Precipitation, microstructure and mechanical properties of maraging steels Wei SHA Professor of Materials Science

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

Fe-19.1Ni-4.4Mo-2.63Ti, 480ºC/3h

Fe-18.9Ni-4.1Mo-1.9Ti, 480ºC. Without ageing: σ 0.2 = 875 MPa, σ b = 1070MPa

Quantification of phase transformation kinetics Particle size

Microstructure Heat treatment: hardness and precipitation Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

Microstructure Heat treatment: hardness and precipitation Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

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

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

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

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

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

Alloy savings CompositionApproximate cost per tonne Fe-18Ni-3.3Mo-8.5Co-0.2Ti-0.1Al£3807 Fe-18.5Ni-3Mo-0.7Ti-0.1Al£2212 Fe-12.94Ni-1.61Al-1.01Mo-0.23Nb£1400 Fe-12Ni-6Cu-0.23Nb-0.03C£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 laboratory- scale manufacturing, mechanical testing and microstructural characterization of steel.

The novelty and contribution of the research This researchPrevious research Improve toughness but with fair strength Improve strength but retain fair toughness Refine grain size  ageing treatment  use Refine grain size  use Achieve fine (<5  m) grainAchieve ultrafine (~1  m) 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-12.94Ni-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

Optical microscopy 500 °C, 0.5 h550 °C, 72 h

Scanning electron microscopy 500 °C, 32 h450 °C, 66 h

Transmission electron microscopy 500 °C, 8 h550 °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-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C Phases450 °C500 °C550 °C600 °C NiAl B BCC NbC0.003 FeNi M6CM6C

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 (%)

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