The mechanical properties of steel are negatively affected by oxygen and hydrogen. Not only is oxygen concentration a factor, but the quantity, type, and distribution of oxygen-containing inclusions also play a significant role. These inclusions include compounds such as metal oxides, silicates, aluminates, and oxygen-containing sulfides. Therefore, deoxidation treatment is necessary during the steelmaking process.
During the solidification of steel, oxygen and carbon in the solution react to form carbon monoxide bubbles, which causes porosity. For this reason, deoxidation is required during the steelmaking process to lower the oxygen content and reduce the formation of bubbles.
Furthermore, during the cooling of steel, oxygen precipitates with oxide inclusions like FeO and MnO, which weakens the steel's hot or cold workability, as well as its ductility, toughness, fatigue strength, and machinability. In addition, oxygen, along with nitrogen and carbon, can cause steel to age at room temperature or undergo a spontaneous increase in hardness.
For cast iron, when the ingot solidifies, oxides and carbon react, leading to the formation of pores and making the product brittle.
Therefore, understanding and controlling the oxygen content in steel, managing oxygen-containing inclusions, and performing proper deoxidation treatment are very important. This can improve the mechanical properties and workability of the steel.
The role of nitrogen in steel cannot be simply categorized as a harmful gaseous element, because nitrogen is intentionally added to some specialty steels. The nitrogen content in steel depends on factors such as the production method, the type and quantity of alloying elements, the method of addition, the casting method, and whether nitrogen is intentionally added.
In some specific grades of stainless steel, adding an appropriate amount of nitrogen can reduce the use of expensive chromium, thereby effectively lowering costs. Nitrogen in steel mainly exists in the form of metal nitrides. For example, when steel has been stored for a period of time and undergoes strain aging under stress, it cannot be used for deep drawing (e.g., for car protective panels) because the steel will tear and cannot be stretched uniformly in all directions. This is caused by grain growth and the deposition of Fe₄N at the grain boundaries.
In addition, the formation of chromium nitride (Cr2N) at the grain boundaries in stainless steel depletes chromium at the interface and can induce intergranular corrosion. By adding titanium, titanium nitride can be preferentially formed, which prevents this harmful effect.
Therefore, the role of nitrogen in steel is complex and cannot be simply reduced to that of a harmful element. Based on specific needs, the nitrogen content in steel can be purposefully adjusted to achieve specific performance requirements, and appropriate measures can be taken to solve the negative effects caused by nitrogen.
When the hydrogen content in steel exceeds 2 ppm, hydrogen plays an important role in the so-called "flake peeling" phenomenon. This flaking is more pronounced during the cooling process after rolling and forging, and is especially common in large sections or high-carbon steel. This defect often leads to internal cracks and fractures, particularly during the use of large rotors, such as those in engines. High hydrogen content can cause "hydrogen embrittlement" which easily forms pores or a general porous structure in cast iron, making the iron brittle. It should be noted that "hydrogen embrittlement" mainly occurs in martensitic steel, is less obvious in ferritic steel, and its specific situation in austenitic steel is currently unclear. In addition, the phenomenon of hydrogen embrittlement generally increases with hardness and carbon content.
Therefore, when the hydrogen content in steel exceeds 2 ppm, a series of problems will appear, such as flake peeling, internal cracks, and fractures. In cast iron, a high hydrogen content leads to the formation of pores and porosity, causing the iron to become more brittle. It is important to note that different types of steel have varying degrees of sensitivity to hydrogen embrittlement, with the specific situation depending on the steel's crystal structure. Furthermore, hydrogen embrittlement is closely related to hardness and carbon content.
