正火，又称常化，是将工件加热至Ac3或Accm以上30~50℃，保温一段时间后，从炉中取出在空气中或喷水、喷雾或吹风冷却的金属热处理工艺。其目的是在于使晶粒细化和碳化物分布均匀化。正火与退火的不同点是正火冷却速度比退火冷却速度稍快，因而正火组织要比退火组织更细一些，其机械性能也有所提高。另外，正火炉外冷却不占用设备，生产率较高，因此生产中尽可能采用正火来代替退火。⑥用于球墨铸铁，使硬度、强度、耐磨性得到提高，如用于制造汽车、拖拉机、柴油机的曲轴、连杆等重要零件。目的是降低硬度，改善切削加工性；消除残余应力，稳定尺寸，减少变形与裂纹倾向；细化晶粒，调整组织，消除组织缺陷。
Normalizing, which is also called normalizing, is a metal heat
treatment process that is heated to Ac3 or more than Accm at 30~50 C.
After a period of heat preservation, it is removed from the furnace in
the air or by water spray, spray or blow cooling. The aim is to refine
the grains and homogenate the carbide distribution. The difference
between normalizing and annealing is that the normalizing cooling rate
is slightly faster than that of annealed cooling, thus the normalizing
tissue is finer than the annealed tissue, and its mechanical properties
are also improved. In addition, the cooling outside the normalizing
furnace does not occupy the equipment and has higher productivity.
Therefore, normalizing is used instead of annealing as far as possible
during the production. It is used for ductile iron to improve hardness,
strength and wear resistance, such as crankshaft, connecting rod and
other important parts for automobile, tractor, diesel engine. The aim is
to reduce the hardness and improve the machinability, eliminate the
residual stress, stabilize the size, reduce the deformation and crack
tendency, refine the grain, adjust the structure and eliminate the
defects of the tissue.
