Title of Invention	"A METHOD FOR IMPROVING FATIGUE AND STRESS CORROSION PROPERTIES OF LEADING EDGE OF STEAM TURBINE BLADES"
Abstract	The invention relates to a method for improving fatigue and stress corrosion properties of a surface layer of a leading edge (4) of a steam turbine blade (8) comprising the steps of: -flame hardening of the leading edge of said steam turbine blade component in a temperature range of 900 to 1100°C. -determining an optimum parameters of a shot peening operation through shot penning of a block sample of said flame hard en en ed surface of the said blade component by using spherical balls of different shot-sizes, peening pressures, and stand off distances; -shot peening of said flame hardened surface of the blade component using spherical steel balls based on said determined optimum peening parameters; -removing the formed material layer on said blade component by a layer through electropolishing; and -measuring a compressive residual stress of said layer by using an X-ray diffraction of said electropolished layer, Characterized in that compressive residual stress of the said layer is maintained atleast 0.5mm below the surface of the blade by shot peening with the said shots of diameter 1.00mm to attain improved fatigue and corrosion properties of the said leading edge of steam turbine blade.

Title of Invention

"A METHOD FOR IMPROVING FATIGUE AND STRESS CORROSION PROPERTIES OF LEADING EDGE OF STEAM TURBINE BLADES"

Abstract

The invention relates to a method for improving fatigue and stress corrosion properties of a surface layer of a leading edge (4) of a steam turbine blade (8) comprising the steps of: -flame hardening of the leading edge of said steam turbine blade component in a temperature range of 900 to 1100°C. -determining an optimum parameters of a shot peening operation through shot penning of a block sample of said flame hard en en ed surface of the said blade component by using spherical balls of different shot-sizes, peening pressures, and stand off distances; -shot peening of said flame hardened surface of the blade component using spherical steel balls based on said determined optimum peening parameters; -removing the formed material layer on said blade component by a layer through electropolishing; and -measuring a compressive residual stress of said layer by using an X-ray diffraction of said electropolished layer, Characterized in that compressive residual stress of the said layer is maintained atleast 0.5mm below the surface of the blade by shot peening with the said shots of diameter 1.00mm to attain improved fatigue and corrosion properties of the said leading edge of steam turbine blade.

Full Text	FIELD OF INVENTION The invention relates to a method for life improvement of low-pressure steam turbine blades. The invention, in particular, relates to a method for improving fatigue and stress corrosion properties of steam turbine blades. BACKGROUND OF THE INVENTION The existing turbine, plant of a thermal power plant, normally has a high pressure turbine, a medium pressure turbine, a low pressure turbine, a steam condenser, a feed water pump, and a boiler. In this system, dry steam coming out from the high pressure turbine, is reheated by a r eh eater and enters the low-pressure turbine through the medium-pressure turbine. The pressure and temperature of this steam decreases as the steam expands. Consequently, the steam at a turbine blade of the last stage of the low-pressure turbine is normally wet. In the low pressure turbine region the zone of condensation shifts as per load condition and gives rise to deposit coated and deposit free areas on the surface of the turbine blades. In the presence of deposits on blades, the water droplets are not able to dissolve the precipitated salt completely but just moistens them resulting in deposits with high salt concentration. These are the starting pints for corrosion related failures. Also leading edge of the blade in this zone undergoes water droplet erosion. For, the best understanding of the invention the leading edge as referred herein is the edge from where steam enters the blade and trailing is the edge from where the steam goes out. As a countermeasure for prevention of this erosion, a strip of hard material such as steiiite is welded to the turbine blade as explained as explained in Japanese patent Nos. SHO-57-28805, dated 16-02-1982 and SHO- 54-101741, dated 10-08-1979. However, number of instances have been noticed wherein the cracks have propagated into the blade material through cracks initiated from these welded strips. Alternatively turbine manufactures have also adopted flame hardening of the leading edge portion of the blade for preventing water droplet erosion. However failures have been reported where cracks initiated from the transition zone of the flame hardened and unhardened areas of the blades. It is therefore an object of the present invention to propose a method for preventing water droplet erosion of the leading edge portion of a steam turbine blade. Another object of the invention is to propose a method for minimizing failures of the turbine blades due to the cracks developed initially at the transition zone and further propagating into the blade material. Yet another object of the invention is to propose a method for introducing compressive stresses on the surface of erosion protective layer provided at the leading edge of a turbine blade. A further object of the invention is to propose a method for providing a surface of erosion protective layer at the leading edge of a turbine blade which derive the advantages of flame-hardening simultaneously preventing development and propagation of cracks from the transition zone. Yet a further object of the invention is to propose a method for improvement of fatigue and stress corrosion properties of steam turbine and thereby improving the life of low-pressure steam turbine blades. These and other objects of the invention will be apparent from the ensuing description of a preferred embodiment with reference to the accompanying drawings. At the outset of the description which follows, it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the invention is not intended to be taken restrictively. SUMMARY OF THE INVENTION Thus, according to this invention there is provided a method for improving fatigue and stress corrosion properties for the surface layer of the leading edge of a steam turbine blade comprising the steps of flame hardening of a turbine blade component in a temperature range of 900° to 1100°C; determining optimum parameters of a shot-peening activity through shot-penning of a block-sample of said flame-hardened surface of the blade component by using spherical balls of different shot-sizes, peening pressures, and stand-off distances: shot peening of the flame-hardened surface of the actual blade component using spherical balls based on said determined optimum peening parameters; removing the material layer of said blade component by a layer through electropolishing; and measuring the compressive residual stress of said layer by using an x-ray diffraction of said electropolished layer, a diameter of said shots used for peening being 1.00 mm, and a depth of said compressive residual stress of said layer produced being 0.5 mm and above. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS: Figure 1 is the line diagram of a steam turbine showing various components in the cycle. Figure 2 and 3 show from different angles the flame hardening operation on the leading edge of the turbine blade. Fig 4, 5 & 6 show fatigue and corrosion properties of the shot peened material. In fig 1, dry steam coming out from a high pressure turbine (1) is reheated by a reheater (7) and the steam enters a low pressure turbine (3) through a medium pressure turbine (2). A steam condenser (4), a feed water pump (5), and a boiler (6) being provided in the system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Martensitic stainless steels having 12% Cr are excellent choice of blade material due to their good corrosion resistance and moderately high strength. These steels are however, susceptible to environmentally assisted cracking in chloride containing media. Also in the last stages of low pressure turbine, due to beginning of condensation, water droplet erosion takes place. Several surface treatments have been employed by the industries to improve fatigue properties of the material. The common ground of different technology used is to change the fatigue strength by means of the modifications of both the microstructure and the residual stress field in the surface layer of the material. Some of the most commonly techniques employed are;- i) Flame Hardening; The region most susceptible for damage is flame hardened thus making the surface hard. ii) Steiiite strip brazing: in this method stellite strip is brazed to the area most susceptible to damage. This strip gets eroded during service and is replaced after a stipulated time thus preventing damage to the blade. However a number of failures have been reported wherein the cracks have initiated from transition zone in cases of flame hardened blades and from stellite strips in other cases thus defeating the purpose for which they are intended. In this method, the blade surface is flame hardened at 900°-11000C followed by shot peening treatment. Prior to shot peening of the actual component, block samples are taken and shot peened using different shot sizes, peening pressure, stand off distance and arrived at optimum peening parameters with respect to depth of peening without sacrificing the surface finish. Residual stress is measured using X-ray diffraction after removing the material layer by layer through electropolishing. Fig 2 and 3 show flame-hardening treatment in the range of Li length from the tip of blade (8), representing the protective layer after flame hardening. This is then followed by shot peening. The conditions for the flame hardening and shot peening can be considered mainly on the basis of corrosion resistance of the material due to the condensate and these may be decided from the view point of minimum sensitivity of stress corrosion crack. Precisely, ideal conditions include flame hardening treatment at 900-1100°C in the range of L1 length of the blade (8) from the steam inlet side's tip, followed by shot peening under the conditions shown in the under mentioned table, on the blade (8). The compressive stress is maintained not only at the surface but at least 0.5mm below the surface of the blade (8) subjected to shot peening treatment. The comparative study on fatigue as well as corrosion properties is carried out in 3,5% NaCI. (Table Removed) Shot peening conditions which have been shown in the table are the optimum ones for producing a compressive residua! sfress layer of sufficient depth (>0.5mm) without affecting the surface finish of the blade. Document SHO-56-33432 dated 03.04.1981 also speaks about shot peening after flame hardening in order to improve corrosion properties however the beneficial effects in this case would not last long as the teaching suggested use of only 06mm to 08mm diameter shots resulting in shallow depths of the beneficial compressive residual stress. In the present invention the depth of compressive stresses is found to be more than 0.50mm and studies have been carried out on influence of pitting corrosion on fatigue and corrosion fatigue properties. The size of the pits introduced were of the order of 0.25mm. In case of document SHO-56-33432 the thickness of compressive residual stress zone would be of the same order as the size of the pit studied in the present invention and it would be easy for a fatigue crack to initiate from the pit which is as deep as the residual stress zone. Figure 4,5 and 6 clearly show improvement of fatigue and corrosion properties. WE CLAIM; 1. A method for improving fatigue and stress corrosion properties of a surface layer of a leading edge (4) of a steam turbine blade (8) comprising the steps of : -flame hardening of the leading edge of said steam turbine blade component in a temperature range of 900 to 1100°C. -determining an optimum parameters of a shot peening operation through shot penning of a block sample of said flame hardenened surface of the said blade component by using spherical balls of different shot-sizes, peening pressures, and stand off distances; -shot peening of said flame hardened surface of the blade component using spherical steel balls based on said determined optimum peening parameters; -removing the formed material layer on said blade component by a layer through electropolishing; and -measuring a compressive residual stress of said layer by using an X-ray diffraction of said eiectropolished layer, Characterized in that compressive residual stress of the said layer is maintained atleast 0.5mm below the surface of the blade by shot peening with the said shots of diameter 1.00mm to attain improved fatigue and corrosion properties of the said leading edge of steam turbine blade. 2. The method as claimed in claim 1, wherein said step of flame hardening comprises flame hardening from the tip of the blade back upto a predetermined length (L,) along the leading edge of said turbine blade L1 being the length of one third of the total blade length from the trio of the blade. 3.The method as claimed in claim 1, wherein said step of shot peening comprises maintaining said flame hardened surface of said blade component at a stand off distance between 135 to 145mm, and selection of and air-pressure of 3.8 kg/mm2 to 4.5kg/mm2 , The method as claimed in claim 1, wherein a material of said blade component selected is preferably marten sitic stainless steel having 10 to 15% chromium.

Full Text

FIELD OF INVENTION
The invention relates to a method for life improvement of low-pressure steam turbine blades. The invention, in particular, relates to a method for improving fatigue and stress corrosion properties of steam turbine blades.
BACKGROUND OF THE INVENTION
The existing turbine, plant of a thermal power plant, normally has a high pressure turbine, a medium pressure turbine, a low pressure turbine, a steam condenser, a feed water pump, and a boiler. In this system, dry steam coming out from the high pressure turbine, is reheated by a r eh eater and enters the low-pressure turbine through the medium-pressure turbine. The pressure and temperature of this steam decreases as the steam expands. Consequently, the steam at a turbine blade of the last stage of the low-pressure turbine is normally wet. In the low pressure turbine region the zone of condensation shifts as per load condition and gives rise to deposit coated and deposit free areas on the surface of the turbine blades. In the presence of deposits on blades, the water droplets are not able to dissolve the precipitated salt completely but just moistens them resulting in deposits with high salt concentration. These are the starting pints for corrosion related failures. Also leading edge of the blade in this zone undergoes water droplet erosion. For, the best understanding of the invention the leading edge as referred herein is the edge from where steam enters the blade and trailing is the edge from where the steam goes out.
As a countermeasure for prevention of this erosion, a strip of hard material such as steiiite is welded to the turbine blade as explained as explained in Japanese patent Nos. SHO-57-28805, dated 16-02-1982 and SHO- 54-101741, dated 10-08-1979. However, number of instances have been noticed wherein the cracks have propagated into the blade material through cracks initiated from these welded strips. Alternatively turbine manufactures have also adopted flame hardening of the leading edge portion of the blade for preventing water droplet erosion. However failures have been reported where cracks initiated from the transition zone of the flame hardened and unhardened areas of the blades.
It is therefore an object of the present invention to propose a method for preventing water droplet erosion of the leading edge portion of a steam turbine blade.
Another object of the invention is to propose a method for minimizing failures of the turbine blades due to the cracks developed initially at the transition zone and further propagating into the blade material. Yet another object of the invention is to propose a method for introducing compressive stresses on the surface of erosion protective layer provided at the leading edge of a turbine blade.
A further object of the invention is to propose a method for providing a surface of erosion protective layer at the leading edge of a turbine blade which derive the advantages of flame-hardening simultaneously preventing development and propagation of cracks from the transition zone.
Yet a further object of the invention is to propose a method for improvement of fatigue and stress corrosion properties of steam turbine and thereby improving the life of low-pressure steam turbine blades. These and other objects of the invention will be apparent from the ensuing description of a preferred embodiment with reference to the accompanying drawings.
At the outset of the description which follows, it is to be understood that the ensuing description only illustrates a particular form of this invention. However, such a particular form is only an exemplary embodiment and the teachings of the invention is not intended to be taken restrictively.
SUMMARY OF THE INVENTION
Thus, according to this invention there is provided a method for improving fatigue and stress corrosion properties for the surface layer of the leading edge of a steam turbine blade comprising the steps of flame hardening of a turbine blade component in a temperature range of 900° to 1100°C; determining optimum parameters of a shot-peening activity through shot-penning of a block-sample of said flame-hardened surface of the blade component by using spherical balls of different shot-sizes, peening pressures, and stand-off distances: shot peening of the flame-hardened surface of the actual
blade component using spherical balls based on said determined optimum peening parameters; removing the material layer of said blade component by a layer through electropolishing; and measuring the compressive residual stress of said layer by using an x-ray diffraction of said electropolished layer, a diameter of said shots used for peening being 1.00 mm, and a depth of said compressive residual stress of said layer produced being 0.5 mm and above.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 is the line diagram of a steam turbine showing various components in the cycle. Figure 2 and 3 show from different angles the flame hardening operation on the leading edge of the turbine blade. Fig 4, 5 & 6 show fatigue and corrosion properties of the shot peened material. In fig 1, dry steam coming out from a high pressure turbine (1) is reheated by a reheater (7) and the steam enters a low pressure turbine (3) through a medium pressure turbine (2). A steam condenser (4), a feed water pump (5), and a boiler (6) being provided in the system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
Martensitic stainless steels having 12% Cr are excellent choice of blade material due to their good corrosion resistance and moderately high strength. These steels are however, susceptible to environmentally assisted cracking in chloride containing media. Also in the last stages of low pressure turbine, due to beginning of condensation, water droplet erosion takes place. Several surface treatments have been employed by the industries to improve fatigue properties of the material. The common ground of different technology used is to change the fatigue strength by means of the modifications of both the microstructure and the residual stress field in the surface layer of the material. Some of the most commonly techniques employed are;-
i) Flame Hardening; The region most susceptible for damage is flame hardened thus making the surface hard.
ii) Steiiite strip brazing: in this method stellite strip is brazed to the area most susceptible to damage. This strip gets eroded
during service and is replaced after a stipulated time thus preventing damage to the blade.
However a number of failures have been reported wherein the cracks have initiated from transition zone in cases of flame hardened blades and from stellite strips in other cases thus defeating the purpose for which they are intended.
In this method, the blade surface is flame hardened at 900°-11000C followed by shot peening treatment. Prior to shot peening of the actual component, block samples are taken and shot peened using different shot sizes, peening pressure, stand off distance and arrived at optimum peening parameters with respect to depth of peening without sacrificing the surface finish. Residual stress is measured using X-ray diffraction after removing the material layer by layer through electropolishing.
Fig 2 and 3 show flame-hardening treatment in the range of Li length from the tip of blade (8), representing the protective layer after flame hardening. This is then followed by shot peening.
The conditions for the flame hardening and shot peening can be considered mainly on the basis of corrosion resistance of the material due to the condensate and these may be decided from the view point of minimum sensitivity of stress corrosion crack. Precisely, ideal conditions include flame hardening treatment at 900-1100°C in the range of L1 length of the blade (8) from the steam inlet side's tip, followed by shot peening under the conditions shown in the under mentioned table, on the blade (8). The compressive stress is maintained not only at the surface but at least 0.5mm below the surface of the blade (8) subjected to shot peening treatment. The comparative study on fatigue as well as corrosion properties is carried out in 3,5% NaCI.

(Table Removed)
Shot peening conditions which have been shown in the table are the optimum ones for producing a compressive residua! sfress layer of sufficient depth (>0.5mm) without affecting the surface finish of the blade. Document SHO-56-33432 dated 03.04.1981 also speaks about shot peening after flame hardening in order to improve corrosion properties however the beneficial effects in this case would not last long as the teaching suggested use of only 06mm to 08mm diameter shots resulting in shallow depths of the beneficial compressive residual stress. In the present invention the depth of compressive stresses is found to be more than 0.50mm and studies have been carried out on influence of pitting corrosion on fatigue and corrosion fatigue properties. The size of the pits introduced were of the order of 0.25mm. In case of document SHO-56-33432 the thickness of compressive residual stress zone would be of the same order as the size of the pit studied in the present invention and it would be easy for a fatigue crack to initiate from the pit which is as deep as the residual stress zone.
Figure 4,5 and 6 clearly show improvement of fatigue and corrosion properties.

WE CLAIM;
1. A method for improving fatigue and stress corrosion properties of a
surface layer of a leading edge (4) of a steam turbine blade (8)
comprising the steps of :
-flame hardening of the leading edge of said steam turbine blade
component in a temperature range of 900 to 1100°C.
-determining an optimum parameters of a shot peening operation through
shot penning of a block sample of said flame hardenened surface of the
said blade component by using spherical balls of different shot-sizes,
peening pressures, and stand off distances;
-shot peening of said flame hardened surface of the blade component
using spherical steel balls based on said determined optimum peening
parameters;
-removing the formed material layer on said blade component by a layer
through electropolishing; and
-measuring a compressive residual stress of said layer by using an X-ray
diffraction of said eiectropolished layer,
Characterized in that compressive residual stress of the said layer is
maintained atleast 0.5mm below the surface of the blade by shot peening
with the said shots of diameter 1.00mm to attain improved fatigue and
corrosion properties of the said leading edge of steam turbine blade.
2. The method as claimed in claim 1, wherein said step of flame
hardening comprises flame hardening from the tip of the blade back
upto a predetermined length (L,) along the leading edge of said
turbine blade L1 being the length of one third of the total blade length
from the trio of the blade.
3.The method as claimed in claim 1, wherein said step of shot peening comprises maintaining said flame hardened surface of said blade component at a stand off distance between 135 to 145mm, and selection of and air-pressure of 3.8 kg/mm2 to 4.5kg/mm2 , The method as claimed in claim 1, wherein a material of said blade
component selected is preferably marten sitic stainless steel having 10
to 15% chromium.

Documents:

1432-del-2003-abstract.pdf

1432-del-2003-claims.pdf

1432-del-2003-correspondence-others.pdf

1432-del-2003-correspondence-po.pdf

1432-del-2003-description (complete).pdf

1432-del-2003-drawings.pdf

1432-del-2003-form-1.pdf

1432-del-2003-form-19.pdf

1432-del-2003-form-2.pdf

1432-del-2003-form-3.pdf

1432-del-2003-gpa.pdf

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Patent Number

216949

Indian Patent Application Number

1432/DEL/2003

PG Journal Number

13/2008

Publication Date

31-Mar-2008

Grant Date

20-Mar-2008

Date of Filing

19-Nov-2003

Name of Patentee

BHARAT HEAVY ELECTRICALS LTD.

Applicant Address

BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	CHIRUMAMILLA RAMAKRISHNA	C/O BHARAT HEAVY ELECTRICALS LTD. BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA.
2	RAWAT MAHIPAL SINGH	C/O BHARAT HEAVY ELECTRICALS LTD. BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA.
3	GOPALAN JAYARAMAN	C/O BHARAT HEAVY ELECTRICALS LTD. BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA.

PCT International Classification Number

f03b 3/18

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA