1. What is ductile iron?

Ductile iron is a type of cast iron with the most compact graphite structure in the form of round spheres  , so it is called ductile iron, therefore it has the highest durability among the types of cast iron containing graphite (graphite or graphite). Spherical graphite is modified by the elements Mg, Ce and rare earth elements.

Ductile iron has English names such as

• Ductile iron
• Ductile cast iron
• Nodular cast iron
• Spheroidal graphite iron
• Spheroidal graphite cast iron
• SG iron

Ductile iron has the same dark gray appearance as gray cast iron, so it is impossible to distinguish between the two types of cast iron with the naked eye, although ductile iron is more expensive than gray cast iron.

Because ductile iron contains more expensive raw materials . But it is a more economical choice than gray cast iron. Because ductile iron castings are often lighter in the same size and design. So overall, ductile iron castings have better quality, but the price is not much different, sometimes even cheaper.

2. What is the standard of ductile iron?

Ductile iron standards are regulated by each country such as the US, China, Japan, France, Vietnam, etc. and different standards such as ASTM, ISO, etc. Accordingly, the composition and content of chemical components in ductile iron, as well as the tensile strength and elongation indexes are also different.

Below are some examples of ductile iron grades and a table of ductile iron standards.

• According to Japanese JSC standards: Ductile iron has the following grades: FCD 450-10 , FCD 500-7 , FCD 600-3 , FCD 700-2 .
• According to American standards:
  • According to SAE standards, there are the following grades of ductile iron: D4018 , D4512 , D5506 , D7003 . In which the first two digits indicate the unit KSI (kilopounds per square inch), the last two digits indicate the unit %.
  • According to ASTM standards, we have cast iron grades: 60-40-18 , 65-45-12 , 80-55-06 , 100-7-3 , 120-90-2 . In which the numbers indicate the minimum values ​​of yield stress , tensile stress and % elongation respectively.
• Russian standards: CЧ10 , CЧ15 ,.. . (Symbol CЧ) is read as Cê-che, meaning cast iron. Cast iron grades have average strength, σb = 150 – 200MPa, with relatively rough ferrite – pearlite base such as CЧ 20 , CЧ25 , used for light load-bearing, low-wear parts such as gear box housings, machine bodies, flanges, and water pipe fittings.
• According to Vietnamese standards: Ductile iron according to TCVN is symbolized by two letters GC   (Abbreviation of Ductile iron ) with two pairs of numbers indicating the minimum value of tensile strength and ductility of the cast iron . Accordingly, ferritic ductile iron grade GC40-10 has a tensile strength > 400 MPA and a relative elongation of 10% . According to TCVN 5016-1987 standard, we have a typical cast iron grade: Ductile iron grade GC45-5 has 2 values ​​expressed as mechanical tensile strength >= 400 MPA and elongation of 5%.
• According to British – European standard BS EN 124: There are popular cast iron grades FCD 500-7 . In which, the values ​​are shown respectively as load-bearing force and tensile strength o. The tensile strength of cast iron grade FCD 500-7 >= 500 MPa and the corresponding elongation is 7%.

3. Characteristics of ductile iron

The microstructure of nodular cast iron is similar to that of gray cast iron, except that its graphite has the most compact spherical form, consisting of three types of metal matrix: ferrite, ferrite-pearlite and pearlite. This determines the very high tensile strength of nodular cast iron compared to gray cast iron.

• Tensile strength: σb = 400 ~ 800 Mpa
• Yield limit: σ0.2 = 250 ~ 600 Mpa
• Ductility: δ = 2 ~ 15%
• Toughness: aK = 300 ~ 600 kJ/m2

Although the mechanical properties of ductile iron are inferior to steel, they are much higher than gray cast iron. Some tensile strength and yield strength indices are considered equivalent to machine-made carbon steel. Therefore, ductile iron is used in industrial valves such as: cast iron globe valves , cast iron gate valves , cast iron butterfly valves, etc.

Although it is very difficult to distinguish between ductile iron and gray cast iron with the naked eye, we can rely on the signs of shrinkage in ductile iron products (ductile iron easily forms concave and porous shrinkage), or by tapping on a product cast from ductile iron, there will be a clear and loud sound (very resonant), while tapping on a product cast from gray cast iron will have a dull, low sound.