Heat treatment of gray cast iron

After heat treatment, gray cast iron can only change the matrix structure and cannot change the morphology of graphite, so it is impossible to significantly improve the mechanical properties of gray cast iron parts. The heat treatment of gray cast iron is mainly used to eliminate internal stress and white microstructure in castings, stabilize dimensions, improve cutting performance, and enhance surface hardness and wear resistance.

Grey cast iron castings

Annealing to eliminate internal stress

Used to eliminate casting stress caused by uneven cooling during solidification, and prevent deformation and cracking of castings. The process involves heating the casting to 500-600 ℃, holding it for a period of time, and then slowly cooling it in the furnace to below 150-200 ℃ before being air cooled. Sometimes, the casting is left in a natural environment for a long time to relax the internal stress of the casting. This method is called "natural aging" and can be used to eliminate casting stress for large gray cast iron parts.

Graphitization annealing

To eliminate white microstructure, reduce hardness, and improve cutting performance. The method is to heat the casting to 850-900 ℃, hold it for 2-5 hours, then slowly cool it in the furnace to 400-500 ℃, and then air cool it out of the furnace to decompose the cementite and form graphite during the holding and slow cooling process.

Surface quenching

Improve surface hardness and extend service life. For the working surfaces of gray cast iron parts such as machine tool guide rails and internal combustion engine cylinder liners, which require high hardness and wear resistance, surface quenching can be used. The commonly used methods include high (medium) frequency induction heating surface quenching and contact resistance heating surface quenching.