Cold worked ferritic alloys and components

This invention relates to liquid metal fast breeder reactor and steam generator precipitation hardening fully ferritic alloy components which have a microstructure substantially free of the primary precipitation hardening phase while having cells or arrays of dislocations of varying population densities. It also relates to the process by which these components are produced, which entails solution treating the alloy followed by a final cold working step. In this condition, the first significant precipitation hardening of the component occurs during high temperature use.

This invention relates to high strength ferritic alloys for use in high 
temperature, and high energy neutron radiation environments. More 
specifically it relates to fully ferritic precipitation hardening alloys 
and their thermomechanical processing. 
Various materials have been considered and are in the process of being 
evaluated for use as heat transfer material (cladding) and structural 
(e.g. ducts) materials in liquid metal fast breeder reactors and steam 
generator turbine applications. These materials have included, for 
example, austenitic solid solution strengthened alloys, austenitic 
precipitation hardening alloys and ferritic alloys. The ferritic alloys 
include, for example, those high strength alloys described in U.S. Pat. 
No. 4,049,431. The ferritic alloys described in this application are 
precipitation hardening materials and have been in the past processed to 
an aged final condition. 
According to the present invention it has been found that precipitation 
hardening ferritic alloys when manufactured to a cold worked final 
condition possess improved swelling properties at elevated temperatures 
when exposed to fast neutron (E&gt;0.1 MeV) fluxes compared to the identical 
material placed in pile in an aged condition. In the present invention a 
ferritic precipitation hardening alloy is solution treated, cold worked, 
and then placed in its intended application, wherein the first significant 
precipitation hardening of said alloy after the last cold working step 
occurs. 
The process is particularly applicable to the fully ferritic precipitation 
hardening alloys described in U.S. Pat. No. 4,049,431. These alloys, 
sometimes described as precipitation hardening delta ferritics, are 
generally characterized by the following chemistry (in weight percent): 
about 9 to 13 chromium; about 4 to 8 molybdenum; about 0.2 to 0.8 silicon; 
about 0.2 to 0.8 manganese; about 0.04 to 0.12 carbon; and the balance 
being essentially iron. Preferably, the alloy chemistry should be as 
follows: about 9.5 to 11.5 chromium; about 5.5 to 6.5 molybdenum; about 
0.04 to 0.07 carbon. In addition, alloys of this type may also include 
about 0.1 to 0.3 vanadium and 0.2 to 0.8 niobium. The niobium being 
preferably held to a range of 0.3 to 0.6. 
For fast breeder reactor applications it is believed that optimum in pile 
properties of long term mechanical stability and swelling resistance will 
be achieved if the precipitation hardening ferritics of U.S. Pat. No. 
4,049,431, especially alloy D57, are modified to include about 0.1 to 1.0 
weight percent nickel, and more preferably about 0.4 to 0.6 weight percent 
nickel, and are processed in accordance with the present invention. 
The above fully ferritic alloys to which the present invention applies may 
in general be melted, cast into ingots, and the ingots initially processed 
to an intermediate size by soaking, forging, and hot rolling, as described 
in U.S. Pat. No. 4,049,431. The material is then typically cold worked to 
final size in one or more cold working steps, having anneals prior to each 
step. These anneals should be at a temperature and time sufficient to 
recrystallize the material and place most precipitates into solution. 
However, the temperature and the time at temperature should not be so 
great as to cause excessive grain growth and significant precipitation at 
the grain boundaries which will lead to a significant reduction in the 
ductility and toughness of the material, making it difficult to further 
cold form without cracking. It is believed that these requirements can be 
met in alloys D57 and D57B if the material is annealed at a temperature 
between approximately 1000.degree. and 1150.degree. C. for about 5 minutes 
to 1-2 hours at temperature. It is however preferred that this anneal be 
performed at a temperature of about 1000.degree. to 1075.degree. C. for 5 
to 30 minutes. According to the present invention there is no annealing or 
aging treatment after the final cold working step which comprises about a 
10 to 50 percent reduction in cross sectional area of the piece after the 
last anneal.

DETAILED DESCRIPTION OF THE INVENTION 
Table I shows the chemistry of the precipitation hardening delta ferritics 
which were processed in accordance with the present invention. Both the 
nominal and analyzed chemistries are shown. It will be noted that the only 
significant chemical difference between alloy D57 and D57B is the addition 
of approximately 0.5 weight percent nickel to the D57B composition. The 
D57 heat shown in Table I is identical to the heat of D57 evaluated in 
U.S. Pat. No. 4,049,431. The cast ingot was soaked at approximately 
1175.degree. C. for 2 hours. It was press forged at about 1175.degree. C. 
to a 0.5 inch thick plate. The plate was then hot rolled at about 
1175.degree. C., with reheats after each reduction, to a hot rolled 
thickness of approximate 0.060 inches. This hot rolled section was vapor 
blasted, and then annealed and cold rolled in a series of steps as shown 
in the FIG. 1 flow diagram. 
The section, was first given a Type I anneal which is a vacuum anneal 
comprising heating the section up to an annealing temperature of 
approximately 1038.degree. C. over a period of about 1.5 hours, soaking it 
at temperature for about 1. hour and then allowing it to furnace cool over 
a period exceeding 4 hours. The material was then given a cold rolling 
reduction of 23%, followed by another Type I anneal and a subsequent cold 
rolling reduction of 29% to an approximate thickness of 0.031 inch. At 
this point the material was then sectioned into two portions, A and B. 
The A portion material was processed as shown in the lefthand column of 
FIG. 1. It was given a Type I anneal, followed by a cold rolling reduction 
of 34 percent, another Type I anneal, and a final cold rolling reduction 
of 44 percent. This material was given a Type III anneal which comprises 
soaking the material at approximately 1149.degree. C. for about 30 
minutes, followed by air cooling. The material was then precipitation 
hardened by aging it about 732.degree. C. for approximately 1. hour, 
followed by air cooling. Samples of the A portion material, now in the 
annealed and aged condition, were exposed to fast neutron (E&gt;0.1 MeV) 
fluxes to determine the materials' swelling characteristics in this final 
condition. 
The B portion material was processed as shown in the righthand column of 
FIG. 1. It was given a Type II anneal which comprises soaking the material 
at approximately 1100.degree. C. for about 15 minutes followed by an air 
cool. The B portion material subsequently received a cold rolling 
reduction of 48 percent, followed by a Type III anneal and a final cold 
rolling reduction of 23%. Samples of the B portion material, now in the 
cold worked condition, according to the present invention, were then 
exposed to fast neutron fluxes to determine the swelling characteristics 
of the material in this final condition. 
Table II lists the swelling data obtained for the two material conditions 
at various temperatures and fluences. It is readily apparent from a 
comparison of the swelling data of the two material conditions that while 
the D57 material in the cold worked condition is still in a densifying 
mode the D57 material in the annealed and aged condition at 427.degree. C. 
and 482.degree. C. is swelling. 
An ingot of D57B Material having the chemistry shown in Table I was cast 
and then worked into a bar of approximately 1.3 inch in diameter. This 
material was then rolled at 1150.degree. C. with reheats after each pass 
to thicknesses of 0.238, 0.150 and 0.067 inches. The 0.067 inch hot rolled 
material was then sandblasted, pickled and processed as shown in FIG. 2. 
This material first received a TYPE 4 anneal in which the material is 
soaked at about 1025.degree. C. for approximately 10 minutes and then air 
cooled. Subsequently the material was given a 40% cold rolling reduction, 
after which it was sectioned into portions, D and C. The D portion 
received the processing showed in the lefthand column of FIG. 2. It was 
given a Type 4 anneal, followed by cold rolling 35 percent, another Type 4 
anneal, and then 38 percent cold rolling reduction. The final anneal this 
material received was a Type 5 anneal in which the material is soaked at 
about 1025.degree. C. for about 5 minutes and then air cooled. This 
annealed material was then cold rolled 25% to a final sheet thickness of 
about 0.012 inch. 
The C portion of the material was processed as shown in the righthand 
column of FIG. 2. It received a Type 5 anneal followed by a cold rolling 
reduction of 25% to a final size of about 0.030 inches. Flat tensile 
specimens having a gauge length of 0.8 inches, and a minimum gauge width 
of 0.06 inches were cut from the final C portion cold rolled sheet and 
tested at a cross head speed of 0.020 inch/minute at the various 
temperatures shown in Table III. 
As finally cold rolled, the C portion material microstructure was 
characterized by a final grain size of approximately ASTM 5 to 6, and was 
essentially free of laves phase precipitates, the precipitates which act 
as the primary ferritic alloy strengthener in the D57 and D57B type delta 
ferritic alloys. 
The preceding embodiments of the invention may be modified as needed within 
the scope of the claims to fabricate the various shapes and sizes of 
components needed for liquid metal fast breeder reactor and steam 
generator components. It is specifically contemplated that rolling 
reductions may be replaced by drawing and/or pilgering operations to 
produce tubing. It is also contemplated that the initial cold reduction, 
and, in some cases, subsequent cold reductions, may be replaced by 
elevated temperature reductions, at up to approximately 500.degree. C., 
preferably below about 350.degree. C., in order to assure fabricability to 
the desired final shape and dimensions, while maintaining the essentially 
laves phase precipitate free, dislocated structure of the final component. 
TABLE I 
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HIGH STRENGTH FERRITIC ALLOYS 
(weight percent; balance essentially iron) 
Alloy 
C Mn Si Cr Ni Mo Nb V Ti Al B Zr N P S 
__________________________________________________________________________ 
D57 
Nominal 
.05 
0.4 
0.3 
10.5 
*-- 
6.0 
0.5 
0.3 
-- -- -- -- -- -- 
Analysis 
.055 
0.45 
0.31 
10.5 
*-- 
5.96 
0.54 
0.33 
-- -- -- -- .048 
.013 
.007 
D57B 
Nominal 
.05 
.5 .3 10.5 
.5 6.0 
.5 .4 -- -- -- -- -- -- -- 
Analysis 
.041 
.49 
.34 
10.25 
.61 
6.22 
.51 
.4 .02 
.05 
.001 
.01 
-- -- -- 
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*Dashed lines indicate elements considered to be impurities in the nomina 
compositions and impurity elements not analyzed in the chemical analysis. 
TABLE II 
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ALLOY D57 SWELLING CHARACTERISTICS 
MATERIAL CONDITION MATERIAL CONDITION 
Annealed & Aged Annealed & Cold Worked 
Temp. .degree.C. 
Fluence (n/sq. cm) 
Percent Swelling 
Fluence (n/sq. cm) 
Percent Swelling 
__________________________________________________________________________ 
400 8.4 .times. 10.sup.22 
-0.69 
427 9.8 .times. 10.sup.22 
+0.52 10.2 .times. 10.sup.22 
-0.70 
454 7.4 .times. 10.sup.22 
-0.76 
482 9.1 .times. 10.sup.22 
+0.07 9.6 .times. 10.sup.22 
-0.92 
510 11.5 .times. 10.sup.22 
-1.22 
538 11.0 .times. 10.sup.22 
-0.43 11.3 .times. 10.sup.22 
-0.74 
593 12.1 .times. 10.sup.22 
-0.20 12.2 .times. 10.sup.22 
-0.78 
649 12.1 .times. 10.sup.22 
-0.89 
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TABLE III 
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TENSILE PROPERTIES OF COLD ROLLED D57-B (UNIRRADIATED) 
0.2% Offset 
Yield Strength 
Ultimate Strength 
Elongation, % 
Reduction 
Hardness 
Temp .degree.C. 
ksi MPa ksi MPa Uniform 
Total 
in Area, % 
DPH 
__________________________________________________________________________ 
RT 105.0 
723.9 
111.1 
766.0 
-- 4.0 64.4 250 
232 97.5 
672.2 
97.5 672.2 
0.7 3.6 69.0 
400 91.4 
630.2 
91.8 632.9 
1.0 3.0 63.7 
450 89.0 
613.6 
89.3 615.7 
1.0 4.1 56.4 
500 82.4 
568.1 
84.0 579.2 
1.1 4.9 57.6 
550 77.1 
531.6 
81.8 564.0 
2.2 5.5 67.8 
600 69.2 
477.1 
75.0 517.1 
4.5 8.0 57.3 
650 67.8 
467.5 
74.8 515.7 
10.7 13.0 
69.2 
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