Mathematical Models and Novelty in Steels www msm
- Slides: 61
Mathematical Models and Novelty in Steels www. msm. cam. ac. uk/phase-trans Tata Steel Jamshedpur
tera giga mega kilo hecto deca T G M k h da deci centi milli micro nano pico d c m m n p 1 000 000 1 000 10 1 0. 01 0. 000 001 0. 000 000 001
Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap ….
Brenner, 1956
Scifer, 5. 5 GPa and ductile Kobe Steel
1 Denier: weight in grams, of 9 km of fibre 50 -10 Denier Scifer is 9 Denier
Morinobu Endo, 2004
Claimed strength of carbon nanotube is 130 GPa Edwards, Acta Astronautica, 2000 Claimed modulus is 1. 2 TPa Terrones et al. , Phil. Trans. Roy. Soc. , 2004
Equilibrium number of defects (1020) Strength of a nanotube rope 2 mm long is less than 2000 MPa
Summary • Strength produced by deformation limits shape: wires, sheets. . . • Strength in small particles relies on perfection. Doomed as size increases.
Smallest size possible in polycrystalline substance?
Yokota & Bhadeshia, 2004
Summary Thermomechanical processing limited by recalescence Need to store the heat Reduce rate Transform at low temperature
Swallow and Bhadeshia, 1996
cementite forced to inherit the substitutional solutes in parent Lord, Bhadeshia, Svensson, 2003
Kozeschnik & Bhadeshia, 2005
Lengthening rate / m s-1 Temperature / °C Bhadeshia, 1985
Solution models Quasichemical approximation, atoms are not distributed at random. Pairs of atoms are treated as independent entities
Distant and near-neighbours
Reference state Chen, Hansip & Bhadeshia, 2004
-0. 17 e. V 2. 17 e. V Chen, Hansip & Bhadeshia, 2004
Bhadeshia, 1981
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
Fe-1. 75 C-Si-Mn wt% Chatterjee & Bhadeshia, 2004
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
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
g g a a a Mateo, 2002 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
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
Stress / GPa Velocity km s-1 Hammond and Cross, 2004
“more serious battlefield threats”
ballistic mass efficiency consider unit area of armour
Charpy fatigue tensile critical stress intensity corrosion
non-linear functions
Brun, Robson, Narayan, Mac. Kay & Bhadeshia, 1998
Kimura et al. , 2001
Components of Creep Strength, 2. 25 Cr 1 Mo 550 °C iron + microstructure solid solution 600 °C precipitates Murugananth & Bhadeshia, 2001
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