ANS:B - cold rolled mild

Square steel keys, which are used in machinery to transmit torque between rotating shafts and components such as gears or pulleys, are typically made of forged steel bars. Explanation:

1. Forged Steel: Forging is a manufacturing process that involves shaping metal using compressive forces, typically applied with a hammer or die. Forged steel bars have superior strength and toughness compared to other forms of steel due to the grain structure developed during the forging process. This makes forged steel an ideal material for keys that are subjected to high stresses and forces during operation.
2. Hot Rolled Mild Steel: While hot rolled mild steel is commonly used in various applications due to its availability and cost-effectiveness, it may not provide the required strength and durability for key applications where failure by shear and crushing must be avoided.
3. Cold Rolled Mild Steel: Cold rolled mild steel is typically used for applications requiring precision dimensions and a smooth surface finish. While it may offer better dimensional accuracy compared to hot rolled steel, it may not provide the necessary strength and toughness for key applications.
4. Machinable Stainless Steel: Stainless steel is known for its corrosion resistance and machinability. While certain grades of stainless steel can offer good mechanical properties, they may not be as commonly used for keys compared to forged steel due to factors such as cost and availability.

In summary, forged steel bars are often chosen for manufacturing square steel keys due to their superior strength, toughness, and resistance to failure by shear and crushing, making them suitable for demanding applications in machinery and mechanical systems.