METHOD FOR MANUFACTURING IT

TECHNICAL FIELD The invention relates to spheroidal graphite iron for cylinder heads and to method for manufacturing it. The invention also relates to a cylinder head of the internal combustion engine manufactured of spheroidal graphite iron. The invention also relates to an internal combustion engine and to a marine vessel, which comprise a cylinder head manufactured from spheroidal graphite iron. Internal combustion engines are used, for example, as main propulsion engines or auxiliary engines in marine vessels or in power plants for the production of heat and/or electricity.

BACKGROUND Internal combustion engines comprise a cylinder head. The cylinder head is the metal part of the engine that encloses and covers the cylinders. The cylinder head is often detachable and contains parts such as valves, valve seats and other e.g. coolant circulation.

The cylinder head helps to keep the engine cool while exposing to mechanical and thermal loads. Therefore there are high mechanical requirements for the cylinder head component and the cylinder head material.

From the prior art is known US2006037675, which discloses a method of preparing and forming parts of spheroidal graphite cast iron having high- grade mechanical characteristics.

From the prior art is known US20031 161 13, which discloses a method for the manufacture of crank cases and cylinder heads from gray cast iron. The method comprises steps of: providing a molten gray iron metal; alloying said molten gray iron metal prior to pouring with tin to a total tin content of about 0.05% to about 0.10% to provide a molten tin-alloyed gray iron metal; inoculating said molten tin-alloyed gray iron metal prior to pouring with a gray iron inoculant to a further silicon addition of from about 0.10% to about 0.12%; and casting an internal combustion engine part as soon as possible after said inoculation.

There are however some disadvantages and drawbacks relating to the known prior art. Modern cleaner fuels have higher combustion temperatures, which yields more efficient combustion. Higher combustion temperatures raise requirements for engine parts in thermal conductivity because of higher thermal loads.

Furthermore, high cylinder pressure is one of the solutions to reduce emissions. To do so, stronger material for the cylinder head is required to stand the high pressure of the engine.

SUMMARY

An object of the invention is to alleviate and eliminate the problems or drawbacks relating to the known prior art. Another object of the invention is to provide an improved cylinder head for internal combustion engine and means to achieve it. Another object of the invention is to provide a spheroidal graphite iron with improved thermal conductivity for use in engines component parts under thermal and mechanical loads. Another object of the invention is to provide enhanced performance against thermal and mechanical loads in cylinder heads and/or also other parts of the engine. Another object of the invention is to provide increased lifetime for cylinder head and/or also other parts of the engine.

The object of the invention can be achieved by the features of independent claims. The invention relates to a spheroidal graphite iron for cylinder head of an internal combustion engine according to claim 1. In addition the invention relates to a method for manufacturing a cylinder head of an internal combustion engine according to claim 9, a cylinder head according to claim 1 1 , an internal combustion engine according to claim 12, a marine vessel according to claim 13. One embodiment of the invention is a spheroidal graphite iron for cylinder head of an internal combustion engine, where spheroidal graphite iron has a tensile strength in the range 350 - 480 Mpa and a thermal conductivity in the range 38 - 45 W/(K ^* m), and where the composition of the spheroidal graphite iron in percentage by weight is: 3,0 to 4,5% of carbon (C ), 0,9 to 1.75% silicon (Si), less than 0,8% manganese (Mn), less than 0,3% copper (Cu), 0,03 to 0,25% Vanadium (V), and 0,01 to 0,1 % niobium (Nb), the rest being iron (Fe) as a ferritic spheroidal cast iron and unavoidable impurities. The spheroidal graphite iron of the present invention comprises a microstructure ferritic ductile iron, which is precipitation hardened. Advantageously, the microstructure is substantially or fully ferritic. The substantially ferritic microstructure is at least 75 % (vol-%) ferritic. The substantially ferritic microstructure advantageously comprises max 25 vol-% perlite.

According to an additional embodiment of the invention, the spheroidal graphite iron has a tensile strength in the range 400 - 480 Mpa.

According to an another embodiment of the invention; a method for manufacturing a cylinder head of an internal combustion engine from a spheroidal graphite iron having a tensile strength in the range 350 - 480 Mpa and a thermal conductivity in the range 38 - 45 W/(K ^* m), the composition of spheroidal graphite iron in percentage by weight is: 3,0 to 4,5% of carbon (C ), 0,9 to 1.75% silicon (Si), less than 0,8% manganese (Mn), less than 0,3% copper (Cu), 0,03 to 0,25% Vanadium (V), and 0,01 to 0,1 % niobium (Nb), the rest being iron (Fe) as a ferritic spheroidal cast iron and unavoidable impurities, said method comprises steps of: a. casting the composition, b. austenitizing at 900 - 1050°C for 1 - 48 hours, c. cooling at rate of 1 - 80 °C/min to temperature 620 - 750 °C d. holding at 620 - 750 °C for 1 - 75 hours e. cooling at rate of 50 °C/hour to temperature of 200 °C f. air cooling to room temperature to complete the precipitation hardening,

In said method, the steps of austenitizing and cooling at rate of 1 - 80 °C/min to temperature 620 - 750 °C is used to create a supersaturated solid solution of Vanadium for precipitation. Essentially precipitation hardened microstructure is achieved by the method. Advantageously the microstructure is substantially or fully ferritic. The substantially ferritic microstructure is at least 75 vol-% ferritic. The substantially ferritic microstructure advantageously comprises max 25 vol-% perlite. The achieved mechanical properties result from precipitation hardening the composition accordingto to an embodiment of the invention.

An additional embodiment of the invention is a cylinder head of the internal combustion engine manufactured of spheroidal graphite iron of the present invention. An additional embodiment of the invention is an internal combustion engine comprising a cylinder head manufactured from spheroidal graphite iron of the present invention.

An additional embodiment of the invention is a marine vessel comprising a cylinder head in an internal combustion engine of the marine vessel, and the cylinder head is manufactured from spheroidal graphite iron of the present invention.

According to a further embodiment of the invention, silicon (Si) is between 1 ,0 - 1 ,5 in percentage by weight.

According to a further embodiment of the invention, carbon (C) is between 3,7 - 4,3 in percentage by weight, to retain needed (normal) carbon equivalency ( CE %).

According to a further embodiment of the invention, vanadium (V) is between 0,04 - 0,1 in percentage by weight, to improve precipitation strengthening effect. According to a further embodiment of the invention, Niobium (Nb) is between 0,03 - 0,05 in percentage by weight, to prevent austenite grain size coarsening during high temperature heat-treatment.

The present invention and its embodiments offers advantages over the known prior art, such as increased thermal conductivity. The present invention and its embodiments provide increased temperature conductivity with relatively high mechanical properties. Increased thermal conductivity provides increased lifetime for cylinder heads and other components of the internal combustion engine by conducting the heat more efficiently and/or because of other mechanical properties. An embodiment of the invention provides enhanced performance against thermal and mechanical loads in cylinder heads and/or also other parts of the engine. Increased thermal conductivity also enables higher temperatures in the engine, which yields more efficient combustion. Cleaner fuels also have higher combustion temperatures. Increased thermal conductivity also reduces more the stresses caused by thermal differences.

The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

Next the invention will be described in greater detail with reference to exemplary embodiments

DETAILED DESCRIPTION

Silicon (Si) is an important and typical alloying element of spheroidal graphite irons because silicon is most important ferrite inducing and a graphite stabilizing alloying element in spheroidal graphite irons.

In typical spheroidal graphite irons silicon levels are usually over 2 % (by weigth). The content of Silicon is in range 0,9 to 1.75 % (by weight). However, preferable content for Silicon in an embodiment of the present invention is 1 ,0 - 1 ,5 % (by weight) in order to ensure a good thermal conductivity. Carbon is essential element relating to spheroidal graphite irons, iron alloys and steels. In spheroidal graphite irons carbon precipitates to graphite on the part that is not in form of carbides or dissolved with iron. The content of carbon is in range 3,0 - 4,5 % (by weight), however, preferable content for carbon is in range 3,7 - 4,3 % (by weight). The carbon content levels are dependent on the content if silicon which is required to achieve intended thermal conductivity.

Vanadium retards grain growth, even after hardening from high temperatures or after periods of extended heating. Vanadium is added to cast iron to stabilize cementite, increase hardness, and increase resistance to wear and heat. Vanadium is also used for precipitation hardening. The content of vanadium is in range 0,03 - 0,25 % (by weight). However, preferable content for vanadium is in range 0,04 - 0,1 % (by weight), in which range the hardening or strengthening effect of vanadium is optimal. Niobium improves mechanical properties including hardness and wear resistance. Niobium is favourable in refining the graphite and is used to prevent austenite grain size coarsening during high temperature heat- treatment. The content of niobium is in range 0,01 - 0,1 % (by weight). However, preferable content for niobium is in range 0,03 - 0,05% (by weight), which is optimal range for inducing the grain size reducing effect.

Copper and manganese are not desired or not useful alloying elements because deteriorating effects to properties spheroidal graphite iron. The content of copper is less than 0,3 % (by weight). The content of manganese is less than 0,8 % (by weight). Copper is perlite inducing alloying element and thereby disturbs manufacturing ferritic or ferritic-perlitic spheroidal graphite irons. Manganese also has effect of inducing perlite or in high concentrations carbides.

The carbide inducing alloying elements should have low enough levels so that the composition of an embodiment of the present invention can be treated to have ferritic or mainly ferritic microstructure.

An example of an embodiment of the invention is a spheroidal graphite iron for cylinder head of an internal combustion engine. The spheroidal graphite iron has a tensile strength in the range 350 - 480 Mpa and a thermal conductivity in the range 38 - 45 W/(K ^* m). The composition of the spheroidal graphite iron in percentage by weight is: 3,0 to 4,5% of carbon (C), 0,9 to 1.75% silicon (Si), less than 0,8% manganese (Mn), less than 0,3% copper (Cu), 0,03 to 0,25% Vanadium (V), and 0,01 to 0,1 % niobium (Nb), the rest being iron (Fe) as a ferritic spheroidal graphite iron. There may be some additional and unavoidable impurities in the composition.

The spheroidal graphite iron comprises a ferritic ductile iron, which is precipitation hardened. Advantageously, the microstructure is fully or mainly ferritic. The mainly or substantially ferritic microstructure is at least 75 vol-% ferritic, and advantageously comprises max 25 vol-% perlite. The precipitation hardened microstructure and desired properties are achieved via heat treatment steps described below.

Another example of an embodiment of the invention is a method for manufacturing a cylinder head of an internal combustion engine from a spheroidal graphite iron having a tensile strength in the range 350 - 480 Mpa and a thermal conductivity in the range 38 - 45 W/(K ^* m). The composition of spheroidal graphite iron of manufactured cylinder head by the method in percentage by weight is: 3,0 to 4,5% of carbon (C ), 0,9 to 1.75% silicon (Si), less than 0,8% manganese (Mn), less than 0,3% copper (Cu), 0,03 to 0,25% Vanadium (V), and 0,01 to 0,1 % niobium (Nb), the rest being iron (Fe) and unavoidable impurities, said method comprises steps of: a. casting the composition, b. austenitizing at 900 - 1050°C for 1 - 48 hours, c. cooling at rate of 1 - 80 °C/min to temperature 620 - 750 °C d. holding at 620 - 750 °C for 1 - 75 hours e. cooling at rate of 50 °C/hour to temperature of 200 °C f. air cooling to room temperature to complete the precipitation hardening,

In the method, the steps of austenitizing and cooling at rate of 1 - 80 °C/min to temperature 620 - 750 °C is used to create a supersaturated solid solution of Vanadium for precipitation. Essentially precipitation hardened microstructure is achieved by the method. The desired properties are achieved via precipitation hardening the composition according to an embodiment of the invention.

The microstructure is at least 75 vol-% ferritic and may comprise max 25 vol-% perlite. Advantageously the microstructure is fully ferritic. Advantageously the spheroidal graphite iron has a tensile strength in the range 400 - 480 Mpa.

Another example of an embodiment of the invention is a cylinder head of the internal combustion engine manufactured from the said spheroidal graphite iron of the present invention. Another example of an embodiment of the invention is an internal combustion engine comprising a cylinder head manufactured from spheroidal graphite iron of the present invention.

Another example of an embodiment of the invention is a marine vessel comprising a cylinder head in an internal combustion engine of the marine vessel, and the cylinder head is manufactured from spheroidal graphite iron of the present invention.

The internal combustion engines may be used, for example, as main propulsion engines or auxiliary engines in marine vessels but the internal combustion engines can also be used in power plants for the production of heat and/or electricity.

Advantageously, the spheroidal graphite iron comprises silicon (Si) between 1 ,0 - 1 ,5 in percentage by weight to achieve higher (than normal) thermal conductivity.

Advantageously, the spheroidal graphite iron comprises carbon (C) between 3,7 - 4,3 in percentage by weight, to retain needed (normal) carbon equivalency ( CE %).

Advantageously, the spheroidal graphite iron comprises vanadium (V) between 0,04 - 0,1 in percentage by weight, to improve precipitation strengthening effect. Advantageously, the spheroidal graphite iron comprises Niobium (Nb) between 0,03 - 0,05 in percentage by weight, to prevent austenite grain size coarsening during high temperature heat-treatment.

The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the characteristics and scope of the inventive thought and the following patent claims.