FRANK ERNST RESEARCH GROUP
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Low-temperature carburization is a new
method for improving the surface properties
of commercial austenitic stainless steels.
With the carbon being supplied from a gas
phase, this process can be applied to parts
of arbitrary shape and generates a uniform
"case" – a thin (30 to
50 μm) homogeneous layer below the
surface, in which the concentration of
interstitially dissolved carbon can reach
more than 10 at%, thus several orders
of magnitude higher than that obtained with
conventional processing [1]. An example of
the conformal nature of this process may be
seen in Fig. 1, where a cross-section
of a carburized 304 stainless steel alloy
is shown. MnS particles ("stringers")
extending to the surface have been etched
out, revealing crack-like features that
have been carburized (seen as an unetched
layer at the surface).
The colossal supersaturation with carbon
resulting from low-temperature
carburization vastly improves the surface
hardness, resistance to wear, fatigue,
erosion, corrosion, and stress-corrosion
cracking [2]. Due to the nature of the
diffusion process by which the carbon case
is formed, the carbon concentration
decreases smoothly from the surface towards
the interior. This gives rise to
considerable stresses and consequently
residual compressive stresses, since the
high carbon concentration in the case
locally expands the lattice, while the
lattice of the supporting, uncarburized
bulk of the material remains unexpanded.
The stresses exciding the yield stress of
the material result in plastic deformation
of the surface (shown in Fig. 2).
X-ray diffractometry (XRD) results indicate
a lattice expansion of up to 2.8 % [1].
Since some of the improvements in surface
properties seem to originate from the
compressive residual surface stresses, it
is of great importance to accurately
measure and engineer the concentration
profile and resulting stress
distribution.
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Fig. 1.
Light-optical micrograph showing a cross of
low-temperature-carburized 304 stainless
steel (etched with Marble's reagent).
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Fig. 2.
SEM micrograph showing the surface relief
of a low-temperature-carburized 316
alloy.
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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.
This
material is based upon work supported by
the Department of Energy, Office of
Industrial Technology (DOE-OIT). Any
opinions, findings, and conclusions or
recommendations expressed in this material
are those of the author(s) and do not
necessarily reflect the views of the
DOE-OIT.
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