Heating Defect of the Heating Furnace
1. Cracking of alloy steel.
At the beginning stage of heating (below 700 ℃), for high carbon tool steel, high manganese steel, bearing steel, high-speed steel and other steel with low thermal conductivity, if the heating rate is too fast, the surface temperature of the heating furnace rises suddenly and the temperature difference of cross-section is too large, thermal stress will be produced, leading to cracks.
2. Overheating and overburning.
If the heating temperature of the heating furnace is too high or the residence time at a high temperature is too long, the grain of the steel will grow excessively, the connection between grains will be weakened, and the steel will become brittle, which is called overheating. Even if there is no cracking in rolling, the mechanical properties of the finished product can not meet the requirements. It can be saved by normalizing the overheated billet. With the further development of overheating, the grains continue to grow up, and the grain boundaries are oxidized or melted, which is often broken or cracked during rolling, which is called overburning. The over burned blank is an irreparable waste. When the rolling operation breaks down suddenly, it is easy to cause overheating or overburning; if the temperature of high carbon steel is not controlled properly, it is also easy to cause overheating or overburning.
3. Oxidation and decarburization of billet.
During the process of billet heating in the heating furnace, metal elements in steel react with oxidizing atmosphere in the heating furnace and produce scale (the inner layer of scale is iron oxide, the middle layer is iron oxide, and the outermost layer is iron oxide). Decarburization is caused by the diffusion of carbon in steel to the surface and reaction with the atmosphere in the furnace. Decarburization of bearing steel, tool steel, spring steel and some other steels is harmful. The surface of decarburized steel can not reach the required hardness during quenching. In addition, the compressive performance, wear resistance and elasticity are reduced.
Oxidation and decarburization are carried out simultaneously, which are related to heating conditions (temperature, furnace time, furnace atmosphere and chemical composition of billet). Generally speaking, oxidation and decarbonization are not obvious when the temperature is less than 750 ℃. But when the temperature is higher than 800 ℃, it increases very fast.
In particular, the heating temperature of carbon steel should not be higher than 1300 ℃. Firstly, the oxide scale on the surface of steel billet will melt (the melting point of pure oxide scale is 1377 ~ 1565 ℃, and it will drop to 1300 ~ 1350 ℃ when it contains impurities), which will fall off the steel surface and expose a new surface, and the burning loss will increase rapidly. Secondly, when the heating temperature of the heating furnaces is higher than 1300 ℃, the billet is likely to overheat or overburnt. The maximum heating temperature should be paid more attention to when producing other kinds of steel.
After the billet is oxidized and burned, it will inevitably affect the yield; the thermal conductivity of the scale is very poor, which has an impact on the heating time of the billet; the scale falls off and accumulates in the heating furnace, which makes manual cleaning very difficult and hard work.
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