Materials science

FERRIT AND PERLITE . As a result of full annealing of pre-eutectoid steel, i.e. heating up to Asz + (20 – 30)º C, and then slowly cooling in the furnace, the grains of the steel are crushed and made of uniformly distributed ferrite and pearlite.
a microstructure consisting of
a

fter softening until softening


CEMENTITE AND PERLITE . As a result of premature annealing of the post-eutectoid steel, i.e. after heating it to As1 + (20-30)ºС and then slowly cooling it in the furnace, a structure consisting of cementite and pearlite is formed in this steel. If cementite-pearlite and secondary cementite have a granular appearance, such a structure is called granular pearlite.

a) after normal softening-granular perlite; b) Mixed structure of granular and plate-like pearlite after improper softening.
If softening has not reached the end, then a mixed structure of granular and plate-like pearlite is formed.


MICROSTRUCTURE OF ROLLED STEELS
MARTENSITE. The main structure of tempered steel is martensite
i
s a structure. Martensite is a supersaturated solid solution of carbon in iron. Martensite has a characteristic needle-like structure.
- the picture shows X500 martensite needles in high-carbon tempered steel:
The size of the martensite needle depends on the size of the austenite grains. The smaller the austenite grains, the smaller the martensite needles. The size of the austenite grains depends on the heating temperature of the steel
as it rises, the grains become larger. Therefore, the higher the heating temperature, that is, the more the steel is heated, the larger the needle martensite structure is formed (Fig. 34) martensite (X500) is depicted:



a) large needle; b) small needles.
When eutectoid (medium carbon) steel is heated to a temperature of Asz + (20-30)ºС after normal tempering, and then quickly cooled in water, fine needle martensite structure is formed in this steel.
Martensite and secondary cementite . When post-eutectoid (high-carbon) steel is heated to a temperature of As1+ (20-30)ºС and then rapidly cooled in water, martensite and secondary cementite structure are formed in this steel.
Secondary cementite is intentionally left in the steel because secondary cementite is so hard that it increases the corrosion resistance of the steel. In order to form a martensite structure without secondary cementite in post-eutectoid carbon steel, it is necessary to heat it to a temperature higher than the Ast line. Such tempering of the steel does not improve its structure, on the contrary, it worsens it, because when heated to a temperature higher than the Ast line, the austenite grains become much larger, which after corresponding tempering, large needle martensite is formed in it. Therefore, steels after the premature eutectoid are normal steels.