Hydrogen Attack A516 G70

i

EFFECTS OF ABSORBED HYDROGEN ON FRACTURE TOUGHNESS OF WELDED SA516 GRADE 70 STEEL

MOHAMAD HAIDIR BIN MASLAN

A project report submitted in partial fulfillment of the requirements for the award of the degree of Master of Mechanical Engineering (Mechanical Engineering)

Faculty of Mechanical Engineering Universiti Teknologi Malaysia

OCTOBER 2007

iii

Dedicated to my parents, Maslan bin Isa and Kaujah binti Yahok, my wife, Noraini binti Kurdi, my daughter, Nazurah Hana, and all my family and friends for their immensurable support and love.

iv

ACKNOWLEDGEMENTS

Thanks to God, the most gracious and the most merciful, for His guidance to accomplish this research. Without His help and mercy, this would not been possible. He is the one who knows the hardship and He is the one I seek his satisfaction and ask His acceptance. I would like to express my deepest gratitude towards my advisor, Professor Dr Mohd Nasir Tamin for his guidance, encouragement, and valuable comments during the research and writing of this dissertation. His attention and technical expertise were key elements to my success. I feel that I gained a deep knowledge from him in area of fracture mechanics and hydrogen embrittlement, which will have a significant impact on my future career. I wish to express my appreciation to my project committee members, Mr Adil Khattak, Mohamad Hafizuddin and Sebastian for their generous cooperation, hospitality, time and insight on related matters during this research. My appreciation also to Mr Rizal bin Khaus, Mr Fadlisah b Abd Kadir , Mr Sazali bin Duki and all technicians that are contribute in this study for their assistant in laboratory work. My appreciation goes to Universiti Teknikal Malaysia Melaka (UTeM) and my colleagues in the Faculty of Manufacturing Engineering for their understanding and support throughout my part time Master study. The working environment here was very pleasant, encouraging and supportive towards my work and study loads. Special thanks goes to my parents for their patient and sacrifice during my academic career. Their concern, encouragement, moral and financial support over the years has always been a source of motivation that enables me to achieve this degree. Finally, and most importantly, special thanks to my beloved wife, Noraini binti Kurdi, for her unconditional love and support during my education. Thanks for taking care of our daughter, Nazurah Hana, and for tolerating my absence from family activities during this challenging time.

v

ABSTRACT

Effects of absorbed hydrogen on structure and properties of welded A516 Grade-70 steel are investigated. Emphasis is placed on ductility measure of the crack-tip plastic zone under Mode I loading. Specimens are cathodically charged in a cell with dilute sulphuric acid and corrosion inhibitor with uniform charging current density of 20 mA/cm2 and at different exposure time. Results indicate a change from coarse- to fine-grained microstructures in the weld region and heat affected zone (HAZ) of hydrogen-charged specimen. Well-defined ferrite-pearlite bands in the base metal are transformed into coarse-grain structure. Hardness variation along radial distance indicates higher values towards the center of the bar, possibly due to faster diffusion rate but limited solubility of hydrogen. Load-COD responses indicate that slow, stable crack propagation occurred in both base metal and HAZ. The measured provisional fracture toughness, KQ is higher for HAZ than that for the base metal. The toughness values decreases significantly for the initial three hours of hydrogen charging. The tensile fracture region in the immediate fatigue pre-crack tip forms a triangular (rough) zone due to limited constraint to free surface deformation in the thin specimen. Fracture surface of HAZ is dominated by intergranular fracture with localized cleavage facets.

vi

ABSTRAK

Kesan penyerapan hidrogen ke atas struktur dan sifat bahagian kimpalan pada besi A516 Gred 70 adalah dikaji. Penekanan diberikan kepada pengukuran kemuluran pada bahagian plastik di hujung retak dengan pembebanan mod 1. Spesimen dicaj pada bahagian katod dalam sel elektrolisis yang menggunakan cecair campuran asid sulfurik dan perencat kakisan, dengan arus caj malar 20 mA/cm2 dan tempoh mengecas yang berbeza. Keputusan mendapati terdapat perubahan dari mikrostruktur kasar kepada mikrostruktur berbijian halus di bahagian kimpalan dan bahagian kesan pemanasan bagi spesimen yang dicaj dengan hidrogen. Jalus ferit pearlit pada logam asas bertukar kepada struktur bijian yang kasar. Variasi kekerasan pada jarak sepanjang jejarian didapati nilainya meningkat ke arah pusat rod. Berkemungkinan ianya disebabkan oleh kadar serapan yang pantas tetapi dengan keterlarutan hidrogen yang terhad. Bebanan perubahan pembukaan retak mendapati berlakunya perambatan retak yang perlahan dan stabil berlaku pada kedua-dua logam asas dan bahagian kesan pemanasan. Pengukuran kekuatan patah sementara, KQ bagi bahagian kesan pemanasan adalah lebih tinggi berbanding bahagian logam asas. Nilai kekuatan berkurangan dengan ketara selepas tiga jam pertama dicaj hidrogen. Bahagian permukaan patah pada pra-retak lesu berbentuk segitiga akibat dari kekurangan pergantungan kepada perubahan bentuk permukaan bebas di dalam spesimen nipis. Permukaan patah bahagian kesan pemanasan di dominasi oleh patah antara bijian dengan celah segi setempat.

vii

TABLE OF CONTENTS

CHAPTER

1

2

TITLE

PAGE

TITLE

i

DECLARATION

ii

DEDICATION

iii

ACKNOWLEDGEMENT

iv

ABSTRACT

v

ABSTRAK

vi

TABLE OF CONTENTS

vii

LIST OF TABLES

x

LIST OF FIGURES

xi

LIST OF SYMBOLS

xiii

LIST OF APPENDICES

xv

INTRODUCTION

1

1.1

Introduction

1

1.2

Background

1

1.3

Research Problems and Hypothesis

3

1.3.1 Statement of Research problem

3

1.3.2 Research Questions

3

1.4

Hypothesis

4

1.5

Objectives

5

1.6

Scope

5

1.7

Significance of Findings

5

LITERATURE REVIEW 2.1

Introduction

6

viii 2.2

Hydrogen Damage

7

2.2.1

Types of Hydrogen Damage

8

2.2.2

Hydrogen Diffusion mechanism

12

2.3

Welding

18

2.3.1 Submerged Arc Welding

19

2.3.2 Weld Stress

20

2.3.3 Post Weld Heat Treatment

20

2.3.4 Weldment Microstructure and Properties

21

2.3.5 Effect of HAZ in Hydrogen Environment

23

Fracture Mechanics

24

2.4.1 Linear Elastic Fracture Mechanics

24

2.4.2 Elastic Plastic Fracture Mechanics

27

2.4.3 Plane Stress and Plane Strain

27

2.4.4 Shear Lip Formation During Crack Growth

29

2.4

3

4

RESEARCH METHODOLOGY

30

3.1

Introduction

30

3.2

Research Design

30

3.3

Material

30

3.4

Sample Preparation

33

3.5

Hydrogen Charging Process

35

3.6

Experimental Design

36

3.6.1 Vickers Hardness Test

36

3.6.2 Microscopic Analysis

37

3.6.3 Tensile Test

37

3.6.4 Fracture Toughness Test

38

3.6.5 Fractography

42

RESULTS AND DISCUSSION

43

4.1

Introduction

43

4.2

Microstructure

43

4.3

Hardness

45

ix

5

4.4

Stress and Strain Curve

47

4.4

Fracture Toughness

48

4.5

Fractographs

50

CONCLUSION

53

6.1

Conclusions

53

6.2

Suggestions for Future Work

54

REFFERENCES

55

APPENDICES

58

x

LIST OF TABLES

TABLE NO.

TITLE

PAGE

2.1

ASTM specification for pressure vessel quality steel plate

7

2.2

Description mode of fracture and types of materials

9

3.1

Composition of A516-Grade 70 pressure vessel steel (wt. %)

32

4.1

Results from Tensile Test

47

4.2

Results from Fracture Toughness Test

48

xi

LIST OF FIGURES

FIGURE NO.

TITLE

2.1

Process of hydrogen evolution and adsorbtion

2.2

Hydrogen discharge process in metal membrane and

PAGE 13

hydrogen concentration through thickness

13

2.3

Hydrogen concentration in pressure vessel steel

14

2.4

Autoclave set up

15

2.5

Electrochemical hydrogen diffusion set-up

16

2.6

High temperature electrochemical hydrogen diffusion set up

17

2.7

Common practice to assemble pressure vessel using fusion welding processes as gas metal arc welding

19

2.8

Post Welding Heat Treatment process

20

2.9

Longitudinal Residual stress at well after post weld heat treatment

21

2.10

Temperature Gradien vs Length in welding process

22

2.11

Stress at crack tip

25

2.12

The cross hatched area represent load that must be redistributed, resulting in a large plastic zone

26

2.13

Stress Triaxiality at crack tip effect from plane stress

28

2.14

Fracture toughness versus thickness

29

2.15

Ductile growth of an edge crack

29

3.1

Research Design

31

3.2

Weld Radiograph of pressure vessel steel

32

3.3

Slice remark on the curve plate to produce straight plate

33

3.4

Flat plate after applying nital

34

3.5

Fracture toughness sample after applying nital

34

3.6

Electrolytic cell for hydrogen charging experiment

35

3.7

Location for hardness test sampling

36

xii 3.8

Shape and dimension For Rectangular Tension Test Specimens

38

3.9

Specimen for Fracture Toughness Test

38

3.10

Principal Types of Force-Displacement (CMOD) Records

40

3.11

Load-displacement curve for an invalid fracture toughness test

42

4.1

Microstructure of welded ASTM A516 steel as received (10x)

44

4.2

Microstructure of welded ASTM A516 steel 3 Hr hydrogen charging (10x)

4.3

Hardness profile along radial locations of 0.30 wt%C steel rod after 6-hour hydrogen charging

4.4

50

SEM of fracture surface of tensile specimen for base metal 3 Hr hydrogen charge

4.9

49

SEM of fracture surface of tensile specimen for base metal as received

4.8

47

Comparison between KQ value for base metal and HAZ vs hydrogen charging time

4.7

46

Stress and strain curve for ASTM A 516 steel before and after charging

4.6

45

Vickers Hardness value distribution in pressure vessel steel non-hydrogenate and 3 Hours hydrogenate time

4.5

44

51

Morphology of fracture surfaces of HAZ in the immediate region of the fatigue pre-crack tip. (a) as-received condition and (b) 3-hour hydrogen charged sample.

52

xiii

LIST OF SYMBOLS

σ YS

Yield strength (MPa)

υ

Poisson ratio

a

Crack length (mm), includes notch plus fatigue pre-crack

Ǻ

Atomic radius

B

Specimens thickness (mm)

BM

Base Metal

C(T)

Compact Test

CH4

Methane

CTOD

Crack tip opening displacement

E

Modulus Young

EPFM

Elastic plastic fracture mechanics

F

Frequency

Fe

Ferrum

Fe3C

Cementite

H2

Hydrogen gas

HAZ

Heat affected zone

KI

Stress intensity factor (MPa m )

KIC

Plane strain fracture Toughness (MPa m )

KQ

Critical stress intensity factor (MPa m )

LEFM

Linear elastic fracture mechanics

P

Load (N)

Pmax

Ultimate Load

PQ

5% secant line to elastic loading slope (N)

R

Load ratio

rρ

Radius of the plastic zone

S

Span (mm)

xiv SAW

Submerged arc welding

W

Specimen width

WM

Weld metal

xv

LIST OF APPENDICES

APPENDIX

TITLE

PAGE

A1

Project Schedule (Gantt Chart) for semester 1

59

A2

Project Schedule (Gantt Chart) for semester 1

59

B1

Stress Strain Curve for as received material

60

B2

Stress Strain Curve for 3 hr hydrogen charging material

60

C1

CTOD curve for Base Metal (as received)

61

C2

CTOD curve for Base Metal (3 hr hydrogen charging)

61

C3

CTOD curve for Base Metal (6 hr hydrogen charging)

62

C4

CTOD curve for Base Metal (9 hr hydrogen charging)

62

C5

CTOD curve for HAZ (as received)

63

C6

CTOD curve for Base Metal (3 hr hydrogen charging)

63

C7

CTOD curve for Base Metal (6 hr hydrogen charging)

64

CHAPTER ONE

INTRODUCTION

1.1

Introduction

Today business sector economics compel industrial units to attain ever-higher capacity factors, yet materials aging and the other forms of degradation are increasing the potential for components failure, derating or outages and higher operation and maintenance costs. Thus managing materials degradation and aging is one of the major technical and economic challenges facing today industry. For plants approaching the license renewal stage, assuring regulators of the continuing reliability and safety of in-service materials adds another dimension to this challenge. The rate of materials degradation, and consequently plant component or system availability, are strongly affected by a plant’s operating environment, including temperatures and corrosiveness. Thus, a comprehensive, integrated understanding of materials characterization with respect to their resistance to load, temperature and corrosive environment are a fundamental consideration in the development of overall plant business and operating strategies.

1.2

Background

Pressure vessel and piping system form a class of components for which particularly high levels of integrity and reliability are required. This is due the potential hazards which are associate with many industrial processes combined with their high capital value. In oil and gas industries, and chemical processing plants, the

2 reactor pressure vessel often operate in aggressive environment. In this study, the environment is focusing the presence of corrosive gases. The loadings constitutes of high pressure with fluctuating as in services operation and shut down. Such condition leads to environment-fatigue interaction of the material. The vessel provides the integrity of the reactor pressure boundary and function as a barrier for preventing the leakage of isolated chemical. In addition, the continued safety of the reactor pressure vessel is a key factor in ensuring the feasibility of implementing plant life extension program. Reactor pressure vessel failures have caused extensive damage to the plant, people and environment. The explosion of Union Oil amine absorber pressure vessel in

1984

has

resulted

in

causing

17

fatalities

and

extensive

property

damage(Challenger et al., 1995). The explosion of boiler/pressure vessel on-board the Mississippi steamship ‘Sultana’ in 1965 have claimed 1238 lives, although more souls were lost when ship sank within 20 min after the explosion. In 1999, 23 percent of a total of 138 explosion and 82 percent of a total of 150 accidents involved failure of boilers, resulting in 21 fatalities(Spence et al., 2004). The situation worsened in 2001 where 158 people died and 342 were injured in boilers, pressure vessel and pressure piping related accidents. Many of these reported mishaps were due to nonconforming design and fabrication of pressurized vessels and components and inadequate in-service inspection. Pressure vessel are often used in the temperature range 480-565oC with the stresses about 15-30 MPa over time periods of some 30 years. The main factor responsible for the good creep resistance of this low alloy steel is the formation of fine and highly stable dispersions of alloy carbides, although a significant contribution also comes from solid solution strengthening (Tsai, 2003). In heavywall vessels operating in similar service environment, it is common to apply welded austenitic steel inlay to low carbon steels, Thereby, taking advantage of the high strength and low cost of the base metal while retaining the superior corrosion resistance of the stainless steel weld inlay(Nasman, 1982). The low carbon steel and stainless steel inlay of the vessel is primarily constructed by welding resulting in different microstructure in the welded zone(Krisnan et al., 2005). The application of heat for the fusion process greatly transforms the microstructure; induce phase changes and mechanical properties of the steel in the vicinity of the welded region. These changes often lead to a decrease in toughness of the weld and heat affected

3 zone (HAZ) resulted in different microstructures throughout the HAZ and the associated residual stresses. Several commercially available steel have been studied for applications in reactor pressure vessel. Although these studies have contributed to better understanding of the microstructures of these welded joints, little information is available in correlating the observed microstructures with the mechanical responses of the alloy. An experimental research, establishing the processing, heat treatmentstructure-properties relationship of the alloy is therefore necessary. The result is essential in generating relevant failure data and quantifying factors that can explain fracture mechanism for both static and fluctuating load at elevated temperatures and in corrosive environment. This research is aimed at discovering and understanding the underlying fracture mechanism with environment interaction of welded connection and to examine the effect of the HAZ in both static and fatigue responses of the welded ASTM SA516 Grade 70 steel connection.

1.3

Research Problem And Hypothesis

1.3.1

Statement of Research Problem

How does the material damage evolve in the process zone of welded ASTM SA 516 Grade 70 steel subjected to hydrogen absorption?

1.3.2

1.

Research Questions

What are the damage parameters for the HAZ of welded ASTM SA516 Grade 70 steel under hydrogen environment?

2.

What are the evolution characteristics of these parameters and its limiting values?

4 3.

What type of hydrogen concentration sequence that should be applied to test specimens in accelerated tests for a given operative conditions?

4.

What are mechanism-base of life prediction models are proposed to represent the Hydrogen interaction failure of welded ASTM SA516 steel?

1.3.3

Hypothesis

Physical-based damage parameters for the HAZ include the different microstructural features of the zone, resulting from steep temperature gradient during the fusion process. These range from grain size, types of microstructures and surface defects. Hardness measurements may also indicated the material damage. In addition, initiation of microcracks may be quantified in terms of crack density (crack length per unit area sampled) Non-linear or logarithmic-type evolution characteristic is expected for the chosen damage parameter. For example, hardness measures decreases rapidly after half design-life of the vessel has been reached. This is expected due to synergistic environment effect especially at elevated temperature when creep might be present. Typical operating conditions for hydrocracking pressure vessel are 380oC455oC along with hydrogen pressure of 17 MPa. This condition has cause critical point in the wall to experience increasing of hydrogen concentration up to 4.28ppm. Temperature is set at ambient temperature as a control process, and different hydrogen concentration will be applied to get the relation of material degradation to the hydrogen concentration. Two approaches are proposed for the mechanics-based life prediction models. In the damage mechanics approach, damage will be plotted against component life(years) throughout designed life of the component. Damage parameters in this approach are hardness variation and microstructure degradation. In the fracture mechanics approach, damage parameters are fracture toughness (KIC) and stress intensity factor (KI). KI can be calculated through non-destructive testing (NDT) during in-service conditions and curve for KIC will be established through experimental setup.

5 1.4

Research Objectives

The objectives of this research are to quantify the effect of absorb hydrogen on crack tip ductility in welded SA516 grade 70 steel

1.5

Research Scope

Scope of this research is: •

Critical literature review on welded ASTM SA516 Grade 70 steel, hydrogen embrittlement of steel, and fracture mechanics

•

Development accelerated hydrogen absorption test cell. Establish absorption characteristics.

•

Mechanical test of ASTM SA516 Grade 70 steel- Tension and fracture toughness tests, fractographic and metallurgical analysis on welded samples and base metal samples.

•

Establish damage evolution characteristics of ASTM SA 516 Grade 70 steel in prolonged hydrogen environment. Damage parameters include fracture toughness, hardness and microstructure.

1.6

Significance of Study

This research directly addresses and ensures safety and integrity of vessel throughout desired life. Moreover this research will help to avoid any expected pressure vessel failure events thereby improving plant capacity, realibility and availability. Last but not least, life-extension program can successfully be achieved through this research.

6

CHAPTER TWO

LITERATURE REVIEW

2.1

Introduction

ASTM A516 Grade 70 steel is one of the most commonly used material in the fabrication of low temperature pressure vessels. Steel used in the fabrication of pressure vessel steel is in two kinds, carbon steel and alloy steel. Those type of pressure vessel steel is grades by ASTM as shown in Table 2.1. In pressure vessel steel, carbon is of prime importance because of its strengthening effect. It also raises the transition temperature, lower the maximum energy values and widens the temperature range between completely tough and completely brittle behaviour. Pressure vessel steel generally contain less than 0.25 wt % C or in other word, those steel are categories in low carbon steel. When two or more parts of pressure vessel steel are welded together, the properties of the metal in weldment vary significantly. Weldment here are heterogeneus structural elements composed of three microstructural region, base metal (BM), weld metal(WM) and heat affected zone(HAZ). Properties in those regions are different due to high heat input during welding, slag inclusions, and disproportionate heat or cooling rate(Kou 2003). During service, these pressure vessel steels, especially 2.25Cr–1Mo steel, are susceptible to temper embrittlement and/or hydrogen damage(Tan, 2005). Several

7 type of hydrogen damage such as hydrogen embrittlement, and hydrogen attack are commonly occur at the pressure vessel material. In this chapter, adequate detail description will be conduct to understanding the hydrogen damage, welding effects, fracture mechanics in order to answer the research question. Table 2.1 ASTM Specification for Pressure Vessel Quality steel Plate (Sharma, 1998) Specification A285 A299 A442 A455 A515 A516 A537 A562 A612 A662 A724 A738

2.2

Steel type and Condition Carbon steel plates of low or intermidiate tensile strength Carbon manganese silicon steel plates Carbon steel plates for applicatios requiring low transition temperature Carbon Manganese steel plates of high tensile strength Carbon silicon steel plates of high tensile strength Carbon silicon steel plates for moderate and lower temperature Heat treated carbon manganese silicon steel plates Titanium bearing carbon steel plates for glass or diffused metallic coatings Carbon steel plates of high tensile strength for moderate and lower temperature Carbon manganese steel plates for moderate and lower temperature Quenched and tempered carbon steel plates for layered pressure vessel not to subject to post weld heat treatment Heat treated carbon manganese silicon steel plates for moderate and lower temperature

Hydrogen Damage

Hydrogen damage refers to the degradation of physical and mechanical properties of metals resulting from action of hydrogen, which may be initially present inside the metal or accumulated through absorbtion. (Chatterjee, 2001, Sinha, 2003, Baecham, 1977). The damage may manifest itself in several way, loss of ductility, tensile strength, fracture toughness, creating microscopic and macroscopic damage beginning with void to internal flaking, blistering, fissuring and cracking (Chatterjee, 2001).

8 Hydrogen can be retained in steels and other metals internally as a result of melting and casting, and present externally in the atmosphere around the alloy material as a gas or constituent of gas as a result of pickling, electroplating, cathoding process, sour environment, contact with water or other hydrogencontaining liquid or gases and so forth (Chatterjee, 2001). Hydrogen damage can develop in a wide variety of environments and circumstances and also variety of materials but it is more predominant in carbon and low alloy steel (Sinha, 2003). Although hydrogen damage presence may have been suspected by makers and users of steel product long ago, but according to Buzzard and Cleaves, in 1873, the first recorded hydrogen damage in the form of hydrogen embrittlement as a cause of metal failure was made by W. H. Johnson (Beachem, 1977).

2.2.1

Types of Hydrogen Damage

Hydrogen damage has been classified into several terminologies base on hydrogen damage encounter under different conditions. The specific types of main hydrogen damage that occur on low strength steel may be categorized as follows:

2.2.1.1 Hydrogen Embrittlement

Hydrogen embrittlement can be describe as a process resulting as materials exposed to hydrogen in low temperature(less then 200oC). Hydrogen embrittlement resulting in the degradation in any one or more of a number of mechanical properties such as ductility, work hardening rate, tensile and yield strength, fracture toughness, and so on, depending on its application. Hydrogen embrittlement damage could be occur in several types of damage such as loss in tensile ductility, hydrogen stress cracking and hydrogen environment embrittlement and hydride formation. Loss in tensile ductility is a damage process when decrease in elongation and reduction in area without the formation of any visible defects, chemical product or cracking cause by hydrogen atoms collecting interstitially between metal atoms

9 causing local distortion of the metal lattice observe in lower strength alloy: steels, stainless steel, nickel base alloy, aluminium alloy. This damage is temporary and could be accomplish by heating. Hydrogen stress cracking or in the other name called hydrogen assisted cracking (HAC) and hydrogen induced cracking (HIC) is a brittle fracture of a normally ductile alloy under sustained load in presence of hydrogen. This damage often initiate at subsurface sites where triaxial stress is highest. Hydrogen stress cracking could be observe in lower strength alloy: steels, stainless steel, nickel base alloy, aluminium alloy, and in ferrous metal but restrict to alloy having hardness of 22 HRC. This damage could be observe in temperature -100 to 100°C, brittle fracture could be avoided if hydrogen if hydrogen is move out from metal by bake the materials. Hydrogen environment embrittlement encounter in an essentially hydrogenfree material when mechanically tested in gaseous hydrogen. These types of hydrogen embrittlement observe in ferritic steels, nickel base alloy, aluminium alloy and some metastable stainless steel. Hydrogen environment occur in gas pressure 3470MPa and most severe in room temperature. The fracture modes for hydrogen embrittlement can vary from intergranular fracture, transgranular ductile failure, cleavage fracture of mixed fracture depends on the materials as shown in Table 2.1. Table 2.2 Description mode of fracture and types of materials(Chatterjee, 2001) Modes of HE Fracture

Types of materials

mixed mode

high strength steel

Intergranular

nickel alloy and steel with segregation of impurities elements such as S, As, Sb

Cleavage

Ti, Nb, Zr based alloy- form stable hydrid, exhibitting a stress-induced hydride formation and cleavage mechanism

Transgranular

nickel alloy and steel with segregation of impurities elements such as S, As, Sb, but with high tensile stress

10 2.2.1.2 Hydrogen Attack

The first report on hydrogen attack of steels appeared in 1933. Inspection reports revealed internal de-carburization and cracking found in plain carbon steel vessels used in ammonia synthesis. Since that time, more hydrogen attacks were observed. In 1940’s, Nelson’s aggregate data regarding the operation experience in Petroleum and petrochemical industry were published. He develops a set of curves known as “Nelson curves”, for design and fabrication of pressure vessel and pumping at moderate temperatures and high hydrogen pressure. The Nelson curve have been accepted by the American Petroleum institute(API 941). The Nelson curve provide operating limits for the combination of temperatures and hydrogen partial pressure above which partial hydrogen attack occur. Hydrogen attack is damage process that are usually occur at petrochemical plant equipment and hydrosulfurization reactor where the materials exposed to high pressure hydrogen and temperature above 200oC(Chatterjee, 2001, Sinha, 2003). The industrial failures due to hydrogen attack are an integral part of the wider category of failures due to hydrogen damage, where related accidents have been estimated to cost equivalent to about 100 billion dollars/year in the USA and are responsible for several deaths and many injuries of workers (Kim, 2002). Hydrogen attack is an irreversible damage and leads to both surface and internal decarburization. The former is encountered on the surfaces directly exposed to high temperature and high pressurized hydrogen(Nasman, 1982). Surface decarburization leads to the formation of a decarburized layer whose thickness depends upon the operational (or testing) conditions; the composition of the steel; the history of the steel (treatment received, eventual cold work, etc.); the geometry of the component (e.g. the thickness); the surface finish (Manna, 2007). Internal decarburization takes place at temperature and pressure below the sursace

decarburization o

process

(partial

pressure>200psia(1379kPa)

and

o

Temperature>430 F (221 C) (Nasman, G. D., 1982). The process start with hydrogen permeates into steel, Hydrogen reacts with carbon atoms in cementite to form methane. 2H2+Fe3C=CH4+3Fe

11 These reactions reduce the content of carbon, a strengthening constituent, in solution in the steel. Accumulate methane at grain boundaries and at voids also contributes to the loss of strength and ductility. Molecular size of methane is too large to diffuse out of the carbon steel lattice (Nasman, 1982). These undesired methane molecules are trapped in cavities (voids) at the grain boundaries, and the high methane pressure which builds up in the cavities is responsible for cavity growth. This results in intergranular fissuring and can lead to catastrophic failure (Schlogl, 2001).

2.2.1.3 Hydrogen Induced Blistering

Hydrogen induced blistering is more common in low strength steels, and it is observed in metals exposed to sufficiently higher hydrogen-charging conditions such as acid pickling, electroplating, cathodic processes, or in service corrosive environment. This damage could occur in low temperature and in the unstressed condition. Hydrogen blistering literally means the formation of surface bulging resembling a blister. This type of damage is caused by the pressure generated by the process of combination of atomic hydrogen into molecular hydrogen. The diffusion hydrogen atoms accumulate at internal micro defects such as voids, lamination, or inclusion-matrix interfaces already present in steel. At sufficiently high concentrations, they tend to combine into molecular hydrogen, exerting an estimated pressure of several thousand atmospheres, which brings about the damage.

2.2.1.4 Shatter Cracks, Flakes and Fish Eyes

Shatter cracks, flakes and fish eyes are common features of hydrogen damage in forgings, castings, and weldments. This types of damage refers to small internal fissures that occur is steels when cooled from high temperature. At higher temperature of melting or welding or heat treatment in austenite range, the solubility of hydrogen in steel is higher than in the solidified or low

12 temperature bcc state. The excess released hydrogen accumulates at internal defects, combine to form hydrogen gas, and cause these types of damage. The cracks produced are detectable by radiographic or ultrasonic inspection, or by visual and microscopic observation of transverse sections. The extent of damage is dependent on the time of exposed to a hydrogen-containing environment.

2.2.2

Hydrogen Diffusion Mechanism

Hydrogen has an atomic radius of 0.25-0.54 Ǻ which compared with the diameter of other metallic atoms is much smaller(Eliaz, 2002). This characteristic gives hydrogen significant mobility (diffusion) in metals. Hydrogen can be introduced in a material through a variety of ways such as by electrochemical, hydrogen gas atmospheres, plating process, etc. Diffusion by electrochemical will be the elaborate in this chapter since the method will be used in this study. In order for hydrogen to go through a material it must be transported to the surface of the material, followed by adsorption, absorption and eventually transported to the material bulk. For electrochemical Hydrogen Diffusion, hydrogen was produced at the cathodic sites when acid solution molecule was receiving electron. H 3O + e − →

1 H 2 + H 2O 2

Hydrogen adsorption into a metal could be explain by characteristic of hydrogen evolution reaction. Figure 2.1 shows the process of hydrogen evolution reaction when a metal is in an acid solution. It shows the distinct steps associated with the entire process. First hydrated atoms are transported to the double layer (surface), a separation of hydrogen proton and water by adsorption, electro donation with the charge of electrons of material thereby producing a discharge; the process of hydrogen combination can occur by two ways atom-atom or ion-atom or both. The ultimate stage are eventually the desorption and entry into the material, with accompanying hydrogen evolution reaction culminating in the formation and hydrogen diffusion(Mamani, 2005).

13 1-Transport 2-Seperation

O H+

O H

H+ O

H

O

H H

H+ H+ -

7-Evolution of hydrogen diffusion

3-Adsorption

H+ H H H

6-Desorption or Entry

H H

H 4-Discharge

5-Combination

Figure 2.1 Process of hydrogen evolution and adsorbtion Rajan then show the hydrogen diffusion process in metal by applies the hydrogen evolution and adsorption information as shown in Figure 2.2. As we can see, at the surface of adsorption, the process of recombination and evolution results a maximum concentration of hydrogen occur in some distance from adsorb surface.

Metal Membrance Hydrogen rich environment

recombination

H

discharge

diffusion

H

H

H

H

evolution

Hydrogen Concentration

0

l1

L

Thickness

Figure 2.2 Hydrogen discharge process in metal membrane and hydrogen concentration through thickness(Mamani, 2005)

14 This theory is support by hydrogen concentration in pressure vessel diagram develop by Askari and Fujii. In his study for hydrogen diffusion in 300mm thick pressure vessel wall, the maximum hydrogen concentration is 4.16 ppm located about 22.85mm inside the base metal.

Figure 2.3 Hydrogen concentration in pressure vessel steel(Askari, 2004) All solid materials contain structural defects; more so crystalline solids such as the metallic ones have certain imperfections such as vacancies, dislocation, grain boundaries areas, voids, inclusions, etc. Although these defects lead the formation of internal corrosion such as blister in the material, its also retard hydrogen diffusion. When hydrogen accumulates on these defects, it becomes difficult for subsequent hydrogen diffusion or transport while the hydrogen resident time on these sites increases correspondingly in comparison to the case for normal lattice distribution. (Mamani S.C 2005).

2.2.2.1 Experimental Hydrogen Adsorption method

There is two general approach used to introduce hydrogen into metal. The first one is using gaseous, and the second one is by using electrochemical.

I.

Gaseous Approach Gaseous hydrogen charge is a traditional approach to introduce hydrogen into

the materials. Usually, the specimens are placed in an oxygen-free, high-conductivity

15 copper chamber with tantalum hydride. The tantalum hydride decomposes and creates a high partial pressure of hydrogen gas at high temperatures (above 973 K) to drive hydrogen diffusing into the specimens. Up to 40 ppm of hydrogen can be introduced into steels and alloys by this technique. (Ming Au, 2007) High-pressure hydrogen autoclave is another technique using gaseous hydrogen to charge materials. Specimens are explored to hydrogen for a long period of time (days or weeks) at high pressure (20–35MPa) and high temperature (above 623 K). At the same pressure and temperature, the hydrogen concentrations of 304 stainless

steel

calculated

by

different

researchers

using

the

equation

C H = kP1 / 2 exp(−∆H / RT ) vary largely from 16 to 118 ppm. The actual hydrogen concentrations measured by the experiments are much less than the calculated values. The main explanation is that hydrogen desorption occurred during autoclave cooling down. Some researchers were able to introduce up to 50 ppm of measurable hydrogen into 304 steel using this technique in their best effort. (Ming Au, 2007)

Figure 2.4 Autoclave set up (Manna, G., 2007)

II.

Electrochemical Method Electrochemical method introduced hydrogen into the specimens by cathodic

charging in a melted salt bath that consisted of sodium bisulfate monohydrate and potassium bisulfate. The mixed salt was melted and maintained at 473K in a glass

16 kettle. In this work, 1500 g mixed salt (2.03 g/ml bulk density) was used. Most evaporated water was collected and dripped back to the salt bath through a condenser. The water was maintained at a constant level through the dynamic evaporation–condensation process and periodical water injection. Cathodic charging was conducted under a 0.850V Ag/Ag+ fixed potential and at 423K with the specimens acting as the cathode.

Platinum (anode)

Reference Material (cathode)

Metal membrane

Figure 2.5 Electrochemical hydrogen diffusion set-up(Rumaih, 2004) After charging, the specimens were removed from the cell and rinsed with distilled water. The specimens remained at 233K in the dry ice box to prevent hydrogen from off gassing. The second method of electrochemical approach is high temperature electrochemical charging technique was developed for effective introduction of hydrogen or tritium into the metallic materials to a high level in a short period of time. The samples of the steels and alloys, as the cathode, were charged in an electrochemical cell consists of Pt anode and molten salt electrolyte. After 3, 6 and 12 h charging, the 304 stainless steel absorbed 25, 45 and 60 ppm of hydrogen, respectively. Correspondingly, the mechanical strength lost 10, 16 and 23%. The plasticity was also reduced to 20, 23 and 38%. The fractography showed the hydrogen embrittlement effect on the fractures. The electrochemical hydrogen charging technique was successfully used for introducing tritium, an isotope of hydrogen, into the super alloys for visualization of hydrogen trapped in the microstructure of the materials. It is found that the hydrogen is trapped at the grain

17 boundaries, in inclusions and carbides. The deformed and twisted grain boundaries trap most hydrogen under stress.

Figure 2.6 High temperature electrochemical hydrogen diffusion set up

2.2.3

Hydrogen Effect on Steel Properties

This study will only focus on effect on Hydrogen effect on steel properties in room temperature and low pressure. The predominant effect of hydrogen on the properties of steel at room temperature is a decrease in ductility. This degradation depends on the hydrogen content and the strength of the material. Several literature found that small amount of hydrogen can lead to a large change in the ductility of a high strength steel (Siddique, 2005). The effect of hydrogen on the yield strength and Young’s modulus of a material is not as significant as on the ductility. Various researchers have suggested that hydrogen did not affect the yield stress and the young modulus in low strength steels but gave marked reduction in both their ductility’s at room temperature. Roger found that the yield strength of SEA 1020 steel charged with hydrogen at 30 mA/cm2

18 was found to increase, compared to uncharged steel without change of young modulus. Zhang et al. Studied the effect of hydrogen on Young Modulus of α-Fe (0.13C charged in H2SO4 for 24 hrs) at room temperature found that hydrogen decreased the modulus of such steel by 0.08-1.83%. They proposed that hydrogen decreased the modulus by increasing the internal stress at accumulation sites rather than decreasing the atomic cohesive force. Ortiz and Ovejero measured the modulus of 1070 and 1005 steels (charged in H2SO4 for 24 hours) and found that hydrogen slightly changed the modulus about 0.4-1.7% of these materials. With the percent change being dependent on the steel hardness (the largest reduction of modulus was found in quenched steels (high hardness) and the smallest in normalized steel (low hardness)). Bastein and Azou found that the modulus of mild steel (0.12C charged in 10% HCl for 48 hours) decreased by 1.9%. It can be conclude that embrittlement of a material due to hydrogen could be determine mostly using fractography compare using tensile data (Tantaseraneewat, 2000).

2.3

Welding

This section of the research considers prediction of thermal effects, and transient and residual stress field in a welded thick plate. Knowledge of basic residual stress formation mechanism is necessary to understanding the environment effect. During welding, a weldment undergoes complex temperature changes that cause transient thermal stresses, and non-elastic strain in regions near the weld. The majority of service failures of industrial equipment made of Cr-Mo steels have been reported to occur in critical parts such as welds, mainly due to the microstructural changes, due to the composition of the alloy in use and to the thermal fields produced by the welding process, which give rise to marked variations in the material properties (Manna, et all, 2007).

19

2.3.1

Submerged Arc Welding

Submerged arc welding (SAW) is an arc welding process in which coalescence of metals is produced by heating them with an arc between a bare consumable electrode and the workpiece, with the arc being shielded by a blanket of granular, fusible material placed over the welding area. In the welding process, flux closest to the arc melts and form slag on the surface of the weld, thus protecting the molten metal from reacting with the air. If the welding parameters are properly set, the appearance of the weld is often very uniform and bright, merging smoothly into the workpiece material. The slag also usually comes away by itself. The flus masks the light from the arc and there is no smoke or spatter from the weld. This improves working conditions as compared to gas metal arc welding (Weman, 2003). The SAW process, owing to relatively large volumes of the molten slag and metal pool, is usually limited to flat position welding and circumferential welding. Also because of the relatively high heat input and the resulting large weld pool, coarse columnar grains often form in the fusion zone. This sometimes results in low toughness or even hot cracking of the weld metal.

Figure 2.7 Common practice to assemble pressure vessel using fusion welding processes as gas metal arc welding (Askari, 2004) SAW have good ductility, uniformity and density. Good impact strengths are obtainable when specific procedures and techniques are evaluated in combination with the electrode and flux. A proper selection of the electrode and flux provides good corrosion resistance depending upon the requirement, and ensures mechanical properties at least equal to that of the base metal(Sharma, 1998).

20 Submerged arc welding is used mainly for large items, such as plates in shipyards, longitudinal welding of large tubes or beams, or large cylindrical vessel.

2.3.2

Weld Stress After two plates are welded together, various kinds of transverse and

longitudinal stresses arise in the weldment due to disproportionate heating and cooling rates. The residual stresses vary from tension on the surface of the weld to compression in the center. Post weld heat treatment helps reduce these non uniform stresses in the weldments, but their existence cannot be completely eliminated.

2.3.3

Post Weld Heat Treatment

A post weld heat treatment is needed to reduce the internal stresses that have developed due to the welding process to an acceptable level. Post weld heat treatment could be defined as heating to a suitable temperature(recrystaallization temperature); holding long enough to reduce residual stresses, and then cooling slowly enough to minimize the development of new residual stress. Heating and cooling must be done slowly and uniformly, usually 150oC/h. Figure 2.8 show the heating and cooling process that usually applied to Cr-Mo pressure vessel wall.

Figure 2.8 Post Welding Heat Treatment process(Al rumaih, A.M., 2004)

21

This treatment has a bearing on the quality of weld or the integrity of the finished weldment, and control of temperature may be rigidly specified. However, the residual stresses in a structure subjected to PWHT still not reduced to zero (Askari, 2003).

Figure 2.9 Longitudinal Residual stress at well after post weld heat treatment (Askari, A. 2003) Figures 2.9 show the effects of PWHT on the longitudinal residual stresses at different location on the weldment. The residual stresses in the welded plate subjected to PWHT were reduced to a

much lower value, but not completely

removed. Also PWHT smoothed out the concentrated residual stress into a wider area. So the high peak values were distributed more uniformly around the welding area, and the effects of high changes of stress in a small area were reduced. In the current work, the material used was SA 516 Grade 70. Hence the welded joint should be post weld heat treated to a temperature of 600oC for one hour at a rate of 200oC/hr. The structure can be cooled slowly at a rate of 230oC/hr. ASME recommends this procedure to obtain minimum residual stresses in the welded joints.

2.3.4

Weldment Microstructure and Properties

Microstructure in heat affected zone due to welding in a carbon steel can be related to the Fe-C phase diagram, as shown in Figure 2.10. Upon high heating rate

22 and short retention time on certain temperature in welding, several types of microstructure was produce at and near to the welding zone.

Figure 2.10 Temperature Gradien vs Length in welding process (Rumaih, 2004) Generally, microstructures on welding zone are divides to several area. Columnar grain was produced at the weld zone. Then at the heat affected zone, coarse grains are produce following by fine grains and partially refined grains. For high strength low alloy steel weld using submerged arc weld, metal microstructures could be dividing in details to five regions. From the closest to the weld interface, the microstructure are constituent of: (Sharma,1998) 1. Proeutectoid ferrite, either in massive equixed form or as thin veins delineating prior austenite grain boundaries 2. Sideplate widmanstatten ferrite(parallel ferite laths) 3. Acicular ferrite(a tough structure found within the body of prior austenite grain that is formed between 592oC to 667oC 4. Retained austenite and twinned or lath mertensite(martensite-austenite phases) 5. Other-pearlite and bainite While for C-Mn Steel, the constituent of the HAZ are(Sharma,1998) 1. Proeutectoid ferite at prior austenite grain boundaries 2. Transgranular widmanstatten ferrite 3. High carbide content microstructures such as pearlite

23 4. Upper bainite and lower bainite 5. martensite Ferrite has a limited solubility of carbon and is thus a relative softer phase. It exists in other forms as acicular ferrite, side plate ferrite, grain boundary ferrite, etc. The formation of even a small volume fraction of grain boundary ferrite, ferrite side plates, or upper bainite is considered detrimental to toughness, since these microstructures provide easy crack propagation paths. Acicular ferrite on the other hand is responsible for high toughness. It is formed intragranularly, resulting in randomly oriented short ferrite needles with basket-weave like structure. This interlocking nature, together with its fine grain size, provides the maximum resistance to crack propagation by cleavage and enhances the yield strength of the metal. Pearlite consists of alternating layers of lamellae of the two phases, ferrite and cementite(Fe3C), that form simultaneously from austenite. Mechanically, pearlite has properties intermediate between the soft, ductile ferrite and the hard, brittle cementite. Bainite is the other microcontituent produced during austenite transformation. It consists of ferrite and cementite phases, and is in the form of needles or plates. Bainite and pearlite formation are competitive processes during transformation from austenite. Since bainite is a finer structure (i.e smaller cementite particles in the ferrite matrix), it is generally stronger and harder than pearlite; yet bainite exhibits a combination of strength and ductility.

2.3.5

Effect of HAZ in Hydrogen Environment

Welding effect with combination of residual stress and heat affected zone microstructure received much attention in failure analysis. However, less data could be achieve from the literature on affected of hydrogen in weldments area. Nelson curves which published for the first time in 1951 provide data available from industrial experience and indicate the pressure–temperature ranges in which certain steels, but not their weldments, may be used without experiencing hydrogen attack.

24 Hydrogen embrittlement in combination with residual stresses is an irreversible process that can cause permanent damage resulting in degradation of the mechanical properties and subsequent failures (Tan, 2005). Several study also have been made to determine effect of microstructure on hydrogen embrittlement. Microstructure have been ranked from the best to worst resistance to hydrogen embrittlement as follows(Banerjee, 2002): 1. Spheroidized or normalized structure 2. Martensite, tempered to precipitate a fine dispersion of alloy carbides 3. Lower bainite plus tempered martensite 4. Martensite, tempered near 500C.

2.4

Fracture Mechanics

Fracture mechanics as an engineering dicipline was introduced in 1950s under the leadership of G. R. Irwin at the Naval Research Laboratory (NRL). The concepts of fracture mechanics were further developed and refined throughout the 1960s(Sanford R.J.2003). The principle of fracture mechanic is to study the fracture behaviour of structure and the influence of parameters such as flaw size, applied stress and fracture toughness of the materials.

2.4.1

Linear Elastic Fracture Mechanics

For many years, linear elastic fracture mechanics (LEFM) has been the primary method for fracture analysis. LEFM applies when the material undergoes only a small amount of plastic deformation. Using LEFM, the stress intensity factor, KI, in the structure is compared with the critical stress intensity factor, KIC, also called the material fracture toughness. The subscript I stands for mode I crack opening mode. Fracture toughness first used when Irwin and Westergaad’s method shows that stress and displacement near crack tip completely characterised in LEFM by a

25 single constant called stress intensity factor, K(Anderson, 2005). This was done by analysing crack near field stress, as shown in Figure 2.11

Figure 2.11 Stress at crack tip

Stress intensity factor describes as

⎛ w⎞ K = σ πa f ⎜ ⎟ ⎝a⎠ Where

σ

= applied stress

a

= crack size

w

= plate width

⎛ w⎞ f ⎜ ⎟ = constant depends on the crack geometry ⎝a⎠ Crack in ideally brittle material plates with infinite size is the easiest crack geometry formula to be develop since the boundary condition could be neglected. Solution for this case was given by Westergaard, Irwin and Koiter, and further development done by Feddersen. The result are:

πa ⎛w⎞ f ⎜ ⎟ = sec W ⎝a⎠ Hence, the formula for stress intensity factor for crack in ideally brittle material plates with infinite size become: K = σ πa sec

πa W

However, this formula gives a infinite stress values occur at crack tip. But, in real structures, stresses at crack tip are finite. Once inelastic deformations occur at

26 the crack tip, stresses at the crack tip relax. Hence, elastic analysis becomes increasingly in accurate as the inelastic region at the crack tip grows.

Figure 2.12 The cross hatched area represent load that must be redistributed,

resulting in a large plastic zone Irwin noted this, and he proposed plastic zone correction to estimate stress intensity factor using LEFM for moderate crack tip yielding.

⎛ w K eff = σ πaeff f ⎜ ⎜a ⎝ eff Where

⎞ ⎟ ⎟ ⎠ 2

a eff

⎛ 1 = a+⎜ ⎝ 2π

⎞⎛⎜ K I ⎟⎜ ⎠⎝ σ YS

⎞ ⎟⎟ For plane stress ⎠

a eff

⎛ 1 = a+⎜ ⎝ 6π

⎞⎛⎜ K I ⎟⎜ ⎠⎝ σ YS

⎞ ⎟⎟ For plane strain ⎠

2

Upon further development, Dugdale and Barenblatt created the strip yield model. This model assumes for a narrow strip plastic zone with the length of p effective intensity factor for through-thickness crack in an infinite plate under plane strain condition become:

⎛ πσ K eff = σ πa sec⎜ ⎜ 2σ ⎝ ys

⎞ ⎟ ⎟ ⎠

In 1966, Further development done by Buderkin. The modified equation to estimation of the strip yield model become

27

⎛ 8 ⎞ ⎛ ⎛ πσ K eff = σ ys πa ⎜ 2 ⎟ ln⎜ sec⎜ ⎝ π ⎠ ⎜⎝ ⎜⎝ 2σ ys

2.4.2

⎞⎞ ⎟⎟ ⎟⎟ ⎠⎠

Elastic Plastic Fracture Mechanics

Linear elastic fracture mechanics (LEFM) can be usefully applied as the plastic zone is smalls compared to the crack size. However, if the plastic zone is large compared to the crack size, LEFM do not apply any longer. Therefore, elastic plastic fracture mechanics(EPFM) analysis is necessary for ductile material such as low carbon steel and alloy steel. EPFM was first proposed by Wells in 1963. Wells proposed that the displacement of the crack faces be an alternative fracture toughness area. As the plastic zone spread through the entire cracked section, plastic deformation at the crack tip can occur freely. This idea became the basis of the crack tip opening displacement (CTOD) method.

2.4.3

Plane Stress and Plane Strain

The radius of the Irwin plastic zone is different depending on whether plane strain or plane stress conditions exist. As in real life, stress and strain exists in all three planes, but three dimensional stress field are very difficult to solve. For this reason the assumptions of plane stress and plane strain were made. Plane strain is used for thick sheets and assumes the stress in z direction (principal direction 3) is equal to υ(σ1 + σ 2 ) . In other words, the material in the z direction resists the material’s tendency to neck (ε Z = 0) . Plane stress is used for thin sheets and that there is no stress in the zdirection. In other words, there is not enough material in z-direction for stress variation to occur (σ Z = 0) .

28 As the plane stress and plane strain effects depending on plate thickness, stress intensity value also different. Figure 2.13 shows a crackes plate with thickness B subjected to in-plane loading. Assuming that the plastic zone is small, the region of the plate that is far from the crack tip must be loaded in plane stress. Material near to the crack tips loaded higher stress then the surrounding material. The high normal stress at the crack tip causes material near to surface to contract, but the material in the interior is constrained, resulting in a triaxial stress state. Material on the plate surface is in a state of plane stress because there are no stresses normal to the free surface.

Stress intensity at the crack tip

B

Figure 2.13 Stress Triaxiality at crack tip effect from plane stress

However, if the thickness of the plate extends to some value, the plane stress condition will change to plane stress condition where stress normal to the free surface start to exist. Plane stress reduces the stress intensity different at the surface and in the interior. This phenomena make the fracture toughness value in the plane strain lower than plane stress condition, and the value is not dependent to the thickness value as shown in Figure 2.14

Fracture Toughness, KQ

29

KIC Plane stress behavior

Plane strain Behavior

Thickness, B Figure 2.14 Fracture toughness versus thickness

2.4.4

Shear Lip Formation During Crack Growth

When an edge crack in a plate grows by microvoid coalescence, the crack exhibits a tunneling effect, where it grows faster in the center of the plate, due to higher triaxiality. The through thickness variation of triaxiality also produces shear lips, where the crack growth near the free surface occurs at a 45o angle from the maximum principal stress as illustrated in Figure 2.10

Figure 2.15 Ductile growth of an edge crack

30

CHAPTER THREE

RESEARCH METHODOLOGY

3.1

Introduction

Chapter three clarifies in adequate detail the method and procedure to perform each essential step in the research in order to obtain accurate and valid results.

3.2

Research Design

The research is an experimental study on material damage accumulated during environment fatigue interaction process. The research design is presented in Figure 3.1

3.3

Material

The material employed in this study is ASTM A516 Grade 70 steel. The material is 16-mm thick pre fabricated plates using to produce the curvature of a cylindrical pressure vessel wall. Each plate is receive with dimension 300mm x 300mm with double vee butt welding at the centre of the plate produced by multiple-

Material

Sample Preparation

Experimental

Figure 3.1 Research Design E

σ σ y

σULT

Tensile Test

ε

Tensile Specimen

KIC

3

9

Charging Time(h)

HAZ

Base Metal

Fracture Toughness Test Hv

Microstructure Comparison, before and after charging

Microstructural Analysis

2 3 4 5 6 7 8Charging Time(h)

Base Metal

HAZ

Vickers Hardness test

Microstructure Specimen at Base Metal and HAZ

Base Metal

Vickers Hardness Specimen

Electrochemical Hydrogen Diffusion Process

CTOD Fracture Toughness Specimen

Welded Part

ASTM SA516 Grade 70 Steel

31

Effects of Absorbed Hydrogen on Fracture Toughness of Welded SA516 Grade 70 Steel

Expected Result

32 pass submerged arc welding(SAW). All the SAW were performed by Hong Weng Sdn Bhd using ANSI/AWS specification and section IX ASME boiler and pressure vessel, welding procedure 135. Post weld heat treatment (PWHT) was carried out at 620oC for 1 hours with the increasing and decreasing rate for 220oC/Hr and 278oC/Hr performed by Pioneer Heat Treatment Sdn Bhd. Radiographic inspection for non destructive test is then performed by Total Sterling (M) Sdn Bhd to indicated complete fusion, free from crack and found to be satisfactory. Figure 3.2 shows the weld radiograph of pressure vessel steel showing complete fusion.

Figure 3.2 Weld Radiograph of pressure vessel steel

A516-Grade 70, plain carbon steel contains two main elements and some other minor ones. The two main constituent elements are carbon (C) and manganese (Mn). Minor alloying elements include sulfur (S), phosphorus (P), silicon (Si), chromium (Cr), nickel (Ni), molybdenum (Mo) and Vanadium (V). The relative amounts of each constituent alloying element shown in Table 3.1

Table 3.1 Composition of A516-Grade 70 pressure vessel steel (wt. %) Element

C

Mn

S

P

ASTM wt%

0.28

1.23

0.035

0.035

Actual wt%

0.26

1.12

0.004

0.015

Si

Cr

Ni

Mo

V

Fe

0.40

-

-

-

-

Bal

0.457

0.018

0.021

0.012

0.005

Bal

33 3.4

Sample Preparation

The first task in preparing for the experimental is cutting process for sample preparation. As mention before, the material come in curve plate shape, hence the material needs to be slice from the curve plate to produce specimen in straight shape as shown in Figure 3.2. This slicing process is conduct using EDM wirecut machine to minimize local heating effect at the material.

Figure 3.3 Slice remark on the curve plate to produce straight plate

The slicing process produce three flat plate with thickness 10mm and width 100mm from each curve plate. Welded location could be define after the slicing process by applying 2% nital on the material surface. Nital will react with the material surface and produce a bright colour at the location of welded zone. Figure 3.4 show the flat plate after applying the nital.

34

This bright zone appears after applying nital

Figure 3.4 Flat plate after applying nital

After the slicing process complete, further cutting process is to produce the specimen. Again, using the EDM wirecut, sample for tensile and fracture toughness is produce. Sample for fracture toughness need to be produce to measure the fracture toughness exactly for the base metal and for the heat affected zone. Welded zone could be determine by applying 2% nital at the plate surface. Figure 3.5(a) show the specimen for fracture toughness after applying 2 % nital. The bright part at the sample is weld metal. Figure 3.5(b) show the assumption that been made to estimate that the edge is locate absolutely on HAZ region.

HAZ Welded Zone Welded Zone (a)

(b)

Figure 3.5 (a) Fracture toughness sample after applying nital (b) Assumption for determining location for HAZ for fracture toughness analysis

35 3.5

Hydrogen Charging Process

The SA516 Grade 70 steel specimen was charged with hydrogen using electrochemical process. Hydrogen charging of the polished test specimen was carried out using a simple set up comprising of an electrolytic cell (Fig. 3.6) with stainless steel plate as the cathod and SA516 Grade 70 steel specimen as the anode. The specimen was mounted in the cell filled with dilute sulphuric acid solution (0.5M H2SO4 solution) containing 10mm of arsenic oxide (As2O3) as a corrosion inhibitor. Previous study showed that this solution with arsenic addition is capable of introducing high total H concentrations in the Cr-Mo base steel, reaching 9 and 10 ppm when charging at intermediate current densities of 1- to 20 mA/cm2 [Rumaih]. A constant current density of 10 mA/cm2 was maintained for the specimen for 24 hour. After charging, each sample was removed and quickly immersed in liquid Nitrogen to minimize H loss. The sample was then quickly cleaned by abrasive-paper grinding to remove any present scale and put back in liquid nitrogen before any further experiment.

Cathode: SA516 Grade 70 Steel

Anode: Stainless Steel

0.5M H2SO4 + 10g As2O3 Figure 3.6 Electrolytic cell for hydrogen charging experiment

36 3.6

Experimental Detail

The aim of this study was to investigate the effect of absorbed hydrogen on properties of SA516 Grade 70 steel such as tensile strength, hardness, chemical composition, microstructure, and fracture toughness for both base metal and HAZ of the A516 Grade 70 steel.

3.6.1

Vickers Hardness Test

The main purpose of the testing was to compare the effect of hydrogen adsorption in term of hardness to the material. Two studies on the effect of hydrogen in material hardness was done to the material. The first study is to evaluate the hydrogen effect in the cross section of 32-mm diameter steel rod subject to 6 hour hydrogen charging. The second study is to determine the hardness different on base metal, HAZ, and weld zone before and after charging. These experiments will be conduct base on standard ASTM E92. Sample hardness variation study in cross section of a 32-mm diameter steel rod subjected to 6 hour charging was prepare by cut the rode after charging process using EDM wirecut. Then the measurement is taken to measure the hardness variation for every thickness. While, for comparison of hardness value in base metal, HAZ and welded zone, cross section for both virgin material and after charged specimen is cut at the base metal, HAZ and the welding zone. Then the hardness value is taken across the welded joint as shown in Figure 3.7

Heat Affected Zone

Welded Zone

Base Metal

Figure 3.7 Location for hardness test sampling

37

For both study, the surface of the cutting surface need to be prepared so that the ends of the diagonals are clearly defined and can be read with precision of 0.0005 mm or 0.5 % of the length of the diagonals as state in the standard. A vickers microhardness tester with a 1kgf load maintained for (10-15) seconds was used. Five data was taken for each sample. The results of this can be seen in chapter four in form of hardness vs hydrogen concentration graph.

3.6.2

Microscopic Analysis

Microstructural study is performed using optical microscope (OM). The following sequence of step was utilized to prepare the specimens for microscopic examination: a. Grinding on 230 grit silicon carbide paper b. Drinding on 4000 grit silicon carbide paper c. Polishing with 15 micron diamond paste on nylon cloth d. Polishing with 1 micron diamond paste on nylon cloth e. Etching with 2% nital and 5% picral

The resulting microstructure of the weldment was optically examined using an (microscope spec) with attached (camera spec). Photomicrographs of various magnifications at different positions of the weldment were taken. The micrographs are shown in chapter four

3.6.3

Tensile Test

Tensile test is a testing to get the value of modulus young, yield strength, and the stress strain plot curve for overall specimen with certain concentration of hydrogen. This experiment will be conduct base on standard ASTM E8M. Figure show the shape and dimension for the specimen.

38 L C

W G

R

T

G - Gage length 50 mm W - Width 40 mm T - Thickness 5 mm R - Radius of fillet 12.5 mm L - Overall length 200 mm Figure 3.8 Shape and dimension For Rectangular Tension Test Specimens

Specimen for tensile test was prepared for both base metal and welding parts. Test was done to measure material properties before and after charging. Loading was done using Instron 100kN with crosshead speed of 2mm/min. Extensometer used to measure elongation on the gage length.

3.6.4

Fracture Toughness Test

These test are performed in accordance with ASTM E399. This test method determines the plane strain fracture toughness (KIC) of the materials. Compact C(T) specimen (Figure 3.9) was chose for this test. This test is very stringent and a valid test has to satisfy several criteria regarding specimens thickness, crack length, and crack length to weight ratio. 1.25W

precrack 1.2W

10 mm