Why is steel tempered after being hardened?

• A. To relieve internal stresses and reduce brittleness
• B. To increase hardness
• C. To improve electrical conductivity
• D. To reduce weight

Answer: (A)

When steel is hardened, it becomes very hard but also very brittle because the rapid transformation to martensite traps internal stresses inside the metal. Tempering heats the steel to an intermediate temperature, allowing those stresses to relax and giving time for diffusion to occur. This changes the microstructure to tempered martensite, which reduces brittleness and improves toughness and resistance to crack propagation, while keeping enough hardness for wear resistance.

In short, tempering doesn’t aim to make the metal harder than after quenching; it aims to balance hardness with ductility and reliability in service by relieving internal stresses and decreasing brittleness. It’s not about changing electrical properties or reducing weight.