Bearing steel - spheroidizing annealing

Before the bearing steel is purchased, the inspection of components, low-magnification, non-metallic inclusions, reticulated carbides, banded carbides, and liquid carbides shall be in the inspection of incoming goods. After the inspection of all the indicators, the subsequent processing can be carried out.

After the bearing steel is thermally deformed by the normal process specification, the fine-grained pearlite is obtained, which has high hardness and is difficult to process. Therefore, spheroidizing annealing is required to reduce the hardness and improve the cutting performance. One of the most important goals in this section is to obtain uniform fine-grained spheroidized pearlite, which is an important part of the organization for the final heat treatment.

The spheroidizing annealing process is shown in the figure. The annealing temperature range of GCr15 is 770-810 ° C, and 790 ° C is considered to be a suitable temperature. If the heating temperature is too high, the amount of undissolved carbide in the austenite is too small, the austenite composition is homogenized, and after cooling, it is a coarse-grained pearlite or a bulky carbide, which is an unqualified superheated structure. If the heating is insufficient, the flaky pearlite is not sufficiently dissolved, and the austenite composition is very uneven. During the cooling process, the carbide is chromatographed along the original piece or has a small chain shape, which is a tissue that is insufficiently heated and has a high hardness. The effect of the holding time is to heat the blank and complete the necessary phase change. Properly extend the holding time to reduce the reticular carbide level. Keeping for 30-40 minutes, the mesh of <3.5 grade can be basically spheroidized. Therefore, starting from the internal organization transformation, it is enough to keep warm for 1 hour. Of course, in actual production, the holding time should be determined according to the furnace loading condition, and the final temperature holding to the temperature workpiece is sufficient.


Figure GCr15 spheroidizing annealing process

a) continuous cooling spheroidizing annealing; b) isothermal spheroidizing annealing

The shape of the carbide depends on the heating temperature, the degree of dispersion of the carbide, depending on the cooling rate.

The cooling rate is high, the carbide has a high degree of dispersion, and the hardness is also high. The cooling rate has a great influence on the pass rate of the spheroidizing annealing. The production is generally controlled in the range of 10-30 ° C, and when the furnace is cooled to 650 ° C, the air cooling can be performed.

Isothermal spheroidizing annealing can shorten the process time. After heating to 790 ° C for a certain period of time, accelerate to 720 ° C isothermal for 3-4 hours. The furnace condition is different, and the isothermal time can be kept for one hour at the end of the warm workpiece. Isothermal spheroidizing annealing allows the spheroidization to be carried out over a narrow temperature range, resulting in a uniform fine spheroidized structure. The isothermal temperature is lowered, the hardness is increased, and the isothermal temperature is increased. Although the hardness can be lowered, the isothermal time needs to be extended accordingly, and the production is rarely used.