Carbon segregation and inclusions effects on surface fracture morphology of a 12% chromium stainless steel |
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Authors: | N Souami D Saidi M Negache A Ati |
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Affiliation: | (1) Départment de Caractérisation DTN CRNA, Centre Recherche Nucléaire d’Alger, B.P. 1016, Alger, Algeria;(2) Département de Métallurgie, DMCN CRND, Centre Recherche Nucléaire de Draria, B.P. 43 Draria, Alger, Algeria;(3) Université de Bejaia, Algeria |
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Abstract: | Martensitic 12% chromium stainless steel is generally used for the manufacture of water vapour turbine blades. This material,
under these environmental conditions, develops fatigue corrosion with failure as a result of the segregation of certain constituent
elements such as phosphorus (P) and sulphur (S),1–3] or the presence of some types of inclusions.2–4] To be able to understand and explain these phenomena, in situ characterization of the fractured surfaces were performed for
two types of samples: steel 1 as manufactured turbine material whose fracture mode is intergranular and steel 2 issued from
last stage turbine blades after 100 000 h service at 40 °C whose fracture mode is transgranular. The techniques used for characterization
were scanning electron microscopy (SEM) coupled with the x-ray analysis by energy dispersive spectroscopy (EDS), and auger
electron spectroscopy (AES). The Auger results enabled the understanding of the brittle to ductile transition for the material
by showing the simultaneous diffusion of carbon from grain boundaries (GB) to grains (G) and chromium from G to GB. Furthermore,
the heavy segregation of phosphorus at the GBs could explain the intergranular crack rupture traces observed in steel 2. SEM
observations coupled with EDS analysis showed the presence of different types of non-metallic inclusions such as silicon-based
complex inclusions and manganese sulfides (MnS). The presence of silicon-based complex inclusions at GB could explain the
intergranular fracture mode previously reported. The characterization of the fracture appearance suggests also that MnS inclusions
can act as nucleation sites for secondary microcracks at the GB level that were observed after service. |
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Keywords: | inclusions segregation stainless steel surface fracture |
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