Chapter 5 - Welding of High-Alloy Steels,

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    5.

    Welding of High-Alloy Steels,

    Corrosion

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    5. Welding of High-Alloy Steels, Corrosion 58

    Basically stainless steels are characterised by a chromium content of at least 12%. Figure

    5.1 shows a classification

    of corrosion resistant

    steels. They can be sin-

    gled out as heat- and

    scale-resistant and

    stainless steels, depend-

    ing on service tempera-

    ture. Stainless steels are

    used at room temperature

    conditions and for water-

    based media, whilst heat-

    and scale-resistant steels

    are applied in elevated

    temperatures and gaseous

    media.

    Depending on their microstructure, the alloys can be divided into perlitic-martensitic, ferritic,

    and austenitic steels. Perlitic-martensitic steels have a high strength and a high wear resis-

    tance, they are used e.g. as knife steels. Ferritic and corrosion resistant steels are mainly

    used as plates for household appliances and other decorative purposes.

    The most important group are austenitic steels, which can be used for very many applications

    and which are corrosion resistant against most media. They have a very high low tempera-

    ture impact resistance.

    Based on the simple Fe-C

    phase diagram (left figure),

    Figure 5.2 shows the ef-

    fects of two different

    groups of alloying elements

    on the equilibrium diagram.

    Ferrite developers with

    chromium as the most im-

    portant element cause astrong reduction of the aus-

    Classification of Corrosion-Resistant Steels

    non-stabilized

    (austenite withdelta-ferrite)X12CrNi18-8

    stabilized

    (austenite withoutdelta-ferrite)

    X8CrNiNb16-13

    ferritic austenitic

    stainlesssteels scale- and heat-resistantsteels

    corrosion-resistant steels

    semi-ferritic ferritic-austenitic

    X40Cr13 X10Cr13 X8Cr13 X20CrNiSi25-4

    perliticmartensitic

    ISF 2002br-er-06-01e.cdr

    Figure 5.1

    Modifications to the Fe-C Diagramby Alloy Elements

    ChromiumVanadiumMolybdenumAluminiumSilicon

    NickelManganeseCobalt

    Alloy elements in %Alloy elements in % Alloy elements in %

    T

    A4

    A3

    T

    A4

    A3

    T

    A4

    A3

    ISF 2002br-er-06-02e.cdr

    Figure 5.2

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    5. Welding of High-Alloy Steels, Corrosion 59

    tenite area, partly with downward equilibrium line according to Figure 5.2 (central figure).

    With a certain content of the related element, there is a transformation-free, purely ferritic

    steel.

    An opposite effect provide austenite developers. In addition to carbon, the most typical mem-

    ber of this group is nickel.

    Austenite developers cause an extension of

    the austenite area to Figure 5.2 (right figure)

    and form a purely austenitic and transforma-

    tion-free steel.

    The table in Figure 5.3 summarises the ef-

    fects of some selected elements on high alloy

    steels.

    The binary system Fe-Cr in Figure 5.4 shows

    the influence of chromium on the iron lattice.

    Starting with about 12% Cr, there is no more

    transformation into the cubic face-centred

    lattice, the steel solidifies purely as ferritic. In

    the temperature range between 800 and

    500C this system contains the intermetallic

    -phase, which decomposes in the lower

    temperature range into a low-chromium -

    solid solution and a chromium-rich -solid

    solution. Both, the development of the -phase and of the unary --decomposition cause a

    Effects of Some Elementsin Cr-Ni Steel

    Element Steel type, no. Effect

    Carbon

    l

    l

    l

    All types

    l

    l

    l

    Increases the strength, supports development

    of precipitants which reduce corrosion

    resistance, increasing C content reduces

    critical cooling rate

    Chromium

    l

    All types

    l

    Works as ferrite developer, increases

    oxidation- and corrosion-resistance

    Nickel

    l

    l

    All types Works as austenite developer, increases

    toughness at low temperature, grain-refining

    Oxygen

    l Special types l

    Works as strong austenite developer

    (20 to 30 times stronger than Nickel)

    Niobium

    l

    1.4511,1.4550,

    1.4580 u.a.

    Binds carbon and decreases tendency to

    intergranular corrosion

    Manganese

    l

    l

    All types

    l

    l

    Increases austenite stabilization, reduces hot

    crack tendency by formation of manganese

    sulphide

    Molybdenum

    l

    l

    1.4401,1.4404,

    1.4435 and others.

    l

    Improves creep- and corrosion-resistance

    against reducing media, acts as ferrite

    developer

    Phosphorus,

    selenium, or

    sulphur l

    1.4005, 1.4104,

    1.4305

    l

    l

    Improve machinability, lower weldability,

    reduce slightly corrosion resistance

    Silicon l

    l

    All types

    l

    l

    Improves scale resistance, acts as ferrite

    developer, all types are alloyed with small

    contents for desoxidation

    Titanium

    l

    l

    1.4510, 1.4541,

    1.4571 and others

    l

    Binds carbon, decreases tendency to

    intergranular corrosion, acts as a grain refiner

    and as ferrite developer

    Aluminium

    l

    Type 17-7 PH

    l

    Works as strong ferrite developer, mainly

    used as heat ageing additive

    Copperl

    l

    l

    Type 17-7 PH,1.4505, 1.4506

    l

    l

    Improves corrosion resistance against certain

    media, decreases tendency to stress

    corrosion cracking, improves ageing

    ISF 2002br-er06-03e.cdr

    Figure 5.3

    Figure 5.4

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    5. Welding of High-Alloy Steels, Corrosion 60

    strong embrittlement. With higher alloy steels, the diffusion speed is greatly reduced, there-

    fore both processes require a relatively long dwell time. In case of technical cooling, such

    embrittlement processes are suppressed by an increased cooling speed.

    Nickel is a strong austenite developer, see Figure 5.5 Nickel and iron develop in this system

    under elevated temperature a complete series of face-centred cubic solid solutions. Also in

    the binary system Fe-Ni

    decomposition processes

    in the lower temperature

    range take place.

    Along two cuts through the

    ternary system Fe-Cr-Ni,

    Figure 5.6 shows the most

    important phases which

    develop in high alloy steels.

    A solidifying alloy with 20%

    Cr and 10% Ni (left figure)

    forms at first -ferrite. -

    ferrite is, analogous to the

    Fe-C diagram, the primary

    from the melt solidifying

    body-centred cubic solid

    solution. However -ferrite

    is developed by transfor-

    mation of the austenite, but

    is of the same structure

    from the crystallographic

    point of view, see Figure

    5.4.

    Binary System Fe - Ni

    30Fe 10 20 40 50 60 70 80 90 Ni

    0

    200

    400

    600

    800

    1000

    1200

    1400

    1600

    Temp

    erature

    C

    %

    Nickel

    Fe Ni3

    FeNi 3

    S+S+

    ISF 2002br-er-06-05e.cdr

    Figure 5.5

    Sections of the Ternary System Fe-Cr-Ni

    700

    800

    900

    1000

    1100

    1200

    1300

    1400

    1500

    1600

    0 5 10 15 20 % Ni

    % Cr30 25 20 15 10

    70 % Fe

    0 5 10 15 20 25700

    800

    900

    1000

    1100

    1200

    1300

    1400

    1500

    1600

    40 35 30 25 20 15

    % Ni

    % Cr

    60 % Fe

    Temperature

    C C SS

    S+

    SSS+

    S+

    S+

    ISF 2002br-er-06-06e.cdr

    Figure 5.6

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    5. Welding of High-Alloy Steels, Corrosion 64

    The ferrite content can only be measured with a relatively large dispersal, therefore DeLong

    proposed to base a measurement procedure on standardized specimens. Such a system

    makes it possible to measure comparable values which don't have to match the real ferrite

    content. Based on these measurement values, the ferrite content is no longer given in per-

    centage, but steels are grouped by ferrite numbers. In addition to ferrite numbers, DeLong

    proposed a reworked Schaeffler diagram where the ferrite number can be determined by the

    chemical composition, Figure 5.13. Moreover, DeLong has considered the influence of nitro-

    gen as a strong austenite developer (effects are comparable with influence of carbon). Later

    on, nitrogen was included into the nickel-equivalent of the Schaeffler diagram.

    The most important feature

    of high alloy steels is their

    corrosion resistance start-

    ing with a Cr content of

    12%. In addition to the

    problems during welding

    described by the Schaeffler

    diagram, these steels can

    be negatively affected with

    view to their corrosion re-

    sistance caused by the

    welding pr

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