Improved Accident Tolerance of Austenitic Stainless Steel Cladding through Colossal Supersaturation
with Interstitial Solutes
Zhen Li, Frank Ernst, and Arthur Heuer
10900 Euclid Ave., Cleveland, Ohio 44106, zxl219@case.edu, fxe5@case.edu, ahh@case.edu.
INTRODUCTION
Presently, LWR (light-water reactor) fuel cladding
(tube) is made from Zr-base alloys (e.g., ZircaloyTM).
However, in steam at temperatures above 1000 K
(727 °C), these alloys oxidize rapidly, stripping oxygen
from water molecules and producing hydrogen, which
may cause explosion. Moreover, Zr-base alloys are prone
to GTRF (“grid-to-rod” fretting) failure, in which flowinduced vibration of fuel rods touching the grid of the
nuclear fuel assembly causes structural damage that can
lead to failure. One tangible alternative to Zr-based alloys
are austenitic stainless steels, which have significantly
better oxidation resistance under accident conditions.
However, they are prone to SCC (stress-corrosion
cracking). Our goal is to improve the properties of
austenitic stainless steels such that they can be used for
nuclear fuel cladding in LWRs and significantly excel on
currently used alloys with regard to performance, safety,
service life, and accident tolerance. The method we
propose for this purpose is based on a new concept of
surface engineering: “case” hardening (i.e., generating a
hard shell) by CSS (colossal super-saturation) with
interstitial solutes infused through the alloy surface
[1,2,3].
CSS with carbon or nitrogen to levels that correspond
to ≈ 100,000 times the equilibrium solubility limit while
suppressing precipitation of undesired carbides or nitrides
can be accomplished if the following conditions are
fulfilled: (i) The alloy must contain metal atoms with a
high affinity for carbon, e.g., Cr in AISI-316L. (ii) The
processing temperature must be chosen such that the
metal atoms (Fe, Ni, Cr), residing in substitutional sites of
the alloy crystal structure, are practically immobile and
therefore cannot precipitate in carbides or nitrides,
whereas the carbon or nitrogen atoms, as they reside in
interstitial sites, can still diffuse over technically useful
distances (a few micrometers) within the processing time.
CSS can significantly enhance the mechanical
properties (hardness, wear resistance, and fatigue life) and
the corrosion resistance of structural alloys. Currently, we
investigate whether these improved properties can be used
to improve accident tolerance of nuclear fuel cladding. Of
particular interest is whether CSS also improves the
resistance to stress-corrosion cracking, and possibly the
resistance to microscopic structural damage caused by
irradiation with electrons, neutrons, or ions. Of critical
importance for success is (i) to optimize the process that
“activates” the alloy surface for infusion of interstitial
solute, i.e., removes the passivating oxide layers, (ii) to
conduct the gas flow such that it can provide even flux
density of interstitial solute through the alloy surface on
the outside as well as at every point on the inside of tubes
with a high aspect ratio, and (iii) to adapt the processing
parameters for the actual infusion of solute to the
particular microstructure of, in this case, AISI-316L
tubes, as it was observed in earlier studies that different
populations of extended structural defects (i.e.,
microstructure) can have a major impact on the results of
this new technique for alloy surface engineering.
EXPERIMENTAL PROCEDURES
Fig. 1. AISI-316L tube sections and flat coupons after
low-temperature-carburization at 653 K (380 °C) for
252,000 s (70 h).
AISI-316L tubes with the exact dimensions of current
fuel cladding and flat AISI-316L coupons were
carburized at different (low) temperatures and for
different periods of time. The inner and outer diameter
and the wall thickness of the as-received tubes are 8.89,
9.50 and 0.254 mm, respectively. The thickness of the as
received coupon is 0.38 mm. XRD (X-ray diffractometry),
metallography, and hardness testing were performed to
investigate the efficacy of the different conditions to
produce a carbon-infused layer. Figures 1 and 2 show
carburized tubes and coupons. As shown in Fig. 1, short
sections of AISI-316L tubes were placed in the CVD
furnace with different orientation to the (vertical) gas
stream and carburized at 653 K (380 °C) for 252 ,000 s
(70 h) This experiment served to investigate the potential
influence of the tube orientation on the flux density of
interstitial solute. In the middle, Figure 1 shows the wire
we used to hang the two specimens in the CVD furnace.
The tubes and coupons shown in Figure 2 were carburized
at 743 K (470 °C) for 79,000 s (22 h).
manifests itself by peak shifts to lower angles, as the
presence of carbon (or nitrogen) on interstitial sites causes
an expansion of the lattice parameter (reflecting the
average distance between the metal atoms Fe and the
substitutional solute atoms, Ni and Cr).
The diffractograms of the as-received, non-treated
AISI-316L tubes and coupons extended (111) and (200)
peak positions that are practically identical, as expected.
For the low-temperature-carburized tubes and coupons, in
contrast, Figure 3 indicates a significant shift of the (111)
and (200) peak positions to the lower 2θ diffraction
angles. This presents strong evidence for successful
infusion of carbon. The absence of additional peaks in the
diffractograms indicates that no carbides have formed in
excess of the detection limit, which is estimated to 1
vol%.
Fig. 2. AISI-316L tubes and flat coupons carburized at
743 K (470 °C) for 79,000 s (22 h).
RESULTS
Fig. 4. Metallographic cross-section showing the wall of
an AISI-316L tube after low-temperature carburization at
743 K (470 °C) for 79,000 s (22 h).
Fig. 3. XRD scans from non-treated (as received) and
treated tubes and coupons, low-temperature-carburized at
743 K (470 °C) for 79,000 s (22 h).
Figure 3 shows XRD scans of several different
specimens, obtained with a Scintag-X1 diffractometer and
vertically placed on top of each other. The data from the
non-treated, as received AISI-316L serves as a reference
for the (111) and (200) peak positions of AISI-316L. As
known from earlier work, successful carburization
Figure 4 presents a light-optical metallographic crosssection showing the wall of an AISI-316L tube after lowtemperature carburization at 743 K (470 °C) for 79,000 s
(22 h). This cross-section was prepared by mechanical
polishing and etching in a reagent containing 25 vol%
HNO3, 50 vol% HCl and 25 vol% H2O. The micrograph
reveals the case as a thin featureless (etch/corrosionresistant) layer at the inner and the outer surface,
separated by a dark line from the grain structure revealed
in the interior of the tube wall. The dark line is
presumably a step in the surface, which originates from
the etch resistance of the case versus the etch attack of the
core.
Generally, the case thickness is nicely uniform
throughout the field of view. The case thickness is on the
order of 10 µm, which roughly corresponds to the case
thickness required for the improvements of alloy
properties observed in earlier work. However, the case on
the inside of the tube is significantly, about 2 times,
thinner than on the outside. This difference is also
reflected in the diffractograms, which generally show
stronger peak shift when acquired from the outside of the
tube. Future work will aim to understand the reason for
this asymmetry. It may result from differences in
microstructure (different populations of extended
structural defects that influence carbon diffusion) at the
inside and the outside of the tubes, from differences in the
gas flow, leading to different flux density of interstitial
atoms through the alloy surface, or geometric differences
owing to different curvature of the inner and outer
surface.
resolution, i.e., small indents, the load of the Vickers
indenter was chosen to only 10 gf. Consistent with
standard properties of AISI-316L, the hardness of the
non-carburized core of the tube wall is about 200 HV10.
At the inner and outer surface, i.e., on the left-hand side
and the right-hand side of the hardness plot, respectively,
the measured hardness is up to 4 times higher. These data,
in spite of the symmetry of the case thickness on the
inside in the outside the two, indicate that significant
improvement of mechanical properties has been obtained.
Future work will address optimization of processing
parameters as well as verification of improvement in
mechanical properties and corrosion resistance.
CONCLUSION
An
established
low-temperature
gas-phase
carburization process has been successfully adapted for
surface hardening of AISI-316L tubes with the
dimensions of nuclear fuel cladding. A corresponding
improvement in surface hardness has been experimentally
verified.
This
constitutes
a
first
important
accomplishment in our research effort aiming to explore
the potential of improving the accident tolerance of LWRs
through surface hardening of nuclear fuel cladding by
colossal supersaturation with interstitial solutes.
ACKNOWLEDGEMENT
We acknowledge financial support for the research
project 12-3451 from the DOE-NEUP program under
contract number 00128081.
REFERENCES
Fig. 5. Microhardness profile across the wall of a lowtemperature-carburized AISI-316L tube. (a) Metallographic cross-section similar to Fig. 4, showing hardness
indents. (b) Microhardness profile.
Figure 5 shows a microhardness profile measured
across the wall of the tube low-temperature-carburized at
743 K for 79,000 s. In order to obtain high spatial
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