Isothermal transformation kinetics

1
Isothermal transformation kinetics
Time to 50 completion
2
Eutectoid decomposition
Steel 0.81C-0.07Si-0.65Mn (wt), Austenitized
at 860C, Transformed at 705C
NOTE Each colony is a complex intertwined
set of one ferrite grain and one cementite grain.
3
Eutectoid decomposition
Steel 0.81C-0.07Si-0.65Mn (wt), Austenitized
at 860C
650C t10 sec
650C t70 sec
650C t20 sec
550C t0.7 sec
550C t0.5 sec
4
Eutectoid decomposition
Steel 0.81C-0.07Si-0.65Mn (wt), Austenitized
at 860C, Transformed at 500C
t0.5 sec
t2 sec
Around the knee of the TTT curve, we see both
pearlite and upper bainite.
5
Summary of eutectoid decomposition
6
Proeutectoid ferrite
Steel 0.55wtC, Austenitized at 860C,
Transformed at 705C
Here we see typical grain boundary nucleation of
ferrite, which thickens and grows as grain
boundary allotriomorphs, followed by
decomposition of austenite to pearlite, as
indicated by the TTT diagram to the right.
7
Proeutectoid ferrite
Steel 0.55wtC, Austenitized at 860C
Below the A1, the amount of proeutectoid
decreases with decreasing isothermal hold
temperature. Notice the dramatic increase in
transformation rate.
8
Proeutectoid ferrite
Steel 0.55wtC, Austenitized at 730C (partial)
Normal Grain Size
Large Grain Size
40 mm
10 mm
Water quenched
Water quenched
650C
650C
500C
550C
500C
For small austenite grains, ferrite nucleates
and grows on the undissolved ferrite, rather than
nucleating new GBAs, resulting in an irregular
structure. For large austenite grains, new GBAs
are nucleated.
For the large grain size, the ferrite forms an
almost continuous network.
9
Proeutectoid ferrite
650C
705C
0.15 wt C
0.25 wt C
0.4 wt C
The amount of ferrite formed decreases with
increasing carbon content. The ferrite
morphology also changes with carbon content.
0.6 wt C
10
Proeutectoid ferrite
11
Proeutectoid ferrite
0.25 wt C, Austenitized at 1150C
Transformed isothermally at 600C.
Transformed at 5C/min.
12
Undissolved ferrite and degenerate pearlite
Austenitized just above the A1
Austenitized just below the A3
Large amounts of undissolved ferrite affect the
subsequent decomposition.
Small amounts of undissolved ferrite have little
effect on subsequent decomposition.
13
Degenerate pearlite
14
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15
Pearlite and Bainite
0.25 wt C
675C
625C
500C
600C