Phase transformations in cast duplex stainless steels

dc.contributor.advisor L. Scott Chumbley
dc.contributor.advisor Brian Gleeson
dc.contributor.author Kim, Yoon-Jun
dc.contributor.department Materials Science and Engineering
dc.contributor.other Materials Science and Engineering
dc.date 2018-08-24T17:10:59.000
dc.date.accessioned 2020-06-30T07:15:26Z
dc.date.available 2020-06-30T07:15:26Z
dc.date.issued 2004-01-01
dc.description.abstract <p>Duplex stainless steels (DSS) constitute both ferrite and austenite as a matrix. Such a microstructure confers a high corrosion resistance with favorable mechanical properties. However, intermetallic phases such as sigma (sigma) and chi (chi) can also form during casting or high-temperature processing and can degrade the properties of the DSS. This research was initiated to develop time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams of two types of cast duplex stainless steels, CD3MN (Fe-22Cr-5Ni-Mo-N) and CD3MWCuN (Fe-25Cr-7Ni-Mo-W-Cu-N), in order to understand the time and temperature ranges for intermetallic phase formation. The alloys were heat treated isothermally or under controlled cooling conditions and then characterized using conventional metallographic methods that included tint etching, and also using electron microscopy (SEM, TEM) and wavelength dispersive spectroscopy (WDS). The kinetics of intermetallic-phase (sigma + chi) formation were analyzed using the Johnson-Mehl-Avrami (JMA) equation in the case of isothermal transformations and a modified form of this equation in the case of continuous cooling transformations. The rate of intermetallic-phase formation was found to be much faster in CD3MWCuN than CD3MN due mainly to differences in the major alloying contents such as Cr, Ni and Mo. To examine in more detail the effects of these elements of the phase stabilities, a series of eight steel castings was designed with the Cr, Ni and Mo contents systematically varied with respect to the nominal composition of CD3MN. The effects of varying the contents of alloying additions on the formation of intermetallic phases were also studied computationally using the commercial thermodynamic software package, Thermo-Calc. In general, a was stabilized with increasing Cr addition and chi by increasing Mo addition. However, a delicate balance among Ni and other minor elements such as N and Si also exists. Phase equilibria in DSS can be affected by local composition fluctuations in the cast alloy. This may cause discrepancy between thermodynamic prediction and experimental observation.</p>
dc.format.mimetype application/pdf
dc.identifier archive/lib.dr.iastate.edu/rtd/1175/
dc.identifier.articleid 2174
dc.identifier.contextkey 6090697
dc.identifier.doi https://doi.org/10.31274/rtd-180813-11426
dc.identifier.s3bucket isulib-bepress-aws-west
dc.identifier.submissionpath rtd/1175
dc.identifier.uri https://dr.lib.iastate.edu/handle/20.500.12876/65041
dc.source.bitstream archive/lib.dr.iastate.edu/rtd/1175/r_3158349.pdf|||Fri Jan 14 18:57:25 UTC 2022
dc.subject Materials science and engineering
dc.title Phase transformations in cast duplex stainless steels
dc.type dissertation
dspace.entity.type Publication
relation.isOrgUnitOfPublication bf9f7e3e-25bd-44d3-b49c-ed98372dee5e
thesis.degree.level Doctor of Philosophy
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