The Role of Residual Stresses with Various Shot Peening Parameters on 12%Cr Blading Steel for LP Turbine Applications

(1)

ISSN (Print): 2321-5747, Volume-1, Issue-1, 2013

90

on 12%Cr Blading Steel for LP Turbine Applications

S. Srilakshmi¹, D. V. Vidyasagar² & S. Devaki Rani³

1&3Metallurgy Department, JNTUH

2Metallurgy Department, Corporate Research and Development Division Bharat Heavy Electricals Limited, Vikasnagar, Hyderabad-500093, India E-mail : [email protected]¹, vidysagar@[email protected]², [email protected]³

Abstract – In fossil fuel power plant, 12% Cr steel is extensively used as blading material for LP turbine component. These steels undergo damage processes like corrosion and fatigue. Fatigue is the main damage process for the failure of blades. Therefore, in order to increase the fatigue life, compressive stresses are induced on surface using shot peening process. In this paper, various shot peened samples with different parameters are co related with the residual stress measurement on the surface.

Residual stress is measured using X-ray Diffraction (XRD) technique. Further, residual stress measurements along the depth profile is also obtained, which are correlated with the shot peening parameters. The developed correlations can be used in assessing the residual life of the 12%Cr steel blades for different shot peening parameters.

Keywords – 12%Cr bladingsteel, Residual Stress, XRD, Shot peening.

I. INTRODUCTION

Thermal Power Plant materials play an important role in our life. In which a coal based Thermal Power Plant will convert the chemical energy into electrical energy. There are varieties of material which will give us power consumption in the thermal systems.

The above materials are extensively used in generator; boiler feed pump shafts, steam turbine blades, control value steam etc. The material of the equipment will depend as a function of service life due to the damage mechanism such as creep, fatigue, erosion, wear etc on many operating conditions. Due to these operating conditions the components may prematurely fail such as fatigue, environment, improper material processing, residual stresses, error in design etc. To ensure the satisfactory performance of the components during the projected service life of the component their effectiveness and failure mechanism have to be understood so that we can prevent or control their manifestation.

Steam turbine blades components are susceptible to premature failure that has a serious economics and safety implication. These steam turbine blades are used for different environment and for different processing conditions. In which they have a particular interest to power industry and have a more importance in the present study. Blading steel is a Martensitic stainless steel. These steels are used in LP turbine blades due to their good corrosion resistance and moderately high strength. Failure statistics of these steels occur in the last stage of the region because of the environmental conditions, loading patterns, vibrations, high temperatures, corrosion, erosion etc. residual life assessment plays a very important role in increasing the life of existing power plant. Among these the major causes of failure point of view is due to fatigue. Failure occurring under condition of dynamic loading is called fatigue failure.

These Blading steels are steam and gas turbines blades which have much susceptible to failure by cracking due to vibrations, high temperatures, corrosion, erosion etc. Fatigue failure initiation takes place on the surface in the form of a crack and even tensile stress also takes place on the surface and which is the major responsible for this initiation to occur on the component.

So the presence of tensile stress in the component is the important fatigue failure on the surface which is unfavourable for the component. So to measure their residual stress there are so many methods can be used.

Among the various methods, X-Ray Diffraction method is the best method and gives accurate reading for measuring the residual stresses on the material.

Determining the suitable measurement parameters, residual stress measurement by X-Ray diffraction helps in various surface hardening process, shot peening, laser peening which are used to deliberately induce compressive stresses in the component which will enhances its fatigue life.

(2)

91 In the present study, Blading steel materials contain 12% Cr and as a microstructure of Martenstic Stainless steel. This material is used in thermal power plant components for making turbines, rotors etc. Fatigue is the main problem in which most of these materials will fail at a point of view. Fatigue occurs when a material is subjected to a repeated loading and unloading. The shapes of the structure will significantly affect the fatigue life. Fatigue life is influenced by a variety of factors such as temperatures, surface finish, microstructure presence of oxidizing or inlet chemicals, residual stresses etc. For some materials there is a theoretical values for stress amplitude below which it will not fail for any number of cycles called fatigue limit or fatigue strength. In fatigue the material will have two types of cycles i.e. High cycle fatigue and low cycle fatigue. We will go with the low cycle fatigue in our method. To improve fatigue resistance we should try to avoid tensile means and have compressive means stresses. To enhance the fatigue resistance of the materials there are many methods of inducing residual stresses. In which they will increase the life of the material of various methods. The various methods are as follows: Mechanical Method, Thermal Method, Plating, Machining etc. For measuring the residual stresses on the material there are many methods such as Hole- drilling, ultrasonic, magnetic method, X-ray diffraction, electron diffraction etc.

X-Ray Diffraction method is chosen for measuring the residual stresses. The residual stresses on the material are measured with the instrument XSTRESS 3000 equipment which is manufactured by Stress-tech Group, Finland. XSTRESS 3000 is an advanced X-Ray stress analyzer that enables converting X-rays into electrical signal. It has been built into a single compact, portable case and safety interlocks. This system has been designed and constructed to meet all the industrial safety requirements for radiation equipment. There are various measuring specification units like measurement unit, control unit, options unit. In measuring unit we will have detectors, X-ray tubes, collimators, Goniometer. In control unit we will have X-ray supply, cooling, computer, cabinet, safety. As in optional unit will have calibrations and measuring options. This method works on the principle of Bragg’s Law.

Shot peening has been chosen for improving the life of the component. Shot peening is carried out in the equipment MECSHOT; Pune. Shot Peening is a cold working process used to produce a compressive residual stresses layers and modified mechanical properties of metals. It entails impacting a surface with a shot with a force sufficient to create plastic deformation. Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. The work done on

the surface mainly depends on a number of factors. Size and material of the spherical shot is important, as is its velocity and the rate and angle at which the blast pattern sweeps across the surface. The relative work done to the surface is called the ‘Peening intensity’. Obviously it is impractical to count and weigh the particles and measure their velocity, so a simpler comparative method has been device to measure peening intensity. As there are many variables in the shot peening method such as shot size, nozzle diameter, velocity, intensity, flow rate, coverage etc. each parameter will have their importance.

The shot size, short hardness, intensity and coverage are the four important variables in this process. The main advantage of this process is it will increase the resistance to fatigue, corrosion; hydrogen assisted cracking increased strength, increased durability. Depth of profile is measured by electro-polishing. This electro polishing is measured by the equipment Struers- Movipal-3. We have chosen this method because of its simple and easily process. With this electro polishing we can measure the residual stresses by X-Ray diffraction method. The gages depth is measured with Digimatic depth gages with Absolute encoder Series 547-217S.

This is a digital readout for error free reading. The fatigue resistance of the material is improved by inducing surface compressive stresses, and in this study the effect of variables on stress in terms of depth is investigated.

II. EXPERIMENTAL PROCEDURE The material and the experimental procedures used in the present work are discussed in the present chapter.

Material Used:

The material for this investigation was taken as 12%Cr Blading steel, widely used in low pressure of steam turbine blades. This type of materials is used in large number of thermal plants in India. The chemical composition was determined with the help of portable alloy analyser XMET 3000 and mechanical properties of the sample are taken by the Tensile tests were carried out using INSTRON servo hydraulic test system as per ASTM-8, Charpy V – notch impact test performed on a 358 Joules Tinius Olsen, USA make machine as per the E-23 standard, hardness test by using Brinell Hardness test:

Procedure:

The experiment consists of basic steps sample preparation, calibration of the equipment and measurement.

Sample Preparation:

The material used was in the form of bar of size 80*20*10mm. The materials were taken for specimen

(3)

92 preparation for carrying out various tests and the processing was done to introduce residual stress for further study.

The sample on which the residual stress measurement is carried out is measured by X-Stress 3000. Residual stresses were measured on the sample using x-ray diffraction technique. Care was taken to ensure that no additional stresses are induced in the sample during the preparation.

Material:

The material under this test is a shot peened hardened blading steel sample. The material was processed mainly to introduce compressive residual stresses for carrying out the test. To study the fatigue properties that will induce the compressive stresses.

Shot Peening:

To induce the surface compressive stresses in the material block samples were shot peened in a peening set up using different combinations of shot sizes, nozzle diameter, shot flow parameters. Peening intensity is estimated by using Almen Strip, and therefore the block samples were peened off.

Measurement:

It is carried out on the shot peened blade samples using the X-Stress 3000 machine. Measurement parameters were finalized. With the stress free samples calibration is carried out. Now stress measurement is carried out on all the shot peened samples including as received. The depth profile is also measured on the shot peened samples by using the equipment Movipal-3 machine for electro-polishing and the material is scooped out. A prototype electrolyte was used as media for electro-polishing the material.

Electro-polishing Parameters:

Time : 30sec Current: 1ampers

Depth was measured by using the Digimatic Depth Gages with Absolute Encoder Series 547-217S and then stress was measured at the sample locations. At the same location every sample up to 0.5mm residual stresses was measured at a standard interval by scooping of the material by the same method.

Calibration of the Equipment:

The purpose of calibration is to determine the distance between the collimator and the sample surface that produces zero stress values in a stress free sample within certain allowed tolerance.

i The computer along with x-ray unit is switched ON and the XSTRESS 3000 software is opened.

ii X-ray run up is carried out the purpose of it is to condition the X-ray tube for operation at full power.

We use Cr tube for this type of samples. The run up will increase the power to the stand by level, 20kV and 2.0mA. The run up takes about a few minutes enabling the equipment ready for operation.

iii Calibration is then carried out by placing the sample in an appropriate position and selecting the option of calibrate in the main menu followed by Measure option the various Measurement Parameters like exposure time, No. of inclinations, Inclination (deg), Angles (manual or auto) are set up in the computer. Rotation is done during calibration.

iv After the pre-set exposure time is completed. The inclination changes according o the input and then when all the inclinations are done the values if stress is calculated in the stress field.

v the last accepted calibration will be valid until the calibration procedure is repeated again and accepted.

Measurement:

i After the calibration is completed. File followed by Stress Measurement is opened. Then the option of measure is selected which opens a window of the Measurement Parameter which must be set according to the requirement.

ii For e.g. Exposure Time (sec) 20 No. of inclinations 4/4 Inclinations (deg) -40 to +40

Angles Auto

Psi-oscillations ±0

iii The measurement is then started. After the pre-set exposure time is completed. The inclinations are changed according to the input and then all the inclinations are done the values of stress is calculated and in the stress field.

iv The rotation angle is then changed according to the pre-set value and the above step is repeated automatically.

v After the stresses at all the rotation angles are determined, the file is then saved with its sample identification number.

vi The results obtained are tabulated in the Table:

(4)

93 vii For the given results graph is plotted between

Residual Stress and Depth Profile for different parameter.

III. RESULTS AND DISCUSSIONS

Blading steel is a steam turbine blade component has a susceptible to premature failure that has a serious economic and safety implication. These steam turbine blades are used for different environment and for different processing conditions. In which there have a particular interest to power industry and have a more important in the present study. Shot penning has been chosen for improving the life of the component by enhancing the surface residual stresses. Shot peening is a cold working process used to produce a compressive residual stresses layers and modified mechanical properties of metals. In the present investigation, an attempt that has been made to understand the role of residual stresses on shot peened blading steel. The analysed chemical composition, mechanical properties like room temperature Tensile test, Impact, Hardness, Microstructure and Residual Stress on the shot peened blading steel samples. The results obtained in this investigation at various stages are incorporated and listed below:

Chemical Composition:

The Chemical composition of the blading steel , which was under investigation is given in Table 1.

Chemical composition of the material was confirming to the standard and meeting to the specified values. The chemical composition results are in conformity with specification:

Table 1:

Specified

(STD) C Cr Mn Mo Nb P S Si Ni Min. 0.12 11.5 0.25 0.20 0.05 0.025 0.025 0.5 0.75 Max. 0.15 12.5 0.65 0.23 0.2 0.027 0.026 0.6 0.75 Actual 0.13 12.1 0.6 0.2 0.07 0.025 0.025 0.5 0.75

Mechanical Properties (Tensile, Impact and Hardness):

Room temperature tensile testing was carried out on all the blading tensile samples in as received condition (room temperature) using a Fast Tract 8800/8803 Instron makes 50tons. Machine. The tensile testing was carried out at room temperature and the results are listed in Table 2.

Table 2:

Specifi ed (STD)

0.2%

Proof StressN/

mm²

Tensile Strength N/mm²

% Elongat

ion

%Reduct ion in

Area

Min. 580 800 15 50

Max. 600 950 18 55

Actual 580 875 17 52

The Charpy V – notch impact test performed on a 358 Joules Tinius Olsen, USA make machine as per the E-23 standard. The impact energy values were given inTable3.

Table 3:

Identification Joules

Sample No. 1 20

Sample No. 2 21

Sample No. 3 20

The hardness test performed using Brinell Hardness tester on the blading steel sample. The hardness values were given in Table 4.

Table 4

Hardness Test Brinell Hardness HB30, max

Sample No. 1 280

Sample No. 2 260

Sample No. 3 270

All the above test results of the material under investigation are in conformity with the standard and meeting the specification.

Microstructure:

The blading steel sample polished and etched with Vellila were observed under optical microscope and photographed at X500 mag.The examined microstructure reveals a martensitic in nature. The microstructures were given in Fig 1 .

Fig.1 : Microstructure of the Blading Steel sample examined (etched in Villela)

X 500

(5)

94 Residual Stress:

The residual stress measurement was carried out on all the shot peened blading samples using XSTRESS3000/Finland machine. Around 73 measurements were done by the varying 3 no. of (three) parameters viz., Shot Size, Nozzle Diameter, Shot Flow.

Shot size is kept constant and the intensity and air pressure are varied at different depths the residual stresses are measured. In the similar manner by varying the other two parameters like nozzle diameter and shot flow are kept constant and other parameters are varied for carrying out the shot peening. On these samples surface residual stresses were measured. The results were tabulated and given below from Table 5 to 7 . The values of residual stresses as a function of depth are represented graphically in Fig. 1 to 9

DISCUSSION:

The present aim of project was to establish shot peening parameters, which will give a compressive residual stress of 300MPa or more at a depth of 0.3mm from the surface. To achieve this requirement without affecting the surface finish a number of experiments has to be performed. Based on the results tabulated all the shot peened samples compressive surface residual stresses were observed. Keeping the shot size as constant and the other two parameters like almen intensity and air pressure are variables, the increased trend of residual stress pattern was observed by increasing the air pressured and almen intensity. By keeping the nozzle diameter as constant and varying almen intensity and air pressure are as variables, the increasing trend of residual stress pattern was observed by decreased the air pressured and almen intensity. Shot flow was kept constant and the other two parameters almen intensity and air pressure are as variables, the increased trend of residual stress pattern observed by decreasing the air pressured and almen intensity.

Table 5:

Constant

Parameter Varied Parameter

Shot Size(4mm)

Almen Intensity

(0.322)

Air Pressure (2.8Kg/sq.cm) Depth (mm)

Residual Stresses (MPa)

0 -436.2

0.02 -416

0.04 -414.2

0.06 -436.1

0.08 -388

0.12 -400.8

0.22 -382.5

0.3 -304.9

Constant

Shot Size (4mm)

Almen Intensity

(0.312)

Air

Pressure(2.5Kg/sq.cm )

Depth (mm)

0 -415.5

0.02 -401.4

0.04 -421.1

0.06 -434.2

0.08 -447

0.12 -428.1

0.2 -380

0.25 -314.5

0.3 -238

Constant Parameter

Varied Parameter Shot Size

(4mm)

Almen Intensity

(0.292)

Air Pressure ( 2.2Kg/sq.cm)

Depth (mm)

0 -415.9

0.02 -403.1

0.04 -417.1

0.06 -420.4

0.08 -399.9

0.12 -429.7

0.2 -392.7

0.25 -284.6

0.3 -286.4

Table 6:

Constant

Nozzle Diameter

(8mm)

Almen Intensity

(0.319)

Air Pressure (2.2kg/sq.cm)

0.01 -486.1

0.03 -467.8

0.08 -472.2

(6)

95

0.12 -476

0.23 -465.3

0.25 -429.5

0.3 -399.9

Constant

Nozzle Diameter

(8mm)

Almen Intensity

(0.302)

Air Pressure (2.1kg/sq.cm) Depth (mm)

0 -540.9

0.01 -527.8

0.03 -432.9

0.08 -459.1

0.12 -438.3

0.23 -452.7

0.25 -441.2

Constant

Nozzle Diameter

(8mm)

Almen Intensity

(0.28)

Air Pressure (1.9kg/sq.cm) Depth (mm)

0 -529.8

0.01 -538.5

0.03 -486.3

0.08 -525.3

0.12 -449.3

0.15 -417

0.23 -414.6

0.25 -427.6

Table7 Constant

Shot Flow(4kg/min)

Almen Intensity

(0.367)

Air Pressure (3.0kg/sq.cm)

Depth (mm)

0 -598.4

0.03 -473.8

0.11 -657.7

0.21 -657

0.29 -598.4

0.3 -509

0.41 -288.8

0.45 -145.7

Constant

Shot

Flow(4kg/min)

Almen Intensity (0.362)

Air

Pressure(2.9kg/sq.cm) Depth (mm)

0 -603.6

0.03 -473.8

0.1 -643.8

0.19 -589.4

0.28 -584.1

0.3 -520.4

0.37 -343.8

0.44 -225.4

Constant

Shot

Flow(4kg/min)

Almen Intensity (0.357)

Air

Pressure(2.8kg/sq.cm) Depth (mm)

0 -614.7

0.07 -593.8

0.19 -642.3

Fig 1:Depth vs Residual stresses of shot size (4mm)with Almen intensity(0.322)and Air pressure(2.8 kg/sq.cm)

Fig 2: Depth vs Residual stresses of shot size (4mm)with Almen intensity(0.312)and Air pressure(2.5 kg/sq.cm)

Fig 3: Depth vs Residual stresses of shot size (4mm)with Almen intensity(0.292)and Air pressure(2.0 kg/sq.cm)

(7)

96

IV. CONCLUSIONS

The following major conclusions are draw on the above experimental results.

 The results will be useful in design, enhancing, and understanding the material behavior.

 The shot peening process, being versatile and economical, can be easily adapted for other application to introduce compressive residual stresses.

 On all the shot peened samples compressive surface residual stresses were observed.

 With shot peening compressive residual stresses can be introduced into the surface of the steel under investigation which enhances fatigue properties.

 By varying the shot peening parameters viz., shot size, nozzle diameter, shot flow the intensity of residual stress depth can be varied.

V. REFERENCES

[1] http://www.eserc.stonybrook.edu/projectjava/

Brag

[2] http://www.eserc.stonybrook.edu/projectjava/

Brag Operating Instructions and Instrumement Document for XStress 3000 system with Floor Stand and X-Y unit

[3] Measurement of Residual Stress measurement by X-Ray Diffraction by pdf

[4] Residual stress Part 1 – Measurement techniquesBy P. J. Withers and H. K. D. H.

Bhadeshia

[5] International conference on Residual life of power plant equipment-Prediction and Extension Jan 23-25, 1989, Hyderabad.

[6] BHEL R&D Literature Resources

[7] Cullity.B .D:Elements of X-ray diffraction;

Addition-Wesley publishing Co.Inc;1978

[8] Macherauch, E: Residual stresses Conf proc:

Application of fracture mechanics to materials and structures

[9] Residual stresses:part1 measurement techniques by p.j.withers and h.k.d.h. bhadeshia



Fig 4: Depth vs Residual stresses of Nozzle Diameter (8mm)with Almen intensity(0.319)and Air pressure(2.2 kg/sq.cm)

Fig5: Depth vs Residual stresses of nozzle diameter (8mm)with Almen intensity(0.302)and Air pressure(2.1 kg/sq.cm)

Fig 6: Depth vs Residual stresses of nozzle diameter (8mm)with Almen intensity(0.28)and Air pressure(1.9 kg/sq.cm)

Fig 7: Depth vs Residual stresses of shot flow (4mm)with Almen intensity(0.367)and Air pressure(3.0kg/sq.cm)

Fig8: Depth vs Residual stresses of shot flow (4mm)with Almen intensity(0.362)and Air pressure(2.9 kg/sq.cm)

Fig 9: Depth vs Residual stresses of shot flow (4mm)with Almen intensity(0.357)and Air pressure(2.8 kg/sq.cm)