The present report is about a vermicular cast iron, of high machinability and free from microporosity.

More specifically, a vermicular cast iron of high machinability and free from microporosities that comprises classes between 350 and 500 MEGA PASCAL of ultimate tensile strength.

As known to experts in the area, the vermicular cast iron represents a group of Fe-C-Si alloys, with Mn, P, S, and controlled additions of Mg, presenting graphite predominately in vermicular form (form IV according to Standard ASTM A247), with up to 20% of nodules (forms I and II as per Standard ASTM A247) in its microstructure. The addition of complementary pearlitic elements, such as Mn, Cr, Cu and Sn allow one to obtain the desired class, which can vary between 350 and 500 Mega Pascal of ultimate tensile strength.

In some cases, additions of Ti are also carried out to increase the resistance to wear out and to improve the lubrication of the cylinder in engine blocks.

Vermicular cast iron has been increasingly applied in automotive components such as internal combustion engine blocks, internal combustion engine heads, disks and brake drums. In general, its application involves the substitution of gray cast iron to increase tensile strength.

This substitution, however, brings some manufacturing problems not yet entirely solved such as the

tendency to form solidification porosities and low machinability of the part.

The said tendency to form solidification porosities results in blanks at places of mass concentration that are usually the last places to solidify. This phenomenon occurs due to the part volumetric shrinkage which takes place when going from a liquid condition to a solid condition.

This problem is partially solved with the use of feeders at places of mass concentration which is not always possible in parts of high geometric complexity such as engine blocks and heads.

The low machinability of conventional vermicular cast iron is already known and reflects in the decrease of a tool life when compared to gray cast iron machining. This decrease depends on the particular machining operation, but in general is not lower than 50%, as in milling and boring operations; it could, though, reach a 95% decrease in the life of the tool in cylinder hole reaming operations.

One of the goals of the present invention is to provide vermicular cast iron of high machinability, free from microporosities that allows the increase in the life of tools to be used with the cast part together with the innovated characteristics.

This and other goals and advantages of the present invention are achieved with the vermicular cast iron of high machinability, free form microporosities that

presents a bigger amount of graphite per volume of vermicular cast iron; this increase being the amount of graphite obtained by the addition of nodules in the microstructure so that the percentage of those nodules be also increased in relation to the volume in the conventional vermicular cast iron.

Particularly, the present invention proposes a vermicular cast iron of high machinability and free from microporosities that presents an amount of graphite between 12-15% in volume of vermicular cast iron; the increase in the amount of graphite is obtained with the addition of nodules to the microstructure, resulting in an amount between 20-40% in the volume of vermicular cast iron.

With this increase in the amount of graphite it is possible to counterbalance the contraction of solidification, producing parts that are free from solidification porosities. At the same time, this increase in the amount of graphite results in the improvement of machinability due to the lubrication effect that the graphite exerts on the cutting tool.

On the other hand, as the increase of the amount of graphite occurs by the addition of a percentage of nodules, the tensile strength properties are not decreased.

In addition, the machinability is improved, if compared to the conventional vermicular iron, with the limitation of elements that form carbide or carbonitrides (Cr, Mn, Ti) and with the formation of pearlite between 50 and 100% pearlite, according to the class (350 to 500 Mega

Pascal), lamellar spacing bigger than 0,5 microns, which is obtained under adequate part cooling at a temperature ranging between 800° and 650°C.

Therefore the vermicular cast iron of high machinability and free from microporosities has a content of carbon between 3,6 and 3, 9%, a content of Si between 2,0 and 2, 6%, a content of Cr lower than 0, 05%, a content of Mn lower than 0,40% and a content of Ti lower than 0,015 %.

The machinability advantages aforementioned are thus increased.

Despite the fact that a preferred constructive concept has been described and illustrated, it is important to mention that changes are feasible and possible to be carried out without deviating from the scope of the present invention.