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

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 2 2008/2009

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.

4.

5. *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, 16100 Kota Bharu, Kelantan

Tarikh: 22 MEI 2009

(EN. MOHAMAD HAIDIR BIN MASLAN) Cop Rasmi: 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.

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

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:

…………………………………………

i

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 K1c 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.

ii

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 K1c 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.

iii

DEDICATION

Dedicate to my parents in the grandest sense that parenting implies

iv

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.

v

TABLE OF CONTENTS

ABSTRACT i

ABSTRAK ii

DEDICATION iii

ACKNOWLEDGEMENT iv

TABLE OF CONTENT v

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF ABBREVATIONS, SYMBOLS & NOMENCLATURE xi

1. INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Objective of Project 3

1.4 Scope of Project 3

2. LITERATURE REVIEW 5

2.1. Pressure Vessel Steel 5

2.2. Fracture Mechanics 6

2.2.1 Linear Elastic Fracture Mechanics (LEFM) 7

2.2.1.1 Irwin Plastic Zone Correction 8

2.2.1.2 Dugdale Approaches 9

2.2.2 Fracture Toughness 10

2.2.2.1 Fracture Toughness Parameters 11

2.2.2.2 Fracture Toughness Testing 13

2.2.3 Elastic Plastic Fracture Mechanics (EPFM) 15

2.2.4 Stress Trixiality and Crack Growth 16

2.3. Welding 17

2.3.1 Fusion Welding Process 17

2.3.2 Distortion and Cracking 18

2.3.3 Post Weld Heat Treatment (PWTH) 19

vi

2.3.4 Thermal Stress Relief 20

2.4. Submerged Arc Welding 21

2.5. Heat Affected Zone (HAZ) 22

2.5.1. HAZ in Welds 22

2.5.2. Thermal Cycle in HAZ 24

2.5.2.1 Heating Rate 25

2.5.2.2 Peak Temperature 25

2.5.2.3 Cooling Rate 25

2.6. Weldment Microstructure and Properties 26

2.7. Hardness Testing 27

2.7.1. Vickers Hardness Test 28

3. METHODOLOGY 29

3.1. Introduction 29

3.2. Research Design 30

3.3. Material Selection 31

3.3.1 ASTM A516 Grade 70 31

3.4. Sample 33

3.4.1 Weld 33

3.4.2 Post Weld Heat Treatment (PWTH) 34

3.4.3 Sample Preparation 34

3.4.4 Cutting 35

3.4.4.1 Cutting Machine 36

3.5. Fracture Toughness 39

3.5.1 Fracture 40

3.5.1.1 Procedures 40

3.5.1.2 Instron Machine 43

3.5.2 Structure 44

3.5.3 Hardness (Vickers Hardness Test) 44

3.5.4 Tensile 46

3.5.5 Fractography 48

vii

4. RESULT AND DISCUSSION 50

4.1 Tensile Test 50

4.2 Hardness 52

4.2.1 Hardness Result 53

4.2.2 Hardness Graphs 53

4.3 Photographs of Structures 54

4.4 Fractographs 56

4.4.1 Base Metal (BM) 56

4.4.2 Heat Affected Zone (HAZ) 59

4.5 Visual Observation 62

4.6 Fracture Toughness 63

4.6.1 Fracture Toughness Data 64

4.6.2 Fracture Toughness Graph 65

5. CONCLUSION 68

5.1 Conclusion 68

5.2 Suggestions for future work 69

REFERENCES 70

APPENDICES

Appendix A

Appendix B

viii

LIST OF TABLES

3.1

3.2

3.3

4.1

4.2

4.3

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

32

32

32

50

51

64

ix

LIST OF FIGURES

2.1 Irwin Plastic Zone 9

2.2 Strip Yield Plastic Zone 10

2.3 Independent modes of crack deformation 11

2.4 Fracture toughness varies with the specimen thickness 13

2.5 Specimen fails in a linear brittle manner 14

2.6 Degree of non-linearity as depicted 14

2.7 3D State of stress 16

2.8 Various microstructural zones formed in fusion weld 18

2.9 Sketch of the Submerged Arc Welding process 22

3.1 Research Design Flow Chart 30

3.2 Dimension of material 31

3.3 Sample 34

3.4 Compact test specimen Design that have been used successful for

fracture Toughness testing

35

3.5 Specimen Plate 36

3.6

3.7

Vertical Band Saw machine

Milling Machine

37

37

3.8

3.9

3.10

3.11

3.12

3.13

3.14a

3.14b

3.15

3.16

3.17

3.18

Specimen after shaped process

EDM Wire Cut

Three types of load-displacement behavior in a K1c 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)

38

39

41

43

44

44

45

45

46

47

48

48

x

3.19

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

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

49

51

52

53

54

54

55

55

56

57

58

58

59

60

61

62

63

65

65

66

xi

LIST OF ABBREVATIONS, SYMBOLS &

NOMENCLATURE

~ – Almost equal to

3D – 3Dimension

a, α – Crack length, includes notch plus fatigue pre-

crack

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

KI – Stress intensity factor (MPa √mm)

KIC – 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

rp – Radius of the plastic zone

xii

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)

1

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

2

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.

3

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

4

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.

5

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.

6

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

7

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 brittle-

elastic 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).

2.2.1 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 (KI), which are

listed in the following formulae:

8

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, rp which represents the radius of the plastic zone. Samit, S.

(1998b).

2.2.1.1 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.