The present invention relates to a ductile iron in accordance with claim 1 . The invention also concerns a method of manufacturing an article of a ductile iron as defined in the other independent claim.

Background of the invention

A cylinder head of a piston engine serves several purposes: it defines one end of a combustion chamber, provides space for intake and exhaust ducts and cooling medium ducts and accommodates intake and exhaust valves and fuel injectors. The shape of a cylinder head is often very complex and cylinder heads need to withstand high temperatures and significant forces. To ensure sufficient strength and other properties of cylinder heads and at the same time allow easy manufacturability and reasonable price, high requirements are set for the selection of a cylinder head material. In cylinder heads of large internal combustion engines, such as ship and power plant engines, cast irons are typically used. While satisfactory properties can be achieved with different known cast irons, usually some properties are optimized at the cost of other desired properties.

Summary of the invention

An object of the present invention is to provide an improved ductile iron, which can be used, for instance, as a cylinder head material. The characterizing features of the ductile iron according to the invention are given in claim 1 . Another object of the invention is to provide an improved method of manufacturing an article of a ductile iron. The characterizing features of the method are given in the other independent claim.

The ductile iron according to the invention comprises, by weight, 3.6-4.3 percent of carbon, at most 1 .75 percent of silicon and at least 92 percent of iron, the microstructure of the ductile iron comprising, by volume, at most 60 percent of pearlite and at least 40 percent of ferrite.

The ductile iron according to the invention has good thermal conductivity and suitable other physical properties for use as a material of cylinder heads of pis- ton engines. The low Si-content of the ductile iron significantly increases the thermal conductivity of the ferrite. In a ferritic-pearlitic microstructure the thermal conductivity of the ferrite dictates the thermal conductivity of the material. The desired properties are thus achieved by the specific combination of the low Si-content and the microstructure where at least 40 percent of the iron is ferritic.

According to an embodiment of the invention, the ductile iron comprises, by volume, at most 45 percent of pearlite and at least 55 percent of ferrite.

According to an embodiment of the invention, the ductile iron comprises at most 0.8 percent of manganese. According to an embodiment of the invention, the ductile iron comprises at least 1 .0 percent of silicon.

The ductile iron can additionally comprise copper and/or tin. With copper or tin, the amount of pearlite can be controlled. The amount of copper can be in the range of 0.005-0.5 percent by weight. The amount of tin can be in the range of 0.001 -0.1 percent by weight, preferably at most 0.07 percent.

According to an embodiment of the invention, the ductile iron consists of Fe, C, Si, Mn and inevitable impurities. Alternatively, the ductile iron can additionally comprise Cu and/or Sn.

The method of manufacturing an article of a ductile iron defined above com- prises the steps of casting the article, austenitizing the article at a temperature of 900-920 °C for 2-4 hours, and converting the microstructure of the article to a ferritic-pearlitic matrix by keeping the article at a temperature of 675- 700 °C for 1 -4 hours.

A cylinder head according to the invention is made of a ductile iron defined above. Description of embodiments of the invention

The present invention concerns a ductile iron. The ductile iron according to the invention can be used, for instance, as a cylinder head material in piston engines, especially in large internal combustion engines, such as main or auxilia- ry engines of ships or power plant engines.

The term "ductile iron" refers to a group of materials, where the common defining characteristic is graphite, which is at least partly in the form of nodules, i.e. small spherical particles. In addition to the term ductile iron, at least the following names can be used: ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron, spheroidal iron and SG iron.

In grey iron, graphite is in the form of flakes. The sharp flakes of grey iron create stress concentration points within the metal matrix, which makes grey iron brittle. The rounded graphite nodules of ductile iron prevent formation of cracks and improve thus ductility of the material. In practice, not all the graphite in a ductile iron is in the form of nodules, but part of the graphite can appear in other forms. The proportion, size and distribution of nodules are significant factors affecting the properties of ductile irons. Nodularity of a ductile iron can be estimated, for instance, using visual inspection, image analysis, or ultrasonic testing. Nodularity is expressed as a percentage. The carbon contents of ductile irons range from 2 percent to over 4 percent. In addition to iron and carbon, ductile irons typically comprise silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S). Copper (Cu) and tin (Sn) can be used for increasing tensile and yield strength. Nickel (Ni), copper and chromium (Cr) can be used for providing corrosion resistance. Ductile irons are available with different microstructures, which provide different properties. The microstructure can be, for instance, ferritic, pearlitic, mar- tensitic or austenitic.

Ferrite provides in general high ductility and toughness, but lower strength and hardness. Pearlite, on the other hand, provides high strength but decreased ductility. With a ferritic-pearlitic ductile iron, properties between ferritic and pearlitic ductile irons can be achieved. In a ferritic-pearlitic ductile iron, good machinability and low production costs can be combined to a material with strength and ductility suitable for many purposes. The microstructure of the ductile iron according to the invention is a ferritic- pearlitic matrix. The ductile iron comprises, by volume, 0-60 % of pearlite, preferably 0-45 %, the balance being ferrite. The desired microstructure can be achieved directly with suitable alloying, via heat treatment, or by a combina- tion of alloying and heat treatment. The carbon content is 3.6-4.3 %. The ductile iron according to the invention comprises at least 92 w-% of iron.

The ductile iron according to the invention comprises at most 1 .75 % of silicon. By keeping the Si-content at 1 .75 % or below, the thermal conductivity of the ferrite can be significantly increased. In a ferritic-pearlitic microstructure, the thermal conductivity of the ferrite controls the thermal conductivity of the material.

With copper and/or tin, the proportion of pearlite in the microstructure can be increased. The amount of copper can be in the range of 0.005-0.5 percent by weight. The amount of tin can be in the range of 0.001 -0.1 percent by weight, preferably at most 0.07 percent.

Chemical composition of the ductile iron according to an embodiment of the invention is shown in table 1 . The ductile iron according to the embodiment comprises, by weight, 3.6^1.3 percent of carbon, at most 1 .75 percent of silicon, at most 0.8 percent of manganese, at most 0.5 percent of copper and at most 0.1 percent of tin. The balance is iron and unavoidable impurities. According to an embodiment of the invention, the ductile iron comprises at least 1 .0 percent of silicon.

Table 1

Table 2 shows the chemical composition of a ductile iron in accordance with one example. With a pearlite content of 45 %, the thermal conductivity of the material was 45 W/mK at a temperature of 300 K. Standard EN 1563:2012 concerning spheroidal graphite cast irons requires thermal conductivity of 35.2 W/mK. An increase of nearly 28 percent was thus achieved. Table 2

According to an embodiment of the invention, an article is manufactured by casting the article, austenitizing the article at a temperature of 900-920 °C for 2-4 hours, and exposing the article to a temperature of 675-700 °C for 1 to 4 hours. At the latter heat treatment, the microstructure of the article is converted from austenite to ferrite and pearlite.

The desired microstructure can also be achieved without heat treatment by cooling the article in the mold. Desired physical properties of the ductile iron include tensile strength of at least 350 MPa and thermal conductivity of at least 38 W/(m- K). A suitable tensile strength range for the material is 350-600 MPa. A suitable thermal conductivity range is 38-60 W/(m- K).

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.