Karl-Heinz Mayer

Requirements for microstructural investigations of steels used in modern power plants

Abstract The thermo-mechanical behaviour of newly developed 9 – 12 wt.% Cr steels used in steam power plants depends strongly on the microstructure. In the present work, a combination of various microscopy techniques is used to study the microstructural features relevant for the creep resistance. These include the subgrain size, the number of free dislocations, the chemical composition and crystallographic structure of the occurring precipitates, and their density and size distribution. Accordingly, the techniques which were applied range from light microscopy to transmission electron microscopy in order to cover all length-scales from the millimeter regime to the nanometer level. These detailed investigations are important for alloy design and as input parameters for thermodynamical calculations and modelling of the microstructural evolution to develop steels for cleaner power stations with high thermal efficiency.

Creep and creep rupture behaviour of 650°C ferritic/martensitic super heat resistant steels

Abstract The investigations of high-temperature creep properties of advanced ferritic/martensitic steels for 650 °C power plant components focus on the chemical composition as well as on the heat treatment conditions. First experiments on various modifications of the 9% Cr model piping steel P92 demonstrate a negative influence of 12% Cr, 5% Co and low tempering conditions on the creep rupture strength up to 104 h operating time. Low tempering conditions promote the precipitation of the modified Z-phase Cr(V, Nb) N during creep. This phase was recently identified as a major cause for premature breakdown in creep strength of some 9 – 12% Cr martensitic steels. The aim “650 °C/100 MPa/100 000 h” in creep life was not achieved in this investigation so far. The effect of boron on the improvement of creep behaviour depends on the interaction of chemical composition and heat treatment. Further activities focus on the stabilization of M23C6, a fine distribution of VN and measures for avoidance of Z-phase.