Heat Treatment Of Steel

Heat Treatment Of Steel :

Heat treatment of steel (1): A steel heating process when the1 eutectoid steel P than in large heating temperature is equal to Ac1, the transformation of A.
The transformation can be divided into the following four stages: ① A ② A nucleus Fe3C dissolution of residual nuclei grow ③ ④ A homogenization
A form must have a certain amount of superheat △ T, provided the phase transition driving force △ G
① A nucleation, nucleation sites are usually two phases in the F and Fe3C interface.
② A nuclei grow up, after the nuclear shape to both F and Fe3C over the two phase interface, F re-constitute the face-centered cubic lattice, Fe3C been dissolved, to A provides C points. Fe3C dissolved faster than the reconstructed fast, so F first dissolved, the remaining C atom diffusion through Fe3C, so A uniform. Sub-eutectoid steel and eutectoid steel were heated to above the Ac3 or Accm to be completely transformed into A.
Heat treatment of steel (2): A grain size
Changes in grain size of the cooling process and the performance of the organization and have received a great impact. Therefore, the master A grain growth grain size of regularity and control A method for the heat treatment has a very important practical significance
1.A of grain growth after
A technology, with the temperature increase or extend the holding time, A will continue to grow grain
2.A grain size of the target
① grain size: the average grain diameter. According to GB6394-86, A grain size is generally 10 stages (standard photo control), the larger the number, the finer grains. 1-4, coarse grain; 5-10 grade fine grain; 10 more ultra-fine, there are also more than a thick 0, -1 level and so on.
② the initial grain size: A form just ended, the grain boundaries just touch each other when the grain size. A tendency to grow up with, but also on chemical composition.
3 A grown affecting factors:
Heat treatment of steel: ① internal: steel composition, the organization decided to grow it has a certain tendency of A.
Tendency to grow up: the same conditions, some grain is easy to grow, due to the different kinds of steel and different, even for the same kind of steel, due to different methods of smelting, in the same heating conditions can show different grain growth bias.

a. Steel C% ↑, easy to grow sub-eutectoid steel, eutectoid steel is not easy to grow, most likely grow up eutectoid steel.
b. alloying elements
In addition to Mn, P, in general, co-crystal element can prevent the A grain growth, such as V Ti Nb Al distribution in the grain boundaries can strongly prevent the formation of insoluble compounds, A grain growth.
c. high-quality structural steel, carbon tool steel, A grain is not easy to grow
Heat treatment of steel: ② external factors: heating conditions
The higher the heating temperature, the longer the holding time, the more likely atomic diffusion, grain boundary migration more easily, A the greater the actual grain.
Grain growth process is actually numerous grains while growing up process, is a large grain process of annexation of small grains.