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 jsuzuki@mach.mie-u.ac.jp

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

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)

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

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

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

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

Equilibrium diagram for Fe-C(Fe3C or graphite) system 　 (Fig.A6)

mass % of Carbon

Tem

pera

ture

℃

Solidification of molten metals 　 (Fig.A7)

(a) (b)

(c) (d)

Molten metal

nucleigrain

grain grain

grain boundary

Liquid

Solid

Growth of grain - dendritic crystal (Fig.A8)

[100]

[111]

[110]

[010]

[001]

[100]

[001]

[010]

[100]

[010]

[001]

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

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

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

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

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%