Which of the following is not a principal alloying element for the structural steel?

ANS:A - Molybednum

Molybdenum is a metallic element that is commonly used as an alloying agent in steel production. When added to steel, molybdenum helps improve several key properties, making it suitable for various structural applications.

1. Strength and Toughness: Molybdenum enhances the strength and toughness of steel. It forms solid solution strengthening and grain boundary strengthening effects, which help prevent dislocations in the crystal lattice, thereby improving the material's overall strength and resistance to deformation.
2. Hardness and Wear Resistance: Molybdenum contributes to the hardness and wear resistance of steel. It forms carbides with carbon atoms in the steel matrix, which increases the material's resistance to abrasive wear and extends its service life in applications subject to friction and abrasion.
3. Corrosion Resistance: Molybdenum improves the corrosion resistance of steel, particularly in environments where corrosion is a concern. It enhances the steel's ability to resist pitting, crevice corrosion, and general corrosion in harsh conditions such as acidic or chloride-containing environments.
4. High-Temperature Strength: Molybdenum improves the high-temperature strength and creep resistance of steel. This makes molybdenum-alloyed steels suitable for applications exposed to elevated temperatures, such as in high-temperature equipment and structures.