The AOD stainless - steel making process begins with the melting of raw materials in a conventional furnace. After melting, the molten liquid is transferred to a converter. Inside this converter, the steel undergoes a refining chemical analysis through a series of redox (reduction - oxidation) reactions.
To ensure continuous mixing of the molten steel, argon and oxygen are blown into the converter using tuyeres (referred to as "sheets" in the original text, likely a misnomer; tuyeres are more appropriate in this context). These redox reactions not only facilitate the mixing but also raise the temperature of the molten steel up to 1650°C.
The main reaction taking place in the AOD process is decarburization. During this reaction, the excess carbon in the steel binds with oxygen, and other gases are evolved. This leads to the formation of carbon monoxide, which helps in reducing the carbon levels in the steel to around 0.015%. One of the key advantages of the AOD process is that the loss of chromium due to oxidation is effectively controlled by carefully calibrating the ratio of oxygen and argon blown into the converter.
Once the chemical analysis of the steel is "centered" (i.e., the desired chemical composition is achieved), the steel can be either continuously cast or poured into ingot molds for further processing.
The initial step of scrap melting in the VOD process is similar to that in the AOD process. However, there is a significant difference in the subsequent handling of the molten steel. In the VOD process, the molten steel is poured into a ladle and then transferred into a special plant (tank). This tank is capable of creating an initial vacuum pressure of 3 mbar, which is then further stabilized at 0.6 mbar.
The vacuum environment in the VOD tank plays a crucial role. It enhances the decarburization reaction while simultaneously protecting chromium from over - oxidation. Argon is introduced into the ladle from the bottom through porous plugs. This argon agitation keeps the steel well - mixed and promotes better contact between the steel and the gases.
Oxygen is then introduced from above using a lance (referred to as a "spear" in the original text). The oxygen spreads over the surface of the molten steel, accelerating the formation of carbon monoxide and thus promoting decarburization. The carbon content in the steel can be reduced to as low as 0.015% through this process.
After reaching the specified carbon level, adjustments are made to other chemical elements in the steel, such as chromium. One of the notable features of the VOD process is that it generates heat through the decarburization reaction. As a result, it requires a relatively smaller amount of additional energy to maintain the high temperatures necessary for the process. Once the refining is complete, the steel is poured into ingot molds for further solidification and processing.
In summary, while both AOD and VOD are important processes for stainless - steel making, they differ in terms of the reaction environment (atmospheric pressure in AOD vs. vacuum in VOD), the way gases are introduced, and the energy requirements for maintaining high temperatures.
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