US2759812A - Method of manufacturing castings of cast iron of refined graphite structure - Google Patents

Description

United States Pate t METHOD OF MANUFACTURING CASTINGS OF CAST IRON OF REFINED GRAPHITE STRUC- TU Hiroshi Sawamura, Sakyo-ku, Kyoto, Japan No Drawing. Application May 15, 1952, Serial No. 288,049

C aims p y appl ion apan M y 9, 1 1

2 Claims. c1. 15-433 This invention relates to a new process of manufacturing eastings of cast iron eharacterized by a highlyrefined eutectic graphite structure, the process involving the use of molten slag containing at least 1.0% of titanium in the form of titanium dioxide whichis caused to eontact molten pig iron or a bath of cast iron under suitable conditions It is an objt ct of this invention to provide a process for producing castings of gray cast iron which possess enhanced mechanical and structural properties.

It is another object of this invention to provide means facilitating a highly economical production method for gray cast iron with pronounced resistance to growth or graphite precipitation at elevated temperature;

It is further an object of this invention to provide means ensuring production of gray east iron of highly refined homogeneous eutectic structure presenting considerably reduced permeability and preventing leakage under severe conditions of use, in particular when articles made of such cast iron are subjected to gas-, airand hydraulic-pressure.

Th P e e t inve o co t l tes he prqvi iqn. o means enabling the use of slag-forming material or molten slag with approximately 1.0% of" titanium or more in h arm f. a ta ium c m o d or e i a a t ea i s mo n P i o o e t i qn unde P s ib o dit sx f. w ere after he t ment asse s sampl -id. he esu a mo en me a ma a in s nd m u ds. meta mo d r t e l k The present invention is further direeted to castings 0f a ron a in a ghly refine eu e t graphite structure, in contra-distinction to the common eoarse graphite structure (exemplified in Graphite FlaheType Chart No A in A F. S.A. S. M Graphite Ciassificatiori in Gray Cast Iron).

In accordance with the practice of the present invention, molten; cast iron, after having been treated with molten slag containing TiOz under strong reducing conditions, was cast in moulds and investigati o n of its structure and composition revealed'the following facts:

(1) Where the titanium absorbed bythe cast iron melt from the molten slag exceeds approximately 0.2% of said cast iron melt, substantially the entire casting presents a eutectic graphite structure.

Where the amount of Ti absorbed is less than about 0.2%, the treated casting is either of entirely common flaky r ph t uc ur a 1 f P r flaky t rh r c u e an a t euteetis rap it structure- (2) The content of N2 in the cast iron has no effect upon the production of a homogeneous eutectic graphite structure throughout the entire casting.

In a word, the production of cast iron. characterized by a homogeneous and substantially completely eutectic graphite structure according to the practice of the present invention appears to reside in the fact that the content of Ti in the treated cast iron slightly exceeds about 0.2%.

Patented Aug, 21, 195.6

Assuming that the temperature of a bath of cast iron which has normal composition aside from Ti is 1450""C., and, at this temperature, comparing the chemical afilnity of N2 or C to the various components contained in the cast iron, the chemical afiinity to Ti is the strongest, (Kelley, U. S. Bureau of Mines Bulletin 407), andthe chemical afiinity of C to Ti is stronger than that of N2 to Ti.

Moreover, the concentration of C in the bath of cast iron is incomparably greater than that of N2.

Therefore, also taking into account the result of analysis of N2 in the cast iron, it can be presumed that, in the bath of cast iron almost all of the Ti exists as TiC.

From the ternary equilibrium diagram of Fe-C-Ti by Tofaute-Biittinghaus (Arch, Eisenhiitten w., 12,193 8,33,), it is seen that the ternary eutectic composed of austenite, TiC, and FesC is solidified at temperatures slightly below 1145 C., corresponding to a composition of C=3.9% and Ti=0. 8%.

Although there is a considerable amount of various impurities" contained in the normally-composed cast; iron q l e ing m re h abou .2 of e is co sidered to take a more complicated solidifying curve than that shown in the said diagram, the major portion of the eutectic formed when the cast iron of this kind is solidified ispresumed to be mixed with the independent solid phase of TiC.

It is understood that the graphite is refined because this TiC is scattered as very fine granules in the eutectic mixture and prevents the development of crystals of Pest? or of crystals of graphite, the latter resulting from decomposition of FesC.

It is still unknown why TiC prevents the development of 'crystals of FesC and of graphite.

However, the latter effect seems to have something in common with phenomenon that when Ti is added to steel forming TiC, the austenite-crystal-grains are refined (See: Comstock-Clark: Metals of Alloys, 8, 1937,42, etc.) or the growth of austenite-crystal-grains containing TiC is impeded. ('See: Comstock: T. A. S. M., 281940608, etc.)

Even if the graphitization of cast iron were primarily brought about by the crystallization of graphite from the melt, the reason for the resulting refined graphite structure can, be .,a5ily understood in terms of theexplauation given hereinabove in connection with the efiect of the addition of titanium to steel.

As for the temperature of the molten pig iron and the molten slag on contact, it is noted that this need not be particularly high since a preferred temperature range of 1 400.C. to 147.0" C. has proven satisfaetory.

The higher temperature limit may be more eifective for obtaining a eutectic structure because it will increase the amount of Ti which is carried from the molten slag to the bath, and, consequently, increases the formation of fine granules of TiC in the bath.

It is further noted that it makes little difference whetherthe molten slag and the bath are brought into contact at the same temperature, or they are brought into contaet at somewhat different temperatures.

Oficourse, the bath of east iron and the molten slag are at substantially the same temperature in a furnace.

Several illustrative examples of the practice of this invention are given as follows:

Braw 1 In ordertomakaa. steel ingot-mould', 50 kg. of molten pig iron, which was melted in a 15-ton cupola, was cast in a green mould of 25 mm. dia.

The composition of the sample is as follows: C 3.7%, Si 1.5%, M11 0.6%, S 0.06%, P 0.24%.

Result: the graphite structure of the sample was of the common very coarse flaky variety and its tensile strength is 14.7 kg./sq.mm.

Example 2 Immediately after 50 kg. of the same molten metal as in Example 1 above was put into a basic electric furnace (capacity: 250 kw.), suitable amounts of clay, silica sand, and lime were charged thereupon, forming a slag of the following composition: SiOz 37.8%, CaO 46.0%, A1203 9.8%, MgO 4.3%, FeO 2.1%.

Then, while maintaining the molten pig iron at approximately 1400" C., the molten slag was kept in contact with the molten pig iron for about 15 minutes, and the resultant molten product was cast in a green mould as above.

The amount of slag was 10% of the molten pig iron.

Result: the graphite structure of the sample tested was almost as coarse and flaky as that of Example 1, and its tensile strength was 14.9 kg./sq.mm.

Example 3 A procedure similar to Example 2 was followed, with the difference that ilmenite was added for the purpose of making a molten slag of the following composition: SiOz 29.4%, CaO 48.3%, A1203 10.2%, MgO 2.8%, TiOz 8.0%, FeO 1.3%.

Result: The sample tested possessed a highly refined eutectic graphite structure, and its tensile strength was as high as 22.8 kg./sq.mm.

Thus, cast iron having a highly refined eutectic graphite structure may be obtained when the molten pig iron is cast in the mould after subjecting it to contact with molten slag at about 1400 C., which slag preferably contains an amount of TiOz corresponding to at least 1% of titanium.

In order to obtain such a cast iron product of highly refined eutectic graphite structure, it is, of course, necessary to vary the contact time in accordance with the basicity of slag and the content of TiOz as well as the total amount of slag.

For example, when the molten slag of normal composition is processed in a 250 kw. electric furnace, the following proportions and contact times are preferred:

Basicity of Contact Amount of molten slag (ratio to TiOz molten slag tune molten pig iron), percent (percent) (OaQ per- (mincent/S102 perutes) cent) The tensile strength of the samples obtained from the above three examples was determined as follows:

Therefore, the tensile strength of the sample which was obtained using the method of the present invention is much higher than that obtained by the normal method.

Comparing the graphite structure of the cast iron of Example 1 with that of the cast iron of Example 3, the latter is much finer than the former and accordingly, it is clear that the resistance of the latter sample of cast iron to growth or graphite precipitation at elevated temperatures is much greater than that of the former.

Consequently, the castings obtained by the practice of the present invention are particularly suitable for a mould for steel ingot which requires rigidity, toughness, and heatresisting qualities.

Furthermore, the present invention is most useful in manufacturing castings of gray cast iron intended for exposure to an oxidizing atmosphere in high temperature range.

Moreover, it is observed that a casting having a finegrained eutectic structure, as provided by the practice of the invention, is of such a dense and compact nature as to minimize gasor water-leakage in comparison with a normal casting subjected to similar conditions of high airor high water-pressure.

Accordingly, this invention can be most eifectively utilized in the manufacture of castings of gray cast iron which are required to be gasor water-proof.

Since certain modifications may be made in the process of the present invention, it is intended that all matter contained in the foregoing specification be interpreted merely as illustrative and not in a limiting sense.

Having thus described the invention what is claimed as new and desired to be obtained by Letters Patent, is:

1. The method of producing articles of gray cast iron comprising the steps of maintaining molten cast iron at a temperature ranging from approximately 1400 C. to 1470 C., and subjecting said molten cast iron within said temperature range to contact with molten slag containing at least 1% of titanium in the form of titanium dioxide, to thereby obtain upon solidification of the resultant cast iron an article of fine-grained eutectic graphite structure and resistance to graphite precipitation at relatively elevated temperatures.

2. The method of producing articles of gray cast iron comprising the steps of maintaining molten pig iron at a temperature ranging from approximately 1400 C. to 1470 C., contacting said molten pig iron within said temperature range with a molten basic slag, said slag containing at least 1% by weight of titanium in the form of titanium oxide and having a calcium oxide to silica ratio in excess of 1, thereafter pouring said treated pig iron into a mold, and allowing said treated pig iron to cool, whereby upon solidification gray cast iron is obtained wherein substantially all of the carbon is present as eutectic graphite.

References Cited in the file of this patent UNITED STATES PATENTS 609,467 Rossi Aug. 23, 1898 830,536 Slocum Sept. 11, 1906 1,955,791 Comstock Apr. 24, 1934 OTHER REFERENCES Stahl und Eisen: vol. 60; Jan. 4, 1940, page 13.

Claims (1)

1. 2. THE METHOD OF PRODUCING ARTICLES OF GRAY CAST IRON COMPRISING THE STEPS OF MAINTAINING MOLTEN PIG IRON AT A TEMPERATURE RANGING FROM APPROXIMATELY 1400* C, TO 1470* C., CONTACTING SAID MOLTEN PIG IRON WITHIN SAID TEMPERATURE RANGE WITH A MOLTEN BASIC SLAG, SAID SLAG CONTAINING AT LEAST 1% BY WEIGHT OF TITANIUM IN THE FORM OF TITANIUM OXIDE AND HAVING A CALCIUM OXIDE TO SILICA RATION EXCESS OF 1, THEREAFTER POURING SAID TREATED PIG IRON INTO A MOLD, AND ALLOWING SAID TREATED PIG IRON IS OBCOOL, WHEREBY UPON SOLIDIFICATION GRAY CAST IRON IS OBTAINED WHEREIN SUBSTANTIALLY ALL OF THE CARBON IS PRESENT AS EUTECTIC GRAPHITE.