HK1128041B - Method for manufacturing spheroidal cast iron mechanical components - Google Patents

Description

Technical field

The present invention relates to a method for manufacturing spheroidal cast iron mechanical components.

Background Art

Spheroidal cast irons of different types and having different structures are currently known and used particularly to provide different types of mechanical components.

Spheroidal cast iron has, as its main characteristic, the shape of the graphite, which is indeed spheroidal, differently from what occurs in conventional gray cast irons with lamellar graphite; the spheroidal structure of the graphite gives the material high ductility.

Spheroidal cast irons subjected to a thermal treatment for normalization have a completely pearlitic matrix. In this case, the material is characterized by a higher wear resistance, although ductility is quite reduced and fatigue strength does not increase due to the thermal treatment. Indeed, with reference to the ISO 1083 standard, pearlitic spheroidal cast iron without thermal treatment, classified by the code JS/800 - 2/S, has a minimum HBW hardness of 245, a minimum tensile strength of 800 MPa, and a typical fatigue strength of 304 MPa.

Pearlitic spheroidal cast iron subjected instead to a thermal treatment for normalization has a minimum HBW hardness of 270, a minimum tensile strength of 900 MPa, and a typical fatigue strength which is unchanged, i.e., equal to 304 MPa.

Spheroidal cast irons subjected to thermal treatment for hardening in water or oil have a bainitic or martensitic structure. They can optionally be subjected, at the end of the cooling process, to a thermal tempering treatment. Such cast irons are generally characterized by a very low ductility accompanied by high surface hardness and consequently are not used in applications which require a certain fatigue strength.

From what has been described above briefly, it can be seen that if a pearlitic spheroidal cast iron is subjected to a thermal treatment in a classic manner, an increase in fatigue strength is not observed.

In order to try to devise a material which would have improved mechanical strength characteristics and especially improved fatigue strength characteristics, the austempered spheroidal cast iron known commercially as ADI (Austempered Ductile Iron) has been devised.

The thermal treatment required to obtain this type of cast iron consists of a complete austenitizing treatment, keeping the component at a temperature which is higher than the upper limit austenitizing temperature (commonly referenced as Ac₃), followed by hardening in a bath of molten salts.

The final structure thus obtained, technically known as ausferritic structure, is composed of acicular ferrite and austenite. This particular structure gives the material high mechanical characteristics and most of all a superior fatigue strength, with lower machinability than traditional spheroidal cast irons.

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

In the mid-1980s, the company applying for the present patent developed, under license from Dr. Horst Muehlberger, a particular thermal treatment which allowed to obtain an austempered cast iron known as GGG 70 B/A: this thermal treatment consists of austenitizing at a temperature lower than Ac₃ (the upper austenitizing limit temperature) and higher than A_C1 (lower austenitizing limit temperature), followed by hardening in a bath of molten salts.

The resulting final structure, technically known as ausferritic structure with proeutectoid ferrite, is composed of proeutectoid ferrite, acicular ferrite and austenite. Since it is essential to prevent the formation of pearlite during cooling, and since the austenitizing temperature used during the first step of the thermal treatment is also relatively low, in this case also it is necessary to alloy the material with alloying elements such as nickel and/or molybdenum in percentages which are higher than in austempered spheroidal cast irons, which as explained earlier have no proeutectoid ferrite.

This particular type of cast iron has been introduced, in the ISO 17804 standard, with the designation JS/800-10 and more recently in SAE standard J2477 May 2004 revision, with the designation AD750. The fatigue strength of this particular type of cast iron is typically equal to 375 MPa.

Recently, spheroidal cast irons known commercially by the acronym MADI (Machinable Austempered Ductile Iron) have also been proposed; this type of cast iron also is obtained as a consequence of a thermal treatment for partial austenitizing at a temperature which is lower than Ac₃ and higher than A_c1 and subsequent hardening in a bath of molten salts. The resulting final structure is different from the structure of the type classified as GGG70 B/A and/or ISO 17804/JS/800-10 and/or SAE J2477 AD750 due to the presence of finally dispersed martensitic needles. However, even MADI cast irons are characterized by the high content of alloying materials such as nickel and molybdenum.

ADI or MADI cast irons ultimately have definitely higher static mechanical characteristics and fatigue limits, but since they are obtained by hardening in salt, as mentioned, they require alloying materials such as nickel and molybdenum in order to ensure their hardenability without the risk of forming pearlite. Currently, therefore, due to the high cost of such alloying elements, these materials, despite being valid in terms of mechanical characteristics, are scarcely competitive on an economical level.

A part from the above, a ductile cast iron containing Ni and Mo can be produced by high temperature isothermal treatement giving or mixed ferritic-pearlitic structure, US-B-4396442 .

Disclosure of the Invention

The aim of the present invention is to provide a new method for the production of spheroidal cast iron which allows to obtain a material which has higher mechanical characteristics than traditional spheroidal cast irons (ferritic, pearlitic, ferritic-pearlitic, et cetera) but has a significantly lower production cost than austempered cast irons (ADI and MADI).

This aim and these and other objects, which will become better apparent hereinafter, are achieved by a method for manufacturing mechanical components made of spheroidal cast iron, characterized in that it comprises the following steps:

• providing a casting of a mechanical component made of cast iron having a structure which is at least partially ferritic and has a carbon content ranging from 2.5% to 4.0% and a silicon content ranging from 2.0% to 3.5%, a manganese content ranging from 0% to 0.15%, a copper content ranging from 0% to 0.15%, a molybdenum content ranging from 0% to 0.15%, and a nickel content ranging from 0% to 0.15%;
• bringing said cast iron casting having an at least partially ferritic structure to a temperature for partial austenitizing, which is higher than the lower limit austenitizing temperature (Ac1) and lower than the upper limit austenitizing temperature (Ac3) for a time required to obtain an at least partially austenitic structure;
• performing a thermal treatment for isothermal hardening at a temperature ranging from 250°C to 400°C in order to obtain a matrix which has at least partially a pearlitic-ferritic structure, wherein said thermal treatment for isothermal hardening is performed in a bath of molten salts.

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:

• Figures 1 and 2 are photographic enlargements, made with an optical microscope, of two regions of a supporting bracket which weighs approximately 70 kg: the photograph of Figure 1 refers to a region having a thermal modulus (volume/cooling surface ratio) of 2.7; the photograph of Figure 2 relates instead to a region which has a thermal modulus of 1.3;
• Figures 3 and 4 are photographic enlargements, made with an optical microscope, of two regions of a spider which weighs approximately 68 kg: the photograph of Figure 3 refers to a region which has a thermal modulus of 2.4 and the photograph of Figure 4 relates to a region which has a thermal modulus of 1.35;
• Figure 5 is a photographic enlargement, made with an optical microscope, of a region of a second spider which weighs approximately 76 kg at a region having a thermal modulus of 1.2;
• Figure 6 is a perspective view of a cylindrical bar;
• Figure 7 is a photographic enlargement (with a magnification factor of 500) of a region of the bar shown in Figure 6.

Ways to carrying out the Invention

With reference to the figures, the present invention relates to a method for manufacturing mechanical components made of spheroidal cast iron, such as for example supports, spiders, hubs and mechanical components in general.

In particular, the method provides for the following steps:

• providing a casting of a mechanical component made of cast iron having a structure which is at least partially ferritic and has a carbon content ranging from 2.5% to 4.0%, a silicon content ranging from 2.0% to 3.5%, a manganese content ranging from 0% to 0.15%, a copper content ranging from 0% to 0.15%, a molybdenum content ranging from 0% to 0.15%,and a nickel content ranging from 0% to 0.15%;
• bringing the cast iron casting having an at least partially ferritic structure to a temperature which is higher than the lower austenitizing temperature (Ac1) and lower than the upper austenitizing temperature (Ac3) for a time required to obtain an at least partially austenitic structure;
• performing a thermal treatment for isothermal hardening at a temperature ranging from 250°C to 400°C in order to obtain a matrix which has a substantially pearlitic-ferritic or perferritic structure, wherein said thermal treatment for isothermal hardening is performed in a bath of molten salts.

In particular, it has been found that it is particularly convenient for the percentage of ferrite in the casting on which the thermal treatment is to be performed to be higher than 20%, preferably higher than 50%.

Experimentally, moreover, it has been found that it is particularly advantageous in terms of the typical mechanical characteristics of the components subjected to the method according to the invention to start from castings of spheroidal cast iron which have a ferrite percentage of more than 80%.

Advantageously, the temperature preferably used to perform isothermal hardening ranges from 350°C to 390°C.

The temperature at which the mechanical components are kept, as mentioned, during the step for partial austenitizing ranges from the temperature referenced technically as A_c1, above which the structure of the cast iron starts to convert to austenite, to the temperature referenced technically as A_c3, or temperature of complete austenitizing; in practice, by bringing the part above the temperature referenced technically as A_c3 one would have a complete transformation of the structure into austenite. By instead keeping, as mentioned, the component at an intermediate temperature between A_c3 and A_c1, not all the structure becomes austenite but part of the ferrite remains as it is (proeutectoid ferrite).

Moreover, it has been observed, as shown in the photograph taken with a 500x optical microscope shown in Figure 7, that the resulting structure has islands which have an ausferritic structure.

The selection of the temperature at which the partial austenitizing is to be performed depends substantially on the amount of austenite that one wishes to obtain at the end of the period of retention at such temperature. It has been found that it is advantageous to maintain the components at a partial austenitizing temperature which allows conversion to austenite in a percentage ranging from 30% to 70% of the structure, preferably substantially equal to 50%; this situation can be obtained by selecting a temperature which lies approximately halfway along the interval comprised between A_c3 and A_c1.

This can be achieved by selecting a temperature of more than 780°C and lower than 840°C and advantageously, depending on the content of carbon and silicon, ranging from 800 to 820°C.

Such temperatures are indications for cast irons which have a carbon content of approximately 3.50% and a silicon content of approximately 2.60%, but of course they may vary according to the percentages of such elements in the casting to be subjected to the thermal treatment.

In order to obtain a predominantly austenitic structure, it has been found experimentally that depending on the dimensions of the mechanical component the retention time of the mechanical component at the austenitizing temperature (a temperature which is intermediate between A_c3 and A_c1) ranges from 90 minutes to 210 minutes, preferably from 120 to 180 minutes.

EXAMPLE 1

A bracket was cast which weighed approximately 70 kg and was made of cast iron having a predominantly ferritic matrix (ferrite in a percentage of more than 50%) with a carbon percentage of 3.55% and a silicon percentage of 2.60%.

The component was brought to a temperature for partial austenitizing (intermediate between A_c3 and A_c1) of 815°C and was kept at this temperature for 150 minutes.

An isothermal hardening treatment in a salt bath at 370°C was then performed.

The finished part was found to have an average hardness of approximately 255-265 HB, while the average mechanical characteristics in regions with a thermal modulus of 2.7 and 1.3 respectively are summarized in table 1. Table 1

	Rm (MPa)	Rp02 (MPa)	A5
Region with modulus 2.7	720	500	7.5
Region with modulus 1.3	820	550	8.5

Figures 1 and 2 are photographs (with 200x magnification) taken with an optical microscope and show the metallographic structure of the part in the regions having a thermal modulus respectively of 2.7 and 1.3.

EXAMPLE 2

A spider was cast which weighed 68 kg and was made of cast iron having a predominantly ferritic matrix (ferrite percentage of more than 70%) with a carbon percentage of 3.55% and a silicon percentage of 2.60%.

The component was brought to a temperature for partial austenitizing (intermediate between A_c3 and A_c1) of 820°C for 140 minutes.

An isothermal hardening treatment in a salt bath at 375°C was then performed.

The finished part was found to have an average hardness of approximately 250-260 HB, while the average mechanical characteristics in regions with a thermal modulus of 2.4 and 1.35 respectively are summarized in table 2. Table 2

	Rm (MPa)	Rp02 (MPa)	A5
Region with modulus 2.4	700	450	5.5
Region with modulus 1.35	800	480	8.0

Figures 3 and 4 further show two photographs (with 200x magnification) taken with an optical microscope, illustrating the metallographic structure of the part in the regions with a thermal modulus of 2.4 and 1.35 respectively.

EXAMPLE 3

A spider was cast which weighed approximately 76 kg and was made of cast iron having a predominantly ferritic matrix (ferrite percentage of more than 80%) with a carbon percentage of 3.55% and a silicon percentage of 2.60%.

The component was brought to an austenitizing temperature (intermediate between A_c3 and A_c1) of 830°C for 160 minutes.

An isothermal hardening treatment in a salt bath at 380°C was then performed.

The finished part was found to have an average hardness of approximately 240-250 HB, while the average mechanical characteristics in a region with a thermal modulus of 1.2 are summarized in table 3. Table 3

	Rm (MPa)	Rp02 (MPa)	A5
Region with modulus 1.2	730	440	8.5

Figure 5 shows a photograph taken with an optical microscope (with 200x magnification), illustrating the metallographic structure of the part in the region with a thermal modulus of 1.2.

EXAMPLE 4

Test pieces were cast which had a diameter of 25 mm and a length of 200 mm; one of these test pieces is shown in Figure 6 and designated by the reference numeral 40; the test pieces were made of cast iron having a predominantly ferritic matrix with a carbon percentage of 3.65% and a silicon percentage of 2.65%.

The component 40 was brought to an (austenitizing) temperature of 810°C for 160 minutes.

An isothermal hardening treatment in a salt bath at 375°C was then performed.

The finished part was found to have an average hardness of approximately 260-270 HB, while the average mechanical characteristics in the region 40a are summarized in table 4. Table 4

	Rm (MPa)	Rp02 (MPa)	A5
Region 40a	890	580	8.5

Figure 7 shows a photograph taken with an optical microscope (with 200x magnification), illustrating the metallographic structure of the test piece in the region designated by the reference numeral 40a.

Notchless test pieces for rotary flexural fatigue tests with a diameter of 6.5 mm were subsequently obtained from these test pieces having a diameter of 25 mm and were found to have a fatigue limit of 368 MPa.

The present invention of course also relates to mechanical components made of spheroidal cast iron having a substantially ferritic-pearlitic structure with islands having an ausferritic structure.

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

In practice, the dimensions may be any according to requirements.

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

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.

Claims (9)

1. A method for manufacturing mechanical components made of spheroidal cast iron, characterized in that it comprises the following steps:

   - providing a casting of a mechanical component made of cast iron having a structure which is at least partially ferritic and has a carbon content ranging from 2.5% to 4.0%, a silicon content ranging from 2.0% to 3.5%, a manganese content ranging from 0% to 0.15%, a copper content ranging from 0% to 0.15%, a molybdenum content ranging from 0% to 0.15%, and a nickel content ranging from 0% to 0.15%;

   - bringing said cast iron casting having an at least partially ferritic structure to a temperature which is higher than the lower austenitizing temperature (A_c1) 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 ranging from 250°C to 400°C in order to obtain a matrix which has at least partially a pearlitic-ferritic structure,

   wherein said thermal treatment for isothermal hardening is performed in a bath of molten salts.
2. The method according to claim 1, characterized in that said casting of a mechanical component made of cast iron having an at least partially ferritic structure has a ferrite percentage of more than 20%.
3. The method according to one or more of the preceding claims, characterized in that said casting of a cast iron mechanical component with an at least partially ferritic structure has a ferrite percentage of more than 50%.
4. The method according to one or more of the preceding claims, characterized in that said casting of a mechanical component made of cast iron with an at least partially ferritic structure has a ferrite percentage of more than 80%.
5. The method according to one or more of the preceding claims, characterized in that said casting has, at the end of the step for holding at the austenitizing temperature ranging from A_c1 to A_c3, a percentage of austenite ranging from 30% to 70%, preferably substantially equal to 50%.
6. The method according to one or more of the preceding claims, characterized in that said isothermal hardening is performed at a temperature ranging from 350°C to 390°C.
7. The method according to one or more of the preceding claims, characterized in that said austenitizing temperature ranges from 780°C to 840°C, preferably from 800°C to 820°C.
8. The method according to one or more of the preceding claims, characterized in that the time for which said casting of a cast iron mechanical component is held at an austenitizing temperature ranging from A_c1 to A_c3 ranges from 90 to 210 minutes, preferably from 120 to 180 minutes.
9. The method for manufacturing mechanical components according to one or more of the preceding claims, characterized in that said matrix having a substantially pearlitic-ferritic structure has islands with an ausferritic structure.