ABSTRACT
C, as-quenched
martensite
decomposes into
tempered martensite
and
ε
-
(or
η
-
)
carbide
(in low-to medium-carbon steels) or
χ
-
carbide
(in highcarbon steels). Above
∼
°
C,
cementite
precipitates from the tempered martensite, whereas the latter becomes
ferrite
and the
ε
- and
η
- (
χ
-) carbides dissolve. In steels alloyed with
carbide-formers
, the
alloying elements
inhibit the carbon diffusion and displace all the previously mentioned phase transitions to higher temperatures. In addition, at temperatures
∼
°
C, the diffusion of the
substitutional
alloying elements becomes possible, which leads to the occurrence of
special carbides
accompanied by cementite dissolution. The phase transformations described are accompanied by the following microstructural changes in martensite and ferrite. Crystallites of tempered martensite retain the shape of as-quenched martensite. Ferrite grains, occurring from tempered martensite, do not change their elongated shape and
substructure
until
coars-
ening
and
spheroidization
of cementite precipitates starts, although
dislocation density
decreases and
subgrains
form inside the ferrite grains. Further heating leads to the subgrain growth and, eventually, to
continuous recrystallization
, which makes the ferrite grains look
equiaxed
. As for carbides,
η
- and
ε
-carbides form inside the martensite laths, the former appearing as rows of
∼
2 nm particles and the latter as thin
Widmannstätten
laths. At higher temperatures, while
ε
- and
η
-carbides dissolve, thin Widmannstätten plates of cementite occur inside the ferrite grains; they nucleate at the
interfaces
between
ε
- or
η
-carbide and ferrite. Simultaneously, a certain amount of cementite occurs at the ferrite grain boundaries. Heating above 300-400
°
C is accompanied by spheroidization and coarsening of cementite particles, especially those at the grain boundaries. Special carbides in alloy steels nucleate at the interfaces between cementite and ferrite and at the grain boundaries and subboundaries in ferrite, as well as on the dislocations inside the ferrite grains. Due to the low
diffusivity
of substitutional alloying elements, the precipitates of special carbides are smaller than cementite particles.
