ML082340114
| ML082340114 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 08/31/2004 |
| From: | Mills W, Wire G Bechtel-Bettis |
| To: | NRC/SECY/RAS |
| SECY RAS | |
| References | |
| 06-849-03-LR, 50-271-LR, Entergy-Intervenor-NEC-JH_66, RAS M-231 | |
| Download: ML082340114 (11) | |
Text
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Gary L. Wire William J. Mills Bechtel Petis, Inc..
AWesl Mitflin, PA 15122-0079 U.S. NUCOLEARl REGULAIORY 00WS~
Cket No.
M Otcal EAMbi No.
fERED by: Applhant~Icensee Intle NP staff Other S
Witro essPanel I
Introduction NEC-JH_66 Fatigue Crack Propagation Rates for Notched 304 0Stainless Steel Specimens In Elevated Temperature Water Fatigue crack propagation (FCP) ratesfor 304 stainless steel (304 SS) were dejermined in 24'C and 288'C" air and 288"C water with 20-60 cc 1-1,/kg 1-1,0 using double-edged notch (DEN) s.pecimens. Tests perfornied at matched loading conditions in air and water provided a direct comparison of the relative crack growth rates over a wide range of test conditions. Crack growth rates of 304 SS in water were about 12 times the air rate for both short cracks (0.03-0.25 num) and long cracks up to 4.06 nin beyond the notch, A
which are consistent with conventional deep crack tests. The large environmentol degra-dation for 304 SS crack growth is consistent with the strong redertion of ftiigue life in high hydrogen wa/e: Further. very similar enviromnpental efl'cts were reported in finigue crack growith tests in hydrogen water chemisot, (HWC). Prior to the recent tests reported by Wire and Mills Il/ and Evats and Wire 12]. most literature data in high hrdrogen water showed only a mild environmental 4ffect for 304 SS. of order 2.5 times air or less.
However, the tests were predominanllv perforned (t high cyclic stress intensities or hiegh frequencies where environmental elfects are saall. The environmental effmct in low oxygen environments at low stress intensir, depends strongly on both the stress ratio. R, and the load rise time. T,. Fractographic. exatmitations were pemJi.'rmed on specimens tested in both air and water to understand the operative cracking inechoaisntis associated with environmental effects. In 288°C water, the fractnre setafces were crispy fiaceted with a crystallographiic appearance, and showed striations under high magnificarion. The clean age-like facets suggest that hydrogen enibrittlentent is the primary coause *f acceler-ated cracking. [DOl: 10.1115/1.767959]
The double-edge notched uniaxial specimen provides two sites 4
stainless steel (304 for crack initiation. It provides an advantage over compact tension termperature aquseous specimens itn that it can be tested in both tensionienlsion and instrumented fatigue tension-compression loading conditions. Tests were performed On-specimens with two der load control it) fully reversed (R = -- I) and tension-tension san, reported by Wire loading (R=0). Alignment was achieved by manually adjusting signed to determine the pull rod to minimize bending stresses, which were monitored itiation but also pro-by strain gages attached to the specimen (Fig. I). Once a satisfac-ow and long cracks.
tory alignment was achieved, the strain gages were removed and ined in air and water the EPD leads were attached. For the tests in water, the assembly tiivalent crack sizes was then enclosed in an autoclave, which was filled with water e environmental ef-and heated to 2880 C. Deaerated water containing 20 to 60 cc that this trend was H /kg H20 was used.in this study. The room temperature pl-was 4 10.1 to 10.3. and the oxygen concentration was less than 20 ppb47g The specimen was cycled until crack initiation was detected,.
based on the electrical potential drop reading corresponding to crack growth of 0.13 mm. Following an interim visual inspection.l4-cycling was continued to obtain crack extension data.
ned front a 127 aun The crack growth rate da/dN was calculated using the secant per ASTM E 8231 method applied to the average extension -curves, as discussed by 46 MPa. The chemi-Wire [I]. Crack growth rates were obtained at extensions as low rovided in [1]. The as 0.013.nm in order to investigate possible short crack effects.
with a grain size of For. conventional deep cracks, rates were calculated over larger increments of crack extension.
performed in air at For shallow cracks, of depth L<p from the notch, the stress d 288'C water. The intensity factor solution developed by Schijve [4.1 for a crack ernia-current reversal was nating from an edge notch was used to compute K. When the s detailedin 1.
crack depth exceeded the notch root radius, the conventional stress intensity factor solution developed by Tada et al. [5] for Divisino fr publicatio DEN specimens, which is based on the total crack depth including Mialoscript received by
.nber 23, 2004. Editor:
the notch depth, was used to calculate K. The transition between the two formulations was made at L=p. It is noted that an inde-Fatigue crack propagation data for Ty'pe 30 SS) were obtained in air and an elevated environment. The data were developed from tests on double-edged notched (DEN) fatigue different notch root radii p of 0.38 and 1.52 r ei at. [3]. The fatigue tests were primarily de the effect of notch radius oil fatigue crack ini vide fatigue crack growth data for both shall Direct comparison of crack growth rates obtai under identical loading condilions.and for eq demonstrates that 304 SS experiences a larg feet, and the. detailed analysis below shows supported by all tests.
2 Experimental The DEN specimens (Fig. 1) were siachin diameter bar forging with an L-C orientation with yield and Ultimate strength of 288 and 54 cal composition of the 304 SS material is p rincrostructure consists of nonssensitized grains ASTM 2.
Load-controlled cyclic fatigue tests were roots) temperature and 288'C and in deaerAle electric potential drop (EPD) technique with c P
1 lised to monitor crack initiation and growth, a
-Contnibuted by the Prssure, Vessels and Piping in the JOURNAL o0' PRESSURE VESSEL "TECnNOLOGy.
- "the PVP Division May 29, 2003; revision received Dee S. Y. Lameik."
- 318 6oý
. 3 6, AUGUST 2004 USNRC August 12, 2008 (11:00am)
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FIg. 1 Double-edge notehod fatigue specimen with EPD (Grip details not shown, all dimensions in mam) p endent K solution for a double-edge notched plate by Yamamoto L6] provided results within 4% for shallow and intermediate crack lengths over the range of the present test$. For both the fully reversed and tensiofi-tenpion tests, the stress intensity factor range (AK) is defined as the difference between K at maximum and minimum loads (i.e., AK=K,ý,-Kmi)'. Crack asymmetry is a potential problem with the DEN specimen. However, the largest difference observed between the two cracks was 2 mm out of an overall crack length (D + L) of about 9 mm. The 2 mm difference is less than 5% of the specimen width of 38 mina, indicating crack asymmetry is not a problem for this data.
Broken specimen halves were examined on a scanning electron microscope (SEM) to characterize the fatigue fracture surface morphology. The crack length associated with each fractograph was detenrmined so fracture surface features could be correlated with crack growth rates and applied 6X levEls. Relative amounts of a' maxtensite on fracture faces were estimated using a com-mercial ferrite measurement instrument (Feritscope MP3C).
While fracture surface roughness and the presence of only a thin layer of rnartensite precluded precise measurements, relative amounts of martermitc were readily determined.
3 Test Results 3.1 Short Crack Effects.
Before examinifng envii0nMetttal effects, it is appropriate to evaluate the cracking behavior for ahort versus long cracks. Crack growth rates for short cracks can be much larger than long crack data due to differences in crack clo-sureaccording to Newman [7). Crack closure in the crack wake reduces the portion of the load that is effective in growing a crack.
However, short cracks have little or no crack wake, and closure is subsequently reduced. To examine for such closure effects, the growth rates in air for the DEN at R = - I were compared directly to growth rates from conventional deep crack tests in air at 288'C, R = 0 per James and Jones 8.]:
daldN= l.40X 10- 9 AK" 7, mm/cycle, AK in MPaIm (2)
The ratio of the DEN rates to rates for long cracks from Eq. (2) are shown in Fig. 2. It was convenient to use only the positive portion of the loading to calculate the air rates for long cracks, as 0 2T608 B,71M Pa, 1.52 n*"
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1T601A,5uMPa.0.3arm 10o 00J 00 0.0 0.2 0.4 0.6 0.o 1.0 e/rho Fig. 2 Short crack growth rates from DEN specimens. Rates normalized by deep crock rates using positive AK, Eq, 2 the ratio in Fig. 2 then decreased to values near unity at large crack depths. The figure shows that the short crack growth rates can exceed the deep crack rates by a factor of tbiry, but that the ratio quickly approaches a stable value as the crack depth be-comes significant compared to the notch radius. The value of the ratio at larger crack depths ranged from approximately one to four for the tests shown, implying that the tensile portion of the loading is largely responsible for the crack propagation at large crack depths. For the particular notch depths studied here, short crack effects are only important below Lip of order 0.2. Therefore, shal-low crack effects can produce an order of magnitude increase in crack growth rates under fully reversed loading conditions, but this acceleration is confined to very small crack extensions, on the order of 0.1 to 0.3 trm. For longer cracks, conventional test data for deeply cracked specimens can be used to predict cracking behavior.
The increased rates observed for short cracks near notches is consistent with increased effective stress intensity, as reviewed in depth by Lalor, Sehitoglu, and McClung [9]. They observed that the crack opening stress. increased rapidly with increasing crack depth and leveled out for crack depths above approximately 40%
of the notch radius. They were able to explain the observed crack opening stresses on the basis of finite element analysis of crack closure effects.
. 3.2 Environmental Effects by Comparisox to Controls.
The effect of environment on fatigue crack gowth can be seen by directly comparing the data from 288"C air and water testS, as controls were run in air at the same or very similar loading con-ditions to the tests in water. This allows a direct assessment of environmental effects down to the smallest detectable crack ex-tensions, while avoiding the treed for an explicit treatment of short crack effects. Hence, the daldN values in air and water are com-pared directly at the same crack extension and cyclic stress. This assures that crack driving forces are the same, without having to explicitly calculate them.
Figure 3 shows conclusively that the crack growth rates in wa-ter are much enhanced over rates in air. The ratio of crack growth rates in water' over air is called the environmental ratio (ER), for convenience. At a stress amplitude of 69 MPa and lowest fre-quency tested of 0.0033 Hz, the BR is 15 (Fig. 3(a)). The large ER in water observed in Fig. 3(s) pcrsjsttd to the end of the test, where the crack extension was 4.1 mm. Hence, large environmen-tal effects continue to crack depths of engineering significance, Journal of Pressure Vessel Technology AUGU5T 20o4, Vol. 126 / 319
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Crmak Eat.nWin, Mm IQ Fig. 3 Environmental effects in DEN tests. Stress amplitude and other test parameters on plots and are not just a short crack phenomenon. Figure 3 shows that the ER was large at the smallest detectable crack extensions of about 0.025 nun. Hence, the increases in crack growth rate 6x-plain the reductions in fatigue life reported in the literature [10],
Several other trends are worthy of note. Higher frequency led to a smnaller ER of IOX, as shown by comparing Mig, 3(b) and Fig.
3(a). The apparent increase in ER with decreasing frequency is consistent with the reduction of fatigue life at low strain rate noted by Cbopra and Smith [10]. The ER for a higher sttres amplitude (Fig. 3(c)) is only about a factor of 8X compared to 15X in Fig.
3(a) at the same low freqbuency, indicating a reduction. in ER at high stress amplitude and crack growth rates. The ER at R = 0 is also smaller, as shown in Fig..3(d), This may be further evidence of an effect of higher effective loading for a given stress ampli-tude provided by R=0 compared to R-= - 1, which has a com-pressive half cycle, Also, the ER in Fig. 3(c,d) decreases at the largest crack extensions. which correspond to the highest stress intensity. Such an effect is consistent with the gq.neral notion that at high loading, mechanical effects will dominate.
3.3 Environmental Effects Using 'Time-Based Plots and Comparison to Literature. From a fundamental point of view, the crack tip strain rate is the appropriate crack driving parameter to correlate environmentally assisted crack propagation tate data, as reviewed by Scott [11]. However, a unique method of crack-tip
.J strain-rate calculation could not be establilshed and variability in calculated values was over a factor of ten between various mod-els. Shoji at al. (12] suggested using the time-based rate in air as a practical correlating parameter representing crack tip strain-rate 320 / Vol. 126, AUGUST 2004 for low alloy steel fatigue crack growth data. This vatiable was used successfully to correlate environmental effects on low alloy steels in water. The time-based crack growth in air (daldt), is defined as (da/dt),=(dc/dN)6, 1 IT, where T, is the load rise time (3a) and the time based environmental rate (daldr), in water is (da~dt),= (daldN),1 T, (3b)
Eq. (3) is appropriate for fatigue crack growth tests with continu-ous cycling, which produce a tineindelen1ent rate such as seen in the present tests. In the event that stress corrosion crAcking or other time-dependent behavior is operative, the total time would be more appropriate-in Eq. 3, The strong environmental effects observed on 304 SS are cor-related reasonably well by utilizing a time-based plot, as shown in Fig. 4, although data variability is large. The air rates for DEN specimens were determined directly from the control tests in air, as shown in Fig. 3. The 304 SS DEN data (diamonds) show a clear increase in crack growth rates relative to those in air at low air rates, and are consistent with the degradation of fatigue ife of up to 15X reported by Chopra and Smith [10] and 13X reported by Leax [13]. As noted above. the large ER did not diminish in one test up to a crack depth of 4.1 mm. This depth iS greater than associated with "hourt crack" effects and is significant from an engineering standpoint. Subsequent tests on conventional compact tension specimens at this laboratory, represented by the circles in Transactions of the ASME M.
2002 19:59 30176233511 NOVERFLO NO\\I~iRFLOPA~GE 03 4
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1 DO 10-6 Air Rale. mn*a FIg. 4 304 SS DEN crack growth rates In water vs air-Torend shows decroeaed ER at high alr rate. Air rates Ere caleulaetdi directly from control tetat Fig. 4, verify that the environmental effect continue3 unabated to a crack growth of 17 mam, as reported by Evans and Wire [2). For 304 SS CT data. the baseline crack growth rate in air, (daldN)r,,
was determined Via [8] for the appropriate test conditions (ie.,
AK, R, and temperature). It is also noted that the agreement be-tween short crack and long crack results indicates that there is no "chemical" enhancement of crack growth of short cracks, such as reported by Gangloff [14) for high strength steel in a NaCl solu-tion.
A review of fatigue crack propagation of austenitic stainless steels was performed recently by Shack and Kassner [15). Data from surface crack tests performed in low oxygen "bydrogen wa-ter chemistry" (HWC) environments by Prater et a]. [ [6] are com-pared with DEN data in Fig. 5. HWC is BWR water chemistrv with hydrogen added to cootrol the electrochemical potential, The literature tests on surface crack spccimens tested in HWC water confirm that the large environmental effects shown here have been observed previously. Overall, the surface crac.k tests from the bt-Air rate, mrnPi Fig. i Comparison o? DEN to CT data in H1WC Largo environ-Mental effeets extend to low air rates eraturc average about 14 times the air rate, very sbrilar to DEN data. Indeed, crack growth rates in HWC at frequencies between 1.67X 10-2 and 5.56X 10-4Hz are identical to those obtained in this study. The fact that the stainless steel studied by Prater was sensitized does not appear to be important, as tlh cracking mode was transgranular. Gordon et al. (17] indicatedl that the fatigue crack growth rates in HWC water were the same for solution annealed and sensitized 304 SS. and )ewett em al. [18) reported very similar rates in these materials as well as welds.
A comparison of crack growth data trends from DEN tests and selected conventional compact tension test data in HWC is pro-yided in Fig. 6. The DEN and CT data are in good agreement in the intermediate growth rate regimes where bomb specimen types were evaluated. Moreover, the data by Ljunberg [19] show even greater enhancement in the low crack growth rate regime. These results provide further support for the observation that environ-mental effects tend to increase in the lower stress intensity regime whore crack growth rates In air are reduced.
The tests by Andresen and Campbell [20] show evidence for a transition to reduced environmental effects at Ifgh equivalent air rates, and more limited data by Gordon et al. (117] are consistent with such an effect. It is noteworthy that the DEN data agree qualitatively with HWC data in Fig. 6, including evidence of a transition to substantially lower environmental offects at equiva-tent air rates above 10- i rm/s, The hydrogen level for the HWC test data mu Figs. 5 and 6 is 150 ppb ox less, much less than several ppm in :the current DEN tests and in PWR water. Although the corrosiou potential in HWC is typically about 0.3 V SHE higher than that in water with higher hydrogen used in the present tests, according to Oilman [21], the overall crack growth rate response in the two environments ap-pears to be similar.
2
'5 3.4 Effects Of Stress Ratio, Stress Intensity,, nod Rise Time.
V 1.*6XI-1-z Fvans and Wire [2) performed a series of tests on a 1.9T CT DEN 1l.t
- p57)1OH ecimieu (thilckness=24.1 mm) of the same heat and water eon-ditions used it the DEN tests. The CT teats sihowed that large cnvirotproental effects occurred in conventional, deeply cracked compact tension specimens with high hydrogem levels and the lo'* *o~s *o"
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attendant lower potential. Results from the DEN' tests and the Air rate, mm/3 compact tension tests by Evans and Wire (2002) are showrn in Fig.
- 7. For DEN data represented in Figs. 7-8, the full cyclic Stress sparlson of DEN to surface crack datp, Surface range and crack extensions increments of 0. 3 2 man or larger were ested In HWC [S]
employed.
I FIg. 5 Con crack data ti JournAl of Pressure Vessel Technology AUGUST 2004, Vol. 126 / 321
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10-4 10-5 10 AK, MPa-m0" 10 6K, MPa-mo' Fig. 7 304 SS crack growth rates In water at 28) °C. Rise time (T,) is In seconds The results in Fig. 7 indicate a strong effect of both T, and A.
The environmental effects can be rationalized in terms of a com-bined mean stress effect on closure or R ratio and a ri.e time or freqtuency effect, consistent with the literature. Bamford [22]
noted larger environmental effects at higher R ratios. He incorpo-rated an effective AKo*=K,,*(I--R) 05, which shifted the high R data more in line with low I? data. Cullen [23] reported strongly increased FCP rates for cast stalrness steel at higher R in PWR uater. Thedata by Bernard et al. [24] on Z3 CNDI17-12, similu to
.,jl 6 SS, showed a clear rise time effect in PWR water. Recently, a correlatiork for FCP of austeoitic stainless steels in BWR water was developed by ttatani et al. [25]. The correlation was of the form da/dN=A(AK)"T/,1(I - R)P with m=3.0, n= 0.5, aidand p=2.12 (4)
Fig. 9 Normalized 304 SS CT FCP Data at Low Potential. Nor-malization: (1-R)-R' 1, "T.ý-O to R0,T,-1SO a This formulation indicates a strong role of T, and R, consistent with results described above in low oxygen water. It is noted that the present crack growth rates are similar to those reported by Itatani et al. t25] in BWR water in the few cases where data are ava*l table at similar T,., R, and A K, Figure 8 shows that the crack growth rates in Fig. 7 can be reasonably well normalized by T11'31 and 1/(J -IR)"., Hence, the form of the correlation developed by Itatani et al, for tension-tension appears to be promising for tension-compression. Further, the presetot rise time and stress ratio R ratio dependence are consistent with all but the very high R (0.95) BWR water data utilized by Itatani et al. [251. Figure 9 shows that normalization in Pig. 8 worked successfully on data from compact tension tests at high R by Evans and Wire [2] and at low R by Bernard et al. [24]. Both data sets include long rise times (450-500 s) where envirotimentaI effects are substantial.
The plot shows that ER reduces to about 2X at larex &K. While the selected parameters values correlate these limited data sets, much ruore data would be required to obtain-a definitive coorelation.
4 Characterization of Fatigue Crack Propagation Mechanisms Frecture surface features for specimens tested in air and water were evaluated to correlate operative cracking mechanisms with environmental cracking behavior, The fracture surface appea-race for specirne1ns tested in roora temperature air was found to be dependent on loading conditions. A faceted morphology (Fig.
10(a)) was observed at crack growth rates less than I X10-t4nV/cyCee, vast fields of well-defined striations were gen-erated betwveen I X 10-4 and I X 10-3 ram/cycle, and a combina-tion of fatigue striations and dimples (Fig. 10(b)) was observed above I X I a0-) m/cycle. The nature of striated fracture surfaces in the intermediate and high crack growth rate regimes resrnbles that typically observed in FCC materials, but the facets formed at low crack growth rates are rather unique, as discussed below, Evidence of rubbing and ftetting (Fig. 11) due to repeated contact between mating fracture surfaces was observed in specimens tested under fully reversed cyclic loading conditions.
Facets generated at low crack growth rates had an irregular appearance that was associated with a quasi-cleavage mechanism Data N6rmallzod tD ROD. T"=150*
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7'3to R=O, T(.Il50 9 crack growth rates were less than I X 10-4 turn/cvcie. Bccause 322 / Vol, 126, AUGUST 2004 TransactIons of the ASME
I Fig. 10 Fractogtaphs of 304 St tested In 241C air. (a) Irregular quaei-cleavage facets at da/dN=ex10-' nam/cycle. Arrow de-notes failed twin boundary- (b) Striations and dimples at i Xi0-2 ramtcycle 304 SS is a metastable alloy at room temperature, the material directly ahead of an advancing crack undergoes a strain-itdueed transformation to a' martensite. Therefore, cracks propagate through martensite, which results in a quasi-cleavage morphology that re.*embles the quasi-cleavage fracture surface appearance in martensitic steels. Forritescope measurements showed that all fa-tigue fracture surfaces generated at room temperature contained a' martensite, with the amount of martensite increasing at higher stress intensity factor levels due to larger plastic zone sizes.
The morphology of the quasi-cleavage facets was consistent with the fracture surface appearance for 304 SS (Gao et al. [261) and high purity Fe-lSCr-I2Ni SS (Wei ct al. [27]) tested in room temperature air, 3.5% NaCI solutions and hydrogen. Strain-induced a' martcnsite forrncd ahead of fatigue cracks in both alloys, which caused a quasi-cleavage mechanisin. Unlike 304 SS, 316 SS fatigue tested in room temperature air (Mills (283) exhib-ited more conventional, cleavage-like facets. Because 316 SS is a more stable alloy due to its higher nickel content, cc' martensite transformation does not occur at room temperature; hence, it ex-hibits classic, cleavage-like faceted growth as cracks propagate through stable austenite, In the tow crack growth rate regime, 304 SS also exhibited localized cracking along annealing twin boundaries, but no evi-dence of intergranular cracking. Localized separation along favor-ably oriented twin boundaries produced fiat, featureless facets that appear as dark islands, surrounded by quasi-cleavage facets. Gao et al. [263 and Wei et at. [27] also reported twin boaindary crack-ing in 304 SS and high purity Fe-ISCr-l2Ni SS, Facets formed in 288°C air had a different morphology. as they took on a more conventional cleavage-like appearance. Compari-Journal of Pressure Vessel Technology Fig. 11 Repeated contact between crack surface, (R=-1). (a)
Rlub mark*s at low AK ltvels in 288C aIr. (b) Striations our-rounded by severely rubbed regions (240C air).
son of Pigs. 12(a) und 12(b) shows that the high temperature facets had more of a cystallographic nature with some evidence of" river patterns, in contrast with the irregular facets generated in room temperature air. The lack of quasi-cleavage facets indicates that 288'C is above the critical temperature where cold working induces a martensrite transformation (i.e., MU, temperature), Based on the composition of 304 SS, the MD temperature associated with 30% cold work is on the order of 100-C (Lacombe [293).
- Indeed, Ferriteseope measurements showed no detectable a'-martensite on fatigue fracture surfaces generatod at 288 0C, At crack growth rates slightly above I X 10-4 mm/cycle, facets formed in 289'C air were poorly defined and parallel fracture markings associated with slip offsets were often superimposed on them, The transition to poorly defined facets is believed to be associated with a transition from beterogeneous-to-homogeneous slip. Fracture surfaces generated in 288'C water were remarkably different than those generated in air. Facets formed in water had a crystallographic appearance with well-defined river patterns, as shown in Fig. 12(c). The sharp, cleavage-like facets formed im-mediately adjacent to machined notches and well away from the notclhes, indicating that the same faceted growth mechanism was operative for shallow and long cracks. Moreover, well-defined crystallographic facets persisted over the entire range of crack growth rates generated in this program, including crack growth rates as high as 8)X 10-4 mm/cycle where fracture surfaces gener-ated in air exhibited poorly defined facets and vast fields of fa-tigpe striations. There was no evidence of either intergranular cracking or annealing twin boundary cracking in 288°C water.
Although fracture surfaces generated in 2880C water exhibited crisp cleavage-like facets, high magnification of facet faces re-'
vealed the presence of fatigue striations (Fig, 13). At crack growth' rates from I X 10- to 3 X 10"' 'nm/cycle, parallel fracture mark-AUGUST 2004, Vol. 126 / 323 t7o 17393Vd 0-1Jd3GAON T19EZ9LI13E 6S:61 BOOZ/TO/90
Fig. 12 FRactographs of 304 SS tatigue tested In (a) 24"C air showing Irregular facets (b) 2M8C air showing cleavage-lIke facets (c) 288WC water with crystallographic facets that nre sharp, cleavage-like, and highly angular.
illg. on the facets wer very straight, but their spacing was iden-tical to macroscopic crack growth rates indicating that they were fatigue striations. At growth rates above 3 X 10-4 mtl/ycle, Stria-tiaras had a ductile or wavy appearance, as shown in Vig-13(b).
Pacet and striation orientations on fracture surfaces generated in 2889M water revealed that local cracking directions. were often very different from the overall cracking direction. Although facets were usually aligned iia the cracking direction, some were aligned normal to the macroscopic cracking direction (Figs. 12(c) and "j(a)). LLkewise, most striations were oriented normal to overall W08stg direction, but in some regions striations had different tations, and in some cars were even parailel to the macro-scopic craclkng direction, These observations indicate that crack advance in water involved a very uneven process, as cracking first 324 / Vol. 126, AUGUST 2004 Fig. 13 Fractographs of 304 8S fatigus tested In 288"C water.
(a) Highly angular fasts persist to 3X10- 4 mm cycle. (b) High magnification of (a) shows fatigue striations superimposed on facet faces occurred in the most Susceptible regions, which left ligaments in the wake of the %dvancing crack front. As the overall crack con-tinued to extend, local stress intensities within the mote resistant ligaments increased to the point where cracking reinitiated and propagated across the ligaments. As a result, local cracking direc-tions within these ligaments were often normal to the overall cracking direction. The rapid crack advance in the more suscep-tible regions is believed to be a significant contributor to the en-vironmental acceleration observed in high temperature water. Spe-cifically, this rapid cracking not only increased the overall crack length, it increased local stress and provided alternate paths for reinitiating local cracks along the more resistant ligaments.
The role of active path dissolution versus hydrogen embrittle-ment in. causing accelerated cracking of stainless steel in high temperature water remains an issue because of the coupled nature of these processes, as electrochemical reactions near the crack tip involve both anodic dissolution of the metal and a cathodic reac-tion that produces hydrogen. The presence of well-defined crys-tallographic features indicates the absence of significant metal dis-solution, thereby suggesting that slip/dissolution is not the primary cause of accelerated cracking. This observation is consis-tent with findings by Chopra and Smith [101 that crack growth rates for 304 SS are greater in low dissolved oxygen water than in high dissolved oxygen water. This observation cannot be recon-ciled with a slip/dissolution mechanism.
The presetnce of sharp, crystallographic facets suggests that a hydrogen ernbrittlemcnt mechanism is responsible for accelerated cracking in 288*C water. This is supported by fractorg,- pbic find-ings by 14aoninea and Hakarainen
[30) where hydrogen-Transactions of the ASME L:
.$.L
19:59 3el7623511 NVRL cG l
NOVERFLO PAGE 05 I
K) 4 precharged 304 SS exhibited cleavage-like facets without any dc-e tectable tx' martensite formation. The facet morphology for the hydrogen-precharged specimens is very similar to that observed in 288gC water, thereby implicating hydrogen in promoting acceler-ated cracking in high temperature water. Moreover, Gao et al. [26]
and Wei et al. [27] demonstrated that a hydrogen embrittlerment mechanism was responsible for accelerated fatigue crack growth rates in stainless steel alloys tested in room temperature aqueous environments. Although r'p martensite formation occurred in.these specimens, Gao and Wei determined that this transformation did not have a critical role in controlling crack growth rates and it was not a prerequisite for hydrogen embrittlement.
Although hydrogen embrittlement is believed to be the primary cause of environmental cracking in 2881C water, it is possible that oxide film formation at the crack tip also affects cracking behavior by restricting slip reversals during the unloading portion of fatigue cycles. The importance of oxide film formation in affecting frac-ture surface morphology is apparent when comparing fracture sur-faces generated in air and vacuum. Fatigue fracture surfaces gen-erated in air possessed crystallographic facets. whereas those generated in vacuum had a nondescript, nonfaceted appearance (Wire [1]). Apparently, the thin oxide hikes that forms in 24'C air serves as a dislocation barrier that impedes slip reversals during unloading cycles. Hence, damage tends to be concentrated along particular slip bands, and eventually local separation along these slip bands produces crystallographic facets, In vacuum, the ab-sence of an oxide film promotes more effective slip reversals that minimizes local damage along any particular slip band. As a re-suit, crystallographic facets do not develop in vacuum. Oxide film I formation in water is also expected to restrict slip reversals and promote facet formation and higher crack growth rates; however, the degree of acceleration is expected to by much less than that[
associated with hydrogen erobrittlement.
In summary, it is unlikely that slip/dissolution is a primaryl cause of environmental cracking in 288'C hydrogenated water because of the presence of crisp crystallographic features and an increase in crack growth rates with decreasing dissolved oxygen levels (Chopra and Smith, (10]). The cleavage-like facets on the fracture surface, which are very similar to facets found in hydrogen-precharged 304 SS (Harninen and Hakatainen [30]),
suggest that hydrogen embrittlement is the primary cause of ac-celerated cracking in high temperature water. It is also likely that the formation of crack tip oxides restricted slip reverials which also contributed to increased crack growth rates, although this effect is expected to be much smaller effect than that associated with hydrogen embrittlemcnt, 5
Summary and Conclusions Instrumented corrosion fatigue tests on 304 SS DEN specimens provided fatigue crack growth rate data in 249 and 2ý88C air and 288°C water Over a wide range of crack growth rates. Results in air and water at the saree mechanical parameters allowed direct assessment of environmental effects, avoiding any concerns for data variability due to materials, test technique, and data correla-lJ tion, Crack growth rates in water are about 12x times the air rate at low speeds where the environmental effects are largest. The large environmental degradation in crack growth is consistent with the strong reduction of fatigue life in commercial PWR wa-ter Further, very similar crack growth rate data were reported in low oxygen HWC. in both s5urface crack and conventional deep eeok tests. The large environmental enhancement in 304 SS (12X) persisted to crack extensions up to 4.1 tmim. Far outside the range associated with short crack effects. The same large environ-mental effects observed in the DEN tests were reproduced in CT upecimens at a high stress ratio and low AK. The overall results can be normalized successfully by incorporating the combined effects of stresS ratio and rise time, qualitatively similar to the formulation developed by Itatani et al. to describe test results in BWR water.
Much of literature data in hydrogenated water chemistry shows an apparently mild environmental effect for 304 SS, with an ER of 2.55 or less. However, based on the current test results, larger environmental effects occur in) bydrogenated water in the low AK regime at long rise times and high R-ratio conditions.
The high crack growth rates in 2880C deaerated water were associated with a faceted growth mechanism. The hightly angular, cleavage-like appearance of the facets suggests that a hydrogen embnittlement mechanism was the primary cause of accelerated cracking in this environment, Acknowledgment This work was performed under U.S. Department of Energy Contract with Bechtel Bettis, Inc. The authors wish to acknowl-edge the efforts of H, K. Shen, A. L. Bradfield, J. T. Kandra, and J.
I. Chasko in performance of these experiments.
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Iý 326 / Vol. 126, AUGUST 2004 Transaction, of the ASME