**Annealing Heat Treatment (3)**
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Source: Bearing Network | Time: July 10, 2013
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The selection of the annealing method is based on a useful thickness of 2mm, with an estimated time of 1 hour. A correction factor is then applied, considering factors such as furnace power, furnace size, loading method, and the amount of material being processed.
For bearing blanks of models 6202–6205:
- Outer ring: 4 hours
- Inner ring: 4.5 hours
- Large set (small): 5 hours
For bearing blanks of models 6206–6406:
- Outer ring: 4.5 hours
- Inner ring: 5 hours
- Large set (small): 5 hours
To achieve the desired holding time, it is recommended to soak the parts at 720°C for 1 hour before raising the temperature to the holding level. This ensures a more uniform microstructure. If using a mud-sealed box for annealing, the operator may add an extra 0.5 hours to the process.
The cooling rate plays a critical role in the final microstructure. The rate of cooling directly affects the formation of spheroidal pearlite. Different cooling rates result in variations in the shape, size, and distribution of carbides. A faster cooling rate produces finer and more dispersed carbides, which generally increases hardness.
To obtain optimal spheroidal pearlite, the cooling rate should be controlled between 10 to 30°C per hour. For better quality assurance, a slower rate of 15 to 20°C per hour is often preferred. In practice, ambient temperature significantly influences the cooling process. Depending on the conditions, the cooling can be adjusted by baking at 500°C and opening the box below 300°C.
Aluminum silicate fiber felt is commonly used for insulation. A partition wall made from this material is placed between the furnace door and the box to reduce heat loss and minimize temperature differences. The box should be sealed with mud, and there must be a gap of at least 20mm between the box and the furnace. Additionally, the distance between the annealing box and the electric heating element should be more than 50mm.
During quality control, samples are taken from three boxes located in front of the furnace, inside the furnace, and at the bottom. A total of nine workpieces—three from each layer—are selected for inspection. Typically, GCr15 bearing steel forgings after annealing exhibit a hardness of HRB 88–94, a fiber structure rating of 2–4 grades, and a decarburization depth of 0.2–0.3 mm. These results meet the requirements specified in JB/T 1255–2001 for high-carbon chromium bearing steel rolling bearing parts.
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