How to realize the high frequency induction surface quenching of the inner hole of martensitic stainless steel workpiece?

  There are two ways of high-frequency quenching heating: the first is simultaneous heating and quenching, that is, the surface of the workpiece that needs to be quenched is heated at the same time, followed by sharp cooling; The second is sequential quenching, that is, by induction heating a small part of the surface of the workpiece, while the workpiece is moved from top to bottom so that the surface successive heating and cooling.

  In the production of multi-variety and small-batch parts, different materials may need to use different quenching mediums, so the quenching method of simultaneous heating is mostly adopted. If the parts with large quenching surface areas are limited by equipment power and other factors, continuous heating is considered for quenching.

  The following to introduce: martensite stainless steel workpiece hole high-frequency surface quenching

(1) Processing difficulties

The high frequency surface quenching of the inner hole of the martensitic stainless steel workpiece adopts the way of simultaneous heating, the processing difficulty lies in the quenching of the stainless steel material and the inner hole surface.

In the process of high-frequency induction heating, when the temperature exceeds the material’s demagnetization point (the temperature of iron and steel’s demagnetization point is generally 700 ~ 800℃), the material’s electromagnetic induction ability decreases, and the heating speed drops several times, making further heating difficult. And stainless steel heat treatment temperature is high, are above 1000℃, heating to the quenching temperature of the material is more difficult. On the other hand, due to its high heat treatment temperature, close to the melting point of the material, although the heating speed above the loss of magnetic point is reduced, the heating speed is still faster than the conventional heat treatment, and difficult to control, there is a risk of surface overheating and melting parts.

The annular effect is one of the three main effects of induction heating and is also the reason for the difficulty of inner hole heating. When the workpiece is heated by the induction coil, the current passing through the induction coil is concentrated on the inner surface of the induction coil. When heating the outer surface of the workpiece, the inner surface of the induction coil should be relative to the outer surface of the workpiece, which is conducive to the heating of the workpiece, while when heating the surface of the inner hole of the workpiece, the direction is just opposite, which will significantly reduce the electrical efficiency of the inductor and is not conducive to the heating of the workpiece. Moreover, when the inner hole induction quenching is carried out, the heating surface is inside the workpiece, so it is not easy for the operator to observe directly from the outside, which increases the operation difficulty to a certain extent.

The spherical bearing of some product requires sf28mm spherical quenching, the material is martensite stainless steel 20Cr13, quenching hardness requires 35 ~ 45HRC.In addition to the above heating difficulties, the heating surface of the workpiece is spherical instead of straight through the inner hole, which will inevitably cause the gap between the sensor and the workpiece heating surface to increase, further reducing the electrical efficiency. In order to overcome the adverse effect of the annular effect on the workpiece heating, magnetic conductivity is set on the inductor to change the distribution of magnetic field and force the electric flow direction to be close to the surface distribution of the workpiece to be heated, so as to improve the heating effect. However, the inner hole of the workpiece is small, so the gap distance between the sensor and the workpiece and the size of the sensor itself is removed. The inner diameter of the sensor is below 13mm, so it cannot be equipped with a magnetic conducting body. The induction quenching of the workpiece can only be done by optimizing the process parameters and improving the heating process to maximize the equipment capacity.

(2) Quenching process scheme

The quenching process scheme includes heating time, quenching temperature, and quenching medium.

Many people think that high-frequency induction hardening belongs to instantaneous heating, which can reach the quenching temperature in just a few seconds. This understanding reflects the general situation, but it is not comprehensive. In some cases, the heating speed will be slower, and in some special cases, by reducing the voltage output and other means to slow down the heating speed of the parts, can meet the needs of some special workpiece or special technical requirements. For the workpiece, due to the existence of many adverse factors, rapid heating is not realistic, considering the need for visual temperature change and prevent overheating or even surface melting phenomenon, in order to ensure the quenching quality, must be based on a slower heating rate. If the heating speed is too slow, the advantages of surface quenching will be lost, and the hardening layer will be too large due to heat conduction. Practice shows that it is more suitable to control the heating time of the workpiece within 2.5 ~ 3min.

The quenching temperature of the workpiece should be determined according to the type of steel, the original structure, and the heating speed in the phase change zone. Under certain conditions of the type of steel and the original structure, the quenching temperature is mainly determined by the heating speed. The faster the heating speed is, the higher the quenching temperature is required. The high-frequency quenching heating speed is much higher than the conventional heat treatment, so the high-frequency quenching temperature is generally higher than the conventional heat treatment. There are many difficulties in heating spherical bearings due to various reasons, and the quenching temperature should not be too high. The higher the quenching temperature is, the more difficult it will be to achieve, which is also one of the reasons for choosing a slower heating speed. Although the slower heating rate is chosen, it is still fast heating. Considering the slower heating rate means that the austenitizing time is longer than the fast heating time. After the comprehensive analysis of many factors, the quenching temperature should be equal to or slightly higher than the conventional heat treatment.

Martensitic stainless steel good hardenability, workpiece size is not very large, air cooling can be completely quenched through. The effective thickness of the spherical bearing is less than 10mm, and the surface is quenched. In theory, air cooling quenching should be selected. At the same time, considering the special situation of choosing low quenching temperature, in order to ensure the quenching effect of the workpiece and meet the hardness requirements, the air cooling quenching inevitably has certain uncertain factors, so it becomes an inevitable choice to choose the quenching medium with faster cooling speed to make up for the possible defects of low quenching temperature. The cooling speed of oil is obviously better than that of air cooling, and it is a kind of slower among all kinds of quenching media. The quenching effect can be achieved by soaking oil immediately after the workpiece is heated to the quenching temperature. The slower cooling rate can meet the technical requirements stably and effectively without cracks and other defects.

(3) Actual effect

After quenching the spherical bearing in accordance with the above scheme, the spherical hardness is above 45HRC. After tempering at 480 ° C, the hardness is still above 40HRC, and the hardness distribution of each workpiece and various parts of the workpiece is uniform and stable, indicating that the workpiece fully meets the quenching requirements. The successful quenching of the workpiece provides a useful reference for the surface quenching of the stainless steel workpiece and the inner hole which is difficult to heat.

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