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Different Quenching and Tempering of GCr15 Bearing Steel Balls

1 Heat treatment process of steel balls

The production process of 10 mm steel ball is: cold sensitivity → stress relief annealing → soft grinding → quenching and tempering → inspection of the hardness of the steel ball after quenching and tempering, structure crushing load, etc. → transfer to grinding after passing the thermal inspection → finished product.

The steel balls are divided into 3 groups (3 in each group), which are represented by groups Ⅰ, ll and Ⅲ respectively. Groups Ⅰ and Ⅱ are quenched in a mesh belt furnace with controlled atmosphere, the quenching process is the same, and the tempering temperature is different; group Ⅲ is the shell Steroid quenching. The specific heat treatment process is shown in Figure 1.

2 Hardness and structure of steel balls

It can be seen from the table that the hardness of group I steel balls is 2~3 HRC higher than that of group II and III steel balls.

In summary, the three groups of steel balls have different quenching and tempering microstructures, but the grain size is basically the same, excluding the influence of grain size on the properties of steel balls. 2.2 Crushing load of steel ball

The steel ball crushing test can not only find the defects of the steel ball heat treatment and raw materials, but also study the variation law of each index under different tempering temperatures. The crushing load test was carried out on the steel balls after heat treatment. The three groups of steel balls were tested at the same time, and the loading speed was the same, which was 1.3 kN/s. The test results are shown in the following table.

It can be seen from the table that the crushing load values of the lower bainite and high temperature tempered martensite are higher and basically equivalent; the crushing load value of the conventional tempering group is lower. The average crushing load value of group II and III steel balls is 1.42 and 1.48 times that of group I respectively, so group I steel balls have higher brittleness, while group II and III steel balls have lower hardness and better toughness.

It can be seen from Figure 4 that in the tempering resistance test, the tempering resistance of the steel balls in the group III is better than that of the steel balls in the group I. The tempering resistance of GCr15 steel ball bainite structure is obviously better than that of conventional martensite structure, that is, under the same tempering process, the hardness drop value of lower bainite structure is smaller than that of martensite structure.

3 Conclusions

1) The microstructures of GCr15 steel balls after tempering at 240℃ and isothermal quenching at 240℃ are different, but the hardness values and crushing load values are similar. .

2) The tempering resistance of the bainite structure obtained after isothermal quenching of GCr15 steel ball is obviously better than that of the conventional martensitic structure.

3) To obtain GCr15 steel balls with high strength and toughness, in addition to high temperature tempering, isothermal quenching can also be used to obtain complete lower bainite structure.