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The difference between vacuum hardening heating temperature and vacuum hardening temperature

The so-called vacuum hardening heating temperature is to set the hardening temperature to 800°C and heat at this temperature; it can also be said that the heating temperature is 800°C. As for the latter so-called 800°C hardening, the temperature during hardening is 800°C, which means that the temperature of the part when it is put into the hardening liquid is 800°C. For this reason, it is estimated that the temperature will be within a short period of time when it is taken out of the heating furnace and then taken to the hardening tank. If it drops, the heating temperature should be increased according to the dropped part.

That is to say, the set hardening heating temperature should be 800℃ +α. For example, if the temperature drops by 100°C, the hardening heating temperature is 800°C + 100°C = 900°C, and the temperature of the hardening liquid is exactly 800°C, which can be interpreted as 800°C hardening. It can be seen that the vacuum hardening heating temperature is 800°C and the hardening temperature is 800°C, and the meanings of the two are quite different.

Therefore, in order to prevent misunderstanding, the term hardening heating temperature is not used, and it is best to distinguish between austenitizing temperature (Ta) and hardening temperature (Tq). The austenitizing temperature is the maximum heating temperature for hardening, and the hardening temperature is the temperature when the hardening liquid is put into it. The two must be distinguished.

The difference between heating time and holding time

In vacuum heat treatment operations, the heating temperature is strictly regulated, but the holding time is very sloppily regulated. In other words, the holding time of vacuum hardening, which was said long ago as “30 minutes for one inch (2.54 mm) square”, has been replaced by accepted. Therefore, it is common sense that large items should be kept warm for a long time and small items should be kept warm for a short time. Is this really a good idea?

Large parts have a long heating time, while small parts have a short heating time. The holding time after the vacuum heat treatment temperature (such as hardening temperature) is reached is no different between large parts and small parts and should be certain. Naturally this is the time it takes for the inside and outside of the treated piece to reach this temperature. To check whether the correct hardening temperature has been reached, it is usually based on the instructions of the thermometer (temperature control meter), but it must be noted that this is the temperature of the thermocouple end, not the temperature of the processed piece. The larger the workpiece is and the more parts are loaded, the greater the difference between the temperature indicated by the thermometer and the actual temperature of the workpiece, which results in a heating lag time.

Hardening is the operation of rapidly cooling and hardening austenitized objects. When steel transforms into austenite, alloy structural steel (pearlite system) only takes an instant (the time required to transform into austenite is zero). As long as it transforms into austenite, the first stage of hardening is completed. Why do we need a What about keeping an inch square warm for 30 minutes? It is estimated that the core of the treated piece has been transformed into austenite and then hardened, which is appropriate. That is, the best holding time is zero.

On the contrary, after the matrix of tool steel (carbide system) transforms into austenite, about 50 to 70% of the proeutectoid carbides in the austenite are not solid solution, so it will not harden during hardening, so it must be Some holding time. However, even so, “30 minutes for one inch square” is too long. At best, about 10 minutes is enough. Of course, the holding time should vary depending on the type, shape, quantity, distribution state, etc. of carbides.

Differences in cooling methods and cooling effects

In vacuum heat treatment, cooling method is very important. The speed with which it cools can make the steel softer or harder. The cooling medium is called coolant, including air, oil, water and other various media.

Generally, the cooling speed of air is slow, followed by oil, and the cooling speed of water is fast. However, the cooling effect caused by the coolant is not absolute and can vary depending on the size of the workpiece. The cooling rate of the coolant is inherent, but the cooling effect of the coolant on the processed part varies depending on the size of the part. In other words, the cooling method and cooling effect of vacuum heat treatment are two different things. The cooling method is cooling from the perspective of the coolant, while the cooling effect is cooling from the processing piece. Even if the cooling method is the same, the cooling effect is different. In vacuum heat treatment, the important thing is not how to cool, but how to obtain a good cold part effect. The two cannot be confused.

The definition of normalizing is cooling in the atmosphere (air cooling). However, the cooling speeds of small parts and large parts are different, that is, the cooling effects are different. Therefore, on the surface, cooling is like normalizing in form, but the essence is very different. If small parts are left to cool, it may become air hardening due to improper technology; conversely, if large parts are left to cool, it may become annealing. Therefore, when normalizing small parts, they must be cooled in a pit or covered; conversely, if large parts are not cooled by blowing air such as a fan, the normalizing effect will not be achieved, and only formal cooling will not achieve vacuum heat treatment. sudden effect.

Vacuum hardening is the same. Although they are both oil hardeners, the cooling effects of large and small parts are different. Therefore, the degree of vacuum hardening hardening also changes. Even if small parts are hardened with oil, they can be hardened through like water hardening, but for large parts, oil hardening can only achieve the same cooling effect as normalizing, and cannot be hardened through.

Thinking about it this way, you can understand: although water cools faster, oil cools slower, and air cools slower, this speed is not absolute. It is related to the size of the processed parts. Therefore, it must be noted that the cooling methods and cooling effects are different.