New Process of Ductile Iron Production
without Nodulizing Treatment

Since late 1940’s when it was invented, ductile iron is one of the materials that enjoyed the most rapid growth in industrial applications because of its attractive combination of the mechanical properties and excellent cast-abilities. In commercial practice, ductile iron has always been produced by treating a base iron melt with nodulizing elements such as magnesium, cerium or other rare earth elements. The conventional production method consumes a lot of natural resources. Tight process control is needed to prevent quality fluctuations due to variations of nodularization and inoculation. It has certain difficulties also in producing thick wall and thin wall castings.

Recently, VTT (Technical Research Centre of Finland, www.vtt.fi) has discovered that nodular graphite can be obtained from industrial grade iron melt by spray forming without adding any nodulizing or inoculation agent. Spray forming is a rapid solidification process, in which metal melt is atomized by gas into droplets of 10 - 200 microns in size, flying at subsonic speed onto a deposition substrate. The cooling rates are between 100 to 100,000 degrees per second, much greater than those in conventional casting solidification processes. Such high solidification rates lead to nodular graphite formation from normal grey iron melts.
Experiments were carried out with an Osprey type spray forming plant. The plant consists of a 50-kg induction furnace, an open tundish with a melt nozzle at the bottom and protruded into a 2-stage nitrogen atomizer, a spray chamber, and a cyclone for collecting over-spray powder. In the spray chamber, a horizontal ram manoeuvres a substrate, i.e. a ceramic mould to receive the spray deposition, as shown by Fig. 1. The sprayformed iron pieces are about 80 mm thick.

Wide ranges of melt compositions have been tested, as listed in Table 1. The melts were made from commercial grades of pig iron, steel scrap and ferro-alloys, melted in an induction furnace with N2 protection. No any melt treatment was carried out before tapping into the tundish for sprayforming. A typical microstructure of the sprayformed iron is shown in Fig. 2.

Table 1. Composition (wt%) ranges of the cast iron melts

The graphite nodules shown in Fig. 2 are about 3 µm thick, and 10 µm long, evenly distributed in a ferrite matrix. The nodularity is not as good as typical ductile iron, but it is much better than that of compacted graphite iron. Compacted graphite is, for example, about 10 µm thick, and 100 µm long.

The nodular graphite structure can be stably obtained, insensitive to the carbon equivalent and to the thickness of the deposition pieces, for the graphite nodules are formed due to the high cooling rate in the solidification achieved through the tiny droplets quenched in the atomising gas. There are potentials that the nodularity can be adjusted by changing the cooling rates and composition.

The invention created a new process for ductile iron production. It does not need any addition of nodulizing elements. It is more environmental friendly, and for certain applications, it has potential of economical benefits, too. Features of the process suggest many potential industrial applications, such as continuous production of cylindrical parts, bars, plates and pipes. The process could be combined with squeeze casting or semi-solid forging for mass production of automobile components, etc.

VTT intends to organize an international research program to extend the invention to mass production processes. The work contents include:

• To explore the limits of processing parameters: the impurity limit, cooling rates, atomizing gases.
• To develop different mass production methods and equipment for different types of products.

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