U.S. patent application number 13/101686 was filed with the patent office on 2012-11-08 for treatment for preventing stress corrosion cracking.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Robin Carl Schwant, Andrew Batton Witney.
Application Number | 20120279619 13/101686 |
Document ID | / |
Family ID | 46045922 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120279619 |
Kind Code |
A1 |
Witney; Andrew Batton ; et
al. |
November 8, 2012 |
TREATMENT FOR PREVENTING STRESS CORROSION CRACKING
Abstract
A treatment for prevention of stress corrosion cracking (SCC)
and a treated component are disclosed. A surface of a relatively
high tensile strength component is heated to a temperature at which
at least one of tempering or annealing occurs. The surface is then
cooled in a controlled manner so as to maintain a reduced tensile
strength at the surface that minimizes SCC while keeping a
relatively high tensile strength in the remainder of the
component.
Inventors: |
Witney; Andrew Batton;
(Schenectady, NY) ; Schwant; Robin Carl;
(Pattersonville, NY) |
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
46045922 |
Appl. No.: |
13/101686 |
Filed: |
May 5, 2011 |
Current U.S.
Class: |
148/567 ;
148/400; 148/559 |
Current CPC
Class: |
C21D 1/30 20130101; C21D
9/00 20130101; F01D 5/286 20130101; Y02P 10/25 20151101; C21D 1/26
20130101; C21D 1/42 20130101; C21D 1/00 20130101; C21D 11/00
20130101; C21D 1/84 20130101; Y02P 10/253 20151101; C21D 1/18
20130101; C21D 2221/00 20130101; C21D 2221/10 20130101; C21D 9/0068
20130101; F05D 2230/41 20130101; C21D 9/0062 20130101 |
Class at
Publication: |
148/567 ;
148/559; 148/400 |
International
Class: |
C21D 1/42 20060101
C21D001/42; C21D 9/00 20060101 C21D009/00 |
Claims
1. A method for treating a component to minimize stress corrosion
cracking (SCC), comprising: heating a surface of a component to a
temperature at which at least one of tempering or annealing occurs
at the surface; and cooling the component in a controlled manner so
as to maintain a surface tensile strength that minimizes SCC,
wherein the resultant surface tensile strength is lower than a
resultant high tensile strength of a remainder of the
component.
2. The method of claim 1, wherein the heating includes induction
heating.
3. The method of claim 1, wherein the temperature is greater than
540 degrees Celsius.
4. The method of claim 1, wherein the heating and cooling produce a
full anneal of the surface.
5. The method of claim 1, wherein the cooling includes: maintaining
a heating apparatus in an operational position with respect to the
component; and adjusting the heating in a manner that causes the
temperature in the surface to decrease along a desired thermal
profile.
6. The method of claim 1, wherein the component includes a
generator component.
7. The method of claim 6, wherein the component is selected from
the group consisting of: a generator rotor or a generator retaining
ring.
8. The method of claim 1, wherein the component includes a turbine
component.
9. The method of claim 8, wherein the component is selected from
the group consisting of: a turbine bucket, a turbine blade, or a
turbine rotor.
10. The method of claim 1, wherein a material of the surface is
chemically homogeneous with a material of the remainder of the
component.
11. A component treated to minimize stress corrosion cracking
(SCC), having a structural metallic layer having a relatively high
structural tensile strength; and a treated metallic layer composed
of a material that is chemically homogeneous with the structural
metallic layer, the treated metallic layer substantially forming at
least a portion of an outer surface of the component and having a
treated tensile strength that is lower than the structural layer
tensile strength, the component formed by the process, comprising:
heating a surface of the component to a temperature at which at
least one of tempering or annealing of the exterior surface of the
component occurs; and cooling the surface of the component in a
controlled manner so as to maintain a resultant surface tensile
strength that minimizes SCC.
12. The component of claim 11, wherein the heating includes
induction heating.
13. The component of claim 11, wherein the temperature is greater
than 540 degrees Celsius.
14. The component of claim 11, wherein the heating and cooling
produce a full anneal of the surface.
15. The component of claim 11, wherein the cooling includes:
maintaining a heating apparatus in an operational position with
respect to the component; and adjusting the heating in a manner
that causes the temperature in the surface to decrease along a
desired thermal profile.
16. The component of claim 11, wherein the component includes a
generator component.
17. The method of claim 16, wherein the component is selected from
the group consisting of: a generator rotor or a generator retaining
ring.
18. The component of claim 11, wherein the component includes a
turbine component.
19. The component of claim 18, wherein the component is selected
from the group consisting of: a turbine bucket, a turbine blade or
a turbine rotor.
Description
BACKGROUND OF THE INVENTION
[0001] The subject matter disclosed herein relates generally to
stress corrosion cracking (SCC) prevention. More specifically, the
present invention relates to a treatment for preventing SCC in
metal parts including turbine and generator components.
[0002] Excessive pressure, heat, and moisture, such as may be found
inside a turbine or generator, can form an extreme environment.
This environment, with the inevitable impurities found within, can
be corrosive to the components that make up a turbine or generator.
Under operational stresses, this environment can lead to SCC in the
components. Components with higher tensile strength tend to be more
susceptible to SCC. However, components having low tensile strength
may not be able to withstand the stresses required for turbine
operation.
BRIEF DESCRIPTION OF THE INVENTION
[0003] A treatment for prevention of stress corrosion cracking
(SCC) and a treated component are disclosed. A surface of a
relatively high tensile strength component is heated to a
temperature at which at least one of tempering or annealing occurs.
The surface is then cooled in a controlled manner so as to maintain
a reduced tensile strength at the surface that minimizes SCC while
keeping a relatively high tensile strength in the remainder of the
component.
[0004] A first aspect of the invention provides a method for
treating a component to minimize stress corrosion cracking (SCC),
comprising: heating a surface of a component to a temperature at
which at least one of tempering or annealing occurs at the surface;
and cooling the component in a controlled manner so as to maintain
a surface tensile strength that minimizes SCC, wherein the
resultant surface tensile strength is lower than a resultant high
tensile strength of a remainder of the component.
[0005] A second aspect of the invention provides a component
treated to minimize stress corrosion cracking (SCC), having a
structural metallic layer having a relatively high structural
tensile strength; and a treated metallic layer composed of a
material that is chemically homogeneous with the structural
metallic layer, the treated metallic layer substantially forming at
least a portion of an outer surface of the component and having a
treated tensile strength that is lower than the structural layer
tensile strength, the component formed by the process, comprising:
heating a surface of the component to a temperature at which at
least one of tempering or annealing of the exterior surface of the
component occurs; and cooling the surface of the component in a
controlled manner so as to maintain a resultant surface tensile
strength that minimizes SCC.
BRIEF DESCRIPTION OF THE DRAWING
[0006] These and other features of the disclosure will be more
readily understood from the following detailed description of the
various aspects of the invention taken in conjunction with the
accompanying drawing that depict various aspects of the invention,
in which:
[0007] FIG. 1 shows a perspective partial cut-away illustration of
a conventional steam turbine pursuant to an aspect of the
invention;
[0008] FIG. 2 shows a fragmentary cross-sectional view of a portion
of a turbine illustrating various stationary and rotational parts
thereof pursuant to an aspect of the invention;
[0009] FIG. 3 shows a partial cut-away illustration of a component
undergoing heat treatment with an induction heater pursuant to an
aspect of the invention; and
[0010] FIG. 4 shows a perspective view of a treated component
pursuant to an aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0011] A treatment for prevention of stress corrosion cracking
(SCC) and a treated component are disclosed. A surface of a
relatively high tensile strength component is heated to a
temperature at which at least one of tempering or annealing occurs.
The surface is then cooled in a controlled manner so as to maintain
a reduced tensile strength at the surface that minimizes SCC while
keeping a relatively high tensile strength in the remainder of the
component.
[0012] Referring to the drawings, FIG. 1 shows a perspective
partial cut-away illustration of a multiple stage steam turbine 10.
Even though FIG. 1 illustrates a steam turbine, it should be
recognized by one skilled in the art that the teachings of this
invention can be applied in any environment in which components may
be susceptible to SCC, including, but not limited to a gas turbine,
a wind turbine, a generator, an aircraft engine, a valve, a tank, a
pipe, a container, a propeller, the hull of a ship, or anything
else that may operate in a harsh environment in which SCC may be a
problem. Nevertheless, in the embodiment illustrated by FIG. 1,
turbine 10 can be a condensing steam turbine or a non-condensing
steam turbine. Turbine 10 includes a rotor 12 that includes a
rotating shaft 14 and a plurality of axially spaced rotor wheels
18. A plurality of rotating blades or "buckets" 20 are mechanically
coupled to each rotor wheel 18. More specifically, blades 20 are
arranged in rows that extend circumferentially around each rotor
wheel 18. A plurality of stationary nozzles 22 extends
circumferentially around shaft 14, and the nozzles are axially
positioned between adjacent rows of blades 20. Stationary nozzles
22 cooperate with blades 20 to form a stage and to define a portion
of a steam or hot gas flow path through turbine 10.
[0013] In operation, gas or steam 24 enters an inlet 26 of turbine
10 and is channeled through stationary nozzles 22. Nozzles 22
direct gas or steam 24 downstream against blades 20. Gas or steam
24 passes through the remaining stages imparting a force on blades
20 causing shaft 14 to rotate. At least one end of turbine 10 may
extend axially away from rotor 12 and may be attached to a load or
machinery (not shown) such as, but not limited to, a generator,
and/or another turbine.
[0014] In one embodiment, turbine 10 may include five stages. The
five stages are referred to as L0, L1, L2, L3 and L4. Stage L4 is
the first stage and is the smallest (in a radial direction) of the
five stages. Stage L3 is the second stage and is the next stage in
an axial direction. Stage L2 is the third stage and is shown in the
middle of the five stages. Stage L1 is the fourth and next-to-last
stage. Stage L0 is the last stage and is the largest (in a radial
direction). It is to be understood that five stages are shown as
one example only, and each turbine may have more or less than five
stages. Also, as will be described herein, the teachings of the
invention do not require a multiple stage turbine.
[0015] Referring now to FIG. 2, there is illustrated a portion 100
of turbine 10 (FIG. 1) having a rotary component 105 and a
stationary component 110. Rotary component 105 includes, for
example a rotor 115 mounting a plurality of circumferentially
spaced buckets 120 at spaced axial positions along the rotor
forming parts of the various turbine stages. Stationary component
110 may include a plurality of diaphragms 125 mounting partitions
130 defining nozzles which, together with respective buckets, form
the various stages of turbine 100. As illustrated in FIG. 2, an
outer ring 135 of diaphragm 125 carries one or more rows of seal
teeth 140 for sealing with shrouds or covers 145 adjacent the tips
of buckets 120. Similarly, an inner ring 150 of diaphragm 125
mounts an arcuate seal segment 155. The seal segment has radially
inwardly projecting high-low teeth 160 for sealing with rotor 115.
Similar seals are provided at the various stages of turbine 100 as
illustrated and the direction of the steam or hot gas flow path is
indicated by the arrow 165.
[0016] Certain components within turbine 10 (FIG. 1) may be
susceptible to SCC, due to their local environment within turbine
10 during operation. These components include, but are not limited
to buckets or blades 20, 120, wheels 18, rotor 12, and/or any
component used to couple one of the above components to another of
the above components and/or to another component within turbine 10.
Previously, alloys with high tensile strength could not be used to
make these components. Instead, these components have been
constructed using alloys that are made to have the lowest tensile
strength possible given the load requirement expected for the
component. Alternatively, costly highly-alloyed metals are used to
mitigate SCC.
[0017] The current invention treats the component to minimize SCC.
This is done by changing, via the treatment, the tensile strength
of the portion of the surface of the component for which SCC
resistance is desired. This surface can include, for example, any
portion of the component that can come into contact with a
corrosive environment. The tensile strength of the remainder of the
component remains relatively unchanged. The result is a component
having a homogeneous chemical composition throughout with an
interior having relatively high tensile strength (e.g., a skeleton
or structural layer) while having a stress corrosion resistant,
lower tensile strength surface (e.g., skin or treated layer) that
is adjacent to and integral with the structural layer.
[0018] Referring now to FIG. 3, there is illustrated one embodiment
of a heating stage 200 of the SCC treatment for a component 210. In
heating stage 200, a surface of component 210 is heated, such as
with an induction heater 220. Prior to this heating, component 210
may be held at any temperature between room temperature and an
elevated temperature below the temperature intended for the heat
treatment. When heating stage 200 uses induction heater 220, such
as illustrated in FIG. 3, an alternating current (current) 230 is
passed through coils 224 of induction heater 220. In one
embodiment, current 230 is in a range of approximately 100-400 kHz
and is produced by a power supply rated at between 50-400 kW. In
any case, current 230 produces a magnetic field 250, which in turn
induces eddy currents 240 in component 210. Induced eddy currents
240, encounter material resistance which causes the metal of
surface 214 to increase in temperature by a mechanism known as
Joule heating. There is an inverse relationship between the depth
of the heating beneath surface 214 and frequency of current 230
passed through coils 224. Higher frequency current 230 through
coils 224 couples with and inductively heats a shallower layer of
surface 214. The portion of component 210 beneath surface 214 is
not directly heated (but may increase in temperature nonetheless as
a result of a secondary mechanism such as conduction of heat from
surface 214). In any case, heating stage 200 is designed such that
the temperature of a remainder 216 of component 210 beneath surface
214 is relatively unchanged. As an example, surface 214 may be on
the order of 0.125 millimeter in thickness.
[0019] In the present invention, induction heater 220 can be used
to heat exterior surface 214 of component 210 to a temperature at
which at least one of tempering or annealing occurs. In the
alternative, any other process of heating a surface of a component
to a temperature that is greater than the tempering or annealing
temperature while relatively maintaining the temperature of the
remainder of the component that is now known or later discovered
may be used, including, but not limited to heating with a laser or
radiant heater.
[0020] Component 210 can comprise a ferrous alloy of any type,
including, but not limited to austenitic, martensitic, bainitic
stainless steels, precipitation hardened steels, etc. In the case
that martensitic stainless steels, bainitic steels or precipitation
hardened steels are used, component 210 may have chromium contents
less than 20%, nickel contents less than 12%, manganese less than
2%, and molybdenum less than 5%. In any case, as the temperature
increases in such alloys tempering of the alloy occurs, producing
softer material. In certain steel alloys, these processes begin to
occur at or above 540 degrees Celsius, such as around 600-800
degrees Celsius. Having achieved such a temperature with induction
heater 220, a treatment of only a few seconds or minutes is
sufficient to achieve some softening of the heated surface layer
212.
[0021] Once heating stage 200 is complete, exterior surface 214 of
component 210 is cooled in a controlled manner so as to maintain a
reduced tensile strength that minimizes SCC. In other fields, in
which a hardened surface with enhanced tensile strength is desired,
the cooling of the heated component is performed in a manner that
causes very rapid cooling, known as quenching. In contrast, in the
current invention, for steels and steel-like materials, the cooling
of component 210 is controlled, but in a manner that typically
avoids such quenching, and that particularly avoids any quenching
that could ultimately result in hardening of exterior surface 214.
Such quenching could ultimately result in hardening at the surface
if, for example, the heating created temperatures in excess of an
alloy's austenitizing temperature at any location on the exterior
surface. The lower tensile strength that is desired for external
surface 214 and achieved during the heating stage can be maintained
during cooling by employing any of several methods of control over
the cooling rate. One such method of controlled cooling involves
leaving induction heater 220 (or alternative heating apparatus) in
its operational position with respect to component 210, but
adjusting the power of the induction current 230 in a manner that
causes the temperature in external surface 214 to decrease along a
desired thermal profile. This approach can also be used to hold the
temperature of external surface 214 at temperatures intermediate
between the heat treatment temperature and room temperature for
some or all of the cooling time. In addition or in the alternative,
all or a portion of the cooling can take the form of "air cooling"
in which cooling of component 210 occurs without use of induction
heater 220 or alternative heating apparatus. In addition or in the
alternative, the frequency and power of the induction current can
be decreased during cooling to control more precisely the thermal
profile of the entire component system (exterior surface and
relatively harder interior) during cooling. These methods of
control over cooling rate can be used individually or in
conjunction with one another to reduce risk of introducing
undesired residual stresses in the component.
[0022] Measurements of hardness (or microhardness) can be taken as
an approximation of tensile strength to determine whether the
desired tensile strength has been achieved. It should be understood
that in some embodiments the heat treatment could lower the tensile
strength in portions of component 210 other than surface 214, such
as by conduction. In these embodiments, a majority of the heat
treatment would occur at surface 214, with a relatively much small
amount occurring in interior metallic layer. As such, in one
embodiment, pre-heating stage 200 component 210 begins with a
tensile strength throughout that is stronger than the desired final
tensile strength for the interior metallic layer. That way, after
treatment that lowers strength throughout (more so at the surface)
the result is a structural layer having a tensile strength that is
weaker than it started, but satisfies final tensile strength
requirements.
[0023] Turning now to FIG. 4, there is illustrated a perspective
view 300 of a component 310 that has been treated to minimize SCC
according to an embodiment of the invention. As illustrated,
component 310 has an interior metallic layer 316 (e.g., the
skeleton or structural layer) and an external metallic layer 314
(e.g., the skin or treated layer). Interior metallic layer 316 of
component 310 has an tensile strength that is relatively high. This
interior metallic layer 316 is generally a large percentage of the
entire component 310. Exterior metallic layer 314 of component 310
is composed of a material that is adjacent to and integral with the
interior metallic layer and has a chemical composition that is
homogeneous with that of the interior metallic layer. As shown,
exterior metallic layer 314 substantially forms at least a portion
of outer surface of component 310. Further, exterior metallic layer
314 can have a tensile strength that is lower than interior
metallic layer 316 and minimizes SCC, having been treated by the
heating and cooling process described above. Further, as the depth
of exterior metallic layer 314 is only a small fraction of the
depth of component 310 as a whole, component 310 maintains an
overall strength that is on the same order as an identical
component without a treated exterior layer.
[0024] To this extent, component 310 is adapted to perform better
in the harsh environment of turbine 10 (FIG. 1). Thus, such
components of turbine 10, which can include, but is not limited to
buckets 120, expansion bellows, wheels 18, blades 20, rotor 12,
generator retaining ring and/or coupling components could be
initially created from materials having relatively high tensile
strength and then treated to form exterior metallic layer 314 that
has lower tensile strength and, as such, is resistant to SCC.
Further, the treatment may be focused so that only the specific
areas of component 310 for which SCC is a concern are treated. Such
areas may include, for example, a dovetail pin hole in a bucket 20
into which a pin is inserted, stressed areas of the rotor 14,
and/or the dovetail area of buckets 120 (FIG. 2).
[0025] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the disclosure. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0026] While various embodiments are described herein, it will be
appreciated from the specification that various combinations of
elements, variations or improvements therein may be made by those
skilled in the art, and are within the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from essential scope thereof. Therefore, it is intended
that the invention not be limited to the particular embodiment
disclosed as the best mode contemplated for carrying out this
invention, but that the invention will include all embodiments
falling within the scope of the appended claims.
* * * * *