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BWR pipe crack remedies evaluation
Nuclear Engineering and Design 108 (1988) 199-210 North-Holland, Amsterdam
BWR PIPE CRACK REMEDIES W.J. SHACK,
T.F. KASSNER,
199
EVALUATION P.S. M A I Y A ,
*
J.Y. P A R K
and W.E. RUTHER
Materials and Components Technology Division Argonne National Laboratory, Argonne, lllionois 60439, USA Received March 1987
This paper presents results on: (a) the influence of simulated BWR environments on the stress-corrosion-cracking (SCC) susceptibility of Types 304, 316NG, and 347 stainless steel (SS), (b) fracture-mechanics crack growth rate measurements on these materials and weld overlay specimens in different environments, and (c) residual stress measurements and metallographic evaluations of conventional pipe weldments treated by a mechanical-stress-improvement process (MSIP) as well as those produced by a narrow-gap welding procedure. Crack initiation studies on Types 304 and 316NG SS under crevice and non-crevice conditions in 289°C water containing 0.25 ppm dissolved oxygen with low sulfate concentrations indicate that SCC initiates at low strains (3%) in the nuclear grade material. Crack growth measurements on fracture-mechanics-type specimens, under low-frequency cyclic loading, show that the Type 316NG steel cracks at a somewhat lower rate ( - 40%) than sensitized Type 304 SS in an impurity environment with 0.25 ppm dissolved oxygen; however, the latter material stops cracking when sulfate is removed from the water. Crack growth in both materials ceases under simulated hydrogen-water chemistry conditions (5 ppb oxygen) even with 100 ppb sulfate present in the water. An unexpected result was obtained in the test on a weld overlay specimen in the impurity environment, viz., the crack grew to the overlay interface at a nominal rate, branched at 90 ° in both directions, and then grew at a high rate (parallel to the nominal applied load). Residual stress measurements on MSIP-treated weldments and those produced by a narrow-gap welding procedure indicate that these techniques produce compressive stresses over most of the inner surface near the weld and heat-affected zones.
1. Introduction Cracking in sensitized austenitic stainless steel (SS) p i p i n g a n d associated c o m p o n e n t s in boiling water reactors ( B W R s ) has b e e n observed since the mid-1960s. Proposed remedies include: (1) procedures that p r o d u c e a favorable residual stress state in the weld regions, (2) r e p l a c e m e n t of the piping with materials that are more resistant to stress corrosion cracking (SCC), a n d (3) modification of the reactor coolant e n v i r o n m e n t . Studies have been carried out t h a t have i m p o r t a n t implications for all three classes of p r o p o s e d remedies. These studies include fracture-mechanics crack-growth-rate tests o n Type 3 1 6 N G SS a n d weld overlay specimens in i m p u r i t y a n d high-purity e n v i r o n m e n t s , metallographic a n d residual stress studies o n w e l d m e n t s treated b y the m e c h a n i c a l - s t r e s s - i m p r o v e m e n t process (MSIP) developed b y O ' D o n n e U a n d Associates, a n d constant-exten-
* Work supported by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission under Interagency Agreement DOE 40-550-75.
sion-rate-tensile ( C E R T ) tests o n the G e r m a n N u c l e a r G r a d e Type 347 SS a n d o t h e r steels to d e t e r m i n e the influence of reactor coolant impurities (e.g., SO 2 - , Cu z+) on SCC susceptibility. In addition, the effect of g a m m a r a d i a t i o n of simulated reactor-coolant environm e n t s on the corrosion potential of stainless steel is being investigated. Together with related studies o n the characterization of the SCC b e h a v i o r of irradiated stainless steels these results will p e r m i t a b e t t e r assessment of the potential for irradiation-assisted SCC of materials in the reactor core.
2. SCC of Type 316NG stainless steel 2.1. Fracture-mechanics crack growth tests F r a c t u r e - m e c h a n i c s crack growth rate tests have b e e n p e r f o r m e d o n Type 3 1 6 N G SS ( H e a t No. P91576), to confirm that the susceptibility to t r a n s g r a n u l a r SCC ( T G S C C ) observed in C E R T tests c a n also occur u n d e r less severe mechanical loading. I n different phases of the tests, the water chemistry conditions were varied
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W.J. Shack et a l . / B W R pipe crack remedies evaluation
Table 1 Chemical composition (wt%) of austenitic stainless steels used in fracture-mechanics and CERT tests Material
Heat No.
C
Mn
P
S
Si
Ni
Cr
Mo
Cu
N
B
Nb
Fe
316NG 316NG 316NG 347 347 304 304
P91576 467958 NDE-28 174100 170162 53319 30956
0.015 0.02 0.014 0.023 0.03 0.06 0.06
1.63 1.51 1.77 1.70 1.70 1.69 1.54
0.02 0.029 0.02 0.036 0.012 0.024 0.019
0.01 0.008 0.002 0.015 0.019 0.013 0.007
0.42 0.64 0.52 0.33 0.43 0.59 0.48
10.95 12.74 13.58 11.00 10.75 8.88 8.00
16.42 17.14 17.79 18.15 18.53 18.33 18.99
2.14 2.43 2.59 0.48 0.35 0.14 0.44
0.16 0.11 0.12 0.11 0.06 0.19
0.068 0.069 0.11 0.029 0.021 0.029 0.10
0.002 0.002 0.0006 0.0005 0.0005 0.0005 -
0.44 0.51 -
Balance
a
-
Material from a 28-in. diam. pipe.
from a reference condition of 0.25 p p m dissolved oxygen plus 0.1 p p m sulfate (as H2SO4) to lower oxygen c o n c e n t r a t i o n s at this sulfate level as well as higher a n d lower sulfate concentrations at a fixed dissolved-oxygen concentration in the feedwater. The experiments were performed in 6-I autoclaves equipped with MTS servo hydraulic load systems. The dissolved-oxygen concentrations of the feedwater were established by b u b bling appropriate o x y g e n / n i t r o g e n gas mixtures through the d e o x y g e n a t e d / d e i o n i z e d feedwater (conductance of < 0 . 2 / ~ S / c m ) contained in 120-I stainless steel tanks. A hydrogen cover gas was used in several experiments at very low dissolved-oxygen concentrations. In tests with impurities, sulfuric acid was a d d e d to the feedwater before sparging with the gas mixture or hydrogen to ensure adequate mixing. The pressure of the cover gas in the feedwater tank was adjusted to provide the desired autoclave effluent dissolved-oxygen concentrations as determined by a Leeds a n d N o r t h r u p Model 7931 oxygen m o n i t o r a n d verified by a colorimettic (Chemetrics ampules) technique. The flowrate of water through the autoclaves was 0.5-1.0 l / h . The tests included a lightly sensitized ( E P R = 2 C / c m 2) Type 304 SS control specimen (Heat No. 30956). The compositions of b o t h materials are given in table 1. Most of the tests were carried out with a m a x i m u m stress intensity of - 3 0 M P a - m l/z, a load ratio R of 0.95, and a frequency of 0.08 Hz, (i.e., a nearly c o n s t a n t load with a small superposed cyclic tipple), since reactors experience small fluctuations in pressure a n d temperature which impose a cyclic stress on the piping a n d other components. R ratios associated with n o r m a l operating conditions are, in general, 1.0 < R < 0.95, viz., - 0 . 9 8 , where a value of 1.0 corresponds to c o n s t a n t load. The crack growth rates for each specimen u n d e r the different water chemistry conditions, i.e., (1) through (6), are shown in fig. 1. The open-circuit corrosion
potential of a Type 304 SS electrode a n d the conductivity of the feedwater for each phase of the test are s h o w n in the insert panel of the figure, where the region above the curve defines the region of susceptibility for intergranular SCC ( I G S C C ) based on C E R T data for this lightly sensitized material. W i t h loading at R = 0.95, b o t h materials cracked u n d e r the reference chemistry [conditions (1) a n d (3)]; the crack growth rates were - 40% higher in the sensitized specimen. In a high-purity e n v i r o n m e n t with 0.25 p p m dissolved oxygen [condition (2)], crack growth ceased in the Type 304 SS specimen, b u t in the Type 3 1 6 N G specimen it continued at almost the same rate as in the impurity environment. U n d e r condition (4), the dissolved-oxygen c o n c e n t r a t i o n of the feedwater was decreased to 0.002 p p m with 1.2 p p m hydrogen a n d 0.1 p p m sulfate. U n d e r these conditions the electrochemical potential of Type 304 SS decreased from + 140 to - 6 3 0 mV(SHE), a n d crack growth ceased in b o t h specimens, even with sulfate present. In the next phase of the test [condition (5)], sulfate was not added to the low-oxygen environment, and the conductivity of the effluent water was reduced to 0.1 # S / c m . As expected, n o crack growth occurred in either specimen during the transition to the high-purity environment. W h e n the e n v i r o n m e n t was then changed to a simulated, high-purity B W R environm e n t with 0.25 p p m dissolved oxygen [condition (6)], crack growth resumed in the Type 3 1 6 N G SS specimen at a rate similar to that observed u n d e r conditions (1) a n d (3), while the crack remained d o r m a n t in the sensitized Type 304 SS specimen. After - 500 h, the loading was changed to c o n s t a n t load ( R = 1) conditions. The crack growth rate in the Type 3 1 6 N G SS specimen decreased by an order of magnitude c o m p a r e d with that observed u n d e r the R = 0.95 load, while the crack growth rate in the sensitized specimen increased to a level close to that observed in the Nuclear G r a d e specim e n u n d e r the R = 0.95 load.
201
W J . Shack et a L / B W R pipe crack remedies evaluation
10' WATERCHEMISTRY
'~
; ,oo ~
E
E
o
~
02, H2, HzSO~
~
LOADINGCONDITION
rr
R = 0.95
~ -3oo -lO
~
lO
f = 8x10 2 Hz
-4oo
Kma:{= 29-35 MPamI/;
0.1
1
10
CONDUCTIVITY
_
100
SENSITIZATION =
at 2 5 ° C ( p S / c m )
EPR,C/cm2
3o-~--qj 2
13~NCd
10
0
~ _ ~ ~ : : ~ _ 1,3
2,6
4
5
WATER CHEMISTRY CONDITION
Fig. 1. Effect of several simulated BWR water chemistries, within and outside of the IGSCC regime for sensitized Type 304 SS, on the crack growth rates of a compact-tension specimen of this material and a Type 316NG SS specimen under low-frequency, moderate-stress-intensity, and high-R loading conditions at 289°C.
Since the test is still in progress, no fractographic information on the mode of cracking is available. However, in previous tests with sensitized and solution-annealed Type 304 SS, where the behavior of the solution-annealed specimen was similar to that observed in the Type 316NG SS specimen in this test, the sensitized specimen cracked intergranularly under these conditions, as expected, while the solution-annealed specimen cracked transgranularly. It has been shown that the dissolution of inclusions in pressure vessel steels, viz., manganese sulfides, can influence the crack-tip environment and thereby the crack growth behavior of the materials [1,2]. This could conceivably occur in our materials as well. However, as shown in table 1, the phosphorus and sulfur levels in the two steels are similar and yet the crack growth rate in the sensitized Type 304 SS shows a consistent dependence on sulfate concentration in the bulk environment, in contrast to the Type 316NG steel. Analogous experiments that compared the crack growth behavior of lightly sensitized and solution-annealed specimens from the same heat of Typ 304 SS yielded similar results [3]. Therefore, differences in the crack-tip chemistry due to dissolution of the materials themselves do not seem to explain the observed behavior. Alternative mechanisms are being explored. 2.2. Crack initiation studies
C E R T experiments were performed to study crack initiation in these materials. The experimental methods
for water chemistry control in these experiments were the same as described above. Previous work has shown that in C E R T tests Type 316NG SS is susceptible to T G S C C in oxygenated water with sulfate additions [4,5]. The sulfate concentration required to produce cracking decreases very rapidly with strain rate, as shown in fig. 2, and at a strain rate of 2 × 1 0 - 7 s - 1 is less than 25 ppb. The C E R T test results also suggest that once a crack initiates, the crack growth rate is only weakly dependent on the concentration of sulfate, as shown in fig. 3. Although the C E R T tests involve large
'
'
'"'"1
'
''"'"1
' ' " ' "
'
TYPE 316NG SS (289*C)
0.1
L o
I
0.01
I I I I
I 0.001 10-8
I
I I 111111 10-7
I
I i I Illll 10-6
I
I I IIII 10-5
STRAIN RATE, s - I
Fig. 2. Variation of critical concentration of sulfate (required for SCC) with strain rate for Type 316NG SS (Heat No. P91576).
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W.J. Shack et al. / B WR pipe crack remedies evaluation
Impurity Effects on T G S C C
-0 f-----
10-9 u)
E
E
l
i
g. t'v
Type 316NG SS ( 289°C )
O
,< 10-10
I
0.0
0.2
,
I
,
0.4
I
0.6
i
I
0.8
,
I
10
,
1 2
Conductivity, pS/cm
Fig. 3. Effect of impurity concentration on average crack growth rate for Type 316NG SS at 289°C and a strain rate of 2 x 10 -7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.025 to 0.1 ppm sulfate.
plastic strains, the general trends predicted from these tests are consistent with the results of our fracturemechanics crack-growth-rate tests. These results have p r o m p t e d a study of the effects of impurities on crack initiation. In earlier work [6], i n t e r r u p t e d C E R T tests performed o n Type 3 1 6 N G SS showed that crack initiation (defined in terms of cracks 5 0 - 2 0 0 lam in length) occurs at relatively low strains. A more sensitive technique involves a C E R T specimen in which a small d i a m e t e r (0.86 m m ) through-hole is drilled in the center of the gage length. This geometry localizes the specimen region in which crack initiation occurs a n d enables the detection of very small cracks by scanning-electron microscopy (SEM). T h e test is i n t e r r u p t e d at low n o m i n a l strains, the specimen is then cross-sectioned as shown in fig. 4, and the surface of the hole is examined b y SEM. T h e strain c o n c e n t r a t i o n of - 8 for this geometry has b e e n estimated by N e u b e r ' s rule a n d finite-element calculations. The n o m i n a l strains can be accurately determined by measuring the distance between two gage m a r k s before a n d after the test. Figs. 5 a n d 6 show micrographs that illustrate the degree of cracking of Type 3 1 6 N G SS (Heat No. P91576) in an impurity e n v i r o n m e n t as a function of the nominal strain. These results suggest that crack initiation occurs at actual strains between 1.5 a n d 3% (assuming a strain concentration factor of 8) at a strain rate of - 2 x 10 -7 s -1. By comparison, results from similar tests on solution-annealed Type 304 SS (Heat No. 53319), which are shown in fig. 7, indicate that crack initiation occurs at strains greater than 3.0% for the same e n v i r o n m e n t s a n d strain rate.
Fig. 4. Cross sectional view of CERT specimen used for crack-initiation studies.
C E R T tests were also p e r f o r m e d o n Type 3 1 6 N G SS in e n v i r o n m e n t s with a n d w i t h o u t impurities u n d e r artificially produced crevice conditions. T h e specimen geometry was similar to that used for the initiation
(c) (p = 1.36 % CRACK INITIATION ;N TYPE ~NG
(d) (p=0.22 % SS
|289"C)
= 2 x 10 - 7 $-I
0.25 ppm 02 + 0.1 plm~ SO2-
Fig. 5. Micrographs of the region on the inner surface of the diametral holes in Type 316NG SS CERT specimens for crack initiation studies at a strain rate of 2 x 1 0 -7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. Nominal plastic strains in the specimen are indicated; localized strains near the hole are a factor of 8 higher.
203
W J . Shack et a l . / B W R pipe crack remedies evaluation
(o) N O ~ - C ~ V ~ , ~
|~t P'm I
~
0z
~b~ CR£VII~0,O.~5 I~pm02
CRACK INITIATION IN TYPE 3H~NG SS 1289"C) ~ = 2 x 1 0 - 7 S-I (Ep • 0.38 %
o . ~
o2 + o., ~
so2, -
Fig. 6. Micrographs of the inner surface of the diametral hole in a Type 316NG SS CERT specimen after a nominal plastic strain of 0.35% at a strain rate of 2 × 10-7 s - ] in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. Localized strain near the hole is a factor of - 8 higher.
re) C ~ E v i ~ , 0 2 ~ ~
Oz÷ O,~ ~ m SO{ -
CRE¥1~ ANO ;Id~tJRtTYI~FE¢1S O~ SCC ;N TYPE 316N6 S$1289%)
Fig. 8. Micrographs of Type 316NG SS CERT specimens with drilled holes after straining at a rate of 2><:10-7 s -1 at 289°C under (a) non-crevice and (b) crevice conditions in high-purity water with 0.25 ppm dissolved oxygen, and (c) crevice conditions in water with 0.1 ppm sulfate at the same oxygen level.
2"
(=) ~ = 5.79 %
(~,) Ep ==.6 %
(c) ~.p = I . O %
(all ,% = O.39 %
--
1
CRACK INITIATION IN SOLUTION ANNEALE0 TYPE 304 SS (289"C1 ~ = 2 x IO-7 s-I 0.25 ppm 02 ÷ O.I l ~ n SO2 -
Fig. 7. Micrographs of the inner surface of the diametral hole in solution-annealed Type 304 SS CERT specimens for crack initiation studies at a strain rate of 2><:10-7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. The nominal plastic strain in the specimens are indicated; the localized strains near the hole are a factor of - 8 higher.
studies, except that two holes were drilled only halfway through the specimen, and Type 304 SS pins (0.84 m m diameter) were inserted in the holes. C o m p a r i s o n tests were p e r f o r m e d without the pins. Test results in highpurity water and in an impurity e n v i r o n m e n t are shown in fig. 8. Without the pins, the specimens failed in a completely ductile fashion in high-purity water, but with the pins in place, SCC occurred b o t h in high-purity water and in sulfate environments. A d d i t i o n a l tests on solution-annealed Type 304 SS and a very resistant heat of Type 316NG SS (Heat No. 467958) also d e m o n strated that SCC occurred under crevice conditions in environments where no SCC was observed u n d e r noncrevice conditions. Since the electrochemical conditions at the tip of a SCC may be similar to those in a crevice of the same composition, these results are consistent with our fracture-mechanics test results in which cracks progagate under environmental conditions where no cracking was observed in s m o o t h specimen tests.
204
14~J. Shack et al./ BWR pipe crack remedies evaluation
2.3. Effect o f weld-induced plastic strains on S C C o f Type 316NG S S
Although no mechanism has been established, it has been reported [7] that cold work (5-20%) promotes intergranular crack initiation in nonsensitized Type 316NG SS. Recent results from a U.S. NRC-sponsored research program at Pacific Northwest Laboratory [8] show that during welding, large amounts of plastic strain occur on the inner surface of large-diameter pipes at relatively low temperatuers. Since the E P R I / G E pipe test qualification program for Type 316NG SS was based primarily on 4-in.-diam pipe weldments [7] in which much less plastic strain occurs during welding than in large-diameter pipes, it has been suggested that the results of the pipe tests may not be conservative for all pipe sizes [9]. In order to test this hypothesis, a 28-in.-diam pipe weldment fabricated from Type 316NG SS was obtained from the EPRI N D E Center. The chemical composition of this material is given in table 1. C E R T tests were performed to determine whether cracks would initiate preferentially in regions close to the inner surface of the pipe where the plastic strains are large and whether the mode of cracking would be intergranular. The tests were performed in an environment containing 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. The results are summarized in table 2. Transgranular cracking occurred in the tests at strain rates of 1 x 10 - 6 s 1; there was no evidence of I G S C C in any test. The cracks initiated on the side of the specimen that was farthest from the inner surface of the weldment. The crack growth rates were somewhat higher than those observed in corresponding tests on our reference heat, but this variation is within the expected heat-to-heat variability. To determine more explicitly the role of the cold work accumulated during welding, comparison tests
Table 2 Effect of strain rate on the crack growth rate of Type 316NG SS from CERT tests in 289°C water containing 0.25 ppm dissolved oxygen and 0.1 ppm sulfate Strain rate (s - l )
28-in.-diam. pipe (0.11 wt% N2)
ANL ref. heat a (0.068 wt% N2)
rl v(m.s -1 ) 1 x 10 6 4x 10 -7 2)<10 -7
No SCC 2.06 X 1 0 - 9 1.52x 10 9
a Heat No. P91576.
1.51 X 1 0 - 9 9.74x 10 -1° 7.35 x 10-10
are now being performed on base material obtianed well away from the welded region.
3. Crack growth resistance of weld overlay material To characterize the inherent crack growth resistance of the Type 308L SS weld metal used for weld overlay repairs, two fracture-mechanics crack growth rate tests are being performed on specimens fabricated from a 10-in.-diam pipe with a weld overlay applied by N U T E C H Engineers and G A P C O Welding in accordance with their standard practice for overlay repairs. The pipe was furnace-sensitized before the overlay was applied. The 1TCT specimens were fabricated such that cracks will propagate through the furnace-sensitized base material into the weld overlay. One specimen is being tested in a high-purity environment with 0.2 ppm dissolved oxygen, the other in a similar oxygenated environment with 0.1 ppm sulfate. The tests are being performed at 2 8 9 ° C under high-R (0.90-0.95) cyclic loads with a maximum stress intensity factor of 28 to 37 M P a - m 1/2. In the ongoing test in the high-purity environment, the crack tip is apparently just at the interface between the pipe and the overlay. N o change in compliance, which would indicate either crack growth through the overlay or crack growth along the interface, was observed during a test period of 840 h at a maximum stress intensity factor of 30 M P a - m 1/2 or during a subsequent 1296-h test period with a maximum stress intensity factor of 33 M P a . n~/2. This may be indicative of an inherent resistance of the weld overlay material to stress corrosion crack growth in this environment. In the impurity environment, compliance measurements indicated that the crack was growing through the overlay. However, as shown in fig. 9, posttest examination revealed that the crack grew to the overlay, branched at 90 ° along the weld interface, and grew - 1 2 mm in each direction along the interface at the side surface parallel to the applied load. The crack length in either direction at the half-thickness location in the specimen was somewhat less (i.e., - 3 . 6 mm). Based on the assumption that the compliance data can be used to determine when the crack reached the overlay, the average crack growth rate along the interface at the side surface is - 6 x 10 -1° m- s -1. This is almost an order of magnitude higher than the crack growth rate for the initial crack in the furnace-sensitized material normal to the applied load (8 x 10 -11 m - s - l ) . Although bending loads are rather high in CT specimens (we have not yet computed the stress intensity factors at
205
W J . Shack et aL / B W R pipe crack remedies evaluation
WATER CHEMISTRY OXYGEN
SULFATE
(Dam)
(ppm)
0.2-0.3
O
t
0.1
CONOx=0.85 gSIcm ECP= 130 mV{SHE) WELD OVERLAY SPECIMEN LOAD CONDITIONS
Klrm~28-38 MPa.mI12 f = 0,08
Hz
R= 0.95 SENSITIZATION EPR ~ 28 Clcm 2
CRACK TIP REGION
SIDE SURFACE
Fig. 9. Micrographs of a compact-tension specimen of sensitized Type 304 SS with an ER 308L weld overlay, showing crack growth along the interface of the overlay (parallel to the nominal applied stress) after low-frequency, moderate-stress-intensity, and high-R loading in 289 °C water containing 0.2-0.3 ppm dissolved oxygen and 0.1 ppm sulfate.
the tips of the branched cracks), it seems intuitive that the stress intensity factors for the branched crack should be substantially lower than those for the crack normal to the applied load, which is contradictory to the apparent high crack growth rate. A future test is planned in which the CT specimen is designed for crack growth along the interface.
4. Metallographic and residual stress studies of M S I P The mechanical-stress-improvement process (MSIP) is a proprietary development of 0'Donnell and Associates, Inc. (0AI). Like induction-heating stress-improvement (IHSI), it is intended to produce a more favorable state of residual stress on the inner surface of pipe
weldments, especially in the vicinity of heat-affected zones (HAZs). Although the two processes have similar objectives, MSIP is a purely mechanical process. The favorable residual stresses are induced by the plastic compression of the weldment produced by a split-ringlike tool mounted on the pipe. The plastic strain imposed on the pipe is controlled by the opening between the split rings, which is adjusted by inserting appropriate shims. Two pipe-to-pipe weldments treated by MSIP were examined. The process did produce favorable residual stress states in both 12- and 28-in.-diam weldments. Unlike most residual stress improvement processes, the effectiveness of the MSIP does not appear to be a strong function of pipe diameter (or thickness), although the peak compressive axial stresses in the smaller
206
W.J. Shack et al./ B W R pipe crack remedies evaluation
pipe weldment are somewhat higher. The 12-in.-diam weldment had been exposed to boiling MgC12 at the E P R I N D E Center before being sent to ANL. Although no cracking was reported by the N D E Center after the MgC12 test, metallographic studies at A N L did reveal some cracking, which we attribute to higly localized tensile stress regions that are susceptible to chloride-induced SCC. N o cracks were observed in the 28-in.-diam weldment that was subjected to higher plastic strains during MSIP, but was not exposed to MgC12. The residual stresses on the inner surface of the MSIP-treated weld in the 12-in.-diam specimen were measured at two azimuthal locations. The stresses at the two azimuths did not differ significantly and the average stresses for a given axial position are summarized in table 3. Both the axial and hoop stresses are compressive at all locations. N o control welds were prepared, but calculations and measurements suggest that the axial stresses in the H A Z s of such welds will typically be tensile with a magnitude of - 2 0 - 3 0 ksi [10,11]. Finite-element analyses by OAI predict tensile stress regions on the portion of the inner surface directly b e n e a t h the tool after MSIP. However, in this case the measured stresses were compressive in this region. Throughwall axial residual stress measurements were m a d e at one azimuth. In the regions near the HAZs, the stress distributions are almost linear across the thickness except near the outer surface, and are similar to the distributions in weldments treated by IHSI [11]. In the region directly under the tool, the profiles are more
Table 3 Average measured residual stresses on the inner surface of a 12-inch-diameter pipe weldment treated by the mechanical stress improvement process Location
Gage position
Tool side HAZ
Under tool
Across weld HAZ
Distance from weld centerline (in.)
Axial stress (ksi)
Hoop stress (ksi)
-0.20 -0.59
- 31 - 35
- 34 - 33 - 13 17
-0.99
- 26
4
-
-
5 6 7
- 2.57 - 3.35 -4.14
-15 - 14 -17
-17 - 15 - 13
8 9
0.20 0.59
- 34 -48
- 36 -31
1.77
16
Table 4 Average measured residual stresses on the inner surface of a 28-inch-diameter pipe weldment treated by the mechanical stress improvement process Location
Gage position
Tool side HAZ
Under tool
Across weld HAZ
Distance from weld centerline (in.)
Axial stress (ksi)
Hoop stress (ksi)
- 0.08 -0.55
- 24 -36
- 53 -30
-
-
-
1.34
22
15
4 5 6 7
-2.52 - 3.70 - 6.06 -8.42
-12 - 1 6 -6
2 8 8 -15
1 2
0.08 0.55
- 22 - 36
- 50 - 36
1.34
-40
-25
nonlinear. The compressive stresses p r o d u c e d by M S I P in the H A Z s persist through a substantial p o r t i o n ( 50%) of the pipe wall. This indicates that the process can provide significant benefits even in the presence of small flaws that may not be detected by nondestructive examination. The results of the strain-gage residual stress measurements on the 28-in.-diam w e l d m e n t are summarized in table 4. There are high compressive stresses in the regions near the weld and heat-affected zones. The stresses become tensile on the inner surface in the region under the tool (i.e., - 3 - 8 in. from the weld centerline) in agreement with the finite-element analyses performed by OAI, but the magnitudes of the stresses are relatively low ( - 5 - 1 0 ksi) c o m p a r e d to the analytical predictions. Throughwall residual stresses measurements were performed at one azimuth on the 28-in.-diam weldment. As in the case of the 12-in.-diam weldment, the compressive stresses p r o d u c e d by M S I P in the H A Z s persist through a substantial portion ( - 50%) of the pipe wall. MSIP appears to be an appropriate technique for improving the residual stress state of piping system weldments and is effective for b o t h large- and small-diameter pipes. The associated plastic strains are unlikely to have detrimental effects either through the production of brittle phases (e.g., martensite) or other mechanisms that increase susceptibility to SCC.
207
W.J. Shack et al. / B W R pipe crack remedies evaluation
5. Studies on modified T y p e 3 4 7 stainless steel 5.1. Stress corrosion tests
T y p e 347 SS h a s b e e n w i d e l y u s e d in G e r m a n y as a p i p i n g m a t e r i a l a n d as a weld c l a d d i n g in P W R s a n d B W R s w i t h n o r e p o r t s o f I G S C C . T h e steel h a s a l o w c a r b o n c o n c e n t r a t i o n (0.04 wt% m a x ) w i t h the m a i n alloying e l e m e n t s a d j u s t e d to give a n o m i n a l ferrite n u m b e r of 4 to 8 a n d the m a x i m u m N b c o n t e n t limited to 0.55 wt% to p r o m o t e weldability. T o e v a l u a t e the S C C s u s c e p t i b i l i t y o f this steel, C E R T tests h a v e b e e n p e r f o r m e d o n t w o h e a t s o f material. T h e m a t e r i a l s w e r e o b t a i n e d as w e l d m e n t s f a b r i c a t e d w i t h m a t c h i n g filler m a t e r i a l s at the E P R I N D E C e n t e r . C E R T tests w e r e p e r f o r m e d o n m a t e r i a l in the a s - w e l d e d c o n d i t i o n a n d
a f t e r a n a d d i t i o n a l h e a t t r e a t m e n t ( 5 0 0 ° C for 24 h) over a r a n g e of s t r a i n rates in 2 8 9 ° C h i g h - p u r i t y w a t e r w i t h 0.25 p p m dissolved o x y g e n a n d w i t h s u l f a t e a d d i t i o n s r a n g i n g f r o m 0.025 to 0.1 p p m . T h e r e s u l t s f o r the t w o h e a t s o f material, are s u m m a r i z e d in t a b l e s 5 a n d 6. F o r b o t h h e a t s of m a t e r i a l T G S C C o c c u r s at s t r a i n r a t e s o f 5 x 10 - 7 s -1 in i m p u r i t y e n v i r o n m e n t s . E v e n f o r s t r a i n r a t e s as low as 2 x 10 - 7 s - I , n o c r a c k i n g is o b s e r v e d in the h i g h - p u r i t y e n v i r o n m e n t . T h e critical s t r a i n rate r e q u i r e d to p r o d u c e T G S C C in these m a t e r i a l s a p p e a r s to be slightly l o w e r t h a n t h o s e r e q u i r e d to p r o d u c e T G S C C in s i m i l a r tests o n T y p e 3 1 6 N G SS. H o w e v e r , the average c r a c k g r o w t h r a t e s are a p p r o x i m a t e l y the same. N o evidence of knife-line a t t a c k h a s b e e n o b s e r v e d , a n d failure a l w a y s o c c u r s in the b a s e metal. N o effect o f the a d d i t i o n a l h e a t t r e a t m e n t w a s o b s e r v e d .
Table 5 Effect of strain rate and environment (0.25 ppm dissolved oxygen and 0 to 0.1 ppm sulfate) a on the SCC susceptibility of Type 347 SS b in 289°C water Sulfate (ppm)
Heat treatment
i ( s - 1)
0.1 0.1
c d
1 × 10 -6 1 x 10- 6
0.1 0.1 0.05
c d d
0.1 0.1 0.05 0 0.1 0.1
a b ¢ d
~f (%)
A A/.40 (%)
°max (MPa)
Failure mode
fi av (m. s - 1)
77.5 65.5
27.9 23.6
62 65
435 432
Ductile Ductile
-
5 x 10 - 7 5 x 10-7 5 × 10 -7
136.6 114.5 126.3
24.6 20.6 22.7
51 47 70
428 417 435
TGSCC TGSCC Ductile
1.40 x 10 - 9 1.63 x 10 -9 -
c d d d
2× 2x 2x 2x
10- 7 10 -7 10 -7 10 -7
328.5 301.5 280.7 313.5
23.7 21.7 20.2 22.6
41 40 47 78
434 448 458 442
TGSCC TGSCC TGSCC Ductile
1.00 x 10 - 9 1.10 x 10 -9 8.31 x 10- lo -
c d
1 X 10-7 1 x 10- 7
676.5 574.5
24.4 20.7
41 47
443 451
TGSCC TGSCC
1.05 x 10-9 7.58 x 10-1o
tf (h)
Steady-state open-circuit corrosion potential: 30 mV (SHE) Heat No. 174100. As-welded. As-welded plus 5 0 0 ° C / 2 4 h.
Table 6 CERT test results for Type 347NG SS at 289°C (Heat No. 170162) Test No.
Oxygen (ppm)
Sulfate (ppm)
SS potential (mV) (SHE)
i ( s - 1)
tf (h)
~f (%)
AA//Ao
ffmax
aav
(%)
(MPa)
(m- s - 1)
310 305 297 310 299 304
0.25 0.25 0.26 0.25 0.005 0.005
0.1 0.1 0 0.1 0.025 0.05
84 91 22 22 - 609 -580
1 X 10- 6 5 X 10-7 2 x 10- 7 2 × 10- 7 2 x 10- 7 2 × 1 0 -7
55.7 114.1 253.8 250.5 243.2 231.8
20.1 20.5 18.3 18.0 17.5 16.7
76 67 80 61 54 69
427 430 444 471 438 442
0 b 0 5.5 × 10-1o 3.5 x 10-10 4.6x 10 -1°
b Evidence of initiation, but the crack length is too short to estimate a velocity.
208
W.J. Shack et aL/ B WR pipe crack remedies eoaluation 10 - 8
I
]
I
I
I
[111
I
TYPE 347 SS,289°C
I
I
I
i
I IE _
10
(TGSCC)
0 . 2 5 ppm 0 2 + 0.1 ppm SO 2 -
L~ E
=-
03 03
1°-9_
°o
-
-10
[] •
0 ° Side 1 0 ° Side 2
[]
24o0° Side 1
-30
03
(SLOPE = O.~3)
-50
io-~O
L 10-8
i
i
L L LLil
I i0 -7
I
t
-70
I I II 10 - 6
'
1'0
' ' o2 ' ' o3 '
4 0'
S T R A I N RATE (s -I)
Fig. 10. Average transgranular crack growth rate as a function of strain rate for Type 347 SS in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate.
[] • [] •
Preliminary e x a m i n a t i o n of six Type 3 4 7 - T y p e 347 a n d Type 3 4 7 - T y p e 304 SS 10-in.-diam weldments received from the N e w York Power A u t h o r i t y has been completed. The weldments were prepared using either the G e r m a n narrow-gap welding process without a n insert or the more conventional welding procedure
0 ° Side 1 0 ° Side 2 240 ° Side 1 90 ° Side 1
03
03 -50
-70
i
I
10
5.2. Characterization of Type 347 stainless steel weldrnents
60
10
-10
As shown in fig. 10, the strain rate and average crack growth rate are related by a power law with an expon e n t of - 1 / 3 , which is consistent with the b e h a v i o r of Type 3 1 6 N G a n d solution-annealed Type 304 SS a n d the model we have developed to describe strain rate effects on various parameters in C E R T tests [12].
' ' 50'
DISTANCE FROM WELD CENTERLINE (ram)
i
I
20
=
I
30
i
I
40
i
I
50
i
60
DISTANCE FROM WELD CENTERLINE (ram)
Fig, 12. Axial (a) and circumferential (b) residual stresses associated with a narrow-gap welding procedure.
developed at the E P R I N D E Center, which uses an insert. Before s h i p m e n t to A N L the weldments were r a d i o g r a p h e d a n d e x a m i n e d ultrasonically; the welds were f o u n d to b e of high quality with relatively few
Fig. 11. (a) Cross section of 10-in.-diam modified Type 347 SS pipe weldment prepared by German narrow-gap welding process. (b) Small defect near weld fusion line on the inner surface.
209
W j . Shack et al. / B W R pipe crack remedies evaluation
potential defects. Metallographic examination of a narrow-gap weldment at A N L [fig. ll(a)] has confirmed the high quality of the weld indicated by the nondestructive examinations. One small defect, which is less than 5 mm long and - 30/~m deep, was noted. A cross section of the defect is shown in fig. ll(b). The defect was observed by dye penetrant examination after the initial sectioning of the weldment. A dye penetrant test before sectioning did not show an indication. The defect appears to have occurred during normal welding operations, i.e., there is no evidence of weld repair in this region. Residual stress measurements were made on one of the narrow-gap weldments. The axial and circumferential residual stresses in the vicinity of the weld are shown in figs. 12(a) and (b), respectively. The stress distributions are substantially different from those associated with conventional welding techniques [10,11]. Tensile stresses are observed only in the axial direction in a very narrow zone near the weld, and the magnitude of these stresses is low. Over most of the inner surface the stresses are as compressive as those generally obtained after a residual stress improvement process such as MSIP or IHSI.
6. Water chemistry effects on the S C C of sensitized stainless steels
The effect of various ions on the SCC behavior of sensitized Type 304 SS is being investigated. Particular attention is being given to both anion and cation impurities that provide alternative cathodic reduction reactions to dissolved oxygen. Although not all impurities affect SCC through this particular mechanism, it does appear important for several impurity species present in reactor coolant water (e.g., sulfate and copper ions). Such impurities are potentially damaging even in lowoxygen environments associated with the hydrogenwater chemistry in BWRs and secondary-system water in PWRs. The influence of the impurities on the S C C of sensitized Type 304 SS (Heat No. 30956) was investigated in C E R T tests in 2 8 9 ° C water at a low dissolved-oxygen concentration (5 ppb) [3]. In these experiments, the crack growth behavior of the materials was correlated with the type and concentration of the impurities and the electrochemical potentials of Type 304 SS and platinum electrodes in the environment. The C E R T results indicate that the crack growth rate of these steels is largely controlled by the rate of cathodic reduction of oxyanions [e.g., SO42-, NO~-, PO 3 - , CIO~-,
1605 140 ~
~ 1202 1002 M.~
80 2
o PM.I
60
F-
4o:
SENSITIZATION EPR = 2 C/cm 2 STRAIN RATE
20
= 1xlO'6s "1
Fig. 13. Effect of several salts at a chloride concentration of 10 ppm in low-oxygen ( < 5 ppb) water on the time to failure of sensitized Type 304 SS CERT specimens (Heat No. 30956) at 289°C.
AsO43-, which have a central atom (S, N, P, C1, and As) that can assume different oxidation states] as well as by cations (e.g., Cu 2+, Cu ÷) that can undergo similar reduction (to metallic Cu) [3]. The observation that BO33- or Zn 2+, Mg 2+, or N a ÷ (added as chloride salts), which cannot undergo cathodic reduction like the other species, do not promote SCC in the low-oxygen environments, as demonstrated by the test results shown in fig. 13, provides further evidence in support of this mechanism. As in the case of dissolved oxygen, the reduction of these species couples with anodic dissolution or oxidation at the crack tip following film rupture. Since cathodic reduction of the various species occurs simultaneously, these impurities contribute to crack growth in a manner determined by their concentrations and the relative rates of the reduction reactions. The C E R T results also indicate different threshold concentrations for each of the species; that is, when their concentrations are below a specific level, which may depend on the strain rate, neither transgranular nor intergranular cracking occurs in high-temperature, lowoxygen water.
7. Effects of irradiation on the corrosion potential of stainless steel in BWR-like environments
Experiments to characterize the effect of high gamma radiation on the corrosion potential of SS have been carried out using a 90K Ci cobalt source. A standard reference electrode was modified to operate satisfactorily in the gamma envir.onment. The effect of the gamma dose rate and total dose on the open-circuit potential of Type 304 SS and the redox potential of a platinum electrode was measured in conventional BWR-like en-
210
W..J. Shack et al. / B WR pipe crack remedies evaluation
v i r o n m e n t s with 0.2-0.3 p p m dissolved oxygen a n d in deoxygenated water (0.005 p p m oxygen). The g a m m a radiation causes a significant increase ( - 2 0 0 mV) in the corrosion potential of the Type 304 SS, b u t no significant changes were n o t e d in the potential of the p l a t i n u m electrode. N o influence of g a m m a dose o n the potentials was found over the range from 0.08 to 10 Mrad.
Acknowledgments The authors wish to acknowledge the assistance of W.F. Burke, D.J. D o r m a n , H. Lund, D.R. Perkins, F.E. Soppet and W.K. Soppet in various aspects of the experimental work. This research was supported b y the Materials Engineering Branch of the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission.
References [1] W. CuUen, G. Gabetta and H. HS.nninen, A review of the models and mechanisms for environmentally-assisted crack growth of pressure vessel and piping steels in PWR environments, NUREG/CR-4422 (December 1985). [2] H. H~inninen, K. TSrrSnen, M. Kemppainen and S. Salonen, On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels, Corros. Sci. 23 (1983) 663-679. [3] W.E. Ruther, W.K. Soppet and T.F. Kassner, Lightwater-reactor-safety materials engineering research programs: Quarterly Progress Report January-March 1985, NUREG/CR-4490 Vol. 1, ANL-85-75 Vol. 1 (March 1986).
[4] P.S. Maiya and W.J. Shack, Environmentally assisted cracking in light water reactors: Annual Report, October 1983-September 1984, NUREG/CR-4287, ANL-85-33 (June 1985). [5] P.S. Maiya and W.J. Shack, Stress corrosion cracking susceptibility of AISI 316 NG and 316 stainless steel in an impurity environment, Corrosion 41(11) (1985) 630-634. [6] P.S. Maiya and W.J. Shack, Environmentally assisted cracking in fight water reactors: Semiannual Report, April-September 1985, NUREG/CR-4667 Vol. 1, ANL86-31, pp. 16-26 (June 1986). [7] J.A. Alexander et al., Alternative alloys for BWR pipe applications, EPRI NP-2671-LD, Electric Power Research Institute, pp. 6-55 to 6-61 (October 1982). [8] D.G. Atteridge, R.E. Page and S.M. Bruemmer, Evaluation of welded and repair-welded stainless steel for LWR service-Annual Report for 1984, NUREG/CR-3613-2 (1985). [9] S. Bruemmer, PNL, Private Communication to W.J. Shack, ANL (July 1986). [10] W.J. Shack, W.A. Ellingson, and L.E. Pahis, Measurement of residual stresses in Type 304 stainless steel piping butt weldments, EPRI NP-1413, Electric Power Research Institute (June 1980). [11] E.F. Rybicki et al., Computational residual stress analysis for induction heating of welded BWR pipes, EPRI NP2662-LD, Electric Power Research Institute (December 1982). [12] P.S. Maiya, Prediction of environmental and strain rate effects on the stress corrosion cracking of austenitic stainless steels, in: Predictive Capabilities in Environmentally Assisted Cracking, PVP-Vol. 99, R. Rungta, ed., Proc. Winter Annual Mtg. of the American Society of Mechanical Engineers, New York, pp. 39-54 (1985).
BWR PIPE CRACK REMEDIES W.J. SHACK,
T.F. KASSNER,
199
EVALUATION P.S. M A I Y A ,
*
J.Y. P A R K
and W.E. RUTHER
Materials and Components Technology Division Argonne National Laboratory, Argonne, lllionois 60439, USA Received March 1987
This paper presents results on: (a) the influence of simulated BWR environments on the stress-corrosion-cracking (SCC) susceptibility of Types 304, 316NG, and 347 stainless steel (SS), (b) fracture-mechanics crack growth rate measurements on these materials and weld overlay specimens in different environments, and (c) residual stress measurements and metallographic evaluations of conventional pipe weldments treated by a mechanical-stress-improvement process (MSIP) as well as those produced by a narrow-gap welding procedure. Crack initiation studies on Types 304 and 316NG SS under crevice and non-crevice conditions in 289°C water containing 0.25 ppm dissolved oxygen with low sulfate concentrations indicate that SCC initiates at low strains (3%) in the nuclear grade material. Crack growth measurements on fracture-mechanics-type specimens, under low-frequency cyclic loading, show that the Type 316NG steel cracks at a somewhat lower rate ( - 40%) than sensitized Type 304 SS in an impurity environment with 0.25 ppm dissolved oxygen; however, the latter material stops cracking when sulfate is removed from the water. Crack growth in both materials ceases under simulated hydrogen-water chemistry conditions (5 ppb oxygen) even with 100 ppb sulfate present in the water. An unexpected result was obtained in the test on a weld overlay specimen in the impurity environment, viz., the crack grew to the overlay interface at a nominal rate, branched at 90 ° in both directions, and then grew at a high rate (parallel to the nominal applied load). Residual stress measurements on MSIP-treated weldments and those produced by a narrow-gap welding procedure indicate that these techniques produce compressive stresses over most of the inner surface near the weld and heat-affected zones.
1. Introduction Cracking in sensitized austenitic stainless steel (SS) p i p i n g a n d associated c o m p o n e n t s in boiling water reactors ( B W R s ) has b e e n observed since the mid-1960s. Proposed remedies include: (1) procedures that p r o d u c e a favorable residual stress state in the weld regions, (2) r e p l a c e m e n t of the piping with materials that are more resistant to stress corrosion cracking (SCC), a n d (3) modification of the reactor coolant e n v i r o n m e n t . Studies have been carried out t h a t have i m p o r t a n t implications for all three classes of p r o p o s e d remedies. These studies include fracture-mechanics crack-growth-rate tests o n Type 3 1 6 N G SS a n d weld overlay specimens in i m p u r i t y a n d high-purity e n v i r o n m e n t s , metallographic a n d residual stress studies o n w e l d m e n t s treated b y the m e c h a n i c a l - s t r e s s - i m p r o v e m e n t process (MSIP) developed b y O ' D o n n e U a n d Associates, a n d constant-exten-
* Work supported by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission under Interagency Agreement DOE 40-550-75.
sion-rate-tensile ( C E R T ) tests o n the G e r m a n N u c l e a r G r a d e Type 347 SS a n d o t h e r steels to d e t e r m i n e the influence of reactor coolant impurities (e.g., SO 2 - , Cu z+) on SCC susceptibility. In addition, the effect of g a m m a r a d i a t i o n of simulated reactor-coolant environm e n t s on the corrosion potential of stainless steel is being investigated. Together with related studies o n the characterization of the SCC b e h a v i o r of irradiated stainless steels these results will p e r m i t a b e t t e r assessment of the potential for irradiation-assisted SCC of materials in the reactor core.
2. SCC of Type 316NG stainless steel 2.1. Fracture-mechanics crack growth tests F r a c t u r e - m e c h a n i c s crack growth rate tests have b e e n p e r f o r m e d o n Type 3 1 6 N G SS ( H e a t No. P91576), to confirm that the susceptibility to t r a n s g r a n u l a r SCC ( T G S C C ) observed in C E R T tests c a n also occur u n d e r less severe mechanical loading. I n different phases of the tests, the water chemistry conditions were varied
0029-5493/88/$03.50 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
200
W.J. Shack et a l . / B W R pipe crack remedies evaluation
Table 1 Chemical composition (wt%) of austenitic stainless steels used in fracture-mechanics and CERT tests Material
Heat No.
C
Mn
P
S
Si
Ni
Cr
Mo
Cu
N
B
Nb
Fe
316NG 316NG 316NG 347 347 304 304
P91576 467958 NDE-28 174100 170162 53319 30956
0.015 0.02 0.014 0.023 0.03 0.06 0.06
1.63 1.51 1.77 1.70 1.70 1.69 1.54
0.02 0.029 0.02 0.036 0.012 0.024 0.019
0.01 0.008 0.002 0.015 0.019 0.013 0.007
0.42 0.64 0.52 0.33 0.43 0.59 0.48
10.95 12.74 13.58 11.00 10.75 8.88 8.00
16.42 17.14 17.79 18.15 18.53 18.33 18.99
2.14 2.43 2.59 0.48 0.35 0.14 0.44
0.16 0.11 0.12 0.11 0.06 0.19
0.068 0.069 0.11 0.029 0.021 0.029 0.10
0.002 0.002 0.0006 0.0005 0.0005 0.0005 -
0.44 0.51 -
Balance
a
-
Material from a 28-in. diam. pipe.
from a reference condition of 0.25 p p m dissolved oxygen plus 0.1 p p m sulfate (as H2SO4) to lower oxygen c o n c e n t r a t i o n s at this sulfate level as well as higher a n d lower sulfate concentrations at a fixed dissolved-oxygen concentration in the feedwater. The experiments were performed in 6-I autoclaves equipped with MTS servo hydraulic load systems. The dissolved-oxygen concentrations of the feedwater were established by b u b bling appropriate o x y g e n / n i t r o g e n gas mixtures through the d e o x y g e n a t e d / d e i o n i z e d feedwater (conductance of < 0 . 2 / ~ S / c m ) contained in 120-I stainless steel tanks. A hydrogen cover gas was used in several experiments at very low dissolved-oxygen concentrations. In tests with impurities, sulfuric acid was a d d e d to the feedwater before sparging with the gas mixture or hydrogen to ensure adequate mixing. The pressure of the cover gas in the feedwater tank was adjusted to provide the desired autoclave effluent dissolved-oxygen concentrations as determined by a Leeds a n d N o r t h r u p Model 7931 oxygen m o n i t o r a n d verified by a colorimettic (Chemetrics ampules) technique. The flowrate of water through the autoclaves was 0.5-1.0 l / h . The tests included a lightly sensitized ( E P R = 2 C / c m 2) Type 304 SS control specimen (Heat No. 30956). The compositions of b o t h materials are given in table 1. Most of the tests were carried out with a m a x i m u m stress intensity of - 3 0 M P a - m l/z, a load ratio R of 0.95, and a frequency of 0.08 Hz, (i.e., a nearly c o n s t a n t load with a small superposed cyclic tipple), since reactors experience small fluctuations in pressure a n d temperature which impose a cyclic stress on the piping a n d other components. R ratios associated with n o r m a l operating conditions are, in general, 1.0 < R < 0.95, viz., - 0 . 9 8 , where a value of 1.0 corresponds to c o n s t a n t load. The crack growth rates for each specimen u n d e r the different water chemistry conditions, i.e., (1) through (6), are shown in fig. 1. The open-circuit corrosion
potential of a Type 304 SS electrode a n d the conductivity of the feedwater for each phase of the test are s h o w n in the insert panel of the figure, where the region above the curve defines the region of susceptibility for intergranular SCC ( I G S C C ) based on C E R T data for this lightly sensitized material. W i t h loading at R = 0.95, b o t h materials cracked u n d e r the reference chemistry [conditions (1) a n d (3)]; the crack growth rates were - 40% higher in the sensitized specimen. In a high-purity e n v i r o n m e n t with 0.25 p p m dissolved oxygen [condition (2)], crack growth ceased in the Type 304 SS specimen, b u t in the Type 3 1 6 N G specimen it continued at almost the same rate as in the impurity environment. U n d e r condition (4), the dissolved-oxygen c o n c e n t r a t i o n of the feedwater was decreased to 0.002 p p m with 1.2 p p m hydrogen a n d 0.1 p p m sulfate. U n d e r these conditions the electrochemical potential of Type 304 SS decreased from + 140 to - 6 3 0 mV(SHE), a n d crack growth ceased in b o t h specimens, even with sulfate present. In the next phase of the test [condition (5)], sulfate was not added to the low-oxygen environment, and the conductivity of the effluent water was reduced to 0.1 # S / c m . As expected, n o crack growth occurred in either specimen during the transition to the high-purity environment. W h e n the e n v i r o n m e n t was then changed to a simulated, high-purity B W R environm e n t with 0.25 p p m dissolved oxygen [condition (6)], crack growth resumed in the Type 3 1 6 N G SS specimen at a rate similar to that observed u n d e r conditions (1) a n d (3), while the crack remained d o r m a n t in the sensitized Type 304 SS specimen. After - 500 h, the loading was changed to c o n s t a n t load ( R = 1) conditions. The crack growth rate in the Type 3 1 6 N G SS specimen decreased by an order of magnitude c o m p a r e d with that observed u n d e r the R = 0.95 load, while the crack growth rate in the sensitized specimen increased to a level close to that observed in the Nuclear G r a d e specim e n u n d e r the R = 0.95 load.
201
W J . Shack et a L / B W R pipe crack remedies evaluation
10' WATERCHEMISTRY
'~
; ,oo ~
E
E
o
~
02, H2, HzSO~
~
LOADINGCONDITION
rr
R = 0.95
~ -3oo -lO
~
lO
f = 8x10 2 Hz
-4oo
Kma:{= 29-35 MPamI/;
0.1
1
10
CONDUCTIVITY
_
100
SENSITIZATION =
at 2 5 ° C ( p S / c m )
EPR,C/cm2
3o-~--qj 2
13~NCd
10
0
~ _ ~ ~ : : ~ _ 1,3
2,6
4
5
WATER CHEMISTRY CONDITION
Fig. 1. Effect of several simulated BWR water chemistries, within and outside of the IGSCC regime for sensitized Type 304 SS, on the crack growth rates of a compact-tension specimen of this material and a Type 316NG SS specimen under low-frequency, moderate-stress-intensity, and high-R loading conditions at 289°C.
Since the test is still in progress, no fractographic information on the mode of cracking is available. However, in previous tests with sensitized and solution-annealed Type 304 SS, where the behavior of the solution-annealed specimen was similar to that observed in the Type 316NG SS specimen in this test, the sensitized specimen cracked intergranularly under these conditions, as expected, while the solution-annealed specimen cracked transgranularly. It has been shown that the dissolution of inclusions in pressure vessel steels, viz., manganese sulfides, can influence the crack-tip environment and thereby the crack growth behavior of the materials [1,2]. This could conceivably occur in our materials as well. However, as shown in table 1, the phosphorus and sulfur levels in the two steels are similar and yet the crack growth rate in the sensitized Type 304 SS shows a consistent dependence on sulfate concentration in the bulk environment, in contrast to the Type 316NG steel. Analogous experiments that compared the crack growth behavior of lightly sensitized and solution-annealed specimens from the same heat of Typ 304 SS yielded similar results [3]. Therefore, differences in the crack-tip chemistry due to dissolution of the materials themselves do not seem to explain the observed behavior. Alternative mechanisms are being explored. 2.2. Crack initiation studies
C E R T experiments were performed to study crack initiation in these materials. The experimental methods
for water chemistry control in these experiments were the same as described above. Previous work has shown that in C E R T tests Type 316NG SS is susceptible to T G S C C in oxygenated water with sulfate additions [4,5]. The sulfate concentration required to produce cracking decreases very rapidly with strain rate, as shown in fig. 2, and at a strain rate of 2 × 1 0 - 7 s - 1 is less than 25 ppb. The C E R T test results also suggest that once a crack initiates, the crack growth rate is only weakly dependent on the concentration of sulfate, as shown in fig. 3. Although the C E R T tests involve large
'
'
'"'"1
'
''"'"1
' ' " ' "
'
TYPE 316NG SS (289*C)
0.1
L o
I
0.01
I I I I
I 0.001 10-8
I
I I 111111 10-7
I
I i I Illll 10-6
I
I I IIII 10-5
STRAIN RATE, s - I
Fig. 2. Variation of critical concentration of sulfate (required for SCC) with strain rate for Type 316NG SS (Heat No. P91576).
202
W.J. Shack et al. / B WR pipe crack remedies evaluation
Impurity Effects on T G S C C
-0 f-----
10-9 u)
E
E
l
i
g. t'v
Type 316NG SS ( 289°C )
O
,< 10-10
I
0.0
0.2
,
I
,
0.4
I
0.6
i
I
0.8
,
I
10
,
1 2
Conductivity, pS/cm
Fig. 3. Effect of impurity concentration on average crack growth rate for Type 316NG SS at 289°C and a strain rate of 2 x 10 -7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.025 to 0.1 ppm sulfate.
plastic strains, the general trends predicted from these tests are consistent with the results of our fracturemechanics crack-growth-rate tests. These results have p r o m p t e d a study of the effects of impurities on crack initiation. In earlier work [6], i n t e r r u p t e d C E R T tests performed o n Type 3 1 6 N G SS showed that crack initiation (defined in terms of cracks 5 0 - 2 0 0 lam in length) occurs at relatively low strains. A more sensitive technique involves a C E R T specimen in which a small d i a m e t e r (0.86 m m ) through-hole is drilled in the center of the gage length. This geometry localizes the specimen region in which crack initiation occurs a n d enables the detection of very small cracks by scanning-electron microscopy (SEM). T h e test is i n t e r r u p t e d at low n o m i n a l strains, the specimen is then cross-sectioned as shown in fig. 4, and the surface of the hole is examined b y SEM. T h e strain c o n c e n t r a t i o n of - 8 for this geometry has b e e n estimated by N e u b e r ' s rule a n d finite-element calculations. The n o m i n a l strains can be accurately determined by measuring the distance between two gage m a r k s before a n d after the test. Figs. 5 a n d 6 show micrographs that illustrate the degree of cracking of Type 3 1 6 N G SS (Heat No. P91576) in an impurity e n v i r o n m e n t as a function of the nominal strain. These results suggest that crack initiation occurs at actual strains between 1.5 a n d 3% (assuming a strain concentration factor of 8) at a strain rate of - 2 x 10 -7 s -1. By comparison, results from similar tests on solution-annealed Type 304 SS (Heat No. 53319), which are shown in fig. 7, indicate that crack initiation occurs at strains greater than 3.0% for the same e n v i r o n m e n t s a n d strain rate.
Fig. 4. Cross sectional view of CERT specimen used for crack-initiation studies.
C E R T tests were also p e r f o r m e d o n Type 3 1 6 N G SS in e n v i r o n m e n t s with a n d w i t h o u t impurities u n d e r artificially produced crevice conditions. T h e specimen geometry was similar to that used for the initiation
(c) (p = 1.36 % CRACK INITIATION ;N TYPE ~NG
(d) (p=0.22 % SS
|289"C)
= 2 x 10 - 7 $-I
0.25 ppm 02 + 0.1 plm~ SO2-
Fig. 5. Micrographs of the region on the inner surface of the diametral holes in Type 316NG SS CERT specimens for crack initiation studies at a strain rate of 2 x 1 0 -7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. Nominal plastic strains in the specimen are indicated; localized strains near the hole are a factor of 8 higher.
203
W J . Shack et a l . / B W R pipe crack remedies evaluation
(o) N O ~ - C ~ V ~ , ~
|~t P'm I
~
0z
~b~ CR£VII~0,O.~5 I~pm02
CRACK INITIATION IN TYPE 3H~NG SS 1289"C) ~ = 2 x 1 0 - 7 S-I (Ep • 0.38 %
o . ~
o2 + o., ~
so2, -
Fig. 6. Micrographs of the inner surface of the diametral hole in a Type 316NG SS CERT specimen after a nominal plastic strain of 0.35% at a strain rate of 2 × 10-7 s - ] in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. Localized strain near the hole is a factor of - 8 higher.
re) C ~ E v i ~ , 0 2 ~ ~
Oz÷ O,~ ~ m SO{ -
CRE¥1~ ANO ;Id~tJRtTYI~FE¢1S O~ SCC ;N TYPE 316N6 S$1289%)
Fig. 8. Micrographs of Type 316NG SS CERT specimens with drilled holes after straining at a rate of 2><:10-7 s -1 at 289°C under (a) non-crevice and (b) crevice conditions in high-purity water with 0.25 ppm dissolved oxygen, and (c) crevice conditions in water with 0.1 ppm sulfate at the same oxygen level.
2"
(=) ~ = 5.79 %
(~,) Ep ==.6 %
(c) ~.p = I . O %
(all ,% = O.39 %
--
1
CRACK INITIATION IN SOLUTION ANNEALE0 TYPE 304 SS (289"C1 ~ = 2 x IO-7 s-I 0.25 ppm 02 ÷ O.I l ~ n SO2 -
Fig. 7. Micrographs of the inner surface of the diametral hole in solution-annealed Type 304 SS CERT specimens for crack initiation studies at a strain rate of 2><:10-7 s -1 in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. The nominal plastic strain in the specimens are indicated; the localized strains near the hole are a factor of - 8 higher.
studies, except that two holes were drilled only halfway through the specimen, and Type 304 SS pins (0.84 m m diameter) were inserted in the holes. C o m p a r i s o n tests were p e r f o r m e d without the pins. Test results in highpurity water and in an impurity e n v i r o n m e n t are shown in fig. 8. Without the pins, the specimens failed in a completely ductile fashion in high-purity water, but with the pins in place, SCC occurred b o t h in high-purity water and in sulfate environments. A d d i t i o n a l tests on solution-annealed Type 304 SS and a very resistant heat of Type 316NG SS (Heat No. 467958) also d e m o n strated that SCC occurred under crevice conditions in environments where no SCC was observed u n d e r noncrevice conditions. Since the electrochemical conditions at the tip of a SCC may be similar to those in a crevice of the same composition, these results are consistent with our fracture-mechanics test results in which cracks progagate under environmental conditions where no cracking was observed in s m o o t h specimen tests.
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14~J. Shack et al./ BWR pipe crack remedies evaluation
2.3. Effect o f weld-induced plastic strains on S C C o f Type 316NG S S
Although no mechanism has been established, it has been reported [7] that cold work (5-20%) promotes intergranular crack initiation in nonsensitized Type 316NG SS. Recent results from a U.S. NRC-sponsored research program at Pacific Northwest Laboratory [8] show that during welding, large amounts of plastic strain occur on the inner surface of large-diameter pipes at relatively low temperatuers. Since the E P R I / G E pipe test qualification program for Type 316NG SS was based primarily on 4-in.-diam pipe weldments [7] in which much less plastic strain occurs during welding than in large-diameter pipes, it has been suggested that the results of the pipe tests may not be conservative for all pipe sizes [9]. In order to test this hypothesis, a 28-in.-diam pipe weldment fabricated from Type 316NG SS was obtained from the EPRI N D E Center. The chemical composition of this material is given in table 1. C E R T tests were performed to determine whether cracks would initiate preferentially in regions close to the inner surface of the pipe where the plastic strains are large and whether the mode of cracking would be intergranular. The tests were performed in an environment containing 0.25 ppm dissolved oxygen and 0.1 ppm sulfate. The results are summarized in table 2. Transgranular cracking occurred in the tests at strain rates of 1 x 10 - 6 s 1; there was no evidence of I G S C C in any test. The cracks initiated on the side of the specimen that was farthest from the inner surface of the weldment. The crack growth rates were somewhat higher than those observed in corresponding tests on our reference heat, but this variation is within the expected heat-to-heat variability. To determine more explicitly the role of the cold work accumulated during welding, comparison tests
Table 2 Effect of strain rate on the crack growth rate of Type 316NG SS from CERT tests in 289°C water containing 0.25 ppm dissolved oxygen and 0.1 ppm sulfate Strain rate (s - l )
28-in.-diam. pipe (0.11 wt% N2)
ANL ref. heat a (0.068 wt% N2)
rl v(m.s -1 ) 1 x 10 6 4x 10 -7 2)<10 -7
No SCC 2.06 X 1 0 - 9 1.52x 10 9
a Heat No. P91576.
1.51 X 1 0 - 9 9.74x 10 -1° 7.35 x 10-10
are now being performed on base material obtianed well away from the welded region.
3. Crack growth resistance of weld overlay material To characterize the inherent crack growth resistance of the Type 308L SS weld metal used for weld overlay repairs, two fracture-mechanics crack growth rate tests are being performed on specimens fabricated from a 10-in.-diam pipe with a weld overlay applied by N U T E C H Engineers and G A P C O Welding in accordance with their standard practice for overlay repairs. The pipe was furnace-sensitized before the overlay was applied. The 1TCT specimens were fabricated such that cracks will propagate through the furnace-sensitized base material into the weld overlay. One specimen is being tested in a high-purity environment with 0.2 ppm dissolved oxygen, the other in a similar oxygenated environment with 0.1 ppm sulfate. The tests are being performed at 2 8 9 ° C under high-R (0.90-0.95) cyclic loads with a maximum stress intensity factor of 28 to 37 M P a - m 1/2. In the ongoing test in the high-purity environment, the crack tip is apparently just at the interface between the pipe and the overlay. N o change in compliance, which would indicate either crack growth through the overlay or crack growth along the interface, was observed during a test period of 840 h at a maximum stress intensity factor of 30 M P a - m 1/2 or during a subsequent 1296-h test period with a maximum stress intensity factor of 33 M P a . n~/2. This may be indicative of an inherent resistance of the weld overlay material to stress corrosion crack growth in this environment. In the impurity environment, compliance measurements indicated that the crack was growing through the overlay. However, as shown in fig. 9, posttest examination revealed that the crack grew to the overlay, branched at 90 ° along the weld interface, and grew - 1 2 mm in each direction along the interface at the side surface parallel to the applied load. The crack length in either direction at the half-thickness location in the specimen was somewhat less (i.e., - 3 . 6 mm). Based on the assumption that the compliance data can be used to determine when the crack reached the overlay, the average crack growth rate along the interface at the side surface is - 6 x 10 -1° m- s -1. This is almost an order of magnitude higher than the crack growth rate for the initial crack in the furnace-sensitized material normal to the applied load (8 x 10 -11 m - s - l ) . Although bending loads are rather high in CT specimens (we have not yet computed the stress intensity factors at
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W J . Shack et aL / B W R pipe crack remedies evaluation
WATER CHEMISTRY OXYGEN
SULFATE
(Dam)
(ppm)
0.2-0.3
O
t
0.1
CONOx=0.85 gSIcm ECP= 130 mV{SHE) WELD OVERLAY SPECIMEN LOAD CONDITIONS
Klrm~28-38 MPa.mI12 f = 0,08
Hz
R= 0.95 SENSITIZATION EPR ~ 28 Clcm 2
CRACK TIP REGION
SIDE SURFACE
Fig. 9. Micrographs of a compact-tension specimen of sensitized Type 304 SS with an ER 308L weld overlay, showing crack growth along the interface of the overlay (parallel to the nominal applied stress) after low-frequency, moderate-stress-intensity, and high-R loading in 289 °C water containing 0.2-0.3 ppm dissolved oxygen and 0.1 ppm sulfate.
the tips of the branched cracks), it seems intuitive that the stress intensity factors for the branched crack should be substantially lower than those for the crack normal to the applied load, which is contradictory to the apparent high crack growth rate. A future test is planned in which the CT specimen is designed for crack growth along the interface.
4. Metallographic and residual stress studies of M S I P The mechanical-stress-improvement process (MSIP) is a proprietary development of 0'Donnell and Associates, Inc. (0AI). Like induction-heating stress-improvement (IHSI), it is intended to produce a more favorable state of residual stress on the inner surface of pipe
weldments, especially in the vicinity of heat-affected zones (HAZs). Although the two processes have similar objectives, MSIP is a purely mechanical process. The favorable residual stresses are induced by the plastic compression of the weldment produced by a split-ringlike tool mounted on the pipe. The plastic strain imposed on the pipe is controlled by the opening between the split rings, which is adjusted by inserting appropriate shims. Two pipe-to-pipe weldments treated by MSIP were examined. The process did produce favorable residual stress states in both 12- and 28-in.-diam weldments. Unlike most residual stress improvement processes, the effectiveness of the MSIP does not appear to be a strong function of pipe diameter (or thickness), although the peak compressive axial stresses in the smaller
206
W.J. Shack et al./ B W R pipe crack remedies evaluation
pipe weldment are somewhat higher. The 12-in.-diam weldment had been exposed to boiling MgC12 at the E P R I N D E Center before being sent to ANL. Although no cracking was reported by the N D E Center after the MgC12 test, metallographic studies at A N L did reveal some cracking, which we attribute to higly localized tensile stress regions that are susceptible to chloride-induced SCC. N o cracks were observed in the 28-in.-diam weldment that was subjected to higher plastic strains during MSIP, but was not exposed to MgC12. The residual stresses on the inner surface of the MSIP-treated weld in the 12-in.-diam specimen were measured at two azimuthal locations. The stresses at the two azimuths did not differ significantly and the average stresses for a given axial position are summarized in table 3. Both the axial and hoop stresses are compressive at all locations. N o control welds were prepared, but calculations and measurements suggest that the axial stresses in the H A Z s of such welds will typically be tensile with a magnitude of - 2 0 - 3 0 ksi [10,11]. Finite-element analyses by OAI predict tensile stress regions on the portion of the inner surface directly b e n e a t h the tool after MSIP. However, in this case the measured stresses were compressive in this region. Throughwall axial residual stress measurements were m a d e at one azimuth. In the regions near the HAZs, the stress distributions are almost linear across the thickness except near the outer surface, and are similar to the distributions in weldments treated by IHSI [11]. In the region directly under the tool, the profiles are more
Table 3 Average measured residual stresses on the inner surface of a 12-inch-diameter pipe weldment treated by the mechanical stress improvement process Location
Gage position
Tool side HAZ
Under tool
Across weld HAZ
Distance from weld centerline (in.)
Axial stress (ksi)
Hoop stress (ksi)
-0.20 -0.59
- 31 - 35
- 34 - 33 - 13 17
-0.99
- 26
4
-
-
5 6 7
- 2.57 - 3.35 -4.14
-15 - 14 -17
-17 - 15 - 13
8 9
0.20 0.59
- 34 -48
- 36 -31
1.77
16
Table 4 Average measured residual stresses on the inner surface of a 28-inch-diameter pipe weldment treated by the mechanical stress improvement process Location
Gage position
Tool side HAZ
Under tool
Across weld HAZ
Distance from weld centerline (in.)
Axial stress (ksi)
Hoop stress (ksi)
- 0.08 -0.55
- 24 -36
- 53 -30
-
-
-
1.34
22
15
4 5 6 7
-2.52 - 3.70 - 6.06 -8.42
-12 - 1 6 -6
2 8 8 -15
1 2
0.08 0.55
- 22 - 36
- 50 - 36
1.34
-40
-25
nonlinear. The compressive stresses p r o d u c e d by M S I P in the H A Z s persist through a substantial p o r t i o n ( 50%) of the pipe wall. This indicates that the process can provide significant benefits even in the presence of small flaws that may not be detected by nondestructive examination. The results of the strain-gage residual stress measurements on the 28-in.-diam w e l d m e n t are summarized in table 4. There are high compressive stresses in the regions near the weld and heat-affected zones. The stresses become tensile on the inner surface in the region under the tool (i.e., - 3 - 8 in. from the weld centerline) in agreement with the finite-element analyses performed by OAI, but the magnitudes of the stresses are relatively low ( - 5 - 1 0 ksi) c o m p a r e d to the analytical predictions. Throughwall residual stresses measurements were performed at one azimuth on the 28-in.-diam weldment. As in the case of the 12-in.-diam weldment, the compressive stresses p r o d u c e d by M S I P in the H A Z s persist through a substantial portion ( - 50%) of the pipe wall. MSIP appears to be an appropriate technique for improving the residual stress state of piping system weldments and is effective for b o t h large- and small-diameter pipes. The associated plastic strains are unlikely to have detrimental effects either through the production of brittle phases (e.g., martensite) or other mechanisms that increase susceptibility to SCC.
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W.J. Shack et al. / B W R pipe crack remedies evaluation
5. Studies on modified T y p e 3 4 7 stainless steel 5.1. Stress corrosion tests
T y p e 347 SS h a s b e e n w i d e l y u s e d in G e r m a n y as a p i p i n g m a t e r i a l a n d as a weld c l a d d i n g in P W R s a n d B W R s w i t h n o r e p o r t s o f I G S C C . T h e steel h a s a l o w c a r b o n c o n c e n t r a t i o n (0.04 wt% m a x ) w i t h the m a i n alloying e l e m e n t s a d j u s t e d to give a n o m i n a l ferrite n u m b e r of 4 to 8 a n d the m a x i m u m N b c o n t e n t limited to 0.55 wt% to p r o m o t e weldability. T o e v a l u a t e the S C C s u s c e p t i b i l i t y o f this steel, C E R T tests h a v e b e e n p e r f o r m e d o n t w o h e a t s o f material. T h e m a t e r i a l s w e r e o b t a i n e d as w e l d m e n t s f a b r i c a t e d w i t h m a t c h i n g filler m a t e r i a l s at the E P R I N D E C e n t e r . C E R T tests w e r e p e r f o r m e d o n m a t e r i a l in the a s - w e l d e d c o n d i t i o n a n d
a f t e r a n a d d i t i o n a l h e a t t r e a t m e n t ( 5 0 0 ° C for 24 h) over a r a n g e of s t r a i n rates in 2 8 9 ° C h i g h - p u r i t y w a t e r w i t h 0.25 p p m dissolved o x y g e n a n d w i t h s u l f a t e a d d i t i o n s r a n g i n g f r o m 0.025 to 0.1 p p m . T h e r e s u l t s f o r the t w o h e a t s o f material, are s u m m a r i z e d in t a b l e s 5 a n d 6. F o r b o t h h e a t s of m a t e r i a l T G S C C o c c u r s at s t r a i n r a t e s o f 5 x 10 - 7 s -1 in i m p u r i t y e n v i r o n m e n t s . E v e n f o r s t r a i n r a t e s as low as 2 x 10 - 7 s - I , n o c r a c k i n g is o b s e r v e d in the h i g h - p u r i t y e n v i r o n m e n t . T h e critical s t r a i n rate r e q u i r e d to p r o d u c e T G S C C in these m a t e r i a l s a p p e a r s to be slightly l o w e r t h a n t h o s e r e q u i r e d to p r o d u c e T G S C C in s i m i l a r tests o n T y p e 3 1 6 N G SS. H o w e v e r , the average c r a c k g r o w t h r a t e s are a p p r o x i m a t e l y the same. N o evidence of knife-line a t t a c k h a s b e e n o b s e r v e d , a n d failure a l w a y s o c c u r s in the b a s e metal. N o effect o f the a d d i t i o n a l h e a t t r e a t m e n t w a s o b s e r v e d .
Table 5 Effect of strain rate and environment (0.25 ppm dissolved oxygen and 0 to 0.1 ppm sulfate) a on the SCC susceptibility of Type 347 SS b in 289°C water Sulfate (ppm)
Heat treatment
i ( s - 1)
0.1 0.1
c d
1 × 10 -6 1 x 10- 6
0.1 0.1 0.05
c d d
0.1 0.1 0.05 0 0.1 0.1
a b ¢ d
~f (%)
A A/.40 (%)
°max (MPa)
Failure mode
fi av (m. s - 1)
77.5 65.5
27.9 23.6
62 65
435 432
Ductile Ductile
-
5 x 10 - 7 5 x 10-7 5 × 10 -7
136.6 114.5 126.3
24.6 20.6 22.7
51 47 70
428 417 435
TGSCC TGSCC Ductile
1.40 x 10 - 9 1.63 x 10 -9 -
c d d d
2× 2x 2x 2x
10- 7 10 -7 10 -7 10 -7
328.5 301.5 280.7 313.5
23.7 21.7 20.2 22.6
41 40 47 78
434 448 458 442
TGSCC TGSCC TGSCC Ductile
1.00 x 10 - 9 1.10 x 10 -9 8.31 x 10- lo -
c d
1 X 10-7 1 x 10- 7
676.5 574.5
24.4 20.7
41 47
443 451
TGSCC TGSCC
1.05 x 10-9 7.58 x 10-1o
tf (h)
Steady-state open-circuit corrosion potential: 30 mV (SHE) Heat No. 174100. As-welded. As-welded plus 5 0 0 ° C / 2 4 h.
Table 6 CERT test results for Type 347NG SS at 289°C (Heat No. 170162) Test No.
Oxygen (ppm)
Sulfate (ppm)
SS potential (mV) (SHE)
i ( s - 1)
tf (h)
~f (%)
AA//Ao
ffmax
aav
(%)
(MPa)
(m- s - 1)
310 305 297 310 299 304
0.25 0.25 0.26 0.25 0.005 0.005
0.1 0.1 0 0.1 0.025 0.05
84 91 22 22 - 609 -580
1 X 10- 6 5 X 10-7 2 x 10- 7 2 × 10- 7 2 x 10- 7 2 × 1 0 -7
55.7 114.1 253.8 250.5 243.2 231.8
20.1 20.5 18.3 18.0 17.5 16.7
76 67 80 61 54 69
427 430 444 471 438 442
0 b 0 5.5 × 10-1o 3.5 x 10-10 4.6x 10 -1°
b Evidence of initiation, but the crack length is too short to estimate a velocity.
208
W.J. Shack et aL/ B WR pipe crack remedies eoaluation 10 - 8
I
]
I
I
I
[111
I
TYPE 347 SS,289°C
I
I
I
i
I IE _
10
(TGSCC)
0 . 2 5 ppm 0 2 + 0.1 ppm SO 2 -
L~ E
=-
03 03
1°-9_
°o
-
-10
[] •
0 ° Side 1 0 ° Side 2
[]
24o0° Side 1
-30
03
(SLOPE = O.~3)
-50
io-~O
L 10-8
i
i
L L LLil
I i0 -7
I
t
-70
I I II 10 - 6
'
1'0
' ' o2 ' ' o3 '
4 0'
S T R A I N RATE (s -I)
Fig. 10. Average transgranular crack growth rate as a function of strain rate for Type 347 SS in 289°C water with 0.25 ppm dissolved oxygen and 0.1 ppm sulfate.
[] • [] •
Preliminary e x a m i n a t i o n of six Type 3 4 7 - T y p e 347 a n d Type 3 4 7 - T y p e 304 SS 10-in.-diam weldments received from the N e w York Power A u t h o r i t y has been completed. The weldments were prepared using either the G e r m a n narrow-gap welding process without a n insert or the more conventional welding procedure
0 ° Side 1 0 ° Side 2 240 ° Side 1 90 ° Side 1
03
03 -50
-70
i
I
10
5.2. Characterization of Type 347 stainless steel weldrnents
60
10
-10
As shown in fig. 10, the strain rate and average crack growth rate are related by a power law with an expon e n t of - 1 / 3 , which is consistent with the b e h a v i o r of Type 3 1 6 N G a n d solution-annealed Type 304 SS a n d the model we have developed to describe strain rate effects on various parameters in C E R T tests [12].
' ' 50'
DISTANCE FROM WELD CENTERLINE (ram)
i
I
20
=
I
30
i
I
40
i
I
50
i
60
DISTANCE FROM WELD CENTERLINE (ram)
Fig, 12. Axial (a) and circumferential (b) residual stresses associated with a narrow-gap welding procedure.
developed at the E P R I N D E Center, which uses an insert. Before s h i p m e n t to A N L the weldments were r a d i o g r a p h e d a n d e x a m i n e d ultrasonically; the welds were f o u n d to b e of high quality with relatively few
Fig. 11. (a) Cross section of 10-in.-diam modified Type 347 SS pipe weldment prepared by German narrow-gap welding process. (b) Small defect near weld fusion line on the inner surface.
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W j . Shack et al. / B W R pipe crack remedies evaluation
potential defects. Metallographic examination of a narrow-gap weldment at A N L [fig. ll(a)] has confirmed the high quality of the weld indicated by the nondestructive examinations. One small defect, which is less than 5 mm long and - 30/~m deep, was noted. A cross section of the defect is shown in fig. ll(b). The defect was observed by dye penetrant examination after the initial sectioning of the weldment. A dye penetrant test before sectioning did not show an indication. The defect appears to have occurred during normal welding operations, i.e., there is no evidence of weld repair in this region. Residual stress measurements were made on one of the narrow-gap weldments. The axial and circumferential residual stresses in the vicinity of the weld are shown in figs. 12(a) and (b), respectively. The stress distributions are substantially different from those associated with conventional welding techniques [10,11]. Tensile stresses are observed only in the axial direction in a very narrow zone near the weld, and the magnitude of these stresses is low. Over most of the inner surface the stresses are as compressive as those generally obtained after a residual stress improvement process such as MSIP or IHSI.
6. Water chemistry effects on the S C C of sensitized stainless steels
The effect of various ions on the SCC behavior of sensitized Type 304 SS is being investigated. Particular attention is being given to both anion and cation impurities that provide alternative cathodic reduction reactions to dissolved oxygen. Although not all impurities affect SCC through this particular mechanism, it does appear important for several impurity species present in reactor coolant water (e.g., sulfate and copper ions). Such impurities are potentially damaging even in lowoxygen environments associated with the hydrogenwater chemistry in BWRs and secondary-system water in PWRs. The influence of the impurities on the S C C of sensitized Type 304 SS (Heat No. 30956) was investigated in C E R T tests in 2 8 9 ° C water at a low dissolved-oxygen concentration (5 ppb) [3]. In these experiments, the crack growth behavior of the materials was correlated with the type and concentration of the impurities and the electrochemical potentials of Type 304 SS and platinum electrodes in the environment. The C E R T results indicate that the crack growth rate of these steels is largely controlled by the rate of cathodic reduction of oxyanions [e.g., SO42-, NO~-, PO 3 - , CIO~-,
1605 140 ~
~ 1202 1002 M.~
80 2
o PM.I
60
F-
4o:
SENSITIZATION EPR = 2 C/cm 2 STRAIN RATE
20
= 1xlO'6s "1
Fig. 13. Effect of several salts at a chloride concentration of 10 ppm in low-oxygen ( < 5 ppb) water on the time to failure of sensitized Type 304 SS CERT specimens (Heat No. 30956) at 289°C.
AsO43-, which have a central atom (S, N, P, C1, and As) that can assume different oxidation states] as well as by cations (e.g., Cu 2+, Cu ÷) that can undergo similar reduction (to metallic Cu) [3]. The observation that BO33- or Zn 2+, Mg 2+, or N a ÷ (added as chloride salts), which cannot undergo cathodic reduction like the other species, do not promote SCC in the low-oxygen environments, as demonstrated by the test results shown in fig. 13, provides further evidence in support of this mechanism. As in the case of dissolved oxygen, the reduction of these species couples with anodic dissolution or oxidation at the crack tip following film rupture. Since cathodic reduction of the various species occurs simultaneously, these impurities contribute to crack growth in a manner determined by their concentrations and the relative rates of the reduction reactions. The C E R T results also indicate different threshold concentrations for each of the species; that is, when their concentrations are below a specific level, which may depend on the strain rate, neither transgranular nor intergranular cracking occurs in high-temperature, lowoxygen water.
7. Effects of irradiation on the corrosion potential of stainless steel in BWR-like environments
Experiments to characterize the effect of high gamma radiation on the corrosion potential of SS have been carried out using a 90K Ci cobalt source. A standard reference electrode was modified to operate satisfactorily in the gamma envir.onment. The effect of the gamma dose rate and total dose on the open-circuit potential of Type 304 SS and the redox potential of a platinum electrode was measured in conventional BWR-like en-
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v i r o n m e n t s with 0.2-0.3 p p m dissolved oxygen a n d in deoxygenated water (0.005 p p m oxygen). The g a m m a radiation causes a significant increase ( - 2 0 0 mV) in the corrosion potential of the Type 304 SS, b u t no significant changes were n o t e d in the potential of the p l a t i n u m electrode. N o influence of g a m m a dose o n the potentials was found over the range from 0.08 to 10 Mrad.
Acknowledgments The authors wish to acknowledge the assistance of W.F. Burke, D.J. D o r m a n , H. Lund, D.R. Perkins, F.E. Soppet and W.K. Soppet in various aspects of the experimental work. This research was supported b y the Materials Engineering Branch of the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission.
References [1] W. CuUen, G. Gabetta and H. HS.nninen, A review of the models and mechanisms for environmentally-assisted crack growth of pressure vessel and piping steels in PWR environments, NUREG/CR-4422 (December 1985). [2] H. H~inninen, K. TSrrSnen, M. Kemppainen and S. Salonen, On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels, Corros. Sci. 23 (1983) 663-679. [3] W.E. Ruther, W.K. Soppet and T.F. Kassner, Lightwater-reactor-safety materials engineering research programs: Quarterly Progress Report January-March 1985, NUREG/CR-4490 Vol. 1, ANL-85-75 Vol. 1 (March 1986).
[4] P.S. Maiya and W.J. Shack, Environmentally assisted cracking in light water reactors: Annual Report, October 1983-September 1984, NUREG/CR-4287, ANL-85-33 (June 1985). [5] P.S. Maiya and W.J. Shack, Stress corrosion cracking susceptibility of AISI 316 NG and 316 stainless steel in an impurity environment, Corrosion 41(11) (1985) 630-634. [6] P.S. Maiya and W.J. Shack, Environmentally assisted cracking in fight water reactors: Semiannual Report, April-September 1985, NUREG/CR-4667 Vol. 1, ANL86-31, pp. 16-26 (June 1986). [7] J.A. Alexander et al., Alternative alloys for BWR pipe applications, EPRI NP-2671-LD, Electric Power Research Institute, pp. 6-55 to 6-61 (October 1982). [8] D.G. Atteridge, R.E. Page and S.M. Bruemmer, Evaluation of welded and repair-welded stainless steel for LWR service-Annual Report for 1984, NUREG/CR-3613-2 (1985). [9] S. Bruemmer, PNL, Private Communication to W.J. Shack, ANL (July 1986). [10] W.J. Shack, W.A. Ellingson, and L.E. Pahis, Measurement of residual stresses in Type 304 stainless steel piping butt weldments, EPRI NP-1413, Electric Power Research Institute (June 1980). [11] E.F. Rybicki et al., Computational residual stress analysis for induction heating of welded BWR pipes, EPRI NP2662-LD, Electric Power Research Institute (December 1982). [12] P.S. Maiya, Prediction of environmental and strain rate effects on the stress corrosion cracking of austenitic stainless steels, in: Predictive Capabilities in Environmentally Assisted Cracking, PVP-Vol. 99, R. Rungta, ed., Proc. Winter Annual Mtg. of the American Society of Mechanical Engineers, New York, pp. 39-54 (1985).