ML20105B920
| ML20105B920 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley |
| Issue date: | 06/30/1992 |
| From: | BROOKHAVEN NATIONAL LABORATORY |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20105B919 | List: |
| References | |
| CON-FIN-L-1529 MT-L1529-6, NUDOCS 9209210297 | |
| Download: ML20105B920 (28) | |
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EVALUATION OF A FAILED RIVERWATER PUMP SHAFT COUPLING FROM THE BEAVER VALLEY POWER PLANT
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. EVALUATION OF A FAILED R.*VEP. WATER .
PUMP SHAFT COUPLING FROM THE BEAVER VALLEY POWER PI?NY P
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C. J. CZAJKOWSKI P
JUNE 1992 F
Nuclear Waste and Materials Technology Division Department of Nuclear Energy Brookhaven National Laboratory Associated Universities, Inc.
Upton, New York 11973 i
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l-This work was performed under the auspices of the United States l Nuclear Regulatory-Commissior.
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- TABLE OF CONTE!GS ;
i I i FAGI !
LIST OF FIGURES . . . . . . . . . . . . . . . . . v '
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l i 1. INTRODUCTION. . . . . . . . . . . . . . I i
- 2. 1 l
VISUAL EXAMINATION /CilEMI. CAL ANALYSIS. . . . . . . . . . . - k l l
- 3. OPTICAL MICROSCOPY, . . . . . . . . . . . . . . . , . . . . 3 i
- 4. SCANNING ELECTRON MICROSCOPY / ENERGY DISPERSIVE SPECTROSCOPi . 3 ;
1
- 5. . . . , . . . . . . . . . 8
- 1 HARDNESS / TENSILE /CllARPY IMPACT TESTING. r l !
! 6. DISCUSSION AND CONCLUSIONS. . . . . . . . 20 ,
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- 7. REFERENCES. . . . . . . . . . . . . . . 22 ,
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LIST OF FIGURES IAGI figuro 1 Sketch of broken riverwater pump shaft coupling from Beaver Valley Plant. . . . . . . . . 2 Figure 2 Low tsagnification photograph of opticil tnount :
-A- (unetched) .B- (etched) . . . . . . . . . . 4 Figure 3 liigher magnification photograph showing cracks following prior austenite grain boundaries (intergranular) . . . . . . . . . . . . 5 Fic.ure 4 Thi.s photomicro,c.raph depicts the structure of
, the coupling as t empero.1 martensite 5 Figure $a ""M photo of fracture showing intergranular cracking. . . . . . 6 l
Figure $b The cracking in this section was also intergre.nular 6 i Figurc $c Definite intergranular cracking was seen in this third aren exemined 6 P
Fracture after deoxidation treatment (intergranular). 7 Figute 6a Figure 6b Second area also showing intergranuir.r failure. 7 Figure 6c- Third area examined - typically intergranular 7 Figure 7 Stress-strain curves for specimens 1 and 2. , . . 9 Figure 8 Stress-strain curves for specimens 3 and 4. . . 10 Fi dure 9a SEM photo tensile specimen 1. . . , 12 Figurc 9b Grain boundary decohesion is evident at higher. ,
magnification . . . . . 12 Figure 10a SEM photo of tensile specimen 4 . . . . 12 i
Figure 10b liighec magnification photo cf specimen 4 show same type failure as seen in specimen 1 (Figure 9b) . . 12 Figure lla The fracture fece from impact specimen C1 .c assentially all intergranule.r . . 13 Figure 11b liigher magnification fractograph showing intergranu).ar features. . . . . 13 Figure 12a The fracture face of C2 was also intergranular 13 t
i Figure 12b liigher magnification fractograph of C2 . 13 v
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hG Figure da C4 fracture face was also intergranu.lar. , . . 14 ,
Firure 13b Intergranular facets are clearly evidant on C4 . . 14 Figure 14a C6 fracture face was also. essentially all intergranular. . . . . . . . . . . . . . . 14 ,
i Pigure 14b liigher magtification fractograph of C6 . . . , 14 Figure 15a Low magnification fractograph of C7.
Area A - Sheer fracture; Area S quasi cleavage area broken in liquid th tecperatures 15 e
Figure 15b Dirapled rupture failure is coaracteristic of Area A. . . . . . , . 15 i
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i Figure 1Sc Quasi-cleavage failure of Area B was typical. 15 i
Figure 16a The fracture face for C6 was almost entirely a shear fracture . . . , 17 l
Figure 16b The ductile nature of the fracture is evident -
in this higher magnificaticn fractograph of CF 17 Figure 17a Low magnification photomicrograph from sample C8 - no grain boundary etching (7 minutes) . 18 Figure 17b Grain boundary etching is evident on saraple C2 specimen. , . . . . . . . . 18 ,
Figure 18a No evidence of grain boundary etching at higher magnificattor, or C8 (7 minutes) . . . , , 18 Figure 18b Grain boundaries are clearly delineated-on sample C2. . . . . 18 Figure 19a Fourteen minutes of etching did not reveal grain boundaries on specimens from C8. . . h l
Figure 19b Grain boundaries are seen on C2 after 14 minutes of etching. . . . . 19 Figure 20a Higher magnification photo showing no boundary etching .
. . 19 Figure 20' Grain boundaries are clearly seen on C2 at higher magnification. .
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1, INTRODUCTJON In October 1991 a riverwat er pump shaft coupling at the Leaver Valley Nuclear Power Plant f ailed during operation. The coupling was used to join two shalts of a Byron Jackson Vertical Circulator river water pump. The pump rotates j at 1170 RPM in river water cf 32-90'F and produces a discharge pressure of 68 psi The coupling was provided 'oy Byron Jackson as part of an order containing l 12 identical couplings.
A preliminary root cause analysis performed by Lehigh University (for the utility) concluded that the failure occurred due to an improper heat treatment being applied to the 410 stainless steel couplings which resulted in a temper embrittled condition (making the component susceptible t.o cracking).
D. order to independently evaluate the iallure, the U.S. huele - r Regulatory Commission (NRC) requested Brookhcven National Laboratory (BNL) co perform a ,
metallurgical failure analysis on a failed coupling from Beaver Valli.f.
Ib investigation consisted of:
a) Visual Examinaticn/ Chemical Analysis b) Optical Microscopy c) Secnning Electron Micrascopy (SEM) d) L:rdness/ Tensile /Chatpy Impact Testing i
This repeat documents the results of the failure $nvestigation.
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- 2. VISUAL EXAMINATION /CllEMICAL ANALYSIS ,
A broken coupling (Figure 1) was received at BNL. The coupling had been sectioned into six pieces by Lehigh University personnel prior to shipment.
Visual inspection of the pieces showed an oxide coating on the fractures and the appearance of worn ti. reads on the inside surface of the coupling, Various sections were cut fram the fracture face for SEM evaluation, while other sections were cut for the mechanical and chemical analysis specimens, Samples sent out for chemical analysis yielded the following results:
TABLE 1 Chemical Analysis kesults Element Wt. t AISI 410 Req trements Ut. %
---wwaams i Carlou 0.12 0.15 max-Manganese 0.42 1.00 max Silicon 0.41 1.00 max Nickel 0.55 No requirement :
I Chromium 12.02 11.5 - 13.5 '
Sulfur 0.012 0.030 man '
Phosphorous Not Tested ,
0.040 max The results are consistent with those of a typical 410 stainless steel. .
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- 3. OPTICAL MICROSCOPY One section was cut from the coupling shich contained two cracks. This sample was toounted and then metallurgically ground and polished. The specirten was etched in an ethereal picral etch (1].
Figure 2 is a low magnification photornie* 7 graph of the area of cracking on the mounted specirt.en. The cracb was intergranular with the cracking following prior austenito grain boundarieu as seen in Figure 3. The inaterial struc ture was that of tempered martensite (Figure 4) which is typical for the 410 stainless steel alloy.
- 4. SCANNING ELECTRON MICROSCOPY (SEM)/ ENERGY OlSPERSlVE SPECTROSCOPY (EDS)
Various fracture areas were examined along the fracture face of the crack (Figure 1). The fracture surfaces were covered by an oxide film (Figures 5a c) and were definit t i.y intergranular. Alter initial exarnination, the specimens were deoxidized using an electrolytic process as follows:
A working solution of Endox 214 is prepared by adding 8 ounces of Endox-214 powder to 1000 ml of cold water and stirring until it is completely dissolved.
A small amount of Photoflow is added to the solution to aid the wetting of the specimen and ellininate some of fecturing during the electrochemical cleaning step. A glass beaker with 500 ml of Endox 214 solution is placed in an ultrasonic cleaner. The specimen is made the cathode, and a platinum wire loop is the anede. A current density of approximately 250 mA/cm is applied for 15 2
seconds. Remove the specimen f rom the electrolyte and ultrasonically wash it in a detergent solution consisting of Alconox and Photoflow for one minute, then rinse in clean water, dip la methanol and dry in hot air. The above procedure i comprises one cycle. It may be necessary to repeat the above cycle several times before removing all the corrosion products. It is not possible to predetermine the exact number of cleaning cycles for any given specimen, since it depends.upon the severity of the oxidation, roughness of surf ace, and the physical si;:e of the sample. Observe the specimen optically cf ter each cycle so that the process can ~
be discontinued after the oxide or the corrosion product is removed and the specimen surface looks clean. Af ter the specimen is thoroughly dry, exacine it lienediately or store ir. a desiccator.
The specimens were reexamined af ter this treatment by SEM (Figures 6a-c).
There was no evidence of f atigue interaction (beach marks, etc . ) nor indication of other than an intergranular mode of failure.
One of the "as received" specimens was also ia' ped by EDS , Four EDS scans were performed on the threaded area of the coup.;ng (2 sc. ins), the outside surface of the coupling and on a cut and ground cross section (bare metal) of the coupling material. There was a clear indication that chromium (Cr) was substantially higher on the interior threaded area and on the exterior coupling surface then was apparent in the cross section examined. This observation supports the hypothesis that the coupling may have been chromium plated prior to being entered into service.
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- 5. IIARDNESS/ TENSILE /CHARPY IMPACT TESTING ,
Hardness'festinc Nine microbardness measurements (500g load) were performed on a specimen cut from f.he failed coupling. The following are the measurement results (in brackets are the equivalent Rockwell "C" scale measurements): TABil 2 Microhardness Results 348 (33) 348 (33) -348 (33) 353 (33) 296 (24) 343 (32) 348 (33) 343 (32) 353 (33) This averages to a R, equivalent of 32 which would equate to a 410 i stainless steel tempered at -1000*F l2). l Tensile Testinc Four subsized tensile specimens were machined from the coupling. Figures 7 and 8 are the stress strain curves generated from the testing. Specimens 13 ' were tested "as received", while specimen 4 had been vacuum degassed at 200'C for > 48 hours prior to testing. Table 3 tabulates the test results: Table 3 Mechanical Test Results Ultivate Stress Reduction Tensile at 0.2% Modulus Elongation in Specimen Stress Yield Area , Number (psi) -(psi) (psi) (%) (%) 1 140600. 93660. 30240000. 14.51 67.40 , 2 141500. 84740. 26950000. 15.81 70.60 . 3 139000. 86650. 30720000. 14.74 69.40 . 4 143200. 89520. 29400000. 15.37 71.20 i Hean 141100. 88640. 29330000. 15.11 69.65 Typical -_ 41055 145,000 115~,000 --- 20- 65 The tensile strength and reduction of area are very similar to that expected [2]-from a 410 stainless steel tempered at 1000*F. Both the stress at 0.2% yield and the elongation were lower than that expected for the same steel. The fracture faces from both tensile specimens 1 and four were examined under the electron microscope. Figures 9a - 10b show the results of - this - examination. It is evident on the f ractographs that elight ductility (dimpled . 8
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rupture) is present on both of the specimens. The secondary cracking and decohesion of apparent grain boundaries are a clear indication that the material was somehow embrittled. Chynv Impact Testine A total of eight subsize charpy impact specimens were machined from the coupling. Specimens Cl C3 were tested "as received"; C4 C6 were tested af ter vacuum degassing for 48 hours at 200'C and C7 and C8 were tested after heat treating the specimens to 1300'F f or 1 hour and then oil quenching. Table 4 list the results of these tests: Table 4 Charpy Impact Test Results .. Absorbed Lateral Specimen n Length Width Thickness Energy Expansion (in.) (in.) (in.) (ft-lbs) (in) C1 2.131 0.2950 0.394 10.0 0.0066 C2 2.126 0.2955 0.395 14.0 0.0126 C3 2.124 0.2960 0.395 15.0 0.0143 C4 2.126 0.2955 0.394 15.0 0.0122 C5 2.131 0.2950 0.394 11.5 0.0116 C6 2.125 0.2950 0.3945 8.0 0.0065 C7 2.118 0.295 0.3935 *DNF on 100ft-lb scale C8 2.118 0.295 0.3945 120 0.078
*DNF - Did Not Fail There was no apparent difference in absorbed energy between the "as received" specimens (C1, C2, C3) and the vacuum degassed specimens (C4, C5, C6) .
There was, however, an eight fold increase in absorbed energy af ter thr cpecimens were heat treated and oil quenched. There was also a six fold increase in lateral expansion after the her.t treatment. The fracture faces of specimens C1, C2, C4, C6, C7, and C8 were examined by the SEM after testing. The "as r e c e ive d" and the vacuum de;;assed specimena (C1, C2, C4 and C6) were virtually 100% intergranular with no apparent area of shear fracture (Figures lla - 14h). The cracked impact specimen C7 was submerged in liquid nitrogen and then broken apart. After warming to room temperature, the fracture face of C7 was examined in the SEM. Figures 15a 15e show the results of this examination. 11
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Ffi . t k A Figure 15a Low magnification fractograph of C7. kbkNhh
- Area A - Thear fracture; 5 Area B - quasi cleavage area broken at liquid N2 temperatures.
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' ~1 gure ISb Dimpled rupture failure is characteristic Figure 15e Quast-cleavage failure of Area B of Area A. was typical.
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This specimen displayed a shear fracture for all of the specimen which broke'in i the impact test;(Figure 15b) with,the remainder of the specimen:(Figure 15b) , exhibiting quasi-cleavage failure as a result of the liquid nitrogen immersion 1 and subsequent separation. This - dimpled rupture . shear fracture was also found on specimen C8. Virtually all-of the fracture face (over 90s) failed by-shear (Figures 16a and 16b). This is a clear indication that the embrittlement process is reversibic. i Since temper embrittlement has a distinct possibility as the cause of the l failure, a section from specimen C2 (as received) and_a seccion from C8 (1300'F 'l ' heat treatment.and oil quench) were munted, polished and etched in Ethernal
-Pieral etch {1}. l This particular solution etches prior austenite grain boundaries of. low -
alloy steels (temper-brittle condition) when the temper brittle condition results from the segregation-of impurities to the prior austenite grain boundaries. The et. chant is prepared in the following manner: I i Picric acid 50 g ams ~ Purified ethyl ether 250 mt11111ters < Zephiran chloride 10 milliliters (12.8% solution) l Water 240 milliliters The solution is prepared by dissolving the picric acid in ether and then adding the 250 milliliters of Zephiran water solution. The resulting mixture is thoroughly shaken up. The solution must be kept in a tightly stoppered bottled to prevent excessive evaporation of the ether. After standing, the mixture-separatea into two layers, the top layer consisting essentially of a saturated ' solution of picric acid in ethyl ether containing Zephiran, and the bottom layer consisa ng cf an aqueous Zephiran picric acid solution. The freshly prepared solution -is allowed to stand overnight prior to use. Af ter _the first thorough shaking, no further agitation .is necessary. To etch the metallographically polished specimens, decant a portion of the top layer into a
- small beaker, and dilute with approximately one-third by volume of ether.
After immersion of the specimens for 7 minutes , they were examined by visible light microscopy. Ini':ial examination of the section= from specimen C8
#1300*F heat . treatment and oil quench) showed no evidence of grain boundary
. ching at two magnifications 100X (Figure 17a) and 300X (Figure 18a). This is
, contrar* *o.the etching in evidence at similar magnificaric s for - the "as-receivee ,,ecimen from C2 (Figures 17b and 18b),
The came results were seen en the two samples af ter 14 minutes of etching (Figures 19a _20b)._ lt is evident that the etchtant has the effect , of highlighting the grain boundaries of embrittled martensitic 410 stainless steel, but by what exact mechanism is yet unclear. Further research should be carried out to determine if this etch can provide a consistent nondestructive test for temper embrittlement in these steels. 16 m_______________________._______._~_____._
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- 6. DISCUSSION AND CONCLUSIONE There-ar a number of ways that a tempered martensitic - steel can suffer from an intergranular (brittle) failure. They are: quench cracking,-stress corrosion cracking, hydrogen induced cracking, temper martensite embrittlement ~
and temper embrittlement. -j Ouench/ Heat Treatment Cracks Quench / heat treatment cracking occurs from stresses produced during the austanite martensite transformation when sufficient martensite has formed in the matrix to provide an internal stress sufficient to exceed the yield stress of the "as quenched" martensite (3) . Since there was no evidence of heat tinting on the . observed - fractures and no evidence of high temperature oxides, this mode of intergranular fracture is not considered probable. Stress Corrosion Crackinn (SCC) Stress corrosion cracking occurs as a result of the synergistic effect of a susceptible material and tensile stresses (either applied or residuali in a corrosive environment, beress corrosion cracks would normally grow until a j critical flaw size is reached. Af ter that poinc, fast fracture would be the l predominant failure mode. Since this mechanism is time dependent it would fit i the facts of the delay between the coupling's installation and subsequent failure. The hw impact energies and intergranular fractures encountered in the coupling and reduced ductility of the 410 stainless steel give a clear indication j ' that the material was embrittled and even though this type of cracking is l possible, the degraded material properties of the 410 stainless steel tend to J minimize this probability. l Hydroren Induced Crackinc l I This-type of cracking occurs in high strength steels which are normally .i under a sustained or slowly increasing load and which are exposed to environments ;! I where hydrogen is available to diffuse into the material. It normally occurs in ferritic alloys in the temperature range from -70*C to +140*C. The source of :l external hydrogen may be localized corrosion (simple Fe oxidation), -H 2 pickup ! from plating operations or decomposition of hydrogen forming compounds (e.g. inclusions). Inclusions (MnS) promote. hydroger, absorption. by the matrix by I acting as windows for hydrogen. Additionally, cracking is time dependent and occ Ars in the temperature range to which the coupling was e>, posed. Since the 200*C vacuum desassing trestment on tensile spe:imens and charpy impact specimens (temperature which would drive off hydrogen, but not induce i temper embrittlement) had no appreciable effect on the material properties of the coupling, this failure mode is considered unlikely. Additionally, hydrogen induced embrittlement normally would reduce the tensile strength of the material which did not occur with the coupling. Temnered-Martensite Embr ttlement This type of embrittlement [5] occurs when steels are tempered in the range of betwt_en 250-400'C and causes a decrease in the steels toughness. This range of toughness decrease occurs at the same temperature where martensite begins its transformation to cementite and ferrite. The embrittlement from this mechanism l 20
d occurs alcng)the grain boundaries where plate like carbides precipitate. This carbide precipitat. ion coupled . with prior impurity segregation during- the - l austenization process causes the intergranular embrittlement. The intergranular fractures on the impact specimens and the tensile specimens suggest this-type of embrittlement. The mechanical properties evident in the "as received": coupling ; does not, however. 410.mc erial tempered in this range has a tensile strength ' of 180-195 ksi with associated Rockwell "C" hardness in the - range of -39 41 (2) . This was not the case, so this type of embrittlement is also discounted. Temper Embrittlement If a low alloy steel is quenched to form martensite and tempered between 600 700*C and then allowed to age at temperatures below 600*C; segregation can .
-l
~
1 occur in the steel to the point where the ductile / brittle transition temperature will be raised, and intergranular failure is possible at room and higher temperature. It has been recorded [5] that the elements phosphorous, tin'and antimony segregate to the grain boundaries and cause this embrittling effect. In Cr Mo steels, . the major embrittling agent is phosphorous. Since the ethereal.picral j etch used during the metallographic portion of this evaluation is extremely l sensitive to phosphorous and it indicated probable grain boundary segregation; this type of embrittlement is considered highly likely. This evidence coupled with the typical tensile and yield strengths recorded and the. significant increase in mechanical properties af ter heat treatment and oil quenching greatly
- enhance the likelihood of this type of embrittlement being the cause of the
- - failure. !
The previous metallurgical investigation and literature review have resulted in the following conclusions: J
- 1. The chemical analysis of the Beaver Valley riverwater coupling verified that the coupling was made from 410 stainless steel.
- 2. The observed cracking was intergranular and appeared to follow prior austenite grain boundaries in the tempered martensitic alloy matrix.
There was no evidence of heat tinting and no high temrerature
.3.
oxides present on the fractures. This precluded the possibility of heat treat cracking.
- 4. The mechanical. properties were reasonably consistent _for a 410 stainless steel tempered at 1000*F. There was a reduction in ductility and yield strength from expected values. The impact energy of the "as received" coupling material was extremely low (8 to 15 f t.lbs. ) and was markedly increased (+100 to- 120 ft.lbs.) after a corrective heat treatment was performed. i
- 5. A new application of ethereal pieral etchtant to 410 stainless steel may provide a nondestructive method of determining temper embrittlement in these steels, but more rescarch is needed.
21 1
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- 6. The root cause of the failure is considered to be temper .
embrittlement of the 410 s tainl e'a s steel coupling . brcught ; about . by improper heat treatment of the coupling ' during i manufacture.
- 7. EXEERENdES
- 1. Cohen, J.B., llurlich, A., Jacobson, cf... Transactions of the A.S.M., Volume 39, 1947, pages 109 138,
- 2. Metals llandbook, Eighth Edition, Volurae 1,1975, page 414 ;
J. Metals llandboqh, Eighth Edition, Volume 10, 1975, page 74'. 4 Engel, L. , Klingelo, H. , An Atins of Metal Damagg, translated-by Stewart Murray, Prtntice Itall, Inc., 1981, page 121.
- 5. Briant, C.L,, . Banerj i, S.K., "Intergranular Fracture in Ferrous Alloys l'n Nonaggressive Environments ," . Tiepjij;.11e _ on Materials Science and Technology, Volume 25, Embritclement of Engineering Alloys, Academic Press, 1983.
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