Microstructural characterization and analysis of inclusions in C-Mn steel and weld metals

Ferritic steels used for the construction of welded pressure vessels may co ntain trace concentrations of impurity elements that can influence their ov erall mechanical properties. Often, the C-Mn ferritic steels and weld metal s used for welded nuclear pressure vessels contain trace concentrations of boron (<10 ppm), and the role of this impurity element could be significant ly different depending upon whether it is present as the free atomic specie s or incorporated into specific microstructural features, such as inclusion s or precipitates, of these materials. In this article, the results of work designed to characterize the microstructure of C-Mn steels and weld metals used for the construction of Magnox nuclear pressure vessels are described . In particular, the type, size, distribution, and chemical composition of inclusions present are considered. A range of techniques are used to charac terize the microstructure, but, in particular, two surface sensitive analyt ical techniques, namely, Auger electron spectroscopy (AES) and secondary io n mass spectroscopy (SIMS), are used to detect and quantitatively analyze i mpurity boron. The results are discussed with respect to the relationship o f the boron to the stable silicate inclusions and the potential influence t his may have on mechanical properties of these materials.