406
S. Papaefthymiou
Fig. 12.18 Upper (a) and lower (b) bainite schematically. Dimensions of ferritic plates depend on
chemical composition and transformation temperature
martensite (250–550 °C). Bainite may form two morphologies: upper and lower
[25]. Figure 12.18 shows the discrepancies between them.
Upper Bainite (550–400
° C)
Upper bainite consists of ferritic platelets with width ~0.2 μm and length 10 μm
separated with low angle misorientation boundaries and cementite. Ferritic platelets
having the same crystal orientation form bainitic packets (Figs. 12.19a, 12.20). Ferrite
laths in upper bainite nucleate at prior austenitic grain boundaries. The development
of each lath is accompanied with plastic deformation of austenite matrix. The regional
increase of dislocation density restricts the interface motion of ferrite platelet. As a
result, the dimensions of ferrite platelets are restricted [25].
Lower bainite (400–250
° C)
(a)
(b)
Fig. 12.19 Microstructure of a upper bainite and b lower bainite in TEM
S. Papaefthymiou
Fig. 12.18 Upper (a) and lower (b) bainite schematically. Dimensions of ferritic plates depend on
chemical composition and transformation temperature
martensite (250–550 °C). Bainite may form two morphologies: upper and lower
[25]. Figure 12.18 shows the discrepancies between them.
Upper Bainite (550–400
° C)
Upper bainite consists of ferritic platelets with width ~0.2 μm and length 10 μm
separated with low angle misorientation boundaries and cementite. Ferritic platelets
having the same crystal orientation form bainitic packets (Figs. 12.19a, 12.20). Ferrite
laths in upper bainite nucleate at prior austenitic grain boundaries. The development
of each lath is accompanied with plastic deformation of austenite matrix. The regional
increase of dislocation density restricts the interface motion of ferrite platelet. As a
result, the dimensions of ferrite platelets are restricted [25].
Lower bainite (400–250
° C)
(a)
(b)
Fig. 12.19 Microstructure of a upper bainite and b lower bainite in TEM