UNIVERSITI TEKNIKAL MALAYSIA MELAKA INVESTIGATION ON THE FRACTURE TOUGHNESS OF WELDED PRESURE VESSEL STEEL

Transcription

1 UNIVERSITI TEKNIKAL MALAYSIA MELAKA INVESTIGATION ON THE FRACTURE TOUGHNESS OF WELDED PRESURE VESSEL STEEL This report submitted in accordance with the requirement of the Universiti Teknikal Malaysia Melaka (Utem) for the Bachelor Degree of Manufacturing Engineering (Engineering Material) with Honours. by SUHAILY BINTI MOHAMAD YUSOF FACULTY OF MANUFACTURING ENGINEERING 2009

2 UTeM Library (Pind.1/2007) UNIVERSITI TEKNIKAL MALAYSIA MELAKA BORANG PENGESAHAN STATUS LAPORAN PSM JUDUL: Investigation on the Fracture Toughness of Welded Pressure Vessel Steel SESI PENGAJIAN: Semester /2009 Saya Suhaily binti Mohamad Yusof mengaku membenarkan laporan PSM / tesis (Sarjana/Doktor Falsafah) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut: 1. Laporan PSM / tesis adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan untuk tujuan pengajian sahaja dengan izin penulis. 3. Perpustakaan dibenarkan membuat salinan laporan PSM / tesis ini sebagai bahan pertukaran antara institusi pengajian tinggi *Sila tandakan ( ) SULIT TERHAD TIDAK TERHAD (Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam AKTA RAHSIA RASMI 1972) (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan) (SUHAILY BT MOHAMAD YUSOF) Alamat Tetap: Lot 107 Kg Kemumin, Pengkalan Chepa, Kota Bharu, Kelantan (EN. MOHAMAD HAIDIR BIN MASLAN) Cop Rasmi: Tarikh: 22 MEI 2009 Tarikh: 22 MEI 2009 * Jika laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

3 DECLARATION I hereby, declared this report entitled Investigation on the Fracture Toughness of Welded Pressure Vessel Steel is the results of my own research except as cited in references. Signature :. Author s Name : Suhaily Binti Mohamad Yusof Date : 22 May 2009

4 APPROVAL This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Manufacturing Material) with Honours. The member of the supervisory committee is as follow:

5 ABSTRACT This report covers the investigation on the fracture toughness of welded pressure vessel steel. Failures occur for many reasons, including uncertainties in the loading or environment, defects in the materials, inadequacies in design, and deficiencies in construction or maintenance. Design against fracture has a technology of its own, and this is a very active area of current research. In this study, fracture toughness test have been conducted for A516 Grade 70 steel and the results has been carried out using K 1c calculations for fracture toughness. It is shown that the ductile fracture occurs by the redirection of the crack propagation from the HAZ to the weld metal. Analysis by optical microcopy and SEM has revealed that the improvement in the toughness, and thus the higher resistance to crack propagation in the HAZ, is due to the presence of a large proportion of fine acicular ferrite. Correlation of the result and failure of pressure vessel phenomenon is also analyzed. i

6 ABSTRAK Laporan ini merangkumi kajian ke atas kekuatan retakan terhadap kimpalan tangki keluli bertekanan. Kegagalan yang berlaku adalah disebabkan banyak faktor termasuk ketidakpastian dalam pemuatan atau alam sekitar, kecacatan dalam material, kekurangan dalam rekabentuk dan kekurangan dalam pembinaan atau penyelenggaraan. Rekabentuk pada retakan mempunyai teknologi yang tersendiri dan merupakan kawasan yang sangat aktif untuk penyelidikan semasa. Dalam pengkajian ini, ujian kekuatan retakan telah dijalankan menggunakan keluli ASTM A516- Gred 70 dan hasilnya telah dilakukan kiraan K 1c untuk nilai kekuatan retakan. Didapati bahawa retakan lentur yang berlaku adalah dari arah penyebaran retakan dari HAZ kepada kawasan kimpalan logam. Analisa menggunakan mikroskop optik dan SEM telah menemui perbaikan di dalam kekuatan dan rintangan yang tinggi terhadap penyebaran retakan didalam HAZ adalah disebabkan oleh kehadiran kadar ferrite yang besar. Kesinambungan daripada keputusan dan kegagalan keluli bertekanan juga dianalisa. ii

7 DEDICATION Dedicate to my parents in the grandest sense that parenting implies iii

8 ACKNOWLEDGEMENT Bismillahirrahmanirrahim, Assalamualaikum, Syukur Alhamdulillah, thanks to God for giving me a chance to finish up my Projek Sarjana Muda (PSM) technical report from the first word until this end of point. First of all, I would like to take this opportunity to express my greatest appreciation to my supervisor, Mr. Mohamad Haidir bin Maslan for his full commitment, support and encouragement, spending some time of their busy schedule to guide me. I would also like to extend my gratitude to Mr Sivarao a/l Subramonian as the PSM Coordinator of Faculty of Manufacturing Engineering of University Technical Malaysia Malacca that had manage and ensure that the final year project was a successful one. Finally, I would like to thank my family especially my parents and my fellow friends in UTeM especially all BMFB s group members for their never ending social support and always lending a helping hand whenever I need them. iv

9 TABLE OF CONTENTS ABSTRACT ABSTRAK DEDICATION ACKNOWLEDGEMENT TABLE OF CONTENT LIST OF TABLES LIST OF FIGURES LIST OF ABBREVATIONS, SYMBOLS & NOMENCLATURE i ii iii iv v viii ix xi 1. INTRODUCTION Introduction Problem Statement Objective of Project Scope of Project 3 2. LITERATURE REVIEW Pressure Vessel Steel Fracture Mechanics Linear Elastic Fracture Mechanics (LEFM) Irwin Plastic Zone Correction Dugdale Approaches Fracture Toughness Fracture Toughness Parameters Fracture Toughness Testing Elastic Plastic Fracture Mechanics (EPFM) Stress Trixiality and Crack Growth Welding Fusion Welding Process Distortion and Cracking Post Weld Heat Treatment (PWTH) 19 v

10 2.3.4 Thermal Stress Relief Submerged Arc Welding Heat Affected Zone (HAZ) HAZ in Welds Thermal Cycle in HAZ Heating Rate Peak Temperature Cooling Rate Weldment Microstructure and Properties Hardness Testing Vickers Hardness Test METHODOLOGY Introduction Research Design Material Selection ASTM A516 Grade Sample Weld Post Weld Heat Treatment (PWTH) Sample Preparation Cutting Cutting Machine Fracture Toughness Fracture Procedures Instron Machine Structure Hardness (Vickers Hardness Test) Tensile Fractography 48 vi

11 4. RESULT AND DISCUSSION Tensile Test Hardness Hardness Result Hardness Graphs Photographs of Structures Fractographs Base Metal (BM) Heat Affected Zone (HAZ) Visual Observation Fracture Toughness Fracture Toughness Data Fracture Toughness Graph CONCLUSION Conclusion Suggestions for future work 69 REFERENCES 70 APPENDICES Appendix A Appendix B vii

12 LIST OF TABLES Chemical composition of 516- Grade 70 Pressure vessel steel Mechanical properties of 516- Grade 70 Pressure vessel steel Table 3.3 Chemical composition of BM, HAZ, WM (wt %) of 516- Grade 70 Pressure vessel Fracture Toughness graph for Comparison between HAZ and BM Tensile Result Fracture Toughness data for specimens viii

13 LIST OF FIGURES 2.1 Irwin Plastic Zone Strip Yield Plastic Zone Independent modes of crack deformation Fracture toughness varies with the specimen thickness Specimen fails in a linear brittle manner Degree of non-linearity as depicted D State of stress Various microstructural zones formed in fusion weld Sketch of the Submerged Arc Welding process Research Design Flow Chart Dimension of material Sample Compact test specimen Design that have been used successful for 35 fracture Toughness testing 3.5 Specimen Plate Vertical Band Saw machine Milling Machine a 3.14b Specimen after shaped process EDM Wire Cut Three types of load-displacement behavior in a K 1c test Instron Machine Models 8802 Specimen setup onto the machine Zones on the specimen plate Hardness grid Illustrations of Hardness graph Vickers Hardness tester Standard Rectangular Tensile Test Specimens Universal Testing Machines Scanning Electron Microscope (SEM) ix

14 3.19 Area of fracture surface Stress-Strain Diagram Hardness Grid position Hardness Graph at v1 position Hardness Graph at v2 position Hardness Graph at v3 position Structure of Base Metal (magnification 20X) Structure of Heat Affected Zone, HAZ (magnification 20X) Structure of Welded (magnification 20X) Fractograph of Pre crack and Fracture areas, BM (magnification 50X) Fractograph of Pre Cracking, BM (magnification 500x) Fractograph of Fracture, BM (magnification 500x) Fractograph of Precrack and Fracture areas, HAZ (magnification 50x) Fractograph of Pre Cracking, HAZ (magnification 500x) Fractograph of Fracture, HAZ (magnification 500x) Fatigue pre cracking onto specimen Visual Inspection of specimens Fracture Toughness graph for Base Metal, BM Fracture Toughness graph for Heat Affected Zone, HAZ Fracture Toughness graph for Comparison between HAZ and BM x

15 LIST OF ABBREVATIONS, SYMBOLS & NOMENCLATURE ~ Almost equal to 3D 3Dimension a, α Crack length, includes notch plus fatigue precrack ASME American Society of Mechanical Engineers ASTM American Society for Testing & Materials BM Base metal COD Crack Opening Displacement CTOD Crack tip opening displacement E Modulus of elasticity in plane stress EDM Electrical Discharge Machining EDX Energy dispersive X-ray microanalysis EPFM Elastic plastic fracture mechanics F Frequency (Hz) FZ Fusion zone G Energy release rate HAZ Heat affected zone HR Rockwell hardness number Hz Hertz J Energy-based estimate of fracture toughness K I Stress intensity factor (MPa mm) K IC Plane strain fracture toughness (MPa mm) LEFM Linear elastic fracture mechanics MARA Majlis Amanah Rakyat MPa Megapascal MSETsc MSET Shipbuilding Corporation Sdn. Bhd PMZ Partially melted zone PWHT Post weld heat treatment r p Radius of the plastic zone xi

16 SAW Submerged Arc Welding SEM Scanning Electron Microscope SENB Single edge notched bend WE Weld electrode WM Weld zone YFM Yielding fracture mechanics γ Gamma Δ Amount of real crack ρ Length of plastic zone σ YS Yield strength (MPa) xii

17 CHAPTER 1 INTRODUCTION 1.1 Introduction A pressure vessel is a closed container designed to hold gases or liquids at a pressure different from the ambient pressure. The pressure differential is potentially dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design, manufacture, and operation are regulated by engineering authorities backed up by laws. The need to protect the public became apparent shortly after the steam engine was conceived in the late 18th century. In the early 1800s, there were literally thousands of boiler explosions in the United States and Europe, each of which resulted in some deaths and a few injuries. The consequences of these failures were not of a catastrophic level that brought a lot of attention to them. It was not until the failures became more catastrophic that attention was brought to bear on the explosions. Canonico, D. A. (2000). For both economic and safety reasons, the pressure vessel steel with sufficient strength and toughness is required in commercial industry. In particular, the WM and HAZ must have sufficient toughness. Effects of mechanical loading, inclusion size, chemical composition and cooling rate on the toughness in pressure vessel steel welds have been extensively investigated for the last two decades. Low fracture toughness has been correlated with the crack propagation behavior of the weld. Cracks have been found in various regions of the weld with different orientation in 1

18 the weld zone, such as centerline cracks, transverse cracks and micro-cracks in the underlying WM or HAZ. 1.2 Problem Statement Failures of welded structures can and do occasionally occur, sometimes with serious human, environmental and economic consequences. Study shows approximate failure rates for various types of welded structure. For example the explosion boiler at USA in year 1900 recorded the failure rate is approximately 400 per year and for year 1970 is approximately 200 per year. For onshore gas pipeline at Western Europe traced the failure rate is 0.6 per 1000 km per year while for petroleum products pipeline at USA give the failure rate 0.55 per 1000 km per year. It shows amongst other things how the use of experience-based engineering codes and standards can reduce failure rate whereas the ASME Boiler Code Committee was established in 1911, when boiler explosions in the USA were occurring at the rate of virtually one per day. Although such occurrences are much less common a century on, the continued prevention of failure requires careful attention to design, materials, construction, inspection and maintenance. A useful way of categorizing failures in welded structures is to distinguish between instant failure modes and time-dependent failure processes. In all cases, the failure occurs when the 'driving force' for failure for example applied stress that exceeds the materials resistance such as fracture toughness. Consequently, instant failure modes are quite likely to occur early in the life cycle of the structure, perhaps due to errors in design, construction, materials or inspection. Smith, T. A. and Warwick, R. G. (1983). The temperature and maximum thickness of plates is limited only by the capacity of the composition to meet the specified mechanical property requirements. However the crack of the material due to life cycle for the required value in the investigation. 2

19 Discontinuities may be classified as defects depending on acceptance criteria in a particular specification or code. Discontinuities are rejectable only if they exceed specification requirements in terms of type, size, distribution, or location. Discontinuities may be found in the weld metal (WM), heat-affected zones (HAZ), and base metal (BM) of weldments. These may eventually lead to final failure by one of the instant failure modes described above. Welded joints are particularly susceptible to fatigue, typically initiating from discontinuities at the weld toe. The region affected is called the heat affected zone that lies outside the fusion zone in pure metals and outside the partially melted zone in alloys similar to the area in the undisturbed tank metal next to the actual weld material. Messler, R. W. (1999a). This area is less ductile than either the weld or the steel plate due to the effect of the heat of the welding process. Literature show that HAZ is frequent where damage start to occur. This zone is most vulnerable to damage as cracks are likely to start here. Thus, the zone is uncovered for exposure to influence the serious damage. 1.3 Objectives of project This research project is to i. Investigate on the fracture toughness of welded pressure vessel steel. In this project, study will be carried on investigating of fracture toughness properties for each zone of welded pressure vessel steel. ii. To differentiate the fracture behavior on area which are Base Metal (BM) and Heat Affected Zone (HAZ) 1.4 Scope of project Pressure vessel steels with good ductility and weldability have been widely used in oil and gas refinery, power generating stations and chemical industries. The weld of pressure vessel steel plates has mechanical and metallurgical inhomogeneity due to 3

20 the weld thermal cycle in the base metal (BM), the heat affected zone (HAZ) and the weld metal (WM). The extent is to differentiate microstructure of base metal zone and heat affected zone on welded structures. Welded structures are subjected to the dynamic loading usually, for example the construction structure during the life cycle on-off. It is necessary to guarantee the base steel and its welded joint for own enough fracture toughness at the loading rate which the structure subjected. Recently, a progressive methodology called as local approach is proposed to address the specimen geometry effect on the fracture resistance. The constituent relation for the structure steel at dynamic loading is decided by the experiment result. The local approach is employed to correlate the fracture toughness at the dynamic loading for HAZ zone. In the project, the fracture crack propagation in the HAZ of commercial pressure vessel steels is studied with regard to the influence of microstructure, inclusion size and distribution, and the hardness distribution. 4

21 CHAPTER 2 LITERATURE REVIEW 2.1 Pressure Vessel Steel Pressure vessel steel is defined as a container with a pressure differential between inside and outside. The inside pressure is usually higher than the outside, except for some isolated situations. The fluid inside the vessel may undergo a change in state as in the case of steams boilers, or may combine with other reagents in the case of a chemical reactor. Pressure vessels often have a combination of high pressures together with high temperatures, and in some cases flammable fluids or highly radioactive materials. In pressure vessel steels, carbon is of prime importance because of it strengthening effect. It also raises the transition temperature, lowers the maximum energy values and widens the temperature range between completely tough and completely brittle behavior. Manganese on the other hand (up to 1.5% improves low temperature properties). Of all the different kinds of steel, those produced in greatest quantity fall within the low carbon classification. These steels generally contain less than about 0.25 wt% C and are unresponsive to heat treatment intended to form martensite; strengthening is accomplished by cold work. Microstructures consist of ferrite and pearlite constituents. As a consequence, these alloys are relatively soft and weak, but have outstanding ductility and toughness; in addition they are machinable, and of all steels are the least expensive to produce. 5

22 They typically have yield strength of 275 MPa, tensile strengths between 415 and 550 MPa and ductility of 25% EL. A516-Grade 70 is one such kind of steel and has applications in low-temperature pressure vessels. Samit, S. (1998a). The important mechanical properties for pressure vessel are: i. Yield Strength ii. Ultimate Strength iii. Reduction of Area (a measure of ductility) iv. Fracture Toughness v. Resistance to Corrosion 2.2 Fracture Mechanics Fracture mechanics is a set of theories describing the behavior of solids or structures with geometrical discontinuity at the scale of the structure. The discontinuity features may be in form of line discontinuities in two-dimensional media such as plates, and shells and surface discontinuities in three-dimensional media. Fracture mechanics has now evolved into a mature discipline of science and engineering and has dramatically changed our understanding of the behavior of engineering materials. One of the important impacts of fracture mechanics is the establishment of a new design philosophy; damage tolerance design methodology, which has now become the industry standard in aircraft design. 'Fracture mechanics ' is the name coined for the study which combines the mechanics of cracked bodies and mechanical properties. As indicated by its name, fracture mechanics deals with fracture phenomena and events. The establishment of fracture mechanics is closely related to some well known disasters in recent history. Several hundreds liberty ships fractured extensively during World War II. The failures occurred primarily because of the changes from riveted to welded construction and 6

23 the major factor was the combinations of poor weld properties with stress concentrations, and poor choice of brittle materials in the construction. Of the roughly 2700 liberty ships built during World War II, approximately 400 sustained serious fracture, and some broke completely in two. The Comet accidents in 1954 sparked an extensive investigation of the causes, leading to significant progress in the understanding of fracture and fatigue. In July 1962 the Kings Bridge, Melbourne failed as a loaded vehicle of 45 tones crossing one of the spans caused it to collapse suddenly. Four girders collapsed and the fracture extended completely through the lower flange of the girder, up the web and in some cases through the upper flange. Remarkably no one was hurt in the accident. Fracture mechanics can be divided into linear elastic fracture mechanics (LEFM) and elastic-plastic fracture mechanics (EPFM). LEFM gives excellent results for brittleelastic materials like high-strength steel, glass, ice, concrete, and so on. However, for ductile materials like low carbon steel, stainless steel, certain aluminum alloys and polymers, plasticity will always precede fracture. Nonetheless, when the load is low enough, linear fracture mechanics continues to provide a good approximation to the physical reality. The purpose of this lecture is to lecture is to provide a broad picture of the theoretical background to fracture mechanics via stress analysis view point. Wang, C. H. (1996) Linear Elastic Fracture Mechanics (LEFM) LEFM applies when the materials undergoes only a small amount of plastic deformation. When characterizing the fracture toughness of these materials they can be evaluated by energy release rate (G), and stress intensity factor (K I ), which are listed in the following formulae: 7

24 Where σ is equal to the yield strength, a is the half crack size and E is the modulus of elasticity. The energy release rate can be related to the stress intensity factor by the following formulae: As state earlier LEFM only applies when very little plastic deformation occurs. To improve the accuracy of this result, several researchers including Irwin; who developed the energy release rate and stress intensity factor. Dugdale, and Barenbatt applied a correction. Irwin corrected for plasticity by assuming the existence of a circular plastic zone ahead of the crack tip. He assumed that the half crack length increases by a factor, r p which represents the radius of the plastic zone. Samit, S. (1998b) Irwin Plastic Zone Correction In order to give a better estimation of the plastic-zone size, Irwin argued that consideration of a larger plastic zone may be taken equivalent to the assumption of a larger crack as shown in figure 2.1 below. Hence, we may define an effective crack length whose length is equal to the size of the actual crack plus a correction ρ. The next step is to repeat the previous procedure for plastic zone size estimation for the effective crack. However, we consider the extra length ρ large enough to carry the extra load ignored by truncating the asymptotic stress distribution. 8