Spherical graphite has little splitting effect on the matrix, so the mechanical properties of ductile iron mainly depend on the matrix structure. Therefore, heat treatment can significantly improve the mechanical properties of ductile iron.

① annealing

The casting internal stress of stress-relieving annealed ductile iron is about twice that of gray iron. For ductile iron castings without other heat treatment, stress relief annealing is needed.

The purpose of graphitization annealing graphitization annealing is to decompose free cementite in as-cast structure and precipitated cementite in pearlite, to obtain ductile iron with high plasticity and ferrite matrix, to eliminate casting stress and improve its machinability.

② Standardization

The purpose of normalizing is to obtain pearlite matrix structure, refine grains and improve the strength, hardness and wear resistance of ductile iron. Normalization can be divided into high temperature normalization and low temperature normalization.

High-temperature normalizing should adopt air cooling or even spray cooling for thick-walled castings to ensure pearlite ductile iron.

Low temperature normalizing is to heat the casting to 840 ~ 860℃, keep the temperature for 1 ~ 4h, and take it out of the furnace to obtain pearlite+ferrite matrix ductile iron.

Ductile iron has poor thermal conductivity and large internal stress after normalizing. Therefore, stress relief annealing should be carried out after normalizing.

③ Isothermal quenching

When the casting shape is complex and requires high strength, good plasticity and toughness, isothermal quenching is needed. Isothermal quenching is to heat the casting to 860 ~ 920℃ (austenite zone), keep the temperature properly (heat penetration), quickly put it into a salt bath furnace at 250 ~ 350℃ for isothermal treatment for 0.5 ~ 1.5h, and then take it out for air cooling to transform undercooled austenite into lower bainite. Isothermal quenching can prevent deformation and cracking and improve the comprehensive mechanical properties of castings. It is suitable for ductile iron castings with complex shape, easy deformation, small section size, complex stress and good comprehensive mechanical properties, such as gears, crankshafts, rolling bearing rings, camshafts, etc.

④ quenching and tempering treatment

Quenching and tempering treatment is a heat treatment method of heating the casting to 860 ~ 920℃, oil cooling after heat preservation, and then tempering at 550 ~ 620℃ for 2 ~ 6h to obtain tempered sorbite and spherical graphite structure. High strength and toughness can be obtained by quenching and tempering treatment, which is suitable for castings with complex stress, large cross-sectional size and high comprehensive mechanical properties, such as crankshaft, connecting rod and other important parts of diesel engine.