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B-1

Equilibrium condition (in general meaning)ferrite(), cementite(Fe3C)

grain size of distribution of cementite(shape, size and number)

Non-equilibrium conditionvariety of microstructure

ferrite, pearlite, bainite, martensite etc.

Equilibrium condition (in equilibrium diagram)ferrite(), cementite(Fe3C)

Important factors affecting the microstructure

a) MaterialChemical compositions

b) Thermal historymainly Cooling rate

Equilibrium condition

Equilibrium and non-equilibrium conditions (Fig.B1)

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B-2

Time-Temperature-Transformation Diagram (TTT diagram) (Fig.B2)

Temperature

time

Coarse pearlite

Fine pearlite

Upper bainite

Lower bainite

Martensite

Austenite

mass % of Carbon

Tem

perature

Eutectoid steel

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B-3Time s

Time s

Temperature

Temperature

Hypo-eutectoidsteel

Hyper-eutectoid

steel

mass % of Carbon

Temperature

Hypo- and Hyper-Eutectoid steel

(Fig.B3)

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B-4

10 102 103 10410-1 1

Time s

800

600

400

200

0

Temperature

Ps

Pf

V1, v2

VIVIIv3

MsM: martensiteT: troostiteP: pearlite

coarse fineM M+T P

v4 VII VIv3

v2

v1

time

tem

perature

TTT CCT (Fig.B4)

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B-5

Microstructure of weld metal cooled with low cooling rate (Fig.B5)

A1 720 a2 630 a3 580 a4 480 a5 room temp.

Te

mperature

Time sec

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B-6

Microstructure of weld metal cooled with medium cooling rate (Fig.B6)

b1 660 b2 602 b3 577 b4 540 b5 320

Temperature

Time sec

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B-7

Microstructure of weld metal cooled with rapid cooling rate (Fig.B7)

c1 578 c2 533 c3 497 c4 410 c5 370

Temperature

Time sec

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B-8

Microstructure of weld metal cooled with various cooling rate (Fig.B8)

R-2F70%P30%

R-4F77%P23%

R-7Zw97%M 3%

R-8Zw96%

M 4%

R-9Zw94%M 6%

R-12Zw83%

M 17%

Temperature

Time sec

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B-9

Transformation of austenite (Fig.B9)

mass % of Carbon

Temperature

900

700

800

600

Temp

erature

mass % of Carbon0 0.2 0.4 0.6 0.8 1.0

Weld metal Heat affected zone

grainboundary

Fine(small) columnaraustenite grain

Coarse(large) polygonalaustenite grain

Start of precipitationof proeutectoid ferriteprobably at austenitegrain boundary

bulk carbon content of the steel

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B-10

Sites for nucleation of ferrite (Fig.B10)

750 660

565 cooling down to

room temperature

Precipitation of ferrite Precipitation of acicular ferrite

0.12C-0.31Si-0.88Mn-0.033Ti-0.036O

Optical microscope

Electron microscope

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B-11

Effect of oxygen content on microstructure of Si-Mn-Ti-B weld metal(Fig.B11)

60ppm 270ppm 440ppm

Oxygen content

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B-13

Classification of microstructure in Heat Affected Zone (Fig.B13)

Ferrite

Grain boundary ferrite GBF

Ferrite allotriomorph FA

Grain boundarypolygonal ferrite

GPF

Ferrite side plate FSP

Bainitic ferrite BF

Intragranular ferrite IF

Intragranularacicular ferrite IAF

Intragranularpolygonal ferrite

IPF

Pearlite

Pearlite P

Lameller pearlite LP

Degeneratedpearlite

DP

Fine colony pearlite FP

Bainite Upper bainite UB

Lower bainite LB

Martensite Lath martensite LM

M-A constituent MA

Ferrite FA GPF FSP IAFIPF

Pearlite LP DP FP

O.M. E.M. O.M. E.M. O.M. E.M.

Bainite UB LB

O.M. E.M. O.M. E.M.

Martensite LM MA

O.M. E.M.

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B-14

Example of ferrite and pearlite morphology (Fig.B14)

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B-15

Example of bainite and martensite morphology (Fig.B15)

Bainite

Martensite