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SUSCEPTIBILITY OF CREEP AGED MATERIAL TO STRESS ©

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SUSCEPTIBILITY OF CREEP AGED MATERIAL TO STRESS ©
SUSCEPTIBILITY OF CREEP AGED MATERIAL TO STRESS
RELIEF CRACKING DURING REPAIR WELDING
HERMAN MOGGEE
© University of Pretoria
SUSCEPTIBILITY OF CREEP AGED MATERIAL TO STRESS
RELIEF CRACKING DURING REPAIR WELDING
by
HERMAN MOGGEE
SUBMITIED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE
MASTERS IN ENGINEERING
{METALLURIGICAL ENGINEERING)
DEPARTMENT OF MATERIALS AND METALLURGICAL ENGINEERING
UNIVERSITY OF PRETORIA
SUPERVISOR AND MENTOR: PROF.
31
0KTOBER
G. T.
1998
© University of Pretoria
VAN ROOYEN
AKNOWLEDGEMENTS
Prof. G.T. van Rooyen for believing in me.
Prof. P.G.H. Pistorius, Prof. P.C. Pistorius, Mr. J. Borman and the rest of the Department of
Material Science and Metallurgy at the University of Pretoria
Eskom
P. Doubell, F van Zyl, P Erasmus, the people at Eskom
G. von dem Bongardt
My mother and the rest of my family
Elri
GOD
When standing to close to the tree
One tends not to see the flowers.
GT
© University of Pretoria
Susceptibility of creep aged material to stress relief cracking during repair
welding
Candidate: Herman Moggee
Mentor: Prof. G.T. van Rooyen
Department of Material Science and Engineering, University of Pretoria
Masters in Engineering
ABSTRACT
The repair welding of main steam pipelines, which involves the welding of new material onto
service-exposed material, are investigated. This paper investigates the literature and
experimental work surrounding this subject. The introduction provides a background to the
applicable welding technology. In section two the heat-affected zone is discussed with
emphasis on the residual stresses that develop in this zone. The mechanical properties of the
heat-affected zone are also investigated. This includes the tensile, toughness and hardness
properties as well as inspecting the relevant microstructures. The effect of post weld heat
treatment on these properties is also investigated.
Section three investigates the
phenomenon of creep. Not only is this important due to the high temperatures at which these
pipelines operate, but creep is also associated with some failures of these weld during post
weld heat treatment. The creep properties of the heat-affected zone are investigated in detail
with the use of weld simulation. Sections four and five detail reasons for weld failure after
welding due to hydrogen and reheat cracking. Hydrogen cracking is investigated with the use
of slow strain rate tensile tests during cathodical charging the specimen with hydrogen. The
phenomenon of reheat cracking is investigated with the use of high temperature tensile tests
as well as a novel approach in which the stress relief of a welded joint is simulated while
measuring crack growth and stress relieved.
SAMEVATTING
Die herstelsweis van die hoofstoompyplyne noodsaak die verbinding van nuwe materiaal aan
diens verouderde materiaal. Die oorsig ondersoek die literatuur beskikbaar oor die besondere
sweis. Die inleiding verskaf inligting in verband met die omgewing waarin die besondere
sweis gemaak word. In afdeling twee word die hitte invloedsone bespreek met klem op die
resspannings in hierdie sane. Die sterkte, hardheid en taaiheid eienskappe van die sane word
ook ondersoek, asook die effek wat nasweishittebehandeling op dit het. Afdeling drie
bespreek die kruipeienskappe van die basismaterial materiaal en die besondere sweisnaat.
Dit word moontlik gemaak deur die intensiewe gebruik van sweis simulasie. Afdeling vier en
vyf ondersoek twee moontlike redes vir die faling van die sweislas direk na sweising of
gedurende nasweis hittebehandeling. Dit sluit in waterstof kraking asook herverhit krake.
Waterstof krake word ondersoek deur gebruik te maak van stadige trektoetse onderwyl die
monster katodies gelaai word met waterstof. Die voorkoms van herverhit krake word
ondersoek deur gebruik te maak van hoe temperatuur trektoetse asook die simulasie van
In hierdie toets word die die spanning verlig asook die
nasweis hittebehandeling.
kraakvorming gedurende nasweis hittebehandeling gemeet.
KEYWORDS
~CR-~MO-%V; CREEP RESISTANT MATERIAL; REHEAT CRACKING; HYDROGEN
CRACKING; STRESS-RELIEF; MAIN STEAM PIPELINE REPAIR WELD
© University of Pretoria
CONTENTS
CHAPTER 1
1.1
INTRODUCTION ...............................................................................................................1
CHAPTER 2 ....................•..•......................•.....••...........•..•........••.••........•.....•...•..•..•........•......3
2.1
HEAT AFFECTED ZONE ....................................................................................................3
2.2
GRAIN GROWTH ZONE ....................................................................................................3
2.3
RECRYSTALLIZED ZONE {GRAIN REFINED ZONE) .................................................•............... 3
2.4
INTERCRITICAL ZONE ......................................................................................................4
2.5
WELD SIMULATION ..................................................................•.........................•............4
2.6
RESIDUAL STRESSES IN WELDMENTS ...............•...................•..............•............•......•.••.•.. .4
2. 7 MICROSTRUCTURE ..................................................................................................•••....6
2.& HARDNESS PROPERTIES
...........................................................................................•..... 9
2.9 TENSILE PROPERTIES .............•.................................................................................•... 10
2.10
TOUGHNESS PROPERTIES .................................................................•......................... 12
2.11
CONCLUSIONS ............................................................................................................ 13
CHAPTER 3 ........................................................................................................................14
3.1
CREEP PROPERTIES OF A %CR-%Mo- %V REPAIR WELD .......................................•........ 14
3.2
CREEP TESTING ............................................................................................•.............• 14
3.3
MECHANISMS OF CREEP DEFORMATION
17
•••••••••••••••••••••••••••••••••••••••.•••••••••••••••••••••••••••••• 17
3.3.1 Dislocation Glide .................................................................................................. 17
3.3.2 Dislocation Creep ................................................................................................. 17
3. 3. 3 Diffusion Creep .................................................................................................... 18
3. 3. 4 Grain Boundary Sliding ........................................................................................ 18
3.4
DEFORMATION MECHANISM MAPS ........................................................................•........• 19
3.5 ACTIVATION ENERGY FOR STEADY STATE CREEP ............................................................ 20
3.6
CAVITIES .............................•..............•.........................................•..••..•..•..........••...•....20
3. 7
MICROSTRUCTURE .......••.............................•.•.............•.•.......•..•...•.•..•..••.••.....•......••••... 23
3.8
REMNANT LIFE ESTIMATION .........................••...•.•...•.........••......•••...•......•.....••........•..•.••. 23
3.9
EXPERIMENTAL WORK .......••..................................••.........•...••.......•..•.•.....••...••......•...... 25
3.10
CREEP RESULTS AND DISCUSSION .......................•.......•...........................•.•....•....•..•.... 26
3.10.1 Creep Rupture Tests .......................................................................................... 26
3.10.2 Steady State Creep Tests. ................................................................................. 28
3.11
CONCLUSION- CREEP ..........................................................................•...................... 30
CHAPTER 4 ........................................................................................................................31
4.1
HYDROGEN CRACKING ...........................•..............••.•.........•.......•..............•.•............•.•.• 31
© University of Pretoria
4.2
TEMPERATURE ............................................................................................................. 31
4.3
STRESS .....................................................•.............................................................•... 32
4.4
INCLUSIONS ...................................................................................................•............. 32
4.5
MICROSTRUCTURE ........•..................................................................... ··••······· .............. 33
4.6
EXPERIMENTAL WORK ...........................................•........•............................................. 34
4.7
RESULTS AND DISCUSSION .......................................................•..................................... 35
4.8
CONCLUSION ...........................................................................•.............•..................... 37
CHAPTER 5 ........................................................................................................................38
5.1
REHEATCRACKING ....•......................................•.....................•..................................... 38
5.1.1. Role of composition ............................................................................................. 38
5.1.2. Role of impurities. ............................................................................................... 38
5. 1. 3. Role of grain size ................................................................................................ 40
5.2
EXPERIMENTAL TESTING METHODS TO ASSES SRC ...•............•.......••............•.................. .40
5.3
EXPERIMENTAL WORK- ELEVATED TEMPERATURE TENSILE TESTS ........•........•.................. 40
5.4
RESULTS AND DISCUSSION- ELEVATED TEMPERATURE TENSILE TESTING ......................... .42
5.4.1. The effect oftemperature .................................................................................... 42
5. 4. 2. Effect of strain rate.............................................................................................. 43
5. 4. 3. Effect of microstructure ....................................................................................... 45
5.5
EXPERIMENTAL WORK- STRESS-RELIEF TESTS ON WELDED SPECIMEN ............................ 47
5.6
RESULTS AND DISCUSSION- STRESS RELIEF SETUP: NEW MATERIAL. ............................... 49
5.6.1. Influence of stress ............................................................................................... 49
5. 6. 2. Influence of heating/cooling rate with post weld heat treatment ........................... 51
5. 6.3. Influence of temperature ..................................................................................... 53
5. 6.4. Influence of successive post weld heat treatment ................................................ 54
5. 6. 5. Influence of microstructure .................................................................................. 55
5.7
STRESS RELIEF TESTING OF SERVICE-EXPOSED MATERIAL. ............................................... 51
5.8.
CONCLUSIONS ............................................................................................................. 59
CHAPTER 6 •••.............................................................................•.•...........•........••..•...•.....••.61
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