US1972103A - Process for treating silicon alloy castings - Google Patents

Description

I Patented Sept. 4, 1934 UNITED STATES- PATENT OFFICE PROCESS FOR TREATING SILICON ALLOY CASTINGS York No Drawing. Application June 9, 1933, Serial No. 675,105

4 Claims. (Cl. 148 21.5)

.The invention relates to a process for treating high silicon iron alloy castings which have a composition rendering them highly resistant to corrosives and particularly chlorides, but which are 5 also normally extremely brittle because of their composition in such manner that the castings are much stronger and also more resistant to heat shock.

High silicon iron castings of approximately 14.35 per cent silicon consist structurally of a chemical compound of iron silicide dissolved in iron. Naturally, a solution of this type is subject to brittleness so highly characteristic of intermetallic compounds. Under the microscope 5 the alloy appears as a typical solid solution with equa-axed grains. If carbon is introduced into the iron-silicon alloy, some of it combines with the iron silicide to form a eutectoid mixture and if an excess of carbon exists, primary graphite is also present.

The rate of solidification, as well as the total amount of carbon present, has a marked influence upon the relative amounts and distribution of the iron silicide, eutectoid and primary graph- 5 ite. For instance, heavy castings poured in green sand, of an alloy consisting of 14.25 per cent-14.50 per cent silicon and about 0.80 per cent carbon, will exhibit under the microscope a solid background of white iron silicide traversed by sinuous lines or flakes of proeutectic or primary graphite.

If the rate of solidification is much greater, as in a thinner section, particles of iron silicide will be found imbedded in a matrix ofthe eutectoid, with some primary graphite also in evidence. It is also possible in castings of heavy section, to develop both types of these structures within the same section, the former type appearing in the center with the latter variety near the edges. Such a duplex structure is prone to develop internal stresses of appreciable magnitude within the casting. Primary graphite, especially when it appears in the form of flakes, sets up planes of additional weakness in the material, so that for maximum strength the preferred structure should consist of a sorbitic nature throughout.

The alloy which I use in my' castings is of the high silicon type, above discussed, containing about 14 per cent of silicon and a maximum of .90 per cent carbon, to which is added about 4 per cent of molybdenum. The presence of the molybdenum materially increases the corrosion resistance of the alloy by reinforcing the silicon. Molybdenum forms an oxide (M003), which is an acid anhydride, very similar to the oxide of silicon (SiOz) Due to these oxides, a compound film is built up on the surface of the casting which is very effective in protecting it from corrosives and particularly from concentrated chlorides. Molybdenum also forms a compound with iron, known as iron molybdide. The molybdenum is thus similar to silicon in several respects, and should, therefore, react in a complementary manner.

Heretofore, in making high silicon iron castings containing molybdenum, difiiculties have arisen due to the fact that the molydenum readily combines with carbon, and the presence of the molybdenum carbides thus formed, further increases the brittleness of the alloy. As a result, it is practically impossible to obtain castings from the sand which are free from cracks, and if free from cracks, the castings are materially weaker than the straight silicon iron alloys. I have found that by proper heat treatment, it is possible to overcomethe difliculties incident to iron silicon alloys containing molybdenum, and to produce castings which are not only superior in corrosion resistance, but which also have superior physical characteristics, as compared with the ordinary high silicon iron alloys.

In practicing the process, the castings are removed from their molds, while still at a red heat, and placed in a furnace previously heated to a temperature above 800 deg. F. The temperature is then elevated to 1500 deg. F., and held at this point for a period of from 15 to 30 hours. The heat supply is then cut off and slow cooling resorted to in the furnace to the point where the castings can be safely and conveniently handled. As a result of this treatment, the casting sections are of entirely uniform structure. In order to obtain this structure, it is necessary to cool very slowly, especially through the lower critical temperature in the neighborhood of 1100 deg. F. In other words, castings, when treated as above set forth, are not subject to the variations in structure with change in section, nor does the structure vary from outside to center in a given section, as is the case in plain iron silicon carbon alloys, as heretofore pointed out. Further, the castings, as produced by the improved process, are more readily cast in intricate shapes, are much stronger, and are more resistant to heat shock.

If the castings were allowed to cool to room temperature in the molds without the above heat treatment, the rate of cooling is so rapid that the transformation temperature is correspondingly reduced, similar to that of steel when quenched. The alpha iron is thereby prevented bides.

(due to its rigid characteristics) from dissolving carbon combined with the molybdenum. The slower rate of cooling which occurs in my process, elevates the transformation point to a value comparable with the theoretical (1100 deg. F.), so that the precipitated carbides are larger, and begin to take on the form of a troostitic or sorbitic formation, rather than the cementitic structure of the casting which is cooled directly in the mold.

The molybdenum carbide is graphitized by the soaking treatment above and below the critical temperature, and the result is a completely homogeneous structure similar to troosite or sorbite. That is, the iron molybdide and iron silicide forms this type of structure with free carbon without the existence of any molybdenum car- In this way, the desirable properties of the alloy, due to the presence of molybdenum, with respect to corrosion resistance are obtained, without the deleterious effects of molybdenum carbides, which would ordinarily materially increase the tendency of the alloy to crack and decrease the strength and resistance to heat shock.

While the proportions of silicon molybdenum and carbon, as heretofore given, are the preferred ones, these proportions may vary over a considerable range, while still retaining in the alloy in a large degree, the improved characteristics as above set forth, when the alloy castings are treated as heretofore described. These proportions may vary within the following limits:

Silicon 7 to 18% Molybdenum .1 to 10% Carbon .25 to 1.5%

In the event that primary or excess graphite is present over and above that necessary for the eutectoid small amounts of nickel added to the alloy serve to control the type and distribution. The invention, therefore, contemplates the addition of nickel to the formula heretofore given, and this may range up to two per cent. The desirable eifect of the heat treatment heretofore described obtains equally well with this addition of nickel.

Vanadium or tungsten may be substituted for molybdenum, and the improvement in the castings under the heat treatment described will be similar, but the molybdenum ispreferred because thecorrosion resistance is greater than where vanadium or tungsten are used. It will also be understood that the invention contemplates combinations of these metals, although the use of the molybdenum alone is best. When the metals tungsten and vanadium are substituted for the molybdenum, the quantity used is the same as stated for the molybdenum.

It will also be understood that the invention comprehends the use of other elements in the alloys, in such relatively small quantities as will not materially affect the character of the alloys. Small amounts of sulphur, phosphorous, manganese and the like, which occur more or less as impurities, will also be understood to be comprehended by this invention.

What I claim is:

1. A process for treating silicon alloy castings containing 7.5 to 18 per cent of silicon, and which also include .1 to 10 per cent of molybdenum and .25 to 1.5 per cent of carbon, which consists in removing the castings from their molds while at approximately a red heat, raising the temperature of such castings to about 1500 deg. F., permitting them to soak at such temperature, and then slowly cooling the castings to handling temperature.

2. A process for treating high silicon iron alloy castings which include .1 to 10 per cent of molybdenum and .25 to 1.5 per cent of carbon, which consists in removing the castings from their molds while at approximately a red heat, raising the temperature of such castings to about 1500 deg. F., permitting them to soak at such temperature, and then slowly cooling the castings to handling temperature.

3. A process for treating high silicon iron alloy castings which include .1 to 10 per cent of molybdenum and .1 to 3 per cent of metal belonging to the'group nickel and cobalt, and .25 to 1.5 per cent of carbon, which consists in removing the castings from their molds while at approximately a red heat, raising the temperature of such castings to about 1500 deg. F., permitting them to soak at such temperature, and then slowly cooling the castings to handling temperature.

4. A process of treating silicon iron alloy castings containing about 14 per cent silicon, and which also includes about 4 per cent of molybdenum and .9 per cent of carbon, which consists in removing the castings from their molds while at approximately a red heat, raising the temperature of such castings to about 1500 deg. F., permitting them to soak at such-temperature, and then slowly cooling the castings to handling temperature.

JAMES A. PARSONS.