__________ addition increases the depth of hardness of steel.

ANS:A - Chromium

When chromium is added to steel, it forms chromium carbides during the heat treatment process. These carbides are very hard and provide additional strengthening to the steel matrix. As a result, the steel becomes harder and more resistant to wear and abrasion. This increase in hardness extends throughout the depth of the material, leading to an overall improvement in its mechanical properties. Therefore, the addition of chromium is commonly used to enhance the depth of hardness in steel components, making them suitable for applications where hardness and wear resistance are critical, such as in cutting tools, dies, and machine parts.