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

Summary The mechanism of transformation is displacive. Transformation temperature higher than martensite. Bainite grows without diffusion. But carbon then escapes into the residual austenite. Shape deformation plastically accommodated. Sub-unit mechanism of growth

DISPLACIVE RECONSTRUCTIVE

Watson and McDougall, 1973

Summary Mechanism displacive but carbon must partition during growth. Pairs of plates grow together to minimise strain.