Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR HEAT TREATING IRON-
NICKEL-CHROMIUM ALLOY
BACKGROUND OF THE INVENTION
The present invention is particularly adapted
~or use with a nickel-chromium-iron alloy such as that
described in U.S. Patent No. 4,236,943 issued on Decem-
5 ber 2, 1980, which has strong mechanical properties and,at the same time, has swelling resistance under the in-
fluence of irradiation and low neutron absorbence~ As
such, it is particularly adapted ~or use as a duct~ng
and cladding alloy ~or fast breeder reactors~
A material of this type is a gamma-prime
strengthened superalloy; and its properties can be altered
drastically by varying the thermomechanical treatment to
which it is sub~ected. For nuclear reactor applications
it is, of course, desirable to subject the alloy to a
thermomechanical treatment which will produce the greatest
irradiation-induced swelling resistance and/or the highest
strength and most importantly the highest post irradiation
ductility.
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SUMMARY OF THE INVENTION
In accordance with the present invention, an
alloy having a composition of about 25% to 45% nickel,
10% to 16~ chromium, 1.5% to 3% of an element selected
from the group cons$sting of molybdenum and niobium,
about 1% to 3% titanium, about 0~5% to 3.0% aluminum and
the remainder substantially all iron, initially heated to
a temperature in the range of about 1000C to 1100C for
a period o~ 30 seconds to one hour; although the preferred
heat treatment is to h~at in the range of 1025C to 1075C
for 2-5 minutes to minimize the time in the furnace. mis
initial heat treatment is followed by a furnace-cool and
cold-working in the range o~ about 20Yo to 60% although cold
working within the range between 10% ~nd 80% is useful.
Thereafter, the alloy i~ heated to a temperature in the
range of 750C to 825C for 4-15 hours and preferably 775C
for 8 hours, followed by an air-cool. mereafter, the alloy
is again heated to a temperature in the range of about 6500C
to 700C for 2-20 hours ~ollowed by an air-cool.
The abo~e and oth~r objects and features of the
invention will become apparent ~rom the following detailed
description taken in connection with the accompanying
drawing which is a plot of percent swelling versus an-
nealing temperature for an alloy within the scope of the
in~ention.
In order to establish the desirable re~ult~ of
the invention, an alloy having the following composition
was subject to uarious thermomechanical treatment~ herein-
after described:
TABLE I
Nickel - 45%
Chromium - 12%
Molybdenum _ 30/O
Silicon - 0.5%
Zirconium - 0.05%
Titanium - 2.5~
Aluminum - 2.5%
Carbon - 0.~3%
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Boron - 0 . 005%
Iron - BalO
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'Ihe foregoing alloy is a gamma-prime strength-
ene(l superalloy. Its properties can be altered drasti-
c~lly b~ varying its thermomechanical treatment prior to
testing. The following Table II sets forth the various
'j thermomechanical treatments to which the alloy set forth
in Table I was subjected; while Table III lists the re-
sultin~ microstructural and mechanical properties of the
alloy after heat treatment:
T~ LE II
Vesi.gna-
tion_ Thermomechanical Treatment
AR 103~C~l hr/FC + 60% CW
IN-I ~982C/1 hr/AC ~ 788C/1 hr/AC + 720C/24 hr/AC
IN-2 ~890C/1 hr/AC + 800C/11 hr/AC + 700C/2 hr/AC
1'j EC -~927C/1 hr/AC + 800C/ll hr/AC + 700C/2 hr/AC
EE *800C/ll hr/AC + 700C/2 hr/AC
ter l~g~C/I hr/FC + 60% CW.
TABLE_LII
Designa- 650C
20 tion Comments UTS (ksi) 80 ksi SR (hr)
AR No gamma-prime, high - 67.9
dislocation density
IN-l Bimodal gamma-prime, 151.5 0.,3
recrystallized above
gamma-prime solvus
lN-2 Trimodal gamma-prime 141.3 81.9
(~islocated)
EC Trimodal gamma-prime - 64.7
recrystallized below
gamma-prime solvus
EE Bimodal gamma-prime, - 235
equiaxed cells
As can be seen from Table III above, the EC
treatment produces higher stress rupture properties than
treatment IN-l. The EC treatment results in a trimodal
distribution of gamma-prime since the recrystallization
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anneal is below the gamma-prime solvus and results in the
prccipitation of a small volume of large (approximately
600 nm) gamma-prime precipitates.
Of the treatments set forth in Tables II and
III, three treatments produced dislocated structures.
These are treatments AR, IN-2 and EE. The stress rupture
data of Table II reveals that heat treatment EE produces a
significantly stronger material. This structure consists
of an interwoven dislocated cell structure which is pinned
by a bimodal gamma-prime distribution. This condition has
the highest strength of any tested and is very stable
because of the pinned nature of the dislocation cells.
The graph shown in the attached figure illus-
trates the swelling behavior of the alloy set forth in
Table I in three thermomechanical conditions, ST, EC and
EE. The swelling versus temperature curves are for radia-
tion doses of 30 dpae, which is equivalent to 203x103 ~-
(i.e., greater than the goal fluence of 120x103 ~-). The
data reveals that the ST and EE treatments produce the
lowest swelling in the alloy set forth in Table II above.
The EC treatment produces an acceptable level of swelling
at goal fluences but the treatment is far from optimum for
in-reactor applications.
In similar tests, an alloy having the following
composition was tested:
TABLE IV
Nickel 60
Chromium 15
Molybdenum 5.0
Niobium 1.5
Silicon 0.5
Zirconium ~ .03
Titanium 1.5
Aluminum 1.5
Carbon 0,03
Boron 0.01
Iron Bal.
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Ihe thermomechanica:l treatalents given to t:he aforesaid
alloy o~` T~ble IV and the microstructures and mechanical
properties of the resulting alloy are given in the follow-
ing Tables V and VI:
TABLE V
Designa-
tion Thermomechanical Treatment*
BP 1038C/1 hr/AC + 800 C/ll hr/AC + 700C/2 hr/AC
BR 927C/1 hr/AC + 800C/ll hr/AC ~ 700C/2 hr/AC
BT 1038C/0.25 hr/AC + 899C/l hr/AC +
749C/8 hr/AC
CT 30%CW at 1038C + 800C/ll hr/AC +
700C/2 hr/AC
CU 890C/1 hr/AC + 800C/ll hr/AC +
700C/2 hr/AC + 30
BU 800C/ll hr/AC + 700C/2 hr/AC
*A~ter TO38~7r-hr/Fc ~ 60% CW.
TABLE VI
Designa- 650C
20 tion Com,ents ~ r)
BP Small gamma-prime, no 136.7
dislocations
BP Bimodal gamma-prime, 152.5 73
gamma cells
25 BT Bimodal gamma-prime 135.3
no dislocations
CT Bimodal gamma-prime, 154.6
non-uniform structure
(long banded cells,
some subgrains)
CU Bimodal gamma-prime, 147.0
elongated cells
BIJ Bimodal gamma-prime, 156.4 74
equiaxed cells
~5 The gamma-prime solvus and the one hour recrystalliza~ion
temperature for the alloy set forth in Table IV are 915C
3~3
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I()"(` al-l(l lO()()QC + 20(`, respectively. Therefore, unlike
thc alloy given in 'I'ab]e I, there is no temperature range
in which recrystallization can be accomplished while
aging. Consistent with this fact, treatments BP and BT
which involve annealing at 1038C and subsequent double-
aging, both produce a dislocation-free austenite matrix
and a bimodal gamma-prime distribution. Structures pro-
duced by treatments CU and BU, which do not induce recrys-
tallization, all contain a highly dislocated cell struc-
l~ ture containing various distributions of gamma-prime.
Table VI is a summary of the observed structures
and corresponding physical properties. Note that the
mechanical property values are grouped into two classes.
These are non-dislocation density, gamma-prime containing
structures having 650C, ultimate ten~ile strengths be-
tween 135 and 137 ksi, and the dislocated gamma-prime
structures, which are much stronger, with ultimate tensile
strengths between 147 and 157 ksi. Because of their
superior strength and because of the benefit of an in-
creased incubation time for swelling, dislocated struc-
tures are preferred.
Treatment CU set forth in Tables V and VI above,
starts with a dislocated cell structure with a trimodal
gamma-prime distribution which is subsequently cold-worked
2~j 30%. The final cold-working operation actually decreases
the strength as indicated by the 650C ultimate tensile
strength data set form in Table VI, apparently by des-
troying the integrity of the dislocation cell walls.
Treatments BR and BU of the alloy set forth in
Table IV both produce a highly dislocated, partially
recrystallized or recovered cell structure with bimodal
gamma-prime size distribution. The BU treatment is pre-
ferre~ since it yields slightly higher stress rupture
~ than the BR treatment. The dislocation and
3~, gamma-prime structures for the BU treatment produce a cell
structure which is much more dispersed and interwoven than
that produced by the EE treatment of the alloy set forth
in Table I. The minimum cell thickness of the BU treat-
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mellt is appro.~imately the satne as the gamma-prime particle
C i ~l g .
Ln order to further demonstrate the improvement
that is obtained by means of the thermomechanical treat-
ment of the present invention, reference may be had to thefollowing Tables VII and VIII which shows that this treat-
ment is very effective in promoting high post radiation
ductility. In this regard it should be pointed out that
there exists a trough in which the ductility of these
materials is materially decreased when tested at a temper-
ature which is 110C above the temperature at which the
material has been irradiated. Thus, the poorest ductility
would be found at a temperature of 805C where the mater-
ial has been irradiated at 695C. This 110 should ac-
1'> count for all transient conditions of operation of forexample a fast breeder reactor. Thus the selection of the
material and the heat treatment or the thermomechanical
heat treatment of the material which when irradiated at
695centigrade should be tested at 805C where the lowest
post irradiation ductility has occurred. Reference to the
following Tables VII and VIII make it abundantly clear for
example that the solution treated condition of alloy D66
when irradiated at 695C and tested at 805C exhibits zero
ductility. In contrast thereto, material which has been
subjected to the treatment set forth in the claims append-
ed hereto of the same alloy irradiated at 695C and tested
at 805C shows that a 1.1% uniform elongation is avail-
able. It is critically important to maintain a greater
than 0.3% ductility under these conditions since this is
necessary to maintain fuel pin integrity during reactor
transient conditions and the tables demonstrate the at-
tainment of those goals. Tables VII and VIII also illus-
trate that the higher ductility of this treatment is also
accompanied by higher strength which is a highly unex-
pected as respects these irradiated materials. Thesehigher strengths also attest to the fact of the excellent
swelling resistance demonstrated by the alloys ~hich are
subjected to the method of this treatment.
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47,107I
Although the invention has been shown in connec-
tion with certain specific embodiments, it will be readily
apparent to ~hose skilled in the art that various changes
in method steps and compositional limits can be made to
suit requirements without departing from the spirit and
scope of the invention.
