KR100417049B1 - A Method of Manufacturing Cast Products Which Are Cast in a Single-Piece Having Controlled Variations of Compacted Graphite Iron and Grey Cast Iron - Google Patents

Abstract

A method of manufacturing a casting having a controlled non-uniform graphite texture as a single structure. The castings can be cast, for example, to obtain a flaky gray iron texture in some areas and a compact graphite texture in other areas, thereby imparting different properties to different parts of the castings.

Description

Technical Field [0001] The present invention relates to a method for manufacturing a single cast product by controlling the change of compact graphite cast iron and gray cast iron,

Background of the Invention [0002] Background art is described in WO-A1-93 / 20969, WO-A1-89 / 04224, US-A-4 667 725, US-A-5 316 068, DE- SE-B-444 817 and JP-A-6/106 331.

Compacted graphite iron (CGI), that is, cast iron with sinuous graphite structure is an intermediate form between a gray cast iron having a flake graphite texture and a soft cast iron having a spherical graphite texture when viewed in a two-dimensional section.

Compact graphite cast iron has desirable and unique properties including good mechanical and physical properties and good machinability, which makes it very suitable for many components of machinery. These components include machine components that are manufactured in large quantities, particularly engines, such as all types of brake discs and hydraulic pumps.

Therefore, compacted graphite cast iron is classified as "Type III" graphite according to ISO / R 945-1975 (E) or graphite defined as "Type IV" graphite according to ASTM A 247, precipitated in the form of sinuous graphite during solidification do. This graphite form was first described in the United Kingdom (1948) and has since been used in the small-scale manufacture of special components. The reason for this small scale manufacturing is that it is impossible to control the characteristics and composition of molten cast iron with sufficient accuracy to assure the composition of castings and graphite structure with sufficient reproducibility.

The characteristics of the compact graphite cast iron are somewhere between the properties of gray cast iron and the characteristics of ductile cast iron. For example, the modulus of elasticity of the compact graphite cast iron is 30-40% higher than that of the cast iron, which means that the modulus of elasticity of the compact graphite cast iron is almost the same as that of the soft cast iron. Compact graphite cast iron has a ductility higher than that of gray cast iron, often more than ten times higher than that of gray cast iron, and has a tensile strength approximately twice that of gray cast iron. The fatigue strength of compact graphite cast iron is 100% higher than that of gray cast iron and is essentially the same as that of soft cast iron. The thermal conductivity of compact graphite cast iron is the same as that of gray cast iron and is 30-50% higher than that of ductile cast iron. The machinability and casting of compact graphite cast iron are also similar to gray cast iron.

It will therefore be appreciated that there is a good reason to use compact graphite cast iron in machine designs that require not only high strength but also good casting, machinability and high thermal conductivity. Due to the difficulties encountered when trying to obtain compact graphite castings in a reproducible manner, it was impossible to produce castings with this type of cast iron earlier.

However, it is possible to determine the nucleator and the concentration of the modifier of the melt by analyzing the temperature data for the time obtained with the aid of the temperature sensor during the solidification of the sample taken from the melt. This makes it possible to accurately determine the manner in which the melt solidifies in the mold, and the contents of the inoculant and modifier can be corrected to impart the desired properties to the casting. See SE-B-469 712, SE-B-444 817 or US-A-4 667 725 for their advantages. According to the patent specification, the values described above are measured with the aid of two temperature sensors located in the sample vessel, where the melt is essentially in thermodynamic equilibrium with the temperature of the sample vessel at the beginning of the coagulation process.

One of these temperature sensors is located in the center of the melt in the sample vessel and the other is located in the melt near the vessel wall. During the solidification process, the subcooling (T * w) of the melt at the vessel wall, the recalescence (recw) at the vessel wall, the difference in the amount (ΔT +) between the temperature at the vessel wall and the vessel center, Values relating to the temperature [(dT / d?) W] at the vessel wall at the growth temperature [(dT / d?) C = 0, T: temperature, With the help of the known reference values for the conditions, the presence and amount of the crystallization nucleating agent and the amount of the tissue regulating agent can be determined and corrected by the introduction of additives or residence time into the melt, This is equivalent to the amount needed to obtain the graphite texture required in the castings.

Tissue regulating additives usually consist of magnesium, optionally in combination with rare earth metals, especially cerium or mis-metal.

If the amount of dissolved magnesium and equivalents of other tissue regulating agents, that is, the amounts of oxides and sulfides in the form of solids present in the solution, excluding the isolated oxides, amount to at least about 0.035%, the graphite may be in spherical form when the melt solidifies And is precipitated. If the content is lowered to about 0.015%, graphite precipitates as compact graphite. If the content is further lowered below about 0.008%, graphite precipitates as flake graphite, and the cast iron solidifies as gray cast iron. It will be apparent from this that compact graphite cast iron is mainly formed between values of about 0.010 and 0.020%.

In some applications it is advantageous to use cast iron products containing non-uniform graphite structure. WO-A-93/20969 discloses a method of making cast iron products, some of which have compact graphite structures and other parts of which have spherical graphite structure.

In addition, for reasons of handling performance, it requires a high thermal conductivity as high as possible with a relatively low modulus of elasticity and, if possible, a flaky gray cast iron in some areas that require good casting and machinability for reasons of productivity and a higher strength and toughness It would also be advantageous to use compact graphite cast iron for other areas that require it.

A similar attempt to obtain non-uniform graphite in the engine block has been previously proposed. JP-A-6/106 331 relates to a method of manufacturing a ductile cast iron engine block for improved strength and toughness by forming a reactive coating on a sand core forming a cylinder bore wall, Of active magnesium to provide elongated graphite flakes, thereby providing good thermal conductivity and machinability to the bore wall.

However, the reactive coating applied to the surface of the cylinder core can only reduce a certain amount of magnesium. Therefore, reproducible results are hardly obtained when the magnesium content of the cast iron to be cast into the mold is constant. The change in magnesium content is typical and manifests itself as a change in the graphite texture within the bore wall.

By starting with a soft base iron as is necessary in JP-A-6/106 331 due to the lack of proper process control, relatively poor casting of the ductile cast iron leads to the formation of complex thin-wall castings such as modern engine blocks and cylinder heads Thus limiting the ability to successfully manufacture.

Initial treatment with magnesium also limits the ability to form thick layers of flake graphite adjacent the reactive core surface. The magnesium content in the cast iron changes from its bulk concentration value in the vicinity of the wall to a sufficiently lower level, which enables flake formation. However, due to the combined effect of the excess magnesium required to produce spheroidal graphite and the magnesium change found in provisionally treated iron, the thickness of the flake layer is rather thin. This is particularly important when it is realized that up to 3 mm of surface can be removed with machining stock and therefore the flake graphite can be almost lost.

Furthermore, it is not clear that the dramatic changes in the physical and mechanical properties between the gray cast iron and the ductile cast iron will benefit the long-term performance of the castings. Large differences in strength, toughness, ductility, and thermal conductivity can lead to abnormal internal stresses and strain gradients, which can ultimately lead to negative effects rather than positive effects.

US-A-5 316 068 also starts with a soft cast iron base, but it transforms the graphite transfer mechanism into a high speed rotation of the mold in the reactive core during solidification to promote gray cast iron in the outer region and compact graphite cast iron in the central bore region. This technique is not only difficult to handle physically, but it also annoys the problem as outlined in the discussion of JP-A-6/106 331, which occurs at a soft starting point.

DE-A-43 08 614 relates to a process for the production of cast iron products, some of which contain flaky gray iron and the other of which contain compacted graphite cast iron. A portion of the mold is coated with an oxygen or sulfur-releasing material to reduce the active magnesium content of the portion of the melt adjacent the coated mold wall.

However, DE-A-43 08 614 does not disclose anything about how to control the composition of the melt in order to be able to reproduce the casting process. The difficulty of casting the compact graphite cast iron in a reproducible manner has already been mentioned in this application and it has been found that the process according to DE-A-43 08 614 can be used to reproducibly cast heterogeneous graphite textures without the possibility of controlling the composition of the melt It should be noted that it is extremely difficult to use. Moreover, when using this method, it is common to use excess magnesium or similar metal. Therefore, up to now, it has been impossible to cast a non-uniform graphite structure composed of a portion containing a flaky gray iron and a portion containing a compact graphite cast iron.

Summary of the Invention

The problem associated with the fabrication of a single cast iron product in which compact and flaky graphite crystals are non-uniformly distributed in different parts of the finished castings

I) A molten cast iron having inherent properties of solidifying with compact graphite cast iron is produced by controlled high Mg Mg or its equivalent component,

II) casting the molten cast iron into a mold having at least one portion having a mold wall or mold core coated with a reactive material that diffuses or penetrates into the molten cast iron to reduce the active Mg or the concentration of the component, A method of solidifying a cast with a flat cast iron in a portion covered with the reactive material and solidifying with a compact graphite cast iron in another portion of the casting,

The inner wall of the vessel is arranged adjacent to and adjacent to the vessel wall in such a manner and manner as to simulate the reduction of the concentration of the active Mg or said component by said reactive substance in the molten cast iron injected into the casting mold in step II In a container made of a material coated with or containing a layer of material which reduces the concentration of active Mg or the percentage of said component in the sample volume corresponding thereto in the vicinity of the positioned temperature responsive means, Extracting a sample volume from the molten cast iron in the sample vessel having two temperature response means located in the center of the vessel wall and in the vicinity of the vessel wall,

Bringing the vessel essentially into thermal equilibrium with the sample,

Recording the temperature registered by the two temperature response means,

- evaluating the properties of the molten cast iron itself from the recorded curve in a known manner and registering a deviation indicative of precipitation of flaky graphite crystals around the temperature responsive means in the vicinity of the vessel wall,

In solidification of molten cast iron, the molten cast iron whose concentration is essentially unaffected by the reactive substance is sufficient to produce compact graphite crystals, and the molten cast iron affected by the reactive substance is a graphite crystal Correcting the content of active Mg in the molten cast iron or the content of the other component (s) in the molten cast iron with the aid of deviations in the device parameters and temperature time curves,

By controlling and correcting the inherent characteristics of the molten cast iron solidifying with the compact graphite cast iron.

Hereinafter, the present invention will be described in more detail with reference to embodiments and drawings.

The present invention relates to a method of making castings cast into a one-piece object having controlled non-uniform graphite texture. According to the invention, this casting can be cast to obtain a gray cast iron having a piecemeal graphite texture in one part and a vermicular graphite texture in another part, thereby giving different properties in different parts of the casting .

Figure 1 is a plot showing percentage of nodularity as a function of magnesium percentage. In Fig. 1, 0% of nominality corresponds to fully compacted graphite cast iron, while nominality 100% corresponds to fully spherical cast iron, that is, ductile cast iron. Finally, the nominal value below 0% is related to gray iron. In fact, the nominal 0% corresponds to 100% compact graphite cast iron, and the lowest part of this axis corresponds to 100% flake gray.

Figure 2 shows a core cross-section of a nonuniform compact gray cast iron / flaky gray iron cylinder head showing graphite texture and general design.

3A-b are photomicrographs showing the transition from a flaky gray cast iron to a compact graphite cast iron. The magnification is 100x.

According to the present invention, it is possible to faithfully reproduce the compact graphite cast iron having the optimum coagulation potential so that a single casting is consistently produced by proper mixing of compact graphite particles and flake graphite. Starting with compact graphite base iron, the thickness of the flaky gray iron which can be produced by a given reactive coating is increased, while the internal stress and strain gradient is reduced because the mechanical and physical properties of gray iron and corrugated cast iron Because it is more similar than that of. The casting and machinability of the finished component is also very good.

In addition, the process according to SE-B-469 712 enables precise measurement of the vicinity of the treated cast iron for rapid transition between compact graphite and flake graphite. By deliberately changing the distance between the coating of the reactive wall and the measuring point of the thermocouple in the sample, or by changing the reactivity of the coating located on the inner wall of the container, the left edge of the stable, serpentine (point A in Figure 1) It is possible to precisely produce compact graphite base iron that is prone to generate a significant amount of flaky gray iron when injected into a mold that is somewhat close to and thus coated with a reactive coating and a core containing the core. On the other hand, the sinuous base point of the vicinity of point B in FIG. 1 requires a further reduction of magnesium and thus does not produce such a broad graphite network. By directly selecting and reproducing the appropriate starting point of the treated liquid iron, a better controllability than the microstructure as cast ensures the optimal coherent layer of flake graphite.

The following embodiments relate to a cylinder head, but the method according to the invention is advantageous if the bulkhead, top desk, fan rail and crankcase section contain compacted graphite cast iron with a higher strength, for example, while the cylinder bore and water jacket cores contain gray iron For example, for use in the casting of engine blocks and also for brake discs containing compacted graphite cast iron such that, for example, the outer flange contains lamellar cast iron with high thermal conductivity and the inner hub provides higher strength.

The maximum heat induction stress developed at the hot face of the cylinder head can be expressed by the following equation (1).

In the above formula

σmax = maximum heat induction stress (MPa)

ΔT = temperature gradient from the open face of the cylinder head to the cooling water channel (° C.)

Eo = elastic modulus (MPa)

α = thermal expansion (° C -1)

μ = Poe ratio (no dimension)

It is now known that the thermal expansion and puffing of sinuous cast iron and gray iron are essentially the same. The only way to minimize the thermal stresses that accumulate on the hot side and ultimately create cracks between the valve ports is to minimize the parameters ΔT and Eo, in this case ΔT being inversely proportional to the thermal conductivity. The aim is therefore to obtain a material with a high thermal conductivity and a low modulus of elasticity on the hot side, which can only be met by gray cast iron. At the same time, the bulk material of the cylinder head and the outer periphery is preferably made of a material having higher strength, toughness and ductility. This objective can be met by high-quality (less than 10% nodality) sinuous cast iron which may result in lower internal deformation than the comparable gray cast iron / ductile iron mixture, and the sinuous cast iron / (Point A in FIG. 1), the solidification behavior initiation point of the base cast iron is the soft cast iron (C in FIG. 1), or the solidification behavior initiation point of the base cast iron It is possible to produce a widescreen graphite network that is wider than that which can be obtained from the < RTI ID = 0.0 >

Where flake graphite is desired, reducing the active magnesium content and consequently growing flake graphite particles rather than compacted graphite particles is accomplished by applying various standard cast coatings to any mold or core surface. In the case of a cylinder head (FIG. 2), a reactive coating can be applied to the surface of the hot-surface mold and the lower half of the water channel core. The coating contains a controlled amount of sulfides and / or oxides that chemically react with the active magnesium to adhere MgS and / or MgO. The amount of coating required can be repeatedly defined for each casting application, depending on the required flake thickness, and will be apparent to those skilled in the art.

The present invention is particularly useful for existing cylinder head designs where it is not possible to redesign the cylinder head because the cylinder head must continue to conform to the existing engine. Recently, as power-up and turbocharging are required for gasoline, especially diesel engines, many designs are recommended for the conversion to stronger materials, which is ideal for compact graphite cast iron. However, when the head is liable to fail due to heat load, compact graphite cast iron having low thermal conductivity and high elastic modulus as compared with gray cast iron effectively increases the heat load, thereby shortening the useful life. Thus, for a compact graphite cast iron cylinder head, the only way to reduce the? T term in Equation 1 would be to reduce the thickness of the flame deck. However, this will make the cylinder head incompatible with existing engine designs. The present invention is an ideal solution in this case because the introduction of flaky gray iron into the flame deck provides the necessary thermal conductivity and low modulus of elasticity to withstand thermal loads while not sacrificing machinability or casting behavior, This is because the cast iron provides the strength, toughness and ductility necessary to resist the mechanical load.

Claims (2)

CLAIMS What is claimed is: 1. A method of making a single cast iron product having non-uniformly distributed graphite crystals in compact and flake forms on different parts of the finished cast product, I) A molten cast iron having inherent properties of solidifying with compact graphite cast iron is produced by controlled high Mg Mg or its equivalent component, II) casting the molten cast iron into a mold having at least one portion having a mold wall or mold core coated with a reactive material that diffuses or penetrates into the molten cast iron to reduce the active Mg or the concentration of the component, A method of solidifying a cast with a flat cast iron in a portion covered with the reactive material and solidifying with a compact graphite cast iron in another portion of the casting, The inner wall of the vessel is arranged adjacent to and adjacent to the vessel wall in such a manner and manner as to simulate the reduction of the concentration of the active Mg or said component by said reactive substance in the molten cast iron injected into the casting mold in step II A container made of material coated with or containing a layer of material that reduces the concentration of the active Mg in the vicinity of the positioned temperature responsive means or the percentage of the corresponding component in the sample volume, Extracting a sample volume from molten cast iron in a sample vessel having two temperature responsive means located in the center of the vessel and in the vicinity of the vessel wall, Bringing the vessel essentially into thermal equilibrium with the sample, Recording the temperature registered by the two temperature response means, - evaluating the properties of the molten cast iron itself from the recorded curve in a known manner and registering a deviation indicative of precipitation of flaky graphite crystals around the temperature responsive means in the vicinity of the vessel wall, In solidification of molten cast iron, the molten cast iron whose concentration is essentially unaffected by the reactive substance is sufficient to produce compact graphite crystals, and the molten cast iron affected by the reactive substance is a graphite crystal Correcting the content of active Mg in the molten cast iron or the content of the other component (s) in the molten cast iron with the aid of deviations in the device parameters and temperature time curves, Characterized by controlling and correcting the inherent characteristics of the molten cast iron solidifying with the compact graphite cast iron. 2. The method of claim 1, wherein the walls of the sample vessel are coated with a layer of material that reduces the concentration of active Mg in the sample by 0.002 to 0.010 weight percent, or by a corresponding percentage of the component (s) ≪ / RTI > material.

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