After metal workpieces are forged and heated in a heat treatment furnace, residual internal stress will be generated, resulting in high hardness and poor plasticity. After heat treatment, metal workpieces can effectively reduce the hardness of steel, improve plasticity, lower residual internal stress, and prevent deformation and cracking of parts during processing. The common process methods for metal heat treatment mainly include:

一、Annealing heat treatment process for metal workpieces: Mainly, metal workpieces are heated to a certain temperature in gas heat treatment, held for a certain period of time, and then slowly cooled to room temperature. After annealing heat treatment, metal workpieces can effectively reduce the hardness of steel and improve plasticity, which is conducive to cutting processing and cold deformation processing. Refine the grains of metal workpieces, homogenize the properties of steel and prepare for subsequent heat treatment; Eliminate the residual internal stress of metal workpieces after heat treatment to prevent deformation and cracking of parts after processing.

(1)Annealing heat treatment: The metal workpiece is heated to the critical temperature in the annealing furnace. The critical temperature varies for different metal workpieces. After being heated and held for a certain period of time, it is then slowly cooled in the furnace or buried in sand for cooling. Annealing can refine grains, homogenize the structure, reduce hardness, and fully eliminate internal stress. Annealing is mainly applicable to forgings or cast steel parts with a relatively low carbon content.

(2) Spheroidizing annealing heat treatment: This mainly involves heating the metal workpiece to approximately 20 to 30 degrees Celsius above the critical temperature, holding it at that temperature, and then slowly cooling it before taking it out of the furnace for air cooling. The purpose is to reduce the hardness of metal workpieces, improve the machinability of metals, and prepare for subsequent quenching to reduce deformation and cracking after quenching. Spheroidizing annealing is suitable for carbon steel and alloy tool steel.

(3) Stress relief annealing heat treatment: This mainly involves heating the metal workpiece, holding it for a certain period of time, and then slowly cooling it (generally using furnace cooling). The purpose is to eliminate the internal stress generated during welding and cold straightening of the metal workpiece, so as to prevent deformation during subsequent processing and use. The stress relief annealing process is mainly applicable to various castings, forgings, welded parts and cold extruded parts, etc.

二、Normalizing heat treatment: Heat the metal workpiece to about 40 to 60 degrees Celsius above the temperature, hold it for a certain period of time, and then cool it in the air. The main purpose is to improve the organizational structure and cutting processing performance. For parts with low mechanical performance requirements, internal stress can be eliminated.

三、Quenching heat treatment: The main process involves heating the metal workpiece to the quenching temperature, holding it for a period of time, and then rapidly cooling it in water, brine or oil. Some metals are cooled in the air. The aim is to endow metal workpieces with high hardness and wear resistance, and to enable metal workpieces to acquire certain special properties after tempering, such as high strength, elasticity and toughness, etc.

（1）单液淬火热处理：主要是将金属工件加热到淬火温度，金属工件经过一定时间保温以后，在淬火剂中冷却。单液淬火只适用于形状比较简单、技术要求不太高的碳素钢及合金钢件。淬火时，对于直径或厚度大于5~8mm的碳素钢件，选用盐水或谁冷却，合金钢件选用油冷却方式。

（2）双液淬火热处理：主要是将金属工件加热到淬火温度，经过保温以后，先在水中快速冷却，然后移入油中冷却；

（3）火焰表面淬火热处理：用乙炔和氧气混合燃烧的火焰喷射到零件表面，使零件迅速加热到淬火温度，然后立即用水想零件表面喷射，火焰表面淬火适用于单件或小批量生产、表面要求硬而耐磨，并能承受冲击载荷的大型中碳钢和中碳合金金属工件，如曲轴、齿轮和导轨等；

四、回火热处理工艺方式：主要是将淬火后的金属工件加热到金属临界温度以下，保温一段时间后，然后在空气或油中冷却。回火热处理是紧接着淬火以后进行的。主要目的是获得所需的金属工件力学性能，在通常情况下，零件淬火后的金属强度和硬度均有很大提高，但塑性和韧性却有明显降低，而零件的实际工作条件要求有良好的强度和韧性。选择适当的回火温度进行回火后，可以获得所需的力学性能，可获得所需的力学性能，稳定金属工件组织，稳定金属工件尺寸，消除热处理残余内应力。

（1）低温回火热处理：将金属工件淬硬的金属工件加热，并在这个温度保温一定时间，然后在空气中冷却，低温回火热处理工艺方式常用于切削工具、量具、模具、滚动  轴承和渗碳零件等，主要目的是消除金属工件因淬火而产生的内应力。

（2）中温回火热处理：主要是将金属工件加热，经保温一段时间冷却下来，一般用于各种弹簧及热冲模等零件，主要目的是使金属工件得到较高的弹性、一定的韧性和硬度；

（3）高温回火热处理：将淬火后的金属工件加热到，经过保温以后冷却，主要用于要求高强度、高韧性的重要结构零件，如主轴、曲轴、凸轮、齿轮和连杆等，主要目的是使金属工件获得较好的综合力学性能，即较高的强度和韧性及足够的硬度、消除金属工件因淬火而产生的内应力。

五、调质热处理工艺方式：主要是将淬火后的金属工件进行高温回火多用于重要的结构零件，如轴类、齿轮、连杆等调质一般是在粗加工之后进行的，主要目的是细化晶粒， 使钢件获得较高韧性和足够 的强度，使其具有良好的综合力学性能。

六、时效热处理工艺方式，主要目的是消除金属工件热处理残余内应，减少零件变形，稳定尺寸

1）人工时效热处理工艺方式：主要是将淬火后的金属工件加热，经过长时间的保温，随后冷却；

（2）自然时效热处理：将铸件放在露天，金属工件放在海水中或长期悬吊或轻轻吊打，要经自然时效的零件，一般需要先进性粗加工；

七、化学热处理：将钢件放到含有某些活性原子的化学介质中， 通过加热、保温、冷却等方法， 使介质中的某些原子渗入到钢件的表层， 从而达到改变钢件表层的化学成分， 使钢件表层具有某种特殊的性能。

（1）金属工件的渗碳热处理：将碳原子渗入金属工件表层，常用于耐磨并受冲击的零件，如；齿轮、轴、轮、活塞销等，目的是使表面具有很高的温度和耐磨性，而中心仍保持高的韧性；

（2）金属工件的渗氮热处理：将将氮原子渗入钢件表层常用于重要的螺栓、螺母、销钉等零件；主要是提高金属工件表层的硬度和耐磨性‘

（3）金属工件的氰化热处理：主要是将碳和氮原子同时渗入到金属工件中，表层适用于低碳钢中碳钢或合金钢等零件，也可适用于高速钢工具，主要目的是提高金属工件表层的硬度和耐磨性；