The effect of microstructure on the performance of nickel based alloys for use in oil and

194 Pages·2016·52.15 MB·English

THE EFFECT OF MICROSTRUCTURE
ON THE PERFORMANCE OF NICKEL
BASED ALLOYS FOR USE IN OIL AND
GAS APPLICATIONS
A thesis submitted to the University of Manchester
for the degree of Doctor of Philosophy
in the Faculty of Science & Engineering
2016
Velissarios Demetriou
School of Materials
Contents
Abstract 10
Declaration 11
Copyright Statement 12
Acknowledgements 13
1 Introduction 14
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2 Problem statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3 Objectives and overall aim . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.4 Structure of the work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 Literature review 19
2.1 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 Materials and properties . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1 Inconel 718 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2 Incoloy 945X . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 Crystal structures and phases of IN718 and IN945X . . . . . . . . . . . 25
2.4 Entry and transportation of hydrogen within the material . . . . . . . . 30
2.5 Form and sources of hydrogen . . . . . . . . . . . . . . . . . . . . . . . 31
2.6 Environmentally assisted cracking (EAC) . . . . . . . . . . . . . . . . . 32
2.7 Hydrogen embrittlement and stress corrosion
cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.7.1 Hydrogen embrittlement, hydrogen induced cracking and step-
wise cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2
2.7.2 Stress corrosion cracking (SCC) . . . . . . . . . . . . . . . . . . 36
2.7.3 Comparison between hydrogen embrittlement and stress corro-
sion cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.8 Hydrogen embrittlement mechanisms . . . . . . . . . . . . . . . . . . . 40
2.8.1 Hydrogen enhanced decohesion (HEDE) mechanism . . . . . . . 41
2.8.2 Hydrogen enhanced (HELP) plasticity mechanism . . . . . . . . 43
2.8.3 Adsorption induced dislocation (AIDE) mechanism . . . . . . . 44
2.8.4 Hydride induced embrittlement (HIE) . . . . . . . . . . . . . . . 44
2.9 Transportation of hydrogen within the material . . . . . . . . . . . . . 45
2.9.1 Reversible and irreversible traps . . . . . . . . . . . . . . . . . . 45
2.10 Reversible traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.10.1 Point defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.10.1.1 Diffusion of hydrogen at interstitial sites . . . . . . . . 46
2.10.1.2 Vacancies . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.10.2 Line defects - dislocations . . . . . . . . . . . . . . . . . . . . . 52
2.10.3 Crystal interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.10.3.1 Grain boundary interfaces . . . . . . . . . . . . . . . . 54
2.10.3.2 Phase interfaces . . . . . . . . . . . . . . . . . . . . . . 54
2.10.4 Three dimensional (3D) volume defects . . . . . . . . . . . . . . 55
2.11 Irreversible traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
2.11.1 Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
2.12 Fundamentals of grain boundaries . . . . . . . . . . . . . . . . . . . . . 58
2.13 Type of grain boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . 58
2.14 Atomic structure of grain boundaries . . . . . . . . . . . . . . . . . . . 60
2.14.1 Low angle grain boundaries (LAGB) . . . . . . . . . . . . . . . 61
2.14.2 High angle grain boundaries (HAGB) . . . . . . . . . . . . . . . 62
2.14.3 Special high angle grain boundaries . . . . . . . . . . . . . . . . 63
2.15 Summary of fundamentals of grain boundaries . . . . . . . . . . . . . . 64
2.16 Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
2.16.1 Gibbs free energy . . . . . . . . . . . . . . . . . . . . . . . . . . 64
2.17 Modelling phase stability . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3
3 Experimental methods 67
3.1 Starting materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.2 Cathodic pre-charging . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.3 Mechanical testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.3.1 Tensile specimen dimensions . . . . . . . . . . . . . . . . . . . . 68
3.3.2 Tensile testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3.3 Hardness testing . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.4 Metallography preparation . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.4.1 Metallographic etching techniques . . . . . . . . . . . . . . . . . 71
3.5 Scanning electron microscopy (SEM) . . . . . . . . . . . . . . . . . . . 71
3.5.1 Energy dispersive X-ray spectroscopy (EDS) analysis . . . . . . 72
3.5.2 Electron backscatter diffraction (EBSD) analysis . . . . . . . . . 72
3.6 Heat treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
3.6.1 Heat treatments of alloy 945X . . . . . . . . . . . . . . . . . . . 73
3.6.2 Heat treatments of alloy 718 . . . . . . . . . . . . . . . . . . . . 75
4 Manuscripts 77
4.1 Manuscript 1: Development of Time-Temperature-Trasformation and
Time-Temperature-Hardness diagrams for alloy 945X . . . . . . . . . . 77
4.2 Manuscript 2: Effect of hydrogen on the mechanical properties of alloy
945X (UNS N09945) and influence of microstructural features . . . . . 78
4.3 Manuscript 3: Study of the effect of hydrogen charging on the tensile
properties and microstructure of four variant heat treatments of nickel
alloy 718 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.4 Manuscript 4: Evolution of the orthorhombic δ phase in alloy 718 . . . 80
5 Summary, final conclusions and future work 81
5.1 Outputs from manuscripts . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2 Direct comparison of alloys 718 and 945X . . . . . . . . . . . . . . . . . 89
5.3 Final conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.4 Suggestions for future work . . . . . . . . . . . . . . . . . . . . . . . . 93
A XRF elemental composition analysis of alloys 94
4
Bibliography 96
5
List of Tables
2.1 Chemical composition of Inconel 718, ‘as received’ [1] . . . . . . . . . . 22
2.2 Mechanical properties of Inconel alloy 718, ‘as received’ [1] . . . . . . . 23
2.3 Chemical composition of Incoloy 945X, ‘as received’ [2] . . . . . . . . . 24
2.4 Mechanical properties of Incoloy alloy 945X, ‘as received’ [2] . . . . . . 25
2.5 Phases within superalloys Inconel 718 and Incoloy 945X . . . . . . . . . 28
2.6 Comparison between typical characteristics of hydrogen embrittlement
and stress corrosion cracking, as presented by Eliaz et al. (2002) [3] . . 40
2.7 Diffusion coefficient of hydrogen for Inconel 718 from different sources . 51
2.8 The four categories of grain boundaries . . . . . . . . . . . . . . . . . . 60
3.1 List of the experiments performed to generate the approximate time-
temperature-transformation (TTT) diagram of alloy 945X. . . . . . . . 74
3.2 The examined heat treatments of alloy 945X . . . . . . . . . . . . . . . 75
3.3 The examined heat treatments of alloy 718 . . . . . . . . . . . . . . . . 76
A.1 Chemical composition of Inconel 718, ‘as received’ (XRF experimental
results) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
A.2 Chemical composition of Incoloy 945X, ‘as received’ (XRF experimental
results) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6
List of Figures
1.1 Schematic of downhole equipment of an offshore exploration system . . 15
2.1 The unit cell of the gamma double prime precipitate which is found in
IN718 and IN945X. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.2 Line schematic of the gamma prime (γ(cid:48)) ordered FCC structure which
is found in IN718 and IN945X. . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 (a)Gaseousand(b)aqueoushydrogenembrittlementmechanisms. Her-
ring, 2010 [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4 Globaldescriptionofhydrogenembrittlementandstresscorrosioncrack-
ing interaction aspects [5] . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5 Schematicillustratingthehydrogenembrittlementmodeofavulnerable
metal in a corrosive environment containing hydrogen. . . . . . . . . . 35
2.6 Hydrogen embrittlement in cast alloy 718 [6] . . . . . . . . . . . . . . . 36
2.7 Stress corrosion cracking failures; (a) SCC crack branching in stainless
steel bar, (b) cracks starting from corrosion pits in a shaft, made of
alloyed tempering steel ASM 6418 [7] . . . . . . . . . . . . . . . . . . . 37
2.8 Stress corrosion cracking in superalloy UNS N09945, after 365 days at
conditions corresponding to ISO 15156/ NACE MR0175 level VII [8] . 38
7
2.9 The effect of hydrogen on the cohesive energy (U) and cohesive stress
(σ) of a metallic material. U◦ cohesion is the cohesive energy needed to
cleavethetwohalfsolidsalongthecleavageplanetoapreparationlarger
thanthecriticaldistance(r)whenhydrogenisnotpresent; UH cohesion
is the cohesive energy in the presence of hydrogen in the microstructure;
σ◦ cohesion is the cohesive stress required to separate the atomic bonds
in the absence of hydrogen, where σH cohesion is the cohesive stress
when hydrogen exist in the solid solution [9]. . . . . . . . . . . . . . . . 42
2.10 Schematic diagrams illustrating the decohesion mechanism, involving
tensile separation of atoms owing the weakening of interatomic bonds
by (i) hydrogen in the lattice, (ii) adsorbed hydrogen and (iii) and
hydrogen at particle-matrix interfaces [10] . . . . . . . . . . . . . . . . 42
2.11 Schematic diagram illustrating the HELP mechanism [10,11]. . . . . . . 43
2.12 Schematic diagram illustrating the AIDE mechanism, as presented by
Lynch (2012) [11]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.13 (a) Octahedral interstitial site of FCC crystal, (b) interstitial sites for
H atoms in FCC crystals. . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.14 (a) Tetrahedral interstitial sites of FCC cell, (b) interstitial sites in FCC
crystal structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.15 An interstitial site on a BCC face. . . . . . . . . . . . . . . . . . . . . . 48
2.16 Solution of the diffusion equation for different values of the diffusion
length, based on equation 2.5 [12]. . . . . . . . . . . . . . . . . . . . . . 50
2.17 Illustration of a vacancy. . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.18 Edge (Left) and screw (Right) dislocations with Burgers and line of the
dislocation vectors shown. . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.19 (a) Coherent interface, (b) two phases with coherent interface and dif-
ferent lattices, (c) a semicoherent interface with an edge dislocation, (d)
an incoherent interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.20 Variables that define a grain boundary [13] . . . . . . . . . . . . . . . . 59
2.21 Schematic of (a) a twist grain boundary, (b) an asymmetrical grain
boundary and (c) a symmetrical grain boundary [14]. . . . . . . . . . . 60
8
2.22 Schematic of (a) a twist grain boundary, (b) an asymmetrical grain
boundary and (c) a symmetrical grain boundary [14]. . . . . . . . . . . 61
3.1 Pictures showing alloy 945X forging, and how the blank specimens were
cut from it. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.2 Schematic diagram of apparatus of the electrochemical cell. W is the
counter electrode (Pt wire), R is the saturated calomel reference elec-
trode and C (specimen) is the working electrode. . . . . . . . . . . . . . 68
3.3 Comparison of 1 x 10−6/s and 3.3 x 10−4/s strain rates on the same
heat treatment of alloy 718 (a) Non-charged specimens; (b) Hydrogen
pre-charged specimens. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.4 Labelled picture of a tensile testing using the Instron 5569 load frame
with a 50kN load cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.5 The FEI Magellan 400 XHR-SEM . . . . . . . . . . . . . . . . . . . . . 72
3.6 The argon tube furnace used for the solution annealing and aging treat-
ments. It is equipped with a Newtronic Micro 96 TP+ temperature
controller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.1 The prpoposed HE mechanism and crack propagation in alloy 945X. (a)
Slip bands form in the microstructure under the uniaxial load. (b) H
diffuses in the microstructure and becomes trapped at defects . Disloca-
tions also transport H atoms during deformation. The cohesive energy
of atomic bonds is reduced. (c) During deformation and in presence of
hydrogen, voids are formed at interfaces of second phase particles, grain
boundaries interfaces and at the intersections of slip bands. (d) Voids
expand, coalesce to form larger voids and, eventually, cracks. . . . . . . 85
5.2 Tensile testing results (non-charged and pre-charged with hydrogen) of
the 718 O&G HT and 945X HT-III, as described in manuscripts two
and three. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
A.1 Spectrum of alloy 718 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A.2 Spectrum of alloy 945X . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
9
The University of Manchester
Velissarios Demetriou
Doctor of Philosophy
The Effect of Microstructure on the Performance of Nickel Based Alloys
for use in Oil and Gas Applications
December 8, 2016
Thisresearchfocused onacomprehensivemicrostructuralandmechanicalproperty
characterisation study of the Ni-Fe-Cr alloys 718 and 945X. The aim of the project
was to better understand the relationship between performance and microstructure
of existing (Alloy 718) and newly developed (Alloy 945X) high strength nickel alloys
focusing on downhole applications. The main difference between the two alloys is that
alloy 945X has lower Nb content than alloy 718, which may minimise the tendency
to form δ when combined with correct processing. Previous studies have related the
hydrogen embrittlement in alloy 718 with the collection of hydrogen by δ phase.
Microstructural characterisation of the new alloy 945X after long term isothermal
exposureupto120hoursinthetemperaturerange650◦Cto900◦Cwasconductedwith
scanning electron microscopy (SEM), to generate a time-temperature-transformation
(TTT) diagram. The TTT diagram was used as a road map for designing two isother-
mal heat treatments of alloy 945X on tensile specimens. Then, the effect of hydrogen
charging on the tensile properties and microstructure of the ‘as-received’ and these
two variant heat treatments was investigated. Fractographic analysis showed that, in
the presence of hydrogen, intergranular fracture occurred for all the heat treatments,
regardless the presence of δ phase at grain boundaries. There was no simple corre-
lation between the volume fraction of δ-phase and susceptibility to hydrogen assisted
embrittlement. Rather, it was demonstrated that the morphology and distribution of
δ-phase along grain boundaries plays a key role and the other precipitate phases also
have an influence through their effect on the ease of strain localisation.
Thisstudyalsoexaminedthehydrogenembrittlementsensitivityofnickelalloy718
given four different heat treatments to obtain various microstructural states. Each
heat treatment leads to differences in the precipitate morphologies of γ(cid:48), γ(cid:48)(cid:48) and δ
phases. Material characterisation and fractography of the examined heat treatments
were performed using a high resolution FEG-SEM. Three specimens of each condition
were pre- charged with hydrogen and tensile properties were compared with those of
non-charged specimens. It was observed that hydrogen embrittlement was associated
withintergranularandtransgranularmicrocrackformation, leadingtoanintergranular
brittle fracture. δ phase may assist the intergranular crack propagation, and this was
shown to be particularly true when this phase is coarse enough to produce crack initia-
tion, but this is not the only factor determining embrittlement. Other microstructural
features play a role, as does the strength of the material.
Finally, the evolution of δ-(Ni Nb) phase in alloy 718 from the early stages of
3
precipitation, with a particular focus on identifying the grain boundary characteristics
that favour precipitation of grain boundary δ phase was investigated. Results showed
that δ phase was firstly formed on Σ3 boundaries after 5 hours at the examined tem-
perature (800◦C). Increasing ageing time at 800◦C was observed to lead to an increase
in size and precipitation of phases γ(cid:48)-γ(cid:48)(cid:48)-δ, an increase in fraction of the special CSL
boundaries and an evolution in the morphology of twins and the growth of grains.
10