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The effects of Arsenic (As) and Bismuth (Bi) on the microstructure and heat treatment of steel

The effects of Arsenic (As) and Bismuth (Bi) on the microstructure and heat treatment of steel

Arsenic (As) and Bismuth (Bi) are elements that have distinct effects on the microstructure and properties of steel. While both elements can improve certain characteristics of steel, their excessive presence can lead to detrimental effects. This article discusses how each element influences the microstructure, heat treatment, and overall performance of steel.

The Role of Arsenic (As) in Steel Microstructure and Heat Treatment

Arsenic (As) is a Group V A element with atomic number 33 and atomic mass 74.92. It exists in three allotropic forms: gray, yellow, and black arsenic. Its density is 5.727 g/cm³, and it sublimates at 613°C. Arsenic typically exists in steel as compounds such as Fe₂As, Fe₃As₂, and FeAs, or as a solid solution. One of the challenges with arsenic is its tendency to segregate during solidification.

Flanged-pipes

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Arsenic belongs to the same group as phosphorus and antimony, and its effects on steel are quite similar. In small amounts, arsenic can improve certain mechanical properties. It increases the tensile strength and yield point of steel, and enhances the material’s resistance to corrosion and oxidation. However, when the arsenic content exceeds 0.2%, its negative effects become more pronounced. Higher levels of arsenic increase the brittleness of steel, reducing its elongation, reduction of area, and impact toughness. Additionally, excessive arsenic can impair the steel’s weldability, making it more susceptible to cracking during and after welding.

The Role of Bismuth (Bi) in Steel Microstructure and Heat Treatment

Bismuth (Bi), another Group V A element, has atomic number 83 and atomic mass 208.98. It has a high density of 9.8 g/cm³, a melting point of 271.4°C, and a boiling point of approximately 1560°C. During the steelmaking process, most of the bismuth evaporates due to its low solubility in steel, resulting in extremely low concentrations of bismuth in the final product. However, even at trace levels, bismuth tends to segregate at grain boundaries and interfaces. The concentration of bismuth at these boundaries can be up to 8,100 times higher than in the bulk alloy.

Bismuth’s presence in steel leads to embrittlement, particularly affecting the hot ductility of stainless steel. When bismuth content is too high, it reduces the material’s plasticity and high-temperature strength, leading to cracking during processes such as extrusion. In most cases, bismuth is considered undesirable in steel due to its embrittling effects.

However, bismuth can be intentionally added to steel in controlled amounts for specific purposes. In particular, the addition of small quantities of bismuth can significantly improve the machinability of steel. This is especially useful in steel grades designed for cutting and forming operations, where enhanced machinability is a key requirement.

Both arsenic (As) and bismuth (Bi) can influence the microstructure and mechanical properties of steel in different ways. Arsenic, in small amounts, improves tensile strength and corrosion resistance but becomes detrimental at higher concentrations, increasing steel’s brittleness. Bismuth, on the other hand, is typically avoided due to its embrittling effects, though it can be beneficial when added in small quantities to enhance machinability. Understanding the roles of these elements allows metallurgists to fine-tune steel properties for specific applications.

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