Precipitation microstructure and mechanical properties of maraging steels

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

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

Fe-19. 1 Ni-4. 4 Mo-2. 63 Ti, 480ºC/3 h

Fe-18. 9 Ni-4. 1 Mo-1. 9 Ti, 480ºC. Without ageing: σ0. 2 = 875 MPa, σb = 1070 MPa

Quantification of phase transformation kinetics Particle size

Microstructure Heat treatment: hardness and precipitation 60 Hardness, HRC 55 50 45 40 440°C 480°C 500°C 540°C 35 30 25 0. 0625 0. 125 0. 5 1 2 4 8 16 32 64 Ageing Time, h Precipitation hardening stainless PH 13 -8 (Fe-0. 97 Al-12. 43 Cr-2. 15 Mo-8. 39 Ni)

Microstructure Heat treatment: hardness and precipitation 60 Hardness, HRC 55 50 45 40 440°C 480°C 500°C 540°C 35 30 25 0. 0625 0. 125 0. 5 1 2 4 8 16 32 64 Ageing Time, h Precipitation hardening stainless PH 13 -8 (Fe-0. 97 Al-12. 43 Cr-2. 15 Mo-8. 39 Ni)

20 n m m Atom maps of Al and Ni 4 h at 510 ºC m m n 17 16 nm

20 n m m Atom maps of Al and Ni 4 h at 510 ºC 16 n m m 17 nm

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

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

New alloy • Composition: Fe-12. 94 Ni-1. 61 Al-1. 01 Mo-0. 23 Nb-0. 046 C • Why reducing nickel?

Alloy savings Composition Approximate cost per tonne Fe-18 Ni-3. 3 Mo-8. 5 Co-0. 2 Ti-0. 1 Al £ 3807 Fe-18. 5 Ni-3 Mo-0. 7 Ti-0. 1 Al £ 2212 Fe-12. 94 Ni-1. 61 Al-1. 01 Mo-0. 23 Nb £ 1400 Fe-12 Ni-6 Cu-0. 23 Nb-0. 03 C £ 1328 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.

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 laboratoryscale manufacturing, mechanical testing and microstructural characterization of steel.

The novelty and contribution of the research This research Previous research Improve toughness but with Improve strength but retain fair strength fair toughness Refine grain size ageing Refine grain size use treatment use Achieve fine (<5 mm) grain Achieve ultrafine (~1 mm) grain Reduce cost by using cheaper alloying elements Reduce cost by choosing the most economical processing routes

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

Low nickel maraging steel Fe-12. 94 Ni-1. 61 Al-1. 01 Mo-0. 23 Nb-0. 046 C Sha, Li, Wilson, Materials Science and Technology, 27, 2011, 983 (half-size) Sha, Ye, Malinov, Wilson, Materials Science and Engineering A, 536, 2012, 129

Optical microscopy 500 °C, 0. 5 h 550 °C, 72 h

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

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

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

Thermodynamic calculations Fe-12. 94 Ni-1. 61 Al-1. 01 Mo-0. 23 Nb-0. 046 C Phases 450 °C 500 °C 550 °C 600 °C Ni. Al B 2 0. 066 0. 063 0. 060 0. 054 BCC 0. 897 0. 919 0. 932 0. 938 Nb. C 0. 003 Fe. Ni 0. 029 0. 000 M 6 C 0. 006 0. 005

Mechanical properties • Impact test: Ductile before ageing (-196 C) Brittle after ageing • Tensile test Ageing time at 500 ºC (h) Tensile strength (MPa) Reduction of area (%) 2 1594 15 6 1577 15

Low nickel maraging steel Fe-12. 94 Ni-1. 61 Al-1. 01 Mo-0. 23 Nb-0. 046 C (-196ºC) Sha, Li, Wilson, Materials Science and Technology, 27, 2011, 983