SCC INITIATION AND GROWTH RATE STUDIES ON TITANIUM GRADE 7 AND BASE METAL, WELDED, AND AGED ALLOY 22 IN CONCENTRATED GROUNDWATER Page: 4 of 18
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very similar stress versus time-to failure dependence for both
sets of data. Because the failure times are similar in air and in
the environment, it is reasonable to conclude that creep was the
primary cause of cracking, although it remains possible that
environmental (SCC) occurs and is accelerated by creep. In
many ductile alloys in aqueous environments, sustained
dynamic strain at the crack tip is considered important, so
accelerated environmental cracking in materials that creep
readily is expected. However, the role of creep in cracking may
be so dominant that the environmental contribution is small. A
comparison of the BSW-exposed failed specimen fractography
(Figure 3c and 3d) with that of an air failed specimen tested at
100*C (Figure A3 in reference ) indicates the air failed
specimen has a more ductile appearing fracture surface with the
presence of dimples resulting from fracture of creep formed
ductile cavities. Fractography is currently underway on the four
105"C air failed specimens tested in this study to obtain a better
assessment of any contributions to the failures due to SCC.
Initial data have also been obtained on a 1 TCT specimen of
titanium Grade 7 tested in 110 *C air under constant load at 30
MPa'm (Figure 9), identical to tests described later in the BSW
environment. The crack length data on the Y-axis represents the
conversion of pd potential drop data to crack length using the
algorithms we use for all CT specimens, but it's possible that
potential drop is affected both by crack advance as well as by
creep alteration of the geometry of the specimen. The initial
growth rate is quite high (4 x I0- mm/s), and it decays
continuously to 1 x 10-8 mm/s, and probably would continue to
decay. The growth rate at 700 hours is a bit slower than the
constant load-growth rate observed in a 110 *C BSW
environment of 1.25 x 10- mm/s, as discussed later.
Crack Growth Data
Two titanium Grade 7 crack growth specimens were tested,
one 1TCT in the as-received condition (Figure 10), and one
0.5TCT in a cold worked (20% reduction in thickness) condition
(Figure I1). References [14,15] describe the overall results of
these tests in more detail. Figure 10 shows that sustained 'SCC'
growth was observed at constant K. However, much of this
apparent growth is likely due to creep effects on the
displacement measurement. While extensive data shows that
cold work can accelerate SCC growth rates [16-18], the .
observed rates on the 20% cold worked specimen (Figure 10)
showed only very low growth rates, even with the use of
"gentle" cyclic unloading once per day. This is consistent with
the high observed resistance of 20% cold worked titanium
Grade 7 to SCC initiation. The cold worked material is also
much more resistant to low temperature creep .
The crack growth rate behavior of titanium Grade 28
(Figure 11) and Grade 29 (Figure 12) were also evaluated under
tandem loading conditions. The titanium Grade 28 exhibited
higher growth rates, which resulted in an increasing K during
the part of the test that kept the Ti Grade 29 specimen at
constant K, then test control was based on Ti Grade 28, and a
slow varying K (-dK/da) was used to monitor the crack growth
rate as the K was slowly decreased. At 25 - 26 ksiJin, the
growth rates of Ti Grade 28 at Grade 29 were similar at about 7
x 10-8 mm/s, but the Ti Grade 28 had grown incrementally
faster during the earlier part of the test, resulting in a somewhat
higher K on that specimen. It then showed a fairly high growth
rate, but this occurred as its K was rising, or maintained constant
at 34 ksi in. As its K was decreased back to ~ 25 ksijin, the
growth rates became similar. A direct comparison of the low
temperature creep behavior of titanium Grade 5 (an analog to
titanium Grade 29) to commercial purity titanium (an analog of
titanium Grade 7) indicates that the higher strength Grade 5
materials is significantly more creep resistant than the
commercial purity titanium materials . Consequently, the
significantly higher crack growth rate observed for the titanium
Grade 29 material, Figure 12 compared to that for the titanium
Grade 7 material, Figure 10 indicates that SCC growth rather
than creep is occurring in the titanium Grade 29 material.
Several alloy 22 specimens were evaluated for crack
growth response, including base metal (c144, c152), aged
material (c200), and 20% cold worked material (c153). The
first base metal specimen (c144) did not exhibit sustained crack
advance even under "gentle" cyclic loading conditions (Figure
13), and the observed rates were at least an order of magnitude
lower than in the as-received titanium grade 7 material (Figure
10). The second base metal specimen (c152) exhibited
sustained crack growth under less aggressive loading conditions,
but still did not exhibit sustained crack advance even under all
"gentle" cyclic loading (much less constant K) conditions
(Figure 14). The corrosion potentials of the specimens were
monitored with a Pt electrode and an Ag/AgCI external
reference electrode filled with 4M KCI. The Ag/AgCI electrode
has a porous ion junction, and so it was expected that after
thousands of hours its reference potential would drift. The long
term corrosion potential falls within the range of 0 to 0.1 V~se
(Figure 15), and the downward drift vs. time in the CT vs. Pt
signal suggests that alloy 22 is not becoming more noble.with
time (based on the reasonable assumption that the potential of Pt
is very stable in this environment).
The response of the 20% cold worked alloy 22 specimen
(c153) is shown in Figure 16. While only very low growth rates
were observed, sustained growth at constant K was observed
over a 700 period before slowing to a much lower rate. The
corrosion potentials for this specimen were also quite stable
(Figure 15b), and were similar to those observed in specimen
c152 (Figure 15a). The response of the aged alloy 22 specimen
(c200) is shown in Figure 17. While the growth rates are again
slow, sustained crack advance at constant K was observed over
a 1000 hours with little evidence of retardation.
A comparison of the effect of 1000 ppm lead under
identical loading conditions is shown in Figures 18 and 19. In
all cases, the growth rate in the presence of Pb is identical or
lower than without Pb, providing very solid evidence that Pb
plays no deleterious role in accelerating SCC growth rates in
these materials under these conditions. This contrasts with the
damaging effects of Pb on nickel alloys (esp. alloy 600) in high
temperature water, e.g., associated with pressurized water
reactor steam generators. The difference is not surprising,
because many characteristics of the passive film and nature of
passivity are very different above about 175 *C.
SCC growth rate tests were also performed at two K levels
on six weld metal specimens of Alloy 22 (machined from about
1-inch thick welded Alloy 22 plate) - as-welded, as-welded +
TCP (650 "C for 200 hrs) and as-welded + LRO (550 *C for 10
hrs). None of the specimens exhibit unusual or more rapid
crack growth rate behavior than the base metal (Figure 20).
SCC Growth Rate Behavior and Predictions
There are a number of indications that the environmental
cracking susceptibility of Alloy 22 is very low. Growth at
constant load was directly observed on specimen c153 (Figure
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Payer, J.H. SCC INITIATION AND GROWTH RATE STUDIES ON TITANIUM GRADE 7 AND BASE METAL, WELDED, AND AGED ALLOY 22 IN CONCENTRATED GROUNDWATER, report, August 1, 2005; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc779941/m1/4/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.