GB2212425A

GB2212425A - Process for the accelerated cooling of cast parts

Info

Publication number: GB2212425A
Application number: GB8727276A
Authority: GB
Inventors: Gerhard Neumann; Gunter Pistol; Jurgen Hubler; Andreas Freudenberg
Original assignee: GISAG JURI GAGARIN VEB
Current assignee: GISAG JURI GAGARIN VEB
Priority date: 1986-10-02
Filing date: 1987-11-20
Publication date: 1989-07-26
Also published as: FR2604644A1; GB8727276D0; DE3730398A1; DD252988A1

Description

1 22 12425 Process for the accelerated cooling of cast parts The present

invention is concerned with a process for the accelerated cooling of castings of cast iron subsequent to the solidification, which process can be 5 used in all foundry plant.

The central point of the casting finishing process is the cast body formation in which the molten melt is brought into the shape corresponding to the geometry of the mould used and is converted into a solid, shapeforming body by the removal of heat. The course of the process at the point of casting determines the capacity of the foundry; it provides the conditions for the material and heat flow of pre- and post-arranged technological moulding and casting plants and has resulted in the high finishing rates and corresponding production capacities of modern foundries. The cooling paths of the shaping plant result for the product from the necessary cooling time of the individual cast bodies, the mould flow time and the longitudinal dimensions of the mould. If the heat transfer conditions between the cast body and the mould are maintained, 0 -2.- then increased finishing speeds lead to proportionally lengtened cooling paths. From this result corres pondingly laborious plaqt-technical solutions to the problem.

With the conclusion of the solidification, the mould has fulfilled its primary task of shaping. After a technologically determined time after conclusion of the solidification, the cast body and the mould are separated. After the separation, other mostly increased heat removal conditions occur. The technically realisable heat removal speed is limited by two criteria: 1. the temperature differences on the basis of the differing wall thicknesses of a cast piece should, during the cooling, not be so great that it results in increased inherent tensions and, in extreme cases, to distortion or stress cracks; 2. in the case of structural changes in the solid state, it should not result in inadmissible increases of hardness.

On the basis of experience, in practical foundry operation, the procedure is usually such that cast parts susceptible to stresses and cracks remain in themould as long as possible, thus, for example, in the case of 0 cast pieces of GGL to temperatures of < 30CC. The extent of the inherent stresses of a cast part are dependent upon its shape and the cooling regime. In the case of parts with a complicated structure, even in the case of cooling in a mould down to temperatures of < 30CC., considerable inherent stresses still occur. In extreme cases, the inherent stresses are -considerably reduced by subsequent stress-free annealing.

Numerous attempts have been made to shorten the cooling times, especially for cast parts of iron carbon casting materials (see German Democratic Republic Patent Specification No. 141,123 and Federal

Republic of Germany Patent Specifications Nos.31 00 028 and 33 23 839).

The proposed solutions to the problem start from accelerated cooling of cast parts of iron-carbon from temperatures of < 600'C. in order to avoid increased inherent stresses and increases of hardness.

In the case of the above-inentioned German Democratic Republic Patent Specification No. 141,123, the cast body and mould are only separated after the eutectoid change. In the case of the above-mentioned Federal Republic of Germany Patent Specification No.

33 23 839, the separation of mould and cast body is admittedly carried out at super-eutectoid temperatures but subsequently a controlled cooling is provided which proceeds slowly until the conclusion of the eutectoid change and only thereafter is it accelerated. On the basis of the course of cooling of comparatively thin parts of the cast body, the eutectoid change for these takesplace markedly more quickly than in the thicker parts. In the case of conventional cooling of a cast body in the mould and also in the case of the suggested in the two above-mentioned Patent Specific ations, the result of tris is that the thinner parts of the cast body display the highest hardness and the thickest parts the lowest hardness.

It is an object of the present invention to obtain a shortening of the process in the case of cooling cast bodies subsequent to the solidification and to achieve improved cast body quality.

The technical problem forming the basis of the present invention is to provide a process which permits, in the case of a conventional manner of working, a substantial shortening of the cooling time and a shortening of the length of the cooling path in modern casting plant with simultaneous reduction of the spread. width of the structural formation of the metallic.

matrix, of the hardness and workability, as well as of the degree of the inherent stresses.

According to the present invention, this problem is solved in that the cast part, immediately after the greatest wall thickness thereof has gone below the solidus temperature, is separated from the mould and subsequently immediately subjected to a controlled cooling regime. The cast part is thereby cooled at an increased rate (18 - 20K/min.) in moving air and, as soon as the temperature in the greatest wall thickness in the temperature range of the transition of the preponderantly plastic to the preponderantly elastic -work material condition has been reached, which for Fe-C alloys lies at 700 to 500'C., it is heated as quickly as possible to temperatures in the region below the eutectic transition. Heating times of < 5 minutes are chosen and there is thereby achieved a temperature adjustment between the thinner and the thicker parts of the cast piece or even high temperatures in the smaller wall thicknesses.

Immediately after reaching the temperature range below the eutectoid transition, there is carried out a rapid cooling of the cast part (70 K/min.) to the manipulation temperature or to ambient temperature.

The following advantages can be achieved with the process according to the present invention: in comparison with conventional technologies, the cooling time can be reduced to about 10%; the residual stresses in comparison with cooling in the mould to 100'C. are reduced to 50%, i.e. to an order of magnitude which otherwise could only be achieved by additional stress-free annealing; in the case of Fe-C casting materials, due to the acceleration of the eutectoid transition in the greater wall thicknesses of the cast part, the formation of the metallic basic structure between the thicker and the thinnr cast body parts becomes s.imilar and thus differences of the mechanical properties, especially of the hardness and thus of the workability, are 5 -reduced.

The followipg Example is given for the purpose of illustrating the present invention:Example.

Cast parts in the form of a tentering frame are cast in moulds for single use from GGL 25. After going below of the cast body temperature below the solidus temperature, the cast body is- separated from the mould at about 10OCC.

In the case of a previously used cooling in the moulding box, the technologically possible unpacking temperature in the thicker cast body parts is achieved after 95 minutes at about 400'C.

After separation of the mould and cast part, the rate of cooling is increased by means of air nozzles from about 0.4 K/s. to about 2.0 K/s.

After the temperature equilibration between the thin and thick parts of the cast body, which is achieved by the most rapid possible heating of the cast body to temperatures below the eutectoid temperature in an annealing furnace, there follows further accelerated cooling. This cooling to ambient temperature is carried out at cooling rates of about 13 K/s. in water with 1% aqua-plast. The total cooling time is thus reduced to about10 minutes.

In the case of a ast body cooled according to the process of the present invention, the inherent stress 5..in a 20 mm. cross-section is reduced to 7.0 MPa, whereas in a 40 mm. cross-section, the decrease of the inherent stresses amounts to about 8.0 MPa.

The difference of hardness between a 20 mm. crosssection and a 40 mm. cross-section is reduced, in the case of a cooling according to the present invention, from 37 HB to 7 HB.

a 1 2 2 12 4 25) -8

Claims

Patent Claims

1. 'Process for the accelerated.cooling of cast parts subsequent to the solidification, wherein the cast part, immediately after the greatest wall thickness thereof ---hasgone below the solidus temperature, is separated from the mould and subsequently cooled at a rate of 18 to 20 K/m., the cast part, upon reaching the temperature range of the transition from the preponderantly plastic to the preponderantly elastic work material condition in the greatest wall thickness is heated as quickly as possible to a temperature in the range below the eutectic transition and thereafter--- immediately cooled at 70 to 90 K/min. to the manipulation temperature or to ambient temperature.

2. Proces according to claim 1 for the accelerated cooling of cast parts, substantially as hereinbefore described and exemplified.

3. Cast parts, whenever produced by the process according to claim 1 or 2.

Published 1989 atThePatentOffice, State House, 66.171 HighHolborn, London WC1R 4TP.Further copies maybe obtainedfrom The PatentOffice. Sales Branch, St Marv Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray. Kent, Con. 1/87