The Impact of H (Hydrogen) and B (Boron) Elements on Steel Properties

Hydrogen (H) is generally considered the most detrimental element in steel, as its dissolution can lead to defects such as hydrogen embrittlement and white spots. Similar to oxygen and nitrogen, hydrogen has extremely low solubility in solid steel. When dissolved in molten steel at high temperatures, it may accumulate in the microstructure upon cooling, forming tiny high-pressure pores that drastically reduce the steel’s plasticity, toughness, and fatigue strength. In severe cases, this can result in cracks and brittle fractures. “Hydrogen embrittlement” is primarily observed in martensitic steels, less prominently in ferritic steels, and generally increases alongside hardness and carbon content.

On the other hand, hydrogen can enhance the magnetic permeability of steel but also increases coercivity and iron loss (coercivity can increase by 0.5 to 2 times after hydrogen addition).

Boron (B) plays a crucial role in steel by increasing its hardenability, thereby enabling the conservation of other more expensive metals such as nickel, chromium, and molybdenum. For this purpose, its content is typically specified within the range of 0.001% to 0.005%. Boron can substitute for 1.6% nickel, 0.3% chromium, or 0.2% molybdenum. However, caution should be exercised when substituting molybdenum with boron, as molybdenum prevents or reduces temper brittleness, while boron slightly promotes it. Therefore, molybdenum cannot be fully replaced by boron.

The addition of boron to medium-carbon steel significantly improves the properties of steel thicker than 20mm after quenching and tempering due to enhanced hardenability. Consequently, 40B and 40MnB steels can replace 40Cr, and 20Mn2TiB steel can substitute for 20CrMnTi carburized steel. However, the effectiveness of boron diminishes or even disappears as the carbon content in steel increases. When selecting carburized boron steel, it is essential to consider that the hardenability of the carburized layer will be lower than that of the core.

Spring steels typically require complete hardenability, and the use of boron-containing steel is advantageous due to their typically small spring areas. However, the effect of boron fluctuates significantly in high-silicon spring steels, making them less suitable for this purpose.

Boron has a strong affinity with nitrogen and oxygen. Adding 0.007% boron to rimmed steel can eliminate aging phenomena.