Ductile iron castings require higher hardness as bearings, and iron castings are often quenched and tempered at low temperature. The process is: the casting is heated to a temperature of 860-900 ℃, the original matrix is ??completely austenitized, and then cooled in oil or molten salt to achieve quenching, and then heated at 250-350 ℃ for heat preservation and tempering, the original matrix is ??converted into a return Fire martensite and retained austenite structure, the original spherical graphite shape remains unchanged. The processed castings have high hardness and certain toughness, retain the lubricating properties of graphite, and improve wear resistance. As shaft parts, ductile iron parts, such as crankshafts and connecting rods of diesel engines, require comprehensive mechanical properties with high strength and good toughness. The iron castings are quenched and tempered. The process is: the cast iron is heated to a temperature of 860-900°C to keep the matrix austenitized, then cooled in oil or molten salt to achieve quenching, and then tempered at 500-600°C to obtain tempered sorbite structure (Generally, there is still a small amount of lumpy ferrite), the shape of the original spherical graphite remains unchanged. After treatment, the strength and toughness match well, which is suitable for the working conditions of shaft parts.