ANS:C - martensite forms in the heat affected zone leading to poor toughness/ductility of the weld.

Steels with high carbon equivalent have poor weldability primarily because martensite forms in the heat-affected zone leading to poor toughness/ductility of the weld. Explanation:

1. High Carbon Equivalent: High carbon equivalent in steels indicates a higher carbon content or the presence of other alloying elements that contribute to increased hardenability.
2. Formation of Martensite: During welding, the high heat input and rapid cooling rates can promote the formation of martensite in the heat-affected zone (HAZ) adjacent to the weld. Martensite is a hard, brittle phase that lacks the toughness and ductility required for most welding applications.
3. Effect on Weld Properties: The presence of martensite in the HAZ can significantly degrade the mechanical properties of the weld joint. Martensite formation leads to increased hardness and reduced ductility, which can result in weld cracking, brittleness, and poor overall performance.
4. Other Factors: While oxidation of carbon and alloying elements can occur during welding, excessive ferrite formation, or segregation of carbon and other elements may also contribute to poor weld quality in certain cases. However, the formation of martensite is particularly detrimental to weld properties in steels with high carbon equivalent due to its inherent brittleness.