[0001] This invention relates to austenitic iron base alloys which find particular use in
nuclear reactors and are characterized by improved swelling resistance and phase stability
in both the annealed as well as the cold work condition in comparison with an AISI
type 316 stainless steel.
[0002] With the advent of the nuclear age and the materials problems associated therewith,
it was believed that the AISI type 316 stainless steel because of its austenitic character
and which is strengthened through a solid solution strengthening addition would prove
to be ideally suited for use in a nuclear reactor. This conclusion was supported by
the fact that the AISI type 316 stainless steel appeared to possess the desired strength
characteristics at elevated temperatures. It was soon found however that even after
low fluid reactor irradiation copious amounts radiation induced percipitation were
evident in the microstructure and the material was subjected to relatively high swelling.
It therefore became apparent that it was necessary to alter the chemical composition
AISI type 316 stainless steel in an attempt to eliminate the phase instabilities and
to provide improved swelling resistance without seriously adversely affecting the
strength characteristics of the fundamental alloy. To this end, the alloys of the
present invention appear to fulfill these primary requisites.
[0003] Accordingly, the present invention resides in an austenitic iron base alloy having
improved structural stability and swelling resistance compared to AISI type 316 stainless
steel and which alloy is suitable for use in an atmosphere subject to neutron irradiation,
characterized in that said alloy consisting essentially of from 14% to 16% nickel,
from 12% to 14% chromium, from 1.2% to 1.7% molybdenum, from 0.5% to 1.1% silicon,
from 1.5% to 2.5% manganese, up to 0.1% zirconium, from 0.2% to 0.5%.titanium, from
0.02% to 0.1% carbon, up to 0.01% boron and the balance iron with incidental impurities.
[0004] It has been found that the desired properties can be achieved by lowering the relative
amounts of nickel, chromium, and molybdenum while still maintaining the austenitic
characteristic of the alloy when the same is subjected to elevated temperature irradiation
of the type normally found, for example, in the case of fuel pins in a nuclear reactor.
More specifically, the alloy will exhibit improved swelling resistance at elevated
temperatures in both the annealed as well as the cold work condition.
[0005] In order that the invention can be more clearly understood, a preferred embodiment
thereof will now be described, by way of example, with reference to the accompanying
drawings in which:
Figure 1 is a plot of- percent swelling verses the temperature of an alloy of the
present invention in comparison with standard AISI type 316 stainless steel, the actual
numbers of the data points being the actual fluence values; and
Figure 2 is a similar plot to Figure 1 but with the alloys in the cold work condition.
[0006] Table 1 set forth hereinafter lists the chemical composition of the AISI type 316
stainless steel as well as the broad range, the preferred range, and the specific
composition of a heat falling within the preferred as well as the broad ranges as
set forth herein.
[0007] By inspection of Table 1 it becomes clear that the alloy of the present invention
has less chromium, less nickel, and less molybdenum than that of a corresponding AISI
type 316 stainless steel. Moreover, as can be seen from Table 1 the larger reduction
of the chromium together with a smaller reduction of the molybdenum and the smaller
reduction or even increase in the nickel is effective for maintaining the austenitic
character of this alloy which austenitic character is strengthened by means of the
molybdenum addition thereto. Note in particular that since the titanium and zirconium
contents are quite limited, the microstructure of the alloy remains substantially
precipitation free after extended exposures to the influence of neutron irradiation
at elevated temperatures. In order to more clearly and graphically depict the improvement
in swelling resistance exhibited by the allow of the present invention, attention
is directed to Figure 1 which directly compares a solution annealed AISI type 316
stainless steel and the alloy of this invention having the composition of heat number
5976 as identified in Table 1 and the effect of the temperature at various fluence
values in relation to the percent swelling. Curve 10 of Figure 1 is a plot of the
AISI type 316 stainless steel material whereas curve 12 is a plot of the identical
values exhibited by-the alloy of the present invention in the solution annealed condition
which alloy has been arbitrarily designated D9B1. As can be seen from the data set
forth in Figure 1, the alloy of the present invention has far superior swelling resistance
to that exhibited by the AISI type 316 stainless steel. This is especially so when
the percent swelling is considered at about the temperature of 600°C and a fluence
value of 5.7
x 10 22 neutrons per square centimeter. These same results are more outstanding when the
data is compared for the material in the cold work condition. Thus in Figure 2, the
curve 20 illustrates the data for AISI type 316 stainless steel in the 20% cold work
condition and curve 22 shows the swelling resistance of alloy D9Bl in the 25% cold
work condition. It is also believed significant to point out that in the cold work
condition, the alloy of the present invention is still densifying while the AISI type
316 stainless steel is into the void swelling regiment regardless of the temperatures
employed. Thus, these data make it clear that the alloys of the present invention
are particularly suitable for use for example in a fast breeder reactor. It has been
found however that the long term stress rupture properties at temperatures greater
than 650°C appear to be weaker than AISI type 316 stainless steel based on the latest
unradiated specimen testing. However, it is believed that comparable results can be
obtained where the material is in the cold worked condition and the degree of cold
working is limited to about 20% for optimum stress rupture and swelling resistance
characteristics. While it will be appreciated that the swelling resistance characteristics
will still be outstanding where the alloy is worked to a degree greater than 20%.
The optimum results appear to be obtained when the cold working is limited to 20%.
For swelling resistance alone, it has been found that cold working the material within
the range between 15% and 40% does not appear to adversely affect the swelling resistance
demonstrated by the alloy of the present invention.
1. An austenitic iron base alloy having improved structural stability and swelling
resistance compared to AISI type 316 stainless steel and which alloy is suitable for
use in an atmosphere subject to neutron irradiation, characterized in that said alloy
consisting essentially of from 14% to 16% nickel, from 12% to 14% chromium, from 1.2%
to 1.7% molybdenum, from 0.5% to 1.1% silicon, from 1.5% to 2.5% manganese, up to
0.1% zirconium, from 0.2% to 0.5% titanium, from 0.02% to 0.1% carbon, up to 0.01%
boron and the balance iron with incidental impurities.
2. An alloy according to claim 1, characterized in that said alloy consists essentially
of from 15.25% to 15.75% nickel, from 13.25% to 13.75% chromium, from 1.4% to 1.6%
molybdenum, from 0.9% to 1.1% silicon, from 1.8% to 2.5% manganese, from 0.4% to 0.06%
zirconium, from 0.2% to 0.3% titanium, from 0.03% to 0.04% carbon, up to 0.01% boron
and the balance iron with incidental impurities.
