Which of the following is NOT a method of hardening steel?

The correct answer is annealing, as it is a process that is typically used to soften steel or other metals rather than harden them. During annealing, steel is heated to a specific temperature where its internal structure can change, then it is slowly cooled down. This process helps to relieve internal stresses, refine the grain structure, and improve ductility, making the metal easier to work with.

In contrast, quenching involves rapidly cooling heated steel in water or oil to increase hardness. Tempering is a subsequent process that involves heating the quenched steel again to reduce brittleness while maintaining a certain level of hardness. Cold working refers to the deformation of metals at room temperature, leading to increased strength and hardness through strain hardening effects. Thus, while quenching, tempering, and cold working are methods that contribute to enhancing the hardness of steel, annealing serves the opposite purpose by softening it.