Strong & Tough Steel Welds

M. Murugananth, H. K. D. H. Bhadeshia

E. Keehan, H. O. Andr₫n

L. Karlsson

10 m

Transition

Cleavage

Flow stress

Temperature

Str

ess

General perception: nickel improves cleavage resistance of ferrite

Fe-0.05C-2Mn-3Ni wt%

Yield strength 850 MPa

Charpy toughness 60 J at -60 oC

Weld A: Fe -0.03C-2Mn-7NiWeld B: Fe -0.03C-2Mn-9Ni

A

B

A = 790 MPaB = 840 MPa

Weld A: Fe -0.03C-2Mn-7NiWeld B: Fe -0.03C-2Mn-9Ni

Conclusion:

Nickel has not improved toughness

Different approach needed !

Neural network created

3300 experimental data22 variableschemical composition, heat treatment, welding conditions test temperature

Charpy

UTS

Weld A: Fe -0.03C-2Mn-7NiWeld B: Fe -0.03C-2Mn-9Ni

Weld C: Fe -0.03C-0.6Mn-7Ni

MPa - 60 oC / J

A 789 15B 841 10C 725 101

How to find the reason ?

-Study temperature dependence of strength.-Retained austenite measurements.-Hardness testing.-Measure transformation temperatures.

Temperature Dependence Study

Trends remain same. Also not able to compare toughness at same strength

Idea was to compare toughness at same strength

But...

Retained Austenite

Volume % Error / %Weld A 1.5 0.1Weld B 0.8 0.1Weld C 2.2 0.1

Not a significant change in volume % to explain large difference in toughness.

Vickers Hardness Measurement

Observation

Large scatter in hardness values along depth of the weld C but not in weld A.

Why large scatter ?

Aoffset

offset distance 5 mm

from weld centerline

Reason for scatter in hardness..

Dilatometric measurements revealed the difference in Ac

1, as expected.

Though, difference in Ac1is less, it is enough to cause a

change in temper effects of reheated zones in a multi-pass weld.

Low Mn = High Ac1

High Mn = Low Ac1

Mechanism of Toughness Improvement

Lower Ac1 means reaustenitising and transforming back to fresh

hard martensite.

Higher Ac1 means tempering of already formed martensite.

Tempering of substrate layers gives a combination of soft and hard final microstructure than a uniformly hard final microstructure.

Assessment of MicrostructureHardness / VHN

Water quenched (H ') As- deposited (H m) Annealed (Ho)

Weld A 356 326 276Weld C 315 279 276

V' V(Vol. fra. of Bainite)

Weld A 0.63 0.37Weld C 0.08 92

Hm=V'

H' + V

o H

o

Future Work and Anticipation

Tempering kinetics of welds A and C may differ. Would be investigated.Charpy sample spans many layers of the weld. Hence, homogenisation weld C should change the toughness.

Conclusion

Nickel does not necessarily improve toughness under all circumstances

A well trained neural network model can predict any complex relationship.

Transformation temperature, Ac1, has a

marked effect on toughness of multipass welds.

http://www.msm.cam.ac.uk/phase-trans/ Murugananth M

http://www.msm.cam.ac.uk/phase-trans/ Murugananth M

http://www.msm.cam.ac.uk/phase-trans/ Murugananth M

http://www.msm.cam.ac.uk/phase-trans/ Murugananth M