The present invention relates to a method for manufacturing mechanical components made of compacted graphite iron or gray cast iron.

Compacted graphite irons or gray cast irons of various types and with different structures are currently known and are particularly used to provide various types of mechanical components.

Compacted graphite iron or gray cast iron has, as its main characteristic, the form of the graphite, which is indeed in flakes or vermicular, differently from what occurs in classic spheroidal graphite irons. The particular shape of the graphite, in flakes or vermicular, gives the material good heat conductivity, higher than can be found in spheroidal graphite irons. This characteristic, combined with certain mechanical properties, allows use in operation even with cyclic stresses of a thermomechanical type. In particular, compacted graphite irons have better mechanical characteristics and higher ductility than gray cast irons. The latter are used in scarcely stressed applications, while compacted graphite irons are also used for relatively stressed components. In both cases, however, the particular configuration of the graphite does not allow to attain the mechanical strengths and the ductility that can be observed in spheroidal graphite irons.

Gray cast irons or compacted graphite irons subjected to a thermal treatment for normalization have a completely pearlitic matrix.

The normalized component is characterized by a higher wear resistance, but ductility is penalized further with respect to non-normalized ones.

In some cases compacted graphite irons or gray cast irons can be subjected to a thermal treatment for hardening in water or oil and have a bainitic or martensitic structure. They can be subjected optionally, at the end of the cooling, to a thermal treatment for tempering. These irons are usually characterized by a very low ductility accompanied by great surface hardness and accordingly are not used in applications requiring a certain fatigue strength. Moreover, as mentioned earlier, gray cast irons or compacted cast irons have the best conductivity among the various types of pig iron. This advantage, in the case of the cited treatments, since they are characterized by a transformation at a low temperature, close to or lower than the martensitic transformation point (Ms), can be utilized only in a certain temperature range, beyond which tempering of the microstructure occurs, with the corresponding loss of mechanical and hardness characteristics.

A material known as IDI has been developed on the basis of austempering technology. This material has a microstructure that offers mechanical and fatigue characteristics that are better than spheroidal graphite irons as cast and is obtained with a thermal treatment that renders it stable at high temperature.

The aim of the present invention is to provide a method for manufacturing gray cast iron or compacted graphite iron that allows to obtain a material that has higher mechanical characteristics than traditional gray cast irons or compacted graphite irons (ferritic, pearlitic, ferritic- pearlitic, etc.) and has a higher fatigue strength even in high-temperature conditions.

Another object of the invention is to devise a method that allows to obtain mechanical components that have a microstructure that is stable as the temperature increases, at least up to 550-600°C.

This aim, as well as these and other objects which will become better apparent hereinafter, are achieved by a method for manufacturing mechanical components made of compacted graphite iron or gray cast iron as provided in appended claim 1.

The present invention relates to a method for manufacturing mechanical components made of compacted graphite iron or gray cast iron.

In particular, the method provides for the following steps: - providing a casting of a mechanical component made of gray cast iron or compacted graphite iron having at least 50% of ferritic structure and having a carbon content comprised between 2.5% and 4.0% and a silicon content comprised between 1.0% and 4.0%;

- bringing the iron casting to a temperature that is higher than the lower austenitizing temperature (A _cl ) and lower than the upper austenitizing temperature (A _c3 ) for a time required to obtain an at least partially austenitic structure;

- performing a thermal treatment for isothermal hardening at a temperature comprised between 230°C and 400°C in order to obtain a matrix that has a substantially pearlitic-ferritic or perferritic structure.

In particular, as a consequence of the method according to the invention, a matrix with a substantially pearlitic-ferritic or perferritic structure having islands with ausferritic structure is obtained.

In particular, it has been found that it is particularly convenient for the ferrite percentage in the casting on which the thermal treatment is performed to be higher than 80%.

Moreover, it has been found experimentally that it is particularly advantageous, from the point of view of the typical mechanical characteristics of the components subjected to the method according to the invention, to start from gray cast iron or compacted graphite iron castings that have a ferrite percentage substantially equal to 100%.

In greater detail, it has been found that it is particularly convenient to perform the above cited thermal treatment for isothermal hardening preferably in a molten salt bath.

Advantageously, the temperature preferably used to perform isothermal hardening is comprised between 300 °C and 390 °C.

The temperature at which the mechanical components are maintained, as mentioned, during the partial austenitization step is comprised between the temperature identified technically as A _cl , above which the structure of the iron begins to transform into austenite, and the temperature identified technically as A _c3 , or complete austenitization temperature: in practice, by raising the part above the temperature identified technically as A _c3 one would have the complete transformation of the structure into austenite. If instead, as mentioned, the component is kept at a temperature that is intermediate between A _c3 and A _cl , not all the structure is transformed into austenite, but part of the ferrite remains as such (pro-eutectoid ferrite).

The choice of the temperature at which the partial austenitization is to be performed depends substantially on the quantity of austenite that one wishes to obtain at the end of the holding period at that temperature. It has been found advantageous to keep the components at a partial austenitization temperature that allows the transformation into austenite of a percentage comprised between 30% and 90% of the structure, preferably comprised between 75% and 85% and, in particular, substantially equal to 80% of the structure: this situation can be obtained by choosing a temperature that is approximately midway along the interval comprised between A _c3 and A _cl .

This can be achieved by choosing a temperature higher than 750 °C and lower than 820 °C and advantageously, depending on the carbon and silicon content, comprised between 780 °C and 810 °C.

These temperatures are indicative for irons having a carbon content of approximately 3.60% and a silicon content of approximately 1.50%, but obviously they can vary depending on the percentages of these elements in the casting to be subjected to thermal treatment.

The gray cast iron or compacted graphite iron with predominantly ferritic structure with which the initial casting is performed conveniently contains magnesium in a percentage comprised between 0.010% and 0.025%, preferably between 0.010% and 0.015%.

The present invention also relates to a mechanical component made of gray cast iron or compacted graphite iron and obtained by means of the method described above. EXAMPLE 1

Test pieces with a weight of approximately 1 kg, a diameter of 25 mm and a length of 200 mm were made with gray cast iron with a completely ferritic structure, having a carbon percentage of 3.60% and a silicon percentage of 1.52%.

Figure 1 is an optical microscope photograph related to the specimen.

A tensile strength of 104 N/mm ² was measured on the specimen.

Surface hardness was not measurable because it was too low for the measurement instrument.

The specimen was brought to a partial austenitization temperature, intermediate between A _cl and A _c3 , equal to 795 °C and held for a time equal to 60 minutes.

An isothermal hardening treatment in salt bath was then performed, holding the specimens for a time of 60 minutes at a temperature of 350 °C.

Figure 2 is an optical microscope photograph related to the specimen after the thermal treatment for isothermal hardening.

A surface hardness of 191 HBW and a tensile strength of 212 N/mm ² was measured on the specimen.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

In practice it has been found that the invention has achieved its intended aim and objects in all of the embodiments.

In practice, the dimensions may be any according to requirements. All the details may further be replaced with other technically equivalent elements.

The disclosures in Italian Patent Application No. 102015000037788 (UB2015A002456) from which this application claims priority are incorporated herein by reference.