Large free forgings and high alloy steel forgings mostly use steel ingots as raw materials. According to the specifications of the steel ingot, they can be divided into large steel ingots and small steel ingots. Generally, steel ingots with a mass greater than 2t~2.5t and a diameter greater than 500mm~550mm are called large steel ingots, and the others are small steel ingots.
According to the temperature of the steel ingot when it is heated and loaded into the furnace before forging, it is divided into cold ingots (generally room temperature) and hot ingots (generally greater than room temperature). This is because cold ingots have poor plasticity when heated below 500°C, and the inside of the steel ingot is undergoing a crystallization process. The residual stress formed in the steel is in the same direction as the temperature stress, and various structural defects will also cause stress concentration. If the heating specifications are not formulated properly, cracks may easily occur. Therefore, during the low-temperature stage of cold ingot heating, the furnace loading temperature and heating speed should be limited.
When using medium frequency induction heating machines to heat large steel ingots, multi-stage heating specifications are often used. Due to its large cross-section size, it is subject to great tensile stress in the center. In addition, the ingot has low strength and poor plasticity, so the temperature stress during heating is easily Cracks will occur. Therefore, the furnace charging temperature cannot be too high and the heating speed must be slow. For example, for carbon structural steel and alloy structural steel, the furnace charging temperature is generally 350℃~850℃. If the steel ingot size is small, the furnace charging temperature should be a larger value and the heat preservation should be carried out. For alloy steel, such as high-speed steel and high-chromium steel, cracks are likely to occur when heated, and the furnace charging temperature should be controlled between 400°C and 650°C. When the temperature is higher than 850°C, the steel ingot can be heated at a faster speed, but not too fast to avoid excessive temperature difference between the inside and outside of the steel ingot. For example, for steel ingots of carbon structural steel and alloy structural steel, the allowable temperature difference between sections is 50°C to 100°C.
When using a medium frequency induction heating machine to heat small steel ingots, due to their small cross-sectional size, the residual stress and the temperature stress caused by heating are not large. When using a medium frequency induction heating machine to heat them, the heating speed can be faster. Therefore, for carbon For small ingots of steel and low-alloy steel, one-stage rapid heating specifications are often used. For small high-alloy steel ingots, due to their poor low-temperature thermal conductivity, multi-stage heating specifications are also used like the heating of large cold ingots, and the furnace can be installed at temperatures between 700°C and 1000°C.
In order to shorten the heating time and save fuel, large ingots are demoulded from the steelmaking workshop and sent directly to the forging workshop for furnace heating. This kind of steel ingot is called a hot ingot. When the hot ingot is installed in the furnace, its surface temperature is generally 550℃~650℃. Since the hot ingot is in a plastic state and the temperature stress is small, the furnace loading temperature can be increased. Depending on the size and material of the steel ingot, it can generally be The furnace is installed at 800"c ~1000℃. The furnace temperature for small ingots can be unlimited. After entering the furnace, the heating speed can be increased.
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