NUREG/CR-5999, Forwards Updated Fatigue Design Curves for Austenitic Stainless Steels in LWR Environments.Design Curves Being Provided in Advance of Future Update of NUREG/CR-5999 to Facilitate Efforts on Resolution of GSI-190
| ML20202C663 | |
| Person / Time | |
|---|---|
| Issue date: | 10/24/1997 |
| From: | Mayfield M NRC (Affiliation Not Assigned) |
| To: | Coffman F NRC (Affiliation Not Assigned) |
| References | |
| RTR-NUREG-CR-5999 NUDOCS 9712040030 | |
| Download: ML20202C663 (7) | |
Text
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d October 24 1997 RECElVED MEMORANDUM TO: Frank Coffman, C i f Generic Safe'y is es bnh, DEW FROM: Michael Mayfiefd!MfefM EE
- Electrical, Materials, and Mechanical 2
Engineering Branch, DET
SUBJECT:
UPDATED FATIGUE DESIGN CURVES FOR AUSTENITIC STAINLESS STEELS IN LWR ENVIRONMENTS The updated fatigue design curves for auste7itic stainless steels in LWR environments are attached. These updated curves were prepared by Argonne National Laboratories based on recont test results for austenitic stainless stec!s, They are being provided in advance of a future update of NUREG/CR-5999 to facilitate your efforts on the resolt tion of GSI-190.
Please note that testing is continuing for the austenitic stainless steels identified in the attachment as well as for aged cast stainless steels. We will revise the NUREG/CR once a larger body of results is available. -
Attachment As stated Distribution:
EMMEB R/F File Center LShao JCraig i
RWessman KWichman EHackett ,
v] b#h I MMcNeil LLuiid oOCuMENT NAME: GEMMEBWEBWayfeldTatgcel wpd To receive a copy of this document, Indicate in the box: "C" = Copy without attachment / enclosure "E" = Copy with attachment / enclosure "N" = No copy ,
i OFFICE u/EMMEB f/j{
NAMEOd L M. MAYJ:lELD / (A/' * * * ' *e *
- DATE jff)M 10/ M /97 OH ICML RECORb COPY RES Fme Ca.a tb-2
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97'12040030 971024 PDR ORQ NRRA Of_ PDR
UPDATED FATIGUE DESIGN LURVES FOR AUSTENITIC STAINLFSS STEELS IN LWR ENVIRONMENTS FATIGUE S N DATA The existing fatigue strain vs. life (S-N) data base for austenitic stainless steels (SSs) is composed of =500 tests in air (240 tests on 28 heats of Type 304 SS,165 tests on 16 heats of Type 31t. 3, and 95 tests on 5 heats of Type 316 NG) and =275 tests in water (120 tests for 8 heats of Type 304,55 tests for 3 heats of Type 316, and 100 for 4 heats of Type 316 NG). Most of the tests in water have been obtained at temperatures
>250 C and at relatively high levels of dissolved-oxygen (DO), e.g.,0.2 ppm or higher.
The models were based on the following data trends.
Air (a) The fatigue lives of Types 304 and 316 SS are comparable and those of Type 316NG are superior.
(b) For all steels, fatigue lives are independent of temperature in the range from room tempeiature to 427 C.
(c) Fatigue data at temperatures of 260400 C indicate that fatigue life decreases with decreasing strain rate. The model assumes strain rate effects at temperatures
>220 C. Based on the tests in water, the effects of strain rate are assumed to saturate below 0.0005%/s.
LWR Environments (a) in LWR environments, the decrease in fatigue life of austenitic SSs depends on strain rate, DO level in water, and temperature.
(b) Environmental effects on fatigue life are comparable for all steels.
(c) Fatigue life decreases with decreasing strain rate. Fatigue dea from the ongoing work at ANL, suggests that the saturation strain rate (i.e., the rate below which strain rate effects satucate) may depend on both steel type and DO level. In low-DO PWR environments, the saturation strain rate is below 0.001%/s, it may be nigher in high-DO water. A saturation strain rate of 0.0005%/s is assumed in the it.odel. A threshold value of 1%/s is also assumed.
(d) Unlike carbon and low-alloy steels, environmental effects are more pronounced in low-DO that in high-DO water. The existing data are inadequate to establish the functional form for the dependence of fatigue life on DO level. Separate correlations are developed for low- and high-DO environments.
1 i
ATTACHMENT
4 (e) The existing data indicate that environmental effects on fatigue life are relatively insensitive to temperature above 260"C. Environmental effects are moderate at temperatures <220"C. ,
ESTIMATION OF FATIGUE 1.lFE Statistical models have been developed to estimate the fatigue lives of austenitic SSs in 1,WR environments (NCREG/CR 6335). These models were based on somewhat limited fatigue data; all tests in water were conducted in high-DO water, i.e.,20.2 ppm DO. The models have been optimized with a larger data base. In air, the fatigue life N, defined as the number of cycles required to fomi a 3-mm-deep crack, of Type > 304 and 316 SS is ext 3ressed as in(N) = 6.715 - 2.079 In(Ca - 0.118) + 0.11 :T t* (la) and that of Type 316NG as in(N) = 7.404 - 1,795 In(ca - 0.116) + 0.11 IT t *, (Ib) where cais the strain amplitude (%), IT si 1 at temperatures 2220"C and 0 at lower temperatures, and t' is transformed strain rate defined as i'* = 0 (t >l %/s) t' = In( t) (0.0005 s t 51 %!s) t* = In(0.0005) (t <0.0005 %/s) (lc)
In low-DO environments (<0.05 ppm DO), the fatigue life of Types 304 and 316 SS is expressed as in(N) = 5.966 - 2.079 In(ca - 0.118) + 0.268 IT t* (2a) and that of Type 31oNG as in(N) = 6.935 - 1.795 In(Ea - 0.116) + 0.268 IT t*, (2b) where ITand c* have been defined in Eq. (Ic).
In high-DO environments (20.05 ppm DO), the fatigue life of Types 304 and 316 SS is expressed as In(N) = 5.966 - 2.079 In(Ea - 0.118) + 0.14 IT t* (3a) and that of Type 316NG as In(N) = 6.935 - 1.795 in(ca - 0.116) + 0.14 IT t *. (3b)
The fatigue S-N curves for Types 304 and 316 SS and Type 316NG in room-temperature air are shown in Fig.1. The results indicate that the S-N curve for Types 304 and 316 SS may also be used for Type 316NG steels; estimates of life me either accurate or conservative for Type 316SG steels. The S-N curves for Types 304 and 316 SS are compared with the current ASME mean curve and the curve calculated from correlation in NUREG/CR-6335 in Fig. 2. The ASME mean curve is not consistent with th; existing fatigue S-N Jata.
The fatij;ue S-N curves for Types 304 and il6 SS in LWR environments are compared with estimates based on the correlation in NUREG/CR-6335 in Fig. 3; the curves were obtair.ed at saturation strain rate, i.e.,0.0X)59/s for updated curves and 0.001%/s for NUREG/CR-6335. For high-DO water, the difference between old and updated curves is due to the differences in saturation strain rate.
DESIGN CURVES The design fatigue curves are obtained by the same procedure that has been used for developing the current ASME Code design curves. For a specific set of environmental conditions, the best-fit data curve is first adjusted for the effect of mean stress using the q Goodman telation and then lowered by factors of 1.5 on stress and 20 on cycles to account for the differences and uncertamties in fatigue life associated with material and loading conditions. The originalintent had been to provide a factor of 2 on stress between the best-fit data curve for spec! mens and the component design curve.
'.towever, because of the differences between the ASME mean curve and the best-fit curve to existing fatigue data (Fig. 2), the margin on stress for the current ASME Code 6
design cune is lower than 2. When the design curve was extended r.cyond 10 cycles, ,,
this was discussed and it was determined that a factor of 1.5 was adequate, although the factor of 2 was maintained for carbon and low-allov steels and even for stainless steels for lives beyond 106cycles. To be cons; stent with the Code, a factor 1.5 rather than 2 was used in developing the design fatigue curves from the updated statistical models.
The design cut .c for Types 304 and 316 SS in air and low- and high-DO w, ar are shown in Figs. 4-6. Note that the design ennas in LWR environments not only account for environmental effects but they also include differences that exist between the current ASME mean air curves and the present mean air curves that have been developed
- from a more extensive data base. Figure 4 shows that although the differencea in the fatigue limit have been reduced or eliminated by reducing the margin on stress from 2 to 1.5, significant differences still exist between the current Code curve and updated design curve in air.
For LWR environments, a minimum threshold strain amplitude of 0.07% (137 MPa stress amplitude) is defined below w hich environmental effects are modest and are represented by the curve corresponding to 1%/s strain rate. Fatigue tests are in progress at ANL to better define this threshold strain limit.
Austenitic Stainless Steels
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