After heat treatment, gray cast iron can only change the matrix structure, but not the shape of graphite, so it is impossible to obviously improve the mechanical properties of gray cast iron. Heat treatment of gray cast iron is mainly used to eliminate internal stress and white structure of castings, stabilize dimensions, improve machinability, and improve surface hardness and wear resistance.

Stress relief annealing

Used to eliminate casting stress caused by uneven cooling during solidification and prevent casting deformation and cracking. The process is to heat the casting to 500 ~ 600℃, keep the temperature for a period of time, then slowly cool it with the furnace to below 150 ~ 200℃, and then take it out for air cooling. Sometimes, the castings are left in the natural environment for a long time to release the internal stress of the castings. This method is called "natural aging" and can be used to eliminate the casting stress of large gray iron castings.

graphitizing annealing

In order to eliminate white structure, reduce hardness and improve cutting performance. The method is to heat the casting to 850 ~ 900℃, keep the temperature for 2 ~ 5 hours, then slowly cool it to 400 ~ 500℃ with the furnace, and then take it out of the furnace for air cooling, so that the cementite decomposes to form graphite during the slow cooling process.

case hardening

Improve surface hardness and prolong service life. For example, for the working surfaces of gray cast iron castings such as the surface of machine tool guide rail and the inner wall of cylinder liner of internal combustion engine, it is necessary to have high hardness and wear resistance, and the surface quenching method can be adopted. The commonly used methods are high (medium) frequency induction heating surface quenching and contact resistance heating surface quenching.