Precipitation, microstructure and mechanical properties of maraging steels

Wei SHAProfessor of Materials Sciencehttp://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-19.1Ni-4.4Mo-2.63Ti, 480ºC/3h

0 4 8 12 16 20 5060

65

70

75

80

85

(c)

KIC ,

MP

a.m

1/2

Aging Time, h

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

Quantification of phase transformation kineticsParticle size

MicrostructureHeat treatment: hardness and precipitation

0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64

25

30

35

40

45

50

55

60

440°C 480°C 500°C 540°C

Ageing Time, h

Ha

rdn

ess

, H

RC

Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

MicrostructureHeat treatment: hardness and precipitation

0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64

25

30

35

40

45

50

55

60

440°C 480°C 500°C 540°C

Ageing Time, h

Ha

rdn

ess

, H

RC

Precipitation hardening stainless PH13-8 (Fe-0.97Al-12.43Cr-2.15Mo-8.39Ni)

Atom maps of Al and Ni4 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 Ni4 h at 510 ºC

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

9 n

m

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

Composition Approximate 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 research Previous 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) grain Achieve 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 steelFe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C

0

100

200

300

400

500

600

0.1 1 10 100 1000

Ageing time (h)

Har

dnes

s (H

V2)

600 °C

550 °C

500 °C450 °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 calculationsFe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C

Phases 450 °C 500 °C 550 °C 600 °C

NiAl B2 0.066 0.063 0.060 0.054

BCC 0.897 0.919 0.932 0.938

NbC 0.003 0.003 0.003 0.003

FeNi 0.029 0.009 0.000 0.000

M6C 0.006 0.006 0.005 0.005

Mechanical properties

• Impact test:

Ductile before ageing (-196C)

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 steelFe-12.94Ni-1.61Al-1.01Mo-0.23Nb-0.046C (-196ºC)

Sha, Li, Wilson, Materials Science and Technology, 27, 2011, 983