13 - Q+T steels & Welding Q+T steels

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    08/06/2004Istituto Italiano della saldaturaENTE MORALE

    WELDABILITY OF LOW ALLOYED

    QUENCHED AND TEMPERD

    STEELS

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    Introduction QUENCHED AND TEMPERED steels

    are designed especially for welded

    construction and fabrication.

    They are furnished from steel producers

    in the heat treated condition

    Yield strengths range from 350 to 1000

    MPa) depending on

    their chemical composition

    Thickness

    type of heat treatment.

    The heat treatment for most of these

    steels consists of austenitizing,

    quenching, and tempering.

    A few are given a precipitation-

    hardening (i.e., aging) treatment

    following hot rolling or some other

    thermal treatment.

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    Metallurgy The carbon content of quenched and tempered steels generally does not

    exceed 0.22% for good weldability.

    The other alloying elements are carefully selected to provide the most

    economically heat-treated steel with the desired properties and acceptable

    weldability:

    Microalloyng elements (Al, Ti, V, Ti, Zr, Nb)

    Quenching elements (Cr, Mo, Cu, Ni)

    Small quantities of Boron, 5 to10 ppm, in order to reduce grain size.

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    Chemical composition EN 10137-2

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    Tensile properties EN 10137-2

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    Toughness EN 10137-2

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    Chemical composition EN 10028-6

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    Tensile properties EN 10028-6

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    Toughness properties - EN 10028-6

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    ASTM designations

    f Q

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    Commercial designations

    T1 (grades T1, T1A, T1B e T1C)

    US Steel patented, is classified

    as

    ASTM A514 structural

    ASTM A517 pressure equipment

    HY

    US Navy specification

    used for submarines

    Good toughness properties at

    low temperature

    USS Skipjack, 1965

    W ld bilit f Q T t l

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    Chemical composition

    W ld bilit f Q T t l

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    Weldability

    Cold cracking

    The metallurgical structure is anon-equilibrium structure

    Hydrogen must be as low as

    possible

    Preheat temperature must be

    properly selected Hot cracking

    Susceptivity is generally low

    Some phenomena may occur

    depending on Ni content

    Lamellar tearing

    The heat treatment generally

    guarantees good properties

    W ld bilit f Q+T t l

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    Weldability

    To obtain the same

    properties of the materialafter welding:

    Low heat input

    Low preheat temperature

    Low interpass temperature

    W ld bilit f Q+T t l

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    Weldability - PWHT

    The mechanical properties of thewelded steel or the weld metal itselfmay be adversely affected by astress relief.

    The alloying elements can causeadverse effects when the weldment

    is postweld heat- treated: A PWHT in the temperature range of

    510 to 650 C, with slow cooling speeds,may decrease the toughness of the weldmetal and the HAZ.

    Reheat cracking can occur whenweldments of many high-strength steelsare heated above about 510 C.Intergranular cracking may take pIace inthe coarse-grained region of the weldHAZ.

    31,83,3 VMoCrCS

    The precipitation of carbides during stressrelaxation at elevated temperature alters thedelicate balance between resistance to grainboundary sliding and resistance to deformationwithin the coarse grains in the HAZ.

    CS Is a Reheat Cracking parameter proposedby Nakamura Reference value is 0

    Weldability of Q+T steels

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    Weldability - PWHT

    With any of the quenched and tempered steels, a postweld

    stress-relief heat treatment should be used only afterassurance that worthwhile benefits can be achieved and

    possible harmful effects can be tolerated. Stress relief is

    desirable for some applications when:

    Dimensional stability must be maintained during close tolerance

    machining,

    The weld is susceptible to stress corrosion after cold forming or

    welding.

    If postweld stress relief is required, the temperature must

    not exceed that used for tempering the steel. A temperatureabout 30 to 60C lower than the tempering temperature is

    desirable to avoid lowering the strength of the steel.

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    Weldability Temper Beads

    Weldability of Q+T steels

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    Filler Metals- MMAW: EN 757

    Weldability of Q+T steels

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    Filler Metals- MMAW: EN 757

    Weldability of Q+T steels

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    Filler Metals GMAW: EN 12564

    Weldability of Q+T steels

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    Filler Metals - MMAW: EN 757

    Weldability of Q+T steels

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