Technical field

The present invention relates to a method for manufacturing mechanical components made of particularly wear-resistant austempered spheroidal cast iron.

Background Art

Currently, austempered spheroidal cast irons of various types and having different structures are known and are used in particular for manufacturing various types of mechanical components.

The thermal treatment required in order to obtain this type of cast iron consists of a treatment of complete austenitization, with holding of the component at a temperature higher than the upper austenitization limit temperature (usually designated by A _c3 ) followed by quenching in a molten salt bath.

The final structure that is obtained, technically termed ausferritic structure, is composed of acicular ferrite and austenite. This particular structure gives the material high mechanical characteristics.

Since it is essential to prevent the forming of pearlite during cooling, it is necessary to alloy the material with alloying elements, such as nickel and/or molybdenum.

With reference to international standard ISO 17804:2005 "Founding - Ausferritic spheroidal graphite cast irons - Classification", Table 1 and Annex C, the mechanical characteristics vary with the structure and therefore with the hardness. In particular, as the hardness increases, so does the minimum tensile strength, while the minimum elongation decreases. Similar relations are found in standard ASTM A 897/A 897/M - 06 "Standard Specification for Austempered Ductile Iron Castings" and in other standards of this family of materials. It is known that, in general, all grades of austempered spheroidal cast irons represented in the above mentioned standard exhibit a good resistance to wear, without the need for specific surface treatments. This characteristic is not explicitly highlighted in the standard, since determining it is difficult and not standardized.

The phenomenon, which has long been described extensively in literature, has to be ascribed to the combined effect of two competing characteristics:

- the proportion of acicular ferrite in the matrix and its hardness

- the transformation of part (up to 8%) of the austenite in martensite, as a consequence of a pressure (PITRAM effect - pressure induced transformation of residual austenite into martensite) and/or of a stress state (SITPvAM effect - stress induced transformation of residual austenite into martensite).

As the hardness of the material increases, so does the contribution of the first characteristic, while the contribution of the second one decreases.

The cited international standard ISO 17804:2005 defines, in Annex A, "Abrasion-resistant grades of ausferritic spheroidal graphite cast iron", grades ISO17804/JS/HBW400 and ISO17804/JS/HBW450. These are the two recommended grades for applications that require high resistance to wear.

In this case, the maximum contribution to wear-resistance is given by the high proportion of acicular ferrite in the matrix and by its hardness.

It is known that generally all grades of austempered spheroidal cast irons represented in the cited standard derive from a process of pouring and subsequent heat treatment for austempering adapted to give the transformed austenite (high part of the Dorazil curve) the maximum possible stability.

Figure 5, taken from "Section IV Ductile Iron Databook for Design Engineers" (http://www.ductile.org/didata/Section4), shows schematically the Dorazil curve (the topmost chart). The curve represents the volumetric fraction of stabilized austenite as a function of the time elapsed from the moment when the material is immersed in the austempering bath. The horizontal portion, which represents the maximum of this curve, corresponds to the maximum stability of the austenite as a consequence of its progressive enrichment with carbon. Accordingly, the Brinell hardness of the material also reaches its minimum value, since the transformation into martensite of an important volume fraction of the matrix subjected to the hardness test is avoided.

The part of the transformation process from the beginning to the attainment of the cited condition of maximum stability of the austenite is termed first stage, since an excessive extension of the holding of the material in the austempering bath would cause an unwanted decomposition of the austenite, stabilized by the carbon enrichment, into acicular ferrite and carbides, a possible part of the transformation process known as second stage.

The condition of maximum stability of the austenite is necessary in order to manage the process in controlled and repetitive conditions. It is also known that the optimum time, with respect to this requirement, of permanence in the thermal bath in order to reach the condition of maximum stability of the carbon-enriched austenite depends essentially on the thickness of the casting, on the austenitization temperature, on the isothermal quenching temperature, and on the content of silicon, manganese and other elements with strong segregation, such as molybdenum.

It is also known that an excessive quantity of strong segregation elements, such as manganese and molybdenum, causes a very long dilation of the required holding time in the thermal bath. For this reason, strong segregation elements, such as manganese and molybdenum, must be limited to values that make the process industrially feasible.

The chart shown in Figure 1, taken from "Section IV Ductile Iron Databook for Design Engineers http://www.ductile.org/didata/Section4/Figures/pfig4_23.htm" , gives evidence of the wear resistance for various materials.

This comparative chart shows the advantageous behavior, in wear resistance, of ADI cast irons with respect to the materials with which they have been compared.

The wear resistance values offered by ADI cast irons, advantageous in many applications in which it has been possible to avoid the induction quenching of cast irons and/or steels, the nitriding of cast irons and/or steels, the cementation of steels, such as for example:

- driving wheels for tracks of earth-moving machines

- epicyclic reduction gear bodies with broached teeth

- driving shafts

- et cetera,

have been found to be insufficient in applications where higher wear resistance is required, such as for example:

- armors for stone mills and for the cement industry

- screw conveyors for the cement industry

- hammers for stone mills

- lamination rollers for clays

- et cetera.

For such applications, ADI cast irons are insufficient and it is necessary to resort to other families of abrasion-resistant cast irons. These materials, besides being penalized by the cost associated with a high content of carbide-forming alloying elements (these are in fact ledeburitic white cast irons in which the carbon is predominantly in a form that is combined with iron), are even more limited in use due to their fragility, which is a consequence of the ledeburitic white structure. In the frequent cases in which a resistance, albeit a limited one, to impacts is required, it is necessary to use wear-resistant steels, which, as is known, are characterized by a foundry process that is more complex and expensive than the typical one of cast iron foundry.

For this reason, attempts have been made to increase the resistance to wear/abrasion by introducing carbides in the pouring step so as to obtain a so-called CADI cast iron (Carbidic Austempered Ductile Iron).

While on the one hand CADI cast irons make it possible indeed to obtain mechanical components that are particularly wear-resistant, on the other hand they are more fragile than normally required in many applications and are also difficult to obtain.

The chart shown in Figure 2, taken from "Agricultural Applications of Austempered Cast Irons" by Kathy L. Hayrynen PhD FASM, Tim Dorn, John R. Keough PE, Vasko Popovski PE, Steven Sumner, Arron Rimmel PhD, illustrates the improvement of the resistance to wear/abrasion of CADI cast irons, compared with abrasion-resistant cast irons, which increases as the carbide content rises.

Correspondingly, by increasing the carbide content fragility increases.

Disclosure of the Invention

The aim of the present invention is to provide a new method for manufacturing austempered spheroidal cast iron that makes it possible to obtain a material that has an optimum wear-resistance and at the same time a sufficient resilience.

This aim and these and other objects that will become better apparent hereinafter are achieved by a method for manufacturing spheroidal cast iron mechanical components according to what is indicated in the independent claims that follow.

Brief description of the drawings

Further characteristics and advantages of the invention will become better apparent from the description of some preferred but not exclusive embodiments of a method for manufacturing spheroidal cast iron according to the present invention, illustrated by way of non-limiting example in the accompanying drawings, wherein: Figure 1 is a comparative chart of the wear resistance of various metallic materials, including various ADI cast irons and various types of hardened and tempered spheroidal cast irons and steels;

Figure 2 is a chart, similar to the previous one, in which the behavior of an ADI cast iron and of CADI cast irons with different percentages of carbides are compared with abrasion-resistant cast irons;

Figure 3 is a perspective view of a hammer subjected to a method according to the invention;

Figure 4 is a micrograph at 500 magnifications of a component subjected to a method according to the invention;

Figure 5 is a schematic diagram showing the effect of austempering time on the amount and stability of austenite and the hardness of ADI;

Figure 6 is a plot of a time/temperature curve of a thermal treatment for interrupted quenching in a salt bath related to the method according to the invention.

Ways of carrying out the Invention

In the exemplary embodiments that follow, individual characteristics, given in relation to specific examples, may actually be interchanged with other different characteristics that exist in other exemplary embodiments.

Moreover, it is noted that anything found to be already known during the patenting process is understood not to be claimed and to be the subject of a disclaimer.

With reference to the figures, the present invention relates to a method for manufacturing spheroidal cast iron mechanical components such as, for example, armors for stone mills and for the cement industry, screw conveyors for the cement industry, hammers for stone mills, lamination rollers for clays and mechanical components in general.

In particular, the method entails the following steps:

- providing a casting of a spheroidal cast iron mechanical component with a percentage by volume of pearlitic structure greater than 70%, having a carbon content by weight comprised between 2.5% and 4.0%, a silicon content by weight comprised between 1.5% and 3.5%, a manganese content by weight comprised between 0.6% and 1.2%, a molybdenum content by weight comprised between 0% and 1%, and a chromium content by weight comprised between 0% and 0.5%;

- bringing said cast iron casting with a percentage by volume of pearlitic structure greater than 70% to a temperature that is higher than the upper austenitization temperature (A _c3 ) for the time required to obtain a fully austenitic structure;

- performing a thermal treatment for interrupted quenching in a salt bath, comprising a step of cooling said mechanical components in a salt bath at a cooling rate that is higher than the critical rate and an isothermal holding step at a temperature comprised between 250°C and 320°C for a time comprised between 10 minutes and 60 minutes in order to obtain a matrix comprising metastable unsaturated austenite in a percentage by volume comprised between 40% and 90% of the total.

The expression "critical rate" is understood as the minimum cooling rate required to prevent pearlitic transformation.

The expression "interrupted quenching in salt bath" according to the provisions of the present application is understood to be a temperating process (as shown in Figure 6) that has a cooling step at a rate that is higher than the critical rate and stops at a temperature higher than that at the beginning of the martensitic transformation MS, followed by a holding period at this temperature for the time sufficient to start the reaction but insufficient to complete it; finally, rapid cooling to ambient temperature is performed.

It should be noted that in the specific case the interrupted quenching in salt bath in practice is interrupted twice: a first time because during the quick cooling step one stops at the isothermal holding temperature, which is higher than MS, the second time because we interrupt the reaction at isothermal temperature at a time that is very early with respect to the condition of maximum stability of carbon-enriched austenite.

The method according to the invention provides for limiting, during the step for producing the casting, the forming of disperse primary carbons (Fe ₃ C) in a percentage by volume of less than 5% of the volume.

Advantageously, the step that consists in limiting the forming of disperse primary carbons (Fe C) comprises a process of post-inoculation in the step for pouring the casting into the die.

In particular, it has been found that it is particularly convenient if the percentage by volume of the pearlite in the casting on which the thermal treatment has to be performed is higher than 80%.

Conveniently, the casting of the spheroidal cast iron mechanical component subjected to the method according to the invention has a nickel content by weight comprised between 0% and 3% and a copper content by weight comprised between 0% and 1.2%.

Preferably, the casting of the spheroidal cast iron mechanical component subjected to the method according to the invention can have a silicon content by weight comprised between 1.5% and 2.5%.

Advantageously, the casting of the spheroidal cast iron mechanical component subjected to the method according to the invention has a molybdenum content by weight comprised between 0.4% and 1%.

Experimentally, moreover, it has been found 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 spheroidal cast iron castings having a percentage by volume of pearlite substantially equal to 85%.

Advantageously, the temperature used preferably to perform the isothermal step of interrupted quenching is comprised between 290 °C and 310 °C.

In particular, the expression "thermal interrupted quenching treatment" designates a cooling with a rate that is higher than the critical one (minimum cooling rate required to prevent pearlitic transformation), keeping the casting at a higher isothermal temperature than the temperature of martensitic transformation MS; the holding at this temperature has a duration sufficient to make the ausferritic reaction start at the edges of the spheroids and is interrupted at such a stage as to obtain the desired proportion of metastable austenite.

Control of the process is possible thanks to the conspicuous slowing of the reaction, caused by the relatively high ratio of high- segregation alloying elements.

The temperature at which the mechanical components are held, as mentioned, during the austenitization step is higher than the temperature technically identified as A _c3 , or temperature of complete austenitization for the time necessary to obtain a fully austenitic structure.

This can be obtained by choosing a temperature higher than 850 °C and lower than 910 °C and, advantageously, according to the carbon and silicon content, comprised between 880 °C and 900 °C.

Such temperatures are indicative for cast irons having a carbon content of approximately 3.5% and a silicon content of approximately 2%, but, obviously, may vary according to the percentages of these elements in the casting to be subjected to the thermal treatment.

In order to obtain a fully austenitic structure, it is observed experimentally that, depending on the dimensions of the mechanical component, the holding time of the mechanical component at the austenitization temperature A _c3 is comprised between 90 minutes and 210 minutes, preferably between 120 minutes and 180 minutes.

Preferably, during the interrupted quenching treatment the isothermal holding step has a duration comprised between 10 minutes and 30 minutes.

The method according to the invention is based essentially on the addition of a high ratio of high- segregation alloying elements, such as manganese and molybdenum. In this manner, the ausferritic reaction starts with a rate of transformation, measurable as volume of matrix transformed in the unit time, which is appreciable from the edges of the spheroids (First To Freeze = First To React), where the concentration of manganese and molybdenum is minimal and the concentration of silicon is maximum, slowing down very quickly when the reaction advances in the direction of the edges of the grains (Last To Freeze = Last to React).

Thanks to this great slowing, caused by the high ratio of segregating elements, it is possible to control the structure composed of high-hardness acicular ferrite and stabilized austenite proximate to the edges of the spheroids, associated with an important fraction of metastable austenite (because it is scarcely enriched with carbon during transformation) toward the edges of the grains.

EXAMPLE 1

A hammer was provided by casting with a weight of approximately

1.74 kg, using cast iron with a predominantly pearlitic matrix (pearlite in a percentage higher than 80%) having a carbon percentage equal to 3.47%, a silicon percentage equal to 2.08%, a manganese percentage equal to 1.07% and a molybdenum percentage equal to 0.94%.

The component was brought to a complete austenitization temperature (higher than A _c3 ) equal to 890 °C and was held at this temperature for 120 minutes.

Then an interrupted quenching treatment in a salt bath at 250 °C was performed.

An average hardness of approximately 520 HB was observed on the finished part.

All the characteristics of the invention indicated above as advantageous, convenient or the like may also be omitted or replaced by equivalents.

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 in all the embodiments the invention has achieved the intended aim and objects.

Comment on the mechanical characteristics of the treated components.

In the specific case, the components made of austempered spheroidal cast iron according to the invention have always shown, for an equal hardness, a higher resistance to abrasive wear (by approximately 10-15%) than traditional austempered cast irons and Q&T steels.

Moreover, these components have always shown a higher impact strength than CADI cast irons or white cast irons (impact strength obtained on a notch- free specimen according to the ISO 148 standard comprised between 10 and 20 J).

In practice, the dimensions may be any according to the requirements, although it has been found that the method according to the invention is particularly effective for treating components with dimensions up to 100 mm.

All the details may further be replaced with other technically equivalent elements.

The disclosures in Italian Patent Application No. VR2010A000124 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.