Grey cast iron with vermicular graphite

Abstract

Grey cast iron with vermicular graphite comprises 3.0-3.3% by weight of C, 3.6-4.0% by weight of Si, 0.3-0.5% by weight of Mn, 0.600-0.800% by weight of Cu, 1.6-1.8% by weight of Ni, 0.3-0.45% by weight of Mo, 0.01-0.02% by weight of Mg, 0.005-0.013% by weight of Ce, 0.001-0.004% by weight of Sb, 0-0.02% by weight of B, the balance being Fe and inevitable impurities.

Description

Subject of invention

[0001] The subject of the invention is grey cast iron with vermicular (compacted) graphite with improved mechanical properties.Prior art

[0002] Grey cast irons are iron-carbon alloys with more than 2.03% of C, to which various alloying elements such as silicon (Si), manganese (Mn), magnesium (Mg) and others are added. Carbon in grey cast iron is precipitated as graphite in various forms. Depending on the form of the precipitated graphite, grey cast irons are divided into grey cast iron with lamellar graphite (GJL), grey cast iron with spheroidal graphite (GJS), also called nodular grey cast iron, and grey cast iron with vermicular (compacted) graphite (GJV). Grey cast iron with lamellar graphite (GJL) has the graphite precipitated in the form of thin flakes which are randomly arranged in a base matrix of ferrite and / or pearlite, depending on the chemical composition and cooling rate. Graphite lamellae are formed using nuclei, which are usually manganese sulphides ((MnX)S). Due to the lamellar shape of the graphite, which causes a notch effect, grey cast iron with lamellar graphite does not have high tensile strength but has good compressive strength and thermal conductivity. It is therefore suitable for components such as machine housings, bearings and brake discs that operate in high temperature environments but are not exposed to high stress forces. Grey cast iron with spheroidal graphite has the graphite precipitated in the form of spheroids which are in a base matrix of ferrite and / or pearlite. The formation of spheroidal graphite requires a nodulation process involving the addition of FeSiMg, which produces magnesium (MgS) and other sulphides that allow the growth of spheroidal graphite. Grey cast iron with spheroidal graphite therefore has high tensile strength and high elongation but lower thermal conductivity. The precipitated spheroidal graphite makes grey cast iron with spheroidal graphite good for machining. Grey cast iron with vermicular graphite has the graphite precipitated in a form being between spheroidal and lamellar graphite. The graphite particles are elongated but shorter than in lamellar graphite, randomly oriented and rounded like the spheroidal graphite. Individual vermicular graphite particles are linked to each other. Vermicular graphite is formed on nuclei that are similar or identical to the nuclei used for the growth of spheroidal graphite, i.e. magnesium sulphide, but not all the conditions for the growth of spheroidal graphite are met. The proportion of Mg in the melt that allows the formation of vermicular graphite is limited to 0.01-0.02% by weight. Also important is the amount of cerium (Ce) which also has an affinity for sulphur and forms important nuclei for the formation of graphite. Ce is also added for its degenerative effect, preventing the formation of spheroidal graphite and extending the tolerance range of Mg. Vermicular grey cast irons exhibit the characteristics of grey cast irons with lamellar as well as spheroidal graphite. Their mechanical properties, such as tensile strength and elongation at break, yield strength, thermal conductivity, which defines thermal stability, are the properties of grey cast irons with lamellar as well as spheroidal graphite. Compared to grey cast irons with lamellar graphite, they can achieve higher mechanical properties, better fatigue resistance but poorer thermal conductivity. Compared to grey cast irons with spheroidal graphite, they exhibit better thermal conductivity, lower mechanical properties and fatigue resistance. They have better castability than grey cast irons with spheroidal graphite but poorer castability compared to grey cast irons with lamellar graphite. Nonetheless, their combination of mechanical properties and machinability is favourable and they are often the choice of engineers when designing various components that are subjected to both mechanical and thermal stresses.

[0003] Due to their mechanical properties, grey cast irons with vermicular graphite are also suitable for highly stressed components, but less so for highly stressed components operating at elevated temperatures. Thus, EN-GJV-450 standardised grey cast iron with vermicular graphite according to SIST EN 16079:2024 has a minimum tensile strength of 450 MPa, a yield strength of min. 315 MPa and a thermal conductivity of 38 W / mK at room temperature, and a minimum tensile strength of 350 MPa, a minimum yield strength of 265 MPa and a thermal conductivity of 36 W / mK at 400°C for the components having a thickness of less than 30 mm. The mechanical values decrease with increasing temperature because said cast iron has a microstructure with a predominantly pearlitic matrix to achieve the required tensile strength. Due to this pearlitic matrix, the cast iron exhibits higher instability at elevated temperatures and also lower elongation values at break. Grey cast irons with vermicular graphite having a predominantly ferritic matrix, such as EN-GJV-300 with a tensile strength of 300 MPa, a yield strength of 210 MPa, and a thermal conductivity of 47 W / mK at room temperature, are more stable at elevated temperatures, but have substantially lower mechanical properties and thus a rather limited applicability to the components that are subjected to both mechanical and thermal stresses.Technical problem

[0004] The technical problem is to formulate grey cast iron with vermicular (compacted) graphite that will exhibit better mechanical properties, i.e. tensile strength, yield strength and elongation at break, while maintaining the thermal conductivity characteristic of grey cast irons with vermicular (compacted) graphite, and will therefore be useful for the components that must exhibit high strength and good thermal conductivity from room temperature up to an elevated temperature of 400°C, without a need for further thermal treatment of the components made of such grey cast iron with vermicular graphite.Solution to the technical problem

[0005] The technical problem is solved by the grey cast iron with vermicular (compacted) graphite as defined in the first independent claim.

[0006] Grey cast iron with vermicular graphite has a ferritic microstructure in which the graphite is arranged in vermicular form, the ferritic structure in the matrix comprising at least 80%, the remainder being the pearlitic structure with carbides.

[0007] Grey cast iron with vermicular graphite has equivalent carbon content in the range of 4.00-4.30 and a CHG factor defining the formation of chunky graphite, which is smaller or equal to 10.0, the CHG factor being defined by the amount of elements Si, Ce, Mg and Sb. The cast iron of the invention has an average value of thermal shock parameter which is indicative of resistance of material to mechanical and thermal stresses in the temperature range of 25°C to 400°C from 0.0895 to 0.0985, depending on temperature.

[0008] Grey cast iron with vermicular graphite has the graphite precipitated in vermicular form, which is distributed in the predominantly ferritic matrix, inside of which molybdenum carbides are arranged. The ferritic matrix represents at least 80%. This type of cast iron provides a good combination of thermal and mechanical properties from room temperatures between 18°C and 25°C up to elevated temperatures of up to 400°C. The grey cast iron with vermicular graphite of the invention exhibits an above-average yield strength / tensile strength ratio and good thermal conductivity, in particular at elevated temperatures and also in elements having thick walls. The ferritic matrix also contributes to better homogeneity of the microstructure at different wall thicknesses and better durability of the microstructure as well as resistance to oxidation at elevated temperatures.

[0009] The grey cast iron with vermicular graphite of the invention will be described in more detail in the following. Fig. 1 Microstructure of grey cast iron with vermicular graphite of the invention, wall thickness d < 30 mm, etched Fig. 2 Microstructure of grey cast iron with vermicular graphite of the invention, wall thickness 30 mm < d < 60 mm, etched Fig. 3 Material properties of the grey cast iron with vermicular graphite of the invention, wall thickness 30 mm < d < 60 mm

[0010] The grey cast iron with vermicular graphite of the invention comprises 3.0-3.3% by weight of C 3.60-4.00% by weight of Si 0.30-0.50% by weight of Mn 1.60-1.80% by weight of Ni 0.30-0.45% by weight of Mo 0.600-0.800% by weight of Cu 0.01-0.02% by weight of Mg 0.005-0.013% by weight of Ce 0.001-0.004% by weight of Sb 0-0.02% by weight of B the balance being Fe and inevitable impurities.

[0011] The grey cast iron with vermicular graphite of the invention has an increased content of silicon, i.e. 3,6-4,0% by weight, and consequently a reduced carbon content, both elements being in a proportion to each other such that the equivalent carbon content amounts to 4.00-4.30. The increased amount of silicon is reflected in the matrix structure which is predominantly formed of ferrite. The latter is represented in the matrix in 80%, which is also reflected in Fig. 1. The remainder amount is represented by graphite, perlite and carbides.

[0012] Silicon (Si) induces stable hardening and precipitation of carbon in the form of graphite. Due to its high proportion in the matrix, because it is a strong ferritising agent and prevents the formation of pearlite, the underlying matrix in which the vermicular graphite is distributed is predominantly ferritic. The amount of ferrite in the matrix is at least 80%. Si is dissolved in the ferritic matrix and hardens the solid solution of ferrite. This provides for the high tensile strength and also increases the yield strength. Due to the high amount of Si, the impact of the perlite-forming elements, such as copper (Cu), nickel (Ni), antimony (Sb), molybdenum (Mo), manganese (Mn), is reduced which prevents the formation of pearlitic matrix. The high amount of Si also has a positive impact on the resistance to oxidation and forms a protective layer of silicon oxide on the surface at elevated temperatures. The amount of Si must not exceed 4.0% since higher amounts lead to a possibility of formation of the undesired form of graphite, i.e. chunky graphite, which causes degradation of mechanical and thermal properties. If the amount of Si is increased, the amount of C is decreased, which has a negative impact on the thermal conductivity of cast iron.

[0013] The amount of carbon (C) in cast iron is lower with respect to the standardised grey cast iron with vermicular graphite and amounts to 3.0-3.3% by weight. The reduced amount of C is a result of the increased amounts of Si. This has a negative impact on the formation of graphite and the thermal conductivity of cast iron but is necessary to reach the desired mechanical properties and provide a good ratio between the mechanical and thermal properties. This is why the Si / C ratio is of importance. The ratio is determined by the equivalent carbon content CE which is here determined by a thermal analysis and is designated as ACEL (Active Carbon Equivalent Liquid). It amounts to 4.00-4.30 and should not be exceeded. Empirically, a rough ACEL can be calculated as ACEL = C + Si / 4 + P / 2.

[0014] The amount of manganese (Mn) is 0.3-0.5% by weight and should not be exceeded. Mn forms with sulphur (S) manganese sulphides (MnS) and creates nuclei for graphite nucleation, which yields a sufficient number of graphite particles. The amount of Mn should not exceed 0.5% by weight, otherwise the likelihood of formation of perlite is increased. As a result, the strength related properties would increase, at the same time decreasing the elongation at break.

[0015] The amount of nickel (Ni) is 1.6-1.8% by weight. Ni contributes to the ferritic matrix hardening and consequently to increased strength and hardness without having a substantial negative impact on toughness and elongation at break. Ni is a necessary additive in the developed cast iron to yield the target material properties in a predominantly ferritic matrix. It improves material toughness even at lower temperatures, this is why it is a useful addition to the components exposed to higher and repeating stresses. It decreases the ductile to brittle transition temperature. It also improves resistance to oxidation at increased temperatures. An addition in said range is necessary to reach the desired mechanical properties. It lowers the temperature of the transition from ductile to brittle fracture.

[0016] The amount of molybdenum (Mo) ranges from 0.30-0.45% by weight, wherein the amount should not be lower, otherwise the effect on the thermal and mechanical properties decreases, and should at the same time not be higher as the concentration or the number of molybdenum carbides in the matrix increases, which reduces the elongation at break and also machinability. Mo in said amount has a positive impact on mechanical properties, particularly at an elevated temperature due to a high melting point temperature. As a result, the components are stable also when operating at elevated temperatures.

[0017] The amount of antimony (Sb) should not exceed 0.004% by weight, because it is a strong perlitizer and an increased amount would cause the formation of perlite structure. At the same time, its presence positively contributes to the formation of several small graphite particles even in thicker walls of a casting and prevents the formation of chunky graphite. As a result, the microstructure is more stable.

[0018] Magnesium (Mg) is a must in grey cast irons with vermicular graphite because it is a key element in the formation of nuclei which allow the formation of vermicular and spheroid graphite. The amount of Mg is 0.01-0.02% by weight. The amount should not be too low as this would lead to the formation of lamellar graphite and not exceeded as this would result in an excessive formation of spheroid graphite.

[0019] The amount of cerium (Ce) should not exceed the threshold of 0.013% by weight since this would lead to the undesired graphite form called chunky graphite or to the so-called exploded graphite. These undesired graphite forms considerably downgrade both the thermal and mechanical properties, particularly resistance to cyclic fatigue.

[0020] The formation of chunky graphite is controlled by the amount of elements which cause its formation: Si, Ce, Mg and Sb. The tendency for the formation of chunky graphite is represented by the CHG factor. CHG = = Si*496 + 800 * Ce 555 / 140,1 − 2 *SB 55 / 121.8 + 50*Mg 55 / 24.3

[0021] The CHG factor should not exceed 10.0 as exceeding this value is likely to result in undesirable chunky graphite. The CHG factor is determined experimentally.

[0022] The grey cast iron with vermicular graphite of the invention has a considerably higher yield strength relative to the hardest conventional EN-GJV-450 cast iron, preserving its thermal conductivity values. As a result, such cast iron has better resistance to thermal and mechanical stresses at both room and elevated temperature up to 400°C. This holds true for thinner walls of up to 30 mm and also for thicker walls of 30-60 mm. The mechanical properties of the grey cast iron with vermicular graphite of the invention are obtained in the cast state, no subsequent heat treatment is required to achieve the target microstructure and material properties. To obtain data on the mechanical properties, a standard tensile test was carried out on a calibrated testing machine at room temperature and elevated temperature up to 400°C. For this purpose, test specimens were made from cast-on samples as defined in SIST EN 16079 for grey cast iron with vermicular graphite. The mechanical properties of the grey cast iron with vermicular graphite of the invention are shown in Table 1. Table 1Wall thickness [mm]T [°C]Rp0.2 [MPa]Rm [MPa]A [%]fromtofromtofromtod < 30254805305406002.55d < 304003504004204702.5530 < d < 60254304805005802.5530 < d < 604003303804004502.55

[0023] The thermal conductivity was measured using the standard TPS (transient plane surface) method. Based on the thermal conductivity and mechanical properties data obtained, a thermal shock parameter is calculated, which is a good indicator of the material's resistance to mechanical and thermal stresses. Thermal shock parameter = λ ∗ Rp0 .2 E ∗ 1000 , λ = thermal conductivity Rp 0.2 = yield strength E = elastic modulus.

[0024] An average value of the thermal shock parameter for the grey cast iron with vermicular graphite of the invention is in the range of 0.0895 to 0.0985, depending on temperature in the range of 25°C to 400°C. The values of the thermal shock parameter as a function of temperature for castings having a thickness 30 mm < d < 60 mm are shown in Table 2. The mechanical properties and thermal conductivity shown in Table 2 were determined from test specimens cut from a casting with a wall thickness of 30 mm < d < 60 mm. Table 2T [°C]E [MPa]Rp0.2 [MPa]Rm [MPa]λ [W / mK]Thermal shock parameterA [%]fromtofromtofromtofromtofromto2515044048050057028320.0820.0972510014740044047052030340.0840.0972520014439043045050033360.0890.1002530014037041043048034370.0900.1002540013234038041046036390.0930.10425

Claims

1. Grey cast iron with vermicular graphite comprises 3.0-3.3% by weight of C 3.60-4.00% by weight of Si 0.30-0.50% by weight of Mn 1.60-1.80% by weight of Ni 0.30-0.45% by weight of Mo 0.600-0.800% by weight of Mo 0.01-0.02% by weight of Mg 0.005-0.013% by weight of Ce 0.001-0.004% by weight of Sb 0-0.02% by weight of B the balance being Fe and inevitable impurities.

2. Grey cast iron with vermicular graphite according to claim 1, characterized by a ferritic microstructure in which the graphite is arranged in vermicular form, the ferritic structure in the matrix comprising at least 80%.

3. Grey cast iron with vermicular graphite according to any of the preceding claims, characterized in that its equivalent carbon content (ACEL) determined by a thermal analysis amounts to 4.00-4.30.

4. Grey cast iron with vermicular graphite according to any of the preceding claims, characterized in that the CHG factor defining the formation of chunky graphite has a value smaller than or equal to 10.0, the CHG factor being defined by the amount of elements Si, Ce, Mg and Sb.

5. Grey cast iron with vermicular graphite according to any of the preceding claims, characterized in that an average value of the thermal shock parameter which is indicative of resistance of material to mechanical and thermal stresses in the temperature range of 25°C to 400°C, is from 0.08950 to 0.0985, depending on temperature.

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