1. A process of steel heating
1. Eutectoid steel P is converted to A when the heating temperature is more than equal to Ac1.
The transformation can be divided into the following four stages: ① A nucleation ② A crystal nucleus growth ③ Residual Fe3C dissolution ④ A homogenization
The formation of A must have a certain degree of superheat △T to provide phase change driving force △G
①A nucleates, and the nucleation position is usually on the interface of F and Fe3C.
② A crystal nucleus grows up, and moves to the two phase interfaces of F and Fe3C at the same time after nucleation. The F crystal lattice reconstitutes face-centered cubic, and Fe3C continues to dissolve, providing A with C points. The reconstitution rate is faster than the dissolution rate of Fe3C, so F dissolves first, and the remaining Fe3C diffuses through C atoms to homogenize A. Hypoeutectoid steel and hypereutectoid steel must be heated to Ac3 or more than Accm to be completely transformed into A.
2. A grain size
The grain size has a great influence on the cooling transformation process and the obtained structure and properties. Therefore, mastering the regularity of A grain growth and the method of controlling A grain size is of great significance for heat treatment practice
1. Grain grows up after A
After A is formed, as the temperature increases or the holding time is extended, the A crystal grains will continue to grow
2. A grain size index
① Grain size: the average value of grain diameter. According to GB6394-86, A grain size is generally divided into 10 levels (standard photo comparison), the larger the number, the finer the grains. Grade 1-4, coarse grains; grade 5-10 fine grains; super fine grains above grade 10, and grades 0 and -1 which are coarser than grade 1.
②Initial grain size: the grain size when the grain boundary of A has just finished forming and when its grain boundaries are in contact with each other. It is related to A’s tendency to grow up, and also related to chemical composition.
3. Factors affecting A’s growth:
① Internal factors: The composition and organization of steel determine that it has a certain tendency to grow up.
Growth tendency: Under the same conditions, some grains are easy to grow up, and they are different due to different steel grades. Even for the same steel, due to different smelting methods, different grains can grow under the same heating conditions. Tendencies.
a. Steel C%↑, hypoeutectoid steel is easy to grow up, hypereutectoid steel is not easy to grow up, eutectoid steel is the easiest to grow up.
b. Alloying elements
In addition to Mn and P, general crystalline elements can prevent the growth of A crystal grains. For example, V Ti Nb Al, etc. distributed in the grain boundary to form insoluble compounds can strongly inhibit