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How should the tempering process be adjusted after workpieces have been quenched using a high-frequency quenching device? Return
Release time:
2023-12-22
After induction hardening, the workpiece must undergo further tempering treatment. The purpose of this step is to relieve the quenching stresses, prevent quenching cracks from forming, and ensure that the workpiece achieves the desired, appropriate hardness.
After induction hardening, the workpiece must undergo further tempering treatment. The purpose of this step is to relieve the quenching stresses, prevent quenching cracks from forming, and ensure that the workpiece achieves the desired, appropriate hardness. Generally speaking, after induction hardening using high-frequency quenching equipment, the workpiece typically undergoes one of three types of tempering processes: self-tempering, furnace tempering, or induction tempering.
1. Self-tempering occurs when the quenching process of a workpiece is interrupted prematurely, before cooling has been fully completed. As a result, residual heat from the core of the adjacent hardened layer is transferred to the hardened layer itself, causing the hardened layer to be reheated to a certain tempering temperature. This allows the hardened layer to achieve the desired microstructure and mechanical properties. Simply put, self-tempering involves the workpiece using the residual heat from its core to reheat and temper the surface of the hardened layer. The self-tempering process is particularly suitable for manufacturers engaged in mass production. It is applicable only to workpieces that retain sufficient residual heat in their cores after heating and that can evenly distribute this residual heat throughout all points of the hardened layer. Otherwise, after self-tempering, the hardness of different areas on the quenched surface will be inconsistent. Moreover, the hardness at the boundary between the quenched and unquenched surfaces will be higher than in the central region; in severe cases, some areas may not undergo self-tempering at all.
The advantage of the tempering-in-place process is that it not only eliminates the need for a separate tempering unit, saving energy and reducing labor costs, but also ensures timeliness. After workpieces are quenched using high-frequency quenching equipment, they generally need to be tempered promptly to prevent cracking caused by delayed tempering.
2. Furnace tempering—this tempering process is suitable only for small workpieces or for workpieces that have been quenched in oil or by immersion quenching. To prevent cracks from forming in the workpiece after quenching due to excessive internal stress, we typically subject it to a tempering treatment. The tempering temperature is determined based on the technical requirements of the workpiece and is carried out in a furnace.
3. Induction Tempering: This process utilizes the induction heating equipment originally used for quenching, but with reduced power, to perform tempering on the workpiece. The advantages of this process include energy savings, compact equipment size, and a shortened production cycle. It allows the quenching and tempering steps to be completed in a single loading and unloading operation, making it well-suited for assembly-line production methods.
Induction tempering offers greater versatility than furnace tempering because it features shorter heating times and higher productivity. It enables workpieces to achieve relatively stable and excellent mechanical properties. Therefore, after being quenched using high-frequency hardening equipment, workpieces are typically subjected to either self-tempering or induction tempering.
The reason why the hardness of workpieces after quenching fails to meet the specifications is that, in general, there are technical requirements for the hardness of quenched workpieces. However, not every workpiece can meet these standards. There are multiple factors contributing to the failure to meet the hardness requirements—for instance, temperature, time, and material all can lead to this issue.
1. The cooling time is too short, and the cooling is insufficient—that is, the cooling rate is lower than the critical cooling rate. After the quenching machine has quenched the workpiece, its internal microstructure will contain not only martensite but also troostite. The higher the content of troostite, the lower the hardness of the workpiece will be. Therefore, during quenching cooling, it is essential to strictly control factors such as the cooling medium, temperature, pressure, and duration.
2. If the quenching temperature is too low—meaning the workpiece doesn't reach the required austenitizing temperature during heating, either because the temperature is insufficient or the heating time is too short—the workpiece will fail to achieve the necessary austenitic structure. For example, in a workpiece made of medium-carbon steel, if there is ferrite still present in the austenite phase—that is, if the internal microstructure contains both martensite and ferrite—then during quenching, the surface of the workpiece will turn blue. At excessively high temperatures, the surface will appear white; a properly quenched workpiece should have a beige-colored surface. Therefore, when quenching a workpiece using a quenching machine, it’s crucial to strictly adhere to the standard requirements and select the appropriate quenching temperature.
3. When the surface of a quenched workpiece turns black, it indicates that the quenched surface has soft spots, which can also affect its hardness. In such cases, we can increase the rotational speed of the workpiece to ensure uniform spray distribution during quenching, thereby achieving even cooling.
4. After quenching, the workpiece surface exhibits decarburization. In this case, if hardness is measured, the surface hardness will be low while the core hardness will be high. Therefore, the decarburized layer on the surface must be removed first.
5. When quenching workpieces, if the heated surface passes through too quickly and the quenched areas don't have sufficient time to cool down, while the cooling medium also fails to promptly cool the quenched areas, the workpiece may experience excessive self-tempering temperatures.
6. When producing workpieces, material selection is one of the key factors. If the carbon content in the material composition is reduced, the hardness of the workpiece will decrease. Therefore, steels with high carbon content should be chosen for manufacturing.
Before machining, some workpieces undergo tempering treatment—also known as preliminary heat treatment. Therefore, it’s crucial to follow the steps precisely. If the order of operations is changed arbitrarily, it could affect the hardness of the workpiece.
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