Abstract
The quantification of key microstructural parameters as a function of aging or creep exposure time is commonplace in the assessment of 9Cr Creep Strength Enhanced Ferritic (CSEF) power-plant steels. In these studies, the sample is either assumed chemically homogenous at the micro-scale or that a material average will be achieved by collecting enough images at random locations. In this paper, the micro-scale chemical homogeneity of two ex-service boiler components, a pipe and a forging, are quantitatively assessed using high sensitivity chemical mapping from µ-XRF. The compositional variation was as expected most pronounced in the larger elements Mo and Nb, where a > 20 pct difference in composition was present between positively and negatively segregated areas. The effect of this micro-segregation on local variations in Laves phase particle characteristics was investigated using SEM images. This showed a factor of two difference in the number of particles and the area coverage between positively and negatively Mo-segregated regions. This result was consistent with the thermodynamic equilibrium predictions of the phase content based on the observed level of segregation.
Highlights
SINCE the international energy crisis in the 1970s, increasing efforts have been made to improve the efficiency of power generation.[1]
An outcome of the drive for increased Fossil fuelled plant efficiency was the adoption and widespread design and fabrication of components using Grade 91 steel. This alloy is a creep strength enhanced ferritic (CSEF) steel that was originally developed over approximately a ten-year period beginning in the mid-1970s by the Oak-Ridge National Laboratory (ORNL) and Combustion Engineering.[2]
The Gr.91 materials used in this study were from an ex-service final superheat outlet header (Figure 1(a)), supplied by the Electric Power Research Institute (EPRI)
Summary
SINCE the international energy crisis in the 1970s, increasing efforts have been made to improve the efficiency of power generation.[1].
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