Steel made from phosphate iron is:
Try it first: pick an option, then read the explanation.
Understanding Impurities in Steel
"Phosphate iron" refers to iron ore that has a high content of phosphorus. When this ore is smelted, the phosphorus is not removed and ends up as an impurity in the final steel. Phosphorus is one of the most detrimental impurities in steel, having a profound effect on its mechanical properties.
Cold Shortness: This is the specific defect caused by excess phosphorus in steel. It means the steel becomes extremely brittle at room temperature or colder, even though it might be perfectly workable when hot.
Detailed Analysis of Phosphorus's Effects
Let's examine how phosphorus affects the properties of steel to determine the correct answer.
(a) Brittle
Why it's correct: Phosphorus does not dissolve uniformly in the iron crystal lattice. Instead, it segregates to the grain boundaries, forming a weak, brittle iron phosphide (Fe₃P) network. This network acts as a continuous path for cracks to travel along. At low temperatures, this makes the steel lose its toughness and fail with very little energy absorption. This severe embrittlement is the most significant effect of phosphorus.
(b) Hard
Why it's not the best answer: Phosphorus does increase the hardness and tensile strength of steel. However, this increase in hardness comes at a catastrophic cost to its toughness and ductility. The primary and most defining characteristic of high-phosphorus steel is its brittleness, not its hardness.
(c) Ductile & (d) Tough
Why they're incorrect: These are the opposite of what happens. Phosphorus severely reduces both the ductility (the ability to stretch) and the toughness (the ability to absorb impact energy) of steel. The steel becomes prone to sudden, catastrophic failure.
Phosphorus vs. Sulfur: The Two Main Villains
It's useful to compare the effects of the two most common harmful impurities in steel:
- Phosphorus (P): Causes Cold Shortness. The steel is brittle when cold.
- Sulfur (S): Causes Hot Shortness. The steel is brittle when hot (during rolling or forging).
Study Tips for Steel Impurities
- Phosphorus Poisons the Steel: Think of phosphorus as a "poison" that makes the steel weak and brittle at normal temperatures.
- Mnemonic: P for "Pops" when cold: A silly way to remember that Phosphorus makes steel "pop" or break easily when it's cold (Cold Shortness).
- Mnemonic: S for "Sizzles" and splits: Sulfur causes problems when the steel is sizzling hot (Hot Shortness).