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Diltheyy
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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
=