Spheroidal graphite cast irons - Application properties

Applications for spheroidal graphite cast irons have been growing since the end of the Second World War.

They have replaced steel, both cast and wrought, and, of course, the malleable cast irons from which they are "intellectually" derived.

The idea of graphitizing carbon in globular rather than lamellar form is indeed a logical deduction from the observation of black-core malleable cast irons.

It's worth remembering that the mechanical and physical properties of cast irons depend on their metallic structure, which contains free carbon. When this carbon is present in its natural, lamellar form, it contributes to the fragility of the whole, as the lamellae, as a solution of continuity, constitute so many rupture initiators. If, on the other hand, the carbon is "collected" in the form of spheroids or nodules, its influence on the behavior of a less embrittled assembly (disappearance of continuity solutions) will be greatly attenuated, leading to the disappearance of embrittlement and an improvement in elongation and fatigue behavior.

This brings us closer to certain properties of steels with the same metal matrix: ferritic, pearlitic, austenitic, martensitic, even bainitic. At the same time, retaining free carbon gives cast irons undeniable advantages:

• good compressive strength, allowing a degree of elasticity;

• in certain environments, good behaviour with regard to corrosion, whose progress is slowed down, or even stopped, by the carbon, which then acts like a filtering mesh ;

• lastly, fatigue behavior, especially for ferrite-pearlitic cast irons, is more than satisfactory: for proof of this, we need only point out that vehicle suspension arms are made of spheroidal graphite cast irons on a daily basis.

Economically, spheroidal graphite cast irons are also attractive, for the following reasons:

• They have casting qualities comparable to those of lamellar cast irons, which are the least expensive of all metal products;

• their pouring temperatures and castability are very similar, which means that they require less work than cast steels with higher pouring temperatures;

• although their shrinkage is somewhat greater than that of lamellar cast irons, it remains lower than that of steels, which reduces the risk of cracking, even if it is prudent to use weights to combat any shrinkage.

As a result, spheroidal graphite cast irons are gaining ground in all industrial sectors, and in all countries, as better control of their production...