To increase hardness in materials, carbon is added to which type of metal?

Adding carbon to steel is a well-established method to increase its hardness. When carbon is combined with iron, which is the primary component of steel, it forms different iron carbides, including cementite (Fe3C). This microstructural change significantly enhances the material's hardness and strength. The process of adding carbon to steel is typically referred to as "carbonization," and it's essential in producing various types of steel with specific properties, such as tool steel, which is specifically designed for applications that require a hard, wear-resistant material.

In contrast, aluminum, copper, and platinum do not gain significant hardness from carbon being added. Aluminum can be alloyed with various elements to enhance its properties, but carbon is not one of the primary alloying agents. Copper can gain strength from processes like work hardening but does not utilize carbon for hardness. Platinum is a noble metal and is generally not alloyed with carbon in a way that enhances its hardness, focusing instead on its corrosion resistance and other unique properties. Thus, steel is distinct in its response to carbon addition, making it the correct choice for increasing hardness.