Which statement about carbon content in ferritic iron is true?

Which statement about carbon content in ferritic iron is true?

• A. It can exceed 1%
• B. It is exactly 0.5%
• C. It is limited to a very small fraction, such as about 0.0025%
• D. It is not soluble at all

Answer: (C)

Carbon in ferritic iron is only able to dissolve in the solid solution in extremely small amounts. The alpha-iron lattice (ferrite) is a body-centered cubic structure that can accommodate only a tiny amount of interstitial carbon before the lattice strain becomes too great and carbide phases begin to form. That makes the solubility of carbon in ferrite very low—roughly a few tens of parts per million, about 0.0025% by weight. Because of this tiny solubility, ferrite tends to stay nearly carbon-free unless the material is designed to contain more carbon, in which case excess carbon precipitates as iron carbides (and you get phases like cementite or pearlite upon cooling). Exceeding 1% or 0.5% would put you well beyond the solubility limit, and saying it’s not soluble at all ignores the actual, albeit tiny, amount that can be in solution.