热处理是将材料加热到一定的温度，保温一定的时间后，以一定的速率降温到室温或更低，从而达到改善材料组织结构获得性能优异的材料，一般是指对金属材料特别是钢材的处理。常用的分类方法是以下四种（“四把火”）：正火（在**称为正常化）、退火、回火和淬火（淬火和高温回火两个过程通常称为调质）。然而在如德国的西方国家，正火（德文：Normalglühen）只是退火（德文：Glühen）的一个子类。

工业生产中，热处理可以视为一系列的用来改变材料的物理性质，偶尔也用来改变材料的化学性质冶金工程步骤。热处理在冶金学方面有非常普及的应用，但是陶瓷、玻璃材料的生产过程中也常有热处理进程的出现。热处理用升高或冷却的方式进行，通常涉及极端的温度，以期改变材料的硬度、韧性等一系列性质。

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.