Tempering of bainite
The extent and the rate of change of the microstructure and
properties during tempering, must depend on how far the
initial sample deviates from equilibrium. The behaviour of
bainite during tempering is therefore expected to be
different from that of martensite.
Unlike martensite, bainitic ferrite usually contains only a
slight excess of carbon in solution. Most of the carbon
in a transformed sample of bainite is in the form of
cementite particles, which in turn tend to be coarser than
those associated with tempered martensite. The effects of
tempering heat treatments are therefore always milder than
is the case when martensite in the same steel is annealed.
Bainite forms at relatively high temperatures where some
recovery occurs during transformation. Consequently, when
low-carbon bainitic steels are annealed at temperatures as
high as 700 ° C (1 hr), there are only minor changes in
recovery, morphology or carbide particles. Rapid softening
occurs only when the plate-like structure of ferrite
changes into equiaxed ferrite. Associated with this change
is the spherodisation and coarsening of cementite. Further
tempering has minimal effects.
In marked contrast with martensitic steels, small variations
in the carbon concentration (0.06-0.14 wt.%) have little
effect on the tempering of bainite. Carbon has a very potent
solid solution strengthening effect. Thus, the strength of
martensite drops sharply as the carbon precipitates during
tempering. With bainite the carbon is mostly present as
coarse carbides which contribute little to strength. It is
not therefore surprising that the tempering response is
rather insensitive to the bulk carbon concentration.
Many bainitic microstructures contain appreciable quantities
of retained austenite. Tempering, usually at temperatures in
excess of 400 ° C, induces the decomposition of this
austenite into a mixture of ferrite and carbides.
Bainitic steels containing strong carbide forming elements
such as Cr, V, Mo and Nb, undergo secondary hardening during
annealing at high temperatures. Secondary hardening occurs
when fine (more stable) alloy carbides form at the expense
of cementite. Because the cementite in bainite is
coarse, the secondary hardening reaction tends to be sluggish
when compared with martensite.
There is considerable interest in the use of copper-bearing
bainitic steels for applications in heavy engineering.
Tempering induces the formation of fine particles of copper
which contribute to strength without jeopardising toughness.
To summarise, there are significant differences in the
tempering behaviour of bainite and martensite, the most
prominent being that there is little carbon in solid
solution in bainite. This has the consequence that bainitic
microstructures are much less sensitive to tempering, since
there is hardly any loss of strength due to the removal of
the small quantity of dissolved carbon. Major changes in
strength occur only when the bainite plate microstructure
coarsens or recrystallises into one consisting of equiaxed
grains of ferrite. Minor changes in strength are due to
cementite particle coarsening and a general recovery of the
dislocation substructure. Bainitic steels containing strong
carbide forming elements tend to exhibit secondary hardening
phenomena rather like those observed in martensitic steels
which depends on the precipitation of fine alloy carbides.