Which technique is characterized by heating and cooling to refine the crystal structure of the work?

Normalising is a heat treatment process that aims to refine the grain structure of a metal. By heating the metal above its critical temperature and then allowing it to cool in a still environment, this technique effectively eliminates internal stresses and creates a uniform microstructure. This controlled heating and cooling helps in producing a consistent and even crystal structure, which leads to improved mechanical properties like toughness and ductility.

The essence of normalising lies in its ability to enhance the material's response to subsequent processes, such as machining or further heat treatment, by ensuring a more isotropic behavior across the material. This is particularly valuable in materials like steel, where the characteristic properties can greatly enhance the performance in various applications.

In contrast, other techniques mentioned serve different purposes: nitriding involves diffusing nitrogen into the surface layers to increase hardness, hardening typically refers to the process of increasing material hardness through heating and rapid cooling, while tempering is a subsequent heat treatment applied to harden materials to reduce brittleness after hardening. These processes do not specifically focus on refining crystal structure in the same manner as normalising.