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Heating speed and heating time of steel forging

Update:27-11-2020
Summary:

The heating speed indicates how fast the temperature ri […]

The heating speed indicates how fast the temperature rises when the metal is heated. It is expressed by the temperature rise per unit time (℃/h), and it can also be expressed by the thickness of the metal layer that is heated per unit time (mm/min). The main factors affecting the heating rate are furnace temperature, steel chemical composition, furnace loading method, steel size, etc.

1) The higher the furnace temperature, the faster the heating rate of steel.

2) The chemical composition of steel determines the thermal conductivity of steel. The higher the carbon content, the worse the thermal conductivity; the higher the alloy content, the worse the thermal conductivity.

3) The denser the furnace is, the more uneven the heating degree of each steel material.

4) The larger the size of the steel, the greater the temperature difference between the inner and outer layers of the steel under the same heating rate. In order to prevent the metal from cracking during the heating process and the temperature difference between the inside and outside of the furnace is too large, the heating speed should be limited. Generally, in the case of ensuring that the steel does not appear overheating, overburning and cracking, the furnace loading temperature should be increased as much as possible. This not only improves the productivity, but also avoids the oxidation, decarburization and grain growth of the steel. Carbon steel, alloy structural steel billets and non-ferrous metal materials, even if the heating speed is fast, will not form large thermal stress or cause internal cracks, so the heating speed is not limited. For steel ingots, high-carbon steel, and high-alloy steel, due to the large cross-section size or low thermal conductivity, too fast heating speed will cause a large temperature difference between the inside and outside of the metal, forming internal cracks, so the heating speed must be slow, or use Two-stage and three-stage heating method.

heating time

The time required to heat the steel material to the specified temperature is called the heating time.

The heating time of the steel in the furnace directly affects the production efficiency and the quality of the forging. Therefore, it is important to determine a reasonable heating time.

The heating time is usually calculated by an empirical formula for reference in actual production. The simplest empirical formula is as follows:

t = KD

In the formula, t——the time required for heating to the initial forging temperature (h);

D——The diameter or side length of the steel material (cm);

K——The influence coefficient of steel's chemical composition on heating time. For low carbon steel K = 0.1 ~ 0.15; for high carbon steel ruler K = 0.2 ~ 0.3; general alloy steel K = 0.15 ~ 0.2.

The way the steel is loaded into the furnace is different, and the heating conditions during heating are also different. The furnace is densely packed, and each steel material is heated unevenly, which requires longer heating time than a single-piece furnace.

If the steel ingot is kept at the forging temperature for longer than the allowable holding time, it will all form a superheated structure with coarse grains. The overheated steel ingots must be kept at a temperature below 650 ~ 750°C before heating. If necessary, the holding time can be extended at the forging temperature, but it cannot exceed the maximum holding time specified in the table.More about:hot forging steel

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