a) Ductile iron: (Spherodial Graphite Iron, SG IRON)
Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron,
spherulitic graphite cast iron and SG iron, is a type of cast iron invented in 1943. While most
varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular
graphite inclusion.
Metallurgy:
Ductile iron is not a single material but is part of a group of materials which can be produced to
have a wide range of properties through control of the micro structure. The common defining
characteristic of this group of materials is the morphological structure of the graphite.
In ductile irons, the graphite is in the form of spherical nodules rather than flakes (as in grey iron),
thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its
name. The formation of nodules is achieved by addition of nodulizing elements, most commonly
Magnesium. Besides the requirement that the graphite be manipulated into the spheroidal shape,
the ferrite and pearlite ratios can be controlled through alloying, shakeout temperature control or
post-casting heat treatment to vary the relative amounts pearlite and ferrite from 0% pearlite and
100% ferrite, to 100% pearlite and 0% ferrite. The control of the pearlite and ferrite ratio manipulates
the tensile, yield and elongation characteristics of the ductile iron to produce numerous standard
grades of material.
Applications:
Ductile iron is specifically useful in many automotive components, where strength needs surpass that of aluminum but do not necessarily require steel. Other major industrial applications include off – highway diesel trucks, Class 8 trucks, agricultural tractors, and oilwell pumps.
Much of the annual production of ductile iron is in the form of ductile iron pipe, used for water and sewer lines. It competes with polymeric materials such as PVC, HDPE, LDPE and polypropylene,which are all much lighter than steel or ductile iron, but which, being flexible, require more careful installation and protection from physical damage.
b) GREY IRON:
Structure:
In grey cast iron the free graphite is in the form of flakes. The matrix may be ferritic for the lower strengths to pearlitic for higher strength.
Metallurgy:
The strength of a grey cast iron varies with the section size of the casting due to solidification effects. Also, the strength in compression is three to four times the tensile strength because of the planes of weakness created by the graphite flakes. The material tends to be brittle compared to steel, but is extremely stiff and deflects little before fracture. This imparts the characteristic damping qualities of grey cast iron. The graphite flakes also have a lubricating effect giving the material advantages in sliding wear applications.
Applications:
- Its easy to machine.
- Classic applications for grey cast iron are engine cylinder heads and cylinder blocks utilizing the thin and complex cast sections for water cooling passages and the damping characteristics for quietness.
- Gear boxes and gears are made in grey cast iron for quietness and the wear properties.
- Machine tools
- General engineering castings
- Many more