1. Strong & Tough Steel Welds M. Murugananth, H. K. D. H. Bhadeshia E. Keehan, H. O. Andr₫n L. Karlsson

2. 10  m Transition Cleavage Flow stress Temperature Stress

3. General perception: nickel improves cleavage resistance of ferrite Fe-0.05C-2Mn-3Ni wt% Yield strength 850 MPa Charpy toughness 60 J at -60 o C Weld A: Fe -0.03C-2Mn-7Ni Weld B: Fe -0.03C-2Mn-9Ni

4. A B A = 790 MPa B = 840 MPa

5. Weld A: Fe -0.03C-2Mn-7Ni Weld B: Fe -0.03C-2Mn-9Ni Conclusion: Nickel has not improved toughness Different approach needed !

6. Neural network created 3300 experimental data 22 variables chemical composition, heat treatment, welding conditions test temperature Charpy UTS

8. Weld A: Fe -0.03C-2Mn-7Ni Weld B: Fe -0.03C-2Mn-9Ni Weld C: Fe -0.03C-0.6Mn-7Ni

10. How to find the reason ? -Study temperature dependence of strength. -Retained austenite measurements. -Hardness testing. -Measure transformation temperatures.

11. Temperature Dependence Study Trends remain same. Also not able to compare toughness at same strength Idea was to compare toughness at same strength But...

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

13. Vickers Hardness Measurement Observation Large scatter in hardness values along depth of the weld C but not in weld A. Why large scatter ? A offset offset distance 5 mm from weld centerline

14. Reason for scatter in hardness.. Dilatometric measurements revealed the difference in Ac 1, as expected. Though, difference in Ac 1 is less, it is enough to cause a change in temper effects of reheated zones in a multi-pass weld. Low Mn = High Ac 1 High Mn = Low Ac 1

15. Mechanism of Toughness Improvement Lower Ac 1 means reaustenitising and transforming back to fresh hard martensite. Higher Ac 1 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.

16. Assessment of Microstructure H m =V  ' H  ' + V o H o

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

18. Conclusion  Nickel does not necessarily improve toughness under all circumstances  A well trained neural network model can predict any complex relationship.  Transformation temperature, Ac 1, has a marked effect on toughness of multipass welds.

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

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

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

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