Prakt. Met. Sonderband 38 (2006) 273 
CHARACTERIZATION OF THE PRECIPITATES IN MODERN 
BORON CONTAINING 9-12 % Cr STEELS BY EFTEM AND 
THEIR CORRELATION TO THE CREEP STRENGTH 
F. Kauffmann*, K. H. Mayer**, S. Straub***, G. Zies*, C. Scheu****, H. Ruoff* and K. Maile* 
2 Materialpriifungsanstalt, Universitat Stuttgart, Stuttgart, Germany 
_. ALSTOM Energie GmbH, Nurnberg, Germany 
_. ALSTOM Power Generation AG, Mannheim, Germany 
Institute for Physical Metallurgy & Materials Testing, Montanuniversitét, Leoben, 
Austria 
ABSTRACT 
Martensitic 9-12 wt% Cr steels currently developed for the usage in steam power plants 
are aimed for service temperatures above 600 °C and steam pressures above 250 bar. 
These high operation conditions would allow to increase the thermal efficiency and to 
decrease CO, emission. A prerequisite for this is a good creep and oxidation resistance of 
the materials. 
This can be achieved by optimizing the microstructural features, especially the subgrain 
size and the precipitation size and distribution. Small precipitations with a high particle 
density and especially a high thermal stability are of great benefit to the creep properties. 
In the present work, a combination of various microscopy techniques, ranging from optical 
microscopy to transmission electron microscopy, is used to investigate the microstructural 
features relevant for the creep-resistance 
1. INTRODUCTION 
In recent years, new creep-resistant ferritic-martensitic 9-12 wt.% Cr-steels were 
developed for applications in steam power plants with the aim to increase the turbine inlet 
temperatures of the steam from approximately 550°C to 600-650°C and simultaneously to 
rise the steam pressure from 180 bar to 280-350 bar [1]. This increased inlet temperature 
would lead to a significantly improved rate of efficiency and reduced CO, emission. 
For these new steels, the oxidation behavior and the creep strength are the most important 
properties, with this paper focusing on the latter. The creep strength depends on the 
microstructure of the material. Of high priority are a small subgrain size and thermally 
stable and fine dispersed precipitates. The most prominent of these beneficial precipitates 
are M23Cg (M: metallic element) and MX (X: C,N). For good creep properties, it is important 
to achieve very small initial precipitation sizes and to impede particle growth [2-9]. During 
creep, the precipitates may grow and partially loose their strengthening effect. Additionally, 
new and larger precipitations can form (e.g. Z-Phase, Laves-phase) and decrease the 
precipitation density by consuming the smaller precipitates [6-10], thereby decreasing the 
strengthening effect.