Structure of the welded joint Lecturer Dr. Jippei Suzuki 鈴鈴鈴鈴 Graduate School of Engineering Mie University Department of Mechanical Engineering Faculty of Engineering, Mie University Kurima-Machiya-cho 1577, Tsu, Mie 514-8507 Tel. 059-231-9372 Fax. 059-231-9663 E-mail [email protected]and Cracking phenomena in steel weld Material for May 6 th 2010
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Structure of welded joint and Cracking phenomena in steel weld
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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
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
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
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
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
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
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
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
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
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
Behavior of growing columnar crystal (Fig.A17)
Stray crystal
Competitive growth
direction ofmaximumthermal gradient
a1 a2 a3a4
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
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
Solidifying range (Fig.A19)
Weld puddle
Welding arc
Tem
pera
ture
→
Tem
pera
ture
→
SoluteConcentration→
Position→
C0
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.
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
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
Substance with low melting point Fe-S system FeS, FeS2