回火 (huí huǒ) is a heat treatment process used to improve the properties of metals, particularly steel. It involves heating the metal to a specific temperature and then allowing it to cool slowly. This process helps to relieve internal stresses, reduce brittleness, and enhance ductility, making the metal more workable and durable. The process of 回火 is often applied after quenching, which hardens the metal. By carefully controlling the temperature and cooling rate during 回火, manufacturers can achieve desired mechanical properties for various applications, such as in tools, machinery, and structural components.