Method and casting mold for the production of cast-iron cylinder liners

Abstract: A method for producing a cast-iron cylinder liner for piston engines in which a metal chill mold (1) is used which has a tubular mold cavity (2) lined with a layer of a hardening molding material or green sand (3). The melt (7) is introduced into the mold cavity from above in such a manner that the cooling effect from the chill mold and the lining provides a frontage of solidification directed upwardly from the bottom.

Description: TECHNICAL FIELD OF THE INVENTION: The present invention relates to a method for the production of a tubular cast-iron component, preferably a cast-iron cylinder liner for use in piston engines according to the preamble of claim 1. The invention further relates to a casting mold for use in said method according to the preamble of claim 7. BACKGROUND OF THE INVENTION: Cylinder liners for car and truck engines are normally centrifugally cast. The reason for this is that the phosphorous-alloyed grey iron which is normally employed is almost impossible to cast in a conventional green sand mold because the iron is particularly susceptible to shrinkage. In centrifugal casting, a heated mold is employed which is made up of rotating tube, the mold cavity of which having a thin layer of an insulating material. Due to the effect of the centrifugal forces, the shrinkage of the cast product is compensated for. Centrifugal casting does however impart limitations as to the strength of the material due to the fact that the quick cooling during solidification i.a. precludes high alloying with carbon-stabilizing alloying elements and low C eq , these being the most common measures which can be employed to increase strength. Other disadvantages are that the centrifugal casting breaks up the first precipitated reinforcing primary austenitic dendrites in the structure and centrifugally separates primary austenite and graphite eutecticum at low C eq . In order to avoid these problems, cylinder liners could, for example, be cast in stable molds of cold hardening mold material or core sand, though such molds are very expensive and detrimental to the environment. DE-C 35 09 015 discloses a method for casting a complete engine cylinder. This method uses a mold consisting of hardened sand material. In the mold cavity a central core is placed which comprises a tubular metal element which on both sides is lined with a layer of hardened sand material, the inner layer having a thinner wall thickness at its upper end than at its lower end. The core forms the bore of the cylinder. Its tubular metal element increases heat transfer from the cylinder wall to the core in order to produce a structure which is free of pores. GB-A-1 368 126 discloses a method for casting cylindrical metal objects. A mold is used which has a mold cavity open at the upper end and a wall covered with a sand layer containing a binding agent. The mold cavity is slightly conical. Molten metal is poured into the upper end of the mold cavity. The variation of the degree of compression of the sand layer due to variation of hydrostatic pressure is compensated by the conical shape of the mold cavity so that a cylindrical object is produced. SUMMARY OF THE INVENTION: It is an object of the present invention to provide a molding method which is particularly, though not exclusively, intended for the production of cast-iron cylinder liners to thereby achieve a low-cost and environmentally-friendly production technique of such cast products which have a higher strength than those obtained by centrifugal casting. This is achieved in accordance with the present invention by a method having the features of claim 1, and by a mold having the features of claim 7. The layer of insulating mold material is preferably a hard and relatively thin (in the order of 5 to 15 mm) sand shell of a hardening molding material with suitable known organic or inorganic binders produced by known methods, or green sand. The shaping is achieved with the help of a pattern having the shape of the object. This is introduced into the mold cavity of the chill mold, whereafter the sand shell is created in the gap between the pattern and the wall of the chill mold by introducing sand using a common core-forming machine or by pressing. In accordance with the invention, by ensuring that the solidification is strongly directed from the bottom of the mold cavity and upwards, the risk of shrinkage porosity in the cast object is eliminated since the lastly solidified iron is located in the header volume. The method has been shown to impart such a high reduction of C eq and increase in the alloying content that the ultimate tensile strength of the cylinder liners is raised by 40% and the modulus of elasticity by 20% compared to centrifugally cast cylinder liners. Despite a high phosphate content, no shrinkage pores are formed. Further scope of applicability of the present invention viii become apparent from the detailed description given hereinafter. However, it is to be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the claimed invention viii become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS: The present invention will be described in greater detail in the following by way of example only and with reference to the attached drawing which shows a longitudinal sectional view through a casting mold for casting cylinder liners. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: In the drawing, reference numeral 1 denotes a thick-walled steel chill mold presenting a tubular mold cavity 2 which is closed at its base and open at its top. The walls of the mold cavity are lined with a layer 3 of insulating material, preferably hardening molding material or green sand. The chill mold 1 is intended for the production of a cylinder liner blank 4. The mold cavity 2 presents a conical profile adapted to the elongated shape of the liner, the upper region of which serves as a header volume 5 for the melt. In the drawing, for the sake of clarity, a finished liner is indicated by dashed lines. In the shown embodiment, casting is effected by pouring the melt 7 from a ladle or from a melting furnace having a pouring basin 8, though the melt may also be poured directly into the mold cavity 2. In one possible production arrangement, four to eight chill molds are positioned along a line or in a circle. Casting takes place via a pouring basin with a runner to each mold. The method according to the invention has been developed primarily for the production of cast-iron cylinder liner blanks having a wall thickness of 8 to 20 mm, in particular grey iron having the following alloying elements and percentage content: C: 2.4-3.2; Si: 1.60-2.20; Mn: 0.5-1.0; S: <0.12; P: 0.3-0.8; Cr: 0.8-1.3; Mo: 0.1-1.0; V: 0.1-0.3. It has been shown to be suitable for cylinder liner blanks with these thicknesses and alloying elements to use a sand shell layer of 5 to 15 mm thickness. By making the layer thinner in the lower region of the mold cavity, preferably 5 to 10 mm, and thicker in the upper region, preferably 10 to 15 mm (as shown in the drawing), the melt will be more quickly cooled in the lower region, which further contributes to the control of the transfer of the frontage of solidification upwardly from the bottom. The slow solidification in the sand-shell insulated chill mold permits greatly reduced C eq and higher content of carbide-stabilizing alloying elements. In this manner, the ultimate tensile strength, fatigue strength and modulus of elasticity can be increased considerably, which implies that the cylinder liners can be dimensioned more thinly, which in turn implies that for a given cylinder block size the cylinder capacity is greater, or that the strength and stiffness margins in the construction are increased. The high alloying content of carbide-stabilizing elements further implies that the volume and the hardness of the wearing phase in the cylinder liner iron, steadite, increases. This is due to the quantity of cementite in the steadite increasing. Compared to normal phosphor-alloyed cylinder liner iron, the quantity is increased from normally circa 4% steadite at 0.6% phosphor to circa 7%, thereby offering improved wear resistance. In traditional centrifugal casting, the thickness of the insulation layer and the mold temperature varies. However, the sand shell insulated chill mold which is employed in the method according to the present invention has a stable, constant insulating effect. This results in even solidification and cooling rates, which in turn provide more constant hardness and strength levels, machinability, etc., in other words generally better quality. With the highest stipulated alloying quantities, the hardness is relatively high because the cooling effect of the chill mold becomes significant at the pearlite transformation temperature, circa 750°C. This can detrimentally affect machining somewhat. In order to avoid the pearlite hardness becoming unnecessarily high, in a further embodiment of the method according to the invention a method has been developed which reduces the cooling rate at just the pearlite transformation so that the pearlite becomes less compacted and thus softer. This is achieved by removing the liner in its austenitic state, 800-1050°C, from the mold and immediately transferring it to, and immersing it in, an insulating medium, preferably vermiculite in powdered form, and maintaining it there until the temperature of the liner has dropped below the pearlite transformation temperature. In this manner, the low C eq and the alloying elements can be fully utilized to achieve a favourable solidification structure which has greatest effect on the desired properties without the pearlite hardness being unnecessarily high.

Claims: Method for producing a cast-iron tubular component, preferably a cast-iron cylinder liner for a piston engine, wherein the walls of a tubular, upwardly open mold cavity (2) in a mold are lined with a layer of an insulating molding material (3) and the cast iron melt is introduced from above, characterized in that the mold is a metal chill mold, the cavity walls of which are lined such that the wall thickness of the insulating molding material is thinner in the lower regions of the mold cavity than in the upper regions and in that the cast-iron melt (7) is introduced in such a manner that the cooling effect from the chill mold and the lining provides a frontage of solidification directed upwardly from the lower end of the lining to a header volume (5) at the top for the lastly solidified iron. Method according to claim 1, characterized in that the tubular mold cavity (2) is lined with a hardening molding material (3). Method according to claim, characterized in that the tubular mold cavity (2) is lined with green sand (3). Method according to any one of claims 1 to 3 for the production of cast-iron cylinder liners, characterized in that the insulating layer (3) is given a wall thickness of 5-15 mm. Method according to any one of claims 1 to 3 for the production of a cylinder liner of high-tensile grey iron, characterized in that the mold cavity (2) is filled with a cast-iron melt (7) having the following alloying elements and percentage content: C: 2.4-3.2; Si: 1.60-2.20; Mn: 0.5-1.0; S: <0.12; P: 0.3-0.8; Cr: 0.8-1.3; Mo: 0.1-1.0; V: 0.1-0.3. Method according to any one of claims 1 to 5 for reducing the hardness of pearlite in a cylinder liner, characterized in that the liner is removed from the mold cavity in the austenitic state and in this state immediately transferred to, and totally surrounded by, an insulating material, preferably vermiculite in powdered form, and maintained there until the liner has cooled to below the pearlite transformation temperature. Mold for molding cast-iron cylinder liners for piston engines, comprising a stationary mold (1) having a mold cavity which is upwardly open and lined with an insulating layer (3), characterized in that the mold is a metal chill mold, the cavity of which is tubular and leads into a header volume and that the mold cavity and the header volume are lined with an insulating layer (3) which has a thinner wall thickness at its lower end than at its upper end. Mold according to claim 7, characterized in that the insulating layer (3) consists of a hardening molding material. Mold according to claim 7, characterized in that the insulating layer (3) consists of green sand. Mold according to any one of claims 7 to 9, characterized in that the insulating layer (3) has a wall thickness of 5-15 mm.

Inventors: DAHLBERG SVEN-ERIK, GYLLENSTEN BERNDT, SANDER BERTIL, SZABO TIBOR, DAHLBERG, SVEN-ERIK, GYLLENSTEN, BERNDT, SANDER, BERTIL, SZABO, TIBOR
Applicants: VOLVO AB, AB VOLVO
Application year: 1993
Main classification: B22D15
IPC codes: B22D15, C22C37, B22D27, F02F1