ML20093H572
| ML20093H572 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 06/30/1984 |
| From: | Brihmadesam J MIDDLE SOUTH SERVICE, INC. |
| To: | |
| Shared Package | |
| ML20093H535 | List: |
| References | |
| NUDOCS 8407250195 | |
| Download: ML20093H572 (25) | |
Text
.,
1(
L,. _, -- to AECM-84/0373 s
METALLURGICAL' EVALUATION OF DIESEL GENERATOR CYLINDER HEAD VALVE SEAT CRACKS AT s
GRAND GULF NUCLEAR STATION - UNIT 1
' PREPARED FOR:
NUCLEAR PLANT ENGINEERING MISSISSIPPI POWER & LIGHT COMPANY PORT GIBSON, MISSISSIPPI 39150 PREPARED BY:
J. S. BRIHMADESAM
. NUCLEAR ENGINEERING DEPARTMENT MIDDLE SOUTH SERVICE, INC.
NEW ORLEANS, LOUISIANA 70112 JUNE, 1984 8
hg72501958 okf6 ADOCM 05 PDR
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's TABLE OF CONTENTS SectIon Page LIST OF FIGURES,
iii
- 1. 0
. INTRODUCTION.
1 2.0 LIQUID PENETRANT EXAMINATION.
1 3.0 FRACTOGRAPHY..
2
.4.0 METhLL0GRAPHICEVALUATION.
3
-5.0 SCANNING ELECTION MICROSCOPIC EVALUATION.
4 6.0-CRACKING. MECHANISM.-.
4 7.0 JACKET WATER LEAK MECHANISM.
6
8.0 CONCLUSION
S.
7
9.0 REFERENCES
8 9
e 9
6 I
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LIST FIGURES Figure No.
Title Page
]
. LIQUID PENETRANT INDICATIONS ON VALVE SEAT.
9 2
LIQUID PENETRANT INDIC'ATION OF SUSPECTED LEAKER.
10 3-LIQUID PENETRANT INDICATIONS OF NUMEROUS STELLITE CRACKS.
11 4
LEAKING CRACK FRACTURE SURFACE.
12 5
CROSS SECTION OF VALVE SEAT SURFACE.
13 6
MICROSTRUCTURE OF CAST STEEL.
14 7
MICROSTRUCTURE BEHIND STELLITE TOP SURFACE.
15 8
MICROSTRUCTURE BEHIND STELLITE BOTTOM SURFACE.
16 9
MONTAGE-SHRINKAGE CRACK IN STELLITE.
17 10 DETAILS OF SHRINKAGE CRACK.
18 11 MICROGRAPH NEAR STELLITE CAST STEEL INTERFACE.
19 12' MICROSTRUCTURE OF STELLITE.
20 13 CRACK IN STELLITE AND MICROSTRUCTURE.
21 14 SEM METALL0 GRAPH OF STELLITE CRACK.
22 h
t 111 I
f
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1.0 INTRODUCTION
Nuclear-Plant Engineering (NPE) forwarded to Middle South Services (MSS) a ring section containing an exhaust valve seat from a cylinder head removed from the Division II standby Diesel Generator Division II.
This specimen forwarded was known to have a crack in the stellite seating surface which had lecked jacket cooling water into the cylinder. Thic specimen had several other radial cracks on the stellite seating surface.
NPE requested MSS to perform a metallurgical evaluation to:
determine the cause for cracks in the stellite seating surface; o
and o
determine the cause for jacket water leak into the cylinder.
The Plant Quality department of Grand Gulf Nuclear Station Plant Staff
-had performed liquid penetrant (LP) examination of all the valve seating surfaces and determined this seating surface to be.the leaking one.
2.0 LIQUID PENETRANT EXAMINATION The specimen was examined using LP.
The indications revealed are shown in Figure 1.
A close up of the suspected leaker is shown in Figure 2 and Figure 3 shows multiple radial cracks on the seating surface.
The LP indications shows that the larger crack was about 90* away from the multiple cracks.
A visual examination of the back side of the ring specimen, which was machined out of the cylinder head, showed a casting defect to be located behind the single large crack.
Thus, it was decided J
I.
to perform the following investigations:
o fractography of fracture surface from the single crack and casting void -- determine mechanism for leak; and, o
light and scanning electron microsecpy of multiple cracks ---
determine reason for cracking.
1 L
- s
-e-43.0.FRACTOGRAPHY The single crack face was opened'in the laboratory-by cutting the ligament from the back side of the ring specimen.
The fracture surface is shown in Figure 4. -The following_ features were revealed:
Th'ere were. two casting defects (voids), one above the other on
- o.
the back side of the seating surface.
' o The stellite fracture surface was covered with bl,ck adherent t
oxide film. Also portions of the casting fracture surface was covered with this oxide film.
o-
- There was a distinct difference in frac; pre surface appearance of lthe laboratory fracture surfaces at the bottom and top of the base material of the' ring.
The appearance at the top was indicative of a ductile weld material.
This indicates that there F
was some weld repair performed in this region.
9 3
o' The crack appears to have propagated from the stellite casting interface to the casting void by corrosion.
The cause for corrosion is the exhaust combustion gases.
i
. As mentioned earlier, it. appeared that the cylinder head, in one area of.this exhausu valve seat was repaired by welding.
The need for weld repair may have been to. fill the casting void such'that an even-surface for weld deposition of stellite was obtained. Though the weld repair achieved the objective-of providing an even surf ace, the weld metal 'did not appear -
to have filled the entire void (See Figure 4).
Furthermore, the casting void below this repaired void was untouched since it did not penetrate the
- exposed surface of the seat area.
A section of this region was obtained using a diamond impregnated metallurgical saw.
The section was used to perform metallography so as to confirm the existence of weld metal (from repair welding). The metallographic section is shown in Figure 5.
Areas where microstructural evaluations were performed are also shown on this figure. No-evidence of a soft butter weld between the cast steel and the stellite was observable.
Figure 6 is a micrograph obtained in a area that shows the microstructurefof the cast steel.
The microstructure consists of a ferrite matrix with pearlite grains.
Figure 7 is a micrograph obtained t
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--from an area on the top side of the stellite.
The microstructure is
- accicular and representative of a weld material.
Similarly, Figure 8, a
-micrograph ~obtained from an area below the stellite, shows a microstructure representative of a weld material. This finding confirms the existence of a weld repair, in the casting..
- 4.0 METAILOGRAPHIC EVALUATION LMetallographic examinations ofLcracks in the stellite were made in the regien where multiple cracks were found.
A section, parallel to the
- horizontal stellite surface, was made which would expose the cracks and its interface with the casting. After metallographic polishing and etching
-(etch.to' reveal casting microstructure) the specimen was examined.
Figure
~
'9 shows one such crack'from the multiple set.
The crack path is jagged.
At'the end of the crack, casting region, a pit is observed.
Similar pits and crack paths were observed on all the cracks in this region.
Figure 10 is a representative micrograph showing the crack in the stellite. The etchant (3% Nical) did not etch the stellite.
The crack path suggests that l
the crack was formed due to shrinkage stresses of the stellite during cooling of the' stellite' veld deposit.
Thus, the cracks were fabrication
= induced-rather than service induced.
Figure 11 shows a close up micrograph of the pit at the end of the. crack.
This pit is located in the cast-material.
The pit formation is due to corrosion from the hot exhaust gases (primarily oxides of nitrogen and sulphur) and not due to any fabrication practice. Also observable in. Figure 11 1s the: intrusion of stellite (white fslender lines) into the: cast' material. The interface between the stellite
~
and cast steel, other than in regions where stellite cracks existed, was clean, and defect f ree..The bond was good, s.
In' order to confirm the causal mechanism for the stellite cracks the I
specimen was re-etched -to reveal the microstructural features of stellite.
I"Hard Surfacing Structures", WRC Publication, Welding Research Council,
\\
United Engineering Center, New York, New York 10017 (No Late).
3
Figure-12 shows the microstructure of the stellite. The microstructure
-shows that the material is a cobalt base, hard facing alloy deposited by a.
shielded arc process (SMAW).
The microstructure is austenitic with a carbide grain boundary network.
Figure 13 is a micrograph showing both the crack and the microstructure.
It can be seen from this micrograph that the crack runs along the grain boundary following the carbide network.
From reference 2 it is observed that brittle hardfacing alloys when deposited by the SMAW process can crack and cross check during cooling to relieve the large strains in the stellite.
This aspect is discussed in a later section. Thus, these cracks form in the regions where the material is still in the liquidus state.
The carbide rich grain boundary is the last to solidify and hence these cracks form along the grain boundaries and are characterized-as shrinkage cracks.
5.0 SCANNING ELECTRON MICROSCOPIC SEM EVALUATION The etched stellite specimen was examined using a scanning electron microscope (SEM). The purpose of this examination was to determine the morphology of the cracks.
Figure 14 shows the SEM micrographs at two.
magnifications.
The grain boundaries are clearly visible as a raised network.
At the higher magnification the cracks shows a tearing type behavior which confirms the causal mechanism to be one of shrinkage cracking. The edges of the cracks are sharp and show no sign of corrosion or errosion.
The extensive cross checking of the crack is also visible.
6.0 CRACKING MECHANISM The metallographic evidence clearly shows that the causal mechanism to be shrinkage cracking.
When the coefficient of thermal expansion for the
" Welding Brazing and Soldering", Metals handbook, Vol. 6, August, 1983, American Society of Metals, Metals Park, Ohio.
3" Steel Castings Handbook", 5th Edition, P. F. Wieser, Editor, Steel Founders' Society of America, Rocky River, Ohio, June, 1980.
4 4
=..-
y 3
2 underlying. cast stee1 and the ste111te are considered it can be shown the cracks are7 ndeed tormed by shrinkage cracking.
that i
1 Rue ' coefficient of thermal expansion are as follows:
-6 Cast Steel -- 7.4 x 10 in/in
'F Stellite
-26.7 x 10~ in/in
- F
~
U This data shows-that.the racio between cast steel and stellite is 1:4.
.Therefore~the stellite would expand considerably more than the cast steel during' welding.- Conversely during cooling the stellite would shrink 4 times)fastercausinglargestrainstobedeveloped.
These shrinkage strains are accommodated by cracking along grain boundaries.
The cracks may or may not expose-themselves to the surface. Therefore when the
.~
. stellite seat is machined some cracks may be-exposed to the free surface and some.stf11 be subsurface and not detectable by LP techniques.
- However, it is not: probable that these cracks are numerous such that a,large area of grain boundary would be expected to be cracked.
This is a valid supposition.and is borne out by metallography which showed that a majority of.the grain boundaries were not cracked..
p The' subsurface shrinkage cracks can open to the surface during service-
.only.during cooling.
t 1This implies, a logical extension considering the differential thermal contraction, that frequent starts and stop' operation of the engine are more apt to open such cracks'than is a continuous. steady-
-operation. Thus wich the given mode of operation (i.e., Surveillance
[~
Testing), it is. expected that pre-existing subsurface cracks would open to the seat surface and be detected in' subsequent LP excms.
The adherence of stellite to the base metal (cast steel) is obtained from the austenite grains and not the carbide rich grain boundary.
~ Reference 2 provides a good discussion on this subject and shows that the
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1 existence of such cracks are not detrimental to the intended performance.
Also, it was observed that the cross sectional examination (Section 4.0) showed a good bond between the stellite and cast steel.
Therefore the cracks in-it-self are not a sufficient condition for stellite failure.
7.0 -JACKET WATER LEAK MECHANISM As previously discussed, shrinkage cracks in the stellite:
~
o do not erode, corrode, or cause crack propagation into the cast steel; and, o
cannot cause separation of stellite from the cast steel.
However, it was also shown (Section 4.0) that corrosion pits existed at the tail end of the stellite cracks and that the fractography (Section 3.0) showed evidence of corrosion on the fracture surface of the cast steel or. the leaking crack.
Therefore, the mechanism that would lead to failure is by corrosion of th'e cast steel by the hot exhaust gases.
Gases formed by combustion of the diesel fuel include; oxides of sulphur and nitrogen.
These oxides and water vapor formed during combustion form respective acids which enter the stellite cracks and attack the underlying cast steel.
Hence, the cracking mechanism that can cause failure is corrosion.
The observed maximum corrosion pit diameter (largest dimension) was 0.005 inches (Figure 2.0).
Furthermore, NPE informed MSS that the engine had operated for about 700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br /> (total) prior to removal of this head.
Since the corrosion mechanism is a surface phenomenon it is reasonable to assume that the rate is proportional to the square root of the measured dimensions. Therefore it is feasible to conservatively estimate the operating time required to extend the corrosion pit to the design minimum of the cast steel wall.
I b
.4'
~
-The:vall thickness (minimum) for this head was 0.400 inches (information obtained from NPE).
Thus we have time required for leak to
~
develop for a design minimum wall to be:
Minimum Time to Propagate Through Wall = 700 x
.400 measured diameter
.'MinimumOperationalTime=700x!.400
.Therefore:
6,300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br />
=
. 005 This implies that'in presence of the stellite crack a conservative approach would.be to devalue the design. life of a cylinder head to 5000 hour0.0579 days <br />1.389 hours <br />0.00827 weeks <br />0.0019 months <br />sLof total. operating life. This crude and very conservative calculation also shows that stellite cracks separated by 0.400 inches could be-. assumed to corrode the underlying cast steel and cause stellite failure.
Hence a safe practice would be to ascertain that secllite cracks in the zexhaust valve seats be at least separated by 0.5 inches.
This extremely conservative approach would allow for assured uneventful operation of 5000
' hours at the.very minimum.
A soft butter veld layer between the stellite and cast steel-would reduce the impact.of differential thermal strain since this much' softer material would be able to. accommodate such strains.
The currently
- manufactured heads by TDI do contain a stainless steel butter which would alleviate' extensive stellite. shrinkage cracking situations.
however, by no
.means is it expected-that this would enable producing a shrinkage crack.
free seating surface.
An austenitic weld butter would, however, prevent the' exhaust gases from corroding the cast steel; since the cast steel, in cheferevice region of the crack, is not exposed to the corrosive exhaust
. gases.
J
8.0 CONCLUSION
S o
. Cracks in the. stellite are created by shrinkage stresses due to
~ differential thermal contraction between the stellite and cast steel.
7
___j
-~
c-6 Subsurface cracks in the stellite, which formed during the o
fabrication, can open to the' surface due to the start-stop mode of operation required for these engines (thermal cycling).
o Shrinkage cracks in the stellite are not detrimental to the intended service of these engines.
o A very conservative approach, that would assure safe uneventful operation of the engine for up to 5000 hours0.0579 days <br />1.389 hours <br />0.00827 weeks <br />0.0019 months <br /> (total operation), is to ascertain that shrinkage cracks in the stellite are at least separated by 0.5 inches.
o The stellite in the cracked region does neither corrode nor erode.
~Hence widening of these cracks is not possible.
In addition, since the valves themselves spin, to some extent, during operation it is highly unlikely that the presence of such stellite cracks would do any damage to valve seating surface.
it.
o-The large number of thermal cycles experienced by the engine and its components (including cylinder head) by virtue of the required testing would tend to open up subsurface shrinkage cracks in the stellite.
Thus, subsequent LP inspections following a baseline may reveal additional cracks in the stellite.
The presence of such cracks during subsequent inspections should not be construed as " cracks initiated and propagated during service".
9.0 REFERENCES
1.
"Hard Surfacing Structures", WRC Publication, Welding Research Council, United Engineering Center, hew York, New York 10017.
(No Date).
2.
" Welding Brazing and Soldering", Metals Handbook, Volume 6, August, 1983 American Society of Metals, Metals Park, Ohio.
3.
" Steel Castings Handbook", Fifth Edition, Wieser, P.
F.,
Editor, Steel Fcunders' Society of America, Rocky River, Ohio, June, 1980.
8-
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- ~.~-w o.a an, Etchant:
3% Nital Magnification: 100x 1 FIGURE 7: Microstructure Behind Stellite Top Surface Microstructurn typical of a weld. 15
e ^Qg s,- y s 7 l 0 ~9 Fv ( 'e e '9 .gn .g + ~., ry ~;b ky , f}}rj (pf:,.. 21T ~..i 4 dW N .- p ' C *; - <se fi, ( A kSE ln : %, E s ' ph;;;i .e 7g c.:.=>HMin';0;P A;~' . - ll ir ,,i MV 3 rst.~% [y?V5. h.Rd,.. Th. f +5 t r ax,,_ ;_,_,,,.,, z .. r.. i Etchant: 3% Nital Magnification: 100x FIGURE 8: Microstructure Behi 3 Stellite Bottom Surface { Microstructure typical of weld metal. 16
Q +,;i.:i.L. :,.x y 4 (G29;g:@.isJz: zy.i3;;c, ~,aq',Xhii.:s,;[, s..
- 1
$gpr 4.q ^. ~D ;1 @6#4 >- w u d~r yMS:aW'"% ntitG&.% W i W c g$ gg 3"d.IpkM Q Ep S!: ^ d' l' '. ...;V i7 'C. h g g '/' .. ' 4 #. ~,,, 9 5. + - A Macnification: 100x g 5 ,~y_--- m.- u s., ;,, u, j ?. -.\\ r.,- .x w. , n ~ ~^~ w 7 y y 4 1 ,,. 3, .1 _+ - ~ + *- 1 74 ,. 2
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- FIGURE 9: Montage - Shrinkage Crack in Stellite
- C l8'5* ' * ' A typical crack in stellite. Jagged path and secondary , cogoss,oM cracks. 7
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[. ' ' I ' ?") q,p i . < 1,g. , 27 . 1 2 ,3 s. ,-s ~. > ~ w.. L.s , e.<,3.;.m m=, -.....t, a,. m w s m -s**+M n.e - = -. i i 5 Et' chant: 3% Nital Magnification: 200x ss I FIGURE 10: Details of Shrinkage Crack \\- .g \\
- 1., '
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O . , f,. s, ~,. ' f ".. ~. 1 L ., (. 3 Etchant: 3" Nital Magnification: 200x FIGURE 11: Micrograph Near Stellite Cast Steel Interface Corrosion pit at the end of shrinkage crack. Evidence of stellite intrusion into cast steel. Fusion line between stellite and cast steel in regions other than cracks and pits is good. 19
e "5 2 r k ~_ -= %h m a ,g ge m n.. s -.-( W e ~ * (l' A'i f g. 9.
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- i r
r ,e ....:. * %A, t J K,7 ~ Magnification: 400x Magnification: 600x Etchant Pet reference 1 l-F IGL'RE 12: Microstructure of Stellite s_ Shows carbide network at grain boundaries. "[ = m h 20 -mm.i.. i =
.= g y g .[ .n tz: w.y;,h 6 %,,= ', Qk },. 2 ,'2 t yl-4 ,.3.-- ,5 6 +.
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-a ?w ' . W. i'b 7' *. h : % Etchant: Per Reference 1 Magnification: 200x FIGURE 13: Crack in Stellite and Microstructure Crack is clearly in carbide network at Crain 3oundaries. 21
m A m, . _ ~ = 'a .m.m. 1m ~3 m
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~u - y Magnificaeion: 400x Magnification: 600x _7 s1 -.i : FIGURE 14: SEM Metallograph of Stellite Crack . 4 Y Metallegraph taken after etching per reference 1. Grain boundary cracks and extensive cross-checking in cracks are evident. i 22 o -.J .}}