Laser-induced breakdown spectroscopy (LIBS) measurements were performed on a steel sample to investigate the feasibility of obtaining fast information on cleanness. A calibration approach was considered to enable the quantification of the elemental composition of non-metallic inclusions; the same data was also used to estimate the size of the inclusions. For the first time, the quantified chemical composition data obtained by LIBS for non-metallic inclusions was compared to the corresponding results obtained from scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, the conventional method used for steel cleanness assessment, and proved successful. Due to the speed of analysis and the systematic recording of the signal from the scanned area, LIBS enables the analysis of larger areas of sample and in significantly shorter time than with automated SEM-EDX. This paper summarizes the hitherto obtained results for the fast analysis of inclusions in steel by LIBS.
A microstructural study of creep failure in grade 91 weld metal has revealed two primary modes of creep failure. In addition to creep fractures along columnar grain boundaries (typical of weld metal creep failure), creep fractures were also found along creep-weak 'white bands' which had formed at the inter-bead boundaries. The white-band regions consisted of material where the M 23 C 6 carbides had dissolved during creep testing; the loss of carbides had allowed recrystallisation of the martensitic structure to ferrite and consequently this material was much softer than the bulk weld metal. The element mapping over the weld metal by laser-induced breakdown spectroscopy demonstrated that there was significant inhomogeneity in the distribution of certain elements, most significantly, chromium. This inhomogeneity resulted in strong activity gradients in carbon (even though the carbon concentration was homogeneous following welding) resulting in carbon loss from the alloy-depleted regions, the associated dissolution of carbides and the recrystallisation that accompanied this, and thus the poor mechanical properties which resulted in creep failure.
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