High strength cast iron: graphite spherical

Ductile iron formally called nodular cast irons, as, eg, in GOST 7293-85. These cast irons are an example of the triumph of metallurgical science in the 1940s.. In English they are called a little differently.: ductile cast iron - ductile or ductile cast irons.

Magnesium or cerium additives

Metallurgical scientists discovered, that by adding very small amounts of magnesium or cerium to the cast iron, as well as maintaining a low level of sulfur and phosphorus impurities in it, you can change the shape of graphite from interconnected petals-plates, which are typical for gray cast iron, in isolated spheres or balls. They are sometimes also called spheroids. (picture 1).

struktura-chuguna-opticPicture 1 - Structure of ductile iron (nodular cast iron). Optical microscope, 100x

On the picture 2 micrograph presented, taken by an electron microscope after polishing and etching a ductile iron specimen, to reveal graphite spheroids. This micrograph shows, that the graphite surface in these cast irons is very smooth and spherical, in contrast to graphite in ductile irons.

elektronnyy-microPicture 2 - Deep etching of ductile iron specimen.
Electron microscope, 950x

The amount of magnesium in cast iron, which is necessary for the formation of nodular graphite, is only from 0,02 to 0,1 % magnesium, and cerium - from 0,02 to 0,04 %. Cerium is a rare earth chemical element with an atomic number 58. Since cerium is much more expensive than magnesium in industry, magnesium is used in the production of ductile iron.

Spheroidal graphite growth

Like gray and white cast irons, ductile iron begins to solidify with the formation of austenitic dendrides.. Graphite spheroids grow directly from the interdendride fluid and are not bonded to each other. Their formation occurs as a result of a special eutectic growth of graphite in austenite.

scheme-formation-spheroidsPicture 3 - Microstructure formation diagram
when solidifying nodular cast iron

Spheroids on 100 % composed of carbon in the form of graphite. The spheroid structure is a set of groups of parallel plates, which are located around its center (cm. picture 1). The spheroid diameter is much smaller, than the structural cell of gray cast iron - in one cell of gray cast iron about 200 nodular spheroids.

Mechanical properties of cast irons according to GOST 7293-85

The mechanical properties of high-strength cast irons are comparable to those of high-carbon steels. Since the graphite spheroids are not bonded and, Besides, have smooth surfaces, means, that they are not stress concentrators.

Minimum GOST requirements 7293-85 to the mechanical properties of the most durable nodular cast iron – cast iron grade VCh 100 - make up:
– tensile strength – 1000 MPa;
– yield point - 700 MPa;
– relative extension - 2;
– Brinell hardness – 270 NV.

The phase structure of the cast iron matrix

Ductile iron phase structure - matrix structure – like gray iron, control the cooling process, addition of alloying elements immediately before casting and heat treatment after casting. As cast, nodular cast iron can be ferritic, pearlite or, more often, some mixture of pearlite and ferrite. On the picture 3 presented a micrograph of nodular cast iron, whose structure is a mixture of a small amount of ferrite-pearlite structure and a predominantly white ferrite structure.

matrica-chugunaPicture 4 - Nodular cast iron with matrix structure predominantly
ferrite. Optical microscope, 250x

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007