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    Microstructure and Impact Toughness of

    C-Mn Weld Meta ls

    The formation of acicular ferrite in over half of the weld

    appears to be the key to improv ing impact toughness

    BY

    L E .

    S V E N S S O N A N D B . G R E T O F T

    ABSTRACT. The effect of va r iat ion in car

    bon and manganese contents on the mi

    crostructure and impact propert ies of al l

    we ld metal samples has been stud ied. The

    welds w er e made using the shielded metal

    arc welding technique. Four dif ferent car

    bon levels , ranging f rom 0.03-0.12 wt -%

    and four dif ferent manganese contents

    (0.8-2.1 wt -%) were used.

    I t was found that signif icant improve

    ments in impact toughness at low t e m

    peratures were achieved with increasing

    amounts of acicular ferrite. High levels of

    acicular ferr ite could be achieved with

    several dif ferent combinat ions of carbon

    and manganese. At excessive amounts of

    alloying addit ions, the impact toughness

    decreased. This is at t r ibuted to the pres

    ence of bands of microphases being

    aligned with the notch in the fracture sur

    face.

    For the lowest carbon content , un

    expectedly low toughness was observed.

    This may be due to the fact that these

    metals contained a somewhat higher ni

    t rogen content .

    I n t roduct ion

    The effect of carbon and manganese

    on the microstructure and mechanical

    prope rt ies of mild steel arc weld s has been

    the subject of many invest igat ions. Vuik

    (Ref. 1) has recent ly summarized the in

    vest igat ions made concerning the ef fect

    of carbon. Evans has published a number

    of papers dealing with the ef fect of car

    bon (Ref. 2), manganese (Ref. 3), silicon

    (Ref. 4), interpass temperature (Ref. 5),

    impurity elements (Ref. 6), molybdenum

    (Ref. 7), heat input (Ref. 8) and heat treat

    ment (Ref. 9) on the microstructure and

    mechanical prope rt ies of m ild steel weld s.

    The welds that Evans examined were of

    the a l l -weld-meta l type, deposi ted wi th a

    shielded metal arc m ultipass techniqu e. In

    the work most comparable to the present

    L-ESvenssona nd

    B.

    Gretoftare with theEsab

    Group, Central

    Laboratories,

    Goteborg, Swe

    den.

    one ,

    Evans found that the opt imum im

    pact toughness propert ies were achieved

    with an a l loy ing combinat ion of 0.07 wt -

    %C-1.4 wt -% Mn, and at t r ibuted th is to

    the compet i t ive act ion between the pro

    gressively f iner ferrite grain sizes obtained

    by increasing alloying addit ions and by the

    simultaneously increasing yield strengths.

    In this paper, an invest igat ion of the mi

    crostructure and impact toughness of 16

    di f ferent welds, wi th vary ing carbon and

    manganese contents, is descr ibed. The

    expe riment t o a large extent mirrors the

    one of Evans (Refs. 2, 3), but the results

    dif fer somewhat. The intent ion of this pa

    per is to clarify the reason for the discrep

    ancy between the dif ferent invest iga

    t ions and to point out some addit ional

    microstructural ef fects that might be of

    importance for the impact toughness.

    However, f irst a short descr ipt ion of the

    role of carbon and manganese in control

    l ing the microstructure and how this may

    inf luence the impact propert ies wil l be

    given.

    B a c k g r o u n d

    With the help of experimental (Refs. 1-

    9) and theoret ical (Refs. 10-12) work, the

    effects of var ious elements on the micro-

    structure of the as-deposited area in a

    weld is now re lat ive ly wel l understood.

    The as-deposited microstructure of

    C-Mn we ld meta ls is com mo nly descr ibed

    with three major microst ructura l compo-

    K E Y W O R D S

    Microst ructure

    Impact Toughness

    C-Mn Weld Meta l

    Acicular Ferrite

    Mechanical Propert ies

    Mild Steel

    Arc We ld ing

    Al loy ing Content

    Microphases

    nents:

    grain boundary ferr ite, ferr ite with

    aligned M-A-C (martensite-austenite-ce-

    ment ite) and acicular ferr ite. The classif i

    cat ion of the var ious microstructures is

    based on the visual impression in the op

    t ica l microscope. However , in the theo

    ret ica l work based on thermodynamics,

    the microst ructura l components are de

    scr ibed f rom the mechanism of fo rmat ion

    point of v iew. The components are then

    called allotr iom orph ic ferr ite (sameasgrain

    boundary fer r i te , but a more cor rect

    name), Widmannstatten ferr ite side plates

    (according to Dube classif icat ion) and ac

    icular ferr ite. I t should be noted that the

    mechanism of format ion of acicular

    ferrite

    is not yet kn ow n. In the fo l low ing text , the

    last ment ioned denotat ion wil l be used.

    The effect of ca rbon is mainly to l imit

    the width of the coarse-grained allotr io

    morphic ferr ite, formed at the pr ior aus

    tenite grain boundaries, and in inf luencing

    the rate of

    Widmannstatten

    fer r ite form a

    t ion . Dur ing the t ransformat ion f rom aus

    tenite to ferr ite, the carbon atoms dif fuse

    into the remaining austenite and the

    growth (or thickening) rate of the allotr io

    morp hic ferr ite is con trolled b y the d i f fu

    sion rate of carbon in austenite. A higher

    carbon content g ives a s lower gro wth

    rate of the ferr ite a nd, thus, a thinner layer

    of ferr ite at the pr ior austenite grain

    boundaries.

    Increasing carbon content leads to

    lower contents of both a l lo t r iomorphic

    and

    Widmannstatten

    fer r i te , g iv ing ro om

    for increasing contents of the f ine-grained

    acicular ferr ite. However, i t is not known

    whether the actual growth rate of ac icu

    lar ferr ite is inf luenced by the carb on con

    tent.

    The manganese atoms, on the other

    hand,

    are not redistr ibuted dur ing the

    transformat ion, but an increased manga

    nese con tent reduces the dr iving force f or

    the transformat ion. Thus, increasingman

    ganese also leads to a thinner layer of al

    lo t r iomorphic fer r ite .

    In

    a way, manganese

    and carbon can be considered as com

    plem entary e lements, and in pr inciple, the

    same microstructure should be at tainable

    454-s

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    DECEMBER 1990

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    Ho we ve r, there is at least one point

    martensite, degenerate pearl i te, bain

    high. The term

    It should b e no ted , that in pract ice there

    f re

    t ly used ones are Ni, M o and B. The

    con

    se the slag system in such a wa y as to

    Turning to the mechanical propert ies

    elationship b etw een microst ruc

    we ld .

    e

    w e l d .

    The rehea ted area unde r a bead

    ained zo ne,

    . The relat ive amounts of these zones

    w i t h ,

    am ong other th ings, chem

    w i l l , of course,

    luence the mechanical proper t ies. H ow

    that the re is a re

    zones and , the refor e, i t is possible to

    When designing weld metals, the most

    s to m eet requ iremen ts o n

    at ional propert ies as good as possible.

    Tensile properties are, at least for mild

    steels, a smaller problem since usually the

    strength of the weld metal is higher than

    the strength of the steel.

    Specif ications on impa ct toughness vary

    substantially, but in many cases the re

    quir em ent is 27 J (20 ft-lb) at a certain

    temperature. For more advanced appl i

    cat ions, higher toughness values are re

    quired,

    e.g., 34 or 40 ) (25 or 30 ft-lb).

    These levels of toughness values are

    achieved wi th on ly a relat ively small f rac

    t ion of the fracture surface of an impact

    toughness test bar having a duct i le, f i

    brous fracture, while the remaining part is

    a br it t le, cleavage type. To achieve ac

    ceptable impact toughness at lower

    t e m

    peratures (which in many cases is the

    t rend in development work today) i t is

    necessary to a void cleavage fracturestart

    ing too near the notch in the impact bar.

    This can be achieved by control of the

    microstructure.

    To improve impact toughness, some

    well-known physical metallurgy pr inciples

    are used. First, increasing the amount of

    acicular ferr ite by the control of al loying

    elements gives a reduc ed grain size. Sec

    ondly, use of basic-type consumables

    gives a lo w amount of oxygen , wh ich

    leads to a low volume fract ion of

    inclu

    sions. Finally, strict control of impurity el

    eme nts like S, P, Sn, As, Sb and N helps to

    prevent embr i t t lement of the s t ructure.

    The applicat ion of the f irst of these

    principles leads us back to the main ques

    t ion of this paper: how can the micro-

    structure be opt imized by changing car

    bon and manganese contents?

    As a contrast to this, Dolby (Ref. 13)

    suggested that weld metals with a very

    lean alloying content, having mainly a

    coarse-grained structure and a low yield

    strength,

    could have good impact tough

    ness.

    Al though there have been major im

    provements in the toughness levels that

    can be achieved in weld metals dur ing the

    last few decades, by applicat ion of the

    principles ment ioned above, there is st i l l

    room for fur ther improvement . A more

    fundamenta l understanding of the mech

    anisms controll ing the onset of cleavage

    fracture and the complex interrelat ionship

    between microst ructure and f racture

    needs to be developed. Major advances

    have indeed already been made in this

    f ield by Knott and coworkers (Refs. 14-

    16) who have studied the f racture beha v

    ior of C-M n w elds in deta il and com bined

    that with their ear l ier experience of f rac

    ture in steels. They concluded that cleav

    age fracture in welds of ten or iginated

    from cracking of oxide inclusions, in par

    t icular those situated in the coarse-grained

    allotr iomorphic ferr ite, and that the size

    distr ibut ion of these inclusions had a

    sig

    nif icant ef fect on the fracture toughness

    results. In steels, where the volume frac

    tion of oxide inclusions is much less, frac

    ture toughness is l inked more to the car

    bides precipitated along grain boundaries,

    nucleating cleavage cracks (Ref. 17). How

    ever, i t should be noted that in test ing

    fracture toughness of weld metals, Knott

    and coworkers used small size notched

    bars and tested them in slow strain-rate

    four-point bending, in a manner similar to

    CTOD test ing. The observat ion of c leav

    age cracks nuclea tingfrominclusions we re

    numerous in these tests but similar obser

    vat ions on impact specimens are, in fact ,

    fairly rare.

    Experimental

    Laboratory-made shielded metal arc

    electrodes , 4 mm (0.16 in.) in diame ter, of

    E7018 type with basic coat ings were used

    for the invest igat ion. The electrode coat

    ings were varied to a systematic series of

    four dif ferent manganese contents (0.8,

    1.1,1.2

    and2.1 wt-%)ateach carbon level

    (0.03, 0.06, 0.09 and 0.12 w t-%). Allw e l d

    ing was made in accordance with ISO

    2560, with a current of 180 A, voltage 23

    V and a maximum interpass temperature

    of 250C (484F). A stringer bead tech

    nique was used g iv ing a weld ing speed of

    about 4 mm/s (9 in. /min). The heat input

    then was around 1k j /mm (25 kj/ in.) .

    The chemical composi t ion of the weld

    deposits was measured using an opt ical

    emission spectrometer (OES), except for

    oxygen and n i t rogen, which were deter

    mined using combust ion furnaces. The

    OES analyses were made on the head of

    the tensile specimen.

    Two longitudinal al l-weld-metal tensile

    specimens (10 mm/0.4 in. in diameter)

    and 25 Charpy V-notch impact specimens

    were taken f rom each w e l d . The speci

    mens were taken f rom the middle of the

    plate. The im pact toughness w as tested at

    five differenttemperatures, wi thfivespec

    imens tested at each temperature.

    The microstructures of the weld metals

    were examined by convent ional meta l

    lography, using l ight opt ical microscopy.

    The etching was m ade usingfirsta solut ion

    of 4% picr ic acid in alcohol, fol lowed by

    2.5% nitric acid in alcohol.

    The quant itat ive assessment of the mi

    crostructu re was m ade using a Swif t p oint

    counter. At least 500 points were mea

    sured on each specimen. The microstruc

    ture const i tuents were ident i f ied accord

    ing to the classif ication of the IIW (Ref. 18).

    The austenite grain size was measured

    normal to the length axis of the grains(i.e.,

    the results are equal to L

    tn

    as denoted by

    Bhadeshia, et al.-R e f . 19) .

    T ofurtherstudy the microphases, t rans

    mission electron microscopy (TEM) was

    used.

    Thin foi ls were prepared by polish

    ing in a S truersTenupolin a 5% solut ion o f

    perchlor ic acid in methanol.

    WELDING RESEARCH SUPPLEMENT

    1455-s

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    Table

    1Chemical

    Compositions of the WeldMetals

    Mn

    Al

    Sn

    As

    Sb

    (a) Al l concen t ra t ions a re in wt -%, excep t fo r ox ygen and n i t roge n , wh ich a re g iven in we ig h t ppm

    o

    Sample No.

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    11

    12

    13

    14

    15

    16

    0.030

    0.032

    0.031

    0.032

    0.059

    0.059

    0.059

    0.065

    0.090

    0.089

    0.088

    0.098

    0.12

    0.12

    0.13

    0.11

    0.45

    0.45

    0.42

    0.45

    0.34

    0.33

    0.30

    0.33

    0.41

    0.35

    0.37

    0.39

    0.43

    0.44

    0.37

    0.36

    0.78

    1.27

    1.71

    2.05

    0.77

    1.09

    1.44

    1.83

    0.78

    1.18

    1.59

    2.25

    0.86

    1.35

    1.83

    2.18

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.010

    0.014

    0.014

    0.014

    0.011

    0.011

    0.013

    0.008

    0.004

    0.002

    0.010

    0.008

    0.006

    0.003

    0.013

    0.011

    0.011

    0.008

    0.011

    0.010

    0.010

    0.008

    0.009

    0.009

    0.008

    0.009

    0.007

    0.007

    0.007

    0.007

    0.003

    0.008

    0.003

    0.003

    0.005

    0.003

    0.003

    0.003

    0.014

    0.013

    0.011

    0.012

    0.011

    0.010

    0.009

    0.009

    0.014

    0.012

    0.012

    0.013

    0.015

    0.015

    0.013

    0.012

    0.006

    0.006

    0.006

    0.006

    0.003

    0.005

    0.004

    0.004

    0.005

    0.005

    0.005

    0.006

    0.005

    0.006

    0.006

    0.006

    0.001

    0.001

    0.001

    0.001

    0.001

    0.001

    0.001

    0.001

    0.004

    0.003

    0.005

    0.005

    0.003

    0.005

    0.005

    0.007

    0.002

    0.003

    0.003

    0.005

    0.001

    0.005

    0.005

    0.005

    0.004

    0.003

    0.004

    0.005

    0.003

    0.006

    0.005

    0.008

    95

    94

    109

    119

    66

    64

    65

    76

    36

    73

    77

    88

    54

    55

    63

    74

    336

    321

    297

    320

    306

    310

    305

    291

    421

    404

    491

    330

    329

    326

    318

    342

    Results

    Chemical Composition

    The chemical composi t ions of the we ld

    metals are given in Table 1. The carbo n

    contents have

    successfully

    been kept close

    to the nominal values. The manganese

    content scat tered somewhat around the

    nominal va lues, the maximum deviat ion

    being around 0.15%.

    The phosphorus content w as re lat ive ly

    constant throughout the invest igat ion,

    typically 0.010%. The sulfur content de

    creased with increasing manganese co n

    tent. This decrease was especially pro

    nounced at the lower carbon contents .

    The othe r im purity e lements (Sn, As and

    Sb) were all on a low level, and their sum

    did not exceed what is considered a safe

    level (Ref. 20).

    The n i t rogen content increased some

    what with increasing manganese content,

    being especia l ly pronounced for the low-

    carbon e lect rodes, whi le the oxygen co n

    tent on the whole was constant. I t should,

    however , be noted that the three lower

    manganese contents in the 0.09%C spec-

    Fig.

    1-Yield

    strength

    as

    a

    function of carbon

    an dmanganese

    content.

    650

    600

    550

    500

    450

    imens

    had an oxygen content approx i

    mately 100 ppm higher than in the other

    specimens.

    Mechanical Properties

    The yield strength measured is shown

    as a funct ion of

    N4n-content

    in Fig. 1. As

    expected, the yield strength increased

    with increasing carbon and Mn-content .

    The inf luence of Mn is relat ively strong,

    while the inf luence of carbon is quite

    small,

    except for the highest carbon con

    tent.

    The Charpy V-notch impact toughness

    curves are p lot t ed in Figs. 2 A- D . First, it

    can be noted that increasing manganese

    content decreased the upper shelf ener

    gies, probably simply due to an increased

    yield strength of the matr ix. The impact

    proper t ies at lowe r temperatures sho wed

    mixed behavior , depending on the com

    binat ion of C and Mn . For the lowe r

    man

    ganese contents, increasing carbon co n

    tent led to signif icant improvement in im

    pact toughness at lower temperatures. At

    the higher manganese contents, the inter-

    400

    0 ,5 1,0 1,5 2,0

    tVIANGANESE(%)

    2,5

    mediate carbon contents gave the best

    impact values at the lower temperatures.

    In

    Fig.

    3,

    impact toughness at

    60C

    (76F)

    is p lot ted as a funct ion of M n

    content, for constant carbon levels. For

    tw o of th e carb on levels (0.09 and 0.12%),

    op t imum con ten ts o f Mn were f ound ,

    whi le for the two lower carbon contents ,

    the opt imum Mn content seemed to be

    higher than the maxim um con tents used in

    this invest igat ion.

    The best impact toughness at

    60C

    was found for the combinat ion 0.12C-

    1.35Mn, but also the intermediate carbon

    levels, combined with a relat ively high

    manganese level, sh ow ed g oo d results. At

    - 4 0 C

    (-40F)

    almost the same pattern

    was fo l lo we d. The best impact toughness

    was achieved wi th the combinat ion

    0.12%C-1.2%Mn. Also, the combinat ions

    0.09%C-1.2%Mn and 0 .06%C-1.4-1 .8%

    Mn gave sat isfactory toughness. Increas

    ing the manganese content above 1.4%

    gave a reduct ion in toughness for the tw o

    highest carbon contents.

    M i c r o s t r u c t u r e

    The austenite grain size, measured in

    the last deposited bead, decreased with

    increasing carbon and manganese

    co n

    tent , except for the 0.06% carbon welds,

    w hic h all had a slightly larger austen ite

    grain size. The austenite grain sizes are

    given in Table 2.

    There is no systematic var iat ion in aus

    teni te gra in s ize wi th oxygen content .

    However , i t should of course be noted

    that the oxygen content var ies wi th in a

    fair ly narrow range.

    The results of the quantitative assess

    ment of the microstructure are given in

    Figs. 4 A- D . For a given ca rbo n c onte nt,

    the am ount of acicular ferr ite increased at

    the expense of both a l lo t r iomorphic fer

    r ite and ferr ite side plates with increasing

    manganese content . The maximum

    456-s|DECEMBER 1990

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    4/8

    I

    D

    2 0 0

    100

    0 . 8 % M n

    1.3 Mn

    1.7 Mn

    2.1% M n

    8 0 - 4 0 - 2 0 0

    T E M P E R A T U R E (C)

    2 0 4 0

    w

    CO

    C3

    O

    h-

    O

    -

    v

    ' / ;

    9-

    -' -

    v

    - -

    >

    ?

    /

    ?

    ^ h ^ i

    ' ^ 5 7 ^ ^ ^

    V

    fl

    ',

    IS. r

    tr~^*r.Vv~

    T

    -

    J

    c^ **' / / / / /

    ^ s :

    < Tfa

    >-

    -\. >./

    ^&&tf$&l& '')

    .

    ^Aiakv.^j.'v.:-jf_v-,

    ';

    V/y ..,

    tf*

    tax

    4 j *

    > - - * -

    Fig.

    5

    Optical micrograph

    of the

    weld metal

    with

    0.03%C

    0.8%Mn. The microstructure

    consists mainly

    of

    a coarse allotriomorphic

    fer

    rite, ferrite side plates andcoarse acicular

    fer

    rite.

    last

    bead .

    How ev er, since the microphases

    appear as retained a usteniteinthe as-de

    posited microstructure, they wil l be either

    ferr ite+ carb ide aggregates, untemp ered

    martensiteorstill retained austenite in the

    reheated mater ia l , depending onreheat

    ing temperature. Allthese phases c ould

    induce br it t leness. Examinat ion of cross-

    sections of impact specimens containing

    segregated bandsof microphases shows

    that in areas whe re the segregat ion bands

    are parallel with then o t c h , the f racture

    surface hasabr it t le appe arance Fig. 12.

    In other cases,inadjacent beads, w here

    the segregat ion bands are inclinedto the

    notch, thef racture surface has a more

    duct i le appearanceFig.13.

    These observat ions cannot be taken as

    str ict evidence for microphase- induced

    cleavage cracking, but comb ined w i t h

    many observat ions made earl ier,wecer

    tainly feel conf ident that the segregated

    microphases are responsibleforthe d rop

    in toughness at higher manganese con

    tents.

    Discussion

    It is com mo nly assumed that a high

    amount of acicular ferr ite should be

    present

    in the

    m icrost ructure

    to

    obta in

    goo d impact toughness through the

    effect

    of thef ine g rain size. The increasein ac

    icular ferrite is usually,butnot a lways,ac

    compan ied by adecreasein theamount

    of coarse-grained allotr iomorphic ferr ite

    and, thus, asdiscussed previous ly, con

    nected to the amounts of carbon and man

    ganese.

    In Fig. 14,theimpact toughness values

    at

    60C

    isp lo t t ed as afunct ion of the

    amountofacicular fer rite . Figure

    15

    shows

    similar informat ion extracted from the

    dataofEvans (Ref.2).

    In bo th invest igat ions, the same general

    t rend is fo un d, that increasing amountsof

    acicular ferr ite improves toughness.In

    deed, thereislarge scatter inthe relat ion

    between acicular ferr ite andtoughness,

    but as

    a

    rule

    of

    t h u m b ,

    it

    c an

    be

    said th at

    more than 50% acicular ferr ite givesac

    ceptable impact toughness. However ,in

    Fig. 14itcanbenoted that the we ld m et

    als with the lowestCcontents g ive m uch

    lower impact toughness values thanex

    pec ted f rom theacicular ferr ite con tent.

    This beha vior is in contradic t iontothe re

    sultsofEvans (Ref. 2), w h o fou ndamuch

    faster improvement inimpact toughness

    with increasing the a m o u n t of acicular

    ferr ite.

    The only element which clear ly isdif

    f e ren t be tween the low-carbon we lds

    and the other weldsinthis s tudy isn i t ro

    gen;being much h igher in the low -carbo n

    welds. Ni t rogeniswe l l k n o w n to

    embr i t -

    458-s|

    DECEMBER 1990

  • 8/11/2019 Manganese Effects 2

    6/8

    Fig. 6 - Optical m icrograph ot the weld metal

    with0.032.1%Mn. Mainly a cicular ferrite and

    thin rims of allotriomorphic ferrite.

    .-.'

    * . : > :

    \ss*-.

    f

    \>

    *ft,

    ;r. ;

    .

    ft;

    fTv77. -vjr ; < ,

    V ^ ' * > V ' - - ^ '-* *

    2

    - ..- * '? - V * .v

    ^ . , - , V K . ^ - < . :

    , j .

    i . a * . .

    , 2 0 p m

    fig. 7 Optical microg raph of the weld metal

    withO

    12%C-0.8%Mn. The rim of allotriomor

    phic ferrite is quite thin. The amo unt of ferrite

    side plates is quite high.

    r

    -r

    2 0 um

    I

    H

    I

    Fig. 8Optical microg raph of the weld m etal

    withO. 12 iC

    2.2%Mn. Shown

    is

    the extremely

    fine microstructure with a high amo unt o f acic

    ular ferrite.

    tie w e l d m e t a l s , a l t h o u g h t h e v a l u e s f o u n d

    h e r e w o u l d n o t b e c o n s i d e r e d a s d a n g e r

    o u s .

    U n f o r t u n a t e l y , E va n s d i d n o t g i v e

    va l u e s o f n i t r o g e n co n t e n t i n h is r e p o r t .

    T h e a g r e e m e n t f o r l o w v a l u e s o f a c i c u la r

    f e r r i t e i s no t su rp r i s ing , s ince th i s is de te r

    m i n e d b y t h e o v e r a l l b r i t t l e m i c r o s t r u c

    t u r e .

    T h e d e v i a t i o n a t t h e h i g h a m o u n t s o f

    ac i cu la r f e r r i t e i s , f o r l ack o f a b e t t e r

    e x p l a n a t i o n , a s s u m e d t o b e d u e t o n i t r o

    g e n . H o w e v e r , t h is is p u r e l y s p e c u l a t i v e

    a n d n e e d s m o r e i n v e s t i g a t i o n .

    A n o t h e r o b s e r v a t i o n th a t c a n b e m a d e

    f r o m bo th F igs . 14 and 15 is t ha t t h e imp ac t

    t o u g h n e ss sh o w s a s l ig h t d e c r e a se f o r t h e

    h i g h e s t a m o u n t s o f a c i cu l a r f e r r i t e . T h i s

    d e c r e a s e i n t o u g h n e s s s e e m s t o o c c u r f o r

    a c i cu l a r f e r r i t e co n t e n t s i n e xce ss o f a b o u t

    7 0 % . C o m p a r i s o n w i t h F ig . 1 2 s h o w s t h a t

    t h e l o w e r t o u g h n e s s is d u e t o i n c r e a s e d

    a m o u n t s o f b r i t t l e c l e a v a g e f r a c t u r e .

    Eva n s ( R e f . 2) a r g u e d t h a t t h e d e c r e a s

    i n g t o u g h n e ss o f h i g h a l l o y i n g co n t e n t

    w e l d s w a s d u e t o i n c r e a s i n g y i e l d s t r e n g t h

    w i t h o u t a c o r r e s p o n d i n g d e c r e a s e in g r a i n

    s i ze .

    A s e x p l a i n e d i n t h e b a c k g r o u n d s e c

    t i o n ,

    t h e m e c h a n i c a l p r o p e r t i e s a r e a

    f u n c t i o n o f a m i x t u r e o f m i c r o s t r u c t u r e s .

    T o a ssess t h e i n f l u e n ce o f e a ch t yp e o f

    m i c r o s t r u c t u r e o n t h e p r o p e r t i e s is a c o m

    p lex t ask . Eve n if i t is a g re a t o ve rs imp l i f i

    c a t i o n t o r e la t e th e m e c h a n i c a l p r o p e r t i e s

    t o t h e a s - d e p o s i t e d m i c r o s t r u c t u r e s , t h i s

    a p p r o a c h s h o u l d g i v e g u i d a n c e t o t h e o p

    e r a t i n g m e c h a n i s m s .

    H o w e v e r , as n o t e d a b o v e , t h e h i g h e s t

    a l l o y e d w e l d s c o n t a i n e d h i g h e r a m o u n t s

    o f a c i cu l a r f e r r i t e t h a n t h e l o w e r a l l o y e d

    w e l d s . T h e y i e l d s t r e n g t h o f t h e se a l l o ys

    a ls o w a s h i g h e r t h a n t h e l o w e r a l l o y e d

    m e t a l s . If t h e a b o v e w a y o f r e a s o n i n g is

    a c c e p t e d , t h e n a h i g h e r a m o u n t o f a c ic u

    l a r f e r r i t e i s e q u i va l e n t t o a d e c r e a s i n g

    g r a i n s i ze i n t h e w h o l e w e l d m e t a l . T h e

    h i g h e r y i e l d s t r e n g t h i s , t h u s , p a r t l y a g r a i n

    s i ze e f f e c t . F i n e r g ra i n s sh o u l d a l so l e a d t o

    b e t t e r t o u g h n e s s , c o n t r a r y t o w h a t is o b

    s e r v e d .

    T h e c la ss ica l m o d e l o f c l e a v a g e f r a c t u r e

    is t h a t th i s o c c u r s a t a t e m p e r a t u r e w h e r e

    t h e y i e l d s t r e n g t h e x c e e d s t h e f r a c t u r e

    s tr e s s. H o w e v e r , b o t h t h e y i e l d s t r e n g t h

    a n d t h e f r a c t u r e s t re ss a r e g r a i n - s ize d e

    p e n d e n t i n su ch a w a y t h a t f i n e r g r a i n s

    l e a d t o b o t h h i g h e r y i e l d a n d f r a c t u r e

    s t r e n g t h . T h u s , t h e a m o u n t o f c l e a v a g e

    f r a c t u r e is n o t e xp e c t e d t o i n c r e a se i n t h e

    h i g h e s t a l l o ye d w e l d m e t a l s , b u t t h is is

    o b v i o u s l y w h a t h a p p e n s w h e n t h e t o u g h

    n e ss f a l l s . O b v i o u s l y , so m e t h i n g i n t h e m i

    c r o s t r u c t u r e o f f se t s t h e b e n e f i c i a l e f f e c t

    o f f i n e r g r a i n s . T h e m o s t l i ke l y f a c t o r

    r e sp o n s i b l e f o r t h is is t h e se g r e g a t e d m i

    c r o p h a s e s , w h i c h is i n l in e w i t h t h e o b s e r

    va t ions in F igs . 12 and 13 .

    **

    -

    k

    i~ S

    K,.. *;

    ~^pt^\

    . ' ' > ' , ? . ' ' ''. ' ' - * ' - . -

    / C , 4 '-i''-' ' *

    :

    5 . . 20 pin

    A - v . . I tlI

    Fig.

    9A

    Optical micrograph showing how the microphases (white small grains) are randomly

    dispersed; Bwith higher alloying content, the micropha ses are becom ing more se gregated. Etch

    ing was made, usingKlemms reagent (R ef. 21), to more easily distinguish the micropha ses.

    WELDING RESEARCH SUPPLEMENT

    1459-s

  • 8/11/2019 Manganese Effects 2

    7/8

    Fig.10 TEM micrographshowingisolated islandsofretained austenite

    (arrowed) andsomegrain boundarycarbides.

    Fig.11

    Centereddark fieldTEMmicrograph of weld metal0.12%C-

    2.2%M n, showing an almost continuous layer of retained

    austenite.

    m * '

    :

    - i-'

    .

    - -j'fi*- ''.7- -J5-:

    K V

    I - - .

    *'

    ' *

    ?*- Y

    fe

    V

    >.

    -rb.- t/v

    A

    -s-

    ,./

    kT '

    r

    H^7 -^,i-'V

    :

    > : fm,- '

    HBt

    v -4 X f

    > *'?s

    ij^i^BK*-y^i.

    '

    --- A -

    |

    jfe

    r

    '*'*- -'

    W i

    I ^ P t : :

    l . _ .

    ij^H>:*4?'SZi

    - f

    '*

    T

    '^ - * ~*

    i ^ M p ^ F*. .\

    .

    50um

    K *

    :

    ' -

    i

    ~

    '

    F/g. 12 Optical micrograph

    of a

    polished

    cross-section through

    an

    impact specimen,

    showing that the britt le appearanceofthe

    fracture surface is parallelwith thesegregation

    band ofmicrophases.

    Fig.

    4 Impact

    toughnessat60C 3

    asa function of

    nm

    v

    : v * :

    :>

    ?,

    ?-ig

    50 pm

    F/g. 7 i

    Opticalmicrographshowinga ductile

    appearance of the fracturesurface in an area

    where thebands ofmicrophasesappear at an

    angle to the fracturesurface.

    acicular ferrite

    o

    content, from this

    investigation.

    C O

    C O

    a

    o


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