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    JOURNAL OF IRON AND STEEL RESEARCH. INTERNATIONAL. 2010. 17(3): 34-39

    Influence   Specimen Geometry   Hot Torsion Test on Temperature

    Distribution

    During

    Reheating Treatment   API X7

    Bahman

    Mirzakhani' •

    Shahin Khoddarn • Hossein

    Arabi ,

    Mohammad

    Taghi

    Salehi •

    Seyed Hossein

    Seyedeirr •

    Mohammad Reza

    Aboutalebi

    (1. Faculty of Engineering . Arak Univers ity, Arak 38156-879 , I ran; 2. Department of Mechanical Engineering.

    Monash

    University. Clayton Campus. Vic 3800. Australia , 3. Department of Mater ials and Metallurgical

    Engineering.

    Iran

    University

    of Science and

    Technology. Tehran

    16844-13114.

    Iran)

    Abstract: The commercial finite element package ANSYSTM was uti lized for prediction of temperature distrihution

    during

    reheating

    treatment

    of hot

    tor sion tes t

    (HTT)

    samples with

    different geometries

    for

    API-X70 microalloyed

    steel. Simulation

    resul ts show tha t

    an

    inappropriate

    choice of

    test

    specimen

    geometry

    and reheating conditions before

    deformation

    could

    lead to non-uniform

    temperature distribution

    within

    the

    gauge section of

    specimen. Therefore.

    fls

    surnptions of

    isothermal experimental

    conditions and zero temperature gradient

    within

    the specimen cross section ap

    pear unjustified and led to uncertainties of f low curve obtained. Recommendations on finding proper specimen geome

    try

    for reducing

    temperature gradient

    along the gauge

    part

    of specimen will be g iven to

    create

    homogeneous initial mi

    crostructure as much as possible before

    deformation

    in order to avoid uncertainty in

    consequent

    results of HTT.

    Key words: hot

    tors ion tcs t

    ,

    simulation;

    specimen geometry; reheating treatment; microalloyed steel

    The hot tor sion tes t (HTT) has

    been

    one of the

    mos t p opul ar mechanical tests for

    assessment

    of

    workability

    of metals

    and

    alloys for

    bulk

    forming

    processes

    during

    last

    decades J-,; .   is

    often

    chosen

    over

    the

    uniaxial tension and compression tests be

    cause

    very

    large strain and strain rates

    can

    be

    a

    chieved

    without t he

    problems

    of

    necking

    and barre

    l ing. respectively' 7 . In

    this

    test the effective strain

    and effective

    stain

    rate are as

    function

    of gauge

    length and

    radius.

    So to overcome

    the te st rig

    limi

    tations

    a wide range of specimen geometr ies and si

    zes

    have

    been

    used

    in different

    studies

    l l

    -

    6

    ] in order

    to obtain the required strain and strain rate

    range.

    The reheating treatment is the first stage of

    thermomechanical processes

    in

    order

    to

    obtain

    hom

    ogeneous composition

    before doing hot

    rolling

    or hot

    forging.

    The effect of pre-deformation

    reheating

    treatment on microstructure evolution such as initial

    austenite

    grain size and behavior of

    microalloying

    el

    ements has

    been

    investigated in several stud

    ies

    L

     ; H - - 12 ] .

    These

    investigations deduce

    t ha t aus ten

    itizing temperature has a great influence

    on

    initial

    microstructure and consequence deformation behav

    ior of

    steels.

    In

    simulation

    of

    thermomechanical

    process via

    HTT.

    before

    s ta rt ing the

    hot t or sio n

    testing,

    the

    material is usually heated to

    the

    given temperature

    as a reheating

    temperature

    and soaked for a

    while

    and then cooled to the deformation t empera tu re . In

    hot tor sion machines th e

    heating energy

    usually is

    supplied by

    an

    induction

    or

    resistance type fur

    nace[J-3.J3-J5].

    The influences

    of

    specimen geometry

    of HTT and reheating condi tions have

    no t

    been

    in

    vestigated systematically. However. no proper con

    sideration of these choices may introduce some er

    rors

    in consequence results of

    test

    due to th e non-u

    niform

    initial

    temperature

    and

    microstructure within

    th e material.

    At the

    present

    study. the commercial f inite ele

    ment

    package ANSYS™ was used to investigate th e

    effects

    of

    specimen geometry and process

    conditions

    on distribution of temperature within the specimen

    during reheating treatment. This approach will ena-

    ble to

    prevent the high

    temperature gradien t within

    Biography:

    Bahman Mirzakhani(

    1977  .

    Male. Doc tor:

    E-mail:

    b-mirzakhani@araku.

    ac. ir , Received Dale: August

    15 .

    200S

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    Issue

    3

    Influence

    of

    Specimen Geometry

    of Ho t Torsion Test on Temperature

    Distribution

    • 35 •

    Fig. 1 Reheating cycle before hot torsion test

    1000

    00

     

    Temperature/ t

    Heating cycle before deformation

    120

    L. .

    ____

    600

    140

    200

    220 .

    Time/s

    Quenching

    I

    880

    1200

    [ 180

    ]

      160

    is

     

    Q

    740

    I

     

    Fig.2 Dilatation

    IL i

    a function of temperature in dilatometry test

    In

    this study

    for

    prediction

    of

    temperature

    dis-

    tribution

    during

    reheating treatment a nd b ef or e

    ho t

    torsion

    testing , the

    commercial

    FEM

    code

    AN

     Y T wa s used. So according to reheating treat

    ments which almost isused in experiments, a rehea

    ting cycle wa s

    designed

    as

    shown

    in Fig.

    1. The

    tem

    peratures 740 an d 880 C in this figure ar e Ad an d

     

    c

    temperatures of present microalloyed steel re-

    spectively

    which were determined by dilatometry

    test

    as presented in Fig. 2. In this

    range rather

    low

    hea ti ng r a te considered in order to give

    time

    for fe r-

    rite and

    perlite

    transformation

    to austenite.

    Th e

    chemical composition of

    th e API-X70

    microalloyed

    s te el u se d in t hi s s tu d y is

    given

    in

    Table 1.

    Th e geometry

    of

    th e H T T specimen used

    in

    this

    investigation is

    shown

    in Fig. 3. A tw o

    dimensional

    FE analysis

    of heat flow ha s

    been

    used here to

    study

    th e effect of in it ial reheating cycle before

    deforma

    tion starts.

    The

    specimen dimensions

    a re s ho wn pa-

    rametrically in order to consider

    th e

    effect of each di-

    mension. Th e d om ai n, b ou nd ar ie s an d the mesh

    w hi ch w er e used in ANSYS™ is

    also presented

    in

    specimen

    by f inding

    opti m um ge om e tr y an d

    rehea

    ting

    conditions

    to obtain homogenous initial

    micro

    structure

    before

    starting

    HTT.

     

    is worth noting

    that

    th e present model devel-

    oped

    on

    th e

    base of a flexible

    h ot t or si on test

    ma

    c hi ne , h as been developed at IROST[15].

    1 Model and Simulation Conditions

    Table 1 Chemical composition of the steel

    c

    Mn

    Si

    p

    S Nb v

    Ti

    N Fe

    0.09

    1. 63

    0.32

    0.009 0.003 0.04 0.05

    0.01 0.002

    Re m

     imm

    4 S3

    S3

    S4 52

    . .  

    s1

    z

    SI

    Fig. 3 Schematic of geometry of a solid hot torsion specimen

      a)

    and domain and boundaries and mesh of ANSYS

    modeling for reheating treatment

      b)

    Fig. 3   b ) w he re o nly

    one-quarter

    of a

    longitudinal

    section

    of Fig. 3   a ) is

    considered

    because of

    th e

    symmetry

    of

    g eo me tr y a nd t he condition with respect

    to

    r a n d

    z axis.

    Th e axisymmetric

    conditions

    duri ng r ehea ti ng

    treatment b et we en t he bo dy

    an d

    its environment im-

    ply

    that

    heat

    flux along th e

    boundaries

    51 is zero

    or

    insulated.

    The boundary 52 is

    directly

    heated by an

    induction coil of 45mm length. Th e h ea ti ng w as

    controlled by a pyrometer

    carefully

    located above

    th e

    center of th e specimen. The boundary

    53

    is exposed

    to

    th e surrounding

    environment

    where th e

    energy

    loss

    is

    considered through

    b ot h b ou nd ar y

    convection

    an d radiation.

    A simpli fi ed

    approach

    for

    radiation

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    • 36 •

    Journal

    of

    Iron

    and Steel

    Research.

    International

    Vol. 17

    (a ) GI ;

      b) G2;

    (c )

    G3; d) G4;

    (e )

    G5;   f G6.

    Fig.4

    Initial temperature distribution in GI. G2.

    G3 ,

    G4, G5,

    and

    G6 after applying reheating cycle

    distribution in various

    geometries

    of HTT specimens

    after applying reheating cycle

    (Fig. 1)

    and before

    starting deformation at 1000  C.

    This

    figure

    illustrates that

    inductive heating

    may produce non uni form temperature along the axis

    of

    the

    specimens and caused temperature

    gradient

    within gauge

    part of the sample.

    In that,

    in s pe ci

    mens having longer gauge, show greater thermal

    gradient. For example in G4 (52 mm gauge length)

    the temperature differential

    between

    center and end

    of

    the gauge

    is the

    largest and

    about

    160 C. It

    should be noted

    that

    for all specimens the induction

    length

    assumed to be constant and was

    equal

    to

    45 mm. Sections of specimens located

    outside

    of in

    duction

    coil

    absorbed the

    heat

    of

    the

    inside

    sections

    and led to lower temperature of ends than

    center

    of

    gauge. This

    result indicates assumption of

    uniform

    temperature distribution in the gauge part of the

    specimen mentioned in previous works[J3 14 17 19] may

    not

    be valid.

    This would

    have a

    negative impact

    on

    the final results of the test.

    The temperature history of two points in the

    gauge section of G3

    during reheating

    treatment is

    presented in Fig. 5.  t shows

    that

    th e desirable

    re

    heating cycle was only obtained at the center of s pe c

    imen.

    The

    ends

    of

    gauge section not only

    did

    not

    reach to

    appropriate

    temperature for

    starting

    deform-

    120 13010

    00

    1000

      >0

     

    90

    Table 2

    Dimensional description of various  Tf specimens

    mm

    Specimen

    L l

    R l

    L2

    R2

    L3

    R3

    R4

    L

    Gl

    62.0

    8

    25.0 6.5

    8

    3.35 1.0 184

    G2

    62.0

    8

    12.5 5. 5

    33

    3. 35 1.5 184

    G3

    62.0

    8

    7.0

    6.5 42 3.35

    1.5 183

    G4 62.0 8

    3. 0 6. 5

    52

    3. 35

    1.5 185

    G5

    57.0 6. 25

    12.5 4. 75

    42

    2. 50 1.5 184

    G6

    66. 5 9. 25

    3. 0

    8.0

    41

    4.25

    2.0

    184

    heat transfer was utilized by using an equivalent con

    vection

    boundary

    condition in

    which

    the

    non-linearity

    is considered through a temperature-dependent con

    vection coefficient which will be designated here as

    h, :

    h,=O €[ T;+T;.) T,+T,.)]n-1 0

    where,

    h

    is equivalent convection coefficient be

    tween

    the body surface 5 and the sur face of

    the

    envi

    ronment enclosing

    the

    body at

    the

    nth

    time step;

    T

    and

    T

    are the

    absolute

    temperatures in degrees Kel

    vin

    between

    these surfaces;   is Steffan-Boltzman

    coefficient;

    e

    is emissivity factor of the surface.

    The

    value of € =   65 was determined according to the

    ASTM standard test[16].

    The

    boundary 54 is in

    contact

    with grippers of

    the HTT machine. For these boundaries, th e t her

    mal

    contact conductance

     kin, of

    the

    interface

    be

    tween the specimen and

    the

    grips was considered and

    heat exchange

    modeled by convection.

    The heat

    transfer

    characteristics

    of the

    steel

    of

    the present work

    were assumed

    to

    be[17 18]

    :

    p=7 800

    kg/rn , c = 6 8 0 J / k g K), k=36.8 W / m K),

    k

    in

    , = 3 740 W / m

    2

    • K),   = 4 8 3 K , T .=368K

    0 =5.67 X

    10-

    8

    W

    /(m

    2

    • K

    1

    ) ,

    h= 4

    W

    /(m

    2

    K) .

    p

    and

    c are

    the

    densi ty and hea t

    capacity of ma

    terial respectively.

    Heat

    convect ion and heat conduc

    tion coefficients are denoted by

    hand k respectively.

    The subscripts

    a,

    g

    refer

    to

    ambient and grip respec

    tively.

    The

    HTT specimens

    with

    different gauge length

    and

    radius were des igned in

    order

    to investigate the

    effect of specimen

    geometry

    on distribution of ther

    mo-mechanical parameters under various deformation

    conditions.  t s ho ul d be n ot ed that the relationship be

    tween the sizes of the specimen, as s ho wn in Fig. 3

    (a),

    were designed

    according to the basic

    stiffness

    re

    quirement

    expressed

    in[19 20]

    Table 2 summarizes the

    dimensions of different specimens used in

    this

    study.

    2 Results  n Discussion

    Fig. 4

    shows the con tour

    plots of

    temperature

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    Issue 3

    Influence of Specimen Geometry of Hot

    Torsion

    Test on Temperature Distribution

    • 37 •

    220

    190

    160

    P

    130

     

    ;l,

    I'

    100

    -

    70

    40

    10

    5 15

    25 35

    45 55

    Gauge length/mm

    [l

    .[]

     

    100

    200 300

    Time/s

    400

    1200 r ~ ; ; ; : : : ; ~ ; : ; : : : ; ; : ; ; : = i l

    o

    P

    800

    1

    o

    Fig. 5 Time history of two poin ts in gauge section of

    G3 during reheating treatment

    Fig. 6 Effect of gauge length on maximum different ia l

    temperature along gauge section

     

    4.0

    .0 3.0

    r

    1.0

    860

    9 0 0 1 ( ~ - - l O l ; o ; ; ; , ; : ; ; ; ; ; ~ ~ ~

    890  

    870 • G5-gauge center 0 G5-gauge end

    .. G3-gauge center A G3-gauge end

    • G6-gauge center

    0

    G6-gauge end

    1000  

    o

    850

    840

      : ~

    .

     

    I

     

    980

     

    960

    1

    950

    OJ

     

    940

    930

    920

    l l ~

     

    1080

    1060

     a 900 C; ( b) 1000 C ; (c ) 1100 C.

    Fig. 7 Radia l va riation of tempera tu re at cente r and end of

    gauge section after reheating and before deformation

    ation bu t also the desired

    reheating

    temperature,

    i.

    e. 1200 C, at

    these points

    did

    not

    achieved. So , it

    is

    expected that

    the

    solution

    of the chemical elements

    will not be occurred completely at these areas.

    This

    would fail achievement of a homogeneous composi-

    t ion within specimen before deformation.

    In

    addi-

    tion,

    different austenitizing temperature at different

    points

    of

    the

    specimen leads to various initial

    grain

    sizes of austenite

    which

    is a c ri ti cal factor that influ-

    ences subsequent deformation behavior and phase

    transformation of steel[8-12].

    For evaluation of

    the

    effec t of gauge

    length

    on

    maximum differential temperature along the gauge

    section

    and obtain opt imum

    sample

    geometries,

    the

    value of temperature difference between points

     

    and

    B et tB along the gauge section

    (Fig.

    5

    was

    cal

    culated

    for various specimens at different

    tempera-

    tures and plo tt ed

    in Fig. 6. This figure

    indicates

    as

    the gauge length increased; the value of  tA t B in -

    creased;

    so that for specimen with 52mm gauge

    l ength this gradi en t

    exceeded

    200 C

    at 1100·C.  

    can

    be

    also observed

    that the

    temperature gradients

    prior

    to start of deformation at different temperature

    ar e not

    similar. On

    t he o the r hand, as

    temperature

    of final step decreased the gradient decreased.

     

    re-

    lates

    to the temperature

    difference between

    gauge

    section and o ther par t of the specimen which is less

    at lower temperatures than higher temperatures.

    For

    specimen with short gauge l ength the t empera tu re

    did not

    show much influence on

    temperature

    gradient

    along the sample.

    . Fig. 7 shows the radial variation of temperature at

    the center and the end of gauge section af ter reheating

    and before

    the s ta rt

    of deformation at 900, 1000 and

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    • 38 •

    Journal

    of

    Iron

    and

    Steel Research.

    International

    Vol . 17

    R es ul t s ho ws that

    no

    p r op e r g e om e tr y an d

    di

    mension selection result in no n

    uniform temperature

    within

    specimen

    an d

    predicted to h av e eff ects on th e

    consequence

    assessment

    of

    m ater ial b eh avi or

    during

    ho t

    deformation as

    seen

    in

    Fig.

    9. In

    addition.

    it

    seems prevention

    of

    an y t e mp e ra t ur e g r ad ie n t d u ri n g

    reheating

    treatment

    of H T T is un-avoidable. bu t

    choosing an

    o pt im um g eo me tr y

    w il l m in im iz e this

    p rob le m. B et we en di ff er en t

    g eo me tr ie s, t he speci

    mens G1

    an d

    G2 showed

    th e

    lo w

    temperature

    gradi

    en t

    in

    both axis an d redial

    directions of

    th e ga uge

    section.

    Therefore it ca n be concluded

    t ha t t he

    opti

    m u m g e om e tr y

    ar e th e

    samples

    having between O. 2

    an d

    O. 7

    length

    of induction coil.

    A numerical modelling

    w as p er fo rm ed

    to

    analy

    sis

    interaction

    of

    geometry

    of

    h ot t or si on test speci

    mens

    an d

    reheating treatment pre-deformation

    of

    API-X70 microalloyed

    steel by

    th e commercial finite

    element package A NS YS ™. R es ul ts s ho we d that

    th e

    specimens

    in

    th e range

    of O. 2 to   7

    length

    of

    induc

    tion

    coil

    e xp er ie nc ed l ow

    temperature

    gradient

    in

    both

    axial an d redial directions a f te r r e he a ti ng

    treat

    ment.

     

    seems

    that

    these

    geometri.es

    ar e

    m o re r el ia

    bl e

    f or dr ivi ng

    accurate constitutive

    parameters of

    3 Conclusion

    Fig.9 Flow localization in G4 specimen deformed at 1100

     

    and

     i =

    10 rad/s after applying reheating cycle

    as

    shown

    in F ig.

    6.

    Fig. 7

    an d

    F ig . 8.

    Various geom

    etries experienced   ~ l f f e r e n t

    temperature

    distribution.

    In Fig. 9

    th e result

    of high

    gradient temperature

    with

    in g aug e sect io n of

    H T T

    sample on

    deformation

    be

    havior

    of G4 is

    illustrated. This

    sample

    deformed

    at

    1100 C an d   10 rad/s after applying t h e r e he a ti n g

    cycle of Fig. 1. As see in

    this

    figure

    interaction

    of

    sever temperature

    gradient

    an d

    deformation condi

    tions

    led to flow localization

    at the center

    of

    speci

    mens.

    It is

    due

    to

    th e

    mi d gauge

    section experiencing

    softening phenomena

    i. e.

    dynamic recrystallization

    during deformation;

    while

    th e

    ends of th e

    gauge

    sec

    tion can not flow as easily as th e center.

    1100

    \   0

    G5-gauge center

    G6-gauge end

    1000

    Temperature/

     t

    00

      -- ,

    15

    45

    35

    Fig. 8 Influ en ce of fin is hi ng t em peratu re of reh eati ng

    cycle on r edi al g ra di en t at c en te r and end of

    gauge section in R Tf specimens

    1100  C . As seen

    in

    th is fig ure. te mp era tu re

    de

    c re as ed a lo ng

    th e

    s pe ci me n r ad iu s be ca us e of heat

    transfer

    via con vecti on and r adi ati on

    from

    surface of

    specimen to

    the surrounding me di um. B ut ra di al

    var

    iation

    of

    temperature

    wa s much

    less at

    th e

    center

    than

    at

    th e

    ends

    of

    g aug e secti on .

      is

    due

    to

    th e

    ends of ga uge s ect ion ar e

    no t

    o nl y a dj ac en t to

    th e

    en d of

    i n d ~ t i o n

    coil

    bu t

    also they

    a r e c o nn e ct ed

    to

    th e m i ni -s h ou l de r s a nd

    shoulders

    which absorb

    a

    considerable amount

    of

    h e at g e ne r at e d   th e gauge

    section.

      is

    worth noting that

    when

    th e

    g au ge r ad iu s

    increased

    th e

    slop of temperature vs   c urve s in

    creased.

    h o we ve r t hi s matter

    is m o re o bv io us at

    th e

    ends

    tha n the center

    of

    gauge. On th e other hand,

    increasing

    in

    gauge radius

    led to

    th e rate

    of

    heating

    l os s a lo ng this d ir ect i on i ncrease. B ecau se acco rd in g

    to

    Table

    2. specimens with lar ge gau ge

    diameter

    have larger

    mini-shoulder

    an d s h ou l de r d ia m et er

    in

    order

    to

    stiffness requirement

    is

    carried out.

    From

    Fig.

    7.

    influence of finishing temperature of

    reheating cycle

      Fig.

    1) on m ax imu m redial gr ad ie nt in

    various H T T specimen after reheating was plotted in

    Fig. 8.

    W h en r eh e at in g treatment finished

    at

    1100

     C

    t he m ax im um redial gradient

    at

    b o th c en te r a nd e nd s

    of ga uge w as greater in comparison of

    1000

     C an d

    900 C.

    However

    at

    th e

    center

    of

    s p ec im e n. t em p er

    ature

    difference between

    surface

    and core

    of gauge

    were

    less

    t han 45 C

    an d 15 C respectively.

    Fo r

    a

    given reheating cycle, geometry

    of

    H T T

    specim en h av e

    a

    great influence

    on temperature distri

    bution after reheating treatment and before

    deformation

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    Issue 3

    Influence of Specimen Geometry of

    Ho t

    Torsion Test on Temperature Distribution

    • 39 •

    the

    steel

    by hot torsion tes t.

    The

    first

    author is

    grateful

    to   rakUniversity

    for

    granting

     

    scholarship

    to him

    The authors

      lso

    acknowledge Sadid

    Industrial

    Group fo r providing

    the steel

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