Method and device for chill molding

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of International Application Number PCT/SE99/02005 filed Nov. 5, 1999 which designates the United States. The disclosure of that application is expressly incorporated herein by reference.

BACKGROUND OF INVENTION

1. Technical Field

The present invention relates to a method and a device for chill molding cast iron.

A method and a device for the manufacture of cast iron parts by casting in a stationary metal mold, which is lined with a layer of hardening molding material or green sand, is shown in SE-C-506508. In that arrangement, a tubular metal mold is used whereby a tubular, upwardly open space in the mold is lined using an insulating form material. Molten cast iron is filled from above in such a way that the cooling effect of the mold and lining gives a directional frontage of solidification from the lower end of the lining and upwards to a feeder volume at the top for the last of the iron to solidify.

The described method and device give excellent results for cast parts of even thickness and relatively thin walls, such as cylinder linings, but are less suitable for casting of parts with varying cross-section and more complex geometry, where the rate of cooling will vary too much between different parts of the casting. Demands for improved mechanical properties combined with good ductility means that alloyed materials, which are traditionally used for improving mechanical properties, can not be used as the workability will be reduced due to the high carbide content and casting becomes difficult due to its tendency to shrink.

SUMMARY OF INVENTION

A general purpose of the present invention is to provide a method and a device for chill molding cast iron parts of varying cross-sectional area and of relatively complex geometry in which the mechanical properties of the cast material is not controlled and limited by the added alloying materials alone.

A further purpose of the casting method according to the invention is to provide increased possibilities for influencing the rate of cooling of the casting, primarily through the pearlite transformation temperature range, which makes it possible to improve the mechanical properties even further. An increased rate of cooling will also increase productivity; that is, a larger number of cast parts per unit of time and production unit.

Still further, the invention fulfills and/or enables: high level environmental requirements such as low emissions of pollutants, reduced use of energy, a clean working environment, reduced use of molding material or sand, calculated per unit of weight for castings with a corresponding reduced need for depositing molding material or sand, and a significantly improved recovery of added energy.

According to the present invention, these purposes are achieved by a device for casting cast iron that includes a chill mold having outer walls and inner walls in which the inner walls are in contact with a mold. The device also includes pressurizing means or arrangements for applying a variable pressure against the outer walls of the mold. A chill mold cooling means or mechanism for variable cooling of the inner walls of said metal chill mold is also provided.

The wall thickness of the mold is chosen so that the desired rate of heat transfer for the required mechanical properties of the cast part is achieved. The mold is preferably made of molding material or green sand.

Furthermore, it is advantageous to include an hydraulic or a pneumatic press in the pressurizing means or mechanism for acting on the outer walls of the metal chill mold.

The chill mold cooling means or mechanism preferably includes a number of cooling circuits arranged in the metal chill mold, a coolant container, a heat exchanger and a coolant pump that circulates a coolant through a coolant conduit interconnecting the cooling circuits with the coolant container, the heat exchanger and the coolant pump.

These purposes are achieved according to the present invention by a method for manufacturing cast iron parts in which a metal chill mold, having outer walls and inner walls and where the inner walls are in contact with a mold, is filled with molten cast iron. The method is characterized in that pressurizing means or mechanism can apply a variable pressure against the outer walls of the metal chill mold and the chill mold cooling means can variably cool the inner walls of the metal chill mold during the cooling of the casting.

The mold is preferably made from a hardening molding material or green sand. The thickness of the walls of the mold is chosen to achieve the required rate of cooling.

The casting method allows casting of materials having a low C-equivalent, as well as materials having high levels of carbide stabilizing alloying materials to be used to obtain castings with a considerably higher flexural strength, fatigue strength and modulus of elasticity, which in all will give good mechanical properties.

By casting materials with a low C-equivalent and by adding moderate amounts of carbide stabilizing alloying materials, a strong material, virtually free of carbides and with a good machinability, can be obtained.

The casting method will also give less dimensional scatter for the casting compared to conventional green sand casting.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the invention will be described in more detail below, with reference to the appended figure, wherein;

FIG. 1

shows a schematic cross-section of a device for chill mold casting of cast iron according to the present invention.

DETAILED DESCRIPTION

FIG. 1

shows an arrangement for chill mold casting a cast iron article according to the present invention. The device includes a rigid, thick-walled metal chill mold

100

, with side elements

200

, a top element

205

and a bottom element

207

. Each of the side elements

200

has an outer wall

210

, facing away from a mold cavity

150

and into which molten cast iron is to be poured, and an inner wall

220

that faces the mold

300

. The top element

205

is provided with a corresponding outer side

206

and an inner side

212

.

The bottom element

207

has an outer side

208

and an inner side

213

. The thickness of he mold wall

330

is chosen so that a desired heat transfer rate is obtained. The mold material, wall thickness, pressure and temperature controls the heat transfer rate; that is, a thin wall will give a fast cooling rate and a thick wall a slow cooling rate. The mold

300

is produced by conventional methods, alternatively in a air-squeezing core machine, a core forming machine or by manual manufacture, using a hardening, insulating mold material, with a suitable known organic or inorganic binding agent, or green sand. The molding is performed using a template which shapes the mold cavity

150

. The thickness of the mold wall

330

is typically generated by conventional means, but may alternatively be established in the core box or by the height of the mold block.

The mold

300

preferably includes a first mold part

310

and a second mold part

320

. The mold parts

310

and

320

are joined by means of an adhesive or a bolt connection after the core has been assembled, should a core be required. The mold

300

is placed in the chill mold

100

whereupon the side elements

200

, the top element

205

and the bottom element

207

of the chill mold

100

closes around the mold

300

by pressurizing one or more pressurizing means or mechanisms

400

. Molten material is poured into the mold through an inlet port

160

which is connected to the mold cavity

150

. The inlet port is made by conventional methods.

In this way it is possible to apply variable pressure on the side elements

200

, the top element

205

and the bottom element

207

of the chill mold, using pressurizing means

400

arranged in connection with the chill mold. The pressurizing means

400

preferably includes hydraulic or pneumatic presses arranged to act on the outer walls,

206

,

208

and

210

respectively, of the chill mold. During solidification of the molten material in the chill mold

100

, volume reductions (e.g. during forming of austenite) and increases (e.g. during forming of graphite) will occur during different phase transformations. These changes in volume will be larger or smaller depending on factors such as the relationship in size between the molten material, the mold and cores, if any, as well as the chemical composition of the basic material, inoculation, treatment of the smelt, etc. By making it possible to control the pressure applied to the outer walls,

206

,

208

and

210

respectively, of the chill mold, it is also possible to partially control the force by which residual molten material is transferred from areas of increasing volume to areas of decreasing volume, without being forced into the mold or core, nor causing shrinkage porosity.

The device according to the invention is also provided with variable cooling by a chill mold cooling means or mechanism

500

with acts on the inner walls of the chill mold

212

,

213

and

220

respectively. The chill mold cooling means

500

includes several, preferably six, cooling circuits

520

arranged in or on the side elements

200

, top element

205

and bottom element

207

of the chill mold. The chill mold cooling means

500

preferably includes a coolant container

530

in which a coolant such as water is stored. A heat exchanger

540

is included for recovering heat from the coolant and a coolant pump

550

is used for circulating the coolant through a coolant conduit to and from the coolant circuits

520

.

The mold cavity

150

is cooled by the coolant in the chill mold

100

during the entire casting process. The rate of cooling is regulated by the heat transfer rate of the mold wall

330

, the heat transfer rate of the inner wall

220

of the chill mold, the mold cavity

150

and the temperature of the coolant. The heat transfer is also affected by the pressurization of the pressurizing means

400

. The rate of cooling is controlled during the entire cooling process, until the pearlite transformation has been completed, to achieve the desired mechanical properties for the casting; a high cooling rate will give a high strength. The cooling rate through the pearlite transformation phase can be increased by opening the chill mold when the temperature of the casting is above the temperature for pearlite transformation. The air cooling which will then occur increases the cooling rate further giving an even higher strength. On the other hand, the cooling rate can also be reduced by opening the chill mold when the temperature of the casting is in the austenite range. Immediately after the opening, the casting is immersed in and covered by an insulating medium and is kept in this state until the temperature of the casting has dropped below the pearlite transformation temperature. This method can also be used for reducing stresses in the cast part, but the casting must then be kept in the insulating medium until its temperature is lower than 200° C., in the case of cast iron. The opening of the chill mold can take place before or after the pearlite transformation phase, depending on the material properties desired.

The invention is not limited to the embodiments shown in the figure or described above, but can be modified within the scope of the appended claims. It is, for instance, possible to construct the mold in more than two mold parts, e.g. by using three or four parts assembled into one mold unit.