applied to determine the size, volume fraction and number frequency of precipitates 
m Ferrit with arbitrary size distribution in samples of varying thickness. The results show growth, 
>mäß im coarsening and dissolution of precipitates during in-service exposure. The dissolution of 
t ist im VN precipitates at long running times was identified as the most likely reason for the drop 
1 durch of creep resistance of the steel CB8. Furthermore, the drop in the number density of VN 
precipitates corresponds with the first appearance of the V- and N- containing modified Z- 
phase Cr(V,Nb)N. 
-0SuUNgs- 1. INTRODUCTION 
Ss darauf 
es teils Creep resistant 9-12% Cr steels are used for high temperature components in fossil power 
ten von plants. Amongst other parameters like oxidation resistance, ductility etc., high temperature 
Sern von creep resistance is one of the key factors for the reliability and long life of these 
etzbaren components. State-of-the-art steels withstand temperatures of 600-620°C at mechanical 
rbunden stresses of 100MPa for over 10 years (10°h). However, further increase of the operation 
temperature allows constructing more efficient power plants with less emission of CO,. 
The European Cooperation in the field of Scientific and Technical Research (COST) is 
aiming at this goal in Action 536 [1]. 
The creep resistance is mainly influenced by the presence and the stability of different 
kinds of precipitates at service conditions. According to the backstress concept [2], a high 
Duplex- number of small precipitates are the best precondition for improved creep properties. Most 
importantly, this high number density must not decrease distinctively during service. 
ic Effects like coarsening and dissolution of precipitates accelerate the number density 
decrease, and thus the drop of creep resistance. 
etik des In the European COST program 522 (the predecessor of 536), several types of Boron 
um alloyed test melts have been developed [3]. One of the most promising candidates for 
creep resistance, the test melt CB8, was chosen for long-term creep tests. It turned out 
that CB8 shows a drop in creep resistance at times greater than 10%h, although the initial