U.S. patent application number 09/825948 was filed with the patent office on 2001-11-15 for manufacturing process of nickel-based alloy having improved hot sulfidation-corrosion resistance.
Invention is credited to Miyasaka, Matsuho, Nakahama, Shuhei, Nonomura, Toshiaki, Ohno, Takehiro, Sawada, Shigeru, Uehara, Toshihiro, Yakuwa, Hiroshi.
Application Number | 20010039984 09/825948 |
Document ID | / |
Family ID | 18621685 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010039984 |
Kind Code |
A1 |
Nonomura, Toshiaki ; et
al. |
November 15, 2001 |
Manufacturing process of nickel-based alloy having improved hot
sulfidation-corrosion resistance
Abstract
A manufacturing method, particularly a heat treatment method of
a Ni-based alloy having sulfidation-corrosion resistance used for
component members of corrosion-resistant high-temperature
equipment, that is, Waspaloy (a trademark of United Technologies)
or its improved Ni-based alloy wherein the high temperature
sulfidation-corrosion resistance of the alloy can be improved while
maintaining hot strength properties is disclosed. A Ni-based alloy
used for the method consists essentially of 0.005 to 0.1% C, 18 to
21% Cr, 12 to 15% Co, 3.5 to 5.0% Mo, not more than 3.25% Ti and
1.2 to 4.0% Al (expressed in mass percentage), with the balance
substantially comprising Ni. In the manufacturing method of a
Ni-based alloy having improved sulfidation-corrosion resistance,
the alloy is, after solution heat treatment, subjected to
stabilizing treatment at a temperature not lower than 860.degree.
C. and not higher than 920.degree. C. for 1 to 16 hours, and aging
treatment at a temperature not lower than 680.degree. C. and not
higher than 760.degree. C. for 4 to 48 hours.
Inventors: |
Nonomura, Toshiaki; (Yasugi,
JP) ; Ohno, Takehiro; (Kuwana, JP) ; Uehara,
Toshihiro; (Yonago, JP) ; Yakuwa, Hiroshi;
(Fujisawa, JP) ; Miyasaka, Matsuho; (Yokohama,
JP) ; Nakahama, Shuhei; (Kisarazu, JP) ;
Sawada, Shigeru; (Tokyo, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Family ID: |
18621685 |
Appl. No.: |
09/825948 |
Filed: |
April 5, 2001 |
Current U.S.
Class: |
148/677 |
Current CPC
Class: |
C22C 19/056 20130101;
C22C 19/055 20130101; C22F 1/10 20130101 |
Class at
Publication: |
148/677 |
International
Class: |
C22F 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 11, 2000 |
JP |
2000-108921 |
Claims
What is claimed is:
1. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance containing 0.005 to 0.1% C, 18 to
21% Cr, 12 to 15% Co, 3.5 to 5.0% Mo, not more than 3.25% Ti, and
1.2 to 4.0% Al (in mass percentage), with the balance substantially
comprising Ni, wherein the alloy is, after solution heat treatment,
subjected to stabilizing treatment at a temperature not lower than
860.degree. C. and not higher than 920.degree. C. for 1 to 16
hours, and aging treatment at a temperature not lower than
680.degree. C. and not higher than 760.degree. C. for 4 to 48
hours.
2. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance as set forth in claim 1 wherein
the alloy contains not more than 2.75% Ti and 1.6 to 4.0% Al (in
mass percentage).
3. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance as set forth in claim 2 wherein
the alloy contains any one type of not more than 0.01% B and not
more than 0.1% Zr (in mass percentage).
4. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance as set forth in claim 1 wherein
the alloy is subjected to secondary aging treatment at a
temperature not lower than 620.degree. C. while not higher than the
aging treatment temperature minus 20.degree. C. for not less than 8
hours.
5. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance as set forth in claim 4 wherein
the alloy contains not more than 2.75% Ti and 1.6 to 4.0% Al (in
mass percentage).
6. A manufacturing method of a Ni-based alloy having improved hot
sulfidation-corrosion resistance as set forth in claim 5 wherein
the alloy contains any one type of not more than 0.01% B and not
more than 0.1% Zr.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a manufacturing method of a
heat-resistant alloy having excellent hot sulfidation-corrosion
resistance suitable for use in apparatuses used in high temperature
corrosion environments, particularly in sulfur-corrosion
environment containing H.sub.2S, SO.sub.2, etc., such as expander
turbines utilizing the energy recovered from exhaust gas from fluid
catalytic cracking unit in a petroleum refining system, for
example.
[0003] 2. Description of the Related Art
[0004] Heat-resistant nickel-based alloys having excellent strength
and corrosion resistance at elevated temperature have heretofore
been widely used for members exposed to high temperatures, such as
expander turbine rotors. A typical example of such alloys is what
is known as Waspaloy (a registered trademark of United
Technologies).
[0005] Heat-resistant nickel-based alloys used for members exposed
to elevated temperatures usually gain their high temperature
strength through the precipitation strengthening of intermetallic
compounds called the .gamma.' phase. Since the .gamma.' phase has
Ni.sub.3(Al, Ti) as its basic composition, Al and Ti are normally
added to these alloys.
[0006] In high-temperature equipment exposed to a combustion-gas
atmosphere, such as boilers, on the other hand, the so-called "hot
corrosion" phenomenon involving molten salts such as sulfates, V,
Cl, etc., is known. It is reported that sulfidation corrosion
caused by the direct reactions of gases not involving molten salts
with metals occurs with nickel-based alloys at approximately
700.degree. C. or higher. This phenomenon is attributable to the
formation of a liquid phase of Ni--Ni.sub.3S.sub.2 eutectics.
[0007] In order to accomplish energy conservation in oil
refineries, on the other hand, a system for recovering energy in
the exhaust gas generated from the fluid catalytic cracking unit
has been developed. When Waspaloy, a typical Ni-based superalloy,
was used for gas-expander turbine blades in such equipment, sulfur
corrosion occurred at the roots of the rotor blades though it was
used in a temperature region far lower than the temperature
heretofore considered critical.
[0008] Closer scrutiny of this phenomenon revealed that although
corrosion developed along grain boundaries, no molten salts were
present at corroded areas, indicating that the corrosion was caused
by the direct reactions of the metal with gases. Such an
intergranular sulfidation corrosion of a Ni-based superalloy in a
sulfur-laden gas environment containing no molten salts in a
temperature region lower than the eutectic point of
Ni--Ni.sub.3S.sub.2 has been scarcely observed in the past.
[0009] To solve this problem, the inventors of U.S. Pat. No.
5,900,078 issued May 4, 1999 studied in detail the effects of alloy
elements on the sulfidation behavior of Waspaloy in a sulfur-laden
gas environment in a temperature region lower than the eutectic
point of Ni--Ni.sub.3S.sub.2, and elucidated that the sulfidation
layer in the alloy including grain boundaries is enriched in Ti, Al
and Mo contained in the alloy, and that the Ti and Al contents of
the alloy have a marked effect on the sulfidation-corrosion
resistance of the alloy.
[0010] As a result, a hot sulfidation-corrosion-resistant Ni-based
alloy containing 12 to 15% Co, 18 to 21% Cr, 3.5 to 5% Mo, 0.02 to
0.1% C, not more than 2.75% Ti and not less than 1.6% Al, with the
balance substantially comprising Ni, excluding impurities, has been
proposed, as disclosed in U.S. Pat. No. 5,900,078.
[0011] The alloy disclosed in U.S. Pat. No. 5,900,078 has attracted
trade attention as a heat-resistant Ni alloy whose hot
sulfidation-corrosion resistance has been dramatically improved by
reducing the Ti content and increasing the Al content among the
known addition elements of Waspaloy.
[0012] The present inventor et al., however, made clear after
further study of the alloy that the sulfidation-corrosion
resistance, particularly corrosion resistance at the alloy grain
boundaries, that is, intergranular sulfidation-corrosion resistance
of even the alloy having improved hot sulfidation-corrosion
resistance, as disclosed in U.S. Pat. No. 5,900,078 could be
changed if manufactured with difference methods. The same hold true
with Waspaloy that has been widely known.
[0013] Since heat treatment conditions for these heat-resistant Ni
alloys have often been determined, placing emphasis mainly upon
strength characteristics and hot workability, the resulting alloys
have not necessarily shown good hot sulfidation-corrosion
resistance.
SUMMARY OF THE INVENTION
[0014] It is therefore an object of the present invention to
provide a manufacturing method, particularly a heat treatment
method for improving the sulfidation-corrosion resistance of the
sulfidation-corrosion-resista- nt Ni-based alloy disclosed in U.S.
Pat. No. 5,900,078 and other Ni-based alloys used for members of
corrosion-resistant high-temperature equipment while maintaining
the same high-temperature strength characteristics as those of
conventional alloys.
[0015] After studying the intergranular sulfidation-corrosion
characteristics of the hot sulfidation-corrosion resistant Ni-based
alloy disclosed in U.S. Pat. No. 5,900,078 and Waspaloy, which were
subjected to various heat treatment processes, the present inventor
et al. discovered that grain boundaries are corroded because
carbides chiefly consisting of Cr are precipitated in the grain
boundaries, causing Cr to reduce in the vicinity of grain
boundaries, and Cr-depleted zones to be formed along the grain
boundaries. Consequently, the present inventor et al. have
conceived the present invention based on the assumption that
sulfidation corrosion at grain boundaries can be controlled by
inhibiting the formation of Cr-depleted zones at the grain
boundaries.
[0016] That is, the present invention is a manufacturing method of
a Ni-based alloy containing 0.005 to 0.1% C, 18 to 21% Cr, 12 to
15% Co, 3.5 to 5.0% Mo, not more than 3.25% Ti, and 1.2 to 4.0% Al
in mass percent, with the balance substantially consisting of Ni,
and a manufacturing method of a Ni-based alloy having improved
sulfidation-corrosion resistance which is, after solid solution
heat treatment, subjected to stabilizing treatment for 1 to 16
hours at not lower than 860.degree. C. and not higher than
920.degree. C., and aging treatment for 4 to 48 hours at not lower
than 680.degree. C. and not higher than 760.degree. C.
[0017] More preferably, the present invention is a manufacturing
method of a Ni-based alloy having improved sulfidation-corrosion
resistance which is subjected to secondary aging treatment for not
less than 8 hours at not lower than 620.degree. C. and not higher
than an aging treatment temperature minus 20.degree. C.
[0018] The present invention is a manufacturing method of a
Ni-based alloy having improved sulfidation-corrosion resistance
whose desirable alloy composition is Ti: not more than 2.75%, Al:
1.6 to 4.0% in mass percent, and more preferably any one type of B:
not more than 0.01%, or Zr: not more than 0.1% in mass percent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows a temperature-time-intergranular corrosion
sensitivity curve in the Streicher test and
[0020] FIGS. 2(A) and (B) are cross-sectional micrographs of
specimens attacked by sulfidation corrosion under stress load
condition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The present invention was made based on the conception that
sulfidation corrosion along grain boundaries can be controlled by
inhibiting the formation of Cr-depleted zones along the grain
boundaries; the conception was derived from the observation results
reached during the study of the intergranular sulfidation-corrosion
characteristics of a hot sulfidation-corrosion-resistant Ni-based
alloy disclosed in U.S. Pat. No. 5,900,078 and Waspaloy that grain
boundaries are corroded because Cr-depleted zones are formed along
the grain boundaries as carbides chiefly consisting of Cr are
precipitated in the grain boundaries.
[0022] In the following, the present invention will be described in
detail.
[0023] The most remarkable feature of the present invention is the
method of precipitating the Cr carbides transformed into solid
solutions during solid solution heat treatment as much as possible
in grain boundaries during the subsequent stabilizing treatment and
recovering Cr-depleted zones through diffusion, thereby inhibiting
the re-precipitation of Cr carbides in grain boundaries and the
formation of the Cr-depleted zones during the subsequent aging (age
hardening) treatment.
[0024] More specifically, the formation of Cr-depleted zones in the
vicinity of alloy grain boundaries is inhibited by setting the
temperature and time of stabilizing treatment after solution heat
treatment to conditions under which Cr carbides can be precipitated
in the grain boundaries and Cr-depleted zones can be recovered
along the grain boundaries, and setting the temperature of aging
(age hardening) treatment to a temperature at which Cr carbides are
hard to precipitate in alloy grain boundaries.
[0025] That is, Cr carbides often tend to be precipitated, thereby
leaving Cr-depleted zones in the neighborhood of grain boundaries
and aggravating the sulfidation-corrosion resistance of the alloy
propensity during stabilizing treatment and aging (age hardening)
treatment that are normally conducted on Waspaloy and other alloys,
as will be described in the embodiments. The simplest way to avoid
this is to subject the alloy to heat treatment at a temperature at
which Cr carbides are not precipitated. In order to attain
stabilized creep properties and adequate strength, on the other
hand, stabilizing treatment and aging (age hardening) treatment to
precipitate the .gamma.' phase and control its shape are necessary,
and precipitation of Cr carbides is inevitable during these
treatments.
[0026] The first key point of the present invention is positively
precipitating Cr carbides by setting stabilizing temperature to a
temperature higher than the normal level, and causing Cr to diffuse
into once-formed Cr-depleted zones because the stabilizing
treatment is set to a temperature and time enough to initiate Cr
diffusion, thereby recovering Cr-depleted zones.
[0027] By recovering Cr-depleted zones during stabilizing treatment
and causing as much Cr carbides as possible to precipitate at this
stage in this way, the precipitation of additional Cr carbides and
the resulting formation of Cr-depleted zones during the subsequent
aging (age hardening) treatment can be minimized.
[0028] If the aforementioned stabilizing treatment is followed by
an inadequate aging (age hardening) treatment, however, the
precipitation of additional Cr carbides and the resulting formation
of Cr-depleted zones could take place a new, aggravating
sulfidation-corrosion resistance of the alloy. The second key point
of the present invention is therefore to inhibit the precipitation
of Cr carbides by setting age hardening conditions to a lower level
than the conventional age hardening conditions.
[0029] Taking into account the fact that stabilizing and aging (age
hardening) treatment conditions greatly affect the strength
properties of alloys, as described earlier, heat treatment
conditions according to the present invention were set so as to
impart adequate strength properties to the alloy. That is, the heat
treatment conditions of the present invention were determined with
primary emphasis placed on the corrosion resistance of the alloy
while carefully studying the conditions that can also ensure
adequate strength, unlike the conventional heat treatment
conditions that had placed emphasis on strength alone.
[0030] The present invention conceived based on the above
considerations is a manufacturing method of a heat-resistant alloy
in which the sulfidation-corrosion resistant Ni-based alloy as
disclosed in U.S. Pat. No. 5,900,078 containing 0.005 to 0.1% C, 18
to 21% Cr, 12 to 15% Co, 3.5 to 5.0% Mo, not more than 3.25% Ti and
1.2 to 4.0% Al, with the balance substantially consisting of Ni,
and other Ni-based alloys, such as Waspaloy, used for members of
corrosion-resistant high-temperature equipment are, after solution
heat treatment, subjected to stabilizing treatment for 1 to 16
hours at temperatures not lower than 860.degree. C. and not higher
than 920.degree. C. and aging (age hardening) treatment for 4 to 48
hours at temperatures not lower than 680.degree. C. and not higher
than 760.degree. C. to inhibit the formation of Cr-depleted zones
in the vicinity of alloy grain boundaries.
[0031] Studies by the present inventor et al. revealed that the
formation of Cr-depleted zones due to the precipitation of Cr
carbides in alloy grain boundaries is markedly facilitated in a
temperature region higher than 760.degree. C. and lower than
860.degree. C. Consequently, the present invention makes it
possible to improve the intergranular sulfidation-corrosion
resistance of the alloy by intergranular precipitating as much Cr
carbides as possible while inhibiting the formation of Cr-depleted
zones by subjecting the alloy to stabilizing treatment at a
temperature higher than this temperature region, and inhibiting the
precipitation of Cr carbides in alloy grain boundaries by
subjecting the alloy to aging (age hardening) treatment at a
temperature lower than the temperature region.
[0032] Stabilizing and aging (age hardening) treatments, on the
other hand, have a role of facilitating the precipitation and
growth of the .gamma.' phase that contributes to the
high-temperature strength of alloys. If the stabilizing treatment
temperature is higher than 920.degree. C., however, the .gamma.'
phase is markedly coarsened, aggravating the high-temperature
strength. Even when stabilizing treatment is carried out at a
temperature not lower than 860.degree. C. and not higher than
920.degree. C. for not longer than 1 hour, then the .gamma.' phase
precipitates and grows inadequately, and if the stabilizing
treatment time is longer than 16 hours, the .gamma.' phase tends to
be coarsened, leading to lowered high-temperature strength.
Consequently, stabilizing treatment conditions were specified as a
temperature range not lower than 860.degree. C. and not higher than
920.degree. C. for 1 to 16 hours.
[0033] As for aging (age hardening) conditions, the .gamma.' phase
is precipitated and grown insufficiently, resulting in insufficient
high-temperature strength in a temperature region lower than
680.degree. C. Even when the temperature region is in the range of
not lower than 680.degree. C. and not higher than 760.degree. C.,
an aging time shorter than 4 hours would lead to insufficient
precipitation and growth of the .gamma.' phase, while an aging time
longer than 48 hours would facilitate the precipitation of carbides
in alloy grain boundaries. Thus, the aging (age hardening)
conditions were specified as follows; an aging temperature not
lower than 680.degree. C. and not higher than 760.degree. C. and
aging time from 4 to 48 hours.
[0034] In the present invention, secondary aging treatment should
preferably be performed at a temperature not higher than an aging
(age hardening) treatment temperature -20.degree. C. and not lower
than 620.degree. C. for not less than 8 hours. In other words,
secondary aging (age hardening) treatment should be performed in a
temperature range lower than aging (age hardening) treatment
temperature.
[0035] With this secondary aging (age hardening) treatment,
precipitation strengthening by the refined .gamma.' phase can be
further facilitated without precipitation of Cr carbides in grain
boundaries, thus making it possible to further improve strength
without sacrificing sulfidation-corrosion resistance.
[0036] A secondary aging (age hardening) treatment temperature
lower than 620.degree. C. would hardly precipitate the .gamma.'
phase, with little effect of increasing strength, whereas a
secondary aging (age hardening) treatment temperature exceeding
-20.degree. C. of aging (age hardening) treatment temperature would
coarsen the .gamma.' phase precipitated during aging (age
hardening) treatment, contributing little to the strength enhancing
effect of the precipitation of the refined .gamma.' phase. It is
for this reason that the upper-limit of the secondary aging (age
hardening) treatment temperature was set to the aging (age
hardening) temperature minus 20.degree. C.
[0037] Since too short a secondary aging (age hardening) treatment
time would reduce the contribution of the precipitation of the
refined .gamma.' phase to precipitation strengthening, the
secondary aging (age hardening) treatment time was set to not less
than 8 hours.
[0038] As described in detail in the foregoing, the manufacturing
method of a Ni-based alloy according to the present invention can
improve the sulfidation-corrosion resistance of the alloy while
imparting excellent strength at elevated temperatures to the alloy.
In order to give full play to the properties of the alloy, however,
it is necessary to optimize the alloy composition needed to improve
the sulfidation-corrosion resistance of the alloy itself.
[0039] In the following, alloy compositions suitable for use in the
present invention will be described. Note that mass percentage is
used throughout this Specification unless otherwise specified.
[0040] C forms carbides of TiC with Ti, and M.sub.6C,
M.sub.7C.sub.3 and M.sub.23C.sub.6 types with Cr and Mo. These
carbides help inhibit the coarsening of grain sizes. Moreover,
M.sub.6C and M.sub.23C.sub.6 are essential elements for the present
invention since they help strengthen grain boundaries as adequate
amounts of them are precipitated at the grain boundaries. The above
effects, however, cannot be expected if the carbon content is not
less than 0.005% of C. C contents over 0.1%, on the other hand, not
only reduce the necessary amount of Ti for precipitation hardening,
but also excessively increases the Cr carbides precipitated in
grain boundaries, thus weakening the grain boundaries and requiring
much longer time for precipitating Cr carbides at the grain
boundaries and recovering Cr-depleted zones. C was therefore
limited to 0.005 to 0.1%.
[0041] Cr forms a stable and dense oxide layer, improving oxidation
resistance in a corrosive environment where oxidation factors such
as atmosphere, oxidizing acids and high-temperature oxidation act
simultaneously. When combined with C, Cr precipitates carbides such
as Cr.sub.7C.sub.3 and Cr.sub.23C.sub.6, showing the effects of
improving elevated-temperature strength. If Cr content is less than
18%, however, oxidation resistance among the aforementioned effects
become insufficient, and a Cr content exceeding 21% facilitates the
formation of harmful intermetallic compounds, such as the .sigma.
phase. Cr was therefore limited to 18 to 21%.
[0042] Co in a Ni-based alloy itself exists in a solid solution
having a matrix strengthening effect, and also has an strengthening
effect as it reduces the amount of solid solution of the .gamma.'
phase in the Ni-based matrix and increases the amount of .gamma.'
precipitation. Co contents less than 12% are insufficient in
showing the above effects, while Co contents exceeding 15% may
produce harmful intermetallic compounds, such as the .sigma. phase,
lowering creep strength. Co was therefore limited to 12 to 15%.
[0043] Mo which mainly solves the .gamma. and .gamma.' phases
enhances high-temperature strength, and also serves to improve
resistance to corrosion from hydrochloric acid. Mo contents less
than 3.5%, however, are insufficient in showing the above effects,
while Mo contents exceeding 5.0% destabilize the matrix structure.
Mo was therefore limited to 3.5% to 5.0%.
[0044] Ti and Al, which form the .gamma.' phase in the form of
Ni.sub.3(Al, Ti), are important elements contributing to
precipitation hardening. With increasing Ti content, however,
sulfidation corrosion in an alloy is facilitated. The upper limit
of Ti content was therefore set to 3.25%. The more preferable upper
limit of Ti content to inhibit the propagation of sulfidation
corrosion is 2.75%. Too low Ti contents, on the other hand, make it
difficult to maintain the required high-temperature strength. The
Ti content not lower than 0.5% is the minimum level.
[0045] When the Ti content is kept within the aforementioned range,
an Al content not less than 1.2% must be added in order to maintain
high-temperature strength by forming a sufficient amount of the
.gamma.' phase. An increase in the Al content is effective in
improving not only high-temperature strength but also sulfidation
corrosion resistance. Excessive addition of Al, however, could
cause small elongation and reduction of area and forgiability at
elevated temperatures. The upper limit of Al content was set to
4.0%.
[0046] To ensure a balance among high-temperature strength,
sulfidation-corrosion resistance, high-temperature ductility and
forgeability, the lower limit of Al content should preferably be
set to 1.6%. By controlling the Ti and Al contents,
high-temperature strength and sulfidation-corrosion resistance can
be improved.
[0047] In the present invention, any one or both of not more than
0.01% of B and not more than 0.1% of Zr can be contained as an
element or elements that are not essential but can inhibit
intergranular fracture by increasing the intergranular
strength.
[0048] If B and Zr are added in quantities exceeding 0.01% and
0.1%, respectively, however, they lower the melting point of grain
boundaries, making the alloy vulnerable to melt fracture. The B and
Zr contents were therefore limited to not more than 0.01% and not
more than 0.1%, respectively.
EXAMPLES
[0049] Alloys were manufactured in a vacuum induction furnace, cast
in vacuum, and forged into 60.times.130.times.1000 mm rectangular
billets and 500 mm-diameter or 1400 mm-diameter discs simulating
discs of the gas expander turbine, which were used as test
specimens. Chemical compositions of the specimens are shown in
TABLE 1. Alloy A was an alloy disclosed in U.S. Pat. No. 5,900,078,
and Alloy B was an alloy commonly known as Waspaloy.
1TABLE 1 (Mass %) C Cr Co Mo Ti Al B Zr Fe Alloy A 0.030 19.58
13.54 4.34 1.35 3.02 0.005 0.05 0.54 Alloy B 0.028 19.43 13.47 4.31
3.10 1.46 0.006 0.06 0.97 Si Mn S P Cu Bi Pb Ni Alloy 0.02 0.01
0.0005 0.002 0.01 0.2 ppm 1 ppm Balance A Alloy 0.03 0.02 0.0010
0.003 0.01 0.1 ppm 2 ppm Balance B
[0050] First, the effects of stabilization treatment temperature
and aging (age hardening) treatment temperature, and hold time on
sulfidation-corrosion resistance were examined. To this end, an
intergranular corrosion region map using Alloy A was prepared to
confirm the optimum stabilization treatment temperature and aging
(age hardening) treatment temperature, and hold time.
[0051] Test specimens used in this test were prepared by sampling
Streicher specimens from disc-shaped forgings, which were subjected
to heat treatments given in TABLE 2 to examine their respective
corrosion weight losses, strength proeprties and
sulfidation-corrosion properties.
2TABLE 2 Solution heat Stabilization treatment or aging Conditions
treatment conditions treatment conditions a 1040.degree. C. .times.
4 h air-cooled 1000.degree. C. .times. 4 h air-cooled b
1040.degree. C. .times. 4 h air-cooled 1000.degree. C. .times. 16 h
air-cooled c 1040.degree. C. .times. 4 h air-cooled 1040.degree. C.
.times. 48 h air-cooled d 1040.degree. C. .times. 4 h air-cooled
900.degree. C. .times. 0.5 h air-cooled e 1040.degree. C. .times. 4
h air-cooled 900.degree. C. .times. 1 h air-cooled f 1040.degree.
C. .times. 4 h air-cooled 900.degree. C. .times. 2 h air-cooled g
1040.degree. C. .times. 4 h air-cooled 900.degree. C. .times. 4 h
air-cooled h 1040.degree. C. .times. 4 h air-cooled 900.degree. C.
.times. 16 h air-cooled i 1040.degree. C. .times. 4 h air-cooled
880.degree. C. .times. 4 h air-cooled j 1040.degree. C. .times. 4 h
air-cooled 843.degree. C. .times. 0.5 h air-cooled k 1040.degree.
C. .times. 4 h air-cooled 843.degree. C. .times. 1 h air-cooled l
1040.degree. C. .times. 4 h air-cooled 843.degree. C. .times. 4 h
air-cooled m 1040.degree. C. .times. 4 h air-cooled 843.degree. C.
.times. 16 h air-cooled n 1040.degree. C. .times. 4 h air-cooled
843.degree. C. .times. 48 h air-cooled o 1040.degree. C. .times. 4
h air-cooled 760.degree. C. .times. 1 h air-cooled p 1040.degree.
C. .times. 4 h air-cooled 760.degree. C. .times. 2 h air-cooled q
1040.degree. C. .times. 4 h air-cooled 760.degree. C. .times. 4 h
air-cooled r 1040.degree. C. .times. 4 h air-cooled 760.degree. C.
.times. 16 h air-cooled s 1040.degree. C. .times. 4 h air-cooled
760.degree. C. .times. 48 h air-cooled t 1040.degree. C. .times. 4
h air-cooled 730.degree. C. .times. 16 h air-cooled u 1040.degree.
C. .times. 4 h air-cooled 730.degree. C. .times. 48 h air-cooled v
1040.degree. C. .times. 4 h air-cooled 700.degree. C. .times. 4 h
air-cooled w 1040.degree. C. .times. 4 h air-cooled 700.degree. C.
.times. 16 h air-cooled x 1040.degree. C. .times. 4 h air-cooled
700.degree. C. .times. 48 h air-cooled
[0052] The Streicher test is designed to examine the degree of the
formation of Cr-depleted zones caused by the precipitation of
intergranular carbides (susceptibility to intergranular corrosion).
As described above, the intergranular sulfidation corrosion put in
question here is attributable to the formation of Cr-depleted zones
in the vicinity of grain boundaries caused by the precipitation of
Cr carbides at grain boundaries. Consequently, the degree of the
Cr-depleted zones evaluated in the Streicher test can be considered
proportional to intergranular sulfidation-corrosion resistance.
This was confirmed by comparing the results of the Streicher tests
and hot sulfidation corrosion tests.
[0053] FIG. 1 shows an intergranular corrosion region map in which
the region of Cr-depleted zone formation is shown by plotting the
corrosion weight loss in the Streicher tests with respect to
temperature and time.
[0054] It is found from FIG. 1 that the temperature zones of the
843.degree. C..times.4 h air-cooled stabilization treatment and the
760.degree. C..times.16 h air-cooled aging treatment that have been
commonly practiced are one of the heat treatment conditions where
susceptibility to intergranular corrosion becomes most remarkable,
and cannot be regarded as the optimum conditions at least for
intergranular sulfidation-corrosion resistance. It is also found
that when stabilization treatment in a higher temperature region
and aging treatment in a lower temperature region are practiced,
susceptibility to intergranular corrosion becomes lower, and
intergranular sulfidation-corrosion resistance is improved.
[0055] As discussed above, the present invention makes it possible
to perform stabilization treatment after solution heat treatment at
higher temperatures than with the conventional treatment
conditions, and aging treatment at lower temperatures than the
conventional conditions, thereby remarkably improving intergranular
sulfidation-corrosion resistance.
[0056] Based on this knowledge, stabilization treatment
temperature, aging (age hardening) treatment temperature, and
treatment time were determined. A list of heat treatment conditions
applied to Alloys A and B as test specimens is shown in TABLE 3.
The alloys shown in the "Alloy" columns in TABLE 3 correspond with
those in TABLE 1. TABLE 4 shows the results of
sulfidation-corrosion tests and strength tests on alloys to which
those heat treatments were applied. The sulfidation-corrosion and
strength test specimens used were prepared from samples of the
aforementioned rectangular billet and disc-shaped forgings.
[0057] Sulfidation-corrosion resistance properties were evaluated
based on the presence/absence of fractures and the depth of the
resulting intergranular sulfidation corrosion observed by
cross-section observation on the test specimens which were
subjected to heat treatments given in TABLE 3, and exposed to an
N.sub.2-3%H.sub.2-0.1%H.sub.2S mixed gas atmosphere at 600.degree.
C. for 96 hours while exerting a 589 MPa tensile stress as a
nominal stress. The strength properties were evaluated based on the
tensile properties at room temperature and 538.degree. C., and on
creep rupture properties at the temperature of 732.degree. C. and a
stress of 518 MPa.
3 TABLE 3 Con- Solution heat Secondary aging dition Alloy treatment
Stabilizing treatment Aging treatment treatment This 1 A
1010.degree. C. .times. 4 h air-cooled 880.degree. C. .times. 4 h
air-cooled 700.degree. C. .times. 16 h air-cooled -- inven- 2 A
1025.degree. C. .times. 4 h air-cooled 880.degree. C. .times. 4 h
air-cooled 700.degree. C. .times. 16 h air-cooled -- tion 3 A
1040.degree. C. .times. 4 h air-cooled 880.degree. C. .times. 4 h
air-cooled 700.degree. C. .times. 16 h air-cooled -- 4 A
1025.degree. C. .times. 4 h air-cooled 880.degree. C. .times. 4 h
air-cooled 700.degree. C. .times. 16 h air-cooled 650.degree. C.
.times. 16 h air-cooled 5 A 1025.degree. C. .times. 4 h air-cooled
880.degree. C. .times. 4 h air-cooled 700.degree. C. .times. 16 h
air-cooled 650.degree. C. .times. 16 h air-cooled 6 A 1025.degree.
C. .times. 4 h air-cooled 900.degree. C. .times. 4 h air-cooled
700.degree. C. .times. 16 h air-cooled 650.degree. C. .times. 16 h
air-cooled 7 A 1025.degree. C. .times. 4 h air-cooled 880.degree.
C. .times. 4 h air-cooled 730.degree. C. .times. 16 h air-cooled
650.degree. C. .times. 16 h air-cooled 8 A 1025.degree. C. .times.
4 h air-cooled 880.degree. C. .times. 4 h air-cooled 700.degree. C.
.times. 32 h air-cooled 650.degree. C. .times. 16 h air-cooled 9 A
1025.degree. C. .times. 4 h air-cooled 880.degree. C. .times. 4 h
air-cooled 760.degree. C. .times. 16 h air-cooled 650.degree. C.
.times. 16 h air-cooled Compara- 10 B 1025.degree. C. .times. 4 h
air-cooled 880.degree. C. .times. 4 h air-cooled 700.degree. C.
.times. 16 h air-cooled -- tive 11 B 1040.degree. C. .times. 4 h
air-cooled 880.degree. C. .times. 4 h air-cooled 700.degree. C.
.times. 16 h air-cooled 650.degree. C. .times. 16 h air-cooled
example 12 A 1040.degree. C. .times. 4 h air-cooled 843.degree. C.
.times. 4 h air-cooled 760.degree. C. .times. 16 h air-cooled
650.degree. C. .times. 16 h air-cooled 13 A 1040.degree. C. .times.
4 h air-cooled 843.degree. C. .times. 4 h air-cooled 760.degree. C.
.times. 16 h air-cooled 14 B 1040.degree. C. .times. 4 h air-cooled
843.degree. C. .times. 4 h air-cooled 760.degree. C. .times. 16 h
air-cooled
[0058]
4 TABLE 4 Hot sulfidation Strength test results corrosion test
Tensile properties at room Tensile properties at elevated Creep
rupture properties results temperature temperature (538.degree. C.)
(732.degree. C./518 MPa) Maximum 0.2% Reduc 0.2% Reduc Reduc
intergranular yield Tensile Elong tion of yield Tensile Elong- tion
of Break Elong- tion of corrosion Condi- strength strength ation
area strength strength ation area time ation area depth under tion
(MPa) (MPa) (%) (%) (MPa) (MPa) (%) (%) (h) (%) (%) tensile stress
This invention 1 894 1339 26.7 30.4 834 1224 21.3 25.8 47.0 16.8
32.1 12 .mu.m 2 874 1287 25.3 29.8 801 1217 22.7 30.1 58.8 17.4
24.3 18 .mu.m 3 852 1257 29.4 34.9 764 1158 24.4 33.8 73.3 15.4
29.2 21 .mu.m 4 885 1269 25.4 30.7 781 1085 24.1 25.5 66.4 21.1
24.7 24 .mu.m 5 838 1231 27.4 33.1 765 1125 22.4 31.5 51.5 24.9
34.2 28 .mu.m 6 870 1281 26.5 35.2 759 1105 24.1 27.7 76.3 28.2
36.9 15 .mu.m 7 874 1310 23.7 27.4 792 1178 20.5 18.8 101.0 36.9
61.5 30 .mu.m 8 884 1308 25.4 29.4 801 1193 26.4 33.5 118.0 25.8
34.1 27 .mu.m 9 866 1301 24.8 29.5 789 1168 21.4 35.6 94.3 30.5
41.1 29 .mu.m 10 877 1261 21.5 28.8 799 1184 23.3 34.3 75.2 22.2
25.7 13 .mu.m 11 862 1263 24.7 30.1 773 1091 21.3 29.8 122.3 13.5
18.1 17 .mu.m Comparative example 12 880 1328 25.1 33.8 787 1188
27.8 36.2 41.2 10.6 14.9 520 .mu.m 13 773 1216 28.5 27.7 806 1101
24.8 31.3 64.2 17.9 17.4 200 .mu.m 14 924 1364 28.9 33.0 836 1214
18.5 22.8 61.7 19.3 16.1 Rupture after 19 h
[0059] It is indicated from the results shown in TABLE 4 that
although no appreciable differences were found in strength
properties at elevated temperature on any test specimens subjected
to any heat treatment conditions, Alloys A and B subjected to
conventional heat treatment conditions (Nos. 12, 13 and 14
Conditions) had deep intergranular corrosion of no less than 200
.mu.m under stress load conditions, or could not withstand 96-hour
exposure tests to rupture, whereas the maximum intergranular
corrosion depth is not more than 30 .mu.m and sulfidation-corrosion
resistance was markedly improved with Alloys A and B subjected to
heat treatments of this invention (Nos. 1 to 11 Conditions).
[0060] Cross-sectional observation results were compared between
the test specimens subjected to the heat treatment of the present
invention (No. 10 Condition) and the comparative alloys subjected
to the conventional heat treatment (No. 14 Condition) that led to
rupture. The results are shown in FIG. 2.
[0061] FIG. 2(A) is a cross-sectional metallographical photograph
of a test specimen treated under No. 10 Condition according to the
present invention in which a white undulated area at the lower
right is the alloy base metal. The photo indicates that the
intergranular corrosion is shallow in depth. FIG. 2(B), on the
other hand, is a cross-sectional metallographical photograph of the
fractured part of a test specimen treated under No. 14 Condition.
The photo indicates that corrosion developed along grain
boundaries, causing severe intergranular sulfidation corrosion.
This seems to suggest that a rupture of the alloy is caused by the
intergranular sulfidation corrosion.
[0062] The above-mentioned test results suggest that hot
sulfidation-corrosion resistance can be remarkably improved while
maintaining almost the same strength properties at elevated
temperature by applying the heat treatment according to the present
invention as conventional heat treatment condition to a Ni-based
heat-resistant alloy having a particular composition.
[0063] As described above, the present invention provides a
Ni-based alloy having improved sulfidation-corrosion resistance,
particularly intergranular corrosion resistance while maintaining
sufficient high-temperature strength properties, compared with
conventional heat treatment methods in which emphasis is placed on
strength alone. Thus, the present invention can provide equipment
components having high reliability in sulfidation corrosive
environment.
[0064] With the lowering quality of fossil fuel resulting from the
needs for reduced loads on the environment and energy conservation,
and increased efficiency of energy equipment in recent years,
service environments of high-temperature equipment, such as
turbines and boilers, are becoming increasingly stringent.
Consequently, inventions concerning the improved corrosion
resistance of equipment components, such as the present invention,
will have great significance in the future.
* * * * *