Ductile iron casting is highly commended in the casting manufacturers because of its strength and corrosion resistance.Ductile iron foundry often take several methods to improve overall performance:
1. Eliminate white cast chilling annealing
Ordinary gray cast iron or ductile casting surface for cooling speed too fast or thin walled in casting process appear white cast,iron can not machining.To eliminate chilling reheated reduce hardness often will this kind of cast iron parts above the eutectoid temperature (generally 880 ~ 900 ℃),insulation and 1~2 h on annealing,cementite decomposition into graphite,then iron slowly cooled to 400 ℃ to 500 ℃ from air cooling.In 700-780 ℃ temperature,namely the eutectoid temperature near the unfavorable cold speed too slow,too much so that cementite into graphite, reduces the strength of cast iron parts.
2. Improve the toughness of ductile cast iron annealing
Nodular cast iron in the process of casting the chilling tendency of ordinary gray cast iron,internal stress is also bigger,cast iron or hard to get pure ferrite pearlite matrix,in order to improve the ductility or toughness of cast iron parts,often used the iron to heat to 900-950 ℃ high temperature annealing and heat preservation time, furnace cooling to 600 ℃ after getting cold again. Cementite decomposition of matrix in the process of the graphite,the precipitation in the austenite,graphite, gathered in the graphite spheroidal graphite surrounding,matrix into ferrite.
3. Improve the strength of ductile cast iron is fire
Ductile cast iron is the purpose of normalizing will be converted into fine pearlite matrix organization. Process is the substrate of ferrite and pearlite ductile iron pieces to heated to 850-900 ℃ temperature,the original ferrite and pearlite transformation to austenite, and a portion of the spheroidal graphite dissolves in austenite,air cooling after heat preservation austenitic transformation of fine pearlite, thus the strength of the castings.
4. Nodular cast iron quenching and tempering treatment
As bearing ductile iron castings need higher hardness, often will cast iron quenching and tempering treatment at low temperature.Ductile cast iron as the axle parts,such as diesel engine crankshaft,connecting rod,high strength and toughness good comprehensive mechanical machinery performance,conditioning treatment on cast iron pieces.
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