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

Abstract

Ferritic steels used for the construction of welded pressure vessels may contain trace concentrations of impurity elements that can influence their overall mechanical properties. Often, the C-Mn ferritic steels and weld metals used for welded nuclear pressure vessels contain trace concentrations of boron (<10 ppm), and the role of this impurity element could be significantly different depending upon whether it is present as the free atomic species or incorporated into specific microstructural features, such as inclusions 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 characterize the microstructure, but, in particular, two surface sensitive analytical techniques, namely, Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), are used to detect and quantitatively analyze impurity boron. The results are discussed with respect to the relationship of the boron to the stable silicate inclusions and the potential influence this may have on mechanical properties of these materials.