1. Isothermal quenching

The structure of high carbon chromium bearing steel after isothermal quenching is: lower bainite + (a small amount of martensite) + undissolved carbide + residual austenite (usually identified as < 3%). Strictly speaking, there is no 100% bainite transformation.

2. Compare strengths

Compared with general (martensitic) quenching, the isothermal quenching of high-carbon chromium bearing steel has the following advantages.

(1) High dimensional stability of products.

(2) Excellent mechanical properties (bending strength, impact toughness), low thermal stress, small deformation.

(3) The surface of the parts after quenching is compressive stress, which improves the service life and reliability of the bearing, and can be used in the working conditions of the railway, rolling mill, mine, coal mining and drilling machine and other conditions of high impact load.

(4) Comparison of martensite and bainite. The product has high martensitic structure strength, but low toughness and large distortion.

3. Blank materials

When isothermal quenching is used, the blank needs to be refined by carbide, usually normalizing and rapid annealing are selected, and the annealing structure is required to be 1~2 grade, and the annealing hardness of GCr18Mo/ GCr15/SKF is 179 ~ 217HBW.

4. Isothermal conditions

Equipment selection of salt bath roller furnace, model 850mm×850mm×300mm. The salt tank capacity is 60t, the salt tank ratio is 50%KNO3+50%NaNO2, the salt tank temperature is (230±5)℃, and the isothermal time is 4 ~ 5h.

5. Radial dimension rule

The relationship between wall thickness and expansion rate is shown in Table 1, and Table 2 is a typical example. The above products have been tested by authoritative institutions, and the product organization and hardness are in line with GB/ T1255-2014 standards.

6. Expand the analysis

It can be seen from Table 1 and Table 2 that the bearing ring of GCr15 steel mill expands along the diameter direction after BL isothermal quenching, and the average expansion rate within 20mm wall thickness is about 3‰. With the increase of product wall thickness, the expansion rate gradually decreases. As long as the production control is well controlled, the expansion is very regular and stable. The reason for the expansion is that the bainite transformation occurs in the isothermal process, the surface of the part and the heart change almost simultaneously, in the transformation process due to the existence of a large number of austenite, the volume expansion of austenite in the transformation to B can be carried out more smoothly, making the overall size of the bearing part B after quenching.

7. Conclusion

At present, many domestic bearing manufacturers have not fully mastered the law of isothermal bainite expansion of bearing rings, leading to inaccurate grasp of the lathe to the grinder processing allowance, increasing the amount of grinding and grinding times, but also increasing the production cost, and even causing product scrap. Therefore, it is of great help to accurately grasp the law of product expansion to reduce production costs and improve production efficiency.

Of course, mastering the law of expansion to reduce the grinding allowance should also be combined with the deformation control of the product after heat treatment, reduce the grinding allowance must be strictly controlled after the oval heat treatment, to combine the two sides in order to effectively control the grinding allowance, reduce the grinding processing cost, improve production efficiency.