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Structure of the welded joint
Lecturer Dr. Jippei Suzuki 鈴木実平Graduate School of Engineering Mie UniversityDepartment of Mechanical EngineeringFaculty of Engineering, Mie UniversityKurima-Machiya-cho 1577, Tsu, Mie 514-8507Tel. 059-231-9372Fax. 059-231-9663E-mail [email protected]
and Cracking phenomena in steel weld
Material for May 6th 2010
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Thermal history of welds (Fig.A1)
Heat source arc laser electron beam
Heat conduction
Heat input H= ×60
H: heat input [ joule/cm]E: arc voltage [V]I: welding current [A]v: welding speed [cm/min]
vEI
Intensity of heating
Heat conduction 1. Thermal conductivity 2. Plate thickness 3. Joint type 4. Preheating temperature
Magnitude of cooling
Heat loss
Arc efficiency ηSielded metal arc welding 77-87%(steel)Submerged arc welding 77-99%TIG welding 68-85%MIG welding 66-69%(steel) 70-85%(aluminum)
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Microstructure of weld depends on the position (Fig.A2)
ab
c
d
δ-ferrite
α-ferrite
γ-austenite
Peritectic reaction L + δ→δ
Carbon content→Time →
Tem
pera
ture
→ Eutectic reaction L→γ + Fe3C
Eutectoid reaction γ→α + Fe3C
Carbon content of the steeld c b a
Unaffectedbase metal
Heat affected zone; HAZ
Weld metal
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Construction of steel weld (Fig.A3)
Meltingelectrode & base metal plate
Heatingbase metal plate
Solidificationweld metal
Cooling (transformation of γ to α)
formation ofWeld metal
formation ofHeat Affected Zone(HAZ)
heat
Weld metal; > melting point composite region unmixed zone
Weld interface
Heat affected zone partially-melted zone true heat affected zone coarse grain zone; > 1250℃ mixed zone; 1250 ~ 1100℃ fine grain zone; 1100 ~ 900℃ granular pearlite zone;900 ~ 750℃
(embrittled zone); 750 ~ 200℃
Un-affected zone; 200 ~ room temp.
Weldmetal
HAZ
UAZ
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Microstructure of steel weld (Fig.A4)
Weld metal
Fusion boundary
Heat affected zone
Weld metal reheated bysubsequent pass
Base metal
C Si Mn P S N O
Base metal 0.12 0.23 1.09 0.019 0.006 0.004 0.008
Weld metal 0.08 0.52 0.83 0.012 0.009 0.008 0.033
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Examples of weld solidification structure of mild steel (Fig.A5)
Axial crystal Stray crystal Columnar crystal Equiaxed dendrite
Weld metal
TIG welding, Heat input 4.3kJ/cm, Welding speed 10 ~ 100cm/minBase metal JIS-SS400, 0.18C-0.06Si-0.60Mn-0.015P-0.02S
Two microstructures Structure formed during solidification Structure formed by solid state transformation
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Equilibrium diagram for Fe-C(Fe3C or graphite) system (Fig.A6)
mass % of Carbon
Tem
pera
ture
℃
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Solidification of molten metals (Fig.A7)
(a) (b)
(c) (d)
Molten metal
nucleigrain
grain grain
grain boundary
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Liquid
Solid
Growth of grain - dendritic crystal (Fig.A8)
[100]
[111]
[110]
[010]
[001]
[100]
[001]
[010]
[100]
[010]
[001]
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Distribution of liquidus temperature near the solid-liquid interface (Fig.A9)
Equilibrium distribution coefficient; k0 = CS/CL
CS CL
Solute concentration→ Solute concentration→
Microsegregation of solute element during solidification
k0>1k0<1
CSCL
solid solid
liquidliquid
C0→
k0C0→Solu
te
conce
ntr
ati
on→
distance→liquid-solid interface
distance→liquid-solid interface
C0→
Solu
te c
once
ntr
ati
on→
CL = →C0
k0
solid liquid
CL = C0 1 + exp - X1 - k0
k0
RDL
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Constitutional supercooling (Fig.A10)
Solu
te c
once
ntr
ati
on
in liq
uid
Tem
pera
ture
Distance from the interface
Distance from the interface
Tem
pera
ture
Solute concentration
CL
C0
CL
C0
Solid-liquid interface
thermal gradient G
TL
constitutionalsupercooling
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Microstructure formed during solidification (Fig.A11)
R
GSome types of solidification structures Planar interface Cellular interface Cellular dendritic interface Columnar dendritic interface Equiaxed crystal
The solidification structure depends on the solidification parameter; and the concentration
of solute element; C0, where G is the thermalgradient, and R is moving speed of solid-liquidinterface.
Weld bead
Solid-Liquidinterface
Welding speed of V
Ri = V cosθi
θi
Thermalgradient
Melting point(liquidus temperature)
Tem
pera
ture
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Planar interface and cellular interface (Fig.A12)
Planar interface
Growth by planar interface near weld center of TIG-arc
weld metal of 99.99% pure Al thin steet
(Welding speed; 25cm/min)
Example of cell in weld metal of HY 80
Base metalplate
Distance from the interface
Distance from the interface
Cellular interface
direction ofGrowth
〈 100 〉
G; thermal gradient
TL; liquidus temperature
Weld metal
tem
pera
ture
tem
pera
ture X G
TL
cross-section of subgrain
liquid-solid interface
liquid-solid interface
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Competitive growth near fusion boundary in TIG arc weld metal of Al thin sheet
Corrugation on surface of TIG arc weld metal of Al thin sheet
Cellular dendritic interface (Fig.A13)
tem
pera
ture
Distance from the interface
G
TL
X
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Columnar dendritic interface and Equiaxed dendritic crystal (Fig.A14)
tem
pera
ture
tem
pera
ture
Distance from the interface
Distance from the interface
G
G
TL
TL
X
Columnar dendritic interface
Equiaxed dendritic crystal
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Characteristics of solidification in the weld pool
Two rules concerning with solidification 1. Molten metal solidifies along the direction of maximum thermal gradient. 2. The solidification rate depends on the crystalline direction. In the case of cubic crystal, the solid crystal grows in the direction of <100>.
Characteristics of solidification of weld pool 1. Epitaxial grow 2. Change in the direction of maximum thermal gradient
Competitive growthStray crystal
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Examples of pitaxial growth
Epitaxial growth and competitive growth (Fig.A15)
Liquid metal
Solid crystals
<100>
Higher temp.
Lower temp.
direction ofmaximumthermal gradient
a1
a2 a3a4
b1 b2
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Epitaxy Growth of one crystal on the surface of another crystal in which the growth of the deposited crystal is oriented by the lattice structure of the substrate.
McGraw-Hill Dictionary of scientific and technical terms
EpitaxyGrowth of a crystalline substance on a substrate crystal, in which the substrate determines the crystal structure adopted. Since crystal structures vary in lattice parameter and crystal type, quite apart from variations in atomic radius, it is obvious that epitaxial growth must be restricted, and that considerable stresses may be generated even when it occurs. In general, the two lattices involved (substrate and deposit) should be reasonably commensurate, the binding energy should not be too dissimilar, and in ionic substances, the arrangement of positive and negative ions should be capable of similar alignment.
Macdonald and Evans, The Metals Society, C.R.Tottle, An Encyclopaedia of Metallurgy and Materials
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Solidification of weld puddle (Fig.A16)
The crystals prefer to growing along the direction of the maximum thermal gradient.While, the crystals have the direction of maximum growing rate, for example, 〈 1 0 0 〉 .
Weld puddle
arcCenter line ofthe weld metal
Weld bond
Speed of solidification = welding speed = constant
Speed of solidification is smaller than welding speed, and increases upto welding speed.And the direction of maximum thermal gradient changes.
Moving speed of liquid-solid interface
θi
θi = 0R = Vcos 0 = V
R = V cosθi
θ =
R = Vcos = 0
2π
2π
Center line ofthe weld metal
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Behavior of growing columnar crystal (Fig.A17)
Stray crystal
Competitive growth
direction ofmaximumthermal gradient
a1 a2 a3a4
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Growth of columnar crystal in the weld puddle (Fig.A18)
Shapes of the puddle
Elliptical shaped (low speed welding)
Parabolic shaped(medium speed welding)
Teardrop shaped(high speed welding)
Low speed welding(25cm/min)
High speed welding(150cm/min)
Examples of growth of columnar crystal in TIG-arc weld metal of Al thin sheet
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Hot cracking
Hot crack occurs in the weld metal during solidification.
Solidification of molten metal
Contraction (shrinkage)
Gap or crevice in the weld metal
New molten metal flows into the gap
Occurrence of thermal stress
Melted region
Base metal plate
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Solidifying range (Fig.A19)
Weld puddle
Welding arc
Tem
pera
ture
→
Tem
pera
ture
→
SoluteConcentration→
Position→
C0
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Borland’s theory (Fig.A20)
Tem
pera
ture
→
Critical SolidificationRange (CSR)
Coherent temperature Critical temperature Stage 1 Stage 2 Stage 3
Wid
th o
f →
Solute content→
Stage 1Solids can movewithout restraint.
Coherent temperature
Critical temperature
Stage 2Solid crystals contacteach other, but liquidregions are connectedthree dimensionally
Stage 3Isolated liquid regionbegins to form, andnew liquid can notflow into that region.
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Tem
pera
ture
℃Sensitivity to hot cracking of aluminum alloys (Fig.A21)
High sensitivity Medium sensitivity Low sensitivityDifficult to weld possible paying attention possible to weld
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Hot cracking in carbon steels (Fig.A22)
Peritectic reaction in Fe-C system L+δ→γ
δ
γ
L
Tem
pera
ture
→
Carbon content→
C0
Microsegregation during solidification
Segregation due to peritectic reaction
δ
L
δ
δ Lγ
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Substance with low melting point Fe-S system FeS, FeS2
Fe-P system Fe3P
溶質濃度 C0 mass%
Sensitivity to hot cracking of steel (Fig.A23)T
empe
ratu
re ℃
Tem
pera
ture
℃
Tem
pera
ture
range
of
solid
ifica
tion
Solute content mass%