Title of Invention	A SHOT PEENING PROCESS TO INTRODUCE A COMPRESSIVE STRENGTH OF 300MPa UP TO A DEPTH OF 0.25MM ON A LOW-PRESSURE STAGE BLADE OF STEAM TURBINES
Abstract	The present invention relates to a process to inroduce a predetermined compressive strength of 300 Mpa upto a depth of 0.25mm from the peened surface on fir-tree root blade for low-pressure steam turbine operating at 3000 rpm. This process comprises bombarding a stream of hardened spherical steel shots through an air jet from a robotic controlled nozzle in a closed chamber. The hardened steel shots impinches the surface of the blade root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases the surface hardness as well as introduce compressive strength.

Title of Invention

A SHOT PEENING PROCESS TO INTRODUCE A COMPRESSIVE STRENGTH OF 300MPa UP TO A DEPTH OF 0.25MM ON A LOW-PRESSURE STAGE BLADE OF STEAM TURBINES

Abstract

The present invention relates to a process to inroduce a predetermined compressive strength of 300 Mpa upto a depth of 0.25mm from the peened surface on fir-tree root blade for low-pressure steam turbine operating at 3000 rpm. This process comprises bombarding a stream of hardened spherical steel shots through an air jet from a robotic controlled nozzle in a closed chamber. The hardened steel shots impinches the surface of the blade root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases the surface hardness as well as introduce compressive strength.

Full Text	FIELD OF INVENTION The present invention relates to a process to introduce predetermined compressive strength of 300 MPa upto a depth of 0.25mm from peened surface on Low Pressure (LP) Fir-tree root blades of Steam Turbine operating at 3000 rpm to prevent failure during operation. BACKGROUND OF INVENTION In large capacity thermal power projects, normally 3 turbines namely high pressure (HP), intermediate pressure (IP) and low pressure (LP) exists. In HP and IP turbines blade heights are relatively small because of high pressure and temperature but in LP turbines blade heights are quite large because of low pressure and high specific volume of steam. In HP and IP turbines normally T-roots are used but in LP turbines because of high centrifugal stress, T-roots are insufficient and normally Fir-tree roots are used in moving blades. In HP and IP turbines because of low blade heights and less centrifugal force, chances of failure from root is negligible, however in LP turbines, due to high CF chances of failure from blade root is very high. Fir Tree roots of Steam Turbine blades have complex profile (Figure-1 and 2) on the root which is inserted in the Rotor of a turbine. These blades are subjected to 3000 RPM (50 Cycles per second), hence heavily stress part of turbine. There has been evidence of stress failure. -2- Steam Turbine Blades are critical component and any blade failure may damage the total thermal set. There is a generic problem of cracking in the last stages of fir tree root blades and from the past data it has been concluded that most of the cracks occur in the top serration radius below the aerofoil. This problem is not limited to anyone manufacturer, turbine speed or operating condition and such failures are reported by several countries including US, Canada, UK, South Africa, Australia etc. Siemens, Germany has implemented Roller pressuring in their LP last stage blade to extend the life of blades. The difficulties envisaged to use the same process are:- i. Long manufacturing cycle as it takes at least 4 months to get the blades back after Roller pressing to Siemens, Germany. ii. Paying opportunity cost to Siemens, Germany for Roller Pressing in case of urgent requirement of blades. iii. Additional work of packing each and every blade for re- exporting. iv. Completing paper formalities for excise and customs clearance. v. To and fro transportation of heavy blades to Mumbai. vi. Risk of misplacement/ damage of blades. To prevent such service failure and extend the blade service life, a special emphasis made by a dedicated technical team in BHEL was on the fina out the key points of failure and accordingly searching out various ways and means for remedical measure of such failure. One of such process is to introduce controlled shotpeening on the blade roots to introduce compressive residual stress of 300 MPa up to a depth of 0.25mm on the intersecting of both side top root flank. -3- The last stage low pressure (LP) steam turbine blades are very highly stressed at the root region because of their size and weight. Cracking of blades at the first serration of the root resulting into failure of the blades has caused serious damage to some turbines. One of the main reasons of last stage blade failure is developing of Fretting stress & stress corrosion cracking (SCC). Fretting is caused by small amplitude cyclical movement of two solid bodies in close contact and under pressure. The debris produced may not escape from their point of origin due to high pressure and small movement. Fretting causes shear peak by means of friction between the contact zones. The minute relative displacement of contact surface subsequently causes the rapture, which then oxidizes. The phenomenon of metallic fragment dislocation will accelerate under the influence of oxidized particles. Severe surface damage will soon appear due to the application of extreme stresses of moving parts and added stress due to friction. Fatigue micro-cracks will develop and propagate leading to eventual part rupture. The mode of failure usually observed is initiation of a stress corrosion crack (SCC) at the highly stressed region in the first (top) serration of the root and subsequent crack propagation by corrosion fatigue failure. The environment required for SCC to take place is provided by wet steam in the last stages of steam turbine. To increase fatigue and stress corrosion cracking resistance at the top serration of the root of advanced design LP Last stage blades, an extensive study reveals that a compressive residual stresses of 300 MPa up to a depth of 0.25 mm on top flank could be an effective solution. The above requirement could be achieved by using a sophisticated shot peening equipment. This operation resulted in the introduction of favourable compressive residual stress in top root flank Refer (Figure 5 to 9 ) -4- Shot Peening is widely recognized as a cost effective process to enhance the fatigue characteristics of metal parts. It is generally recognized that one of the reason for improvement on fatigue durability by shot peening is that the surface compression residual stress is increased. Shot peening is also applied to the improvement of stress corrosion cracking. Keeping this in mind it was envisaged that the required depth of compressive residual stress as required in Siemens advanced design LP blades for improving SCC and fatigue properties can also be obtained by controlled shot peening in the first (top) serration of the root without sacrificing other properties such as fretting resistance, surface finish, dimensional accuracy etc. The Shot peening was carried out to introduce compressive stresses, to a predetermined depth, in the Blade root of LP Fir tree to enhances fatigue strength, retard fretting and improve resistance to stress corrosion cracking and corrosion fatigue. The peening process consists of bombarding the exposed surface of fir tree root with a stream of hardened spherical metallic shots, delivered from a robotic controlled nozzle by applied air pressure. OBJECTS OF THE INVENTION It is therefore an object of the present invention to propose a process of to introduce compressive strength of LP Fir-tree blade root by shotpeening which eliminate the disadvantages of prior art. Another object of the present invention is to propose a process to introduce compressive strength of LP Fir-tree blade root by shot peening which increases the service life of the blade. -5- A further object of the present invention is to propose a process to introduce the compressive strength of LP Fir-tree blade root by shot peening which reduces maintenance cost due to failure of blades. A still further object of the present invention is to propose a process to introduce the compressive strength of LP Fir-tree blade root by shot peening which prevents all of a sudden failure of blade. A yet further object of the present invention is to propose a process to introduce compressive strength of LP Fir-tree blade root by shot pinning which is economic compared to other process of remedy. SUMMARY OF THE INVENTION The present invention relates to a process to inroduce a predetermined compressive strength of 300 Mpa upto a depth of 0.25mm from the peened surface on fir-tree root blade for low-pressure steam turbine operating at 3000 rpm. This process comprises bombarding a stream of hardened spherical steel shots through an air jet from a robotic controlled nozzle in a closed chamber. The hardened steel shots impinches the surface of the blade root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases the surface hardness as well as introduce compressive strength. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING Fig-l(A) : shows the side view of blade which is subjected to shot peening at root.. Fig-l(B) : shows the front view of the blade. -6- Fig-2(A) : shows the Fir-tree blade root. Fig-2(B) : shows the Fir-tree blade groove. Fig-3 : shows the nozzle angle of the robot with respect to the top flank of the blade root. The orientation of the nozzle should be perfectly normal to the plane of attack for best result. Fig-4 : shows the General arrangement of shot peening equipment displaying major parts/items. Fig-5,6,7&8 : shows the variation of compressive residual stress with depth. Fig-9 : shows the depth profile of compressive residual stress. Table: -1 : shows residual measurement on shot peened samples for depth profile. DETAILED DESCRIPTION OF A DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig-1 shows the side view and the front view of the Fir-tree blade (1) and the Fir tree blade root (2) which is inserted in a rotor groove for firmly holding the blade. During the operating condition of turbine, a small crack generates from the crack prone zone (3). To protect such failure, the present invention emphasize to insert some compressive strength in the crack prone zone by impinching shots through robotic nozzle with a high pressure air jet having a pressure 3kg/m2. The blades shall be shot peened after complete root machining, flame hardening and stress relieving operations. Prior to shot peening, the blades are dried, free from grease, oil, soiling, corrosion and paint coating. All the edges shall be deburred and filleted as per the drawing specifications. The steel shots are injected through a nozzle having a diameter 8mm and impinched shots generates compressive strength on the surface 300 MPa upto a depth of 0.25mm as shown in Fig 5to 9. -7- Fig-3 - shows central axis of root curvature along which the nozzle rotates from right to left direction and the blade rotates in left to right direction. The shots impinches the surface 8& 9 and the nozzle direction acts according to direction 11 i.e. to the center of the root curve to achieve the peening intensity range 0.35 to 0.35 Almen A with 125% to 200% coverages. RESULT Table 1 shows test result of the two samples. KEY FEATURES Reference Numerals Character Figure 1 Working profile of Blade 1(A) 2 Fir tree root 1(A) 3 Crack proon zone 1(A) 4 Rotor groove 2(B) 5 Center Axis of Root Curvature 3 6 Nozzle direction outlet 3 7 Nozzle direction inlet 3 8 Pressure side 3 9 Suction side 3 10 Smooth passage 3 11 Nozzle Direction 3 -8- WE CLAIM 1. A process to introduce predetermined compressive strength of 300 N/m2 up to a depth of 0.25mm on LP Fir-tree blade root by shot peening to prevent failure. Comprising:- - a peening of shots having size S-330 by an air jet of 3kg/m2 on the surface of the blade root from a distance 200mm for a duration of 40- 50 sec time; - the hardened spherical metallic shots delivered from a robotic controlled nozzle through an air jet in a closed chamber. - the peening of shots takes place from right to left direction motion while the blade roots rotates left to right direction and vice-versa. - the flow of shots per cycle during peening is 3.5 to 4.0 kg -9- The present invention relates to a process to inroduce a predetermined compressive strength of 300 Mpa upto a depth of 0.25mm from the peened surface on fir-tree root blade for low-pressure steam turbine operating at 3000 rpm. This process comprises bombarding a stream of hardened spherical steel shots through an air jet from a robotic controlled nozzle in a closed chamber. The hardened steel shots impinches the surface of the blade root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases the surface hardness as well as introduce compressive strength.

Full Text

FIELD OF INVENTION
The present invention relates to a process to introduce predetermined
compressive strength of 300 MPa upto a depth of 0.25mm from peened
surface on Low Pressure (LP) Fir-tree root blades of Steam Turbine operating
at 3000 rpm to prevent failure during operation.
BACKGROUND OF INVENTION
In large capacity thermal power projects, normally 3 turbines namely high
pressure (HP), intermediate pressure (IP) and low pressure (LP) exists. In HP
and IP turbines blade heights are relatively small because of high pressure
and temperature but in LP turbines blade heights are quite large because of
low pressure and high specific volume of steam. In HP and IP turbines
normally T-roots are used but in LP turbines because of high centrifugal
stress, T-roots are insufficient and normally Fir-tree roots are used in moving
blades. In HP and IP turbines because of low blade heights and less
centrifugal force, chances of failure from root is negligible, however in LP
turbines, due to high CF chances of failure from blade root is very high.
Fir Tree roots of Steam Turbine blades have complex profile (Figure-1 and 2)
on the root which is inserted in the Rotor of a turbine. These blades are
subjected to 3000 RPM (50 Cycles per second), hence heavily stress part of
turbine. There has been evidence of stress failure.
-2-

Steam Turbine Blades are critical component and any blade failure may
damage the total thermal set. There is a generic problem of cracking in the
last stages of fir tree root blades and from the past data it has been
concluded that most of the cracks occur in the top serration radius below the
aerofoil. This problem is not limited to anyone manufacturer, turbine speed or
operating condition and such failures are reported by several countries
including US, Canada, UK, South Africa, Australia etc.
Siemens, Germany has implemented Roller pressuring in their LP last stage
blade to extend the life of blades.
The difficulties envisaged to use the same process are:-
i. Long manufacturing cycle as it takes at least 4 months to get
the blades back after Roller pressing to Siemens, Germany.
ii. Paying opportunity cost to Siemens, Germany for Roller
Pressing in case of urgent requirement of blades.
iii. Additional work of packing each and every blade for re-
exporting.
iv. Completing paper formalities for excise and customs clearance.
v. To and fro transportation of heavy blades to Mumbai.
vi. Risk of misplacement/ damage of blades.
To prevent such service failure and extend the blade service life, a special
emphasis made by a dedicated technical team in BHEL was on the fina out
the key points of failure and accordingly searching out various ways and
means for remedical measure of such failure. One of such process is to
introduce controlled shotpeening on the blade roots to introduce compressive
residual stress of 300 MPa up to a depth of 0.25mm on the intersecting of
both side top root flank.
-3-

The last stage low pressure (LP) steam turbine blades are very highly stressed
at the root region because of their size and weight. Cracking of blades at the
first serration of the root resulting into failure of the blades has caused
serious damage to some turbines. One of the main reasons of last stage blade
failure is developing of Fretting stress & stress corrosion cracking (SCC).
Fretting is caused by small amplitude cyclical movement of two solid bodies in
close contact and under pressure. The debris produced may not escape from
their point of origin due to high pressure and small movement. Fretting
causes shear peak by means of friction between the contact zones. The
minute relative displacement of contact surface subsequently causes the
rapture, which then oxidizes. The phenomenon of metallic fragment
dislocation will accelerate under the influence of oxidized particles. Severe
surface damage will soon appear due to the application of extreme stresses of
moving parts and added stress due to friction. Fatigue micro-cracks will
develop and propagate leading to eventual part rupture. The mode of failure
usually observed is initiation of a stress corrosion crack (SCC) at the highly
stressed region in the first (top) serration of the root and subsequent crack
propagation by corrosion fatigue failure. The environment required for SCC to
take place is provided by wet steam in the last stages of steam turbine.
To increase fatigue and stress corrosion cracking resistance at the top
serration of the root of advanced design LP Last stage blades, an extensive
study reveals that a compressive residual stresses of 300 MPa up to a depth
of 0.25 mm on top flank could be an effective solution. The above
requirement could be achieved by using a sophisticated shot peening
equipment. This operation resulted in the introduction of favourable
compressive residual stress in top root flank Refer (Figure 5 to 9 )
-4-

Shot Peening is widely recognized as a cost effective process to enhance the
fatigue characteristics of metal parts. It is generally recognized that one of
the reason for improvement on fatigue durability by shot peening is that the
surface compression residual stress is increased. Shot peening is also applied
to the improvement of stress corrosion cracking.
Keeping this in mind it was envisaged that the required depth of compressive
residual stress as required in Siemens advanced design LP blades for
improving SCC and fatigue properties can also be obtained by controlled shot
peening in the first (top) serration of the root without sacrificing other
properties such as fretting resistance, surface finish, dimensional accuracy
etc.
The Shot peening was carried out to introduce compressive stresses, to a
predetermined depth, in the Blade root of LP Fir tree to enhances fatigue
strength, retard fretting and improve resistance to stress corrosion cracking
and corrosion fatigue. The peening process consists of bombarding the
exposed surface of fir tree root with a stream of hardened spherical metallic
shots, delivered from a robotic controlled nozzle by applied air pressure.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process of to
introduce compressive strength of LP Fir-tree blade root by shotpeening
which eliminate the disadvantages of prior art.
Another object of the present invention is to propose a process to introduce
compressive strength of LP Fir-tree blade root by shot peening which
increases the service life of the blade.
-5-

A further object of the present invention is to propose a process to introduce
the compressive strength of LP Fir-tree blade root by shot peening which
reduces maintenance cost due to failure of blades.
A still further object of the present invention is to propose a process to
introduce the compressive strength of LP Fir-tree blade root by shot peening
which prevents all of a sudden failure of blade.
A yet further object of the present invention is to propose a process to
introduce compressive strength of LP Fir-tree blade root by shot pinning
which is economic compared to other process of remedy.
SUMMARY OF THE INVENTION
The present invention relates to a process to inroduce a predetermined
compressive strength of 300 Mpa upto a depth of 0.25mm from the peened
surface on fir-tree root blade for low-pressure steam turbine operating at
3000 rpm. This process comprises bombarding a stream of hardened
spherical steel shots through an air jet from a robotic controlled nozzle in a
closed chamber. The hardened steel shots impinches the surface of the blade
root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases
the surface hardness as well as introduce compressive strength.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig-l(A) : shows the side view of blade which is subjected to shot peening
at root..
Fig-l(B) : shows the front view of the blade.
-6-

Fig-2(A) : shows the Fir-tree blade root.
Fig-2(B) : shows the Fir-tree blade groove.
Fig-3 : shows the nozzle angle of the robot with respect to the top flank
of the blade root. The orientation of the nozzle should be
perfectly normal to the plane of attack for best result.
Fig-4 : shows the General arrangement of shot peening equipment
displaying major parts/items.
Fig-5,6,7&8 : shows the variation of compressive residual stress with depth.
Fig-9 : shows the depth profile of compressive residual stress.
Table: -1 : shows residual measurement on shot peened samples for depth
profile.
DETAILED DESCRIPTION OF A DESCRIPTION OF A PREFERRED
EMBODIMENT OF THE INVENTION
Fig-1 shows the side view and the front view of the Fir-tree blade (1) and the
Fir tree blade root (2) which is inserted in a rotor groove for firmly holding the
blade. During the operating condition of turbine, a small crack generates from
the crack prone zone (3). To protect such failure, the present invention
emphasize to insert some compressive strength in the crack prone zone by
impinching shots through robotic nozzle with a high pressure air jet having a
pressure 3kg/m2. The blades shall be shot peened after complete root
machining, flame hardening and stress relieving operations. Prior to shot
peening, the blades are dried, free from grease, oil, soiling, corrosion and
paint coating. All the edges shall be deburred and filleted as per the drawing
specifications. The steel shots are injected through a nozzle having a diameter
8mm and impinched shots generates compressive strength on the surface
300 MPa upto a depth of 0.25mm as shown in Fig 5to 9.
-7-

Fig-3 - shows central axis of root curvature along which the nozzle rotates
from right to left direction and the blade rotates in left to right direction. The
shots impinches the surface 8& 9 and the nozzle direction acts according to
direction 11 i.e. to the center of the root curve to achieve the peening
intensity range 0.35 to 0.35 Almen A with 125% to 200% coverages.
RESULT
Table 1 shows test result of the two samples.
KEY FEATURES
Reference Numerals Character Figure
1 Working profile of Blade 1(A)
2 Fir tree root 1(A)
3 Crack proon zone 1(A)
4 Rotor groove 2(B)
5 Center Axis of Root Curvature 3
6 Nozzle direction outlet 3
7 Nozzle direction inlet 3
8 Pressure side 3
9 Suction side 3
10 Smooth passage 3
11 Nozzle Direction 3
-8-

WE CLAIM
1. A process to introduce predetermined compressive strength of 300 N/m2
up to a depth of 0.25mm on LP Fir-tree blade root by shot peening to
prevent failure.
Comprising:-
- a peening of shots having size S-330 by an air jet of 3kg/m2 on the
surface of the blade root from a distance 200mm for a duration of 40-
50 sec time;
- the hardened spherical metallic shots delivered from a robotic
controlled nozzle through an air jet in a closed chamber.
- the peening of shots takes place from right to left direction motion
while the blade roots rotates left to right direction and vice-versa.
- the flow of shots per cycle during peening is 3.5 to 4.0 kg
-9-

The present invention relates to a process to inroduce a predetermined
compressive strength of 300 Mpa upto a depth of 0.25mm from the peened
surface on fir-tree root blade for low-pressure steam turbine operating at
3000 rpm. This process comprises bombarding a stream of hardened
spherical steel shots through an air jet from a robotic controlled nozzle in a
closed chamber. The hardened steel shots impinches the surface of the blade
root for duration of 40-50 sec with an airjet of pressure 3 kg/m2 and increases
the surface hardness as well as introduce compressive strength.

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Patent Number

272187

Indian Patent Application Number

558/KOL/2008

PG Journal Number

14/2016

Publication Date

01-Apr-2016

Grant Date

21-Mar-2016

Date of Filing

20-Mar-2008

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

REGIONAL OPERATIONS DIVISION (ROD), PLOT NO: 9/1 DJBLOCK, 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA-700091 BHEL HOUSE, SIRI FORT, NEW DELHI- 110049

Inventors:

#	Inventor's Name	Inventor's Address
1	T.K. GHOSH	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403
2	C.R.K. PRASAD	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403
3	S.K. GUPTA	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403
4	B. DAS	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403
5	A. MOOKHERJEE	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403
6	H.P. CHOUDHURY	BHARAT HEAVY ELECTRICALS LTD HEAVY ELECTRICAL EQUIPMENT PLANT, RAINIPUR, HARIDWAR-249 403

PCT International Classification Number

F01D5/14; F01D5/32; F01D5

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA