Mechanism of the Bainite Transformation in Steels IIT Kharagpur

Bundy (1965)

body-centred cubic cubic close-packed

DISPLACIVE RECONSTRUCTIVE

1 µm upper bainite

lower bainite

Surface 1 Surface 2 Srinivasan & Wayman, µm

c r  s 1

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

Temperature / °C Decarburisation time / s Fe-0.4C wt%

Ae3' T' o x Carbon in austenite Temperature

Growth is diffusionless. Strain energy must be accounted for.

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

Oka and Okamoto

Ohmori and Honeycombe

1 µm 0.4 C 2 Si 3 Mn wt%

Very poor toughness!

50 µm

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

Carbide-free alloys wt %

0.4C-3Mn-2Si 0.4C-4Ni -2Si 0.2C-3Mn-2Si Impact Energy Test temperature / °C Charpy impact / J Temperature / °C

Pearlite Martensite Bainite kilocycles to Crack Initiation Yates, Jerath

austenite

Chang and Bhadeshia 15 mm thick plate, without Mo 550°C for 5 h 0.2C 1.25Si 1.55Mn 0.5Cr 0.15V 0.15Mo wt%

U.K.

Ultimate tensile strength / MPa Fracture toughness / MPa √m 18 wt% Ni maraging steel QT Caballero & Bhadeshia, 2001

hexagonal close-packedcubic close-packed Christian, 1951

Brooks, Loretto and Smallman, 1979

Olson & Cohen, 1976

Carbon / wt% Temperature / K Fe-2Si-3Mn-C wt% B S M S

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

20 nm     

excess carbon in solid solution in ferrite !

Peet, Babu, Miller, Bhadeshia, 2004