Method of Producing Thin-Walled Castings

The present invention relates to foundry practice and, more particularly, to methods of producing thin-walled castings.

Widely known is a method of producing castings from high-strength cast iron with nodular graphite by pouring inoculated cast iron into moulds and subsequently cooling the cast iron.

However, the high susceptibility of cast iron with nodular graphite to shrinkage defects (shrinkage holes and pores) necessitates the use of shrinkage heads which results in serious technological problems and is actually impractical for intricate castings of the cylinder-block type.

Moreover, for producing castings by the above-described method cast iron with a large content of expensive magnesium (at least 0.025%) must be used.

The object of the present invention is to provide a method of producing thin-walled castings from high-strength cast iron which ensures, the production of more light-weight (by 25-40%) castings and a lower consumption of materials required for their production.

Another object of the invention is the production of dense castings free from shrinkage holes and pores without using shrinkage heads, and featuring high mechanical properties.

These and other objects are accomplished by the provision of a method of producing thin-walled castings by pouring inoculated cast iron moulds and subsequently cooling the cast iron. in which method, according to the invention, the mould is poured with inoculated cast iron containing, by weight percent: carbon 3.6-3.8, silicon 2.8-3.2, manganese up to 0.4, magnesium not more than 0.01 with the melt temperature ranging from 1420 to 1450.degree. C and wall thickness chosen so that the initial cooling rate is 10.degree.-14.degree. C/sec for a preset heat conductivity.

The herein-proposed method enables the production of intricate castings without using shrinkage heads, with thin, about 3 mm thick, walls made from high-strength magnesium cast iron with dense massive portions (without cavities) and rugged thin walls.

The present invention will be better understood from a consideration of a detailed description of an exemplary embodiment thereof, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the rate of cooling of a casting, according to the invention;

FIG. 2 shows the dependence of mechanical properties on wall thickness of the casting;

FIG. 3 shows the structure of thin walls of the casting produced according to the invention; and

FIG. 4 illustrates the structure of thick walls of the casting produced according to the invention.

The herein-proposed method consists in the following:

Cast iron is melted by the known methods in melting units, its composition being brought up to the following weight percent: carbon 3.6-3.8, silicon 2.8-3.2, manganese up to 0.4 and phosphorus up to 0.08. This composition is characterized by a higher content of carbon and silicon. Then the cast iron is inoculated either by dopants in a ladle or by magnesium in an autoclave. Peculiar to the inoculation process is the use of a spheroidizing modificator in reduced doses so that the contents of residual magnesium in the castings is not more than 0.01% by weight with the sulphur contents amounting up to 0.002% by weight. Usually for producing cast iron with nodular graphite the contents of residual magnesium is assumed to be not less than 0.025% by weight with the above-specified sulphur contents. To prevent chilling, the spheroidizing modificator is introduced into the pouring ladle, and the moulds, for example, of an engine cylinder block with walls 3.+-.0.3 mm thick are poured at a temperature of 1430.degree.-1450.degree. C. As a rule, the minimum wall thickness of the automobile cylinder blocks amounts to 5-6 mm.

For obtaining castings with excellent mechanical properties and lower weight, it is necessary to have thin walls (which determine the strength of the castings) made from cast iron with nodular graphite. The thick parts of the castings, which are usually subject to machining and serve for fastening other elements to the castings, are made from cast iron with flake graphite. This will eliminate the formation of shrinkage holes and enhance the machinability of the castings as compared with those having nodular grapite throughout their sections.

It is common knowledge that high cooling rates contribute to the formation of nodular graphite. Therefore with a reduction in the wall thickness (from 5-6 mm to 3 mm) the process of heat transfer is carried out at a higher rate with an ensuing increase in the cooling rate. The formation of nodular graphite requires high cooling rates during the first two minutes, insofar as on completion of the process of spheroidization of the cast iron the rate of cooling of the castings does not affect the structural state of graphite.

The initial rate of cooling of the casting walls, 3 mm thick, therefore amounts to 10.degree.-14.degree. C/sec and that of the thick portions is equal to 1-4.degree.C/sec.

FIG. 1 shows cooling rate curves for the walls of different thickness. Plotted against the horizontal axis is time .upsilon. in minutes.

Plotted against the vertical axis is the rate of cooling (.degree.C/sec) of the walls of various thicknesses. Curve A gives the cooling rate for a wall 3 mm thick, curve B for a 10-mm thick wall and curve C for a wall 40 mm thick. A comparison of these curves shows that maximum difference in the wall cooling rates is clearly manifest during the first 2 minutes, i.e. at the moment when the structural state of graphite is formed in both the thin and thick walls of the casting.

Subsequently the cooling rate curves converge abruptly, the difference between them being negligibly small, and reach the rate of cooling amounting to 0.5.degree.-2.0.degree. C/sec. With low magnesium contents in the cast iron a high cooling rate will contribute to the formation of nodular graphite in thin walls, and with a considerably lower cooling rate, gray cast iron with flake graphite will form in the thick walls. As a result, the thin walls of the casting will have the structure of high-strength inoculated cast iron and the thick walls that of gray cast iron which excludes shrinkage appearance of shrinkage defects in massive units of the casting without using the heads. The material of the thin walls of such a casting features excellent mechanical properties which ensures high strength and rigidity of the part with the wall thickness of up to 3 mm instead of 5-6 mm usually employed.

Referring to FIG. 2, a graph showing the dependence of the resistance to rupture and hardness on the wall thickness of the casting is depicted. Plotted against the X-axis is the wall thickness of the casting in mm. The left-hand vertical axis shows the resistance to rupture in MH/m.sup.2 and the right-hand vertical axis shows hardness H.sub.b. Curve D shows the dependence of hardness on the wall thickness and curve E is a strength-wall thickness curve.

FIG. 3 shows the microstructure of cast iron with nodular graphite in 3 mm thick walls of the casting.

Shown in FIG. 4 is a microstructure of cast iron with flake graphite in the thick walls of the casting.

Hence, the herein-proposed method enables a 20-40% reduction in weight of the castings due to reduction in the wall thickness of 3 mm so as to eliminates shrinkage holes and pores without using shrinkage heads. It also enables the consumption of the spheroidizing modificator to be decreased and the machinability of the castings to be enhanced as compared to those of the castings in high-strength cast iron with nodular graphite produced by the prior-art methods.