chapter
12 Pages

T

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.