The yield point phenomenon observed in annealed low carbon steel is due to the presence of the following element.

ANS:B - Carbon

Strength and Hardness: One of the primary effects of carbon in steel is its contribution to strength and hardness. Carbon atoms occupy interstitial spaces within the iron lattice, distorting it and making it more difficult for dislocations to move. This impedes plastic deformation, leading to increased strength and hardness of the steel.

1. Tensile Strength: Carbon content influences the tensile strength of steel. Generally, higher carbon content results in higher tensile strength. However, this relationship may reach a peak before declining due to the formation of brittle phases like cementite (iron carbide) at high carbon levels.
2. Hardenability: Carbon content also affects the hardenability of steel, which is its ability to be hardened through heat treatment processes like quenching and tempering. Higher carbon content generally improves hardenability, allowing steel to achieve greater hardness and depth of hardness after quenching.
3. Ductility and Toughness: While carbon contributes to increased strength and hardness, excessive carbon content can lead to reduced ductility and toughness. Steel with higher carbon content tends to be more brittle and prone to fracture, especially under impact loading.
4. Weldability: Carbon content influences the weldability of steel. Higher carbon content increases the likelihood of weldability issues such as cracking and distortion during welding. Proper selection of welding techniques and preheating measures may be necessary to mitigate these issues.
5. Corrosion Resistance: Carbon itself does not significantly affect the corrosion resistance of steel. However, carbon steel can be alloyed with other elements like chromium and nickel to enhance its corrosion resistance.

In summary, carbon is a critical alloying element in steel, affecting its mechanical properties such as strength, hardness, ductility, and toughness. The carbon content in steel must be carefully controlled to achieve the desired balance of properties for specific applications.