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Dissimilar Metal

Jun 02, 2018




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    Dissimilar Metal Welds


    Karl E. Dawson

    Thesis submitted in accordance with the requirements of the University

    of Liverpool for the degree of Doctor in Philosophy.

    July 2012

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    PREFACEThis dissertation is submitted for the degree of Doctor of Philosophy at the University of

    Liverpool. The work undertaken and described herein was carried out under the supervision

    of Professor Gordon J. Tatlock in the School of Engineering and Material Science, at the

    Centre for Materials and Structures, University of Liverpool, between September 2008 and

    April 2012.

    This work is original except where acknowledgment and references are made to the previous

    work. Neither this nor any substantially similar dissertation has been or is being submitted

    for a degree, diploma or other qualification at any other university.

    The work has been presented in the following publications:

    i. Dawson, K. and Tatlock, G. J.,Creep Strengthening Nano-Precipitate Distributions

    in the Carbon Depleted Region of Dissimilar Ferritic Power Plant Steel Welds, The

    Eighth International Charles Parsons Turbine Conference, IOM3, Portsmouth, UK.


    ii. Dawson, K. and Tatlock, G. J., The Stability of Fine, Sub-Grain Microstructures

    Within Carbon Depleted Regions of Dissimilar Metal, Ferritic, Creep Resistant

    Welds, ASME Pressure Vessel and Piping Conference, Baltimore, US, paper-57868,


    iii. Dawson, K. and G.J. Tatlock, Observations of native oxides on ion beam polished

    heterogeneous Cr Mo weld TEM foil specimens.Materials at High Temperatures,2011. 28(4): p. 259-263.

    Karl Dawson

    Date: 07/2012

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    I would like to express my sincere gratitude to my supervisor Professor Gordon J. Tatlock

    for his support, guidance and encouragement.

    I gratefully acknowledge Dr. Simon Romani for the training on, and provision of,

    meticulously maintained transmission electron microscopes at the University of Liverpool. I

    would also like to thank Mr. David Atkinson for his help in laboratory and Dr. Bob Murray

    for sharing his vast knowledge of electron microscopy.

    I would like to thank Doosan Power Systems, of Renfrew, Scotland, for their financialassistance to the project and the provision of materials used for experimental investigations

    during this work. Particular thanks go to Les Buchanan, Matt Barrie, Kuangnan Chi and Pete

    Barnard for the discussions and sharing of thoughts at the series of meetings in Renfrew.

    I would like to express my appreciation to all members of the materials groups in Liverpool

    for their support, help and friendship. To friends and colleagues, Dr. Ziwen Fang, Dr. Darren

    Potter, Dr. Adam Clare, Dr. Pete Beahan, Dr. Ashwin Rao, Thomas Boegelein, Joe Roberts

    and in particular to Mr. Peter King.

    I would like to express my deepest gratitude to my family; to my loving wife Nina, for her

    continued support throughout all aspects of life during my time at Liverpool. I am also

    grateful to my beautiful children, Jessica and Thomas; your smiles and laughter give me a

    welcome break from my working life. Finally, to my parents, to whom I am completely

    indebted for their encouragement and the support given to me in all walks of my life.

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    ABSTRACTThis dissertation details the findings of experimental investigations of welds made between

    ferritic creep resistant steels that differ in chromium content. Analysis of the microstructural

    evolution during the application of post weld heat treatments is reported. Particular attention

    was paid to the key alloy strengthening mechanisms and the manner in which they were

    affected by carbon redistribution which takes place when these welds are exposed to high


    The fusion interface regions of transition joints, made between P91 parent alloy and P22,

    P23 and P24 type weld consumables, were analysed in as received and post weld heat

    treated conditions. Carbon redistribution from the low to higher alloyed material, which

    resulted in its depletion from weld alloy adjacent to the fusion line, was confirmed in allweld systems subsequent to post weld heat treatment (PWHT). The effect of tempering

    treatments, carried out at 730C for two and eight hour durations, on carbide populations in

    partially decarburised weld alloy was explored. The consequential microstructural changes,

    which were affected by the dissolution of M23C6and M7C3carbides, were compared to those

    observed in regions of weld alloy unaffected by carbon depletion.

    High resolution transmission electron microscopy (HRTEM) and field emission scanning

    electron microscopy (FESEM) were used extensively in the analysis of weld metal and heat

    affected zone (HAZ) microstructures. Electron diffraction and x-ray energy dispersive

    spectroscopy were exploited in the crystallographic and chemical characterisation of

    precipitates. Their evolution as a function of thermal exposure is presented for each alloy.

    Chemical signatures for each precipitate species, which enabled their identification, were

    determined for carbides in the different alloys. However, due to variations in the

    compositions of fusion interface M23C6 carbides, some permutations of which overlapped

    with compositions of M7C3, satisfactory identification demanded classification of their

    crystal structure.

    A significant difference between the microstructures of P23 and P24 alloys, in the weld

    specimens tested, was observed. Although vanadium and niobium carbonitrides (MX) were

    identified in both alloys, their distributions were not the same. Retention of carbonitride

    particles within partially decarburised P23 and P24 weld materials, subsequent to 8 hours

    post weld heat treatment, has been substantiated.

    Diffraction intensity distributions in Debye-Scherrer ring patterns, which were generated

    from MX precipitation, indicated lattice parameters varied. Microanalysis revealed that MXprecipitates were present over a wide range of compositions. A combination of the

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    composition analysis and diffraction studies indicated that MX precipitation was stable over

    a range of compositions in the carbon depleted regions of P24 alloy.

    Recrystallisation of the bainitic P22 weld alloy adjacent to the fusion line, which was

    accompanied by a loss of material hardness, was observed in 2 and 8 hour PWHT P91/P22welds. It has been shown that the microstructural stabilisation of carbon depleted T/P23 and

    T/P24 alloys was conferred by a dispersion of MX precipitates. Retention of these stable

    particles, which in many cases are less than 10 nm in diameter, in carbon depleted material,

    resulted in the complete avoidance of any recrystallisation in 2 hour post weld heat treated

    T/P23 and T/P24 welds and only isolated occurrences in 8 hour tempered specimens.

    Subgrain size distributions were determined from electron channeling contrast images of

    various regions of the dissimilar metal welds. Results showed that, although recrystallisation

    of MX forming alloys did not occur, destabilisation of lath boundaries, due to the dissolution

    of M23C6and M7C3carbides, results in a coarser subgrain microstructure in carbon depleted

    P24 weld alloy.

    The loss of resistance to plastic deformation as a result of recrystallisation, which has been

    shown to take place in decarburised P22 alloy, was not observed in the alloys which

    precipitated the MX phase.

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    ABSTRACT iii




    1.1 Power Plant Operation 1

    1.2 Materials Selection 3

    1.3 Dissimilar metal welds 6

    1.4 Thesis Structure 9



    2.1 2 Cr-1Mo Steel (T/P22) 11

    2.2 2 Cr-1.6W-V-Nb Steel (T/P23) 15

    2.3 2 Cr-1Mo-V-Ti-B Steel (T/P24) 16

    2.4 9Cr-1Mo-V-Nb Steel (T/P91) 18



    3.1.1 Shielded Metal Arc Welding 20

    3.2 Preheat, Interpass Temperature and Post Weld Heat Treatment 21

    3.3 Microstructures of Bainitic and Martensitic Creep Resistant Steel Welds 22

    3.3.1 Solidification of the Weld Pool 23

    3.4 Decomposition of Austenite 24

    3.4.1 Diffusional Growth of Ferrite 24

    3.4.2 Widmansttten Ferrite 24

    3.4.3 Bainite 25

    3.4.4 Precipitation in Bainite 273.4.5 Martensite 28

    3.6 Dislocation Densities 30




    4.1 Introduction 32

    4.1.1 MX 34

    4.1.2 M2X 35

    4.1.3 M2C 35

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    4.1.4 M3C 35

    4.1.5 M6C (-carbide) 36

    4.1.6 M7C3 37

    4.1.7 M23C6 (-carbide) 38

    4.1.8 Laves Phase 38

    4.1.9 Z-Phase 394.2 Alloying Additions 39

    4.2.1 Boron 39

    4.2.2 Carbon 40

    4.2.3 Nitrogen 40

    4.2.4 Silicon 40

    4.2.5 Titanium 40

    4.2.6 Vanadium and Niobium 41

    4.2.7 Chromium 42

    4.2.8 Manganese 42

    4.2.9 Cobalt 434.2.10 Nickel 43

    4.2.11 Molybdenum and Tungst