Precipitation, microstructure and mechanical properties of maraging steels Wei SHA

1. Precipitation, microstructure and mechanical properties of maraging steels Wei SHA Professor of Materials Science http://space.qub.ac.uk:8077/cber/Sha

2. What are maraging steels? Sha, Leitner, Guo, Xu, Phase transformations in steels. Vol. 2, Chapter 11, 2012

3. Fe-19.1Ni-4.4Mo-2.63Ti, 480ºC/3h

4. Fe-18.9Ni-4.1Mo-1.9Ti, 480ºC. Without ageing: σ0.2 = 875 MPa, σb = 1070MPa

5. Quantification of phase transformation kineticsParticle size

6. 60 55 50 45 Hardness, HRC 40 440°C 480°C 35 500°C 540°C 30 25 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 Ageing Time, h MicrostructureHeat treatment: hardness and precipitation Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

7. 60 55 50 45 Hardness, HRC 40 440°C 480°C 35 500°C 540°C 30 25 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 Ageing Time, h MicrostructureHeat treatment: hardness and precipitation Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

8. 20 nm 20 nm 17 nm 17 nm 16 nm 16 nm Atom maps of Al and Ni4 h at 510 ºC

9. 20 nm 20 nm 16 nm 16 nm 17 nm 17 nm Atom maps of Al and Ni4 h at 510 ºC

10. 9 nm 9 nm 13 nm 13 nm 10 nm 10 nm Iso-surface of Al (left) and Ni (right)6 min at 593 ºC

11. Applications of artificial neural network to modelling Sha, Malinov, ASM Handbook, 22A, 2009, 553

13. New alloy • Composition: Fe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C • Why reducing nickel?

14. Alloy savings About £2400 per tonne savings are possible on 12% nickel steels compared with standard 18% nickel steels. About £800 per tonne savings on 12% nickel steels compared with cobalt-free 18% nickel steels.

15. Aim & objectives • The aim of the project is to develop novel maraging steel with reduced nickel content, for high strength applications with good toughness at a reduced steel cost. • The objectives are to complete laboratory-scale manufacturing, mechanical testing and microstructural characterization of steel.

16. The novelty and contribution of the research

17. Experimental process • Hardness test • Microstructure: optical microscopy, scanning electron microscopy, transmission electron microscopy • X-ray diffraction • Tensile test • Impact test • Fractography at room temperature • Thermodynamic calculations

18. Low nickel maraging steelFe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C Sha, Li, Wilson, Materials Science and Technology, 27, 2011, 983 (half-size) Sha, Ye, Malinov, Wilson, Materials Science and Engineering A, 536, 2012, 129

19. Optical microscopy 500 °C, 0.5 h 550 °C, 72 h

20. Scanning electron microscopy 500 °C, 32 h 450 °C, 66 h

21. Transmission electron microscopy 500 °C, 8 h 550 °C, 256 h

22. X-ray diffraction No reflection peaks for fcc austenite were observed, after ageing at 550°C and 600°C.

23. Thermodynamic calculations Fe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C

24. Mechanical properties • Impact test: Ductile before ageing (-196C) Brittle after ageing • Tensile test

26. Low nickel maraging steelFe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C (-196ºC) Sha, Li, Wilson, Materials Science and Technology, 27, 2011, 983