Complementarity of SANS,TEM and APT for the study of NbC and MnS precipitates in a direct strip cast steel
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Date
2016-02-04
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Australian Microscopy and Microanalysis Society
Abstract
One of the main alloy design strategies to provide strengthening to low alloy steels is the deliberate addition of strong carbide forming elements such as Nb or Ti to form nano-precipitates with a carbide or carbo-nitride chemistry 1 . The addition of Mn is also common commercial practice to remove free sulphur from the steel. Thus, low alloy steels contain at least two chemically distinct precipitate populations, coarse manganese
sulphides and fine carbo-nitrides. When steel alloys are processed by thin slab or direct strip casting (DSC), a significant decrease in the size of the sulphides is observed 2 and this is a direct consequence of the higher cooling rates experienced in these processes 3 . Since DSC is a relatively new processing technology 4 , the precise measurement of these nano-sulphides has not before been required, and very little information on this topic is available in the open literature. Small angle neutron scattering (SANS), atom probe tomography and electron microscopy have been used to investigate the MnS and Nb(C,N) precipitate populations in a low alloy steel processed by direct strip casting. Rapid cooling refined the sulphides, and both the SANS and atom probe
tomography data indicate that sulphur is retained in solid solution after rapid cooling. A similar result is observed for the Nb-carbonitrides. The rapid cooling supresses precipitate formation, and only small Nb and N enriched clusters are able to form. These NbN clusters have a low volume fraction compared to the equilibrium condition in which classical Nb(C,N) precipitation is complete. Finally, we present a method to extract
precipitate chemistry from the SANS data in order to validate the atom probe measurements on a statistically robust specimen volume.
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Keywords
Transmission electron microscopy, Niobium, Cast iron, Steels, Titanium, Cooling, Small angle scattering, Tomography
Citation
Dorin, T., Wood, K., Stanford, N., Taylor, A., & Hodgson, P. (2016). Complementarity of SANS,TEM and APT for the study of NbC and MnS precipitates in a direct strip cast steel. Paper presented to ACMM24 : Australian Conference on Microscopy and Analysis : Melbourne Convention and Exhibition Centre, 31 Jan-4 Feb 2016, (pp. 80-81).