The present paper addresses the experimental assessment of the concentration dependent nitrogen diffusion coefficient in stress free expanded austenite foils from thermogravimetry, the numerical simulation of nitrogen concentration depth profiles on growth of expanded austenite into stainless steel during gaseous nitriding, a qualitative discussion of the role of stress on local equilibrium conditions of growing expanded austenite and a discussion of the erroneous concentration dependent diffusivity of nitrogen in expanded austenite as obtained from applying the Boltzmann-Matano method to composition-depth profiles.
White etching crack (WEC) networks were characterized in heavy loaded bearings for wind turbines. Both conventional techniques as reflected light microscopy and (scanning and transmission) electron microscopy as well as electron backscatter diffraction and ion channelling contrast imaging were applied. The complementary use of the techniques in unravelling the complicated failure mechanisms is explored in the present work.
Laboratory testing of selected chromia-forming alloys was performed to rank the materials and gain further knowledge on the mechanism of KCl-induced high temperature corrosion. The investigated alloys were stainless steels EN1.4021, EN1.4057, EN1.4521, TP347H (coarse-grained), TP347HFG (fine-grained), Sanicro 28 and the nickel-based alloys 625, 263 and C276. Exposure was performed at 600 8C for 168 h in flowing N 2 (g)þ5%O 2 (g)þ15% H 2 O(g) (vol.%). Samples were covered with KCl powder prior to exposure. A salt-free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was observed that in the salt-free exposure, stainless steels TP347H (coarse-grained) and EN1.4521 failed to form a thin protective oxide layer compared to the oxide formed on the other alloys. In the presence of solid KCl, all the alloys showed significant corrosion. Measurement of corrosion extent indicated that alloys EN1.4057, Sanicro 28 and 625 show a better performance compared to the industrial state of the art material TP347HFG under laboratory conditions. An additional test was performed with KCl vapor in static air for the same duration and at the same temperature. This was undertaken to investigate the role of the vapor phase and revealed that KCl vapor at 600 8C can initiate attack.
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