High frequency quenching defects and prevention

1. Quenching crack

Quenching crack is the most common defect in high frequency quenching. There are many causes, such as overheating, too fast cooling speed and high frequency quenching before the microstructure is not appropriate. In addition, the carbon content of the steel also has a great impact, such as the carbon content in about 0.30% rarely produced quenching crack, but when the carbon content in about 0.50% is very easy to produce quenching crack. In addition, the grain size and carbide morphology of steel should be paid attention to.

In order to prevent quenching cracking, pay attention to the shape of the workpiece, especially to pay attention to the edges of the workpiece, round hole edge, wall thickness and chamfering, etc. In addition, if there is slag in the steel, of course, the emergence of quenching cracks will also have a great impact.

2. Insufficient thickness of soft spot and hardened layer

It is generally believed that the insufficient thickness of soft spot and hardened layer is caused by quenching temperature, heating time and cooling method. In addition, the current frequency high and low, sensor shape should also be paid attention to. Hardening hardness and thickness of hardened layer are also affected by the presence or absence of reticulated carbides and spheroidization size in steel before high frequency quenching. In order to prevent the occurrence of such defects, the steel used should be normalized and tempered as required. In addition, it is also important to select the appropriate current frequency (if adjustable) for the required thickness of the hardened layer.

3. Burns

Overheating caused by the shape of the workpiece, the shape of the sensor, and the frequency of the current can cause burns. In order to prevent the occurrence of burns, attention should be paid to the keyway, the edge of the round hole and the gap between the sensor and the workpiece.

4. Grinding cracks

For high frequency quenching or ordinary quenching workpiece, when it is under quenching and low temperature tempering state during grinding, due to certain grinding heat generated in the local area first and second phase of the tempering contraction, the results make it around the metal by certain tensile stress and forming soft point, because the present heat tint in soft point position, so also called the phenomenon of grinding burn. In addition, in the grinding process due to the residual austenite to martensite transformation or due to excessive grinding heat caused by local secondary quenching of the workpiece surface, sometimes also lead to grinding cracks.

There are two types of grinding cracks: one is that the grinding heat makes the workpiece temperature rise to about 180℃ (corresponding to the first stage of tempering), and the cracks are vertical and parallel to the grinding feed direction, which is called the first type of grinding crack; The other kind is the grinding heat that makes the workpiece temperature rise to about 250 ~ 300℃ (corresponding to the second stage of tempering), the crack is a network, this crack is called the second type of grinding crack.