ANS:D - plain carbon

Plain carbon steel, as the name suggests, primarily consists of iron and carbon. It typically contains less than 2% carbon, with trace amounts of other impurities such as manganese, silicon, sulfur, and phosphorus. The absence or very low concentration of alloying elements like nickel, chromium, and tungsten distinguishes plain carbon steel from alloy steels. The properties of plain carbon steel are largely influenced by its carbon content. Increasing the carbon content generally results in higher hardness, strength, and wear resistance, but decreased ductility and weldability. Conversely, decreasing the carbon content improves ductility and weldability but reduces hardness and strength. Plain carbon steel is commonly classified based on its carbon content into three main categories:

1. Low Carbon Steel: Also known as mild steel, low carbon steel typically contains up to 0.3% carbon. It is relatively soft and ductile, making it suitable for applications requiring formability, such as automotive panels, construction materials, and sheet metal fabrication.
2. Medium Carbon Steel: Medium carbon steel contains carbon concentrations ranging from 0.3% to 0.6%. It offers a balance between strength and ductility and finds applications in components requiring higher strength and wear resistance, such as gears, shafts, and machinery parts.
3. High Carbon Steel: High carbon steel contains carbon concentrations above 0.6%, often up to 1.5%. It possesses high hardness and wear resistance but limited ductility. High carbon steel is commonly used in cutting tools, springs, and knives where hardness and edge retention are critical.

The absence of significant alloying elements in plain carbon steel makes it cost-effective and easy to manufacture through traditional steelmaking processes like melting, casting, and rolling. However, its properties can be tailored through heat treatment processes such as quenching and tempering to achieve specific mechanical properties required for various applications. Overall, plain carbon steel remains one of the most widely used materials in engineering and construction due to its versatility, affordability, and a wide range of mechanical properties that can be achieved through simple processing techniques.