Basic knowledge of bearing steel spheroidization organization

- Oct 15, 2020-

1. Spheroidizing annealing organization:

The original structure before annealing is hot-rolled, hot-forged or normalized, which is the P between the flakes of K and F. When it is heated to between AC1~AC3 and kept warm, the body-centered cubic F becomes face-centered cubic. A. Part of the flake K dissolves into A, and the remaining K gradually transforms from flake to granular or spherical. The higher the heating temperature, the longer the holding time, and the more K infiltrates into A. If the heating temperature is high enough and the holding time is long enough, K will all penetrate into A. In the subsequent cooling process, if the cooling rate is slow enough or it is cooled to 790°C ~ 810°C (GCr15) for isothermal, the dissolved K will be precipitated on the undissolved K or in a new position in granular form, and A will be transformed into F. , Granular K is distributed on the matrix, and granular P is the normal structure of spheroidizing annealing. The larger the cooling rate, the smaller the precipitated K. Slow cooling will produce coarse K, which is characterized by coarser K particles and poor uniformity (4~5 Level organization). However, if the cooling rate is too fast, the heating temperature is too high, and the holding time is too long, all or part of the dissolved K will be precipitated in the form of flakes, and become a mixed P() with all or part of the flake K distributed on the F matrix. Level 6 organization).


2. Evaluation principles of annealing organization:

The ideal annealed structure is fine spheroidized or granular K uniformly distributed on the F matrix.

1. The particle size of K;

2. The uniformity of K distribution;

3. The degree and shape of K's spheroidization.


3. Level and description of annealing organization:

Level 1:

Fine dot K+fine granular P+partial flake P

Explanation: The K particles are small in point and granular shape, with diffuse distribution, and local flakes. It is the unqualified structure, the reason is insufficient heating, and part of the forged structure is retained.

level 2:

Punctate P+Fine P

Description: K particles are small in point and granular, with good roundness and uniform distribution. It is an excellent qualified organization.

Level 3:

Spherical P

Note: K particles are larger than the second level, the spheroidization is intact, the distribution is relatively uniform, and it is a good qualified organization.

level 4:

Spherical P

Note: K particles are coarser, with poor uniformity, K is not uniformly distributed, some areas are dense, some areas are sparse, and are qualified organizations.

Level 5:

Uneven P

Explanation: K particles are uneven in size, poor roundness, angular and spherical K, K is unevenly distributed, some areas are dense, some areas are sparse, and are unqualified organizations. The formation of this structure is not only caused by the coarse and uneven original structure, but also the high heating temperature, slow cooling or repeated annealing.

Level 6:

Uneven coarse-grained P+flaky P

Note: The size of K particles is unevenly distributed, and there are obvious flake P in some areas, accounting for about 11.8% of the field of view, which is unqualified tissue. The heating temperature is too high, the original structure is coarse, the cooling is too slow, and the cooling is too fast after being out of the furnace.


4. Some brief descriptions of the assessment organization:

1. The fine granular or punctate P is beneficial to obtain evenly distributed M content after quenching, but too fine K is often accompanied by high hardness, which is not conducive to subsequent cutting and processing, and the quenching heating process is more stringent; K If the particles are too thick and the hardness is too low, it is also not conducive to cutting and easy to stick to the phenomenon. Moreover, due to the increase of K spacing, the carbon concentration distribution of the subsequent quenched structure is uneven, which affects the performance.

2. The uniformity of K distribution directly affects whether uniform M can be obtained after quenching. The excessively uneven K distribution, in addition to the uneven hardness after annealing and affecting the cutting performance, also makes the M content and size after quenching uneven. Even produce local overheated tissue.