ML20112G803
| ML20112G803 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 12/31/1984 |
| From: | Hellner R, Orlin T, Thurgood B GENERAL ATOMICS (FORMERLY GA TECHNOLOGIES, INC./GENER, PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20112G800 | List: |
| References | |
| NUDOCS 8501160531 | |
| Download: ML20112G803 (28) | |
Text
{{#Wiki_filter:T' METALLURGICALEXAMINATf0N OF CONTROL R00 DRIVE CABLES Prepared By: Ob Prepared By: r,r:# Rick Hellner Brian Thurgooy ( Public Service Company GA Technologies Inc. of Colorado
./ /
Approved By: Approved By: 7 ! N'.Orlinc/N T. F
~
0 . Roberts
- l. Public Service Company GA Technologies t of Colorado Approved By: N Approved By:
T. C. Pringdr R. Rosenberg T l- - Public Service Company GA Technologies of Colorado l i 8 Mk koh27 PM
-1
~
r INTRODUCT10N { Recently during a scheduled exercise of CRD #25 one of the cables failed. In addition, the second cabl~e which had not failed was observed to have one broken strand (7 wires per strand 19 strands per cable). The cables are manufactured from type 347 austenitic stainless steel wire which was cold drawn to 1/4 bard. The wire diameters vary from 0.016" to 0.020" depending upon location in the cable. The wire was certified as having a coating of a molybdenum disulfide lubricant applied on the final draw pass. The object of the work described in this report was to determine the metallurgical aspects which cent.ributed towards the failure and make recommendations as to the future course of action. INIITIAL EXAMINATION Initially, the only materials available for examination were two 2" ' sections from the broken strand of the cable which had not failed on CRD #25. The samples were decontaminated and then excained at the Universty of Denver in a scanning electron microscope (SEM). This examination indicated that the wires all failed in an apparently brittle manner (Figures 1 & 2) with little or no reduction in cross sectional area being observed. Pitting was also observed at or adjacent to the fractures. The pitting was more prevalent where individual wires were touching or overlapping, i.e., in crevices (Figures 3 and 4). In addition, there were considerable numbers of cracks present in the wires. In some cases the cracks were associated with the pits, (Figures 5, 6, 7, and 8), and some were not associated with pitting, (Figures 9 and 10). A section was taken from a typical cracked area and prepared for SEM examination. Cracks were observed emanating from pits. The cracks exhibited " delta" branching typical of stress corrosion cracks. The general plane of the cracks in all cases was perpendicular to the direction of tensile loading, (Figures 11 and 12). Approximately 12 ft. of the failed cable from the CRD #25 was removed to a location where a more thorough examination utilizing a visual . binocular (X 20) microscope could be performed. It was observed that t the cable had areas of discoloration and other areas of normal appearance. The cable in its contaminated condition showed pitting, cracking and large amounts of surface debris; the majority of which , was associated with areas of discoloration. Flat spots at various I locations on individual wires were also observed. Examination of i original virgin cable showed identical flat spots (Figures 13 and l 14). The cable which had a broken strand (2" sections examined initially) was removed in its entirety from CRD #25 for macroscopic examination. , The examination found similar observations to those previously described. The areas that showed discoloration were reported to be below the seal (Figure 15).
1 B For a more detailed examination, a 12" section from the failed cable of CR0 #25 was removed. In audition, a section of virgin cable utilized to fabricate th,e original cables was examined. The examinations utilized the following methods:
, 1) Optical Metallography f .
- 2) SEM/EDAX
- 3) Chemical analysis of surface corrosion products OPTICAL METALL0 GRAPHIC EXAMINATION Wires from the 12" length of the failed cable from CRD #25 were ~
sectioned, mounted, and polished for examination. This examination found transgranular cracks that exhibited " delta" branching (Figures . 16 and 17). Many cracks were observed to start from the base of pits. A standard microhardness test was performed on a cross section from a failed wire that exhibited no reduction in cross sectional area and from a wire that exhibited a ductile fracture. The results showed that both wires had the same hardness (DPH 349 and 350 respectively). SCANNING ELECTRON MICROSCOPIC EXAMINATION Scanning electron microscopic examination of samples removed from the 12" section showed extensive debris on the surface of the cable (Figures 18 and 19). Extensive pitting was also observed at various locations. The pitting was more prevalent in the locations where individual wires are in close proximity of each other f.e. crevice areas (Figures 20 and 21). Some of the pits were observed to have oxide whiskers (Figures 22 and 23). Cracks were also observed at various locations adjacent to and remote from fractures (Figures 24 , and 25). Brittle appearing fractures with little or no reduction in cross sectional area were observed on the outer strands of the cable, and some of the fracture faces also exhibited oxidation indicating old . fractures (Figures 26 and 27). Brittle appearing and ductile fractures were observed on the inner strands (Figures 28 and 29). No evidence of pitting or cracking was found on the wires with ductile l failures. Pitting and cracking was observed only on those wires with brittle appearing fractures. A scanning electron photomicrograph of a crack emanating from the base of a pit with extensive " delta" branching is shown in Figures 30
- and 31. The cracking propagation was transgranular as shown in
- l. Figure 32.
I f t
- EDAX analysis was performed on several samples from the 12" section which was removed from the failed cable of CR0 #25. The location of the areas analyzed and constituents found are listed in Table 1. The analysis indicated that the major corrosion product constituents found in the surface debris, pits and cracks are C, 0, C1, Fe, and Ni. The constituent of most interest is. Cl which was found to be as high as 14% wt. in the corrosion whiskers, 0.6% wt. in the pits and cracks and 4% wt. in the surface debris. A black appearing grease substance observed at various locations on the virgin cable was analyzed with EDAX (Figure 33). This analysis indicated the major constituent is carbon with small amounts of 0, Na, Si, S, and C1.
This substance is likely some type of hydrocarbon.
SUMMARY
OF EDAX ANALYSIS TABLE 1 I I I I I l SAMPLE l LOCATION OF l CONSTITUENTS l APPLICABLE EDAX l l l AREA EXAMINED l FOUND l X-RAY SPECTRA l l l l l l l l l l l l12" section l Corrosion Whisker l C, 0, Cl, Cr l Figure 34 l lfrom CRD #25l Figures 22 and 23 l Fe, Ni l l l l l l l l l l l l l12" section l Crack tips of l Cl up to 0.5% wt.l l lfrom CRD #25l Figure 31 l observed l l l l l l l l l l l l l12" section l From fracture surfacel C, 0, C1, Cr, l Figure 35 l lfrom CRD #25l Figures 26 and 27 l Fe, Ni l l l l l l l l l l l l l12" section l Surface debris l C, 0, C1, Cr i Figure 36 l l l l Fe, Ni l l ~ l l l l l l l l Major constituent l l l Virgin cable l Black Grease l found was C; I Figure 37 l l l l substance Figure 33 l small amounts of l l l l l 0, Na, SI, S, l l l l l and Cl l l l l l l l l l t
Chemical Analysis
-a Chemical analysis of surface corrosion product was performed by immersing a strand from the fractured end of the contaminated CRD #25 cable in boiling demineralized water for 24 hours. A similar strand taken from a virgin cable in storage in San Diego was tested in the same way for comparison purposes. The virgin cable was found to have 2 ppm chlorides and the failed cable had 135 ppm chlorides. It may be significant that the virgin cable has 2 ppm chlorides and the broken cable has-135 ppm chlorides. However, the mechanism for increased chlorides in the failed cable is not known at this time.
DISCUSSION The observations of brittle appearing fractures, pitting and transgranular cracks with extensive " delta" branching indicates a stress. corrosion cracking mechanism, and is considered _a major factor - towards failure of the cable wires. In order for stress corrosion cracking to occur, the following conditions have to be satisfied. The combination of:
- 1) Sufficient tensile stress (applied, residual, or thermal)
- 2) Corrosive environment
- 3) Susceptible material The CRD cables have sufficient tensile stress as a result of original cold drawing and applied loads during operation to cause stress corrosion cracking. Chlorides which were found in significant quantities cause pitting and transgranular stress corrosion cracking in 347 austenitic stainless steels. Moisture in the form of liquid
. and small amounts of oxygen are also required for stress corrosion
!' cracking to occur. The historical operational environments that the CR0 cables have been exposed to indicate that both these constitutents have been present. l l -It was also observed that inner strands from the same location along l the cable failed in both ductile and brittle manners. The brittle appearing failures appear to have been initiated due to stress corrosion. The ductile failures were typical cup cone tensile failures. This indicates ~at this point that not all strands within a given cable have been exposed to environmental conditions necessary for stress corrosion cracking to occur. The source of the chlorides is unclear at this time. The original manufacturing practice does not appear to be a source of chlorides. ( However, further investigation to' confirm this is required. The - source of chlorides .to cause stress corrosion cracking is not required to be massive. High local concentrations of chlorides can occur in systems where continual moisture, condensation, and subsequent drying take place. The operational environment historical data of the CRD's indicate that wetting and drying of the CR0 cables has likely taken place. L
~ ~ - . _ _ _ _ . --
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- CONCLUSTONS
( We have determined the existence of chloride stress corrosion
'l cracking and pitting on a considerable number of individual wires from the failed and the broken strand from the unfailed #25 CR0 cable. These stress corrosion cracks and/or pits could well have weakened the cable leading to failure.
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7 i Fig. 20 Pitting occurring in crevice areas between ,
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l i P Fig. 21 , Higher magnification of Figure 20 ' showing pi,ts. X640 ,
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i l Fig. 24 i f Typical crack in wire from failed cable. ! CR0 #25 Failed cable I i gg y .:.<,eg.74g. < .c -
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[ ' ic': ' ___ . ___/ --. _ --- l .y',! .f ' *. / l ( (' < i i l Fig. 26 ) Brittle appearing fractured wire from outer strand of 12" section from failed CRD 25 cable. l {- X160
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l ~: ! Fig. 28 Broken inner strand from failed cable showing
/ ductile fractures.
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/ .to determine constituents of corrosion product at locations indicated.
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Stress corrosion crack propagating in a transgranular direction.
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/ Black grease like substances on surface of
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. FIGURE 34
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, FIGURE 35
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( G1-NOV-84 s 13: 43: 48 RATE: CPS TINE 73LSEC S G --2 G K E V: 1GEV/CH PRST: OFF A: BULK LUBRICANTS: tiEW WIRE FS= 4101 NEM: A FS= 299 G2 G4 06 08 10 i
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