dilthey wt2 - welding metallurgy

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2005 ISF – Welding and Joining Institute RWTH – Aachen University Lecture Notes Welding Technology 2 Welding Metallurgy Prof. Dr. –Ing. U. Dilthey

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  • 2005

    ISF Welding and Joining Institute RWTH Aachen University

    Lecture Notes

    Welding Technology 2 Welding Metallurgy

    Prof. Dr. Ing. U. Dilthey

  • Table of Contents

    Chapter Subject Page

    1. Weldability of Metals 3 2. TTA / TTT Diagrams 8 3. Residual Stresses 21 4. Classification of Steels, Welding of Mild Steels 31 5. Welding of High-Alloy Steels, Corrosion 57 6. Welding of Cast Materials 76 7. Welding of Aluminium Alloys 83 8. Technical Heat Treatment 94 9. Welding Defects 107 10. Testing of Welded Joints 125

  • 1.

    Weldability of Metals

  • 1. Weldability of Metals 4

    DIN 8580 and DIN 8595 classify welding into production technique main group 4 "Joining, group 3.6 "Joining by welding, Figure 1.1.

    Weldability of a component is determined by three outer features according to DIN 8528, Part 1. This also indicates whether a given joining job can be done by welding, Figure 1.2.

    ISF2002br-er01-01-E.cdr

    ClassificationofProduction TechniquestoDIN8580

    Maingroup6

    Changingmaterialcharacteristics

    Maingroup5

    Plating

    Maingroup4

    JoiningDIN8593

    Maingroup3

    Separating

    Maingroup2

    Deforming

    Maingroup1

    Forming

    Production TechniquesDIN8580

    Group4.1

    Assembling

    Group4.3

    Pressing

    Group

    Joiningby4.4

    forming

    Group4.6

    Joiningbywelding

    Group4.7

    Joiningbysoldering

    Group

    Joiningby4.5

    deforming

    Group4.8

    Bonding

    Group4.2

    Filling

    Sub-group4.6.2

    Fusionwelding

    Sub-group4.6.1

    Pressurewelding

    Figure 1.1

    ISF2002br-er01-02-E.cdr

    Influencing Factors onWeldability to DIN 8528 Part 1

    Weldability

    ofa

    component

    MaterialWeldingsuitability

    Welding possibility

    Manufacture W

    elding

    safet

    yDe

    sign

    Figure 1.2

  • 1. Weldability of Metals 5

    Material influence on weld-ability, i.e. welding suitabil-ity, can be detailed for a better understanding in three subdefinitions, Figure 1.3.

    The chemical composition of a material and also its met-allurgical properties are mainly set during its produc-tion, Figure 1.4. They have a very strong influence on the physical characteristics of the material. Process steps on steel manufacturing, shown in Figure 1.4, are the essential steps on the way to a processible and usable material.

    During manufacture, the requested chemical composition (e.g. by alloying) and metallurgi-cal properties (e.g. type of teeming) of the steel are obtained.

    Another modification of the material behav-iour takes place during subsequent treatment, where the raw material is rolled to processible semi-finished goods, e.g. like strips, plates, bars, profiles, etc.. With the rolling process, material-typical transformation processes, hardening and precipitation processes are used to adjust an optimised material charac-teristics (see chapter 2).

    ISF2002br-er01-04-E.cdr

    Important Process StepsDuring Steel Production

    Blastfurnace:Reductionoforeto

    IntakeofC,S,andPrawiron

    Converter:RemovalofCandPthroughoxygenandCaO

    Top-blow(BOF)-,bottomblow(OBM)-,stirrer-converter

    Injectionofsolidmaterialorfeedingcoredwires

    Ladletreatment:Alloyingandvacuumdegassing(removalofN ,H ,CO/CO )2 2 2

    Ladletreatmentelectricallyheated

    Continuouscasting:castingofbillets,blooms,slabs

    Figure 1.4

    Figure 1.3

  • 1. Weldability of Metals 6

    A survey from quality point of view about the influence of the most important alloy elements to some mechanical and metallurgical properties is shown in Figure 1.5.

    Figure 1.6 depicts the deci-sive importance of the car-bon content to suitability of fusion welding of mild steels. A guide number of flawless fusion weldability is a carbon content of C < 0,22 %. with higher C contents, there is a danger of hardening, and welding becomes only pos-sible by observing special precautions (e.g. pre- and post-weld heat treatment).

    ISF2002br-er01-05-E.cdr

    Influenceof AlloyElementsonSomeSteelProperties

    Tensilestrength

    Charpy-V-toughness

    Formationofseggregations

    Formationofinclusions

    Criticalcoolingrate

    +

    +

    -

    +(-400C)

    Hardness

    Creepresistance

    Hotcracking

    -

    +

    +

    +

    (+)

    -

    ++

    -

    +withMn

    +

    +

    ++

    +

    -

    --

    +withS

    +

    (+)

    -

    ++

    +

    (-)

    (-)

    -

    +

    ++

    +

    +

    -

    +

    +with Al

    + +

    ++

    + +

    (-)

    -

    ++

    -

    +

    +

    C Si Mn P S O Cr Ni Al

    +Increaseofproperty++Strongincreaseofproperty

    -Decreaseofproperty--Strongdecreaseofproperty

    Figure 1.5

    ISF2002br-er01-06-E.cdr

    FusionWeldabilityofUnalloyedQualitySteels

    S185(St33)[EN10025]

    Material C-content(%)(Meltanalysis) Fusionweldability

    unlimited(upto0,30)

    Notguaranteed,howevermostlynoproblemwithlowC-content

    S250GT (St34),S235JR(St37),S275JR(St42)[EN10025]L235GT (St35),L275GT (St45)[SteelsfortubingEN10208]P235GH(HI),P265GH(HII),P285NH(HIII)[SteelsforpressurevesselconstructionEN10028]C10(C10),C15(C15),C22(C22)[CasehardeningandtemperingsteelsEN10083]

    upto0,22%C:goodweldable(exception:platethicknesscondtions),aslongascontentofimpurities(P,Setc.)nottoohigh

  • 1. Weldability of Metals 7

    In addition to material behaviour, weldability is also essentially determined through the design of a component. The influence of the design is designated as welding safety, Figure 1.7.

    The influence of the manufac-turing process to weldability is called welding possibility, Figure 1.8. For example, a pre- and post-weld heat treatment is not always possi-ble, or grinding the weld sur-face before welding the subsequent pass cannot be carried out (narrow gap weld-ing).

    ISF2002br-er01-07-E.cdr

    WeldingSafety

    (Weldingsafetyduetodesign)

    WeldingSafety

    Design

    e.g.PowerflowinworkpieceArrangementofjointsMaterialthicknessNotcheffectStiffnessdifferences

    Stresscondition

    e.g. TypeandLevelofstrainDimensionaldegreeofstrainStressspeedTemperatureCorrosion

    inworkpiece

    Figure 1.7

    ISF2002br-er01-08-E.cdr

    WeldingPossibility

    Welding Executionofwelding

    e.g.WeldingmethodConsumbletypeandauxiliariesJointtypeGrooveshapePreheatingActionsinthecaseofunfavourableweatherconditions

    Post-treatment

    e.g.HeatcontrolHeatinputWeldingsequence

    e.g.Post-weldheattreatmentGrindingPickling

    WeldingPossibility

    (weldingpossibilityduetomanufacture)

    preparation

    Figure 1.8

  • 2.

    TTA / TTT - Diagrams

  • 2. TTA / TTT Diagrams 9

    An essential feature of low alloyed ferrous materials is the crystallographic trans-formation of the body-centred cubic lattice which is stable at room tempera-ture (-iron, ferritic struc-ture) to the face-centred cubic lattice (-iron, aus-tenitic structure), Figure 2.1. The temperature, where this transformation occurs, is not constant but depends on factors like

    alloy content, crystalline structure, tensional status, heating and cooling rate, dwell times, etc..

    In order to be able to understand the basic processes it is necessary to have a look at the basic processes occuring in an idealized binary system. Figure 2.2 shows the state of a binary system with complete solubility in the liquid and solid state. If the melting of the L1 alloy is cooling down, the first crystals of the composition c1 are formed with reaching the temperature T1. These crystals are depicted as mixed crystal , since they consist of a compound of the components A (80%) and of B (20%). Further, a melting with the composi-tion c0 is present at the temperature T1. With dropping temperature, the remaining melt is en-

    ISF2002br-eI-02-01.cdr

    Body- and Face-CenteredLattice Structures

    Latticeconstant0.286nm

    atroomtemperature

    Latticeconstant0.364nmat900C

    a -Ironbody-centered

    g -Ironface-centered

    Figure 2.1

    ISF2002br-eI-02-02.cdr

    Binary System With Complete Solubilityin Liquid and Solid Phase

    1

    23

    45

    S

    Li

    So

    A(Ni)

    B(Cu)

    L1 L1

    TsA

    T1

    T2

    TsB

    c1 c2 c3 c4c0 Timet

    Tem

    pera

    ture

    T

    Tem

    pera

    ture

    T

    Concentrationc

    S+ a

    a -

    ba

    ss

    Figure 2.2

  • 2. TTA / TTT Diagrams 10

    riched with component B, following the course of line Li (liquidus line, up to point 4). In paral-lel, always new and B richer -mixed crystals are forming along the connection line So (solidus line, points 1, 2, 5). The distribution of the components A and B in the solidified struc-ture is homogeneous since concentration differences of the precipitated mixed crystals are balanced by diffusion processes.

    The other basic case of complete solubility of two components in the liquid state and of com-plete insolubility in the solid state shows Figure 2.3 If two components are completely insolu-ble in the solid state, no mixed crystal will be formed of A and B. The two liquidus lines Li cut in point e which is also designated as the eutectic point. The isotherm Te is the eutectic line.

    If an alloy of free composition solidifies according to Figure 2.3, the eutectic line must be cut. This is the temperature (Te) of the eutectic transformation: S A+B (T = Te = const.). This means that the melt at a constant temperature Te dissociates in A and B. If an alloy of the composition L2 solidifies, a purely eutectic structure results. On account of the eutectic reaction, the temperature of the alloy remains constant up to the completed transformation (critical point) (Figure 2.2).

    Eutectic structures are normally fine-grained and show a characteristic orientation between the constituents. The alloy L1 will consist of a compound of alloy A and eutectic alloy E in the

    solid state. You can find further in-formation on transforma-tion behaviour in relevant specialist literature.

    The definite use of the principles occurs in the iron-iron carbide diagram. Transformation behaviour of carbon containing iron in the equilibrium condi-tion is described by the

    ISF2002br-eI-02-03.cdr

    Binary System With Complete Solubility in Liquid Phaseand Complete Unsolubility in Solid Phase

    TsA

    Te

    2

    L1 L1L2 L2

    1

    2

    34

    S+A

    So

    S

    S+B

    Li Li

    A+E E B+E

    B

    TsB

    c1 ce

    Tem

    pera

    ture T

    Tem

    pera

    ture

    T

    Concentrationc TimetA

    Figure 2.3

  • 2. TTA / TTT Diagrams 11

    stable phase diagram iron-graphite (Fe-C). In addition to the stable system Fe-C which is specific for an equilibrium-close cooling, there is a metastable phase diagram iron cementite (Fe-Fe3C). During a slow cooling, carbon precipitates as graphite in accord with the stable system Fe-C, while during accelerated cooling, what corresponds to technical conditions, carbon precipitates as cementite in agreement with the metastable system (Fe-Fe3C). Per definition, iron carbide is designated as a structure constituent with cementite although its stoichiometric composition is identical (Fe3C). By definition, cementite and graphite can be present in steel together or the cementite can decompose to iron and graphite during heat treatment of carbon rich alloys. However, it is fundamentally valid that the formation of ce-mentite is encouraged with increasing cooling rate and decreasing carbon content. In a dou-ble diagram, the stable system is shown by a dashed, the metastable by a solid line, Figure 2.4.

    The metastable phase diagram is limited by the formation of cementite with a carbon content of 6,67 mass%. The strict stoichiometry of the formed carbide phase can be read off at the top X-coordinate of the molar carbon content. In accordance with the carbon content of Fe3C, cementite is formed at a mo-lar content of 25%. The solid solutions in the phase fields are designated by Greek charac-ters. According to convention, the transition points of pure iron are marked with the character A - arrt (stop point) and distinguished by subjacent indexes. If the transition points are de-termined by cooling curves, the character r = refroidissement is additionally used. Heat-up curves get the supplement c - chauffage. Important transition points of the commercially more important metastable phase diagram are:

    - 1536 C: solidification temperature (melting point) -iron, - 1392 C: A4- point - iron,

    StableandMetastableIron-Carbon-Diagram

    ISF2002br-eI-02-04.cdr

    melt+-solidsolutiond

    d -+g-

    solidsol.

    d -

    solidsol.melt

    melt+graphite

    Fe C(cementite)

    3

    melt+cementite

    melt+austenite

    austenite

    austenite+graphiteaustenite+cementite

    lede

    burit

    e

    austenite+ferrite

    ferrite

    perli

    te

    stableequilibriummetastableequilibrium

    ferrite+graphiteferrite+cementite

    Mass%ofCarbon

    Tem

    pera

    ture

    C

    Figure 2.4

  • 2. TTA / TTT Diagrams 12

    - 911 C: A3- point non-magnetic - iron, with carbon containing iron: - 723 C: A1- point (perlite point). The corners of the phase fields are designated by continuous roman capital letters.

    As mentioned before, the system iron-iron carbide is a more important phase diagram for technical use and also for welding techniques. The binary system iron-graphite can be stabi-lized by an addition of silicon so that a precipitation of graphite also occurs with increased solidification velocity. Especially iron cast materials solidify due to their increased silicon con-tents according to the stable system. In the following, the most important terms and transfor-mations should be explained more closely as a case of the metastable system.

    The transformation mechanisms explained in the previous sections can be found in the bi-nary system iron-iron carbide almost without exception. There is an eutectic transformation in point C, a peritectic one in point I, and an eutectoidic transformation in point S. With a tem-perature of 1147C and a carbon concentration of 4.3 mass%, the eutectic phase called Le-deburite precipitates from cementite with 6,67% C and saturated -solid solutions with 2,06% C. Alloys with less than 4,3 mass% C coming from primary austenite and Ledeburite are called hypoeutectic, with more than 4,3 mass% C coming from primary austenite and Lede-burite are called hypereutectic.

    If an alloy solidifies with less than 0,51 mass percent of carbon, a -solid solution is formed below the solidus line A-B (-ferrite). In accordance with the peritectic transformation at 1493C, melt (0,51% C) and -ferrite (0,10% C) decompose to a -solid solution (austenite).

    The transformation of the -solid solution takes place at lower temperatures. From -iron with C-contents below 0.8% (hypoeutectoidic alloys), a low-carbon -iron (pre-eutectoidic ferrite) and a fine-lamellar solid solution (perlite) precipitate with falling temperature, which consists of -solid solution and cementite. With carbon contents above 0,8% (hypereutectoidic alloys) secondary cementite and perlite are formed out of austenite. Below 723C, tertiary cementite precipitates out of the -iron because of falling carbon solubility.

  • 2. TTA / TTT Diagrams 13

    The most important distinguished feature of the three described phases is their lattice struc-ture. - and -phases are cubic body-centered (CBC lattice) and -phase is cubic face-centered (CFC lattice), Figure 2.1.

    Different carbon solubility of solid solutions also results from lattice structures. The three above mentioned phases dissolve carbon interstitially, i.e. carbon is embedded between the iron atoms. Therefore, this types of solid solutions are also named interstitial solid solution. Although the cubic face-centred lattice of austenite has a higher packing density than the cu-bic body-centred lattice, the void is bigger to disperse the carbon atom. Hence, an about 100 times higher carbon solubility of austenite (max. 2,06% C) in comparison with the ferritic phase (max. 0,02% C for -iron) is the result. However, diffusion speed in -iron is always at least 100 times slower than in -iron because of the tighter packing of the -lattice.

    Although - and -iron show the same lattice structure and properties, there is also a differ-ence between these phases. While -iron develops of a direct decomposition of the melt (S ), -iron forms in the solid phase through an eutectoidic transformation of austenite ( + Fe3C). For the transformation of non- and low-alloyed steels, is the transformation of -ferrite of lower importance, although this -phase has a special importance for weldability of high alloyed steels. Unalloyed steels used in industry are multi-component systems of iron and carbon with alloy-ing elements as manganese, chromium, nickel and silicon. Principally the equilibrium dia-

    gram Fe-C applies also to such multi-component sys-tems. Figure 2.5 shows a schematic cut through the three phase system Fe-M-C.

    During precipitation, mixed carbides of the general composition M3C develop. In contrast to the binary system Fe-C, is the three Descriptionofthe Terms Ac , Ac , Ac1b 1e 3

    Ac3

    Ac1e

    ISF2002br-eI-02-05.cdr

    Figure 2.5

  • 2. TTA / TTT Diagrams 14

    phase system Fe-M-C characterised by a temperature interval in the three-phase field + + M3C. The beginning of the transformation of + M3C to is marked by Aclb, the end by Acle. The indices b and e mean the beginning and the end of transformation. The described equilibrium diagrams apply only to low heating and cooling rates. However, higher heating and cooling rates are pre-sent during welding, con-sequently other structure types develop in the heat affected zone (HAZ) and in the weld metal. The struc-ture transformations during heating and cooling are described by transformation diagrams, where a temperature change

    is not carried out close to the equilibrium, but at different heating and/or cooling rates. A representation of the transformation processes during isothermal austenitizing shows Figure 2.6. This figure must be read exclusively along the time axis! It can be recognised that several transformations during isothermal austenitizing occur with e.g. 800C. Inhomogeneous austenite means both, low carbon containing austenite is formed in areas, where ferrite was present before transformation, and carbon-rich aus-tenite is formed in areas during transforma-tion, where carbon was present before transformation. During sufficiently long an-nealing times, the concentration differences are balanced by diffusion, the border to a ho-

    TTA DiagramforIsothermal Austenitization

    ISF2002br-eI-02-06.cdr

    s

    C

    Figure 2.6

    TTA-Diagram forContinuous Warming

    ASTM4;L=80m ASTM11;L=7m

    20m 20m

    ISF2002br-er02-07.cdr

    Tem

    pera

    ture

    Time

    Figure 2.7

  • 2. TTA / TTT Diagrams 15

    mogeneous austenite is passed. A growing of the austenite grain size (to ASTM and/or in m) can here simultaneously be observed with longer annealing times.

    The influence of heating rate on austenitizing is shown in Figure 2.7. This diagram must only be read along the sloping lines of the same heating rate. For better readability, a time pattern was added to the pattern of the heating curves. To elucidate the grain coarsening during aus-tenitizing, two microstructure photographs are shown, both with different grain size classes to ASTM.

    Figure 2.8 shows the rela-tion between the TTA and the Fe-C diagram. It's obvi-ous that the Fe-C diagram is only valid for infinite long dwell times and that the TTA diagram applies only for one individual alloy.

    Figure 2.9 shows the dif-ferent time-temperature passes during austenitizing and subsequent cooling down. The heating period is com-posed of a continuous and an isothermal section.

    During cooling down, two different ways of heat con-trol can be distinguished: 1. : During continuous temperature control a cooling is carried out with a constant cooling rate out of

    DependenceBetween TTA-DiagramandtheFe-M-CSystem

    Ac3

    Ac1e

    Ac1b

    ISF2002br-eI-02-08.cdr

    Figure 2.8

    HeatingandCoolingBehaviourWithSeveralHeat Treatments

    Ac3

    Ac1e Ac1b

    continuous

    isothermal

    ISF2002br-eI-02-09.cdr

    Figure 2.9

  • 2. TTA / TTT Diagrams 16

    the area of the homogeneous and stable austenite down to room temperature. 2. : During isothermal temperature control a quenching out of the area of the austenite is carried out into the area of the metastable austenite (and/or into the area of martensite), fol-lowed by an isothermal holding until all transformation processes are completed. After trans-formation will be cooled down to room temperature.

    Figure 2.10 shows the time-temperature diagram of a isothermal transforma-tion of the mild steel Ck 45. Read such diagrams only along the time-axis! Below the Ac1b line in this figure, there is the area of the me-tastable austenite, marked with an A. The areas marked with F, P, B, und M represent areas where fer-rite, perlite, Bainite and martensite are formed. The

    lines which limit the area to the left mark the beginning of the formation of the respective structure. The lines which limit the area to the right mark the completion of the formation of the respective structure. Because the ferrite formation is followed by the perlite formation, the completion of the ferrite formation is not determined, but the start of the perlite formation. Transformations to ferrite and perlite, which are diffusion controlled, take place with elevated temperatures, as diffusion is easier. Such structures have a lower hardness and strength, but an increased toughness.

    Diffusion is impeded under lower temperature, resulting in formation of bainitic and marten-sitic structures with hardness and strength values which are much higher than those of ferrite and perlite. The proportion of the formed martensite does not depend on time. During quenching to holding temperature, the corresponding share of martensite is spontanically formed. The present rest austenite transforms to Bainite with sufficient holding time. The right

    Isothermal TTT-DiagramofSteelC45E(Ck45)

    ISF2002br-eI-02-10.cdr

    Figure 2.10

  • 2. TTA / TTT Diagrams 17

    detail of the figure shows the present structure components after completed transformation and the resulting hardness at room temperature. Figure 2.11 depicts the graphic representation of the TTT diagram, which is more important for welding techniques. This is the TTT diagram for continuous cooling of the steel Ck 15. The diagram must be read along the drawn cooling passes. The lines, which are limiting the individual areas, also depict the beginning and the end of the respective transformation. Close to the cooling curves, the amount of the formed structure is indicated in per cent, at the end of each curve, there is the hardness value of the structure at room temperature.

    Figure 2.12 shows the TTT diagram of an alloyed steel containing approximately the same content of carbon as the steel Ck 15. Here you can see that all trans-formation processes are strongly postponed in rela-tion to the mild steel. A completely martensitic transformation is carried out up to a cooling time of about 1.5 seconds, com-pared with 0.4 seconds of Ck 15. In addition, the completely diffusion con-trolled transformation proc-esses of the perlite area are postponed to clearly longer times.

    The hypereutectoid steel C 100 behaves completely different, Figure 2.13. With this carbon content, a pre-

    Continuous TTT-DiagramofSteelC15E(Ck15)

    Time

    27

    4019

    370 235 220 170

    ISF2002br-eI-02-11.cdr

    Figure 2.11

    MS

    M

    A+C F

    B

    5

    55 22 P25

    2

    23

    Ac3

    Ac1

    Chemicalcomposition%

    SiC Mn P S Al Cr Mo Ni V0,13 0,31 0,51 0,023 0,009 0,010 1,5 0,06 1,55

  • 2. TTA / TTT Diagrams 18

    eutectoid ferrite formation cannot still be car-ried out (see also Figure 2.3). The term of the figures 2.9 to 2.11 "austenitiz-ing temperature means the temperature, where the workpiece transforms to an austen-itic microstructure in the course of a heat treatment. Dont mix up this temperature with the AC3 temperature, where above it there is only pure austenite. In addition you can see that only martensite is formed from the aus-tenite, provided that the cooling rate is suffi-ciently high, a formation of any other microstructure is completely depressed. With this type of transformation, the steel gains the highest hardness and strength, but loses its toughness, it embrittles. The slowest cooling rate where such a transformation happens, is called critical cooling rate.

    0

    100

    200

    300

    400

    500

    600

    700

    800

    900

    1000C

    Tem

    pera

    ture

    10-1 100 101 102 103 104 1050

    100

    200

    300

    400

    500

    600

    700

    800

    900

    1000

    s

    C

    Tem

    pera

    ture

    Time

    P

    2 15

    100100 100 100 100 100 100

    AC1e

    AC1b100

    MS

    M RA 30

    914 901 817 366 351 283 236 215 214 177

    180

    A+C

    AC1e

    AC1b

    MS

    M

    A PC

    100 100

    100

    1005

    100 100 100 100 100

    RA 04

    876 887 867 496 457 442 347 289 246 227 200

    194

    Continuous TTT-Diagramof Steel C100U (C 100 W1)

    Chemicalcomposition%

    Mn P S Cr Cu Mo Ni V1,03 0,17 0,22 0,014 0,012 0,07 0,14 0,01 0,10 traces

    C Si

    austenitizingtemperature790Cdwelltime10min,heatedin3min

    austenitizingtemperature860Cdwelltime10min,heatedin3min

    ISF2002br-er02-13.cdr

    Figure 2.13

    Influence of Alloy Elementson Transformation Behaviour of Steels

    Tem

    pera

    ture

    Transitiontime

    Lownumberofnucleiduetomelting,hightemperature,longdwelltime,coarseaustenitegrain,C-increaseupto0,9%,Mn,Ni,Mo,Cr

    Highnumberofnuclei,lowhardeningtemperature,C-increaseabove0,9%

    Ar1

    Ar3

    Perlite 100%

    Cr,V,Mo

    Cr,V,Mo

    Bainite

    C,Cr,Mn,Ni,Mo,hightemperature,ferriteprecipitationinperlite

    Lowhardeningtemperature(specialcarbides),austeniteabovebainite

    C,Mn,Cr,Ni,Mo,V,highhardeningtemperature,pre-precipitationinbainite

    Martensite

    Co, Al,deformationofaustenite,lowhardeningtemperature

    Ms

    ISF2002br-er02-14.cdr

    Figure 2.14

    Temperature Influence onTransformation Behaviour of Steels

    Tem

    pera

    ture

    1000

    800

    A

    600

    400

    200

    C

    MS

    M

    B

    FP

    900C1300C

    Stru

    ctur

    edi

    strib

    utio

    n

    100

    75

    50

    25

    0

    %M M

    B B

    10-1 1 10 102 103sCoolingtime(A to500C)3

    ISF2002br-er02-15.cdr

    Figure 2.15

  • 2. TTA / TTT Diagrams 19

    Figure 2.14 shows schematically how the TTT diagram is modified by the chemical compo-sition of the steel. The influence of an increased austenitizing temperature on transformation behaviour shows Figure 2.15. Due to the higher hardening temperature, the grain size of the austenite is higher (see Figure 2.6 and 2.7).

    This grain growth leads to an extension of the diffu-sion lengths which must be passed during the trans-formation. As a result, the "noses" in the TTT diagram are shifted to longer times. The lower part of the figure shows the proportion of formed martensite and Bainite depending on cool-ing time. You can see that with higher austenitizing temperature the start of Bainite formation together with the drop of the mart-ensite proportion is clearly shifted to longer times. As Bainite formation is not so much impeded by the coarse austenite grain as with the completely diffu-sion controlled processes of ferrite and perlite forma-tion, the maximum Bainite proportion is increased from about 45 to 75%.

    Welding TTT-DiagramofSteelS355J2G3(St52-3)

    0

    100

    200

    300

    400

    500

    600

    700

    800

    900

    1 2 4 6 8 10 20 40 60 80 100 200 400sTime

    Tem

    pera

    ture

    449 420 400 363 334 324 270 253 251 249

    222 215

    243

    C

    Chemicalcomposition%

    SiC Mn P S Al N Cr Cu Ni0,16 0,47 1,24 0,029 0,029 0,024 0,0085 0,10 0,17 0,06

    Max.temperature1350C Weldingheatcycle

    48

    75

    S355J2G3(St52-3)

    55

    ISF2002br-eI-02-16.cdr

    B

    Figure 2.16

    Welding TTT -DiagramofSteel15Mo3(15Mo3)

    0

    100

    200

    300

    400

    500

    600

    700

    800

    900C

    1 2 4 6 8 10 20 40 60 80 100 200 400sTime

    Tem

    pera

    ture

    440 431 338 285 255 234 224 210

    208 200 178

    A =861Cc3A =727Cc1

    MSB

    F

    HV30

    P

    M

    14 74 8795

    99 83 77 60 38 15

    1 7 19 4 832 45

    1753

    32

    Chemicalcomposition%

    SiC Mn P S Mo0,16 0,30 0,68 0,012 0,038 0,29

    15Mo3 Max.temperature1350C Weldingheatcycle

    ISF2002br-eI-02-17.cdr

    Figure 2.17

  • 2. TTA / TTT Diagrams 20

    Due to the strong influence of the austenitizing temperature to the transformation behaviour of steel, the welding technique uses special diagrams, the so called Welding-TTT-diagrams.

    They are recorded following the welding temperature cycle with both, higher austenitizing temperatures (basically between 950 and 1350C) and shorter austenitizing times. You find two examples in Figures 2.16 and 2.17.

    Figure 2.18 proves that the iron-carbon diagram was developed as an equilib-rium diagram for infinite long cooling time and that a TTT diagram applies al-ways oy for one alloy.

    RelationBetween TTT-DiagramandIron-Carbon-Diagram

    FP

    0

    200

    400

    600

    800

    C

    1000

    10-1 100 101 102 103 104s

    B

    M

    0

    200

    400

    600

    800

    C

    10000

    0,451

    %C2

    MSTe

    mpe

    ratu

    re

    Tem

    pera

    ture

    Time

    0,5

    ISF2002br-eI-02-18.cdr

    Figure 2.18

  • 3.

    Residual Stresses

  • 3. Residual Stresses 22

    The emergence of residual stresses can be of very different nature, see three examples in Figure 3.1. Figure 3.2 details the causes of origin. In a pro-duced workpiece, material-, production-, and wear-caused residual stresses are overlaying in such a way that a certain condition of residual stresses is cre-ated. Such a workpiece shows in service more or

    less residual stresses, and it will never be stress-free!

    Figure 3.3 defines residual stresses of 1., 2., and 3. type. This grading is independent from the origin of the residual stresses. It is rather based on the three-dimensional extension of the stress conditions.

    Based on this definition, Fig-ure 3.4 shows a typical distri-bution of residual stresses. Residual stresses, which build-up around dislocations and other lattice imperfections (III), superimpose within a grain causing stresses of the 2nd type and if spreading around several grains, bring out residual stresses of the 1st type. The formation of residual stresses in a transition-free

    VariousReasonsofResidualStressDevelopment

    grindingdisk

    pressure tension

    tens

    ion

    pres

    sure

    weld

    ISF2002br-eI-03-01e.cdr

    Figure 3.1

    DevelopmentofResidualStresses

    relevantmaterial

    e.g.polyphasesystems,

    non-metallicinclusions,griddefects

    AnalysisofResidualStressDevelopment

    wearproduction

    mechanical

    e.g.partial-plasticdeformationof

    notchedbarsorcloseto

    inclusions,fatiguestrain

    thermal

    e.g.thermalresidualstresses

    duetooperational

    temperaturfields

    chemical

    e.g.H-diffusion

    underelectro-chemical

    corrosion

    changingmaterialcharacteristics

    inductionhardening,casehardening,

    nitriding

    separating

    residualstressesdueto

    machining

    forming

    e.g.thermalresidualstresses

    joiningresidual

    stressesduetowelding

    plating

    layer residualstresses

    deforming

    residualstressesdueto

    inhomogenuousdeformation-anisotropy

    ISF2002br-eI-03-02e.cdr

    Figure 3.2

  • 3. Residual Stresses 23

    steel cylinder is shown in Figures 3.5. and 3.6. During water quenching of the homogeneous heated cylinder, the edge of the cylinder cools down faster than the core. Not before 100 seconds have elapsed is the temperature across the cylinder's cross section again

    homogeneous. The left part of Figure 3.5 shows the T-t- curve of three different meas-urement points in the cylinder. Figure 3.6 shows the results of quenching on the stress condition in the cylinder. At the beginning of cooling, the cylinder edge starts shrinking faster than the core (upper figure). Through the stabilising effect of the cylinder core,

    DefinitionofResidualStresses

    GeneralDefinitionoftheTermResidualStresses

    Residual stresses of the I. type are almost homogenuous across largermaterial areas (several grains). Internal forces related to residualstresses of I. type are in an equilibrium with view to any cross-sectional

    throughout the complete body. In addition, the internal torquesrelated to the residual stresses with reference to each axis disappear.When interfering with force and torque equilibrium of bodies underresidual stresses of the I. type,

    .

    Residual stresses of the II. type are almost homogenuous across smallmaterial areas (one grain or grain area). Internal forces and torquesrelated to residual stresses of the II. type are in an equilibrium acrossa sufficient number of grains. When interfering with this equilibrium,

    Residual stresses of the III. type are inhomogenuous across smallestmaterial areas (some atomic distances). Internal forces and torquesrelated to residual stresses of the III. type are in an equilibrium acrosssmall areas (sufficiently large part of a grain). When interfering with thisequilibrium, .

    plane

    macroscopic dimension changes

    always develop

    macroscopic dimension changes may develop.

    macroscopic dimension changes do not develop

    ISF2002br-er03-03e.cdr

    Figure 3.3

    Definition of Residual Stresses ofI., II., and III. Type

    s

    I+

    -

    0

    tens

    ion

    s

    s

    II

    s

    III

    x

    x

    grainboundaries

    0

    y

    s

    I

    s

    II

    s

    III

    residualstressesbetweenseveralgrainsresidualstressesinasinglegrainresidualstressesinapoint

    =