Note: Descriptions are shown in the official language in which they were submitted.
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SURFACE TREATMENT OF AUSTENITIC NI-FE-CR BASED ALLOYS
Field of the Invention
This invention relates to a process for the surface
treatment of articles fabricated of austenitic iron-
nickel-chromium alloys, to resist and deter the onset of
intergranular cracking and corrosion and to enhance the
concentration of special grain boundaries. The process
comprises at least one cycle of working to induce
deformation of the near surface region, for example by
high density shot peening, followed by recrystallization
heat treatment. The novel process can be applied to
wrought, cast or welded materials, and is particularly
suited for in-situ or field application to components
such as steam generator tubes, core reactor head
penetrations of nuclear power plants, recovery boiler
panels used in the pulp and paper industry, closure welds
on canisters for the storage of nuclear waste and storage
battery components.
Description of Prior Art
The prior art primarily describes the use of surface
cold work, for example by "shot peening", as a means to
effect a state of residual compression at the surface of
a material, and thus render the material resistant to the
initiation of cracks which require a tensile stress for
initiation and propagation. Shot peening is a method of
cold working, inducing compressive stresses on and near
the surface layer of metallic parts. The process
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consists of impinging the test article with a stream of
shot, directed at the metal surface at high velocity
under controlled conditions.
Although peening cleans the surface the major
purpose is to impact and enhance fatigue strength. The
peening process is known to relieve tensile stresses that
contribute to stress-corrosion cracking. Yamada in U.S.
5,816,088 (1998) describes a surface treatment method for
a steel work piece using high speed shot peening.
Mannava in U.S. 5,932,120 (1999) describes a laser shock
peening apparatus using a low energy laser. Harman and
Lambert in US 4,481,802 (1984) describe a method of
peening the inside of a small diameter tube in order to
relieve residual tensile stresses.
Friske and Page in US 3,844, 846 (1974) describe a
surface deformation treatment by shot peening, which is
applied to austenitic Cr-Fe-Ni alloys without subsequent
heat treatment, in order to render the surface region
highly deformed, and subsequently more resistant to
intergranular corrosion in the event that the article
becomes exposed to sensitization temperatures, i.e.,
400 -700 C, during service.
Kinoshita and Masamune in US 4,086,104 (1978) also
describe a surface deformation treatment for austenitic
stainless steel components, applied following final mill
annealing or hot rolling treatments, which renders the
surface of the stainless steel more resistant to oxide
scale formation during subsequent exposure to high
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temperature steam.
Anello in US 4,495,002 (1985) describes a three step
process for martensitic stainless steels to increase
their resistance to chloride corrosion, wherein, an
article is subjected to surface deformation via shot
peening, followed by an ageing treatment at 527 C-549 C,
and followed by a final lower intensity shot peening. In
such manner, a homogeneous near surface region consisting
of aged martensite is obtained which is resistant to
chloride corrosion and cracking.
Polizotti in US 4,424,083 (1984) discloses a method
for enhancing the protection of cast austenitic stainless
steel tube against carburization when such tubes are
employed in high temperatures carburizing atmospheres,
such as in the steam cracking of hydrocarbons. The
diffusion of carbon into the alloy steel causing
formation of additional carbides, resulting in
embrittlement of the tubes, is avoided by heating the
cold-worked inner surfaces of such a tube for an
effective amount of time, at a temperature between the
recrystallization temperature and its melting
temperature, in an atmosphere where the oxygen partial
pressure is at least oxidizing with respect to chromium.
These temperatures used by Polizotti are stated to be
420 -1150 C, preferably 420 -800 C with the treatment
time at such temperatures being about 200 to about 500
hours. Suitable atmospheres include hydrogen or steam.
The treatment time required depends on the oxygen partial
pressure, longer treatment times are required if the
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oxygen partial pressure is low.
Palumbo in US 5,702, 543 (1997) and 5, 817, 193
(1998), describes thermomechanical mill processes
involving the application of bulk cold work followed by
recryetallization heat treatment to improve the grain
boundary microstructure of austenitic Ni-Fe-Cr alloys and
thereby effect significant improvements in intergranular
corrosion and cracking resistance.
Studies have shown that certain "special" grain
boundaries, described on the basis of the "Coincident
Site Lattice" mpdel of interface structure (Kronberg and
Wilson, Trans. Met. Soc. AIME, 185, 501 (1949)) as lying
within A6 of E, where E<29 and A6<15 _E-0.5 (Brandon, Acta
Metall., 14, 1479, (1966)) are highly resistant to
intergranular degradation processes such as corrosion,
cracking, and grain boundary sliding; the latter being a
principal contributor to creep deformation.
We have discovered that finished and semi-finished
articles made of austenitic Ni-Fe-Cr alloys, whether in
the wrought, forged, cast or welded condition, may be
subjected to working to induce deformation of the near
surface region by a technique such as shot peening,
followed by annealing of the article at a temperature
below the melting point for a time sufficient to induce
recrystallization in the cold-worked near surface region
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and increase the frequency of special low E CSL grain
boundaries.
In this specification, "the near surface region"
refers to the surface layer of the article to a depth in
the range of 0.01mm to about 0.5mm. "Working" will
hereinafter be used in this specification as a shorthand
reference to working to induce deformation.
Summary of the Invention
It is a principal object of this invention to
provide a surface treatment methodology which will alter
the recrystallized structure in the near surface region
of a finished article or component made austenitic Ni-Fe-
Cr alloys to impact significant resistance to
intergranular corrosion and cracking during the service
of the article or component, without the need for bulk
deformation thereof by a process of rolling, extruding,
forging or the like. The hardness of the surface layer
after the recrystallization treatment is lower than the
hardness of the article before the processing.
It is a further object of this invention to provide
a surface treatment process as aforesaid, which may be
used to treat and improve the degradation and corrosion
resistance of finished parts of complex shape and parts
which may already be in service, in particular, nuclear
steam generator tubes, nuclear reactor head penetrations
and the like. Suitably treated parts also include weld
clad components such as recovery boiler wall panels for
the pulp and paper industry, and closure welds on
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canisters for nuclear waste storage.
The. method of the present invention enhances the
concentration of special grain boundaries in the surface
of metallic articles. This is achieved without invoking
conventional strengthening mechanisms, such as
precipitation or age-hardening, and without substantially
altering the tensile strength or hardness of the
material. Typically the layer in which the special grain
boundary fraction has been increased, exhibits a
reduction in tensile strength, when compared to the as
received material or the bulk of the material, which has
not been subjected to this process.
Our experiments and reviews of the literature
indicate that conventional surface cold working of
articles of the kind with which we are herein concerned
produces a special grain boundary fraction no greater
than 10 to 15%. The method of the present invention
allows this to be improved significantly more than 20%.
Enhanced resistance to intergranular corrosion and
cracking results when the special grain boundary fraction
goes above 30% and typically 40% to 50%.
The treatment time required to achieve the desired
properties varies, depending on the material, but
typically ranges from 1 minute to 75 hours, and
preferably from 5 minutes to 50 hours.
With a view of achieving these objects, there is
provided a method for improving intergranular corrosion
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and cracking resistance of an article fabricated of an
austenitic Ni-Fe-Cr alloy by subjecting the alloy to at
least one cycle comprising the steps of:
(i) working the surface region of the article to a
depth in the range of from 0.01mm to about
0.5mm; and
(ii) annealing the article at a temperature below
the melting point of said alloy for a time
sufficient to induce recrystallization in said
surface region.
Brief Description of the Drawings
Preferred embodiments of the invention are described
in detail below, with reference to the drawings. The
three figures are comparative cross-sectional optical
micrographs of an austenitic alloy, in which:
Figure 1(a) is a micrograph of as-received Alloy
625;
Figure 1(b) is a sample of the same Alloy 625
material but subsequent to treatment by a single cycle of
surface deformation (shot peening) and recrystallization,
according to the present invention; and
Figure 1(c) is an optical micrograph for the same
alloy, which has been treated according to two cycles of
the process according to the present invention.
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Preferred Embodiments of the Invention
As is known by those skilled in the metallurgical
art, cold working involves mechanical deformation of an
article at a low enough temperature that dislocations are
retained, leading to a structure of non-recrystallized,
deformed grains. Hot working, on the other hand, results
in an article having primarily recrystallized grains.
This invention relies on working the surface layer
of the article, followed by an annealing treatment, which
results in recrystallization of the deformed region.
Shot peening is a non-conventional method of cold-working
in which compressive stresses are induced in the exposed
surface layers of metallic parts by impingement of a
steam of shot, directed at the surface at high velocities
under controlled conditions. When shot in a high-
intensity stream contacts the test article surface, they
produce light, rounded depressions in the surface,
causing a plastic flow to extend up to 0.5mm (0.02")
below the surface. The metal beneath this layer remains
unaffected. The penetration depth of the peening into
the exposed surface of the article can be controlled by
the hardness, weight and size of the shot and the impact
velocity.
In carrying out the method of the invention,
working is typically carried out below or near room
temperature, e.g. between -20 C and 45 C. We have found
that the deformation treatment can also be successfully
applied at higher temperatures, e.g. from about 40 C up
to a temperature of about 50% of the melting point of the
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article as expressed in degrees Kelvin (50% of Tm K) , and
preferably below 25% of Tm K. In any event, the maximum
deformation temperature must be below the
recrystallization temperature of the alloy being treated.
Heat-treatment of the austenitic Ni-Fe-Cr article,
following peening, is carried out at temperatures and
times sufficient to allow complete recrystallization to
occur, and which are sufficient to ensure that chromium
carbides remain dissolved and that elemental chromium and
carbon are retained in solid solution. We have found
that suitable annealing temperatures fall in the range
between 50% of Tm K and up to but less than Tm K(0.5 to
< 1.0 TM K) , typically between 0.6 and 0.99 Tm K and
preferably between 0.7 and 0.95 Tm K.
The peening and heat treatment steps can optionally
be repeated a number of times to achieve optimum
homogeneity in near-surface microstructure.
Also, a final lower intensity surface deformation
may be applied following heat treatment in order to
impart compressive stresses in the near surface of the
treated article. In the case of precipitation hardenable
austenitic Ni-Fe-Cr alloys, the final recrystallization
treatment or reduced intensity peening treatment may be
followed by an ageing heat treatment to effect the
precipitation of strengthening phases.
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Example
A section of austenitic weld overlay Alloy 625
(chemical composition: 61.0%Ni, 21.4%Cr, 8.2%Mo and
9.4%Fe) was obtained in the as-cast condition. Samples
of the material were treated according to the preferred
embodiments of this invention, whereby exposed surfaces
were shot peened according to the conditions outlined in
Table 1. Following each peening cycle, the samples were
recrystallized at a temperature of 1000 C (1832 F) for 5
minutes and air-cooled. Figure 1 shows cross-sectional
optical micrographs of (a) the as-received material (F),
and (b), (c) material treated by the preferred
embodiments of this invention, in one and two cycles (G-
1, G-2), respectively. As noted in these micrographs,
the treated materials display a recrystallized surface
layer extending approximately 0.127mm (0.005 in) into the
specimens. Table 2 summarizes the final microstructural
characteristics obtained by applying the method of the
present invention.
Treated samples and the as-received materials were
subsequently subjected to a `sensitization' heat
treatment which simulates a manufacturing stress relief
protocol; this treatment was applied as follows: samples
were heated to a target temperature of 1650 F (899 C) at
a heating rate of 400 F (204 C) per hour from room
temperature; the samples were held at 1650 F (899 C) for
20 minutes, and subsequently furnace cooled to a
temperature of 600 F (315 C), and then air cooled to room
temperature.
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All samples were subsequently corrosion tested as
per ASTM G28A to evaluate resistance to intergranular
corrosion arising from sensitization. The test involves
120-hour exposure in boiling ferric sulfate - 50% aqueous
sulfuric acid. Replicated samples of approximately
0.0615 in x 0.5 in. x 2 in. were accurately dimensioned
to determine exposed surface area and weighed to 1 mg
accuracy prior to, and following exposure in order to
establish mass loss, and corrosion rate in units of mils
per year.
Table 2 summarizes the measured corrosion
performance. As-received and sensitized material (F),
not treated according to the preferred embodiments of
this invention display a corrosion rate of 393 mils per
year. Material treated by the preferred embodiments of
this invention and subsequently sensitized displays a
marked reduction in sensitization and improvement in
corrosion resistance with G-1 and G-2 specimens
displaying similar average corrosion -rates of 40 and 41
mils per year respectively.
Table 1: Details of applied shot peening parameters
Shot Peening Time Hardened Steel Air Pressure
Peening Shot Size (psi)
One Cycle 7 minutes 0.028 in. 80
Two Cycles (1) 7 minutes 0.028 in. 80
(2) 5 minutes 0.028 in. 80
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Table 2: Summary of the microstructural characteristics
Sample Process Fraction of Grain Average
conditions Special Grain Size Corrosion
Boundaries (o) ( m) Rate (mils
/year)
F As Received + Z15 >100 393
Sensitization
Treatment
G-1 Single cycle + =50 =3 40
Sensitization
Treatment
G-2 Two cycles + =58 z5 41
Sensitization
Treatment
By using the process of the invention, a wide
variety of articles may, without bulk deformation, be
treated to increase significantly their resistance to
corrosion.
