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Low-temperature (470 °C)
gas-phase carburization treatment,
developed by the Swagelok Company, vastly
improves the mechanical and electrochemical
surface properties of 316-type austenitic
stainless steels [1-4]. Normally, the
precipitation of carbides restricts the
carbon concentration in the austenite of
316-type steels to <0.015 at.% at
470 °C, the Swagelok treatment
generates a colossal supersaturation of up
to 12 at.% carbon in solid solution ? 600
times the equilibrium solubility at the
carburization temperature (thus an even
larger supersaturation at room
temperature). Upon extended treatment,
however, carbide precipita- tion does
eventually occur [1,5]. The majority of the
carbide particles have the structure of
Fe5C2, known as
"Hägg" or "χ" carbide (space
group: C2/c) [5].
Since the precipitation of carbides
constitutes the ultimate limit to
low-temperature, "para-equilibrium"
carburization, it is important to
understand the details of precipitate
nucleation and growth under these
particular non-equilibrium conditions. For
this purpose, we employ
high-spatial-resolution imaging and
analysis by advanced methods of TEM
(transmission electron microscopy).
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1. Y. Cao, F. Ernst, and G.M. Michal:
Colossal Carbon Supersaturation in
Austenitic Stainless Steels Carburized at
Low Temperature. Acta Materialia 51
(2003) 4171.
2. G. M. Michal, F. Ernst, H. Kahn, Y.
Cao, F. Oba, N. Agarwal, and A.H. Heuer:
Carbon Supersaturation due to
Paraequilibrium Carburization: Stainless
Steels with Greatly Improved Mechanical
Properties. Acta Materialia 54 (2006)
1597.
3. F. Ernst, G. M. Michal, H. Kahn, A.
H. Heuer: Paraequilibrium Surface
Alloying with Interstitial Solutes: A New
Concept for Improving the Performance of
Medical Devices. Materials for Medical
Applications and Devices, ASM International
(2006), in press.
4. G. M. Michal, F. Ernst, A. H. Heuer:
Carbon Paraequilibrium in Austenitic
Stainless Steel. Metallurgical and
Materials Transactions (2006), in
press.
5. F. Ernst, Y. Cao, and G.M. Michal:
Carbides in Low-Temperature-Carburized
Stainless Steels. Acta Materialia 52
(2004) 1469.
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