Note: Descriptions are shown in the official language in which they were submitted.
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Ni-Al BASE MATERIAL HAVING OPTIMIZED OXIDATION RESISTANCE AT HIGH
TEMPERATURES AND FURNACE TRANSFER ROLLS MADE THEREFROM
Cross-Reference to Related Applications
This Application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional
Application No. 61/585,087 filed January 10, 2012.
Field of the Invention
The present invention relates generally to Ni-Al compositions. More
specifically,
the Ni-Al compositions have optimized oxidation resistant at high
temperatures. Most
specifically, the invention relates to Ni-Al compositions useful in producing
austenitizing
furnace transfer rolls .
Background of the Invention
The most common transfer roll alloy material in use today in austenitizing
furnaces is an H-series austenitic alloy that provides limited high-
temperature strength,
wear and oxidation resistance. After a short service of a few months the rolls
show
deterioration. Finally, after two to three years inside the annealing furnace
the transfer
rolls need to be removed from service because of a variety of major issues.
First, the
rolls tend to sag at the current operating temperatures becoming eccentric in
their
rotation, which also limits efficiency for operating at even higher processing
temperatures. The rolls at temperatures and loading condition undergo local
distortion
(bulges) which requires hand-grinding the bulges. The iron oxide on the plates
are
transferred to the rolls and then back onto the plates. The performance of the
rolls
(which have bulges, distortions and oxidation) cause the plate to undergo
quality
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degradation. To avoid such degradation, the furnace is frequently shut down
and the
rolls are ground or replaced to minimize the defects. The energy used to
restart the
furnace after the shutdown is also an important factor in maximizing energy
savings.
A number of years ago, the use of nickel aluminide alloys (specifically, IC-
221M
developed by ORNL)to form transfer rolls was proposed as a solution to the
issues with
H-series austenitic alloy rolls because of Ni-Al's superior high temperature
strength,
wear and oxidation resistance, as well as for better plate surface quality
control.
Unfortunately, after about 4 years in service, the Ni-aluminide rolls develop
a green
scale on the surface thereof. Furthermore, scale in the form of protrusions
from the
surface causes indentations on the bottom surface of the plate during heat
treatment.
Since these indentations on the plate are a quality concern, the present
inventors
examined the cause of this problem.
The study was dedicated to understanding the Ni-Aluminide alloy and its
oxidation behavior through microstructurel changes and oxidation behavior of
the
Ni-aluminide rolls. The mechanisms and kinetics of oxidation of the rolls
subjected to
the prolonged exposure to the hardening temperature was established and
validatd
through laboratory simulations. An extensive metallographic investigation
using optical
microscopy, SEM, EDS and Micro Raman spectroscopy was carried out on samples
from the rolls in as-cast condition, after use in the hardening furnace for
more than 4
years and after laboratory oxidation simulations.
The results of this study reveal long term oxidation phenomena at high
temperature as the cause of the surface deterioration. The oxidation mechanism
of the
Ni aluminide rolls can be summarized as follow: (1) at 900 C in air the oxides
form in
a manner that follows the microsegregation patterns in the as-cast
microstructure; (2)
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the y + Ni5Zr eutectic colonies provide a fast diffusion path; (3) the first
oxide nodules
to form protrude from the surface in the vicinity of they + Ni5Zr eutectic
regions; (4) the
dominant oxide of the nodules is NiO, but A1203 and NiA1204 are present in
significant
quantities; (5) NiO nodules protrude above the surface and an Al-depleted zone
grows
beneath the surface oxide; (6) internal oxides stringers mainly composed of Zr
extend
from the alloy surface into the parent matrix.
Two types of oxides were detected on the rolls after service in the hardening
furnace. In general, the surface of the rolls is covered by numerous round
shaped
green nodules referred to as primary oxides that tend to coalesce and create
dimples.
These oxide nodules present a dense external NiO layer above a subscale
consisting
of a mixture of NiO, A1203 and Ni(Cr,A1)204 oxides.
Black oxides that protrude from the surface referred to as secondary oxides
are
difficult to remove as they are well attached to the surface. These nodules
consisted
of an exterior layer of Fe304 and Fe2O3 followed by an inner and thicker layer
of a
mixture of NiO, A1203 and Ni(Cr,A1)204 oxides. In general, the outer layers of
the
secondary oxides where Fe is present exhibit higher hardness values than the
primary
oxides. The Fe oxide layer develops through contact at high temperature
between the
plates and the primary oxides that protrude from the rolls.
The appearance of oxide scales, in the form of dimples or nodules, on the
surface of the nickel aluminide rolls is inevitable with the present alloy
used to make the
rolls and the required service conditions.
Thus, there is a need in the art for austenitizing furnace rolls formed from
material that retains the superior high temperature strength, wear and
oxidation
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resistance of the present Ni-Al material, but avoids the formation of oxide
scales, in
the form of dimples or nodules, on the surface of the rolls.
Summary of the Invention
The present invention comprises a high temperature oxidation resistant
nickel-aluminide alloy composition and furnace rolls formed therefrom. The
nickel-
aluminide alloy may comprise 0.15 wt % or less Zr, and preferably may comprise
from about 0.08 - 0.1 wt% Zr. The alloy may further comprise from about 2.5 to
3.0
wt. % Mo, and preferably may comprise about 2.8 wt% Mo. The alloy may further
comprise from about 7.5 to 8.5 wt.% Al, and from about 7.5 to 8.5 wt.% Cr. The
nickel-aluminide alloy may further comprise less than about 0.015 wt.% B,
preferably
about 0.01 wt.% B. The alloy may further comprise, in wt.%: C - 0.05 max; Si -
0.1
max; Fe - 0.3 max; S - 0.005 max; Mn - 0.1 max; P - 0.01 max; and Cu - 0.3
max.
The alloy may contain no more than trace amounts of the other elements from
group
IVB, VB and VIB of the periodic table.
The present invention comprises a furnace roll for a high temperature furnace
comprising a cast roll of a nickel-aluminide alloy comprising 0.15 wt.% or
less Zr,
between 2.5 to 3.0 wt.% Mo, between 7.5 to 8.5 wt.% Al, and between 7.5 to 8.5
wt.% Cr, wherein said furnace roll has an increased resistance to oxidation
when
compared to an identical furnace roll but in which the Zr content is above
0.15 wt.%.
The present invention comprises a furnace roll for a high temperature furnace
comprising a cast roll of a nickel-aluminide alloy comprising 0.15 wt.% or
less Zr,
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between 2.5 to 3.0 wt.% Mo, between 7.5 to 8.5 wt.% Al, and between 7.5 to 8.5
wt.% Cr.
Brief Description of the Drawings
Figures la-li are cross sectional SEM images of samples having varied Zr
content (M-0 = Owt.%Zr, M-2 = 0.3wt.%Zr, and the prior art alloy IC-221M =
1.8wt.%Zr), oxidized at 900 C for 1500, 3000 and 18000 hours inside a
hardening
furnace.
Figure la: prior art alloy IC-221M oxidized at 900 C for 1500 hours;
Figure lb: M-2 = 0.3wt.%Zr oxidized at 900 C for 1500 hours;
Figure lc: M-0 = Owt.%Zr oxidized at 900 C for 1500 hours;
Figure Id: prior art alloy IC-221M oxidized at 900 C for 3000 hours;
Figure le: M-2 = 0.3wt.%Zr oxidized at 900 C for 3000 hours;
Figure if: M-0 = Owt.%Zr oxidized at 900 C for 3000 hours;
Figure lg: prior art alloy IC-221M oxidized at 900 C for 18000 hours;
Figure lh: M-2 = 0.3wt.%Zr oxidized at 900 C for 18000 hours;
Figure Ii: M-0 = Owt.%Zr oxidized at 900 C for 18000 hours;
Detailed Description of the Invention
The inventive Ni-Al compositions provide the superior strength and creep
properties of the Ni aluminide family and solve the oxidation problems that
the prior
composition/rolls experienced in high temperature service. The new Ni
aluminide
alloy
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composition comprises 0.08 - 0.1 wt.% Zr, 2.5 - 3.0 wt.% Mo, 7.5 -8.5 wt.% Al,
7.5 - 8.5
wt.% Cr, about 0.01 wt.% B and the balance being substantially nickel. This
new
composition will extend the life of the Ni-aluminide transfer rolls used in
the plate mill
austenitizing furnaces and will sustain the use of Ni-aluminide rolls for
superior
temperature strength, wear, oxidation resistance and better plate surface
quality control.
Thus, the new alloy composition will reduce the number of plates rejected due
to
surface marks. Further, in terms of energy costs, there are five major
benefits of using
Ni-aluminide rolls in comparison with HP-type of rolls: (1) energy savings due
to the
elimination of shutdowns and restarts for roll repair and maintenance, (2)
energy
savings due to straight through processing, (3) cost savings due to the
elimination of
roll maintenance labor, (4) fewer plates downgraded or rejected as the result
of
elimination of HP-type roll bulging and the oxide protrusions in the prior art
Ni-Al rolls,
(5) cost savings due to the reduction in roll inventory because of longer roll
life.
The present inventors conducted an extensive investigation to understand the
oxidation behavior of the prior art Ni-Al alloy through the microstructural
changes and
oxidation behavior of the Ni-aluminide rolls. The mechanisms and kinetics of
oxidation
of the rolls subjected to the prolonged exposure to the hardening temperature
was
established through the analysis of rolls in service and oxidation laboratory
simulations.
The results of the study showed that the presence of Zr in the alloy was
detrimental to
the oxidation properties at operation temperatures due to preferential
oxidation of Zr
which in turn creates a non-uniform oxidation of the surface.
The study also showed that nickel-oxide nodules are formed as protrusions on
the roll surface in a manner that follows the micro-segregation patterns in
the as-cast
microstructure. It was seen that internal oxidation that extended from the
roll surface
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into the matrix was highly concentrated in the vicinity of the zirconium
inclusions or the
eutectic zones. Further, NiO nodules were responsible for the formation of the
hard
protrusions on the rolls and hence to the rolls surface deterioration due to
their growth,
coalescence and/or spallation.
Despite the oxidation problems exhibited by the prior art alloy, Ni-aluminide
alloys, in general, provide excellent strength and creep properties at high
temperature
with a roll life 3 times longer than HP alloy roll. Therefore, the present
inventors set
about redesigning the Ni-aluminide roll chemistry to develop an alloy that
prevents
formation of detrimental oxide nodules.
The first phase of the study investigated Ni aluminide alloys with variable Zr
(0-1
wt.%) and Mo(0-3 wt.%). Samples were produced for oxidation simulations in
laboratory and industrial environments. The oxidation behavior of the samples
in the
laboratory conditions were examined after 72, 900, 1500, 3000 and 5000hrs at
900 C
to down-select the most promising alloys. Afterwards, long-term oxidation
experiments
were performed with selected alloys inside an actual furnace environment for
up to
18,000 hours and a correlation with the laboratory results was established.
Figures la-1i are cross sectional SEM images of samples having varied Zr
content (M-0 = Owt.%Zr, M-2 = 0.3wt.%Zr, and the prior art alloy IC-221M =
1.8wt.%Zr),
oxidized at 900 C for 1500, 3000 and 18000 his inside a hardening furnace.
Figures
la-ic are the results of the three samples, IC-221M, M-2, and M-0,
respectively,
oxidized at 900 C for 1500 hours. As can be seen from Figure la, even at this
sort of
service time, the prior art alloy (with 1.8 wt.% Zr) has developed significant
NiO nodules
on the surface thereof. Further, it can be seen from Figure lb that the alloy
with
0.3wt% Zr starts to form small NiO nodules as well. Significantly, the alloy
with no Zr
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does not form any NiO nodules, but instead forms a protective A1203 surface,
see Figure
1c.
As the oxidation time is increased to 3000 and finally 18,000 hours it can be
seen that NiO nodules of the sample having 1.8 wt.% Zr and the sample having
0.3
wt.% Zr grow significantly. This can be seen in figures ld, 1 g (1.8 wt.% Zr)
and le, 1h
(0.3 wt.% Zr). In contradistinction thereto, the alloy with no Zr does not
form any NiO
nodules even at oxidation times of 3000 hours and 18,000 hours.
The results of the long term oxidation experiments showed that NiO dominates
the oxidation products in samples with more than about 0.15wt.%Zr. Internal
oxidation
was highly concentrated in the vicinity of the Zr inclusions and the eutectic
zones. A
protective continuous A1203 layer does not form, rather, the surface oxide
consist of a
discontinuous mixture of NiA1204, NiO and A1203. The protective Al2O3 layer
was found
to be formed on the surface of the alloys with about 0.15%Zr or less. Mo was
added
in order to improve the high temperature strength and did not affect the
oxidation
behavior of the alloys.
The conclusions of the investigation show that the most suitable composition
in
order to avoid oxidation deterioration of transfer rolls are Ni aluminides
that contain:
zirconium ranging from 0 to 0.15 wt. %, preferably about 0.08 - 0.1 wt% Zr;
molybdenum ranging from 2.5 to 3.0 wt. %, preferably about 2.8 wt% Mo;
aluminum ranging from about 7.5 to 8.5 wt.%;
chromium ranging from about 7.5 to 8.5 wt. `)/0;
boron maximum of 0.015 wt.%, but preferably about 0.01 wt.%,
C, Si, Fe, S, Mn, P and Cu should be kept as low as possible, with aimed
maximum
concentrations indicated in the Table I; and
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other elements from the group IVB, VB and VIB of the periodic table should be
kept as
low as possible.
TABLE 1
Element Weight percent (wt. %) Atomic percent (at. %)
Aim composition I Range Aim composition I
Range
Ni balance balance balance balance
Al 8 7.5-8.5 15.9 14.9-16.8
Cr 7.7 7.5-8.5 7.9 7.8-8.7
Zr 0.1 0.05 - 0.15 0.05 0.03-0.09
Mo 2.8 2.5-3.0 1.6 1.4-1.7
0.01 0.015max 0.050 0.05-0.07
0.05 max
Si 0.1 max
Fe 0.3 max
0.005 max
Mn 0.1 max
0.01 max
Cu 0.3 max
Ni-aluminide rolls with inventive alloy composition were centrifugally cast
for
production trial. Additional rolls with different chemical composition,
including the prior
art IC-221M chemistry, were also produced for the benchmarking of the new
alloy. The
tensile properties of the rolls were determined at varying temperatures up to
1000 C in
round 35mm gauge section specimens. Table 2 lists the tensile properties of
the
inventive and prior art alloys.
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TABLE 2
Temp. Temp. Tensile Strength (ksi)
C F Experiment Experiment Experiment Experiment Production Production
Production
roll 2.1%Zr roll 1.2%Zr roll 0%Zr roll 0.1%Zr Roll 131 Roll 156 Roll
157
0.1%Zr 0.1%Zr 0.1%Zr
25 70 100 100 132 122.8 105.3 107 98
700 1292 110 104 77.5 84.15 86.3 72.05 85.35
925 1697 80 77 29.3 42.55 31.1 30.25 26.25
982 1800 47 40 16.1 32.7 30.25 14 14
1038 1900 15.125
It is to be understood that the disclosure set forth herein is presented in
the form
of detailed embodiments described for the purpose of making a full and
complete
disclosure of the present invention, and that such details are not to be
interpreted as
limiting the true scope of this invention as set forth and defined in the
appended claims.
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