Post on 02-Jan-2016
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