Evaluation of Oxidation and Hydrogen Permeation of Al Containing Duplex Stainless Steels Page: 3 of 8
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WSRC-MS-2005-00393
3% were subsequently permeation tested, to the base
compositions were made and the as-cast ingots were processed via
the following working schedule: 1) the 4-in square ingots were
hot forged at 1000 C down to 1.5 inches square sections at 0.5-in
reduction per pass. The 1.5-in sections were hot-rolled @10000 in
multiple passes down to approximately 1-in thick sections. A
final thermal treatment of the 1-in sections was performed at
950 C for 1 hour followed by a water quench.
Analysis of the chemical composition of the final 1-in sections
was performed for the four heats typical values for 3wt%Al
doped alloys provided in Table 1. Characterization of the as-
processed microstructure was performed using light optical
microscopy on polished sampled electrolytically etched in a
solution of 10% oxalic acid. Scanning electron microscopy and
energy dispersive x-ray spectroscopy were performed to
characterize the phase structure and chemical analysis. In
addition, X-ray diffraction was used to determine the phases
present in the alloys.
The ferrite content of the samples was determined using a Fischer
Ferrite Scope. The instrument was calibrated and the ferrite
content ascertained from the relative magnetic strength of the
ferrite.
Oxidation kinetic testing was performed via thermogravimetric
analysis in a TA-TGA system operated at 600 C and 1000 C with
isothermal holds of 2-hrs at temperature. Additional testing to
examine the cyclic oxidation behavior was performed at 10000C
in air for cycles of nominally 24 and 48 hrs with a total exposure
time of approximately 1000-hrs. Weight change of the cyclic
oxidation samples was measured using a five-place balance.
The evaluation of oxide scale chemistry was performed on
samples oxidized at 450 C and 600 C for 2-hrs and 30-mins
respectively. Following these oxidation treatment the surface
oxide layer chemistry was analyzed via Auger Electron
Spectroscopy using a Perkins-Elmer PHI 660 Scanning Auger
Mulitprobe.
Preliminary hydrogen permeation testing was conducted at the
Savannah River National Laboratory using the permeation test
rig shown in Figure X. Tube samples, 19 mm diameter and 0.89
mm thick, were brazed into 2.12" diameter Conflat (CF) flanges.
The sample assemblies were placed in a 1" OD vacuum system
fabricated with 2.12" CF flanges. Copper gaskets were used to
seal the samples. The samples were evacuated to 1 x 10-6 Torr for
a period of at least six hours at room temperature. The samples
were then heated to 100C for 8 to 16 hours to outgas the system.
The sample was finally heated to the test temperature of 300C. A
leak rate test was conducted. If the leak rate was not linear, the
sample was evacuated for additional time, after an acceptable leak
rate was obtained, the sample section valves were closed and 400
Torr of deuterium was introduced. It took approximately 2-3
minutes for the pressure to reach the target value. The pressure
rise on the low pressure side of the system was monitored. The
data were logged at a 30 second interval. The data were reduced
to estimate the Diffusivity and Permeability. The data were
plotted as a function of time. The data exhibit three distinct
regions, the background in-leakage region, a transition region, and
a steady state region, nearly linear region. The diffusivity was
estimated by calculating the slope and the intercept of the linear
region using a least squares method. These two variables were
*contact author: Thad Adams, SRNL, thad.adams@srnl.doe.govthen used to determine the lag time (t), i.e., the time at which the
line crossed the Y-axis at zero. Lag time, t, time was used in the
equation t = x2 / 6 D, to determine D. The permeability was
estimated from the slope of the curve, the expansion volume, the
sample area, and the test pressure.
Results and Discussion
Alloy Characterization
Austenitic stainless steel alloys are primarily comprised of a
single phase FCC microstructure; however some BCC ferritic
phase also occurs in these alloys with the volume fraction being
dependent on alloy composition and processing history.
Characterization of the as-processed microstructures of 304L and
347H modified with 3.Owt% Al using optical microscopy is
shown in Figure 1. Previous work on Fe-Ni-Al alloys has
indicated that up to approximately 0.5wt% Al can be dissolved in
austenite. From this same study the remaining 2.5wt% Al would
be expected to be found in multicomponent intermetallic phases.
However, examination of the 3.Owt% Al alloys clearly show a
two-phase structure in the arc melted buttons. X-ray diffraction
and scanning electron microscopy data for these alloys has
indicated a multi-phase microstructure consisting of austenite,
ferrite and an Fe-Ni-Cr-Al intermetallic. The Fe-Ni-Cr-Al
intermetallic is believed to be occur as (Fe, Ni, Cr)3A1. Energy
Dispersive X-ray spectroscopy (EDS) analysis performed in the
SEM shows Al in the austenite matrix thereby confirming the
solubility of Al in austenite. The ferrite in the microstructure was
usually found in the interdendritic areas adjacent to the Al;-
containing intermetallic in the arc melted buttons. Examination
of the microstructure of the other arc melted button alloys and
conventionally cast and forged alloys with Al-concentration up to
7wt% has shown similar features with increasing ferrite content.
Analysis of these alloys using a Fischer Ferritescope to provide a
measure of total ferrite content (Table 2) has shown that as the Al
concentration increases up to 7wt% so does the ferrite content to
the point where the dominate phase in the microstructure is ferrite.
The stabilization of the ferrite phase can be attributed to the
relative size of the Al atoms in comparison with the austenite
(FCC) and Ferrite (BCC) structures. Since the Al atom is rather
large the openness of the BCC structure allows for easier
accommodation and the Al [16]. The resulting duplex(austenite-
ferrite) microstructures from the addition of Al is potentially
detrimental for hydrogen service applications since ferrite alloys
are typically more susceptible to hydrogen embrittlement and
increased permeation. However, the focal point of this program is
to understand the nature of the oxide formed on these alloys and
as such the duplex structure may not be as damaging as when
compared to unoxidized materials.
The hardness of the Al-doped 304L and 347H alloys was
measured using a Rockwell Hardness tester to evaluate the
influence of Al-content. The hardness of the both alloys increased
from approximately 45HRA to 70HRA with increasing Al content
up to 7wt%. This 35% (25 point) increase is attributed to the
increase in ferrite and Al-containing intermetallic phase volume
fraction with increasing Al concentration. Hardness data from
samples given homogenization treatments at 900 C for times up
to 18 hours show only moderate decreases in hardness. For the
3wt% Al-doped 304L alloy the hardness decreased approximately
11% over the 18-hr treatment whereas the for the 3wt% Al-doped
347H the hardness decreased only 7%. These results are
indicative of a reasonably stable microstructure, especially since
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Adams, Thad M.; Korinko, Paul & Duncan, Andrew. Evaluation of Oxidation and Hydrogen Permeation of Al Containing Duplex Stainless Steels, article, June 17, 2005; Aiken, South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc874255/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.