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Welding Technology

for Welding Engineers

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Fe-C Phase diagram

Steel is an alloy

of Iron and carbon

Iron C < 0.02%

Steel 0.02C 0.21%

Cast iron : 0.21% < C

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Phase transformation in cooling - I

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Phase transformation in cooling - II

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Diffusion of carbon plays an important role in

phase transformation

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Heattreatment of steels

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Weldability of steels

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Welding heat input

Energy Input (AWS D1.1), Arc Energy(EN standard)

EI(J/mm) = 60 (EI/v)

E(V), I(A), v(mm/min)

6025170/150 1700 (J/mm), 1.7(kJ/mm)

Heat Input

HI(J/mm) = hEIh: Arc thermal efficiency 1.0 for SAW

0.8 for SMAW, GMAW

0.6 for autogenus TIG

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Welding cooling rate, cooling time

CR(oC/s) at 540oC

t8/5(s):

Cooling time between800oC and 500oC

1.7kJ/mm on 20mm thick

7s in t8/5

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Cooling rate, Cooling time

Heat input

Plate thickness

Joint shape (Butt-joint, fillet-joint)

Preheat temperature

Prediction of cooling time, t8/5

JWES IT-Center

(http://www-it.jwes.or.jp/index_e.jsp)

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Microstructure of HAZ

Normalizing heat treatment

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Cooling curve (log-scale)

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HAZ maximum hardness

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Prediction of HAZ hardness

HAZ hardness

Welding conditions

Heat input

Plate thickness

Preheat temperature

t8/5

Chemical composition of steel

C

Carbon Equivalent

JWES IT-Center

(http://www-it.jwes.or.jp/index_e.jsp)

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Carbon equivalent

CEIIW= C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5

CEWES= C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14

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Weld cracking

Hot cracking (>1200oC)

Solidification crackingLiquation cracking

Cold cracking (

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Hot cracking

Solidification crack Liquation crack

Stainless steel, Al

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Weld metal cracking

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Segregation of impurities during solidification

Phase diagram Residual liquid phase

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Direction of solidification growthH/W

Welding velocity

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Cold cracks

Root crack (HAZ) Toe crack (HAZ) Transverse crack(Weld metal)

Under-bead crack(HAZ)

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Generation and diffusion of hydrogen

Generation of hydrogen Hydrogen diffusion in weld

Mineral water in flux, Moisture in flux

Moisture in atmosphere,

Rust, oil, grease in groove

Arc

H (hydrogen)

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Effect of preheat on HAZ hydrogen

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Cause of hydrogen-assisted cold cracking

Cold cracking

Diffusible hydrogen

Weld metal hydrogen

Preheat temperature

Hardness (HAZ, Weld metal)

Steel chemical composition

t8/5 HI, thickness

Tensile residual stressYield strength of weld metal

Notch concentration factor

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Determination of necessary preheat temperature

AWS D1.1 Annex IHardness control method (CEIIW) C>0.11%

Hydrogen control method (Pcm) C

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Flux type of covered electrode

Basic type CaCO3 CaO + CO2

lime

Decrease of partial pressure of H

High basicity Low oxygen in weld metal

Low hydrogen