Theoretical Basics of Scale Formation in Steel Reheating
When steel is heated in the presence of oxidizing agents like oxygen or water vapor, it undergoes chemical reactions that form a layer of oxides, known as "scale." This process is unavoidable in reheating furnaces, and the scale layer must be removed before further processing to ensure consistent product quality. The scale formation process leads to a loss of steel during reheating, but it also helps improve surface quality by reducing defects.
For pure iron, the scale layer consists of various iron oxides, such as wustite, magnetite, and hematite. However, for alloyed steels, especially high-alloy steels, the process is more complex due to the interference of alloying elements with the scale formation reactions. Initially, a metal oxide layer forms on the steel surface, separating the steel from the oxidizer. Over time, as the scale thickens, internal oxidation slows down, and the rate of scale formation follows a parabolic law.
The rate of scale formation depends on factors such as temperature, the type of oxidizing agent, and the specific steel grade. Additionally, decarburization can occur, which lowers the carbon content and affects the physical properties of the steel. The oxidation rate is influenced by the furnace atmosphere, temperature, and time.
The phase diagrams for pure iron and the 1.2379 steel grade illustrate the complex nature of oxidation at high temperatures. These diagrams show that while the oxidation processes are relatively straightforward for pure iron, they become more complex for alloyed steels, involving multiple phases and oxide formations depending on the alloying elements.
In summary, understanding the scale formation process is crucial for controlling steel quality in reheating, and different furnace atmospheres significantly impact the final product.