Note: Descriptions are shown in the official language in which they were submitted.
SPECIFICATION
This invention relates to nickel-iron base alloys,
and, more particularly, to an alloy containing nickel, iron,
chromium, molybdenum, titanium, aluminum and columbium criti-
cally balanced to provide good sulfidation resistance combined
with a high degree of hot strength at el~vated temperatures in
the heat treated condition.
A number of alloys have hitherto been developed which
were suitable for use under conditions requiring good hot
strength and corrosion resistance at the elevated temperatures
encountered in internal combustion engines. With the increasing
use of fuels containing larger amounts of sulfu-, it is becQming
more important that such alloys also have good resistance to
sulfidation. Thus, at the present time, heavy duty diesel
engines, which may burn high sulfur content fuels, require
valves and valve components made of an alloy which not only has
good hot strength at operating temperatures of up to about
1500F, but also has high resistance to sulfidation at such
elevated temperatures. Alloy A, having a nominal composition
of about 15% chromium, 7~ iron, 2.5% titanium, 1~ aluminum, 1%
columbium and the balance nickel, has been used as a valve
alloy for diesel engines because of its high strength in the
1300-1500F temperature range. However, as the sulfur content
of fuel has increased, Alloy A has shown poor resistance to
sulfidation attack. This is a type of corrosion in which
sulfides form at the surface of the alloy part, and, especially
103~6SS
when chromium is removed from the alloy matrix by this sulfidation
corrosion, can result in the catastrophic failure of the part.
Alloy B, having desirable properties for use under
stress in a sulfur-bearing atmosphere at elevated temperatures
has a nominal composition of about 0.05% carbon, 0.30% manganese,
0.20% silicon, 29% chromium, 46% nickel, 20% cobalt, 2.30%
titanium, 1.20% aluminum, 0.70~ columbium, 0.006% boron and
0.50% maximum iron. The good hot strength at elevated temperatures
and high resistance to sulfidation exhibited by this alloy make
it especially desirable for use in making valves for diesel
engines. However, the high percentage of cobalt and the rela-
tively high expense involved in using iron-free alloying additions
make this alloy relatively expensive.
Alloy C has a nominal composition of about 27% chromium,
37% nickel, 8% manganese, 2~ titanium, 1% aluminum and 25% iron.
This alloy is more resistant than X-751 alloy to sulfidation
attack, but has a much lower strength in the 1300-1500F tempera-
ture range. For this reason, Alloy C is not a good material for
parts which must operate at such temperatures in diesel engines.
It is, therefore, a principal object of this invention
to provide an alloy which has high strength and good resistance
to sulfidation.
A more specific object is to provide a nickel-iron
base alloy for making valves and valve components for use in
heavy duty diesel engines and which is especially resistant to
attack by sulfidation which occurs when high sulfur content
fuels are used.
The foregoing, as well as additional objects and
advantages of the present invention will be apparent from the
30 following description of a preferred embodiment of this invention
and the accompanying drawing in which
FIGURE 1 is a micrograph prepared from a specimen
made from the alloy of the present invention, and which has
undergone a sulfidation resistance test; and
` 1038655
FIGURE 2 is a similar micrograph of a specimen made
of Alloy B; and
FIGURE 3 is a similar micrograph of a specimen made
of Alloy C.
In accordance with the present invention, there is
provided a nickel-iron base alloy resistant to sulfidation at
elevated temperatures in the range of about 1300 to 1500F and
which has good hot strength and stress rupture life at elevated
temperatures up to about 1500F when heat treated which con-
sists essentially of the following elements in about the amountsindicated in the broad and preferred ranges given in approximate
weight percent below. It is to be noted that it is not intended
to be limited by the form of the following tabulation which
has been used for convenience. It is intended that the upper
and/or lower limits of one or more of the elements included
in the broad range can be used with the upper and/or lower
limits of one or more of the elements as included in the
preferred range.
BroadIntermediatePreferred
C 0.02-0.080.04-0.065 0.04-0.065
Mn 2 MaxØ25 Max. 0.20 Max.
Si 0.25 Max. 0.20 Max. 0.20 Max.
P 0.01 Max. 0.01 Max. 0.01 Max.
S 0.01 Max. 0.01 Max. 0.01 Max.
Cr 21-2622.0-24.5 22.0-23.5
Ni 52-58 53-56 54-56
Mo 1-3.51.5-2.5 1.5-2.5
Ti 1.75-3.252.25-2.75 2.25-2.75
Al 0.75-2.251.25-1.75 1.25-1.75
Cb 0. 50-20.75-1.50 0.75-1.50
B up to 0.02 0.002-0.008 0.002-0.008
The balance of the composition is iron, except for
incidental impurities which, preferably, are kept low. The
elements manganese, silicon, phosphorus and sulfur are impurities
which should be present, preferably in the smallest amounts
possible. Particularly, silicon is ~ept below about 0.25%, or
preferably below about 0.20%, since higher amounts adversely
affect the mechanical properties of the alloys. For best
mechanical properties, particularly stress rupture life and
ductility, manganese is kept below about 0.25% and better yet
below about 0.20%. However, when the use for which the alloy
is intended does not preclude it, then larger amounts of manganese
. r~
103B65S
up to about 1% and even up to about 2% can be present. Phosphorus
and sulfur are limited to about 0.01% each because greater amounts
adversely affect the mechanical properties, cleanliness, and
forgeability of the alloy.
In the alloy of this invention, a minimum of about 0.02%
carbon is required to provide the desired deoxidation and the
desired formation of carbides in the grain boundaries during aging.
Carbon ranging from about 0.04-0.065% is preferred. Because the
main strengthening reaction of this alloy is the formation of gamma
prime which is believed to be mainly composed of Ni3(Al,Ti), ex-
cessive carbon tends to detract from the strength of this alloy by
tying up titanium. Therefore, no more than about 0.08~ carbon
should be present.
A minimum of about 21% chromium is required to provide
the desired sulfidation resistance, particularly necessary in the
environment to which valves are exposed in heavy duty diesel engine
cylinders where sulfur-containing fuel oil is combusted. Too much
chromium results in the formation of a chromium-rich phase, tenta-
tively identified as a body centered cubic alpha phase, too much of
which adversely affects the elevated temperature stress rupture
life as well as the ductility at room temperature. Therefore, the
chromium content is limited to about 26%, and is preferably kept in
the range from about 22.0-24.5%.
Nickel is required to minimize the presence of phases
other than the desired austenite and to take part in the reaction
by which the alloy attains its desired strength. A minimum of
about 52%, or preferably about 53-56%, is used for this purpose,
while beyond about 58~, larger amounts of nickel will needlessly
increase the cost of the alloy without providing any significant
offsetting advantages.
For best all-around properties, that is, microstructural
stability, sulfidation resistance and mechanical properties, the
` 103~655
chromium and nickel contents are adjusted to about 22-23.5~ chromi-
um and about 54-56% nickel.
Molybdenum acts as a solid solution strengthener and, for
this purpose, is present from about 1-3.5%, preferably 1.5-2.5%.
When present in amounts above about 3.5%, molybdenum may have an
adverse effect on the sulfidation resistance and hot workability of
the alloy.
As was noted, titanium is required for the formation of
the gamma prime phase by which this alloy is strengthened, and, for
this purpose, there should be at least about 1.75%. However, more
than about 3.25% may adversely affect the hot workability of the
alloy. Preferably, the titanium is present in the range of about
2.25-2.75%.
Aluminum, which also takes part in the main strengthening
reaction, should be present in the amount of at least about 0.75%
to ensure that the gamma prime phase is stable at such elevated
temperatures as 1300-1500F, and the preferred range for aluminum
is about 1.25-1.75%. Best results are obtained when the titanium/
aluminum ratio is greater than 1Ø More than about 2.25% aluminum
adversely affects the hot workability of the alloy.
To form stable carbides which nucleate early in the
solidification process, columbium is added, usually in amounts
about 10 to 12 times the percent carbon present. A minimum of
about 0.50~ columbium is used, and, preferably about 0.75-1.50%.
More columbium than that which forms carbides can be tolerated, and
some small amount of columbium may be in the gamma prime phase, but
above a total of about 2% merely adds to the cost of the alloy.
A small amount of boron, up to about 0.02%, contributes
to the improved elevated temperature stress rupture life and duc-
tility of this alloy. Preferably at least 0.002% is used, and bestresults are obtained with about 0.004-0.008%.
The alloy of this invention can be prepared using con-
ventional practices, but it is preferably melted and cast into
` ~03B655
ingots by a multiple melting technique. For example, a heat can
be first melted and cast as an ingot under vacuum in an induction
furnace, and then that ingot used as a consumable electrode and
remelted under vacuum. Alternatively, an electroslag remelting
technique can be used.
The alloy is forged from a furnace temperature above
about 1900F, preferably from about 2100 to 2150F, followed by
solution treatment at about 1875 to 2100F for about 1 to 4
hours, or longer if necessary, preferably at about 2000F for 4
hours. After quenching in oil, or faster if desired, the alloy is
aged by heating at about 1200 to 1550F for about 16 to 48 hours.
Preferably aging is carried out at about 1300F for 24 hours, but
other aging treatments can be used including double aging treat-
ments. By double aging is meant aging for about 2 to 8 hours near
the upper end of the range, followed by a final age for about 16
to 48 hours at a temperature near the lower end of the 1200-1550F
range. As solution treated and aged, the alloy is fully austenitic.
The heat treatment of this alloy brings out a gamma
prime phase which is a face centered cubic (FCC) structure, which
helps give the alloy its good strength in the temperature range of
1300 to 1500F. There may also be a small amount of chromium
rich alpha phase which is a body centered cubic structure similar
to ferrite. Excessive amounts of this phase adversely affect room
temperature ductility as measured by percent elongation in room
temperature tensile tests.
As a further illustration of the present invention, two
experimental vacuum induction heats, Examples 1 and 2, were pre-
pared having compositions in accordance with this invention. The
ingot of Example 2 was remelted as a consumable electrode under
vacuum. For comparison, small heats of prior Alloys A, B and C
were prepared as was Example 1. The compositions of these five
heats are given in Table I.
~'
` ~03~655
TABLE I
Ex. 1 Ex. 2 A B C
C.063 .062 .03 .09.055
Mn.17 .17 .05 .358.11
Si.17 .17 .05 .16.10
P<.005 <.005 .006 <.005 <.005
S.006 .003 .003 .005 .008
Cr23.30 23.78 15.63 28.3926.86
Ni54.13 54.84 Bal. Bal.37.50
Co - - - 19.45
Mo2.03 1.87
Ti2.55 2.50 2.49 2.281.97
Al1.43 1.48 1.18 1.16.99
Cb1.02 .99 1.01 .63
B.0062 .0053 .0024 .0057 .0056
FeBal. Bal. 7.65 .90Bal.
In each instance, the balance was iron or nickel, as indicated,
except for incidental impurities.
To demonstrate and compare the sulfidation resistance of
the alloys, specimens of Examples 1 and 2 and Alloys A, B and C
were machined from forgings to provide 0.300 in. diameter, 0.750 in.
long cylinders. Each was heat treated as shown in Table II.
TABLE II
_ S ol. q reat. ~
Temp Time Primary Final
(F) (hrs) Cooll F hrs Cool F hrs Cool
Ex.l 2000 4 OQ1500 4 AC 1350 24 AC
Ex.2 2000 4 OQ_ _ _ 1300 24 AC
A 2100 4 OQ1550 24 AC 1300 20 AC
B 1975 8 AC_ _ _ 1300 16 AC
C 2100 4 OQ_ _ _ 1300 24 AC
1 OQ - oil quenched
AC - air cooled
The specimens were placed vertically in one-inch diameter crucibles
containing 7.0 grams of a molten salt mixture of 90% Na2SO4 and
10% NaCl, and allowed to stand for 100 hours at 1700F exposed to
an air atmosphere. Then the samples were removed, and examination
clearly demonstrated that only the specimens of Examples 1 and 2
and Alloy B had good resistance to sulfidation. In order to
prepare micrographs of the tested specimens of Example 1, Alloy B
and Alloy C, cross-sectional sl~ces were taken at the height of
1038655
the air/salt interface and mounted on plastic supports. Optical
micrographs at 100x magnification were then taken of the outer
edge of each slice and are shown respectively in FIGURES 1, 2 and
3.
No micrograph was prepared from the specimen of Alloy A
because it was catastrophically attacked by the hot salt. The
micrographs of FIGURES 1 and 2 show that the specimens of Example
1 and Alloy B were attacked only slightly, if at all, by the
molten salt. On the other hand, the micrograph of FIGURE 3 shows
that Alloy C suffered severe intergranular attack. These accel-
erated sulfidation tests clearly show that the alloy of the present
invention has about the same resistance to sulfur attack as Alloy B
and much greater resistance than Alloy C.
Standard A.S.T.M. stress rupture test specimens and
tensile test specimens were prepared from each of the analyses of
Table I except that tensile tests were not carried out in the case
of Example 1 and, in the case of Alloy C, because the stress
rupture life obtained was so low. Heat treatment of the specimens
was as indicated in Table II. Stress rupture testing was carried
out at 1350F under a load of 50,000 psi (50 ksi) and at 1500F
under a load of 30,000 psi (30 ksi), and the results are given in
Table III. In each case, the duration of the test before failure
is indicated in hours (hrs) under "Life", and percent elongation
(El. %) and percent reduction in area (R.A. %) are also given. In
Table IV, the results of tensile tests carried out at 70F and
1500F are indicated. In each instance, after the test temperature
there is indicated the ultimate tensile strength (U.T.S.) followed
by .2% yield strength (Y.S.), percent elongation and percent
reduction in area.
103~655
TABLE III
Stress Rupture Data
1350F/50 ksi 1500F/30 ksi
Life El. R.A. Life El. R.A.
(hrs) (%) (%) (hrs) (%) (%)
Ex. 1 129 9.512.4 99.7 9.410.0
Ex. 2 133.2 4.5 4.4 61.2 3.97.7
Alloy A 100.0 8.0
Alloy B 198.5 6.9 6.9110.4 7.615.2
Alloy C 1.3 1.9 2.0 1.9 6.77.6
TABLE IV
Tensile Data
Test Temp. U.T.S. .2~Y.S. El. R.A.
F (ksi) (ksi) %
. .
Ex. 2 70 175 111 30.638.2
Alloy A " 153 89 23 25
Alloy B " 183 120 30 41
Ex. 2 1500 104.5 91.5 12.414
Alloy A " 80 70 25 33
Alloy B ~I 109 92 11 14
An additional stress rupture specimen of Example 1,
when aged by a single instead of a double heat treatment, had a
stress rupture life at 1350F under 50 ksi of 277.6 hours, with a
7.4% elongation and a 12.4% reduction in area. Because of its
combination of high strength at elevated temperatures and good
resistance to sulfidation, the alloy of this invention is par-
ticularly well suited for use in the fabrication of parts which
must withstand stress and sulfur-bearing corrosive atmospheres at
elevated temperatures. This alloy is considerably less expensive
than Alloy B because Alloy B contains about 20~ cobalt and must be
made with the more expensive iron-free forms of the alloying
elements.
The terms and expressions which have been employed are
used as terms of description and not of limitation, and there is
no intention in the use of such terms and expressions of excluding
any equivalents of the features shown and described or portions
thereof, but it is recognized that various modifications are
possible within the scope of the invention claimed.
