Why is steel tempered after being hardened?

Tempering is a crucial process performed after hardening steel, primarily to relieve internal stresses and reduce brittleness. When steel is hardened through processes like quenching, it undergoes significant changes in its microstructure, which can lead to residual stresses within the material. These stresses may make the steel very hard but also quite brittle, meaning it can easily fracture under impact or load.

By tempering, the steel is reheated to a temperature below its critical point, allowing some of the hard microstructures, such as martensite, to transform into a more stable structure, typically troostite or softer phases. This transformation not only alleviates internal stresses but also allows the steel to retain much of its hardness while improving its toughness and ductility. As a result, tempered steel can better withstand impacts and mechanical loads without breaking, making it more usable in a wide variety of applications.

The other options do not accurately reflect the primary purpose of the tempering process. While increasing strength, enhancing corrosion resistance, or improving electrical conductivity are important properties of materials, they are not the primary intents of tempering steel after the hardening step.