What does tempering refer to in metalworking?

Tempering in metalworking specifically refers to a heat treatment process used to alter the hardness and elasticity of metal, particularly steel. This process involves heating the metal to a specific temperature below its critical point and then allowing it to cool, which results in a reduction of brittleness while enhancing toughness. This method is particularly important after hardening processes, where metals can become extremely hard but also very brittle. By tempering, the desired balance of hardness and ductility can be achieved, making the metal more suitable for various applications where flexibility and strength are required.

The other options do not accurately describe tempering. Cooling metals is a general term that does not capture the selective adjustments in properties that tempering entails. Joining metals pertains to methods like welding or soldering, and increasing the weight of metal does not relate to the tempering process, which focuses on optimising material characteristics rather than changing its mass.