How does tempering change the properties of martensitic steel?

Martensite is the hardest and most brittle form of steel, created by rapidly quenching the metal to trap carbon atoms in a strained crystal structure. Because fresh martensite is often too brittle for practical use, it undergoes tempering to improve its mechanical properties.123

Tempering involves heating the martensitic steel to a specific temperature below its melting point and holding it there for a set duration. This process changes the material in the following ways:31

• Reduces Brittleness and Hardness: Tempering decreases the extreme hardness of the martensitic structure, making the steel less prone to fracturing.12
• Increases Toughness: By coarsening the carbide precipitates, the process transforms the brittle martensite into a tougher, more durable material.12
• Relieves Internal Stresses: The heat treatment reduces residual stresses that were introduced during the rapid phase transformation of quenching.32

The final balance of properties is controlled by the tempering temperature: lower temperatures are used when high strength is required, while higher temperatures are preferred when maximum toughness is the primary goal.2