Title: "A process for producing a superalloy and superalloy obtained by said process’*

Field of the invention

The present invention relates to a method of producing metal superalloys as defined in the preamble of claim 1.

Background art

The term superalloys is intended to designate materials that are able to withstand high temperatures, possibly above 1000°C, and are mainly composed of Iron and Nickel, with the addition of variable amounts of Chromium, Cobalt, Niobium, Titanium and other elements.

Superalloys are known as materials that must have a high degree of purity with respect to elements that are considered as impurities, such as, for example, Sulfur (S), Lead (Pb) and Bismuth (Bi) whose presence would jeopardize the mechanical, corrosion-resistance and thermal performances required of superalloys.

Materials having the aforementioned performances may be used for a variety of applications namely, but without limitation, in the aircraft industry, for example for the manufacture of propellers and rotor blades for turbojets.

Such materials are produced using many types of manufacturing methods including the steps of melting, cooling in suitable ingot molds, further melting and refining, for improving mechanical performances but also for reducing harmful element, namely Sulfur, Lead and Bismuth, as mentioned above.

In current methods, the production of superalloys by a“triple melt” process includes causing a charge of base materials, whose total amount by weight does not exceed twenty-five tons, to undergo a first melting step in a vacuum induction furnace, known as Vacuum Induction Melting (V.I.M.) furnace, then casting the melted material into round ingot molds from which the ingots are transferred to a second melting step, known as E.S.R. (Electro Slag Remelting) which attempts to eliminate as many impurities as possible, particularly the amount of S (Sulfur), Pb (Lead), Sn (Tin) and Bi (Bismuth). Such second melting step is followed by a third melting step, known as V.A.R. (Vacuum Arc Remelting).

Thus, this process is known to include three melts of the initial charge, with impurity removal being carried out in the second melt, known as ESR (Electro Slag Remelting).

Prior art drawbacks

Despite the provision of three melting steps, one of which is E.S.R. , the final result of the process is the production of a superalloy in which impurities, albeit in minor amounts, are present in uneven amounts in the commercial product, as they differ between charges of materials obtained from the initial melt V.I.M., each not exceeding twenty-five tons, as is known in the art.

Also, this quantitative limit imposed on the charge does not allow the use of a much more effective purification procedure known as A.O.D. (Argon Oxygen Decarburization), which would lead to a further reduction of the impurities, because a charge amount of about twenty-five tons, as mentioned above, is insufficient to accommodate the reactions required to remove impurities such as S, Pb, Sn and Bi in the AOD procedure.

Object of the Invention The object of the present invention is to provide a triple-melt method for the production of superalloys, which can provide a high degree of removal of S-, Sn-, Pb- and Bi-based impurities in the final product, as well as a high degree of evenness thereof, both in a single charge and in different charges of the material that composes the superalloy.

These and other objects, as better explained hereafter, are fulfilled by a method of producing a superalloy characterized by claim 1 hereinafter.

Advantages of the Invention

The present invention can provide a method of producing a superalloy that results in a superalloy having a high homogeneity due to turbulent stirring of the liquid bath caused by gases blown during the A.O.D. process.

Furthermore, the present invention can provide a method of producing a superalloy that results in a superalloy having extremely high levels of desulfurization (less than 5 ppm sulfur) and deoxidation, minimization of impurities (Bi and If are volatilized and reduced to levels of less than 1 ppm) and high inclusion cleanness.

Also, the present invention can provide strict control and high reproducibility of the chemical composition in the method of producing a superalloy due to the stoichiometry of the reactions involved.

Finally, the present invention can provide a method of producing a superalloy having a highly stable remelting rate and minimized gas, inclusions and segregation.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will appear more clearly from the illustrative, non-limiting description of a preferred, non- exclusive embodiment of a method of producing superalloys, as well as superalloys obtained with the method as shown in the accompanying Figure 1 , which shows a flowchart of the production of a superalloy according to the present invention.

DETAILED DESCRIPTION

Even when this is not expressly stated, the individual features as described with reference to the particular embodiments shall be intended as auxiliary to and/or interchangeable with other features described with reference to other exemplary embodiments.

Referring to the accompanying figure, numeral 1 designates the method of producing a metal superalloy 10, i.e. a metal alloy that is mainly composed of Iron and Nickel with the addition of variable amounts of Chromium, Cobalt, Titanium and other elements.

In particular, the method 1 is a triple-melt method including, as further explained below, a triple-melt process for melting and remelting a charge of base material 2.

The method 1 comprises a first step 3 in which the aforementioned charge of materials 2 is provided in an amount ranging from forty to sixty tons, preferably of fifty tons.

This first step 3 includes melting the aforementioned charge of materials in an electric-arc furnace to obtain a first melt 3A.

The electric-arc furnace is a conventional furnace and will not be further described.

Once the first melt 3 A of the entire charge of materials 2 has been obtained by means of the electric-arc furnace 3, the same melt, in a liquid state, undergoes an A.O.D. (Argon Oxygen Decarburization) treatment 4, which is known in the art and will not be further described.

In other words, the first melt 3A molten by the electric furnace 3 undergoes the A.O.D. treatment 4 to obtain a refined first melt 4A.

According to an advantageous aspect of the present method, the treatment A.O.D. treatment 4 is carried out on the first melt 3A when the first melt 3A is still in the liquid state.

Namely, the A.O.D. treatment 4 can provide a decarburized and refined melt 4 A with minimized impurities such as S, Pb and Sn and extremely high deoxidation.

It shall be noted that the first melt 4 A has the same amount as the charge of materials 2 introduced into the electric furnace 3, that is if the charge of materials 2 is fifty tons then also the first melt 4A. will be fifty tons.

During the A.O.D. treatment 4, the liquid mass of the first melt 3A is subjected to vigorous stirring due to gases (argon, nitrogen and oxygen in varying proportions depending on the stage of the A.O.D. treatment) blown during treatment, which imparts high homogeneity to the first melt 4A.

The A.O.D. treatment 4 is followed by a step of solidification 5 of the first melt 4A.

In one aspect the solidification step 5 comprises a step in which the melt is cast into ingot molds and later cooled to obtain ingots 5 A, preferably having a cylindrical shape.

Then, the ingots 5A undergo melting in a V.I.D.P. (Vacuum Degassing Induction and Pouring) furnace 6 to obtain a second melt 6A.

At the end of the treatment in the V.I.D.P furnace 6, the second melt 6 undergoes a step of solidification 7, still in the V.I.D.P furnace.

In one aspect the solidification step 7 comprises a step in which the melt is cast into ingot molds and later cooled to obtain ingots 7A, preferably having a cylindrical shape.

The ingots 7A as obtained from the solidification step 7 undergo melting in a V.A.R. (Vacuum Arc remelting) furnace 8 to obtain a third melt 8A.

Once melting in the V.A.R. furnace 8 is completed, the third melt 8A undergoes a step of solidification 9 in the V.A.R. furnace to obtain the metal superalloy 10.

The superalloy 10 obtained by the triple-melt 4, 6 and 8 later undergoes homogenization, thermomechanical processing and thermal treatment in prior art plants and with prior art equipment.

Namely, after the solidification step 9, the superalloy 10 undergoes thermomechanical processing, which comprises a press forging step, preferably with a hydraulic press.

If a thickness of less than 250-300 mm is desired, after the solidification step and the press forging step, the superalloy 10 undergoes additional thermomechanical processing which comprises a radial four-die forging step using a hydraulic RUMX machine, and a continuous radial deformation forging step in a rolling plant, preferably of hydraulic type.

This superalloy 10 has a high chemical homogeneity and uniform mechanical, thermal and corrosion-resistance properties, as required for their use.

The superalloy 10 obtained with the method 1 is found to have high chemical homogeneity, a very high degree of desulfurization with less than 5 ppm residual sulfur, an equally high degree of deoxidation and Bi and Se impurity levels of less than 1 ppm.

This is believed to result from the fact that the initial charge 2 ranging from forty to sixty tons establishes the chemico-physical conditions required for an effective A.O.D. Treatment 4, leading to a high impurity elimination degree. In addition, the A.O.D. treatment after the first melt 3 A is carried out with a high turbulence of the molten mass generated by blowing process gases, which results in a high homogeneity of chemical process reactions, and hence of the chemical composition of the molten alloy.

According to a variant embodiment of the method of the invention, the charge that leaves the A.O.D. treatment 4, for the subsequent V.I.D.P. and V.A.R. melting steps 6, 7, may be divided into multiple portions depending on the required supply of the alloy 10, thereby ensuring highly homogeneous supplies of the material even after some time.

Those skilled in the art will obviously appreciate that a number of changes and variants as described above may be made to fulfill particular requirements, without departure from the scope of the invention, as defined in the following claims.