Fundamentals and Applications of Bainitic Steels Thermodynamics Simulation of kinetics “Superbainite” Mechanical properties Future

upper bainite 1 µm

lower bainite

Surface 1 Surface 2 50 µm Srinivasan & Wayman, 1968

s d c r 1

50 µm

Carbon supersaturated plate Carbon diffusion into Carbon diffusion into austenite austenite and carbide precipitation in ferrite Carbide precipitation from austenite UPPER BAINITE LOWER BAINITE (High Temperature) (Low Temperature)

1 2 Fe-0.4C wt% Decarburisation time / s 500 400 300 Temperature / °C

Temperature Ae3' T' o x Carbon in austenite

Growth is diffusionless. Strain energy must be accounted for.

Takahashi and Bhadeshia Carbon supersaturated plate Carbon diffusion into Carbon diffusion into austenite austenite and carbide precipitation in ferrite Carbide precipitation from austenite UPPER BAINITE LOWER BAINITE (High Temperature) (Low Temperature) Takahashi and Bhadeshia

Oka and Okamoto

Ohmori and Honeycombe

T h G N

Each point represents a different steel Nucleation function G N Each point represents a different steel Bhadeshia, 1981

The nucleation of bainite must involve the partitioning of carbon Why does the required free energy vary linearly with T?

hexagonal close-packed cubic close-packed Christian, 1951

Brooks, Loretto and Smallman, 1979

Olson & Cohen, 1976

Nucleation of bainite must involve the partitioning of carbon. Mechanism of nucleation is otherwise identical to that of martensite.

Fe-2Si-3Mn-C wt% 800 B 600 Temperature / K 400 M 200 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1 1.2 1.4 Carbon / wt%

Fe-2Si-3Mn-C wt% 1.E+08 1 month Time / s 1.E+04 1.E+00 0.5 1 1.5 1 year 1 month Time / s 1.E+04 1.E+00 0.5 1 1.5 Carbon / wt%

wt% Low transformation temperature Bainitic hardenability Reasonable transformation time Elimination of cementite Austenite grain size control Avoidance of temper embrittlement wt%

Homogenisation Austenitisation Isothermal transformation Temperature 1200 o C 2 days 1000 o C 15 min Temperature Air 125 o C - 325 o C slow cooling hours - months cooling Quench Time

g g a a a Caballero, Mateo, Bhadeshia 200 Å

Low temperature transformation: 0.25 T/Tm Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV) Carbide-free Designed using theory alone

g g a a a Very strong Huge uniform ductility No deformation No rapid cooling No residual stresses a Cheap Uniform in very large sections a 200 Å

Stress / GPa Velocity km s-1 Hammond and Cross, 2004

“more serious battlefield threats”

ballistic mass efficiency consider unit area of armour

Caballero, Mateo, Bhadeshia

Caballero, Mateo, Bhadeshia

Sherif, 2005, Ph.D. thesis, Cambridge

Below percolation threshold Above percolation threshold

Geometrical percolation threshold of overlapping ellipsoids

0.4 C 2 Si 3 Mn wt% 1 µm

Very poor toughness!

50 µm

stress transfer length Fe-1C-1.5Si…… wt% periodic cracking stress transfer length Chatterjee & Bhadeshia, 2005

Carbide-free alloys wt %

Impact Energy Charpy impact / J Temperature / °C Test temperature / °C 10 20 30 40 50 60 70 0.4C-3Mn-2Si 0.4C-4Ni -2Si 0.2C-3Mn-2Si Charpy impact / J 200 100 -100 -200 Temperature / °C Test temperature / °C

kilocycles to Crack Initiation Pearlite Martensite Bainite 1200 1000 800 600 400 200 kilocycles to Crack Initiation Yates, Jerath

Yates, Jerath

U.K.