Investigation of postweld heat treatment of quenched and tempered pressure vessel steels

Transcription

1 University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2003 Investigation of postweld heat treatment of quenched and tempered pressure vessel steels Zoran Sterjovski University of Wollongong, Recommended Citation Sterjovski, Zoran, Investigation of postweld heat treatment of quenched and tempered pressure vessel steels, PhD thesis, Materials Engineering, University of Wollongong, Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library:

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3 INVESTIGATION OF POSTWELD HEAT TREATMENT OF QUENCHED AND TEMPERED PRESSURE VESSEL STEELS A thesis submitted in fulfilment of the requirements for the award of the degree Doctor of Philosophy from The University of Wollongong by Zoran Sterjovski, M.E. (Hons), B.E. (Hons) Materials Engineering 2003

4 ii CANDIDATE S CERTIFICATE This is to certify that the work presented in this thesis has been carried out by the candidate whilst enrolled as a full time postgraduate student in the Department of Materials Engineering, at the University of Wollongong. The results obtained in this study and the conclusions drawn are those of the candidate. The work contained in this thesis has not been submitted to any other university or institution for a higher degree... Zoran Sterjovski

5 iii ACKNOWLEDGEMENTS I wish to acknowledge my supervisor, Professor Druce Dunne, for his guidance, support and inspiration during my PhD candidature. It was a great pleasure working with him again and I will miss him when I re-enter the workforce. I also wish to thank the WTIA, CRC for Welded Structures, Bisalloy Steels, BHP Steel, The Lincoln-Electric Company (Australia) and ANSTO for their expertise and extensive use of facilities. A special thanks to: Stan Ambrose (WTIA) for his co-supervision, input and encouragement, Geoff Hancock (BHP Steel) for his support and in-kind contributions, Andrew Croker (ANSTO) for his input into my research, David Carr (ANSTO) for teaching me everything about CTOD testing, Russel Barnett (Bisalloy Steels) for his committed in-kind support and technical expertise, and Milo Dumovic and Chris Jones (Lincoln Electric) for their in-kind support and technical expertise. I am indebted to CCI Pope Pty Ltd for their financial and technical support during my candidature. A special thankyou to Dr Chris Tabrett and Mike O Brien. I am also grateful to all the members of the Faculty of Engineering at the University of Wollongong. In particular, I must thank Jose Gonzales for his quality workmanship in the preparation of samples for mechanical testing and Greg Tillman for his acumen in the metallography laboratory. Last but not least, I must thank my beautiful wife, Anne-Marie, and my beautiful daughters, Mikaela and Neve, for putting up with me during the good and the bad that are part of doing a PhD.

6 iv ABSTRACT Longitudinal and circumferential welds in transportable pressure vessels are produced by submerged-arc welding using a single vee preparation and multiple weld runs. Quenched and tempered (QT) steels, which are commonly used for transportable pressure vessels, require mandatory postweld heat treatment (PWHT) regardless of the plate thickness. During their life transportable pressure vessels may have up to four PWHT cycles, and concerns have emerged about possible effects on material properties such as hardness, tensile strength, impact toughness and fracture toughness. This thesis reports on the weld procedure, microstructural evaluation and various mechanical properties (bend, yield strength, tensile strength, elongation, fatigue, impact toughness, CTOD fracture toughness and hardness) for 11 mm, 12 mm and 20 mm QT steel weldments. The 11 mm and 20 mm base plates investigated were BIS80PV, which is a pressure vessel grade steel. The 12 mm plate was BIS80, which is structural grade steel but deemed a possible candidate for pressure vessels due to superior impact and fracture toughness properties over currently used QT pressure vessel steels. The parent metal, heat affected zone and weld metal regions of each weldment were examined, and then exposed to temperatures and times in the PWHT range. Although there was no apparent change in microstructure at an optical level and little change in hardness for up to four postweld heat treatments, there was a marked decrease in hardness of the parent metal for more extensive heat treatments (increasing Holloman parameter). There was also evidence of minor secondary hardening in the 11 mm and 20 mm BIS80PV parent plate following short heat treatment times. The weld metal (WM) and HAZ hardness typically decreased with one PWHT cycle and subsequently stabilised with further PWHT cycles. For all test plates, results are also presented for Charpy V-notch impact tests in the parent metal, HAZ and weld metal region, and CTOD fracture toughness tests in the PM region. The effect of exposure to multiple PWHT cycles on these properties is discussed. A decrease in impact energy and fracture toughness with an increase in the

7 v number of heat treatments was evident in the parent metal. In contrast, the weld metal showed a decrease in impact energy after two PWHT cycles, and then an increase towards the original impact energy after a further two cycles. In PM samples, which have been extensively tempered in the manufacturing process, the mechanism by which toughness properties are affected by cumulative PWHT holding time is through the coarsening and coalescence of second phase carbide particles. These particles decrease impact energy by the formation of larger voids in the plastic zone ahead of the crack tip or by the initiation of cleavage fracture. These two phenomena decrease the energy required for fracture. In WM with an as-solidified structure, ductile failure in the form of void coalescence is initiated by non-metallic inclusions. This leads to an initially high impact toughness, which then decreases after PWHT because of the nucleation and coarsening of metastable Fe 3 C precipitates that promote quasi-cleavage type fracture. Upon exposure to further PWHT cycles the impact energy begins to increase again due to the dissolution of the metastable carbides and the formation of finer, more stable carbides based on elements such as Cr, Mo, Ni and Nb. This change in the form of the carbide promotes localised fracture by micro-plasticity, with void formation and coalescence. Additionally, cross-weld root bend (180 ) and tensile tests were carried out before and after PWHT. None of the bend samples showed any evidence of cracking or tearing, hence confirming the ductility of the weldment. The tensile properties of the BIS80PV cross-weld samples complied with the Australian Standards, but the tensile properties of the BIS80 cross-weld samples only complied in the as-welded condition. Tensile properties of the PM showed no significant trends with the number of PWHT cycles and the fatigue crack growth rate increased slightly in relation to the number of PWHT cycles or PWHT holding time. One of the primary roles of PWHT is to reduce residual stresses caused by the welding process. Residual stress measurements using the hole drilling method were made to

8 vi ascertain the need for PWHT. Residual stresses, measured in the weld centre-line by the hole drilling technique, were compressive along both the longitudinal and transverse directions and were no greater than 250 MPa (between times the yield strength at room temperature). Although, the presence of residual stresses (tensile and compressive) of greater magnitude is not excluded, the residual stress measurements demonstrated that PWHT significantly reduced the magnitude of the residual stresses. The ability to predict the impact energy of a pressure vessel steel subjected to various cumulative PWHT holding times is beneficial to pressure vessel inspectors, who are misguidedly instructed to be wary of the mechanical properties of vessels after 6 hours of cumulative PWHT holding time. The original impact energy of the steel, which is determined by its chemical composition, microstructure and thermo-mechanical treatment determines the cumulative PWHT time before the impact properties of the QT steel decrease below the specified 40 J limit at 20 C. An artificial neural network (ANN) model has been developed using an artificial neural network to predict the effect of PWHT time and other variables such as composition and test temperature on impact energy. This ANN model provides a valuable design tool for predicting impact toughness as a function of composition, heat treatment and testing conditions. Finally, the performance of BIS80 was evaluated and its superior impact toughness and fracture toughness properties over BIS80PV were confirmed. However, the tensile properties of postweld heat-treated 12 mm BIS80 cross-weld samples did not comply with AS3597, the Australian Standard for QT steels; and the PM stress ratio exceeded the specified limit of These deficiencies could be remedied by (i) discarding PWHT or (ii) using a high strength WM; and (iii) abandoning or relaxing the stress ratio requirement, which is of questionable significance, under conditions of high strain rate impact loading (road collision).

9 vii ABBREVIATIONS BM CGHAZ CTOD δ DBTT FAS FGHAZ FL GTAW n HAZ HP IE ICHAZ LE MMAW No. OM PE PM PS (0.2%) PWHT Base metal Coarse grained heat affected zone Crack tip opening displacement Crack tip opening displacement Ductile to brittle transition temperature Fracture acceleration site Fine grained HAZ or grain refined HAZ Fusion line Gas tungsten arc welding Resistance to fatigue crack growth Heat affected zone Holloman parameter or Holloman-Jaffe parameter Impact energy Intercritical heat affected zone Lateral expansion Manual metal arc welding Number Optical microscopy Processing element Parent metal 0.2% proof stress Postweld heat treatment

10 viii QT RIA RMS RR σ SAW SEM t T Quenched and tempered Reduction in area Root mean square Reduction ratio Stress Submerged arc welding Scanning electron microscopy Time Temperature T-L, L-T, T-S, L-S Sample orientation (see Figure 3.6) UTS VHN W WM YS Ultimate tensile strength Vickers hardness number Delta rule Weld metal Yield strength

11 TABLE OF CONTENTS ix

12 x ACKNOWLEDGMENTS ABSTRACT ABBREVIATIONS CONTENTS iii iv vii ix PART A INTRODUCTION AND LITERATURE REVIEW 1 CHAPTER 1 INTRODUCTION 2 CHAPTER 2 LITERATURE REVIEW POSTWELD HEAT TREATMENT & RESIDUAL STRESSES Postweld heat treatment Residual stresses Residual stresses related to welding Effect of PWHT on residual stresses Stress relieving to reduce residual stresses Heat treatment as a form of stress relief Mechanical methods of stress relief Overloading Vibrational Techniques Shot Peening Temper beading as a method of stress relief PWHT in Australian Standards and Codes Current Overview Historical Overview Methods for measuring residual stresses Stress relaxation techniques X-ray diffraction & neutron scattering techniques Observing cracks to measure residual stresses Ultrasonic residual stress measurements

13 xi 2.2 PRESSURE VESSELS- DESIGN, MANUFACTURE & CASE STUDIES Introduction to pressure vessels Parts of a transportable pressure vessel Case studies of failures in transportable pressure vessels 2.3 MATERIALS IN TRANSPORTABLE PRESSURE VESSELS Quenched and tempered Steels (BIS80PV) Effect of alloying elements on BIS80PV Carbon Manganese Silicon Molybdenum Chromium Boron Titanium Nickel Copper Calcium, Sulphur & Phosphorous Welding of QT pressure vessel steels Typical welding processes used in manufacturing processes Manual metal arc welding Gas metal arc welding Fluxed core arc welding Submerged arc welding The different zones as a result of welding Parent metal Heat affected zone Weld metal Desirable properties for transportable pressure vessel steels Fracture & impact toughness Other desirable properties Techniques for measuring fracture & impact toughness CTOD test Charpy V-notch impact test Ductile to brittle transition temperature (DBBT) 2.4 SUMMARY OF LITERATURE

14 xii PART B EXPERIMENTAL INVESTIGATION CHAPTER 3 EXPERIMENTAL PROCEDURE MATERIALS SELECTION AND WELDING Materials selection Welding process and weld procedures 3.2 SIMULATED PWHT Box furnace heat treatment Dilatometer heat treatment 3.3 MECHANICAL TESTING Hardness testing Bend testing transverse guided bend test of weldment Impact testing Charpy V-notch Tensile testing parent plate and cross-weld specimens Fatigue testing crack growth rates Testing of 11 and 12 mm samples Testing of 20 mm samples Fracture toughness testing - CTOD Sample preparation Fatigue pre-cracking CTOD the test Post test procedure CTOD value qualification 3.4 MICROSCOPY Low magnification microscopy Optical microscopy Scanning electron microscopy (SEM) 3.5 RESIDUAL STRESSES Hole drilling technique for measuring residual stresses Stress relaxation testing CHAPTER 4 EXPERIMENTAL RESULTS STRUCTURAL ANALYSIS OF MATERIALS Weld macrographs Parent metal (PM) microstructures Weld metal (WM) microstructures HAZ microstructures Effect of PWHT on microstructure

15 xiii 4.2 HARDNESS Parent metal region Weldment (cross-weld samples) 4.3 BEND TEST RESULTS CROSS-WELD SAMPLES 4.4 IMPACT TESTING CHARPY V-NOTCH Parent metal region PM test results PM microstructure Weld metal region WM test results WM microstructure Heat affected zone HAZ test results HAZ microstructure Comparison of the weldment zones Test results 4.5 TENSILE TESTING Parent metal region Cross-weld samples Comparison of PM and cross-weld specimens 4.6 FATIGUE CRACK GROWTH DATA (PM) 4.7 FRACTURE TOUGHNESS (CTOD) DATA PM mm BIS80PV PM mm BIS80 PM mm BIS80PV PM Comparison of CTOD values for the three plate steels SEM fractography of CTOD PM samples 4.8 RESIDUAL STRESSES Hole drilling technique Stress relaxation data

16 xiv CHAPTER 5 DISCUSSION THE NEED FOR PWHT IN ACHIEVING WELDMENT PROPERTIES Microstructure and hardness Bend testing Impact and tensile testing 5.2 THE EFFECT OF MULTIPLE PWHT CYCLES ON WELDMENT PROPERTIES Microstructure and hardness Impact toughness Parent metal Weld metal HAZ Tensile testing Fatigue crack growth and CTOD fracture toughness 5.3 THE NEED FOR PWHT IN REDUCING RESIDUAL STRESSES 5.4 SUITABILITY OF BIS80 AS PRESSURE VESSEL STEEL PART C MODELLING CHAPTER 6 NEURAL NETWORK MODELLING PREAMBLE 6.1 INTRODUCTION TO NEURAL NETWORKS 6.2 REVIEW OF LITERATURE 6.3 PROCEDURE-BUILDING, TRAINING, TESTING, AND VALIDATING THE MODEL Data file preparation Building the model Training the model Testing the model Results (experimental versus predicted and sensitivity analysis) 6.4 RESULTS Actual versus predicted impact toughness Sensitivity analysis of the model 6.5 DISCUSSION Actual versus predicted impact toughness Sensitivity analysis PART D OVERALL CONCLUSIONS AND REFERENCES CHAPTER 7 CHAPTER 8 CONCLUSIONS REFERENCES

17 xv PUBLICATIONS APPENDICES Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Appendix H Appendix I