05_Phase Transformation in Welding

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Transcript of 05_Phase Transformation in Welding

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    Solidification and phaseSolidification and phase

    transformations in weldingtransformations in weldingSubjects of Interest

    Suranaree University of Technology Sep-Dec 2007

    Part I: Solidification and phase transformations in carbon steel

    and stainless steel welds

    Part II: Overaging in age-hardenable aluminium welds

    Part III: Phase transformation hardening in titanium alloys

    Solidification in stainless steel welds

    Solidification in low carbon, low alloy steel welds

    Transformation hardening in HAZ of carbon steel welds

    Tapany Udomphol

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    ObjectivesObjectives

    This chapter aims to:

    Students are required to understand solidification and

    phase transformations in the weld, which affect the weld

    microstructure in carbon steels, stainless steels, aluminiumalloys and titanium alloys.

    Suranaree University of Technology Sep-Dec 2007Tapany Udomphol

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    IntroductionIntroduction

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    Suranaree University of Technology Sep-Dec 2007

    Part I:Solidification in carbon

    steel and stainless steel welds

    Carbon and alloy steels withhigher strength levels are more

    difficult to welddue to the risk of

    hydrogen cracking.

    Fe-C phase binary phase diagram.

    Austenite to ferrite transformationin low carbon, low alloy steel

    welds.

    Ferrite to austenite transformation

    in austenitic stainless steel welds. Martensite transformation is not

    normally observed in the HAZof a

    low-carbon steel.

    Carbon and alloy steels are more frequently welded than any other materials

    due to their widespread applications and good weldability.

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    Solidification in stainless steel weldsSolidification in stainless steel welds

    Suranaree University of Technology Sep-Dec 2007

    Ni rich stainless steelfirst

    solidifies as primary dendrite

    of austenite with

    interdendritic ferrite.

    Cr rich stainless steelfirst

    solidifies as primary ferrite. Upon

    cooling into ++++region, the outer

    portion (having less Cr) transformsinto austenite, leaving the core of

    dendrite as skeleton (vermicular).

    This can also transform into lathly

    ferrite during cooling.

    Solidification and post solidification

    transformation in Fe-Cr-Ni welds

    (a) interdendritic ferrite,

    (b) vermicular ferrite (c ) lathy ferrite

    (d) section of Fe-Cr-Ni phasediagram

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    Solidification in stainless steel weldsSolidification in stainless steel welds

    Suranaree University of Technology Sep-Dec 2007

    Weld microstructure of high Ni

    310 stainless steel(25%Cr-

    20%Ni-55%Fe) consists of primary

    austenite dendrites andinterdendriticferrite between

    the primary and secondary dendrite

    arms.

    Weld microstructure of high Cr309 stainless steel(23%Cr-

    14%Ni-63%Fe) consists of primary

    vermicular or lathyferrite in an

    austenite matrix.

    The columnar dendrites in both

    microstructures grow in the

    direction perpendicular to the tear

    drop shaped weld pool

    boundary. Solidification structure in (a) 310 stainlesssteel and (b) 309 stainless steel.

    Austenite dendrites and

    interdendritic ferrite

    Primary vermicular or lathy

    ferrite in austenite matrix

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    Solidification in stainless steel weldsSolidification in stainless steel welds

    Suranaree University of Technology Sep-Dec 2007

    Quenched solidification structure near the pool of an

    autogenous GTA weld of 309 stainless steels

    Primary ferrite

    dendrites

    A quenched structure of ferritic

    (309) stainless steel at the weld pool

    boundary during welding shows

    primary ferrite dendrites beforetransforming into vermicular ferrite

    due to transformation.

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    Mechanisms of ferrite formationMechanisms of ferrite formation

    Suranaree University of Technology Sep-Dec 2007

    The Cr: Ni ratio controls the

    amount of vermicular and lathy ferrite

    microstructure.

    Cr : Ni ratio

    Vermicular & Lathy ferrite

    Austenite first grows epitaxially from

    the unmelted austenite grains at thefusion boundary, and ferrite soon

    nucleates at the solidification front in the

    preferred direction.

    Lathy ferrite in an

    autogenous GTAW of

    Fe-18.8Cr-11.2Ni.

    Mechanism for the formation of vermicularand lathy ferrite.

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    Prediction of ferrite contentsPrediction of ferrite contents

    Suranaree University of Technology Sep-Dec 2007

    Schaefflerproposed ferrite content prediction from Crand Ni

    equivalents (ferrite formers and austenite formers respectively).

    Schaeffler diagram for predicting weld ferrite content and solidification mode.

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    Effect of cooling rate on solidification modeEffect of cooling rate on solidification mode

    Suranaree University of Technology Sep-Dec 2007

    Cooling rate

    Low Cr : Ni ratio

    High Cr : Ni ratio

    Ferrite content decreases

    Ferrite content increases

    Solid redistribution during solidification is reduced at high cooling rate

    for low Cr: Ni ratio.

    On the other hand, high Cr : Ni ratio alloys solidify as ferrite as the

    primary phase, and their ferrite content increase with increasing cooling

    rate because the transformation has less time to occur at high

    cooling rate.

    Note: it was found that if N2 is introduced into the weld metal (by adding

    toAr shielding gas), the ferrite contentin the weld can be significantly

    reduced. (Nitrogen is a strong austenite former)

    High energy beam

    such as EBW, LBW

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    Ferrite to austenite transformationFerrite to austenite transformation

    Suranaree University of Technology Sep-Dec 2007

    At composition Co, the alloy

    solidifies in theprimary ferrite mode

    at low cooling rate such as in

    GTAW.

    At higher cooling rate, i.e., EBW,

    LBW, the melt can undercool below

    the extended austenite liquidus (CL)

    and it is thermodynamically possible

    forprimary austenite to solidify. The closer the composition close to

    the three-phase triangle, the easier

    the solidification mode changes from

    primary ferrite to primary austenite

    under the condition of undercooling.

    Cooling rate Ferrite austenite

    Section of F-Cr-Ni phase diagram showing

    change in solidification from ferrite to

    austenite due to dendrite tip undercooling

    Weld centreline austenite in an autogenous GTA weld of

    309 stainless steel solidified as primary ferrite

    Primary

    ferrite austenite

    At compositions close to

    the three phase triangle.

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    Ferrite dissolution upon reheatingFerrite dissolution upon reheating

    Suranaree University of Technology Sep-Dec 2007

    Multi pass weldingor repaired

    austenitic stainless steel weld consists

    of as-depositedof the previous weld

    beads and the reheated region of theprevious weld beads.

    Dissolution of ferrite occurs

    because this region is reheated to

    below the solvus temperature.

    This makes it susceptible to

    fissuring under strain, due to lower

    ferrite and reduced ductility.

    Effect of thermal cycles on ferrite

    content in 316 stainless steel weld (a)

    as weld (b) subjected to thermal cycle

    of 1250oC peak temperature three times

    after welding.

    Primary austenite dendrites (light)

    with interdendriticferrite (dark)

    Dissolution of ferrite after thermal

    cycles during multipass welding

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    Solidification in low carbon steel weldsSolidification in low carbon steel welds

    Suranaree University of Technology Sep-Dec 2007

    The development of weld microstructure in low carbon steels

    is schematically shown in figure.

    As austenite is cooled down from

    high temperature, ferrite nucleates

    at the grain boundary and grow inward

    as Widmansttten. At lower temperature, it is too slow for

    Widmansttten ferrite to grow to the

    grain interior, instead acicular ferrite

    nucleates from inclusions

    The grain boundary ferrite is also

    called allotriomorphic.Continuous Cooling Transformation

    (CCT) diagram for weld metal of low

    carbon steel

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    Weld microstructureWeld microstructure

    in lowin low--carbon steelscarbon steels

    Suranaree University of Technology Sep-Dec 2007

    A: Grain boundary ferrite

    B: polygon