WELDABILITY INVESTIGATIONS OF DISSIMILAR METAL JOINTS
FOR
NUCLEAR PLANT APPLICATIONS
DINESH WASUDEV RATHOD
DEPARTMENT OF MECHANICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY DELHI
HAUZ KHAS, NEW DELHI
JANUARY 2015
© Indian Institute of Technology Delhi (IITD), New Delhi, 2015
WELDABILITY INVESTIGATIONS OF DISSIMILAR METAL JOINTS
FOR
NUCLEAR PLANT APPLICATIONS
by
DINESH WASUDEV RATHOD
DEPARTMENT OF MECHANICAL ENGINEERING
Submitted
in fulfilment of the requirements of the degree of
DOCTOR OF PHILOSOPHY
to the
INDIAN INSTITUTE OF TECHNOLOGY DELHI
HAUZ KHAS, NEW DELHI
JANUARY 2015
i
CERTIFICATE
This is to certify that the thesis entitled, ‘Weldability Investigations of Dissimilar Metal Joints for Nuclear Plant Applications’ being submitted by Mr. Dinesh W.
Rathod to the Indian Institute of Technology Delhi for the award of the degree of DOCTOR OF PHILOSOPHY is a record of the bonafide research work carried out by him. Mr. Dinesh W. Rathod has worked under our supervision for the submission of this thesis, which to our knowledge has reached the requisite standard.
The thesis or any part thereof has not been presented or submitted to any other University or Institute for any degree or diploma.
Dr. Sunil Pandey Professor
Department of Mechanical Engineering Indian Institute of Technology Delhi
Hauz Khas, New Delhi - 110016 INDIA
Dr. S. Aravindan Associate Professor
Department of Mechanical Engineering Indian Institute of Technology Delhi
Hauz Khas, New Delhi - 110016 INDIA
Date:
Place: New Delhi
ACKNOWLEDGEMENTS
I would like to express my gratitude to all the people who helped me over the last four years in the work leading to this dissertation.
I express my profound gratitude towards my supervisors, Prof. Sunil Pandey and Dr. S.
Aravindan, Department of Mechanical Engineering, IIT Delhi for their inspiring guidance, continuous support and constant encouragement both in research and personal life. I really have insufficient words to express my gratitude. Their vast experience in the area and willingness to impart their knowledge has helped me during the period of research. It is because of their tender care and exceptional interest that this thesis could be brought to the present form.
Prof. Sunil Pandey has given me an opportunity in very interesting and challenging project of Bhabha Atomic Research Centre (BARC), Mumbai for the weldability investigation of dissimilar alloy steels in Nuclear plants. During the work of project, he offered me the plenty of freedom and the responsibility including pipe welding, Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) assessment and various special testing at BARC, Mumbai and DMRL, Hyderabad with his exceptional working attitude, which cannot be explained in the words.
Prof. Pandey also allowed me to travel, to meet people in the profession, and to gain knowledge and expertise outside the bounds of my dissertation research. I will forever benefit from these experiences. It has been an honour and a privilege to work not only for the great professor, but also the great man.
I am grateful for the guidance provided by my Student Research Committee members, Prof.
Naresh Bhatnagar, Prof. D. Ravi Kumar and Prof. D. K. Sehgal. I would like to thank Dr.
Sudarshan Ghosh, Prof. R. K. Pandey, and faculty members for their valuable suggestions and discussions. I express my thanks to the former and present Head, Department of Mechanical Engineering, for providing the departmental facilities.
I would also like to thank Prof. Rajesh Prasad for his valuable and indeed support for the research. His indeed decisions facilitate the project research work to finish by the time.
I am thankful to Mr. P.K. Singh (Scientist ‘G’, BARC) and Mr. J.K. Jain (Head, Quality Control NPCIL) for discussions during the Research activities in project sponsored by BARC (BRNS, DAE, India) which helped me a lot for my PhD work and for project work also. They have also provided the valuable guidance and facilities for fracture toughness testing and the expertise of ASME sections required for the work.
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I would like to express my gratitude to Mr. Suresh Meshram (Scientist ‘D’, DMRL) for the EPMA and XRD facilities at the DMRL Hyderabad. I could not miss the help and support offered by my colleagues and seniors Mr. Ratnesh Raj Singh, Mr. Dhruv Anand, Dr. Arshad Noor Siddiquee, Dr. D K Shukla, Dr. Manoj Kumar, Dr. Hariom Choudhary, Dr. Zaheer Khan, Mr. Abhishek Pandey, Mr. Mayank, Mr. Durga, Mr. Rajesh, Mr. Deepak, Lt. Col.
Sanjay Pandey, Mr. Dinesh Shetti, Mr. Aryajyoti Goswami, and Mr. Anirudh Jaswal. The support extended by them during the critical stages of this research work is gratefully acknowledged and I am also thankful for their interest and involvement in the technical discussions.
The help provided by staff of the Mechanical Engineering Division Laboratories especially Mr. Ayodhya Prasad, Mr. Tulsiram, Mr. Ramchandar, Mr. Ram Mehar, and Mr. Tiwari is appreciated. The help extended on personal fronts by department of Applied Mechanics staff particularly Mr. Anil Kumar is gratefully acknowledged.
My gratitude’s are due to Guru-Mata Mrs. Anita Pandey, cute sister Ms. Srishti and loving brother Mr. Kshitij Pandey for the homely environment they set up among all of us students.
It was the second home to me during these four years of research. We students had lot of fun and enjoyed celebrating most of the Indian festivals at my supervisor’s home. I would never forget those joyful moments.
I am most grateful to my family. I express my deepest gratitude’s to my parents, wife, son, brother, and sister. I love them with all of my heart. I am thankful to my parents for the principles they instilled in me as a young man and for their continued support, guidance, and love for our family. The technical discussions with my elder brother Dr. Ganesh, and care taken by him during entire research work has helped me a lot. It would have been impossible to complete this work without my family’s understanding, patience, and sacrifices.
Date: Dinesh W. Rathod
ABSTRACT
In power generation industries especially in nuclear power plants, the dissimilar metal weld (DMW) joints are extensively used in pressure vessel components and piping systems to accommodate the varying temperature, pressure and the corrosive environments at different locations. Due to several weldability related problems, the austenitic stainless steel consumables are replaced by Ni-base consumables due to considerable advantages. Therefore, at present the dissimilar metal weld joints between ferritic steel and austenitic stainless steel in such applications are fabricated using Ni-base (ERNiCr-3) buttering with GTAW process and welding with SMAW process using Ni-base (ENiCrFe-3) consumables. The design life of these joints considerably increased than those fabricated with stainless steel consumables.
However, these joints are still not performing the required design life due to premature failures like leakages and brakes in weldment regions. For such premature failures, the number of researchers has reported several reasons and the carbon migration is considered to be one of the major problems contributing to the failure.
In the present study, carbon migration is considered as the major objective and it has
been tried to control the carbon migration, which in turn can significantly control the
rest of the problems associated with it. Carbon migration from ferritic steel to Ni-base
weld metal is often observed. Therefore, the adjacent buttering deposit to the ferritic
steel is highly prone to the carbon migration than the weld metal. Despite the critical
concern on the buttering deposits, no extensive study with respect to carbon migration
in buttering is reported in the literature. Good amount of work was carried out on the
DMW joints but specific attempt on carbon migration and metallurgical deterioration
in buttering is not carried out yet. Hence, the specific comparative investigations for
vi
carbon migration and metallurgical studies have been carried out by using the Ni-Fe buffer layer and by varying the buttering deposition processes. Detailed metallurgical investigation with an emphasis on thermal aging with the carbon diffusion model has been carried out. The optimized buttering deposits with number of layers, their thickness, process of deposition and the effect of buffer layer are confirmed with investigation. By using Ni-Fe alloy buffer layer and the SMAW process for buttering, the carbon migration is significantly controlled and the associated problem with it has been alleviated for a great extent.
The optimized buttering deposits were chosen for fabrication of the DMW joints and the joints were studied for the integrity assessment. The optimized four buttering deposits were used with GMAW and SMAW processes by varying the consumables.
In the study, sixteen plate joints were fabricated and investigation was carried out to quantify the local metallurgical and mechanical properties across the weldment of joints for integrity assessment. The austenitic stainless steel consumables were used to fabricate the joints with Ni-base buttering. The fabricated joints had observed with solidification cracks in bulk weld metals. Therefore, austenitic stainless steel consumables cannot be recommended for DMW joint applications.
The weld qualification tests, NDT inspections, metallurgical studies and mechanical
testing were conducted for the weldment regions and at the interfacial regions. The
local mechanical and metallurgical properties for these joints were investigated and
integrity assessments were carried out. Furthermore, the experimental results
indicated that the Ni-Fe buffer layer in buttering (with SMAW process) and the joint
fabrication using GMAW process and Ni-base consumables could be the best
combination for DMW joints in terms of mechanical properties and joint integrity.
TABLE OF CONTENTS
CERTIFICATE i
ACKNOWLEDGEMENTS iii
ABSTRACT v
TABLE OF CONTENTS vii
LIST OF FIGURES xv
LIST OF TABLES xxxi
LIST OF ABBRIVATIONS xxxiii
LIST OF SYMBOLS xxxv
Chapter 1 Introduction 1
1.1 The Nuclear Power Plants 1
1.2 Dissimilar Metal Weld Joints 2
1.3 Problems Associated with Dissimilar Metal Welds 3
1.3.1 Carbon Migration 3
1.3.2 Cyclic Thermal Stresses 4
1.3.3 Metallurgical Deterioration 6
1.3.4 Low Oxidation Resistance of Ferritic Steel 7
1.3.5 Residual Stresses 8
1.4 Techniques to Resolve the Problems 9
1.4.1 Intermediate Buffer Layer to Control Carbon Migration 10
1.4.2 Welding Processes and Consumables Variations 11
1.5 Organization of the Thesis 12
1.6 Summary 14
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Chapter 2 Literature Review 17
2.1 Transition Metal Joints / Dissimilar Metal Weld Joints 17
2.2 Problems and Failures in Dissimilar Metal Welds 22
2.3 Heat Flow during Welding 28
2.4 Carbon Migration 30
2.5 Metallurgical and Mechanical Properties of DMW Joints 35
2.6 Research Gap in Dissimilar Metal Weld Joining 38
2.7 Objectives of the Present Research Work 40
Chapter 3 Preliminary Investigations 41
3.1 Selection of Consumables 41
3.2 Selection of Buttering and Welding Processes 45
3.3 Buttering Deposit Selection 46
3.4 Weld Joint Design 48
3.4.1 Half ‘V’ design 48
3.4.2 Single ‘V’ design (included angle 60o) 48
3.4.3 Single ‘V’ design (included angle 90o) 49
3.4.4 Single ‘V’ design (included angle 75o) 49
3.4.5 Modified single ‘V’ design (compound bevel) 49
3.5 Summary 51
Chapter 4 Buttering Deposition 53
4.1 Materials and Methods 53
4.1.1 GTAW Deposit with ERNiCr-3 54
4.1.2 GTAW Deposit with Ni-Fe alloy Buffer Layer and ERNiCr-3 55 4.1.3 GMAW Deposit with Ni-Fe alloy Buffer Layer and ERNiCr-3 56 4.1.4 SMAW Deposit with Ni-Fe alloy Buffer Layer and ENiCrFe-3 57
4.2 Process Parameters of Buttering Deposition 58
4.2.1 GTAW Deposit of ERNiCr-3 59
4.2.2 GTAW Deposit of Ni-Fe and ERNiCr-3 60
4.2.3 GMAW Deposit of Ni-Fe and ERNiCr-3 61
4.2.4 SMAW Deposit of Ni-Fe and ENiCrFe-3 62
4.3 Heat Flow Analysis 63
4.3.1 Heat Flow during GTAW Process 66
4.3.2 Heat Flow during GMAW Process 69
4.3.3 Heat Flow during SMAW Process 71
4.4 Thermal Aging and Specimen Coding 74
4.4.1 Thermal Aging and PWHT of Weld Pads 74
4.4.2 Specimen Coding 78
4.5 Summary 80
Chapter 5 Metallurgical Evaluation of Buttering Deposits 83
5.1 Chemical Composition and Carbon Diffusion Analysis 83
5.1.1 Chemical Composition in Buttered Deposits 83
5.1.2 Carbon Diffusion and Estimation 85
5.1.2.1 Analysis of GTAW with ERNiCr-3 (GT deposits) 92 5.1.2.2 Analysis of GTAW with Ni-Fe buffer layer and ERNiCr-3 (NT deposits) 97 5.1.2.3 Analysis of GMAW with Ni-Fe buffer layer and ERNiCr-3 (NM deposits) 101 5.1.2.4 Analysis of SMAW with Ni-Fe buffer layer and ENiCrFe-3 (NS deposits) 105
5.1.3 Comparative Evaluation of the deposits 109
5.2 Martensite Formation Analysis 111
5.2.1 Analysis of GTAW with ERNiCr-3 (GT41 Deposit) 112
5.2.2 Analysis of GTAW with Ni-Fe buffer layer and ERNiCr-3 (NT41 Deposit) 117 5.2.3 Analysis of GMAW with Ni-Fe buffer layer and ERNiCr-3 (NM41 Deposit) 120 5.2.4 Analysis of SMAW with Ni-Fe buffer layer and ENiCrFe-3 (NS41 Deposit) 123 5.2.5 Comparative Evaluation for Martensite formation 126
x
5.3 Micro-hardness Evaluation 130
5.3.1 GTAW with ERNiCr-3 buttering (GT deposits) 131
5.3.2 GTAW with Ni-Fe alloy buffer layer and ERNiCr-3 buttering (NT deposits) 135 5.3.3 GMAW with Ni-Fe alloy buffer layer and ERNiCr-3 buttering (NM deposits) 138 5.3.4 SMAW with Ni-Fe alloy buffer layer and ENiCrFe-3 buttering (NS deposits) 142
5.3.5 Comparative evaluation of the deposits 146
5.4 Microstructure Evolution 148
5.4.1 Microstructure of GTAW with ERNiCr-3 (GT deposits) 150 5.4.2 Microstructure of GTAW with Ni-Fe alloy and ERNiCr-3 (NT deposits) 156 5.4.3 Microstructure of GMAW with Ni-Fe alloy and ERNiCr-3 (NM deposits) 161 5.4.4 Microstructure of SMAW with Ni-Fe alloy and ENiCrFe-3 (NS deposits) 168
5.4.5 Comparative evaluation of the deposits 173
5.5 X-Ray Diffraction (XRD) Analysis 175
5.5.1 GTAW with ERNiCr-3 buttering (GT4X deposits) 176
5.5.2 GTAW with Ni-Fe buffer layer and ERNiCr-3 buttering (NT4X deposits) 178 5.5.3 GMAW with Ni-Fe buffer layer and ERNiCr-3 buttering (NM4X deposits) 180 5.5.4 SMAW with Ni-Fe buffer layer and ENiCrFe-3 buttering (NS4X deposits) 181
5.5.5 Comparative evaluation 183
5.6 Summary 184
Chapter 6 Plate Joints Fabrication 187
6.1 Materials and Methods 187
6.2 Fabrication Details of the Joints 188
6.2.1 Buttering deposition on SA508 ferritic steel plates 188
6.2.2 Process Parameters during Buttering deposition 191
6.2.2.1 GT buttering deposition for weld joints 192
6.2.2.2 NT buttering deposition for weld joints 193
6.2.2.3 NM buttering deposition for weld joints 194
6.2.2.4 NS buttering deposition for weld joint 194
6.2.3 Joint Geometry for Plate Joint Welding 195
6.2.4 Joint Fabrication Experiments / Runs 197
6.3 Welding Setup and Plate Joint Welding 198
6.3.1 Welding setup 198
6.3.2 Process Parameters during welding 203
6.3.3 Radiograph Testing (RT) 204
6.4 Test Specimens Fabrication and Testing Procedure 206
6.4.1 All Weld Tensile Test 206
6.4.2 Composite Tensile Test 208
6.4.3 Charpy ‘V’ notch Impact Toughness Test 209
6.4.4 Fracture Toughness (CTOD) specimens 211
6.4.5 Testing Procedure for Metallurgical Testing 212
6.5 Macro-hardness (Indentation) and Angular Distortion 213
6.5.1 Macro-hardness (indentation) 213
6.5.2 Angular Distortion 214
6.6 Summary 218
Chapter 7 Metallurgical Testing of Plate Joints 219
7.1 Chemical Composition across the Weldments 219
7.2 Micro-hardness Evaluation 224
7.2.1 Plate Joints (PJ-1 – PJ-4) with GT buttering deposit 224 7.2.2 Plate Joints (PJ-5 – PJ-8) with NT buttering deposit 225 7.2.3 Plate Joints (PJ-9 – PJ-12) with NM buttering deposit 226 7.2.4 Plate Joints (PJ-13 – PJ-16) with NS buttering deposit 228
7.2.5 Comparative Evaluation of Plate Joints 229
7.3 Microstructure Evolution 230
7.3.1 Base Metals used in the study 230
7.3.2 HAZ Ferritic Steel Region 231
7.3.3 HAZ 304LN Austenitic Stainless Steel Region 233
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7.3.4 Buttering and Weld Metal Regions 235
7.3.4.1 Plate Joint – 1 (GT buttering – ENiCrFe-3 welding) 235 7.3.4.2 Plate Joint – 2 (GT buttering – SS-E308L16 welding) 237 7.3.4.3 Plate Joint – 3 (GT buttering – ERNiCr-3 welding) 238 7.3.4.4 Plate Joint – 4 (GT buttering – SS-ER308LSi welding) 239 7.3.4.5 Plate Joint – 5 (NT buttering – ENiCrFe-3 welding) 240 7.3.4.6 Plate Joint – 9 (NM buttering – ENiCrFe-3 welding) 242 7.3.4.7 Plate Joint – 13 (NS buttering – ENiCrFe-3 welding) 243 7.3.4.8 Plate Joint – 15 (NS buttering – ERNiCr-3 welding) 244 7.3.4.9 Plate Joint – 16 (NS buttering – SS-ER308LSi welding) 245
7.3.5 Comparative Evaluation for Microstructure 246
7.4 Summary 248
Chapter 8 Mechanical Testing of Plate Joints 249
8.1 Composite Tensile Test 249
8.1.1 Composite Tensile Test - Results and Analysis 249
8.1.2 Comparative evaluation of Joints based on composite tensile test 252
8.2 Tensile Properties Variations 253
8.2.1 All Weld Tensile Test – Results and Analysis 254
8.2.2 Fracture Surface Analysis of Tensile Test Specimens 275
8.2.2.1 Base Metals - Ferritic steel and SS304LN 275
8.2.2.2 HAZ Ferritic steel region fracture surface 276
8.2.2.3 Buttering region fracture surfaces 278
8.2.2.4 Weld Metal region fracture surfaces 280
8.2.3 Comparative evaluation of Tensile Properties 282
8.3 Charpy Impact Toughness 287
8.3.1 Impact Toughness – Results and Analysis 288
8.3.2 Fracture Surface Analysis of Impact Toughness Test Specimens 293
8.3.2.1 Base Metals - Ferritic steel and SS304LN 294
8.3.2.2 HAZ Ferritic steel region fracture surface 295
8.3.2.3 Buttering region fracture surfaces 297
8.3.2.4 Weld Metal region fracture surfaces 299
8.3.3 Comparative evaluation of impact toughness properties 301
8.4 Fracture Toughness 301
8.4.1 CTOD (δc) Testing – Procedure 302
8.4.2 CTOD (δc) Testing – Results and Analysis 303
8.4.3 Comparative evaluation of fracture toughness (δcmax) 309 8.5 Influence of Mechanical Properties on Integrity Assessment of DMW Joints 310
8.6 Summary 313
Chapter 9 Conclusions 315
9.1 Buttering Deposition Investigations 315
9.2 Plate Joint Investigations 318
9.3 Further Work 320
9.3.1 Carbon diffusion modelling 321
9.3.2 Metallurgical investigation at fusion boundaries 321
9.3.3 Fracture Toughness 321
9.3.4 Stress corrosion cracking (SCC) susceptibility 322
REFERENCES 323
VITAE 337
