**Annealing Heat Treatment (3)**
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Source: Bearing Network | Time: July 10, 2013
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In the process of annealing, the selection of heating methods is based on a useful thickness of 2mm for 1 hour. A correction factor is then added, considering factors such as furnace power, furnace size, charging method, and load capacity.
For bearing blanks of models 6202–6205:
- Outer ring: 4 hours
- Inner ring: 4.5 hours
- Large set: 5 hours
For models 6206–6406:
- Outer ring: 4.5 hours
- Inner ring: 5 hours
- Large set: 5 hours
To achieve the required holding time, it is common to soak at 720°C for 1 hour before raising the temperature to the desired holding level. This helps ensure uniform microstructure transformation within a controlled range. If the annealing box is sealed with mud, operators may add an extra 0.5 hours to the process.
The cooling rate plays a critical role in determining the final microstructure. It directly affects the formation of pearlite during spheroidizing annealing. Different cooling rates lead to variations in the shape, size, and distribution of carbides. A faster cooling rate results in finer and more dispersed carbides, which generally increases hardness. On the other hand, slower cooling can lead to coarser structures.
For optimal spheroidized pearlite formation, the cooling rate should typically be controlled between 10–30°C/h. To ensure consistent quality, a cooling rate of 15–20°C/h is often preferred. The ambient temperature significantly influences this process. Depending on the situation, it may be necessary to perform a pre-bake at 500°C before cooling, and the box should be opened below 300°C.
Aluminum silicate fiber felt is commonly used for insulation. In a box-type electric furnace, a partition wall made of this material is placed between the furnace door and the box to reduce heat loss and minimize temperature differences. The annealing box should be sealed with mud, ensuring a gap of more than 20mm. Additionally, there should be a minimum clearance of 50mm between the box and the electric heating elements.
During sampling, three boxes are inspected—one in front of the furnace, one inside the furnace, and one in the middle. A total of nine workpieces, taken from the top, middle, and bottom layers, are examined for routine evaluation.
Typically, GCr15 bearing steel forgings after annealing have a hardness of HRB 88–94, a fiber structure rating of 2–4 grades, and a decarburization layer depth of 0.2–0.3 mm. These specifications meet the requirements outlined in JB/T 1255–2001 for high-carbon chromium bearing steel rolling bearing parts.
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This article is originally published on China Bearing Network.
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