What are "magnesium-treated irons?"


Shareholder, name withheld

The conventional cast iron used for cylinder blocks and heads, grey iron, is not treated with magnesium. Grey iron is characterised by having the graphite particles in the shape of elongated flakes. These flakes provide good thermal conductivity and vibration damping, but also make the iron relatively weak and brittle.

The addition of magnesium changes the shape of the graphite particles. A small addition (approx. 0.010~0.015% Mg) changes the flakes into compacted graphite. A further addition to approximately 0.035~0.055% Mg will cause the graphite particles to grow as spheres, as in ductile iron. CGI typically has less than 20% of the graphite particles present as nodules (>80% compacted) while ductile iron typically requires that more than 90% of the graphite particles are spheroidal. The differences between grey iron, CGI and ductile iron, and the foundry production processes are explained in more detail in the "CGI section" of our web site.

The term "magnesium-treated irons" would therefore refer to the larger ’family’ of CGI, ductile iron, and everything in between (ie, 20~90% nodularity). The term "magnesium-treated iron" is not common in the industry and it is better to use the accepted names of either CGI or ductile iron to avoid confusion. While there may be some confusion among the general public, the discussion between foundries and their customers are very clear. The automotive companies specify their material requirements in great detail, including the graphite shape requirement, minimum material properties, chemistry limits and test locations within the casting. A normal specification is at least two pages in length and its purpose is to avoid any possibility of confusion.

The differences between CGI and ductile iron are completely understood in the industry. CGI will be used for components that require a combination of strength and thermal loading while ductile iron will continue to be used for mechanical components such as suspension parts and steering knuckles. The heat transfer ability of irons decreases quickly as the nodularity increases above 20%. Therefore, materials with intermediate nodularity (20-60%) will generally be limited to non-thermal applications such as bedplates.