Hard Bainite www msm cam ac ukphasetrans Acknowledgment

Hard Bainite www. msm. cam. ac. uk/phase-trans Acknowledgment Francisca Garcia Caballero

Minimum temperature at which bainite can be obtained? Mechanism of nucleation and growth of bainite Low temperature bainite Hard bainite Future

Thermodynamics Gibbs free energy DG m a g ga DG a xm x Carbon

GN Th

Each point represents a different steel Nucleation function G N 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.

Swallow and Bhadeshia, 1996

50 µm

Growth is diffusionless. Strain energy must be accounted for.

Bainite-start temperature

Fe-2 Si-3 Mn-C wt% Temperature / K 800 BS 600 400 MS 200 0 0 0. 2 0. 4 0. 6 0. 8 1 Carbon / wt% 1. 2 1. 4

Fe-2 Si-3 Mn-C wt% 1. E+08 1 year Time / s 1 month 1. E+04 1. E+00 0 0. 5 Carbon / wt% 1 1. 5

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

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

Vickers Hardness 700 600 500 400 T = 200°C 300 1. E+04 1. E+05 1. E+06 Time/ s 1. E+07

700 Temperature/ o. C 600 500 400 BS ~ 350 o. C 300 200 MS = 120 o. C 100 0 1. E+02 1. E+04 Time / s 1. E+06 1. E+08

Percentage of phase 100 80 retained austenite X-ray diffraction results 60 40 bainitic ferrite 20 0 200 250 300 Temperature/o. C 325

a g g a g 50 nm

g g a a a 200 Å

Story so far Low temperature transformation: 0. 25 T/Tm Fine microstructure: 20 -40 nm thick plates Carbide-free Designed using theory alone Typical mechanical properties:

Mateo, Caballero and Bhadeshia, 2004

Faster Transformation Cobalt (1. 5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite Refine austenite grain size

200 o. C 250 o. C 300 o. C

700 650 H V 600 550 500 450 400 30 min 60 min 24 h 300 350 400 450 500 550 600 650 o Temperature / C

excess carbon in solid solution in ferrite !

Peet, Babu, Miller, Bhadeshia, 2004

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