Note: Descriptions are shown in the official language in which they were submitted.
~z~
PC-~823
The invention is directed to the provision of
articles and parts which are resistan~ ~o stress corrosion
cracking in aqueous environments such as those encountered
in light wateE reactors.
. It has been ound that certain relatively highly
stressed parts employed in light water nuclear reactor
environments are subject to stress corrosion cracking wherein
the ~art failed catastrophical~y without apparent damage.
Thus, relatively highly stressed parts ~uch as springs,
bolts; valve stems, etc, produced of age hardened nickel-
base alloys such a~ theses ~isclosed in U.S. Patents No.
2,57~193 and No~ 2~570,1g4 having a tensile yield strength
of 100,000 lbs/in2 or higher have been subject to failure
attrihuted to stress ~orrosion cracking in deaerated water
at pH 10 at temperatures up to 360~C.
The failures encountered in ~ervice were quite
unexpected and dismayingO It was known that over-aging
h~at treatments were beneficial in improving resistance to
stress corro~ion ~ra~king of age hardenable alloys and it
was postulated that compo~itional changes in the alloy used
could also improve stress corxosion cracking but no leads
were available which would indicate the direction in which
~o proceed or the ingredient in the alloy which should ~e
controlled in order to improve resistance to stress corrosion
craclcing of age~hardenable alloys heat treated to provide a
room temperatu~e yi~ld strength (0~2% offset) oP at least
about lQ0,0~0 psi. The problem was Purther complicated in
that not only was it desirable to obtain the property of
resistance to stress corroslon cracking ~ut in addition the
capability of proYiding a yield strength of design interest
~'
~2~ 2~32
was still to be retainedO It was known rom testing of
wedge opening l~ading (WOL) ~tress corrosion specimens made
of an es~ent1ally non-~a~in~ nickel-base alloy containing
15.7~ chromium~ 8.1~ iron, 00014% carbon, 0.29~ aluminum,
Q.007% titanium, 0~0005~ sulfuY, 0.15% silicon, 0~ copper
0.006% boronJ 0~008% pho~porous~ 0.006~ nitro~en, 0.21~
zirconIum, subjected ~o deaerated, deionized water at p~I lQ
and a temperature of 360C tha~ cracking was observed a~ 7
and 16 weeks i~ two specimens of material whi~h had been
annealed (solution treated) one hour at 1120C and water
quenched but that when the same annealed material was then
heated ("L" treatment) for 7 hours at 608C and alr cooled
that no cracking was observed in the full 36~week course of
the test. The alloy in the ann~aled condition would typically
have a yield strength ak room temperature below 40/000 psi,
and it was con~idered that no valid compar;son could be
drawn from such an alloy to an age-hardenable alloy heat
treated to provide a yield ~rength at room temperature o
100,000 psi or mor~.
~UM ~ RY OF THE VENTION
I~ has been discovered that age-hardenable alloys,
aged to provide a yield strength (0.2~ ofset~ of at least
about 100,000 pounds per square inch) containing, by weight,
0.08~ max~ carbon~ 1% max mangane e~ 5%-9~ iron, 0~01%
max. sulfur, 005~ maxO silicon, 005% maxO copper, 14~-17
chromium~ 0~4~ to 1~ aluminum~ 2.25%~2.75% titanium, 0.7~-
1.2~ columbium and the balance essentially nickel, have
much greater resistance to stress corrosion cracking in
testing at 360C in deaerated, deionized water containing
less than 50 parts ~er billion of oxygen and saturated with
-3 PC-~823
hydrogen, when the zirconium content of the alloy is at least 0.07% and
up to 0.15% or 0.2%.
DETAILED DESCRIPTION OF THE INVENTION
The invention is based on the discovery that age hardened
nickel-base alloys having a yield strength oE at least about 100,000
lbs/in2 are much better protected against stress corrosion cracking by
including in the alloy at ]east about 0.07% of zirconium.
This discovery i9 illustrated in the following examples.
LX~MPLE 1
Sixteen laboratory size heats of an alloy containing nom:inally
15% chromium, 7.5% iron, 1% columbium, 0.75% aluminum3 and 2.7% titanium
with a balance essentia]ly nickel were produced and reduced to 0.5"
thick by 5" wide hot rolled plate. Certain of the alloys contained
about O.U8~ zirconium while the others were essentially zirconium-free.
Half ~nch thick WOL specimens were prepared from each heat. The WOL
samp]es were fatlgue pre-cracked at room tempera~ure and bolt loaded to
various starting stress intensities determined by a crack opening
displacement gage inserted at the outer edge. Two or three WOI, samples
were tested from each heat. Prior to machining, specimen blanks oF the
WOL samples were subjected to a heat treatment comprising a solution at
1093C for 2 hours followed hy water quenching and an aging at 704C for
20 hours. The WOL samples were tested in deaerated pH 10 water at
360C. The samples were removed from test at Eour-week intervals and
the crack lengths
~;;
'i .1
measured on the sîdes of the samples. These tests and earlier
studies showe~ tha~ visihle crack propaga~ion halted after
approximately six weeks of testing~ The total test time
was 1~ weeksO Compositions of the heats are given on Table
I and the results of the WOL test are given in Table XI.
The stress corrosion cracking resistance is measured in
terms o the ~tress intensity at which s~ress corrosion
crack propagation stopped, as measured on the frackure
surfaces of samples that were mechanically broken open at
room temper~ture after the stress corrosion test was comple
ted~ The higher the KI~C~ value the greater is the resist-
ance to stress corrosion cracking. The yield strength val.ues
were determin~d by tensile tests at room temperature.
The results of Table II demonstrate that sta~isi-
cally the alloys containing 0.08~ zirconium were signi~icantly
higher in KISC~ value than were the low zirconium heats.
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T~I.E 2
5~RE:SS CORROSIO~ l~EST RESULTS
~ .
Yield Strength
HeatZr KXSCC ~ksi in3 680F Water (ksi)
31. 9, 34 . 5 112
2 L 42 ., 4 ~ 45 .1 . 110
3 L 39 .1, 45 O 0, 41. ~ 116
4 L 52 . 0, 51. 3 lQ8
~ 5~ 9.1, 60.1 112
6 ~ ~ 55 7 49 . 3 107
7, E~ 55 ~ 56 . 1 115
8 E~ 57, S~.5 112
9 L 52 l 53 11
10. L 47 J 52 0 6, d~8 0 8 112
11 L . 53 " 55 . 9 112
12 L 54 .1~ 52 . 5 112
13 H 53, 56 . 7 l.l4
14 H 52 . 5, 61. 8 ~ 54 110
15 ~I 53 . ~, 61 . 1 ~ 52 , 115
16 H 54 ~ 5 ~ 5U .1 114
L = Low ~r O . 0059~'br
H ~ High ~ O . 08%Zr
XAMPL~ II
~ even heats having composi~ions set forth in Table
III were pxoducedO
Tensile specimens and WOL tes~ specimens were
prepared fro~ each oE these seven al~oys. All samples were
solution trea~ed at 1093C or ~wo hours and water quenched
and were then given an aging treatment at either 704C for
20 hours followed by air cocling ~treatment ~ or at 760C
for ~6 hours followed by air cooling (treatment B~. The
results of the tensile and WOL testing are given in Table
IVo The test conditions for WOL test were the same as those
set forth in Example I hereinbefore.
Again the signiicance of increasing the zirconium
content of the alloy is c~early set forth in comparing the
RIScc values given in Table IV~
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~AiBLE 4
Heat ~ield Stren~th KIS(:C
Treatment tksi)
17 A 111 50.3, 39O7p 33
18 A 110 49 . 3, 47
~19 ~ ~10 56.2p Ds7.8
, A 115 ~ 6 2 . 4
B 102 >61. 6
9 >50.9, 61~5
22 A 126 23~4, 34.. 6
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A = 1093C/2 hr. WQ ~ 704C~20 hr~ AC
B = 1093~C,/2 hr. WQ ~ 760C/~6 hr. AC
. g _
32
EX~MPLE III
Plate stock of co~mercial origin of an alloy having
the ~omposition ~et forth .in Table V was obtained.
Tensile specimens and WOL test specimens of the
type described hereinbefore in the Example I were prepared
rom the alloy material~ -
The results of the tensile tests on ~he heat treatedalloy specimens together With the KIScc results obtained
for the various heat treatments are set forth in Table VT.
The heat treatment accorded each specimen is also included
in Table VIo
The data of Table VI demonstrate that the heat
treatments produced underaged~ peaked aged and overage micro-
structuresu The data of Table VI indicate however that
neither underaging nor overaging produces a distinctively
better combination of strength and cracking resistance.
Only the specimens aged for 96 hours at 760C appear tg
possess distinctly better properties. The results of the
heat treatment experiments indicate that the flexibility
accorded through the heat treatment route is far less than
that provided by changes in ailoy compositionD Wh.ile it is
true that a substantial improYement in propert.ies was pro-
duced by a 9~ hvur asing treatment, such a treatment is not
considered to be ccmmercially practical~
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T~ 6
PRC)PERrrIES OF ALLOY 24 AS A
FUNC'rION OF HEAT TREATMENT
(All samples originally
annealed for 2 hours at
1093nC and wat2r ~uenched)
Yield
P~ging H[eat Btrength KISCC
Treatment ~1) (ksi~ (ksi ~.
.~ . _ .. .. _
593C/2û h 74 53, '18
649 C/20 h 99 ` ~12 ~ 41
704nc/20 h 105 42, 48
760C,/20 h 95 44 ;r 47
816~C/20 h 77 36 ~ 50
7~0~C/96 ~ ~0 58, ~1
Although the present invention has been described
in conjunction wi~h preferred em~odi~nts, it is to be ~nde~-
stood ~ha~ modi~ications and variations may be resorted to
without departing from the spirit and scope of the inven
tion, as those skilled in ~he art will readily unders~and.
Such modifications and variations are considered to be within
the purview and scope of the inven~ion and appended claims.
~ ~3 -
