Gray cast iron refers to cast iron containing flake graphite. It is called gray cast iron because its fracture surface is dark gray. The main components of gray cast iron are iron, carbon, silicon, manganese, sulfur, and phosphorus. It is the most widely used type of cast iron, accounting for over 80% of total cast iron production.
Ductile iron is produced by spheroidizing and inoculating graphite to obtain spheroidal graphite, effectively improving the mechanical properties of cast iron, especially its plasticity and toughness, thus achieving strength higher than carbon steel. Developed in the 1950s, ductile iron is a high-strength cast iron material with comprehensive properties approaching those of steel. Based on its excellent properties, it has been successfully used to cast parts subject to complex stresses and requiring high strength, toughness, and wear resistance. Ductile iron has rapidly developed into the second most widely used cast iron material after gray cast iron.
The castings for motor products are mainly made of gray cast iron, but ductile iron is used for high-voltage motor bearing sleeves and end covers for special purpose motors. However, ductile iron is relatively more expensive than gray cast iron.
The difference between cast iron and ductile iron
● Graphite shape. In gray cast iron, graphite exists in flake form, while in ductile iron, graphite exists in spherical form.
●Performance differences. The flake graphite in gray cast iron fractures the matrix, causing stress concentration, resulting in low strength and plasticity, and high brittleness, but good vibration damping performance. It is mainly used to produce various boxes, bases, etc., that do not require high strength and mainly bear compressive stress, such as small and medium-sized motor bases and end covers.
In ductile iron, the spherical graphite minimizes the cutting effect on the matrix and reduces stress concentration, resulting in high strength comparable to medium carbon steel. It fully utilizes the matrix's properties and possesses good plasticity and toughness, making it ideal for manufacturing complex-shaped parts requiring high strength and toughness. Furthermore, its casting performance is superior to steel. Examples include crankshafts and connecting rods in internal combustion engines.
●Strength Properties. Ductile iron can generally be strengthened through heat treatment, while gray cast iron generally cannot be heat-treated to improve its strength. Compared to cast iron, ductile iron has an absolute advantage in strength. The tensile strength of ductile iron is 60k, while that of cast iron is only 31k. The yield strength of ductile iron is 40k, while cast iron does not exhibit yield strength and eventually fractures. The strength of ductile iron is far superior to that of cast iron.
●Corrosion resistance. Ductile iron has the same corrosion resistance as cast iron.
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