TREATMENT FOR PREVENTING STRESS CORROSION CRACKING
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.
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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.
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 INVENTIONA 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.
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.
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.
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:
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.
Referring to the drawings,
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.
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.
Referring now to
Certain components within turbine 10 (
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.
Referring now to
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.
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.
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.
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.
Turning now to
To this extent, component 310 is adapted to perform better in the harsh environment of turbine 10 (
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.
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.
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.
Type: Application
Filed: May 5, 2011
Publication Date: Nov 8, 2012
Applicant: GENERAL ELECTRIC COMPANY (Schenectady, NY)
Inventors: Andrew Batton Witney (Schenectady, NY), Robin Carl Schwant (Pattersonville, NY)
Application Number: 13/101,686
International Classification: C21D 1/42 (20060101); C21D 9/00 (20060101);